Renewable Energy Sources by M A Qadeer

M A Qadeer Siddiqui Renewable Energy Sources

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By

Md Abdul Qadeer Siddiqui


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RENEWABLE ENERGY SOURCES

Md Abdul Qadeer Siddiqui B-Tech (Automobile Engineering)

Bhaskar Engineering College (JNTU Hyderabad)


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Dedicated to... My friends Azeem, Akram and Imran for motivating me and ………………….

To my teacher Sandhya Rani Mam for Her blessings……….


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Preface

This book ―Renewable Energy Sources‖ caters the need of JNTU-H specially. Each

topic is explained in simple way to make student understand and comprehend the

subject.

Effects of environmental, economic, social, political and technical factors have led to the

rapid deployment of various sources of renewable energy-based power generation. The

incorporation of these generation technologies have led to the development of a broad

array of new methods and tools to integrate this new form of generation into the power

system network. This book, arranged into eight sections, tries to highlight various

renewable energy based generation technologies.

Renewable Energy Resources is a numerate and quantitative text covering subjects of proven

technical and economic importance worldwide. Energy supply from renewables is an

essential component of every nation‘s strategy, especially when there is responsibility for

the environment and for sustainability. This book considers the timeless principles of

renewable energy technologies, yet seeks to demonstrate modern application and case

studies. Renewable Energy Resources supports multi-disciplinary master degrees in science

and engineering, and also specialist modules in science and engineering first degrees.

Moreover, since many practicing scientists and engineers will not have had a general

training in renewable energy, the book has wider use beyond colleges and universities.

Each chapter begins with fundamental theory from a physical science perspective, then

considers applied examples and developments, and finally concludes with a set of

problems and solutions. The whole book is structured to share common material and to

relate aspects together. Therefore the book is intended both for basic study and for

application.

The corrections, suggestions and feedbacks from the readers are always appreciated and duly acknowledged. You can reach the author at [email protected]


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CONTENTS

1) PRINCIPLE OF SOLAR RADIATION ………………………………….5

2) SOLAR ENERGY COLLECTION ………………………….………….13

3) SOLAR ENERGY STORAGE AND APPLICATION ………..21

4) WIND ENERGY ………………………………………………………35

5) BIO MASS …………………………………………………………45

6) GEROTHERMAL ENERGY………………………63

7) OCEAN ENERGY………………………….70

8) DIRECT ENERGY CONVERSION ………………88


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CHAPTER 1)

PRINCIPLES OF SOLAR RADIATION

ROLE AND POTENTIAL OF NEW AND RENEWABLE ENERGY

ROLE

A broad range of different global energy scenarios confirms that the exploitation of

energy efficiency potentials and the use of renewable energies play a key role in reaching

global CO2 reduction targets. In scenarios aiming at the stabilisation of greenhouse gas

concentration at 450 ppm equivalent, the contribution of renewables to global primary

energy supply reaches between 31% (Greenpeace/EREC, Energy [R]evolution) and 23%

(IEA World Energy Outlook 2008, 450 Policy Scenario) in 2030. By 2050, renewables

are expected to contribute between 56% (Greenpeace/EREC, Energy [R]evolution) and

35% (IEA Energy Technology Perspectives, BLUE Map) to primary energy supply.

Differences in the share of renewables across scenarios are partly due to different

assumptions on the potentials for increasing energy efficiency. Compared to other

scenarios, the more ambitious reduction of energy demand in the Greenpeace/EREC

Energy [R]evolution scenario facilitates higher shares of renewables in energy

consumption. In the IEA scenarios, also nuclear and fossil technologies with carbon

capture and storage (CCS) are considered to be essential elements for achieving the

climate protection targets. In particular the use of CCS in the IEA WEO scenario leadsto

high CO2 abatement costs, as CCS is not expected to gain economic competitiveness

before 2030. It remains unclear what constraints the market uptake of more cost effective

renewable options in the IEA scenarios. While all scenario studies analysed provide a

wealth of detailed information on various technical and economic issues, there is a

general lack of reporting key assumptions in a comprehensive and transparent way,

which sometimes makes comparison across studies difficult. All studies analysed are

particularly weak in providing data on the heating sector, which in spite of its large

contribution to fuel consumption and CO2 emissions in general is treated as a second

priority only. A more transparent documentation of basic assumptions and constraints is

desirable for any future scenario work.

POTENTIAL

The largest electricity generation potential on a global scale is seen for the solar

technologies concentrating solar thermal power plants (CSP) and PV, followed by wind


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onshore and ocean energy. The global potential for direct thermal use of solar and

geothermal energy several times exceeds global low temperature demand.

The potential for CSP and PV electricity generation is particularly large in Africa.

Wind onshore potentials are high in North America, while Latin America has abundant

biomass resources. Current global final energy consumption is less than 5% of the overall

projected technical renewable energy potential.

SOLAR ENERGY OPTION

ENVIRONMENTAL IMPACT OF SOLAR POWER

The sun provides a tremendous resource for generating clean and sustainable electricity

without toxic pollution or global warming emissions.


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The potential environmental impacts associated with solar power — land use and habitat

loss, water use, and the use of hazardous materials in manufacturing — can vary greatly

depending on the technology, which includes two broad categories: photovoltaic (PV)

solar cells or concentrating solar thermal plants (CSP).

The scale of the system — ranging from small, distributed rooftop PV arrays to large

utility-scale PV and CSP projects — also plays a significant role in the level of

environmental impact.

Physics of Sun

The sun is a huge ball of hot gas subject to the action of gravitational forces that tend to

make it shrink in size. This force is balanced by the pressure exerted by the gas, so that

an equilibrium size prevails. The core of the sun which extends from the centre to about

20% of the solar radius is at an extremely high temperature of around 15.7×106 K and

pressure of 340 billion times earth‘s air pressure at sea level. Under these extremes, a

nuclear fusion reaction takes place that merge four hydrogen nuclei or protons into an α-

particle (helium nucleus), resulting in the production of energy from the net change in

mass due to the fact that the alpha particle is about 0.7%less massive than the four

protons. This energy is carried to the surface of the sun in about a million years, through

a process known as convection, where it is released as light and heat (Hufbauer 1991).

Figure shows the internal structure of the sun: the radiative surface of the sun, or

photosphere, is the surface that emits solar radiation to space and has an average

temperature of about 5,777 K. Localized cool areas called sunspots occur in the

photosphere. The chromosphere (around 10,000 K) is the region where solar flares

composed of gas, electrons, and radiation erupts.


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The solar constant

The solar constant is defined as the quantity of solar energy (W/m²) at normal incidence

outside the atmosphere (extraterrestrial) at the mean sun-earth distance. Its mean value is

1367.7 W/m². The spectral distribution is given in the figure. At shorter wavelengths

you can see the Fraunhofer absorption lines.

Extraterrestrial and Terrestrial Solar Radiation

Extraterrestrial radiation

Extraterrestrial radiation ( ) is the intensity (power) of the sun at the top of the Earth‘s

atmosphere. It is usually expressed in irradiance units (Watts per square meter) on a

plane normal to the sun. It varies throughout the year because of the Earth‘s elliptical

orbit, which results in the Earth-Sun distance varying during the year in a predictable

way. This effect can be represented empirically with the following equations:

, where is the solar constant ( ). is the

mean sun-earth distance and is the actual sun-earth distance depending on the day of

the year.

, where ,

where is the day of the year (integer).


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Terrestrial radiation

energy emitted from the Earth and atmosphere detectable both day and night

http://pvpmc.org/wp-content/uploads/2012/04/Ea_DOY.png


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Earth's ambient temperature - 300K Earth radiates 160,000 times less than the sun

essentially all energy is radiated at (invisible) thermal infrared wavelengths between 4-25um

maximum emission occurs at 9.7um

Solar Radiation on Tilted Surface

The power incident on a PV module depends not only on the power contained in the

sunlight, but also on the angle between the module and the sun. When the absorbing

surface and the sunlight are perpendicular to each other, the power density on the surface is equal to that of the sunlight (in other words, the power density will always be

at its maximum when the PV module is perpendicular to the sun). However, as the angle between the sun and a fixed surface is continually changing, the power density on a fixed

PV module is less than that of the incident sunlight.

The amount of solar radiation incident on a tilted module surface is the component of

the incident solar radiation which is perpendicular to the module surface. The following figure shows how to calculate the radiation incident on a tilted surface (Smodule) given either the solar radiation measured on horizontal surface (Shoriz) or the solar radiation

measured perpendicular to the sun (Sincident).

Instruments for measuring solar radiation

Instrumentation Actinometer is the general name for any instrument used to measure the intensity of

radiant energy, particularly that of the sun. Actinometers are classified according to the

quantities that they measure:


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A pyrheliometer measures measures the intensity of direct solar radiation. It is so

designed that it measures only the radiation from the sun's disk (which has an apparent

diameter of ½°) and from a narrow annulus of sky of diameter 5° around the sun's disk.

A pyranometer measures global radiation (the combined intensity of direct solar

radiation and diffuse sky radiation). It measures solar irradiance from the solid angle 2pi

onto a plane surface. When mounted horizontally facing upwards it measures global

solar irradiance. If it is provided with a shade that prevents beam solar radiation from

reaching the receiver, it measures diffuse solar irradiance.

A pyrgeometer measures the effective terrestrial radiation. It measures the atmospheric

infrared radiation spectrum that extends approximately from 4.5 µm to 100 µm.

A multi-filter rotating shadowband radiometer measures direct normal, diffuse

horizontal and total horizontal solar irradiance.

A radiometer is an instrument designed to measure the radiated electromagnetic power.

When used in solar energy applications, it is usually desirable for radiometers to respond

the same to equal amounts of energy at all wavelengths over the wavelength range of the

radiation to be measured. Most radiometers therefore work by using a thermopile to

measure the temperature rise of a sensitive element whose receiving surface is painted

dull black. Instruments for measuring solar irradiance using a photovoltaic cell as the

sensitive element have a non-uniform spectral response.

An alternative method of measuring solar radiation, which is less accurate but also less

expensive, is a sunshine recorder. Sunshine recorders measure the number of hours in

the day during which the sunshine is above a certain level (typically 200 mW/cm2). Data

collected in this way are used to determine the solar insolation by comparing the

http://www.theenergylibrary.com/node/419


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measured number of sunshine hours to those based on calculations and including several

correction factors.

Campbell Stokes Recorders are the instruments that are used to measure sunshine. These

are one of the most important tools in meteorology. They are just important as the rain

gauge for example.

SOLAR ENERGY RADIATION DATA

According to Mathias Aarre Maehlum, Energy Informative, one year‘s worth of solar

energy (radiant light and heat from the sun) reaching the surface of the earth would be

twice the amount of all non-renewable resources, including fossil fuels and nuclear

uranium. The solar energy that hits the earth every second is equivalent to 4 trillion 100-

watt light bulbs. Furthermore, the solar energy that hits one square mile in a year is

equivalent to 4 million barrels of oil. Thus, the potential of solar energy is immense.


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CHAPTER 2)

SOLAR ENERGY COLLECTION

Glazed flat-plate collectors

Glazed flat-plate collectors are very common and are available as liquid-based and air-

based collectors. These collectors are better suited for moderate temperature

applications where the demand temperature is 30-70C and/or for applications that

require heat during the winter months. The liquid-based collectors are most commonly

used for the heating of domestic and commercial hot water, buildings, and indoor

swimming pools. The air-based collectors are used for the heating of

buildings, ventilation air and crop-drying.

Glazed flat-plate collector

In this type of collector a flat absorber efficiently transforms sunlight into heat. To

minimize heat escaping, the plate is located between a glazing (glass pane or transparent

material) and an insulating panel. The glazing is chosen so that a maximum amount of

sunlight will pass though it and reach the absorber.

Unglazed flat-plate solar collectors

In North America unglazed flat-plate collectors currently account for the most area

installed per year of any solar collector. Because they are not insulated, these collectors

are best suited for low temperature applications where the demand temperature is below

30C. By far, the primary market is for heating outdoor swimming pools, but other

markets exist including heating seasonal indoor swimming pools, pre-heating water for

car washes and heating water used in fish farming operations. There is also a market

potential for these collectors for water heating at remote, seasonal locations such as

summer camps.


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Unglazed flat-plate collectors

Unglazed flat-plate collectors are usually made of black plastic that has been stabilized to

withstand ultraviolet light. Since these collectors have no glazing, a larger portion of the

sun‘s energy is absorbed. However, because they are not insulated a large portion of the

heat absorbed is lost, particularly when it is windy and not warm outside. They transfer

heat so well to air (and from air) that they can actually ‗capture‘ heat during the night

when it is hot and windy outside!

Unglazed perforated plate collectors

The key to this type of collector is an industrial-grade siding/cladding that is perforated

with many small holes at a pitch of 2-4 cm. Air passes through the holes in the collector

before it is drawn into the building to provide preheated fresh ventilation air. Efficiencies

are typically high because the collector operates close to the outside air temperature.

These systems can be very cost-effective, especially when they replace conventional

cladding on the building, because only incremental costs need be compared to the energy

savings.The most common application of this collector is for building ventilation air

heating. Other possible components for this system are: a 20-30cm air gap between the

building, a canopy at the top of the wall that acts as a distribution manifold, and by-pass

dampers so that air will by-pass the system during warm weather.Another application for

this collector is crop drying. Systems have been installed in South America and Asia for

drying of tea, coffee beans, and tobacco. Currently there is a project underway with

funding through the Technology Early Actions Measures (TEAM) for the monitoring of

10 crop drying systems in 8 countries in South and Central America and Asia.


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Back-pass solar collectors

Air-based collectors use solar energy to heat air. Their design is simple and they often

weigh less than liquid-based collectors because they do not have pressurized piping. Air-

based collectors do not have freezing or boiling problems. In these systems, a large solar

absorber is used to heat the air. The simplest designs are single-pass open

collectors. Collectors that are coated with a glaze can also be used to heat air for space

heating. This type of collector may be integrated in the building and combined with

thermal mass such as in the Trombe wall described at the National Renewable Energy

Laboratory.

Concentrating solar collectors

By using reflectors to concentrate sunlight on the absorber of a solar collector, the size of

the absorber can be dramatically reduced, which reduces heat losses and increases

efficiency at high temperatures. Another advantage is that reflectors can cost

substantially less per unit area than collectors.This class of collector is used for high-

temperature applications such as steam production for the generation of electricity and

thermal detoxification. These collectors are best suited to climates that have an

abundance of clear sky days and therefore are not so common in Canada. Stationary

concentrating collectors may be liquid-based, air-based, or even an oven such as a solar

cooker.There are four basic types of concentrating collectors:

1. Parabolic trough

2. Parabolic dish

3. Power tower

4. Stationary concentrating collectors

Air based solar collectors

The energy collected from air-based solar collectors can be used for ventilation air

heating, space heating and crop drying. The most common application in Canada is for

ventilation air heating. Three types of air-based collectors :

Type of collector Ventilation Air Heating Space Heating Crop Drying

Unglazed perforated plate Very Good Poor Very Good

Glazed flat-plate Good Poor Good

Back Pass Fair No Fair-Good

Trombe wall No Good No


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Designs for the first three collector types are simple. The collectors usually weigh less

than liquid-based collectors because they do not have pressurized piping. Another

advantage of air-based collectors is that they do not have freezing or boiling problems.

All four of these air-based collectors can be integrated into buildings and form part of a

building‘s envelope. These first three collectors are described in more detail on each of

their own pages; the trombe wall is described in detail at the U.S. National Renewable

Energy Laboratory web site. NRCan has developed two free software programs for

analysis of solar air heating systems. SWIFT (Solar Wall International Feasibility Tool)

can be used for job quotations and system design for ventilation air heating and crop

drying applications for the first three system types. RETScreen is useful for conducting

quick pre-feasibility studies of the cost effectiveness of proposed unglazed perforated

plate collector installations.

Batch solar collectors

One hundred years ago, water tanks that were painted black were used as simple solar

residential water heaters. Today their primary market is for residential water heating in

warm countries. In Canada, they can be effectively used in campgrounds and for

residential water heating in temperate climates such as Vancouver Island; during winter

the tanks must be protected from freezing or they must be drained. Modern batch

collectors have a glazing that is similar to the one used on flat plate collectors and/or a

reflector to concentrate the solar energy on the tank surface. Because the storage tank

and the solar absorber act as a single unit, there is no need for other components. On an

area basis, batch collector systems are less costly than glazed flat-plate collectors but also

deliver less energy per year.

Solar cookers

Though there are many types of solar cookers, all of them have a couple of basic

components:

Concentrator or lens to increase the available solar energy and insulation to

reduce heat loss.

Often there is also an oven type cavity to place food into for cooking. Hot dog cookers,

which do not need an 'oven' can also be made.Solar cookers are commonly able to reach

cooking temperatures of 90-150 C (200-300 F) and some can even reach 230 C (450 F)!

With these temperatures, it is possible to cook virtually any food as long as it is sunny

outside.Making and using solar cookers can be a fun and educational school project.

Liquid-based solar collectors

Liquid-based collectors use sunlight to heat a liquid that is circulating in a "solar loop".

The fluid in the solar loop can be water, an antifreeze mixture, thermal oil, etc. The solar

loop transfers the thermal energy from the collectors to a thermal storage tank. The type

of collector you need depends on how hot the water must be and the local climate. The

most common liquid-based solar collectors are:

http://www.retscreen.gc.ca/


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Glazed flat-plate

Unglazed flat-plate

Vacuum tube

Batch collector

Concentrating

Parabolic dish systems

A parabolic dish collector is similar in appearance to a large satellite dish, but has mirror-

like reflectors and an absorber at the focal point. It uses a dual axis sun tracker.

A parabolic dish system uses a computer to track the sun and concentrate the sun's rays

onto a receiver located at the focal point in front of the dish. In some systems, a heat

engine, such as a Stirling engine, is linked to the receiver to generate electricity.

Parabolic dish systems can reach 1000 °C at the receiver, and achieve the highest

efficiencies for converting solar energy to electricity in the small-power capacity range.

Parabolic dish collector with a mirror-like reflectors and an absorber at the focal

point

Parabolic trough system

Parabolic troughs are devices that are shaped like the letter ―u‖. The troughs concentrate

sunlight onto a receiver tube that is positioned along the focal line of the trough.

Sometimes a transparent glass tube envelops the receiver tube to reduce heat

loss. Parabolic troughs often use single-axis or dual-axis tracking. In rare instances, they

may be stationary. Temperatures at the receiver can reach 400 °C and produce steam for

generating electricity. In California, multi-megawatt power plants were built using

parabolic troughs combined with gas turbines.


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Parabolic trough system

Power tower system

A heliostat uses a field of dual axis sun trackers that direct solar energy to a large

absorber located on a tower. To date the only application for the heliostat collector is

power generation in a system called the power tower. A power tower has a field of large

mirrors that follow the sun's path across the sky. The mirrors concentrate sunlight onto a

receiver on top of a high tower. A computer keeps the mirrors aligned so the reflected

rays of the sun are always aimed at the receiver, where temperatures well above 1000°C

can be reached. High-pressure steam is generated to produce electricity.

Power tower system

Stationary concentrating solar collectors

Stationary concentrating collectors use compound parabolic reflectors and flat reflectors

for directing solar energy to an accompanying absorber or aperture through a wide

acceptance angle. The wide acceptance angle for these reflectors eliminates the need for

a sun tracker. This class of collector includes parabolic trough flat plate collectors, flat

plate collectors with parabolic boosting reflectors, and solar cookers. Development of the

first two collectors has been done in Sweden. Solar cookers are used throughout the

world, especially in the developing countries.


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Vacuum tube solar collectors

Vacuum (also ―evacuated‖) tube solar collectors are amongst the most efficient and most

costly types of solar collectors. These collectors are best suited for moderate temperature

applications where the demand temperature is 50-95C and/or for very cold climates such

as in Canada‘s far north. Like with glazed flat-plate solar collectors, applications of

vacuum tube collectors include heating of domestic and commercial hot

water, buildings, and indoor swimming pools. Due to their ability to deliver high

temperatures efficiently another potential application is for the cooling of buildings by

regenerating refrigeration cycles. Vacuum (also “evacuated”) tube solar

collectorsVacuum tube solar collectors have a selective absorber for collecting sunlight

that is in vacuum-sealed tube. Their thermal losses are very low even in cold climates.

Orientation of Solar Collector

For best performance, you need around 100 square ft of unshaded south-facing space for every kilowatt of electricity. The

amount of solar radiation will be less if your roof faces SE or SW.

Photo Credit: NREL.gov

The condition of your roof is important, too. Ensure the roof can hold the weight of the system (about 3-5lb per square foot) and

that the roofing material will last as long as the PV system (25-30 years).

http://solartribune.com/wp-content/uploads/2011/07/NREL.gov-location-of-solar-panels.png


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Solar thermal energy collector

Glazed plate solar collector


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CHAPTER 3)

SOLAR ENERGY STORAGE AND APPLICATIONS

Solar energy Storage

1) Thermal 2) Electrical 3)Chemical 4) Mechanical 5)Electromagnetic

1-Capacitor

2-Inductor 3-Battery 1-Chemical 2-Thermochemical

1-Sensible: a- Water storage

b- Pebble bed Storage 1-Pumped hydro electric 2- Compressed air 2-Latent Heat 3- Flywheel

LATENT HEAT STORAGE SYSTEM

Scheme of of a solarthermal power plant with

integrated storage capacity

.


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Latent heat storage system use the energy

absorbed or released during the isothermal phase

change of materials. A first selection of suited

storage materials is based on the melting

temperature which should be close to the

saturation temperature which is dependent on the

pressure of the steam. Since systems using direct

steam generation are operated at pressures

between 30 and 100bar the melting temperature

of phase change materials (PCM) should be in the range of 230°C and

330°C. Due to cost aspects metallic materials like tin or lead can‘t be used,

instead salts like sodium nitrate or potassium nitrate are typical candidate

materials. A direct contact between storage material and steam is not

possible, so thermal energy has to be transferred within the storage material.

Cost effective storage systems demand high internal heat fluxes, which

depend mainly on the heat conductivity of the storage material. Non-

metallic storage materials usually show low heat conductivities, especially

the solid phase behaves like a thermal isolator. Essential for the

development of cost effective storage systems is the increase of the effective

heat conductivity in the storage material. DISTOR include research on

three different concepts for PCM-storage:

innovative composite materials made from PCM and materials

showing a high thermal conductivity

minimization of the average distance for heat transfer within the

storage material, e.g. by encapsulation of the storage material

application of an intermediate heat transfer medium for transfer of

thermal energy between the pipes of the steam system and the storage

material

T-s diagramm power plant with integrated storage

capacity


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Stratified Storage

Theoretical principles:

How can the physical properties of the heating water in a stratified storage tank be optimally utilised?

Quite simple – by considering the following principle demands:

1. The layer system or other inserts built into the tank must not consist of metallic

materials. Metals are good heat conductors and would quickly compensate for/mix up different temperature layers again.

2. No heat exchangers may be located inside the heat exchanger: This "thermal stirrers" would generate unwanted currents and destroy temperature layers, or mix up layers with different temperatures.


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4. A largely calm ("laminar") inflow of relatively large amounts of water into the tank volume is only possible through multi-chamber systems: Depending on the velocity,

flows into pipes always generate overpressure or underpressure (venturi effect).

5. Preliminary sorting into different temperature levels (hot, warm, cool) is generally

necessary, as there would otherwise be dynamic, turbulent currents within the tank because of excessive temperature differences and at the same time great height

differences, also caused by thermal lift and descent rates (natural law of gravity).

Perfect implementation

Stratified storage tank OSKAR-10/... from ratiotherm The thermal hydraulic 5-chamber layer insert of OSKAR-10/... comprises a heat-resistant but bad heat conducting, that is, heat insulating plastic, and depending on the

tank size and required volume flow one or more basic main pipe/s with correspondingly sized internal "inflow and stratification chambers".

The supply or withdrawal of heat flows takes place via this layer insert with its sophisticated connection system, exactly suited to OSKAR-10/... and its areas of

application and also made of heat resistant, thick-walled plastic pipes with good thermal insulation properties; in other words: the charging and discharging of OSKAR-10/…

Because of the significantly larger cross-sections of this chamber system, compared to the

connecting pipes, the flow rate (dynamic, kinetic energy) of the storage medium water is

reduced to a minimum.

Due to the extremely slow flow of the water, which is hardly moving, and the integrated water deflections in the chambers, the effect of a calm lift or descent according to the

physical law of gravity (warm water is lighter than cold water), can develop without interference.

The classical problem:


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The classic problem:

Each supply flow mixes the entire tank content to a smaller or greater degree. This

quickly causes the tank to have a uniform temperature level. The temperature level is often not sufficient to operate the heating system or heat the domestic water.

The costly consequence of these systems is being reheated by the burner.

Why temperature layers?

The different heat generators (solar, wood, etc.) largely vary in their output rating and thus also deliver very different temperatures.

If they are not mixed but stored in layers, their energy content is fully preserved and can

be used advantageously for heating or domestic hot water heating.

Mechanical stratified storage tanks

often work with external tank connections at different heights.

The heat flows are then supplied directly to the tank content with different temperatures

using control technology.

The many lateral connections thereby constitute a drawback: They penetrate the tank insulation thereby causing increased "heat loss" of the tank.

Furthermore, detrimental effects on existing temperature layers in the tank can hardly be avoided. The necessary control technology for "mechanical" stratified storage tanks is

extremely expensive, depending on the system, and is the basis for the entire range of problems regarding maintenance, failure and wear.

Running costs for servicing and maintenance are inevitable.


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Thermal hydraulic stratified storage tanks

with single chamber systems and integrated heat exchangers for utilising solar power function relatively well.

The handling of larger volume flows and/or different return temperatures from

consumer circuits represents a problem, however, often causing a mixing of the temperature layers


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The thermal hydraulic stratified storage technology developed by ratiotherm

- with the 5-chamber layer inserts in OSKAR-10/...

or special layer inserts in ratiotherm special stratified storage tanks

- sorts out different temperature levels using a connection system especially suited

exactly to the respective storage tank type.

All connections are inserted into the tank at the bottom from underneath and emerge in

the corresponding temperature zone of the multi-chamber system of the layer insert

These calming and layer chambers let the heat flows rise and descend in the centre of the tank like in a "lift" in a completely calm and current-free manner.

The supply and withdrawal of heat flows into or out of the corresponding temperature layers of the tank volume range takes place according to temperatures.

In practice, most of the temperature zones are reached with this method.


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SOLAR POND

The sun is the largest source of renewable energy and this energy is abundantly available in all parts of the earth. It is in fact one of the best alternatives to the non-renewable sources of energy.

One way to tap solar energy is through the use of solar ponds. Solar ponds are large-scale

energy collectors with integral heat storage for supplying thermal energy. It can be use for various applications, such as process heating, water desalination, refrigeration, drying

and power generation. The solar pond works on a very simple principle. It is well-known that water or air is

heated they become lighter and rise upward e.g. a hot air balloon. Similarly, in an ordinary pond, the sun‘s rays heat the water and the heated water from within the pond

rises and reaches the top but loses the heat into the atmosphere. The net result is that the pond water remains at the atmospheric temperature. The solar pond restricts this

tendency by dissolving salt in the bottom layer of the pond making it too heavy to rise. A solar pond has three zones. The top zone is the surface zone, or UCZ (Upper

Convective Zone), which is at atmospheric temperature and has little salt content. The bottom zone is very hot, 70°– 85° C, and is very salty. It is this zone that collects and

stores solar energy in the form of heat, and is, therefore, known as the storage zone or LCZ (Lower Convective Zone). Separating these two zones is the important gradient

zone or NCZ (Non-Convective Zone). Here the salt content increases as depth increases, thereby creating a salinity or density gradient. If we consider a particular layer in this zone, water of that layer cannot rise, as the layer of water above has less salt content and

is, therefore, lighter. Similarly, the water from this layer cannot fall as the water layer below has a higher salt content and is, therefore, heavier. This gradient zone acts as a

transparent insulator permitting sunlight to reach the bottom zone but also entrapping it


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there. The trapped (solar) energy is then withdrawn from the pond in the form of hot brine from the storage zone.

Though solar ponds can be constructed anywhere, it is economical to construct them at

places where there is low cost salt and bittern, good supply of sea water or water for filling and flushing, high solar radiation, and availability of land at low cost. Coastal

areas in Tamil Nadu, Gujarat, Andhra Pradesh, and Orissa are ideally suited for such solar ponds.

Solar Pond at Bhuj

The Bhuj Solar Pond is a research, development, and demonstration project. The construction of the 6000 m2 pond started in 1987 at Kutch Dairy, Bhuj as a collaborative

effort between Gujarat Energy Development Agency, Gujarat Dairy Development Corporation Limited, and TERI under the National Solar Pond programme of the Ministry of Non-Conventional Energy Sources. TERI carried out execution, operation,

and maintenance of the Bhuj Solar Pond.

The solar pond is 100 m long and 60 m wide and has a depth of 3.5 m. To prevent seepage of saline water, a specially developed lining scheme, comprising locally available

material, has been adopted. The pond was then filled with water and 4000 tonnes of common salt was dissolved in it to make dense brine. A salinity gradient was established and wave suppression nets, a sampling platform, diffuses for suction and discharge of hot

brine, etc. were also installed. This pond has been successfully supplying processed heat to the dairy since September 1993, and is, at present, the largest operating solar pond in

the world.

SOLAR APPLICATIONS

SOLAR HEATING/COOLING Solar heating & cooling (SHC) technologies collect the thermal energy from the sun and

use this heat to provide hot water, space heating, cooling, and pool heating for

residential, commercial, and industrial applications. These technologies displace the

need to use electricity or natural gas. Today, Americans across the country are at work

manufacturing and installing solar heating and cooling systems that significantly reduce

our dependence on imported fuels. We need smart policies to expand this fast‐growing,

job‐producing sector.


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SOLAR DISTILLATION

Solar distillation is a relatively simple treatment of brackish (i.e. contain dissolved salts)

water supplies. Distillation is one of many processes that can be used for water purification and can use any heating source. Solar energy is a low tech option. In this

process, water is evaporated; using the energy of the sun then the vapour condenses as pure water. This process removes salts and other impurities.

Solar distillation is used to produce drinking water or to produce pure water for lead acid batteries, laboratories, hospitals and in producing commercial products such as rose

water. It is recommended that drinking water has 100 to 1000 mg/l of salt to maintain electrolyte levels and for taste. Some saline water may need to be added to the distilled

water for acceptable drinking water. Solar water distillation is a very old technology. An early large-scale solar still was built

in 1872 to supply a mining community in Chile with drinking water. It has been used for emergency situations including navy introduction of inflatable stills for life boats.

There are a number of other approaches to desalination, such as photovoltaic powered

reverse-osmosis, for which small-scale commercially available equipment is available; solar distillation has to be compared with these options to determine its appropriateness to any situation. If treatment of polluted water is required rather than desalination, slow

sand filtration is a low cost option.

Energy requirements for water distillation The energy required to evaporate water, called the latent heat of vaporisation of water, is

2260 kilojoules per kilogram (kJ/kg). This means that to produce 1 litre (i.e. 1kg as the density of water is 1kg/litre) of pure water by distilling brackish water requires a heat input of 2260kJ. This does not allow for the efficiency of the system sued which will be

less than 100%, or for any recovery of latent heat that is rejected when the water vapour is condensed.

It should be noted that, although 2260kJ/kg is required to evaporate water, to pump a kg of water through 20m head requires only 0.2kJ/kg. Distillation is therefore normally

considered only where there is no local source of fresh water that can be easily pumped

or lifted.

Solar dryers

Solar dryers are devices that use solar energy to dry substances, especially food.

These are of two types

Direct: Direct solar dryers expose the substance to be dehydrated to direct sunlight. They

have a black absorbing surface which collects the light and converts it to heat; the

substance to be dried is placed directly on this surface. These driers may have enclosures;

glass covers and/or vents to in order to increase efficiency.


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Indirect: In indirect solar dryers, the black surface heats incoming air, rather than

directly heating the substance to be dried. This heated air is then passed over the

substance and exits through a chimney, taking moisture from the substance with it.

PHOTOVOLTAIC ENERGY CONVERSION

Solar energy is leading the green revolution. If you're considering installing a solar

photovoltaic (PV) system on your home, you don't need to know how the PV cells work.

Your solar contractor knows the details, and they know which types of panels to use in a

given application.

But PV systems cost a lot of money, and customers are generally interested in knowing

as much as possible about the details. The more you understand, the better your own

decision-making process will be.

A standard PV cell is a thin semiconductor sandwich, with two layers of highly purified

silicon. Photovoltaic arrays are nothing more than huge matrices of interconnected

semiconductor sandwiches. Usable PV systems comprised all sorts of equipment that

protects the user from electrical shock, stores the electricity in battery banks, and

converts the direct current (DC) into alternating current (AC), which is what people use

in their houses. But at the heart of each system is a simple conversion process.

A photovoltaic cell changes light into electricity.

http://www.dummies.com/how-to/content/how-do-photovoltaic-cells-convert-sunlight-into-el.html#glossary-PV_cell

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A basic cell is around 1/100th of an inch thick, with a wide range of surface areas. A

typical life span is over 25 years, and there are cells in place that have produced

electricity reliably for over 40 years. This longevity is ultimately due to the coatings and

framing structures that protect the cells.

A module is an assembly of individual cells, connected in series and parallel

arrangements designed to yield optimum performance.

In a series connection, voltage is additive, while in a parallel connection, current is additive.

A typical PV cell produces around half a volt of electrical output. When 36 PV cells are

connected in series, the result is an 18-volt module. A module or panel is a number of

individual cells interconnected and housed into a finished product. A typical PV module

in a residential application measures around 2.5 feet by 5 feet, in either bluish or black.

Frames are either aluminum-colored or black, with the latter being the overwhelming

choice of most homeowners these days (they just look better).

It's possible to achieve a wide range of voltage and current outputs, depending on how

the individual cells are connected together. The amount of power a module can produce

is a function of the total surface area, as well as the amount of sunlight that strikes the

module.

Typical modules are rectangular and are available in a variety of sizes and

configurations. Small modules (the kind used in hand-held calculators) output less than a

single watt of power, while a typical residential module produces around 200 watts of

power, more or less.


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Modules are characterized by:

Cell material, or the type of silicon process that is used

Glazing material

Frame and electrical connections

The most important feature of a cell is the composition of the silicon structure. Single

crystalline cells may be cast into an ingot of multiple crystals. Or the crystalline materials

may also be deposited as a thin film, which is referred to as amorphous silicon.

Individual silicon wafers used to manufacture PV cells are embedded with metallic

contacts (wires). The cells are coated with an anti-reflection material so that the

maximum amount of sunlight is absorbed into each cell.

Single crystalline cells are more efficient than polycrystalline because, in polycrystalline

cells, inter-grain boundaries introduce resistance to current flow (which consumes

energy). Amorphous silicon is much less expensive to manufacture, but it's only around

half as efficient in converting sunlight into useable electrical energy. In practical terms,

this means that an amorphous system requires twice the surface area to output the same

amount of power. Depending on how much surface area is available, this may or may

not be a problem. In most residential applications, suitable roof space is limited, so

efficiency is an important factor.


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CHAPTER4)

WIND ENERGY

SOURCES

The energy from the wind can be harnessed by wind turbines to generate electricity and

also windmills to pump water.

During the 1990s, wind turbines evolved dramatically to become very large and

increasingly efficient machines, as a direct result of government policies set in Denmark

enabling communities and co-ops to develop wind turbines and encouraging research

and development.

Chart: Global Annual Wind Installations (MW) 1980-2005

Capturing and Using Wind

Modern wind turbines generate electricity typically around 80% of the time. The output

varies depending on wind speed, but over the course of a year, a turbine is designed and

expected to generate about 30% of its theoretical maximum output. This is known as its

capacity factor, which for conventional power plants is typically 50%.

Although the wind does not always blow, one region may be calm while another one is

windy. Therefore, overall fluctuations can be significantly reduced if wind turbines are

spread out across a country or region.

Wind turbines tend to generate more power during the day when it is needed most and

less at night, a pattern that corresponds well to electricity demand. Wind power therefore

combines well with existing power plants in Canada that can be used only when needed,

such as hydro plants, or must-run power plants like biomass that tend to have excess

power at night.

The energy that is available in the wind is cubically related to the speed at which it is

moving. In other words, doubling the wind speed means there is eight times more


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energy. A good wind power site therefore needs to be consistently windy. Even small

differences in wind speed can have a large impact on the performance a project.

Wind energy can also be used for mechanical work such as water pumping and small-

scale power systems for homes and small businesses. Canada has a unique expertise and

manufacturing ability for medium-scale electrical systems that are ideally suited for

remote community applications and hybrid systems such as wind-diesel.

Benefits

Large-scale wind energy is becoming cost competitive with traditional power generation.

Very low cradle-to-grave impacts.

Wind is abundant all over the world.

Wind patterns tend to follow consumption patterns.

Turbines co-exist nicely with farms, supplying additional income with minimal impact

on the usable land.

Wind turbines are very quiet and are less likely to be struck by a bird than a downtown

building, a bay window or a car is.

Challenges

Significant local visual impact.

Moving parts require maintenance and upkeep.

A consistent and considerable amount of wind is needed.

Bats can be affected or struck by turbines.

Power output is variable and needs overall system integration.

POTENTIAL

Global wind power has doubled over the last 3 years, which now accounts for 2% of the

world‘s electricity production, and as much as 20% in some countries. It is estimated that

13% of the worlds land area has wind speeds greater than 6.9 m/s at commercial wind

turbine heights, this could theoretically produce 40 times the world's current electricity

production. Although the total quantity of wind energy potentially available is

considerable, there remain obstacles to the substantial expansion of this industry.

Critics claim that wind power cannot replace conventional power sources since these still

need to be available for when the wind isn‘t blowing, and these are expensive to keep in

reserve and waiting on part load, reducing overall energy efficiency. For low

contributions, wind power can be considered largely additional, since reserves are always

needed to cater for unexpected unavailability of the largest single power source and

having a large number of smaller generating units can sometimes be beneficial.


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In addition to providing replacement power, wind plants reduce emissions by forcing the

most polluting and inflexible power plants offline with more efficient and flexible types

of generation. However, with increasing use of wind energy in the system the proportion

of conventional plant wind replaces reduces, and consistent wind power generation

requires high voltage connections to wind farms over a wide area.

Further expansion of the industry may also require energy storage technologies to

balance fluctuations of supply and demand; these include hydroelectricity, compressed

air storage and electro-chemical batteries. The latter option is particularly interesting in

view of the potential use of battery-electric vehicles in the future since their batteries

could be charged overnight when power demand is low and used to supply power during

peak demand periods. Peak electricity demand could also be reduced through pricing

structures and switching off non-essential appliances.

In addition to to replacing carbon intensive electricity production, wind power can drive

heat pumps for space heating or charge vehicle batteries for transport, reducing natural

gas and petroleum use. These are more efficient methods of producing heat and work

than the traditional methods they replace, so direct comparisons of energy cannot

usually be made . For example, one popular book grossly underestimated the potential

contribution of wind power, partly because it compared the electricity generated from

wind turbines with the primary chemical energy in fossil fuel directly.

HORIZONTAL AXIS TURBINE

In the wind turbine business there are basically two types of turbines to choose from,

vertical axis wind turbines and horizontal axis wind turbines. They both have their

advantages and disadvantages and the purpose of this article is to help you choose the

right system for your application.

Horizontal axis wind turbine dominate the majority of the wind industry. Horizontal

axis means the rotating axis of the wind turbine is horizontal, or parallel with the

ground. In big wind application, horizontal axis wind turbines are almost all you will

ever see. However, in small wind and residential wind applications, vertical axis turbines

have their place. The advantage of horizontal wind is that it is able to produce more

electricity from a given amount of wind. So if you are trying to produce as much wind as

possible at all times, horizontal axis is likely the choice for you. The disadvantage of


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horizontal axis however is that it is generally heavier and it does not produce well in

turbulent winds.

VERTICLE AXIS TURBINE

In comes the vertical axis wind turbine. With vertical axis wind turbines the rotational

axis of the turbine stands vertical or perpendicular to the ground. As mentioned above,

vertical axis turbines are primarily used in small wind projects and residential

applications.

This niche comes from the OEM‘s claims of a vertical axis turbines ability to produce

well in tumultuous wind conditions. Vertical axis turbines are powered by wind coming

from all 360 degrees, and even some turbines are powered when the wind blows from

top to bottom. Because of this versatility, vertical axis wind turbines are thought to be

ideal for installations where wind conditions are not consistent, or due to public

ordinances the turbine cannot be placed high enough to benefit from steady wind.

Wind turbine power ouput variation with steady wind speed.

The figure below shows a sketch a how the power output from a wind turbine varies with steady wind speed.


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Cut-in speed.

At very low wind speeds, there is insufficient torque exerted by the wind on the turbine

blades to make them rotate. However, as the speed increases, the wind turbine will begin

to rotate and generate electrical power. The speed at which the turbine first starts to

rotate and generate power is called the cut-in speed and is typically between 3 and 4

metres per second.

Rated output power and rate output wind speed.

As the wind speed rises above the cut-in speed, the level of electrical ouput power rises

rapidly as shown. However, typically somewhere between 12 and 17 metres per second,

the power output reaches the limit that the electrical generator is capable of. This limit to

the generator output is called the rated power output and the wind speed at which it is

reached is called the rated output wind speed. At higher wind speeds, the design of the

turbine is arranged to limit the power to this maximum level and there is no further rise

in the output power. How this is done varies from design to design but typically with

large turbines, it is done by adjusting the blade angles so as to to keep the power at the

constant level.


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Cut-out speed.

As the speed increases above the rate output wind speed, the forces on the turbine

structure continue to rise and, at some point, there is a risk of damage to the rotor. As a

result, a braking system is employed to bring the rotor to a standstill. This is called

the cut-out speed and is usually around 25 metres per second.

Wind turbine efficiency or power coefficient.

The available power in a stream of wind of the same cross-sectional area as the wind

turbine can easily be shown to be

If the wind speed U is in metres per second, the density ρ is in kilograms per cubic metre

and the rotor diameter d is in metres then the available power is in watts. The efficiency,

μ, or, as it is more commonly called, the power coefficient, cp, of the wind turbine is

simply defined as the actual power delivered divided by the available power.

The Betz limit on wind turbine efficiency.

There is a theoretical limit on the amount of power that can be extracted by a wind turbine from an airstream. It is called the Betz limit .The limit is μ=16/27≈ 59%

Betz Criterion


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What is Betz Law?

We all feel the wind on us when we go for a ride on a motorcycle or on a windy day

where you almost are swept off your feet. Wind can be very strong and we experience

this power on windy days or storms. With all this wind energy flowing around us,

scientists decided to put it to good use specially in these days with more emphasis on

global warming there is a lot of pressure to use renewable sources of energy. Wind

energy is flowing freely all around us and has not been utilized much. Well, with that in

mind, wind farming is being tried and to some extent has been harnessed very

successfully. Wind farming makes use of turbines which turn with the speed of the wind

and create kinetic energy. This energy is converted to electricity by an electrical grid or

generator.

Many experiments are being conducted to find the most cost effective

and energy efficient solution. But one of the most important inventions was made way

back in 1919 by a German physicist; Albert Betz demonstrating the limitations of wind

turbines is worrying the scientists. Even today these findings known as Betz Law or Betz

limit have been found to be true irrespective of man‘s effort to create different energy

efficient wind turbines. Betz law is quite interesting and how a calculation made almost

100 years back holds true even today.

What is Betz Law or Betz Limits?

According to Betz law even when all the ideal conditions of energy generation are

prevalent we can only derive 59% energy from wind turbines. 100% wind energy

generation is simply not possible. Herein the capacity or ability of a generator to

convert kinetic energy into electric energy is not under question. Rather the structure and

mechanism of wind turbine has limitations in converting the wind energy into

100% kinetic energy owing to which we cannot take full benefit of wind energy.


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Although we are always inclined to get 100% of everything, it‘s not always possible and

most of the times nature has its genuine reasons for confining man from achieving his

own will. Betz law although was invented in 1919 has been known unknowingly to

mankind but in a simpler way. How? Well what happens when you extract 100% energy

from any source? The energy becomes empty or dried out. Similarly, if we are successful

in deriving 100% kinetic energy from wind energy will there be any air or wind left? No!

And then what will rotate the wind turbine? So at least to keep the velocity enough so as

to make the wind turbine rotate for energy generation it‘s important that 100% efficiency

is not achieved.

What is the fundamental basis of Betz law?

Betz law basically talks about how a wind turbine cannot extract more than 59.3 %

of Kinetic energy from the wind. Under Ideal conditions or theoretically the maximum

energy that can be extracted from the wind is called the Power coefficient which is a

ratio between the amounts of energy that can be extracted by a Wind turbine to the total

energy in the Wind.

Power Coefficient (Beth's Coefficient) = Kinetic Energy that is extracted by a Wind

turbine/Total energy in the Wind

What are the limitations for achieving high efficiency?

Usually the most important intention of energy generation is to achieve maximum

efficiency within reasonable operational costs. So as to make the turbines cost effective,

there would be a need to adapt design trade-offs but this can affect the overall efficiency

of the system. So the idea is to get more power from stronger wind by sidelining the

efficiency a bit. It has been found that less efficient rotor blades have been found to give

more output. Moreover, the speed of wind is always a variable, while the turbines

installed will always be of same efficiency and capacity. All these problems are currently

being faced by the engineers and hence even today Betz law seems to be relevant.


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CHAPTER 5)

BIOMASS

The term Bioconversion, also known as biotransformation refers to the use of live

organisms often microorganisms to carry out a chemical reaction that is more costly or

not feasible nonbi ologically. These organisms convert a substance to a chemically

modified form. An example is the industrial production of cortisone. One step is the

bioconversion of Progesterone to 11-alpha-Hydroxyprogesterone by Rhizopus nigricans.

Another example of this is the conversion of organic materials, such as plant or animal

waste, into usable products or energy sources by biological processes or agents, such as

certain microorganisms, some Detritivores or enzymes.

The conversion of organic materials, such as plant or animal waste, into usable products

or energy sources by biological processes or agents, such as certain microorganisms.

The Bioconversion Science and Technology group performs multidisciplinary R&D for

the Department of Energy's (DOE) relevant applications of bioprocessing, especially

with biomass. Bioprocessing combines the disciplines of chemical engineering,

microbiology and biochemistry. The Group 's primary role is investigation of the use of

microorganism, microbial consortia and microbial enzymes in bioenergy research.

New cellulosic ethanol conversion processes have enabled the variety and volume of

feedstock that can be bioconverted to expand rapidly. Feedstock now includes materials

derived from plant or animal waste such as paper, auto-fluff, tires, fabric, construction

materials, municipal solid waste (MSW), sludge, sewage, etc.

AEROBIC AND ANAEROBIC DIGESTION

Aerobic Digestion

Aerobic digestion of waste is the natural biological degradation and purification process

in which bacteria that thrive in oxygen-rich environments break down and digest the

waste.


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During oxidation process, pollutants are broken down into carbon dioxide (CO 2 ),

water (H 2 O), nitrates, sulphates and biomass (microorganisms). By operating the

oxygen supply withaerators, the process can be significantly accelerated. Of all the

biological treatment methods, aerobic digestion is the most widespread process that is

used throughout the world.

Biological and chemical oxygen demand

Aerobic bacteria demand oxygen to decompose dissolved pollutants. Large amounts of

pollutants require large quantities of bacteria; therefore the demand for oxygen will be

high.

The Biological Oxygen Demand (BOD) is a measure of the quantity of dissolved

organic pollutants that can be removed in biological oxidation by the bacteria. It is

expressed in mg/l.

The Chemical Oxygen Demand (COD) measures the quantity of dissolved organic

pollutants than can be removed in chemical oxidation, by adding strong acids. It is

expressed in mg/l.

The BOD/COD gives an indication of the fraction of pollutants in the wastewater that is

biodegradable.

Advantages of Aerobic Digestion

Aerobic bacteria are very efficient in breaking down waste products. The result of this is;

aerobic treatment usually yields better effluent quality that that obtained in anaerobic

processes. The aerobic pathway also releases a substantial amount of energy. A portion

is used by the microorganisms for synthesis and growth of new microorganisms.

Path of Aerobic Digestion


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Aerobic Decomposition

A biological process, in which, organisms use available organic matter to support

biological activity. The process uses organic matter, nutrients, and dissolved oxygen,

and produces stable solids, carbon dioxide, and more organisms. The microorganisms

which can only survive in aerobic conditions are known as aerobic organisms. In sewer

lines the sewage becomes anoxic if left for a few hours and becomes anaerobic if left for

more than 1 1/2 days. Anoxic organisms work well with aerobic and anaerobic

organisms. Facultative and anoxic are basically the same concept.

Anoxic Decomposition

A biological process in which a certain group of microorganisms use chemically

combined oxygen such as that found in nitrite and nitrate. These organisms consume

organic matter to support life functions. They use organic matter, combined oxygen

from nitrate, and nutrients to produce nitrogen gas, carbon dioxide, stable solids and

more organisms.


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Anaerobic Digestion

Anaerobic digestion is a complex biochemical reaction carried out in a number of steps

by several types of microorganisms that require little or no oxygen to live. During this

process, a gas that is mainly composed of methane and carbon dioxide, also referred to

as biogas, is produced. The amount of gas produced varies with the amount of organic

waste fed to the digester and temperature influences the rate of decomposition and gas

production.

Anaerobic digestion occurs in four steps:

• Hydrolysis : Complex organic matter is decomposed into simple soluble organic

molecules using water to split the chemical bonds between the substances.

• Fermentation or Acidogenesis: The chemical decomposition of carbohydrates by

enzymes, bacteria, yeasts, or molds in the absence of oxygen.

• Acetogenesis: The fermentation products are converted into acetate, hydrogen and

carbon dioxide by what are known as acetogenic bacteria.

• Methanogenesis: Is formed from acetate and hydrogen/carbon dioxide by

methanogenic bacteria.

The acetogenic bacteria grow in close association with the methanogenic bacteria during

the fourth stage of the process. The reason for this is that the conversion of the

fermentation products by the acetogens is thermodynamically only if the hydrogen

concentration is kept sufficiently low. This requires a close relationship between both

classes of bacteria.


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The anaerobic process only takes place under strict anaerobic conditions. It requires

specific adapted bio-solids and particular process conditions, which are considerably

different from those needed for aerobic treatment.

Path of Anaerobic Digestion

Advantages of Anaerobic Digestion

Wastewater pollutants are transformed into methane, carbon dioxide and smaller

amount of bio-solids. The biomass growth is much lower compared to those in the

aerobic processes. They are also much more compact than the aerobic bio-solids.

Anaerobic Decomposition

A biological process, in which, decomposition of organic matter occurs without oxygen.

Two processes occur during anaerobic decomposition. First, facultative acid forming

bacteria use organic matter as a food source and produce volatile (organic) acids, gases

such as carbon dioxide and hydrogen sulfide, stable solids and more facultative

organisms. Second, anaerobic methane formers use the volatile acids as a food source

and produce methane gas, stable solids and more anaerobic methane formers. The

methane gas produced by the process is usable as a fuel. The methane former works

slower than the acid former, therefore the pH has to stay constant consistently, slightly


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basic, to optimize the creation of methane. You need to constantly feed it sodium

bicarbonate to keep it basic.

BIOGAS DIGESTERS TYPES:

1) Floating drum type

2) Fixed Dome type

1) Fixed-dome Plants

A fixed-dome plant consists of a digester with a fixed, non-movable gas holder, which

sits on top of the digester. When gas production starts, the slurry is displaced into the

compensation tank. Gas pressure increases with the volume of gas stored and the height

difference between the slurry level in the digester and the slurry level in the

compensation tank.

The costs of a fixed-dome biogas plant are relatively low. It is simple as no moving parts

exist. There are also no rusting steel parts and hence a long life of the plant (20 years or

more) can be expected. The plant is constructed underground, protecting it from physical

damage and saving space. While the underground digester is protected from low

temperatures at night and during cold seasons, sunshine and warm seasons take longer

to heat up the digester. No day/night fluctuations of temperature in the digester

positively influence the bacteriological processes.

The construction of fixed dome plants is labor-intensive, thus creating local employment.

Fixed-dome plants are not easy to build. They should only be built where construction

can be supervised by experienced biogas technicians. Otherwise plants may not be gas-

tight (porosity and cracks).


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The basic elements of a fixed dome plant (here the Nicarao Design) are shown in the

figure below.

Fixed dome plant Nicarao design: 1. Mixing tank with inlet pipe and sand trap. 2.

Digester. 3. Compensation and removal tank. 4. Gasholder. 5. Gaspipe. 6. Entry

hatch, with gastight seal. 7. Accumulation of thick sludge. 8. Outlet pipe. 9.

Reference level. 10. Supernatant scum, broken up by varying level.

picture:Basic function of a fixed-dome biogas plant, 1

Mixing pit, 2 Digester, 3 Gasholder, 4 Displacement

pit, 5 Gas pipe

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Function

A fixed-dome plant comprises of a closed, dome-shaped digester with an immovable,

rigid gas-holder and a displacement pit, also named 'compensation tank'. The gas is

stored in the upper part of the digester. When gas production commences, the slurry is

displaced into the compensating tank. Gas pressure increases with the volume of gas

stored, i.e. with the height difference between the two slurry levels. If there is little gas in

the gas-holder, the gas pressure is low.

Digester

The digesters of fixed-dome plants are usually masonry

structures, structures of cement and ferro-cementexist.

Main parameters for the choice of material are:

Technical suitability (stability, gas- and liquid

tightness);

cost-effectiveness;

availability in the region and transport costs;

availability of local skills for working with the

particular building material.

Fixed dome plants produce just as much gas as floating-drum plants, if they are gas-tight.

However, utilization of the gas is less effective as the gas pressure fluctuates

substantially. Burners and other simple appliances cannot be set in an optimal way. If the

gas is required at constant pressure (e.g., for engines), a gas pressure regulator or a

floating gas-holder is necessary.

Gas-Holder

The top part of a fixed-dome plant (the gas space) must be gas-tight. Concrete, masonry

and cement rendering are not gas-tight. The gas space must therefore be painted with

a gas-tight layer (e.g. 'Water-proofer', Latex or synthetic paints). A possibility to reduce

the risk of cracking of the gas-holder consists in the construction of a weak-ring in the

masonry of the digester. This "ring" is a flexible joint between the lower (water-proof)

and the upper (gas-proof) part of the hemispherical structure. It prevents cracks that

develop due to the hydrostatic pressure in the lower parts to move into the upper parts of

the gas-holder.

Fixed-dome plant in Tunesia.

The final layers of the

masonry structure are being

fixed.[3]

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Types of Fixed-dome Plants

Chinese fixed-dome plant is the archetype of all fixed dome plants. Several million

have been constructed in China. The digester consists of a cylinder with round

bottom and top.

Janata model was the first fixed-dome design in India, as a response to the Chinese

fixed dome plant. It is not constructed anymore. The mode of construction lead to

cracks in the gasholder - very few of these plant had been gas-tight.

Deenbandhu, the successor of the Janata plant in India, with improved design, was

more crack-proof and consumed less building material than the Janata plant. with a

hemisphere digester

CAMARTEC model has a simplified structure of a hemispherical dome shell based

on a rigid foundation ring only and a calculated joint of fraction, the so-called weak /

strong ring. It was developed in the late 80s in Tanzania.

Fixed dome plant CAMARTEC design[1]

Climate and Size

Fixed-dome plants must be covered with earth up to the top of the gas-filled space to

counteract the internal pressure (up to 0,15 bar). The earth cover insulation and the

option for internal heating makes them suitable for colder climates. Due to economic

parameters, the recommended minimum size of a fixed-dome plant is 5 m3. Digester

volumes up to 200 m3 are known and possible.

https://energypedia.info/wiki/Biogas_Technology_in_China

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Summary

Advantages: Low initial costs and long useful life-span; no moving or rusting parts

involved; basic design is compact, saves space and is well insulated; construction creates

local employment.

Advantages are the relatively low construction costs, the absence of moving parts and

rusting steel parts. If well constructed, fixed dome plants have a long life span. The

underground construction saves space and protects the digester from temperature

changes. The construction provides opportunities for skilled local employment.

Disadvantages: Masonry gas-holders require special sealants and high technical skills for

gas-tight construction; gas leaks occur quite frequently; fluctuating gas pressure

complicates gas utilization; amount of gas produced is not immediately visible, plant

operation not readily understandable; fixed dome plants need exact planning of levels;

excavation can be difficult and expensive in bedrock.

Disadvantages are mainly the frequent problems with the gas-tightness of the brickwork

gas holder (a small crack in the upper brickwork can cause heavy losses of biogas).

Fixed-dome plants are, therefore, recommended only where construction can be

supervised by experienced biogastechnicians. The gas pressure fluctuates substantially

depending on the volume of the stored gas. Even though the underground construction buffers

temperature extremes, digester temperatures are generally low.

Fixed dome plants can be recommended only where construction can be supervised by

experienced biogas technicians.

Variations: Some companies are now looking into small pre-fab fixed dome plants made

of fibreglass which appears to be a low cost alternative to construction

intensive masoned plants. A custom made plant can be produced in 2 days and -after

transport- installed in less than 1 day!

2) Floating-drum Plants

Floating-drum plants consist of an underground digester and a moving gas-holder. The

gas-holder floats either directly on the fermentation slurry or in a water jacket of its own.

The gas is collected in the gas drum, which rises or moves down, according to the

amount of gas stored. The gas drum is prevented from tilting by a guiding frame. If the


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drum floats in a water jacket, it cannot get stuck, even in substrate with high solid

content.

Floating-drum plant in Mauretania[3]

The Drum

In the past, floating-drum plants were mainly built in India. A floating-drum plant

consists of a cylindrical or dome-shaped digester and a moving, floating gas-holder, or

drum. The gas-holder floats either directly in the fermenting slurry or in a separate water

jacket. The drum in which the biogascollects has an internal and/or external guide frame

that provides stability and keeps the drum upright. If biogas is produced, the drum

moves up, if gas is consumed, the gas-holder sinks back.

Size

Floating-drum plants are used chiefly for digesting animal and human feces on a

continuous-feed mode of operation, i.e. with daily input. They are used most frequently

by small- to middle-sized farms (digester size: 5-15m3) or in institutions and larger agro-

industrial estates (digester size: 20-100m3).

Disadvantages: The steel drum is relatively expensive and maintenance-intensive.

Removing rust and painting has to be carried out regularly. The life-time of the drum is

short (up to 15 years; in tropical coastal regions about five years). If fibrous substrates are

used, the gas-holder shows a tendency to get "stuck" in the resultant floating scum.

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Water-jacket plant with external guide

frame: 1 Mixing pit, 11 Fill pipe, 2

Digester, 3 Gasholder, 31 Guide frame, 4

Slurry store, 5 Gas pipe[4]

Water-jacket Floating-drum Plants

Water-jacket plants are universally applicable and easy to maintain. The drum cannot

get stuck in a scum layer, even if the substrate has a high solids content. Water-jacket

plants are characterized by a long useful life and a more aesthetic appearance (no dirty

gas-holder). Due to their superior sealing of the substrate (hygiene!), they are

recommended for use in the fermentation of night soil. The extra cost of the masonry

water jacket is relatively modest.

Material of Digester and Drum

The digester is usually made of brick, concrete or quarry-stone masonry with plaster. The

gas drum normally consists of 2.5 mm steel sheets for the sides and 2 mm sheets for the

top. It has welded-in braces which break up surface scum when the drum rotates. The

drum must be protected against corrosion. Suitable coating products are oil paints,

synthetic paints and bitumen paints. Correct priming is important. There must be at least

two preliminary coats and one topcoat. Coatings of used oil are cheap. They must be

renewed monthly. Plastic sheeting stuck to bitumen sealant has not given good results.

In coastal regions, repainting is necessary at least once a year, and in dry uplands at least

every other year. Gas production will be higher if the drum is painted black or red rather

than blue or white, because the digester temperature is increased by solar radiation. Gas

drums made of 2 cm wire-mesh-reinforced concrete or fiber-cement must receive a gas-

tight internal coating. The gas drum should have a slightly sloping roof, otherwise

rainwater will be trapped on it, leading to rust damage. An excessively steep-pitched roof

is unnecessarily expensive and the gas in the tip cannot be used because when the drum

is resting on the bottom, the gas is no longer under pressure.

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Floating-drums made of glass-fiber reinforced plastic and high-density polyethylene have

been used successfully, but the construction costs are higher compared to using steel.

Floating-drums made of wire-mesh-reinforced concrete are liable to hairline cracking and

are intrinsically porous. They require a gas-tight, elastic internal coating. PVC drums are

unsuitable because they are not resistant to UV.

Guide Frame

The side wall of the gas drum should be just as high as the wall above the support ledge.

The floating-drum must not touch the outer walls. It must not tilt, otherwise the coating

will be damaged or it will get stuck. For this reason, a floating-drum always requires a

guide. This guide frame must be designed in a way that allows the gas drum to be

removed for repair. The drum can only be removed if air can flow into it, either by

opening the gas outlet or by emptying the water jacket.

The floating gas drum can be replaced by a balloon above the digester. This reduces

construction costs but in practice problems always arise with the attachment of the

balloon to the digester and with the high susceptibility to physical damage.

Types of Floating-drum Plants

There are different types of floating-drum plants:

KVIC model with a cylindrical digester, the oldest and most widespread floating

drum biogas plant from India.

Pragati model with a hemisphere digester

Ganesh model made of angular steel and plastic foil

floating-drum plant made of pre-fabricated reinforced concrete compound units

floating-drum plant made of fibre-glass reinforced polyester

low cost floating-drum plants made of plastic water containers or fiberglass

drums: ARTI Biogas plants

BORDA model: The BORDA-plant combines the static advantages of hemispherical

digester with the process-stability of the floating-drum and the longer life span of a

water jacket plant.

Summary

Advantages: Advantages are the simple, easily understood operation - the volume of

stored gas is directly visible. The gas pressure is constant, determined by the weight of

the gas holder. The construction is relatively easy, construction mistakes do not lead to

major problems in operation and gas yield.

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Disadvantages: Disadvantages are high material costs of the steel drum, the

susceptibility of steel parts to corrosion. Because of this, floating drum plants have a

shorter life span than fixed-dome plants and regular maintenance costs for the painting of

the drum.

GAS YEILD

Feedstock Dry

Matter % Biogas

Yield m3/tonne Feedstock

Dry Matter %

Biogas Yield m3/tonne

Cattle slurry 10 15-25 Potatoes - 276-400

Pig slurry 8 15-25 Rye grain - 283-492

Poultry 20 30-100 Clover grass - 290-390

Grass silage 28 160-200 Sorghum - 295-372

Whole wheat crop

33 185 Grass - 298-467

Maize silage 33 200-220 Red clover - 300-350

Maize grain 80 560 Jerusalem artich.

- 300-370

Crude glycerine 80 580-1000 Turnip - 314

Wheat grain 85 610 Rhubarb - 320-490

Rape meal 90 620 Triticale - 337-555

Fats up to 100 up to 1200 Oliseed rape - 340-340

Nettle - 120-420 Reed canary grass

- 340-430

Sunflower - 154-400 Alfalfa - 340-500

Miscanthus - 179-218 Clover - 345-350

Whole maize crop

- 205-450 Barley - 353-658

Flax - 212 Hemp - 355-409

Sudan grass - 213-303 Wheat grain - 384-426

Sugar beet - 236-381 Peas - 390

Kale - 240-334 Ryegrass - 390-410

Straw - 242-324 Leaves - 417-453

Oats grain - 250-295 Fodder beet - 160-180

Chaff - 270-316


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COMBUSTION CHARACTERSTICS OF BIO GAS

Fuel types have experienced a transition process since the beginning of the industrial era,

from solid fuels, liquid fuels, to the modern era where the objective is to disseminate the

utilization of gas fuels. The objective is to find a renewable, cleaner source of energy,

easy to manage and with low environmental impact not only at a local level but also

globally. Biogas is one of these renewable sources of energy.

Biogas is produced from the anaerobic fermentation of organic matter in sanitary

landfills and in anaerobic biodigestors of organic solid wastes, from plants and animals.

The environmental benefit of the production and use of this fuel is deeply appreciated

through the reduction of gas emissions that exhibit a greenhouse effect since it is

produced by the organic matter decomposition in agricultural and animal wastes as well

as in sanitary landfills.

Regarding its use, biogas can be utlized directly as a fuel in heating or power generation

processes, as it is stated by Desideri, Di Maria, Leonardo & Proietti (2003) and

Zamorano, Pérez, Aguilar & Ridao (2007). The first authors studied the energy potential

of biogas produced at a sanitary landfill located in the center of Italy that serves a 400

000 - population town. This work concludes that 60% of the biogas produced at the

sanitary landfill can be utilized in internal combustion engines for power generation due

to its lower calorific effect, close to 19 000 KJ / Nm3 and given its methane content

(greater than 40%). The utilization of the produced biogas represents, according to this

publication, a yield close to 100 GW-h/ year. Zamorano et al (2007) conducts a similar

work in the city of Granada in the South of Spain, whose landfill serves 300 000 people.

An approximate energy potential estimation of 4 500 GW-h/year, greater than the

values found in Italy, is reported in this work. This difference is produced because the

biogas analysis revealed a 45% methane content in all the biogas produced in the landfill.

These works indicate that it is feasible, from the economic standpoint, to utilize the

biogas produced in landfills according to the type of technology, being mainly utilized in

internal reciprocating combustion engines.

The biogas utilization potential in this type of engines reveals that purification is

necessary before considering its use as vehicle fuel (The Society of Motor Manufactu-rers

and Traders Ltd. - SMMT - 2002). This purification consists in the elimination of certain

components such as hydrogen sulphide, chlorine compounds, and ammonia, among

others, and the decrease in the percentage of carbon dioxide. Despite the above, its

utilization also depends on the knowledge available about its behavior in combustion

systems. Therefore, a phenomenological study of the biogas combustion pattern in

premixed systems is required, about its combustion properties and characteristic

phenomena in premixed systems, in order to gain enough information to be applied in

the design, redesign, or optimization of systems operating with this type of fuel.

Furthermore, criteria about optimum atmospheric burner design are necessary so biogas

can be utilized efficiently and safely in rural areas where organic materials are available.


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The objective in this case is to substitute wood logs in the preparation of food, thus

contributing to decrease the pressure imposed on rainforest deforestation and avoid

respiratory diseases resulting from a deficient wood combustion. Definition of these

properties and characteristic phenomena are presented during the development of this

work and the methodology described as well.

Combined heat and power (CHP) is the simultaneous production of useable heat and electricity. As

the process of AD(Anaerobic digestion) requires some heat it is suited to CHP. Whilst coal and gas-

fired power stations have an efficiency of around 34% and 55% respectively, CHP plants can

achieve overall efficiencies in excess of 70% at the point of use.

The ratio of heat to power varies dependent on the scale and technology, but typically 30-35% is

converted to electricity, 40-45% to heat and the balance lost as inefficiencies at various stages of the

process. This typically equates to over 2kWh electricity and 2.5kWh heat per cubic meter, at 60%

methane.

BIOGAS AND IC ENGINE

Biogas and Internal Combustion Engines

Biogas is a mixture of approximately 60 per cent methane (CH4), 40 per cent carbon

dioxide (CO2), and traces of hydrogen sulphide (H2S).

Biogas has a number of problems as a fuel:

High CO2 content reduces the power output, making it uneconomical as a

transport fuel. It is possible to remove the CO2 by washing the gas with water.

However, this is expensive (the capital cost in the mid 1980s was US$1200/kW in

the UK), although the equipment lifetime can be expected to be much longer than

the engine. The solution produced from washing out the CO2 is acidic and needs

careful disposal.

H2S is acidic and if not removed can cause corrosion of engine parts within a

matter of hours. It is easy to remove H2S, by passing the gas through iron oxide

(Fe203 -rusty nails are a good source) or zinc oxide (ZnO). These materials can be

re-generated on exposure to the air, although the smell of H2S is unpleasant.

There is a high residual moisture which can cause starting problems.

The gas can vary in quality and pressure.

Engine operation

Biogas can be used in both spark ignition and compression ignition engines. The exact

amount needed depends upon the methane content of the biogas. However, an

approximate substitution figure is:

1.3 -1.9m3 biogas is approximately equivalent to 1 litre petrol/gasoline

1.5 - 2.1m3 biogas is approximately equivalent to 1 litre diesel


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Since the engine will run hotter than with a liquid fuel, the cooling system needs to be

efficient or there will be problems with engine wear. The waste heat from the cooling

water can be used to heat the digester.

Compression / ignition engines

Biogas will not self-ignite in a diesel engine. Therefore, it is necessary to use a little

diesel (approximately 20 per cent) to ignite the fuel. The biogas enters the engine via the

air inlet system, after the air filter. This needs a small modification to the air intake

system (see figure3 ). This option also allows for the engine to be operated on diesel fuel

alone in periods when there is insufficient biogas.

An engine requires 0.6- 0.7m3 of biogas/kWh. Therefore, a 5kW generator running for

three hours needs about 10m3. This is a large volume of biogas.

Spark ignition engines

These are not commonly used as stationary engines. They can, however, operate on 100

per cent biogas, since the spark ignites the biogas/air mixture. The same modifications

to the air intake system are required as for a diesel engine.

Operating experience

A large amount of operating experience has now been built up in India, China and

Nepal on using biogas engines. There are conflicting views on the problems of long-term

operation and engine wear. Problems may be encountered with the valves sticking, since

these normally rely on the liquid fuel to provide some lubrication. Likewise, the piston

also obtains some lubrication from the fuel and without this there is cylinder wear, which

eventually leads to loss of power.

The use of gas engines would avoid these problems, although these are expensive

compared to an internal combustion engine. However, evidence suggests that many of

the most common problems encountered with all engines, irrespective of fuel, could be

avoided if people were trained in simple maintenance; for example, in tightening bolts

and changing or topping up lubricating oil. Loss of lubrication oil is one of the most

common causes of engine failure and manufacturers are now incorporating an automatic

cut-off device to protect the engine.


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ECONOMIC ASPECTS

A Biogas unit can yield a whole range of benefits for their users, the world, society and

the environment in general, the key benefits include;

(1) Production of energy (heat, light, electricity).

(2) Transformation of organic wastes into high quality fertilizer.

(3) Improvement of hygienic conditions through reduction of pathogens, worm eggs and

flies.

(4) Workload reduction; mainly for women, in firewood collection and cooking.

(5) Environmental advantages through protection of forests, soil, water and air.

Production of energy (heat, light, electricity)

Methane is the valuable component under the aspect of using biogas as a fuel. Biogas

use, replacing conventional fuels like kerosene or firewood, allows for the conservation

of environment. It therefore, increases its own value by the value of i.e. forest saved or

planted.

Biogas is able to substitute almost the complete consumption of firewood in rural

households.

1 m3 Biogas (approximately 6 kWh/m3) is equivalent to:

Diesel, Kerosene (approx. 12 kWh/kg) 0.5 kg

Wood (approx. 4.5 kWh/kg) 1.3 kg

Cow dung (approx. 5 kWh/kg dry matter) 1.2 kg

Plant residues (approx. 4.5 kWh/kg d.m.) 1.3 kg

Hard coal (approx. 8.5 kWh/kg) 0.7 kg


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City gas (approx. 5.3 kWh/m3) 1.1 m3

Propane (approx. 25 kWh/m3) 0.24 m3

Large units and/or communal units produce biogas in large quantities and can be used

to power engines and generators for mechanical work or power generation.

Transformation of organic wastes into high quality organic fertilizer

The biogas digester is fed with cow dung slurry at a design rate, which is governed by

local parameters. The output from the digester (digested manure) is actually a high

quality organic fertilizer. This fertilizer is very important where the farmers do not have

the resources to buy chemical fertilizers frequently. If you compost chicken manure, for

example, the finished compost will have in it only 1.58 to 2%o nitrogen. The same

manure digested in a bio-gas plant will analyze 6% nitrogen.

The digested slurry must be immediately utilized - and properly applied - as fertilizer,

each daily kg can be expected to yield roughly 0.5 kg extra nitrogen, as compared with

fresh manure. If the slurry is first left to dry and/or improperly applied, the nitrogen

yield will be considerably lower. This nitrogen is already present in the manure. The

nitrogen is preserved when waste is digested in an enclosed bio-gas plant, whereas the

same nitrogen evaporates away as ammonia during open air composting.

The bio-gas plant is the perfect fertilizer-making machine and it has been tested all over

the world. There is no better way to digest or compost manure and other organic

material than in a bio-gas plant. One can compare the bacteria in a digester tank to fish

worms. Fish worms help the soil by eating organic matter, passing it through their bodies

and expelling it as very rich fertilizer. They live by breaking waste material down into

food for plants. It is the same with the bacteria in a methane digester. It is all very

natural.


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CHAPTER 6)

GEOTHERMAL ENERGY

RESOURCES

Geothermal energy is the heat from the Earth. It's clean and sustainable. Resources of

geothermal energy range from the shallow ground to hot water and hot rock found a few

miles beneath the Earth's surface, and down even deeper to the extremely high

temperatures of molten rock called magma.

Almost everywhere, the shallow ground or upper 10 feet of the Earth's surface maintains

a nearly constant temperature between 50° and 60°F (10° and 16°C). Geothermal heat

pumps can tap into this resource to heat and cool buildings. A geothermal heat pump

system consists of a heat pump, an air delivery system (ductwork), and a heat exchanger-

a system of pipes buried in the shallow ground near the building. In the winter, the heat

pump removes heat from the heat exchanger and pumps it into the indoor air delivery

system. In the summer, the process is reversed, and the heat pump moves heat from the

indoor air into the heat exchanger. The heat removed from the indoor air during the

summer can also be used to provide a free source of hot water.

In the United States, most geothermal reservoirs of hot water are located in the western

states, Alaska, and Hawaii. Wells can be drilled into underground reservoirs for the

generation of electricity. Some geothermal power plants use the steam from a reservoir to

power a turbine/generator, while others use the hot water to boil a working fluid that

vaporizes and then turns a turbine. Hot water near the surface of Earth can be used

directly for heat. Direct-use applications include heating buildings, growing plants in

greenhouses, drying crops, heating water at fish farms, and several industrial processes

such as pasteurizing milk.

Hot dry rock resources occur at depths of 3 to 5 miles everywhere beneath the Earth's

surface and at lesser depths in certain areas. Access to these resources involves injecting

cold water down one well, circulating it through hot fractured rock, and drawing off the

heated water from another well. Currently, there are no commercial applications of this

technology. Existing technology also does not yet allow recovery of heat directly from

magma, the very deep and most powerful resource of geothermal energy.

Many technologies have been developed to take advantage of geothermal energy - the

heat from the earth. NREL performs research to develop and advance technologies for

the following geothermal applications:

Geothermal Electricity Production

Generating electricity from the earth's heat.


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Geothermal Direct Use

Producing heat directly from hot water within the earth.

Geothermal Heat Pumps

Using the shallow ground to heat and cool buildings.

The centre of the Earth is around 6000 degrees Celsius - easily hot enough to melt rock. Even a few

kilometers down, the temperature can be over 250 degrees Celsius if the Earth's crust is thin. In

general, the temperature rises one degree Celsius for every 30 - 50 meters you go down, but this does

vary depending on location .In volcanic areas, molten rock can be very close to the surface.

Sometimes we can use that heat. Geothermal energy has been used for thousands of years in some

countries for cooking and heating. The name "geothermal" comes from two Greek words: "geo"

means "Earth" and "thermal" means "heat".

GEOTHERMAL WELLS

Geothermal wells are wells which tap into the natural geothermal energy found beneath

the Earth‘s crust. Geothermal energy is thermal energy generated and stored in the

Earth. Thermal energy is the energy that determines the temperature of matter. Earth‘s

geothermal energy originates from the original formation of the planet, from the

radioactive decay of minerals, from volcanic activity, and from solar energy absorbed at

the surface. Geothermal wells have been used for several decades as an adjunct to

existing heating and cooling systems. The systems are designed to use the Earth‘ s

relatively constant subsurface temperature along with a heat exchanger to either add to

or remove heat from a dwelling.

What is Geothermal Electricity?

Even though the surface of Earth can get quite cold at times, the area beneath the Earth‘s

crust has a relatively stable temperature and it is usually very hot. Geothermal energy

utilizes this heat to generate electricity called the ‗Geothermal electricity‘ and to provide

heating to various structures. The energy can either be used directly, in the form of

geothermal wells which connect to sources of water and steam heated by the earth, or

indirectly, in the case of systems which pump water through hot regions under the

Earth‘s crust.


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What are the different types of geothermal wells?

Geothermal wells also known as geo exchange systems have 2 basic designs. They are:

Open looped system: in an open looped system, ground water is pumped from water

well into a heat exchanger located in a surface dwelling. The water drawn from the earth

is then pumped back into the aquifer through a different well, or in some cases the same

well. Alternatively, the ground water could be discharged to a surface of water body. In

the heating mode, cooler water is returned to the earth, while in the cooling mode,

warmer water is returned.

Closed loop system: In a closed loop system, an opening (either borehole or trench) is

made in the earth. A series of pipes are installed into the opening and connected to a

heat exchange system in the dwelling. The pipes form a ―closed loop‖, and are filled

with a heat transfer fluid. The fluid is circulated through the piping from the opening

into the heat exchanger and back. The system functions in the same manner as the open

looped system, except there is no pumping of ground water.

How is geothermal well constructed?

The methods and equipment used to drill geothermal wells are similar to those used to

drill oil and gas wells.

High-temperature and Steam-dominated Reservoirs: The drilling fluid-also called drilling

mud-is circulated through the well to bring the cuttings back to the surface and to cool

the well. After a well is drilled, often beyond 5,000 feet, steel pipe, called casing, is

cemented in place by pumping cement into the annulus (space between the casing and

the rock formation). The casing and cement prevent fluids in different zones from mixing

with each other or with the produced fluids. It is essential to cement all of the annular

space behind the casting. Cement prevents the casing from expanding when heated and

helps prevent corrosion.

Low-temperature, Water-dominated Reservoirs: Wells in low-temperature, water

dominated reservoirs are usually less than 1,000 feet deep. Typically, the well is cased

and cemented to appoint above the production zone to prevent polluted surface waters

coming from contaminating shallows, water-bearing zones. The standard for this type of

well resemble those for other water wells.

How many geothermal wells are there and how much water is produced?

There are about 470 steam wells and 230 high-temperature, hot wells in 10 high-

temperature geothermal fields in California. In 2005, over 324 billion kilograms of water

was produced from these wells. In addition, there are several hundred low-temperature

geothermal wells in the state for which the Division has no records.


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TYPES OF GEOTHERMAL SYSTEMS

There are two basic geothermal system types: Hydrothermal and Hot Rock or Enhanced

Geothermal Systems.

Normally conventional hydrothermal reservoirs are found in fractured volcanic rocks

where temperatures are relatively high near the surface such as in New Zealand.

However, geothermal reservoirs are found in non volcanic areas where the crustal heat

flow is sufficient to produce high temperatures and the rocks are permeable to allow the

production of large volumes of fluid.

Greenearth Energy's domestic and international exploration and development focus is

on conventional geothermal (Hydrothermal) systems. In Australia the focus is on Hot

Sedimentary Aquifer (HSA) systems. In Indonesia the focus is on Volcanic Systems.

FIGURE 1: Schematic of Geothermal Systems

The extraction of heat from hot rocks is achieved by pumping water into the rocks at

depth using an injection well, and subsequently withdrawing it from a production well at

a much higher temperature after it has flowed under pressure through fractures in the hot

rocks. Hot rocks are hot due to the heat which is generated at depth being trapped by the

insulating effect of overlying rocks and sedimentary cover usually over 3000 metres in

thickness.


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USES FOR GEOTHERMAL ENERGY

Direct uses for low and moderate temperature resources typically up to 150oC involves

using the heat in the water directly (without a heat pump or power plant) for such things

as heating of buildings, industrial processes, greenhouses, aquaculture (growing of fish)

and resorts. Figure 2 provides an overview of such applications at varying temperatures.

FIGURE 2: Temperature ranges for Direct Heat Applications

POWER GENERATION USING GEOTHERMAL ENERGY

Most geothermal power plants operating today are "flashed steam" power plants using

high temperature water from production wells. Released from the pressure of the deep

reservoir, part of the water flashes (explosively boils) to steam and the force of the

steam is used to spin the turbine generator. In a binary power plant the working fluid

(usually isobutane or isopentane) boils and flashes to a vapor at a lower temperature than

water does, so electricity can be generated from reservoirs with lower temperatures.


67

Binary power plants have virtually no emissions but are relatively less efficient. Figure 3

provides a schematic of a typical binary power plant arrangement.

Figure 3 Schematic of Binary Cycle Power Plant Source: Oregon Institute of Technology

(modified by Greenearth Energy) to take account of the change of the cooler to air

cooling rather than vapour (evaporative) cooling.

Conventional hydrothermal systems have well-demonstrated economic and

technological viability. Some geothermal fields have been supporting cost-effective,

dependable electrical power generation for over 50 years. Ongoing advances in binary

geothermal power-plant technology have lowered the temperature requirement for

electrical power generation, and also improved the bottom-line economic viability of

many conventional hydrothermal resources.

TECHNOLOGIES HARNESSING ENERGY

Technology

Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and

very hot water that drive turbines that drive electricity generators. Four types of power

plants are operating today:


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Flashed steam plant

The extremely hot water from drill holes when released from the deep reservoirs high

pressure steam (termed as flashed steam) is released. This force of steam is used to rotate

turbines. The steam gets condensed and is converted into water again, which is returned

to the reservoir. Flashed steam plants are widely distributed throughout the world.

Dry steam plant

Usually geysers are the main source of dry steam. Those geothermal reservoirs which

mostly produce steam and little water are used in electricity production systems. As

steam from the reservoir shoots out, it is used to rotate a turbine, after sending the steam

through a rock-catcher. The rock-catcher protects the turbine from rocks which come

along with the steam.

Binary power plant

In this type of power plant, the geothermal water is passed through a heat exchanger

where its heat is transferred to a secondary liquid, namely isobutene, iso-pentane or

ammonia–water mixture present in an adjacent, separate pipe. Due to this double-liquid

heat exchanger system, it is called a binary power plant. The secondary liquid which is

also called as working fluid, should have lower boiling point than water. It turns into

vapor on getting required heat from the hot water. The vapor from the working fluid is

used to rotate turbines. The binary system is therefore useful in geothermal reservoirs

which are relatively low in temperature gradient. Since the system is a completely closed

one, there is minimum chance of heat loss. Hot water is immediately recycled back into

the reservoir. The working fluid is also condensed back to the liquid and used over and

over again.

Hybrid power plant

Some geothermal fields produce boiling water as well as steam, which are also used in

power generation. In this system of power generation, the flashed and binary systems are

combined to make use of both steam and hot water. Efficiency of hybrid power plants is

however less than that of the dry steam plants.

Enhanced geothermal system

The term enhanced geothermal systems (EGS), also known as engineered geothermal

systems (formerly hot dry rock geothermal), refers to a variety of engineering techniques

used to artificially create hydrothermal resources (underground steam and hot water)

that can be used to generate electricity. Traditional geothermal plants exploit naturally

occurring hydrothermal reservoirs and are limited by the size and location of such

natural reservoirs. EGS reduces these constraints by allowing for the creation of

hydrothermal reservoirs in deep, hot but naturally dry geological formations.EGS

techniques can also extend the lifespan of naturally occurring hydrothermal resources.

Given the costs and limited full-scale system research to date, EGS remains in its

infancy, with only a few research and pilot projects existing around the world and no

commercial-scale EGS plants to date. The technology is so promising, however, that a

number of studies have found that EGS could quickly become widespread.


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Potential In India

It has been estimated from geological, geochemical, shallow geophysical and shallow

drilling data it is estimated that India has about 10,000 MWe of geothermal power

potential that can be harnessed for various purposes. Rocks covered on the surface of

India ranging in age from more than 4500 million years to the present day and

distributed in different geographical units. The rocks comprise of Archean, Proterozoic,

the marine and continental Palaeozoic, Mesozoic, Teritary, Quaternary etc., More than

300 hot spring locations have been identified by Geological survey of India (Thussu,

2000). The surface temperature of the hot springs ranges from 35 C to as much as 98 C.

These hot springs have been grouped together and termed as different geothermal

provinces based on their occurrence in specific geotectonic regions, geological and

strutural regions such as occurrence in orogenic belt regions, structural grabens, deep

fault zones, active volcanic regions etc., Different orogenic regions are – Himalayan

geothermal province, Naga-Lushai geothermal province, Andaman-Nicobar Islands

geothermal province and non-orogenic regions are – Cambay graben, Son-Narmada-

Tapi graben, west coast, Damodar valley, Mahanadi valley, Godavari valley etc.

Puga Valley (J&K)

Tatapani (Chhattisgarh)

Godavari Basin Manikaran (Himachal Pradesh)

Bakreshwar (West Bengal)

Tuwa (Gujarat)

Unai (Maharashtra)

Jalgaon (Maharashtra)


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CHAPTER 7)

OCEAN ENERGY

The ocean can produce two types of energy: thermal energy from the sun's heat, and

mechanical energy from the tides and waves.

Oceans cover more than 70% of Earth's surface, making them the world's largest solar

collectors. The sun's heat warms the surface water a lot more than the deep ocean water,

and this temperature difference creates thermal energy. Just a small portion of the heat

trapped in the ocean could power the world.

Ocean thermal energy is used for many applications, including electricity generation.

There are three types of electricity conversion systems: closed-cycle, open-cycle, and

hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working

fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a

turbine. The turbine then activates a generator to produce electricity. Open-cycle systems

actually boil the seawater by operating at low pressures. This produces steam that passes

through a turbine/generator. And hybrid systems combine both closed-cycle and open-

cycle systems.

Ocean mechanical energy is quite different from ocean thermal energy. Even though the

sun affects all ocean activity, tides are driven primarily by the gravitational pull of the

moon, and waves are driven primarily by the winds. As a result, tides and waves are

intermittent sources of energy, while ocean thermal energy is fairly constant. Also,

unlike thermal energy, the electricity conversion of both tidal and wave energy usually

involves mechanical devices.

A barrage (dam) is typically used to convert tidal energy into electricity by forcing the

water through turbines, activating a generator. For wave energy conversion, there are

three basic systems: channel systems that funnel the waves into reservoirs; float systems

that drive hydraulic pumps; and oscillating water column systems that use the waves to

compress air within a container. The mechanical power created from these systems

either directly activates a generator or transfers to a working fluid, water, or air, which

then drives a turbine/generator.


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What is OTEC

Ocean Thermal Energy Conversion (OTEC) is a marine renewable energy technology

that harnesses the solar energy absorbed by the oceans to generate electric power. The

sun‘s heat warms the surface water a lot more than the deep ocean water, which creates

the ocean‘s naturally available temperature gradient, or thermal energy.

OTEC uses the ocean‘s warm surface water with a temperature of around 25°C (77°F) to

vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor

expands and spins a turbine coupled to a generator to produce electricity. The vapor is

then cooled by seawater that has been pumped from the deeper ocean layer, where the

temperature is about 5°C (41°F). That condenses the working fluid back into a liquid, so

it can be reused. This is a continuous electricity generating cycle.

The efficiency of the cycle is strongly determined by the temperature differential. The

bigger the temperature difference, the higher the efficiency. The technology is therefore

viable primarily in equatorial areas where the year-round temperature differential is at

least 20 degrees Celsius or 36 degrees Fahrenheit.

Resource

The oceans cover more than 70% of Earth‘s surface and capture a large part of the sun‘s

heat in the upper layers, making them the world‘s largest solar collectors and energy

storage system. Utilizing just a small portion of this energy, can cover the global energy

need.

The energy source of OTEC is free, available abundantly and is continually being

replenished as long as the sun shines and the natural ocean currents exist. Various

renowned parties estimate the amount of energy that can be practically harvested to be in

http://www.otecnews.org/what-is-otec/working-principle/

http://www.otecnews.org/what-is-otec/resource/


72

the order of 3 to 5 terawatts (1 terawatt is 1012 watts) of baseload power generation,

without affecting the temperature of the ocean or the world‘s environment. That‘s about

twice the global electricity demand. The oceans are thus a vast renewable resource, with

the potential to contribute to the future energy mix offering a sustainable electricity

production method.

The technology is viable primarily in equatorial areas where the year-round temperature

differential is at least 20°C (36°F).

Benefits

The distinctive feature of OTEC is the potential to provide baseload electricity, which

means day and night (24/7) and year-round. This is a big advantage for for instance

tropical islands that typically has a small electricity network, not capable of handling a

lot of intermittent power.

OTEC benefits

Next to producing electricity, OTEC also offers the possibility of co-generating other

synergistic products, like fresh water, nutrients for enhanced fish farming and seawater

cooled greenhouses enabling food production in arid regions. Last but not least, the cold

water can be used in building air-conditioning systems. Energy savings of up to 90% can

be realized.

The vast baseload OTEC resource could help many tropical and subtropical (remote)

regions to become more energy self-sufficient.

Synergetic products

Fresh Water: The first by-product is fresh water. A small hybrid 1 MW OTEC is capable

of producing some 4,500 cubic meters of fresh water per day, enough to supply a

population of 20,000 with fresh water. OTEC-produced fresh water compares very

favourably with standard desalination plants, in terms of both quality and production

costs.

Food: A further by-product is nutrient rich cold water from the deep ocean. The cold

―waste‖ water from the OTEC is utilised in two ways. Primarily the cold water is

discharged into large contained ponds, near shore or on land, where the water can be

used for multi-species mariculture producing harvest yields which far surpass naturally

occurring cold water upwelling zones, just like agriculture on land.

Cooling: The cold water is also available as chilled water for cooling greenhouses, such

as the Seawater Greenhouse or for cold bed agriculture. The cold water can also be used

for air conditioning systems or more importantly for refrigeration systems, most likely

linked with creating cold storage facilities for preserving food. When the cold water has

been used it is released to the deep ocean.


73

Cold water applications

PRINCIPLE UTILIZATION

1. Absorption of waves means generation of waves.

A body oscillating in water will produce waves. A big body and a small body may

produce equally large waves provided the smaller body oscillates with larger amplitude.

This may be utilised for the purpose of wave energy conversion, for instance by a small

floating body heaving in response to an incident wave, in particular so if it can be

arranged that the body oscillates with a larger amplitude than the wave amplitude.

2. Optimum oscillation for maximum energy capture.

In order to obtain maximum energy from the waves it is necessary to have optimum

oscillation of the wave-energy converter (WEC). For a sinusoidal incident wave there is

an optimum phase and an optimum amplitude for the oscillation.

3. Phase control by latching.

In order to obtain the optimum oscillatory motion for maximising the absorbed energy

or the converted useful energy it may be necessary to return some energy back into the

sea during some small fractions of each oscillation cycle and profit from this during the

remaining part of the cycle. For this reason ―optimum control‖ of WECs has also been

termed ―reactive control‖. To achieve this in practice it is required to utilise a reversible

energy-converting machinery with very low conversion losses. It could, for instance be a

high-efficiency hydraulic machinery which can work either as a motor or as a pump.11

To realise the optimum control in practice, a computer with appropriate programme

software, and with input signals from sensors measuring the wave1 and/or the WEC‘s

oscillatory motion10, is required. It is also necessary to predict the wave some seconds

into the future.


74

SETTING OF OTEC PLANTS

The Earth's oceans are continually heated by the sun, and cover nearly 70% of the earth‘s

surface. The secret to harvesting the ocean‘s stored solar energy lies in exploiting the

difference in temperature between the warmer water at the surface, and the colder water

at greater depth.

If the extraction could be made cost-effective, it could provide two to three times more

energy than other ocean-energy options, such as wave power. But the small magnitude

of the temperature difference makes energy extraction, so far, relatively difficult and

expensive.

How Does Ocean Thermal Energy Conversion Create Electrical Energy?

Perhaps the easiest way to understand ocean thermal energy conversion (OTEC) is by

looking at the three primary types of OTEC plant: (1) open-cycle, (2) closed-cycle, and

(3) hybrid.

All three plants make use of a ―heat engine‖ – a device placed between deep, cold ocean

water and shallow, warmer water. As heat flows from the warm water to the cold water,

the heat engine uses the energy of the transfer to drive a generator that creates electricity.

Closed-cycle Ocean Thermal Energy Conversion

Warm surface seawater is pumped through a heat exchanger that vaporizes a fluid with a

low boiling point (e.g., ammonia). The expanding vapor turns a turbo-generator to

produce electricity.

Open-cycle Ocean Thermal Energy Conversion

Warm seawater is placed in a low-pressure container, where it boils. The expanding

steam drives a turbine attached to an electrical generator. When the ocean water turns to

steam, it leaves behind its salt and other contaminants. The steam is then exposed to cold

ocean water, condensing it into fresh water for drinking or irrigation.

Hybrid Ocean Thermal Energy Conversion

Warm seawater enters a vacuum chamber, where it is flash-evaporated into steam

(similar to the open-cycle process). The heat of the steam vaporizes ammonia in a

separate container, and the vaporized ammonia drives a turbine to produce electricity

(similar to the closed-cycle process). Vaporizing the seawater removes its salt and other

impurities. When the steam condenses in the heat exchanger, it emerges as fresh, pure

water for drinking or agriculture.


75

Where Are the Best Locations for OTC Plants?

OTEC plants can produce more power where the temperature difference between warm

and cold ocean water is greatest. This generally occurs within 20° north and south of the

equator, in the tropics.

What Is the Record Power Output From an OTEC Plant?

In May 1993, an experimental open-cycle OTEC plant at Keahole Point, Hawaii

produced 50,000 watts of electricity, breaking the record of 40,000 watts set by a

Japanese system in 1982.

Has Ocean Thermal Energy Conversion Been Tried in the Past?

In 1881, French physicist Jacques Arsene d‘Arsonval proposed tapping the thermal

energy of the ocean. A student of d‘Arsonval‘s, Georges Claude, built the first OTEC

plant in Cuba in 1930. The system generated 22 kW of electricity using a low-pressure

turbine.

The Natural Energy Laboratory of Hawaii Authority, established in 1974, is one of the

world's leading test facilities for OTEC technology. Hawaii is often said to be the best

U.S. location for OTEC, because of warm surface water, excellent access to very deep,

very cold water, and because Hawaii has the highest electricity costs in the U.S.

Japan has been a major contributor to the development of OTEC technology, primarily

for export to other countries. In the 1970s, the Tokyo Electric Power Company built a

100 kW closed-cycle OTEC plant on the island of Nauru. The plant became operational

in 1981 and produced about 120 kW of electricity (90 kW was used to power the plant,

and the remaining electricity was used to power a school and several other facilities in

Nauru). This set a world record for power output from an OTEC system where the

power was sent to a real power grid.

What Share of the World’s Energy Needs Could OTEC Supply?

Some experts believe that if OTEC became cost-competitive, it could provide gigawatts

of electrical power, and in conjunction with electrolysis, could produce enough hydrogen

to completely replace all projected global fossil fuel consumption.

What Barriers Stand in the Way OTEC Power Production?

Managing costs remains a huge challenge. OTEC plants require expensive, large-

diameter intake pipes, submerged at least a kilometer deep in the ocean to bring very

cold water to the surface. Cold seawater is a requirement for all three types of OTEC


76

systems. The cold seawater can be brought to the surface by direct pumping, or by

desalinating the seawater near the sea floor, lowering its density and causing it to ―float‖

through a pipe to the surface.

Has a Closed-cycle OTEC Plant Ever Been Built?

In 1979, the Natural Energy Laboratory and several private-sector partners developed a

mini OTEC experiment that achieved the first successful at-sea production of net

electrical power from closed-cycle OTEC. (Net power is that which remains after

subtracting the power required to run the plant.) The mini OTEC vessel was moored 1.5

miles off the Hawaiian coast and produced enough net electricity to illuminate the ship's

light bulbs and run its computers and televisions.

In 1999, the Natural Energy Laboratory tested a 250 kW pilot closed-cycle plant, the

largest of its kind. Since then, no further tests of OTEC technology have been conducted

in the U.S., largely because the costs of energy production today have delayed financing

of a permanent, continuously operating plant.

What OTEC Projects are on the Drawing Board?

Planned OTEC projects include a small plant for the U.S. Navy base on the island of

Diego Garcia in the Indian Ocean, to replace existing diesel generators. The plant would

also provide 1,250 gallons of drinking water to the base per day.

A private firm has proposed building a 10-MW OTEC plant on Guam. And Lockheed

Martin‘s Alternative Energy Development team is in the final design phases of a 10-MW

closed cycle OTEC pilot system that will become operational in Hawaii in 2012 or 2013.

The system will be designed to expand to 100-MW commercial systems in the near

future.

Does OTEC Have Benefits Beyond Producing Power?

Yes, indeed. For example, the cold seawater from an OTEC system can provide air-

conditioning for buildings. If such a system operated 8000 hours per year in a large

building, and local electricity sold for 5¢-10¢ per kilowatt-hour, it could save $200,000-

$400,000 in annual energy bills (U.S. Department of Energy, 1989). The

InterContinental Resort and Thalasso-Spa on Bora Bora now uses OTEC technology to

air-condition its buildings. The system passes cold seawater through a heat exchanger,

where it cools fresh water in a closed-loop system. The cool freshwater is then pumped

to buildings for cooling (no conversion to electricity takes place).

Another application is chilled-soil agriculture . When cold seawater flows through

underground pipes, it chills the surrounding soil. The temperature difference between


77

plant roots in the cool soil and plant leaves in the warm air allows many plants that

evolved in temperate climates to be grown in the subtropics.

Aquaculture, another viable OTEC offshoot, is considered one of the best ways to reduce

the financial and energy costs of pumping large volumes of water from the deep ocean.

Deep ocean water contains high concentrations of essential nutrients that are depleted in

surface waters due to consumption by animal and plant life. This ―artificial upwelling‖

mimics natural upwellings responsible for fertilizing and supporting the largest marine

ecosystems, and the largest densities of life on the planet. Cold-water delicacies such as

salmon and lobster, and microalgae such as spirulina can also be cultivated in the

nutrient-rich cold water from OTEC plants.

As described earlier, open-cycle and hybrid OTEC plants produce desalinated wate.

System analysis indicates that a 2-megawatt (net) plant could produce about 4300 cubic

meters of desalinated water per day (Block and Lalenzuela 1985).

OTEC plants can produce hydrogen via electrolysis, using electricity generated by the

OTEC plant. Also, minerals can be extracted from seawater pumped by OTEC plants.

Japanese researchers have recently found that developments in materials sciences and

other technologies are improving the ability to extract minerals efficiently, using ocean

energy.

What Barriers Stand in the Way of Ocean Thermal Energy Conversion?

The obstacles to OTEC as a viable power source are considerable, but probably not

insurmountable. Political concerns include the legal status of OTEC facilities located in

the open ocean. Costs, of course, also remain uncertain, because so few OTEC facilities

have been deployed. One study estimated OTEC power generation costs as low as US

$0.07 per kilowatt-hour, compared with $0.05 - $0.07 for subsidized wind systems.

How Positive is the Outlook for OTEC Power Generation?

Positive factors include the fact that OTEC is a renewable resource without waste

products or limited fuel supplies; the vast area in which it is available (within 20° of the

equator); freedom from dependence on petroleum; possible development of alternate

ocean power sources, such as wave energy, tidal energy, and by extracting methane

hydrates; and the possibility of combining OTEC with solar energy, aquaculture, air

conditioning, and mineral extraction.

Do Technical Difficulties Stand in the Way of OTEC?

Unfortunately, yes. For example, OTEC plants often use direct contact heat exchangers,

which generate gases that can degrade a plant‘s efficiency. Since the theoretical


78

maximum efficiency of OTEC plants is 6% to 7%, and present plants operate at slightly

lower efficiencies, anything that degrades performance must be considered significant.

Other problems include microbial fouling, which lowers thermal conductivity; improper

sealing; and parasitic power consumption by exhaust compressors. However, these

obstacles are the focus of ongoing research, and seem likely to be solved in the near

future.

Thermodynamic Cycle


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TIDAL ENERGY AND WAVE ENERGY

What is Wave and Tidal Energy? In addition to its abundant solar, wind and geothermal resources, the Pacific Northwest

is also uniquely situated to capture the renewable energy of the ocean. Special buoys, turbines, and other technologies can capture the power of waves and tides and convert it into clean, pollution-free electricity. Like other renewable resources, both wave and tidal

energy are variable in nature. Waves are produced by winds blowing across the surface of the ocean. However, because waves travel across the ocean, their arrival time at the

wave power facility may be more predictable than wind. In contrast, tidal energy, which is driven by the gravitational pull of the moon and sun, is predictable centuries in

advance. The technologies needed to generate electricity from wave and tidal energy are at a

nascent stage, but the first commercial projects are currently under development, including some in the Pacific Northwest. Like most emerging energy technologies, wave

and tidal technologies are currently more expensive than traditional generating resources, but with further experience in the field, adequate R&D funding, and proactive

public policy support, the costs of wave and tidal technologies are expected to fol-low the same rapid decrease in price that wind energy has experienced.

Potential Worldwide potential for wave and tidal power is enormous, however, local

geography greatly influences the electricity generation potential of each technology.

Wave energy resources are best between 30º and 60º latitude in both hemispheres,

and the potential tends to be the greatest on western coasts. The United States receives 2,100 terawatt-hours of incident wave energy along its

coastlines each year, and tapping just one quarter of this potential could produce as much energy as the entire U.S. hydropower system. Oregon and Washington have the

strongest wave energy resource in the lower 48 states and could eventually generate

several thousand megawatts of electricity using wave resources.2 Several sites in

Washington‘s Puget Sound with excellent tidal resources could be developed, potentially yielding several hundred megawatts of tidal power.3

While no commercial wave or tidal projects have yet been developed in the United States, several projects are planned for the near future, including projects in the

Northwest. AquaEnergy Group, Ltd is currently designing and permitting a one-megawatt demonstration wave power plant at Makah Bay, Washington. Ocean Power

Technologies has received a preliminary permit to explore construction of North America‘s first utility-scale wave energy facility off the coast of Reedsport, Oregon. With the support of the Oregon Department of Energy, Oregon State University is also

seeking funding to build a national wave en-ergy research facility near Newport, Oregon. Several tidal power projects are also being explored in the region. Tacoma Power has

secured a preliminary permit to explore a tidal power project at the Tacoma Narrows, one of the best locations for tidal power in the country, and Snohomish County Public

http://www.rnp.org/node/wave-tidal-energy-technology#end2



80

Utility District has received preliminary permits for seven other potential tidal power sites in the Puget Sound.4

Wave Energy Technologies There are three main types of wave energy technologies. One type uses floats, buoys, or

pitching devices to generate electricity using the rise and fall of ocean swells to drive hydraulic pumps. A second type uses oscillating water column (OWC)devices to

generate electricity at the shore using the rise and fall of water within a cylindrical shaft. The rising water drives air out of the top of the shaft, powering an air-driven turbine.

Third, a tapered channel, or overtoppingdevice can be located either on or offshore. They concentrate waves and drive them into an elevated reservoir, where power is then

generated using hydropower turbines as the water is released. The vast majority of

recently proposed wave energy projects would use offshore floats, buoys or pitching devices.

The world‘s first commercial offshore wave energy facility will begin operating by the

end of 2007 off the Atlantic coast of Portugal. The first phase of the project, which Scottish company, Ocean Power Delivery (OPD) developed, features three ‗Pelamis‘ wave energy conversion devices and generates a combined 2.25 MW of electricity. OPD

plans to expand the facility to produce 22.5 MW in 2007.5

Tidal Power Technologies Until recently, the common model for tidal power facilities involved erecting a tidal dam, or barrage, with a sluice across a narrow bay or estuary. As the tide flows in or out,

creating uneven water levels on either side of the barrage, the sluice is opened and water flows through low-head hydro turbines to generate electricity. For a tidal barrage to be

feasible, the difference between high and low tides must be at least 16 feet. La Rance Station in France, the world‘s first and still largest tidal barrage, has a rated capacity of 260 MW and has operated since 1966. However, tidal barrages, have several

environmental drawbacks, including changes to marine and shoreline ecosystems, most notably fish populations.6

Several other models for tidal facilities have emerged in

recent years, including tidal lagoons, tidal fences, and

underwater tidal turbines, but none are commercially operating. Perhaps the most promising is the underwater

tidal turbine. Several tidal power companies have developed tidal turbines, which are similar in many ways

to wind turbines. These turbines would be placed offshore or in estuaries in strong tidal currents where the tidal flow

spins the turbines, which then generate electricity. Tidal turbines would be deployed in underwater ‗farms‘ in waters 60-120 feet deep with currents exceeding 5-6 mph.

Because water is much denser than air, tidal turbines are smaller than wind turbines and can produce more

electricity in a given area.7 A pilot-scale tidal turbine facility – the first in North America – was installed in New York‘s East River in December 2006. The developer, Verdant

Power, hopes to eventually install a 10 MW tidal farm at the site.8







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Environmental Impacts Unlike fossil-fueled power plants, wave and tidal energy facilities generate electricity

without producing any pollutant emissions or greenhouse gases. Since the first wave and tidal energy facilities are currently being deployed, the full environmental impacts of wave and tidal power remain uncertain but are projected to be small. Concerns include

impacts on marine ecosystems and fisheries. Environmental impact studies are currently underway and several pilot and commercial projects are undergoing environmental

monitoring. The East River tidal turbine pilot project includes a $1.5 million sonar system to monitor impacts on fish populations, for example.9 Careful siting should

minimize impacts on marine ecosystems, fishing and other coastal economic activities. Wave and tidal facilities also have little or no visual impact, as they are either submerged

or do not rise very far above the waterline.

HYDRO POWER PLANT

There are three types of hydropower facilities: impoundment, diversion, and pumped

storage. Some hydropower plants use dams and some do not. The images below show

both types of hydropower plants.

Many dams were built for other purposes and hydropower was added later. In the

United States, there are about 80,000 dams of which only 2,400 produce power. The

other dams are for recreation, stock/farm ponds, flood control, water supply, and

irrigation.

Hydropower plants range in size from small systems for a home or village to large

projects producing electricity for utilities. The sizes of hydropower plants are described

below.

IMPOUNDMENT

The most common type of hydroelectric power plant is an impoundment facility. An

impoundment facility, typically a large hydropower system, uses a dam to store river

water in a reservoir. Water released from the reservoir flows through a turbine, spinning

it, which in turn activates a generator to produce electricity. The water may be released

either to meet changing electricity needs or to maintain a constant reservoir level.


http://energy.gov/eere/water/types-hydropower-plants#sizes


82

DIVERSION

A diversion, sometimes called run-of-river, facility channels a portion of a river through

a canal or penstock. It may not require the use of a dam.

PUMPED STORAGE

When the demand for electricity is low, a pumped storage facility stores energy by

pumping water from a lower reservoir to an upper reservoir. During periods of high

electrical demand, the water is released back to the lower reservoir to generate electricity.


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SIZES OF HYDROELECTRIC POWER PLANTS

Facilities range in size from large power plants that supply many consumers with

electricity to small and micro plants that individuals operate for their own energy needs

or to sell power to utilities.

Large Hydropower

Although definitions vary, DOE defines large hydropower as facilities that have a

capacity of more than 30 megawatts.

Small/Mini Hydropower

Although definitions vary, DOE defines small hydropower as facilities that have a

capacity of 100 kilowatts to 30 megawatts.

Micro Hydropower

A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro-

hydroelectric power system can produce enough electricity for a home, farm, ranch, or

village.


84

MINI HYDEL POWER PLANT AND Their ECONOMICS

Hydropower is using water to power machinery or make electricity. Water constantly

moves through a vast global cycle, evaporating from lakes and oceans, forming clouds,

precipitating as rain or snow, then flowing back down to the ocean. The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for

mechanical tasks like grinding grain. Hydropower uses a fuel—water—that is not reduced or used up in the process. Because the water cycle is an endless, constantly recharging system, hydropower is considered a renewable energy.

When flowing water is captured and turned into electricity, it is called hydroelectric

power or hydropower. There are several types of hydroelectric facilities; they are all powered by the kinetic energy of flowing water as it moves downstream. Turbines and

generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses, and by industry.


85

Mini hydel power station Mangrol is situated on Right Main canal of Chambal project

near R.D. 106070.4 meter emanating from Kota Barrage.

The plant have three units of capacity 2 MW each. The main supplier & erector

of main generation equipment by M/s BHEL and Civil contractor of power house

building is M/s RSBCEE Jaipur.

SALIENT FEATURES

S.NO. Particulars

a. Type of Power House 9 Surface Power House.

b. Installed Capacity 6 MW

c. No. of Unit & Capacity 3 Units of 2 Mw each.

d. Location of Power House Near R.D. 106070.4 meter on RMC of Kota Barrage.

e. Nearest Village MUNDLA, Tehsil Mangrol Distt. Baran.

f. Nearest Railway Station Baran (25 Km.)

g. Supplier & Main erector of

Main generation equipment. M/S BHEL.

h. Civil Contractor of PH

Building M/S RSBCC Jaipur.

i. Consultants CEA for Electrical works WNPCOS for Civil works.

j. Design head 7.32 meter

k. Design discharge of RMC At

PH site. 4050 cusecs.


86

l. No. of falls & height 5 Nos total head 24.27 ft.

m. Max. Power potential 6.94 MW

n. Average Annual Power

potential 4.31 MW

o. Max. tail water level EL 241.12 meter.

p. Type of Turbine and Name Propeller Axial Flow Horizontal type Kaplan turbine

make :-BHEL.

q. Type of Generator & make Synchronous Make- BHEL.

GATES & TRASH RACK.

a Intake gate 3 Nos. of 4028.4x5049 mm each.

b Stop-log gate 1 Set of 4028.4 x13800 mm.

c Draft Tube gate 1 No. of 5278.4x 4167 mm.

DISCHARGE CANAL NEAR POWER HOSUE:

a Full supply level EL 241.12 m.

b Bed Level EL 237.92m

c Section Bed width 26.70m, FSD 3.2m

d Velocity 3 ft/sec.

e Length 1800 m.

CAPITAL COST

a a cost of power production 11.76 crore.

b Cost of transmission 5.78 crore.

c Total capital cost 17.54 crore.

d Installation cost per MW Rs. 29,233/-

TRANSMISSION LINE

a Ist Feedr Mangrol to ayana 33 KV 16 Km.

b IInd Feeder Mangrol to Baran 33 KV 25 Km.

GENERATION SINCE COMMISSINING.

S.No. Financial year Generation in (LU)

1. 92-93 31.749

2 93-94 62.388

3. 94-95 98.767

4. 95-96 116.100

5. 96-97 110.747

6. 97-98 78.221

7. 98-99 78.323

8. 99-2000 19.242

9. 2000-2001 23.963

10. 2001-02 56.266

11. 2002-03 NIL

12. 2003-04 34.8825

13. 2004-05 83.6985

14. 2005-06 59.199

15. 2006-07 80.5815

16. 2007-08 63.732


87

17. 2008-09 39.135

18. 2009-10 22.05350

19. 2010-11 32.62350


88

CHAPTER 8)

DIRECT ENERGY CONVERSION

Need for Direct Energy Conversion

In direct energy conversion system energy source directly converted into electricity

without and working fluid or steam. Direct conversion systems have no moving parts.

Need:

1. No conversion of energy into mechanical and to electricity.

2. Less losses in conversion process.

3. More efficient process

4. Cost also reduced but technology required to improve it.

CARNOT CYCLE

If an engine is continually to convert thermal energy into mechanical energy, it must operate

cyclically. At the end of each cycle it must return to its initial configuration, so it can repeat

the process of conversion of heat into work over and over again. Steam engines and

automobile engines are obviously cyclic -- after one (or sometimes two) revolutions, they

return to their initial configuration. These engines are not 100% efficient. The condenser of a

steam engine and the radiator and exhaust of an automobile engine eject a substantial amount

of heat into the environment; this waste heat represents lost energy. Besides, there are

frictional losses.

Any device that converts heat into work by means of a cyclic process is called a heat

engine. The engine absorbs heat from a heat reservoir at high temperature, converts this heat

partially into work, and ejects the remainder as waste heat into a reservoir at low temperature.

In this context, a heat reservoir is simply a body that remains at constant temperature, even

when heat is removed from or added to it. In practice, the high-temperature heat reservoir is

often a boiler whose temperature is kept constant by the controlled combustion of some fuel,

and the low-temperature reservoir is usually a condenser in contact with a body of water or in

contact with the atmosphere of the Earth, whose large volume permits it to absorb the waste

heat without appreciable change of temperature.

Figure 21.5 is a flow chart for the energy, showing the heat Q1 flowing into the engine from

the high-temperature reservoir, the heat Q2 (waste heat) flowing out of the engine into the

low-temperature reservoir, and the work generated. The work generated is the difference

between Q1and Q2,

The efficiency of the engine is defined as the ratio of this work to the heat absorbed from the

high-temperature reservoir,

https://curricula2.mit.edu/pivot/book/showgfx.html?images/ch21/lrgimg/ph21eq004.gif

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This says that if Q2 = 0 (no waste heat), then the efficiency would be e= 1, or 100%. If so, the

engine would convert the high-temperature heat totally into work. As we will see later, this

extreme efficiency is unattainable. Even under ideal conditions, the engine will produce some

waste heat. It turns out that the efficiency of an ideal engine depends only on the

temperatures of the heat reservoirs.

CARNOT CYCLE

Whether it is coal, oil, gas or nuclear power, 80% of the worlds electricity is derived from

heat sources and almost all of the energy conversion processes used convert the thermal

energy into electrical energy involve an intermediate step of converting the heat energy

to mechanical energy in some form of heat engine. To satisfy this need a wide range of

energy conversion systems has been developed to optimise the conversion process to the

available heat source.

Despite over 250 years of development since James Watt's steam engine was first fired

up, the best conversion efficiency achieved today is only around 60% for combined

cycle steam and gas turbine systems. Efficiencies in the range of 35% to 45% are more

common for steam turbines, 20% to 30% for piston engines and as low as 3%

for OTEC ocean thermal power plants. This page describes some thermodynamic

aspects of a variety of representative heat engines. More detailed descriptions of these

engines can be found on other pages on this site via the links below.

The efficiency of heat engines was first investigated by Carnot in the 1824 and expanded

upon by Clapeyron who provided analytical tools in 1834 and Kelvin who stated the

Second Law of Thermodynamics in 1851 and finally by Clausius who introduced the

concept of entropy in 1865.


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The Carnot Cycle

Figure 1. An ideal gas-piston model of the Carnot cycle.

The Carnot cycle consists of the following four processes:

I. A reversible isothermal gas expansion process. In this process, the ideal gas in the system absorbs qin amount heat from a heat source at a high temperature Th, expands

and does work on surroundings.

II. A reversible adiabatic gas expansion process. In this process, the system is thermally insulated. The gas continues to expand and do work on surroundings,

which causes the system to cool to a lower temperature, Tl.

III. A reversible isothermal gas compression process. In this process, surroundings

do work to the gas at Tl, and causes a loss of heat, qout.

IV. A reversible adiabatic gas compression process. In this process, the system is thermally insulated. Surroundings continue to do work to the gas, which causes the temperature to rise back to Th.

Thermoelectric Generators

How Thermoelectric Power Generation Works

The basic concept of thermoelectric Generators TEG‘s (Seebeck effect) is outlined below

to explain to the Layman and also Engineers who are not familiar with the technology.

We have been manufacturing TEG Power Generators for the last 10 years. A full 70-

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80% of the interest comes from persons whom have only minimal knowledge of the

technology. Based on the requests for pricing and products available, interest in this field

has exploded in the last three years. Thermoelectric modules work on two different

principal :

Peltier Effect: This effect introduce power to the module with a resultant cooling of one

side and heating of the other these type of modules are low amp typically in the 6 amp

range and are designed for low temperature exposure of NO MORE THAN 100°C to

110°C hot side. Higher temperature exposure will cause the module to either break apart

or the soldered couples to melt from high heat making them poor choices for power

generation!

Seebeck Effect: This effect is created by temperature differential across the module from

heating one side and cooling the other side by moving the heat flux away as fast as it

moves through the module . You cannot describe a peltier module and say it produced

power as many laymen do in the BLOGS. Please describe a Seebeck effect module as a

power generator and a Peltier module as a cooling module.

Thermoelectric Generators using the Seebeck Effect work on a temperature differentials.

The greater the differential (DT) of the hot side less the cold side, the greater the amount

of power (Watts) will be produced. Two critical factors dictate power output :

The amount of heat flux that can successfully move through the module.

The temperature of the hot side less the temperature of the cold side Delta Temperature

(DT).

Great effort must be placed on the heat input design and especially the heat removal

design (Cold Side). The better the TEG Generator construction is at moving heat from

the hot side to the cold side and dissipating that heat as it moves thru the module array

to the cold side the more power will be generated. Unlike solar PV which use large

surfaces to generate power. Thermoelectric Seebeck effect modules are designed for very

high power densities, on the order of 50 times greater than Solar PV!

Thermoelectric Seebeck Generators using liquid on the cold side perform significantly

better then any other method of cooling and produce significantly more net additional

power than the pump consumes.

For any thermoelectric power generator (TEG), the voltage(V) generated by the TEG is

directly proportional to the number of couples (N) and the temperature difference (Delta

T) between the top and bottom sides of the TE generator and the Seebeck coefficients of

the n and p- type materials.


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The standard material we work with is BiTe. The best efficiency that can be achieved

with this material is approximately 6%. But once the material is placed into a

constructed module the efficiency drops to 3 to 4% depending on DT because of thermal

and electrical impedance!

No other semiconductor material can perform as well as BiTe as far as efficiency is

concerned at temperatures below 300°C.

Other material like PbTe are used but are far less efficient at lower temperatures, and

must be used at significantly higher temperatures in the 600°C hot side range and are

commercially available but very expensive!

The standard material we work with is BiTe. The best efficiency that can be achieved

with this material is approximately 6%.

But once the material is constructed into a module, efficiency drops to 3 to 4% because

of thermal and electrical impedance. No other semiconductor material can perform as

well as BiTe as far as efficiency is concerned. Other material such as PbTe are used but

are far less efficient, and must be used at significantly higher temperatures (450°C-

600°C) hot side and are not commercially available!

Power output based on (DT) is very predictable and well documented, but access to this

information is difficult to find. With power generation the thinner the length or thickness

of the module the greater the amp output or rating.

You can have a 25 amp * module the same size typically 40 mm x 40 mm as a 3 amp

module * in module size, but length or height of the pellet or element determines how

much heat can pass thru the module. The ratio of the length compared the actual width x

depth determines the overall amperage of the module. As the height of the pellet is

shortened ability of heat flux to pass more quickly thru the module allows for greater

power generation as long as DT can be maintained. That same 25 amp modules will

produce over 8 times the amount of power as the 3 amp module. But 8 times the watts

will need to pass thru that 25 amp module in order to produce that power. It is

imperative that a DT be maintained. The module simply acts as a bridge. The larger the

bridge area to length the greater the flow of heat and resulting power output.

Our low temperature modules (TEG2) are high amp modules with contacts that are

soldered using AgTn solder on both sides. Although, the temperature of the solder has a

240°C melting point the solder begins to degrade at about 190-200°C . Therefore we

recommend the hot side stay below 190C to allow for small temperature variations.

Our High Temperature Modules (TEG1) use flame spraying high temperature metal

Aluminum on the hot side and can withstand much higher temperatures in the range of

300°C hot side and have considerably larger tolerances when it comes to incidental


93

higher temperature over shouts. So much so that you can expose the hot side to 320°C

intermittently with very little module degradation. This technique is much more

expensive to implement and therefore the cost is reflected in the price of the modules.

Temperature of the hot side is probably the most critical component when considering

Thermoelectric Generators. (DT) Delta T needs to be in the 100°C range to get a viable

power output from each modules.

Joule Thomson Effect

Joule-Thomson effect, the change in temperature that accompanies expansion of

a gas without production of work or transfer of heat. At ordinary temperatures and

pressures, all real gases except hydrogen and helium cool upon such expansion; this

phenomenon often is utilized in liquefying gases. The phenomenon was investigated in

1852 by the British physicists James Prescott Joule and William Thomson (Lord Kelvin).

The cooling occurs because work must be done to overcome the long-range attraction

between the gas molecules as they move farther apart. Hydrogen and helium will cool

upon expansion only if their initial temperatures are very low because the long-range

forces in these gases are unusually weak.

The Joule–Thomson coefficient

The change of temperature ( T ) with a decrease of pressure ( P ) at constant enthalpy

( H ) in a Joule–Thomson process is the Joule–Thomson coefficient denoted as μJT and

may be expressed as:

μJT≡(∂T∂P)H

The value of μJT is typically expressed in K/Pa or °C/bar and depends on the specific

gas, as well as the temperature and pressure of the gas before expansion.

For all real gases, it will equal zero at some point called the inversion point and, as

explained above, the Joule–Thomson inversion temperature is the temperature where the

coefficient changes sign (i.e., where the coefficient equals zero). The Joule–Thomson

inversion temperature depends on the pressure of the gas before expansion.

In any gas expansion, the gas pressure decreases and thus the sign of ∂ P is always

negative. With that in mind, the following table explains when the Joule–Thomson effect

cools or heats a real gas:


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For some gases, the Joule–Thomson inversion temperatures at atmospheric pressure are

very low: for helium, about 51 K (−222 °C), and for hydrogen, about 202 K (-71 °C).

Thus, helium and hydrogen will warm during a J–T expansion at typical room

temperatures. On the other hand, nitrogen has an inversion temperature of 621 K (348

°C) and oxygen has an inversion temperature of 764 K (491 °C). Hence, the two most

abundant gases in atmospheric air can be cooled by a J–T expansion at typical room

temperatures.[10]

It should be noted that μJT is always equal to zero for ideal gases. In other words, they

will neither heat nor cool during an expansion through an insulated throttling device.

Applications

In practice, the Joule–Thomson effect is achieved by allowing the gas to expand through

a throttling device (usually a valve) which must be very well insulated to prevent any

heat transfer to or from the gas. External work must not be extracted from the gas during

the expansion (meaning that the gas must not be expanded through a turboexpander).

The effect is applied in the Linde cycle, a process used in the petrochemical industry for

example, where the cooling effect is used to liquefy gases, and also in many cryogenic

applications (e.g., for the production of liquid oxygen, nitrogen and argon). Only when

the Joule–Thomson coefficient for the given gas at the given temperature is greater than

zero can the gas be liquefied at that temperature by the Linde cycle. In other words, a gas

must be below its inversion temperature to be liquefied by the Linde cycle. For this

reason, a simple Linde cycle cannot normally be used to liquefy helium, hydrogen and

neon.

MHD (Magneto Hydro Dynamic) Generators:

Magnetohydrodynamic power generation provides a way of generating electricity

directly from a fast moving stream of ionised gases without the need for any moving

mechanical parts - no turbines and no rotary generators. Several MHD projects were

initiated in the 1960s but overcoming the technical challenges of making a practical

system proved very expensive. Interest consequently waned in favour of nuclear power

which since that time has seemed a more attractive option.

MHD power generation has also been studied as a method for extracting electrical power

from nuclear reactors and also from more conventional fuel combustion systems

Working Principle

The MHD generator can be considered to be a fluid dynamo. This is similar to a

mechanical dynamo in which the motion of a metal conductor through a magnetic field

http://chemengineering.wikispaces.com/Joule%E2%80%93%C2%ADThomson+effect#cite_note-10

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95

creates a current in the conductor except that in the MHD generator the metal conductor

is replaced by a conducting gas plasma.

When a conductor moves through a magnetic field it creates an electrical field

perpendicular to the magnetic field and the direction of movement of the conductor. This

is the principle, discovered by Michael Faraday, behind the conventional rotary

electricity generator. Dutch physicist Antoon Lorentz provided the mathematical theory

to quantify its effects.

The flow (motion) of the conducting plasma through a magnetic field causes a voltage to

be generated (and an associated current to flow) across the plasma , perpendicular to

both the plasma flow and the magnetic field according to Fleming's Right Hand Rule

Lorentz Law describing the effects of a charged particle moving in a constant magnetic

field can be stated as

F = QvB

where

F is the force acting on the charged particle

Q is charge of particle

v is velocity of particle

B is magnetic field

The MHD System

The MHD generator needs a high temperature gas source, which could be the coolant

from a nuclear reactor or more likely high temperature combustion gases generated by

burning fossil fuels, including coal, in a combustion chamber. The diagram below shows

possible system components.

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96

The expansion nozzle reduces the gas pressure and consequently increases the plasma

speed (Bernoulli's Law) through the generator duct to increase the power output .

Unfortunately, at the same time, the pressure drop causes the plasma temperature to fall

(Gay-Lussac's Law) which also increases the plasma resistance, so a compromise

between Bernoulli and Gay-Lussac must be found.

The exhaust heat from the working fluid is used to drive a compressor to increase the

fuel combustion rate but much of the heat will be wasted unless it can be used in another

process.

The Plasma

The prime system requirement is creating and managing the conducting gas plasma

since the system depends on the plasma having a high electrical conductivity. Suitable

working fluids are gases derived from combustion, noble gases, and alkali metal

vapours.


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The Gas Plasma

To achieve high conductivity, the gas must be

ionised, detaching the electrons from the atoms or

molecules leaving positively charged ions of the

gas. The plasma flows through the magnetic field

at high speed, in some designs, more than the

speed of sound, the flow of the charged particles

providing the necessary moving electrical

conductor.

Methods of Ionising the Gas

Various methods for ionising the gas are available,

all of which depend on imparting sufficient energy

to the gas. It may be accomplished by heating or

irradiating the gas with X rays or Gamma rays. It

has also been proposed to use the coolant gases

such as helium and carbon dioxide employed in

some nuclear reactors as the plasma fuel for direct

MHD electricity generation rather than extracting

the heat energy of the gas through heat exchangers

to raise steam to drive turbine generators. Seed

materials such as Potassium carbonate or Cesium

are often added in small amounts, typically about

1% of the total mass flow to increase the

ionisation and improve the conductivity,

particularly of combustion gas plasmas.

Containment

Since the plasma temperature is typically over

1000 °C, the duct containing the plasma must be

constructed from non-conducting materials

capable of withstanding these high temperatures.

The electrodes must of course be conducting as

well as heat resistant .

Note that 90% conductivity can be

achieved with a fairly low degree of

ionisation of only about 1%. (Note

also logarithmic scale)

The Faraday Current

A powerful electromagnet provides the magnetic field through which the plasma

flows, and perpendicular to this field are installed the two electrodes on opposite sides

of the plasma across which the electrical output voltage is generated. The current

flowing across the plasma between these electrodes is called the Faraday current. This

provides the main electrical output of the MHD generator.

The Hall Effect Current

The very high Faraday output current which flows across the plasma duct into the

load itself reacts with the applied magnetic field creating a Hall Effect current

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98

perpendicular to the Faraday current, in other words, a current along the axis of the

plasma, resulting in lost energy. The total current generated will be the vector sum of

the transverse (Faraday) and axial (Hall effect) current components. Unless it can be

captured in some way, the Hall effect current will constitute an energy loss .

Various configurations of electrodes have been devised to capture both the Faraday

and Hall effect components of the current in order to improve the overall MHD

conversion efficiency.

One such method is to split the electrode pair into a series of segments physically side

by side (parallel) but insulated from eachother, with the segmented electrode pairs

connected in series to achieve a higher voltage but with a lower current. Instead of the

electrodes being directly opposite eachother, perpendicular to the plasma stream, they

are skewed at a slight angle from perpendicular to be in line with the vector sum of the

Faraday and Hall effect currents, as shown in the diagram below, thus allowing the

maximum energy to be extracted from the plasma.

Power Output

The output power is proportional to the cross sectional area and the flow rate of the

ionised plasma. The conductive substance is also cooled and slowed in this process.

MHD generators typically reduce the temperature of the conductive substance from

plasma temperatures to just over 1000 °C.

An MHD generator produces a direct current output which needs an expensive high

power inverter to convert the output into alternating current for connection to the grid.

Efficiency

Typical efficiencies of MHD generators are around 10 to 20 percent mainly due to the

heat lost through the high temperature exhaust.

This limits the MHD's potential applications as a stand alone device but they were

originally designed to be used in combination with other energy converters in hybrid

applications where the output gases (flames) are used as the energy source to raise


99

steam in a steam turbine plant. Total plant efficiencies of 65% could be possible in

such arrangements.

Experience

Demonstration plants with capacities of 50 MW or more have been built in several

countries but MHD generators are expensive. Typical use could be in peak

shaving applications but they are less efficient than combined-cycle gas turbines which

means there are very few installations and MHD is currently not considered for

mainstream commercial power generation.

HALL EFFECT

If an electric current flows through a conductor in a magnetic field, the magnetic field

exerts a transverse force on the moving charge carriers which tends to push them to one side of the conductor. This is most evident in a thin flat conductor as illustrated. A

buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of

this measurable transverse voltage is called the Hall effect after E. H. Hall who discovered it in 1879. Note that the direction of the current I in the diagram is that of conventional current, so

that the motion of electrons is in the opposite direction. That further confuses all the "right-hand rule" manipulations you have to go through to get the direction of the forces.

The Hall voltage is given by

Show

The Hall effect can be used to measure magnetic fields with a Hall probe.

Faraday’s Law-

Faraday's 1st Law of Electrolysis - The mass of a substance altered at

an electrode during electrolysis is directly proportional to the quantity of

electricity transferred at that electrode. Quantity of electricity refers to the quantity

of electrical charge, typically measured in coulomb.

Faraday's 2nd Law of Electrolysis - For a given quantity of D.C electricity (electric

charge), the mass of an elemental material altered at an electrode is directly

proportional to the element's equivalent weight.

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100

FUEL CELL

What is a fuel cell?

A fuel cell is a device that generates electricity by a chemical reaction. Every fuel cell has two electrodes, one positive and one negative, called, respectively, the anode and cathode. The reactions that produce electricity take place at the electrodes.

Every fuel cell also has an electrolyte, which carries electrically charged particles from one electrode to the other, and a catalyst, which speeds the reactions at the electrodes.

Hydrogen is the basic fuel, but fuel cells also require oxygen. One great appeal of fuel

cells is that they generate electricity with very little pollution–much of the hydrogen and oxygen used in generating electricity ultimately combine to form a harmless byproduct, namely water.

One detail of terminology: a single fuel cell generates a tiny amount of direct current

(DC) electricity. In practice, many fuel cells are usually assembled into a stack. Cell or stack, the principles are the same.

How do fuel cells work?

The purpose of a fuel cell is to produce an electrical current that can be directed outside the cell to do work, such as powering an electric motor or illuminating a light bulb or a city. Because of the way electricity behaves, this current returns to the fuel cell,

completing an electrical circuit. (To learn more about electricity and electric power, visit "Throw The Switch" on the Smithsonian website Powering a Generation of

Change.) The chemical reactions that produce this current are the key to how a fuel cell works.

There are several kinds of fuel cells, and each operates a bit differently. But in general

terms, hydrogen atoms enter a fuel cell at the anode where a chemical reaction strips them of their electrons. The hydrogen atoms are now "ionized," and carry a positive electrical charge. The negatively charged electrons provide the current through wires to

do work. If alternating current (AC) is needed, the DC output of the fuel cell must be routed through a conversion device called an inverter.


101

Graphic by Marc Marshall, Schatz Energy Research Center

Oxygen enters the fuel cell at the cathode and, in some cell types (like the one illustrated above), it there combines with electrons returning from the electrical circuit and

hydrogen ions that have traveled through the electrolyte from the anode. In other cell types the oxygen picks up electrons and then travels through the electrolyte to the anode, where it combines with hydrogen ions.

The electrolyte plays a key role. It must permit only the appropriate ions to pass between the anode and cathode. If free electrons or other substances could travel through the electrolyte, they would disrupt the chemical reaction.

Whether they combine at anode or cathode, together hydrogen and oxygen form water,

which drains from the cell. As long as a fuel cell is supplied with hydrogen and oxygen, it will generate electricity.

Even better, since fuel cells create electricity chemically, rather than by combustion, they

are not subject to the thermodynamic laws that limit a conventional power plant (see "Carnot Limit" in the glossary). Therefore, fuel cells are more efficient in extracting

energy from a fuel. Waste heat from some cells can also be harnessed, boosting system efficiency still further.

So why can't I go out and buy a fuel cell?

The basic workings of a fuel cell may not be difficult to illustrate. But building

inexpensive, efficient, reliable fuel cells is a far more complicated business.

Scientists and inventors have designed many different types and sizes of fuel cells in the search for greater efficiency, and the technical details of each kind vary. Many of the

choices facing fuel cell developers are constrained by the choice of electrolyte. The design of electrodes, for example, and the materials used to make them depend on the


102

electrolyte. Today, the main electrolyte types are alkali, molten carbonate, phosphoric acid, proton exchange membrane (PEM) and solid oxide. The first three are liquid

electrolytes; the last two are solids.

The type of fuel also depends on the electrolyte. Some cells need pure hydrogen, and therefore demand extra equipment such as a "reformer" to purify the fuel. Other cells can

tolerate some impurities, but might need higher temperatures to run efficiently. Liquid electrolytes circulate in some cells, which requires pumps. The type of electrolyte also

dictates a cell's operating temperature–"molten" carbonate cells run hot, just as the name implies.

Each type of fuel cell has advantages and drawbacks compared to the others, and none is

yet cheap and efficient enough to widely replace traditional ways of generating power, such coal-fired, hydroelectric, or even nuclear power plants.

The following list describes the five main types of fuel cells. More detailed information

can be found in those specific areas of this site.

Different types of fuel cells.

Alkali fuel cells operate on compressed hydrogen and oxygen. They generally use

a solution of potassium hydroxide (chemically, KOH) in water as their electrolyte. Efficiency is about 70 percent,

and operating temperature is 150 to 200 degrees C, (about 300 to 400 degrees F).

Cell output ranges from 300 watts (W) to 5 kilowatts (kW). Alkali cells were used in

Apollo spacecraft to provide both electricity and drinking water. They require pure hydrogen fuel, however, and

their platinum electrode catalysts are expensive. And like any container filled

with liquid, they can leak. Drawing of an alkali cell.


103

Molten Carbonate fuel cells (MCFC) use high-temperature compounds of salt (like

sodium or magnesium) carbonates (chemically, CO3) as the electrolyte.

Efficiency ranges from 60 to 80 percent, and operating temperature is about 650

degrees C (1,200 degrees F). Units with output up to 2 megawatts (MW) have been constructed, and designs exist for

units up to 100 MW. The high temperature limits damage from carbon

monoxide "poisoning" of the cell and waste heat can be recycled to make

additional electricity. Their nickel electrode-catalysts are inexpensive compared to the platinum used in other

cells. But the high temperature also limits the materials and safe uses of MCFCs–they would probably be too hot for home use. Also, carbonate ions from the electrolyte are

used up in the reactions, making it necessary to inject carbon dioxide to compensate.

Phosphoric Acid fuel cells (PAFC) use phosphoric acid as the electrolyte. Efficiency ranges from 40 to 80 percent, and operating temperature is between 150 to 200 degrees C

(about 300 to 400 degrees F). Existing phosphoric acid cells have outputs up to 200 kW, and 11 MW units have been tested. PAFCs tolerate a carbon monoxide concentration of about 1.5 percent, which broadens the choice of fuels they can use. If gasoline is used,

the sulfur must be removed. Platinum electrode-catalysts are needed, and internal parts

must be able to withstand the corrosive acid.

Drawing of how both phosphoric acid and PEM fuel

cells operate.

Proton Exchange Membrane (PEM) fuel cells work with a polymer electrolyte in the form of a thin, permeable sheet. Efficiency is about 40 to 50 percent, and operating temperature is about 80 degrees C (about 175 degrees F). Cell outputs generally range

from 50 to 250 kW. The solid, flexible electrolyte will not leak or crack, and these cells

Drawing of a molten carbonate cell


104

operate at a low enough temperature to make them suitable for homes and cars. But their fuels must be purified, and a platinum catalyst is used on both sides of the membrane,

raising costs.

Solid Oxide fuel cells (SOFC) use a hard, ceramic compound of metal (like calcium

or zirconium) oxides (chemically, O2) as electrolyte. Efficiency is about 60 percent,

and operating temperatures are about 1,000 degrees C (about 1,800 degrees F). Cells output is up to 100 kW. At such

high temperatures a reformer is not required to extract hydrogen from the

fuel, and waste heat can be recycled to make additional electricity. However, the

high temperature limits applications of SOFC units and they tend to be rather large. While solid electrolytes cannot leak,

they can crack.

Drawing of a solid oxide cell

Documents

Renewable Energy Sources by M A Qadeer