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A Broad Survey on Non-Conventional Energy Resources
Shruthi Rajagopal
Electrical Engineering Departmant
State University of Newyork,
Binghamton,NY
1. INTRODUCTION:
Energy in the world is increasing day by day. The conventional energy resources such as hydro electric power, thermal power, and nuclear power are coping with this increasing demand. But many non-conventional energy which were in research stage a few years ago have established their usefulness over the years. Though they may in smaller quantity and less stable compared to conventional energy resources when included in the power grid can take a considerable amount of load. This survey makes an attempt to study the process and utilization of non conventional energy resources such as solar energy, wind energy, biomass energy, geo thermal energy and energy from the ocean. Though a brief survey here we try to shed light on the basic principles of the above mentioned non- conventional energy resources.
2. INTRODUCTION TO ENERGY SOURCES:
Energy is the basic power source to perform work. All energy sources follow the law “energy can neither be created nor be destroyed”. Thus the input energy is converted from one form to another in the process work is done and losses are accounted.
The energy sources can be divided into 3 categories:
Primary energy source:
The energy sources, which provide net supply of energy, are called the Primary energy sources. The examples for this type of energy are coal, oil, natural gas, uranium etc., the energy required to extract energy from these sources are much smaller than what
they yield. Though their yield ratio is high and accelerated their supply is very limited.
Secondary energy source:
They produce no net energy. They are necessary for the economy but does not produce energy per se. Agriculture is the example for this kind.
Supplementary energy source:
These are defined as the energy sources whose net energy yield is zero. These require highest investment is high. Thermal energy is an example.
3. WORLD ENERGY TREND:
If the present energy trend continue the conventional sources of energy will deplete soon. Non-conventional energy resources such as renewable energy may be the future trend.
The study on alternative energy strategies result in these conclusions:
The supply of oil will fail to meet the increasing energy demand.
Demand for energy will grow in spite the actions to conserve energy.
Renewable sources of energy will be demand more than ever.
Energy efficiency will play a very big role in energy utilization.
An eminent need of non-conventional energy arises in the future.
Continuing in this direction we conduct a broad study on the non-conventional energy resources available, their feasibility and constraints are discussed in the next sections.
4. conventional energy resources:
Before we study non-conventional energy resource, we first mention briefly the conventional energy resources available.
Major sources of energy include:
Fossil fuel: solid fuels available from inside the earth consist of fossil fuel. Coal, petroleum, coke etc., are this type.
Potential energy of water: mainly the hydropower generated from the dams constitutes these.
Nuclear energy: the energy from nuclear fission is this form.
5. NON-CONVENTIONAL ENERGY RESOURCES:
The available non-convectional energy is listed below:
SOLAR ENERGY
WIND ENERGY
BIO-ENERGY
GEOTHERMAL ENERGY
ENERGY FROM THE OCEAN
In the following section we discuss each of above mentioned non-conventional sources in detail:
6. SOLAR ENERGY:
a. .Introduction:
The energy produced and radiated by the sun that reaches the earth is called the solar energy. Solar energy received in the form of radiation can be directly or indirectly converted into heat and electricity, which can be utilized in many ways. The main advantage is that the solar radiation is unlimited but the intermittent nature makes it not so popular choice in some applications.
b. Solar constant:
The sun is a large hot sphere of gas. The diameter is 1.39 x 106km while earth is 1.27 x 104km with a mean distance of 1.50x 104km. Though sun is a large body it subtends an angle of 33 min at earth’s surface. The rate at which at which the solar energy arrives at the surface of the earth is call as solar constant.
By definition the amount of energy received in unit time in unit area perpendicular t sun s direction at the mean distance of earth’s surface from
the sun. By NASA standard solar constant is give by 1.353KWatts per sq meter.
Due to the variation in the distance between the sun and the earth the intensity of radiation I is also is sinusoidal and is given by:
I/Isc= 1+0.33 cos(360n/365)
Where n is the day of the year.
The spatial distribution of solar radiation intensity received on earth spread over different wavelength is given by figure 1 below. Thus maximum wavelength occurs at 480nm range with maximum intensity value at 2074 W/m2.this measurements are taken at the outer atmosphere.
Figure 1: The spatial distribution of solar radiation intensity.
c. Solar radiation at the earth’s surface:
The solar radiation reaching the earth s surface is entirely not direct sunlight. Part of it is reflected back, some of them are absorbed by molecules in the air and some are scattered by the droplets in the cloud. The solar radiation received at the surface of the earth is of two types:
Beam radiation:
Solar radiation that directly reaches the earth surface without being scattered or absorbed is called beam radiation or direct radiation. The shadows are produced by this radiation.
Diffused radiation:
Solar radiation received from the sun after its direction altered due to scattering or reflection by the earth s atmosphere is called diffused radiation.
Thus the total radiation received at any point on the earth surface is the sum of beam radiation and diffused radiation.
Figure 2: Total radiation form the sun.
d. SOLAR ENERGY COLLECTORS:
Solar energy collectors are the device used to collect solar radiation and pass the energy to a fluid flowing through it and in contact with it.
There are two types are:
Flat plate (non-concentrating) type solar collectors: here the collector area is same as the absorber area.
Focusing (concentrating) type solar collectors: the collector area is very large compared to absorbing area.
These collectors are essential part of any solar energy conversion system. In the following section these are explained in detail.
e. PRINCIPLE OF SOLAR ENERGY CONVERSION:
The basic principle is conversion of radiation energy to heat energy at the site of contact through green house effect.
A glass plate is provided at the contact of radiation which acts as a heat trap; with generally more than one glass covers with top cover painted black.The black painting plate absorb the sunlight and as it s temperature increases, it emits excessive of heat in the form of infrared rays.
Figure 3: Total radiation from the sun.
Black glass plate being a blackbody has he properties of highest absorbing rate as well as highest emission rate. This emission increases at a rate of T4
law. The re-emitted light is progressively shorter wavelength and greater energy as the temperature of the black body increases.
f.FLAT-PLATE COLLECTOR :
These are used where the temperature is below 90oC flat-plate collector. These are the most popular type of solar energy utilizes. There are two types:
Liquid heating collectors are used for heating water and non freezing solutions.
Solar air heaters are used for heating air or gas.
Main components of flat plate collector-
I. A transparent cover with one or more layer transparent film.
II. Tubes, fins, channels which carries water, air or other liquid to transfer heat.
III. An absorbent plate made of metal colored black can be used as heaters.
IV. Insulation made of fiber to minimize any heat losses.
V. Container to hold all the components together.
g. APPLICATIONS OF FLAT PANEL COLLECTORS:
A. They are mainly used to provide hot water for domestic purposes.
B. Supplying hot water for industrial heaters.
C. Steam heated building heating systems.
A typical flat panel collector is shown below:
Figure 4: A typical flat panel liquid heating solar collector.
h. SOLAR AIR HEATER :
Figure 5: A typical flat panel solar air heater.
The figure 5 shows a typical flat panel solar heater which uses air as heat transferring medium.
The air heaters are further classified into two types:
a. Non porous absorber:
These are cooled by air stream flowing other both sides of the plate. Though heat losses are high from flowing air it is the most commonly used design.
Performance of these heaters can be improved by:
b. Roughing the back side of plate to improve turbulence
c. Adding fins to increase the transfer of heat.
d. Porous absorber:
These are heaters with a plate with fins to absorb heat. They work better than the porous type but cost is increased due to increased components.
i.APPLICATIONS OF SOLAR AIR HEATERS:
o Heating buildings
o Drying lumber
o Heating green house
o Drying agricultural produce
o Air conditioning buildings
o Absorption refrigeration process
o Air heaters in heat engine.
j.ADVANTAGES OF FLAT-PLATE CLLECTORS:
o These utilize both direct and diffused radiation.
o Orientation of sun is not a problem.
o Little maintenance.
o No moving parts so less wear and tear.
k. CONCENTRATING TYPE SOLAR COLLECTOR :
The solar collector where the radiation beam is focused at a point to heat up the transfer medium is called the concentrating type solar collector. The focusing is done through the use of optical systems such as mirrors, lens etc.,
The different types of concentrating collectors are:
1. Parabolic trough collector.
2. Mirror strip reflector.
3. Fresnel lens collector.
4. Flat plate collector with adjustable mirrors.
5. Compound parabolic concentrator (CPC).
A typical concentrating type solar collector is shown in figure 6.
Figure 6: Concentrating type solar collectors.
A brief study of solar energy conversion and utilization is made here. Solar energy being a very large topic cannot be covered entirely in this short study.
7. WIND ENERGY:
The wind is the result of moving air. The solar radiation eats up the air in the equator, this reduces the density of air. This low density air raises above and the cooler air form the top replaces it. Thus resulting in movement of air form pole to pole. This forms the wind.
Wind energy is America s one of the greatest natural resource. Windmills are used I several countries like Denmark where high wind is available. America ramped up wind-power capacity to 25GW in 2008, overtaking the previous leader, Germany, according to new data from the Global Wind Energy Council. America added 8.4GW of installed power in 2008, more than any other country. China is also investing heavily in wind power, nearly doubling its capacity for the fourth year running. Global capacity grew by 29% last year, the highest annual increase for six years. The global wind energy distribution is shown in below figures:
Figure 7 : Global distribution of wind energy as in 2008.
Figure 8 : USA ranking second in wind energy production.
a) PRINCIPLE OF WIND ENERGY CONVERSION SYSTEM (WECS):
The circulation of air due to non uniform heating of air as explained above results in wind. The nature of the terrain, the degree of the could cover angle of sun in the sky all influence the nature of the wind. The distance from costal region, planetary winds also affect the strength and direction of wind. Despite the intermittent nature of the wing energy the wind patterns in a region remains constant over the years.
Wind possesses energy by virtue of its motion. Any device capable of converting this kinetic energy into a different form of energy can do useful work. The main factors determining the outputs from wind energy are:
1. The wind speed
2. The cross section of wind swept rotor.
3. Conversion efficiency of rotor, transmission system and generator.
The typical wind energy set up is shown below. The power from the wind mill is calculated by the concepts of kinetics.
The power is equal to energy per unit time. The kinetic energy of a particle is given by ½ m V2.
Mass of air passing in unit time with velocity V and density p in a area A is given by m=pAV.
Substituting the kinetic energy is given by = ½ pAV3 Watts.
An aero turbine with a large swept area is placed normally circular to wind direction the power generated is given by:
Pa= ½ p (pi/4)D2V3 Watts
Figure 9 :Wind energy principle.
Thus wind generators intended for generating large amount of power should have large rotors and be located in a area of high wind energy. The dependence of wind rotor power on wind speed and wind speed on height is shown below:
Figure 10: dependence of wind rotor power on wind speed.
Figure 11: Wind speed with height
Almost all wind turbines producing electricity for the national grid consist of rotor blades which rotate around a horizontal hub. The hub is connected to a gearbox and generator, which are located inside the nacelle. The nacelle houses the electrical components and is mounted at the top of the tower. This type of turbine is referred to as a 'horizontal axis' machine. Wind turbines can have three, two or just one rotor blade. Most have three. Blades are made of fiberglass-reinforced polyester or wood-epoxy.
Power is controlled automatically as wind speed varies and machines are stopped at very high wind speeds to protect them from damage. Most have gearboxes although there are increasing numbers with direct drives. The turbine includes a "yaw mechanism" which turns the turbine automatically so that it always faces the wind. The picture of a wind turbine with it s components is shown below.
Turbine arrangement:
Turbines in wind farms must be carefully arranged to gain the maximum energy from the wind - this means that they should shelter each other as little as possible from the prevailing wind.
Figure 12: Wind turbine with it s components.
b) SITE SELECTION TO SET UP A WIND TURBINE GENERATOR:
a. High annual average wind speed:
An anemometer data for several years is carefully studied and analyzed at correct height and area.
The following strategy is considered while choosing the site:
1. Survey of historical wind data.
2. Maps of the terrain and wind data at required elevation studied.
3. Inspection of proposed sites.
4. Probable sites are monitored for one year.
5. Optimal site considering all the above steps.
b. Availability of anemometer data:
A reliable historical data for several years should e available at required elevation. The anemometer height above the ground, location of the tower, accuracy, linearity, shadowing are to be carefully considered before considering the actual installation.
c. Availability of wind V(t) curve at the site:
This is a very important curve which gives the idea of the max power that can be generated at different speed. This gives analysis of thr reliability at the point of interest. The below figure shows a v(t) curve for a T-100 type turbine.
Figure 13: V(t) curve for a T100 turbine set up.
d. Altitude of the site:
The altitude of the site affects the air density which in turn decides the wind power which varies the output of the generator. The altitude is not same as the height above the ground.
e. Aerodynamics of the terrain:
Terrain should be easily accessible for installation and maintenance. The place should be near road or railways so that cost of installation is not too high. The rotor should be placed such that the air flow is horizontal for maximum power.
f. Local ecology:
The local ecology should not be disturbed at the same time dense forest may alter the wind pattern. Other factors like icing, heavy monsoon sand storms etc., should be considered carefully.
c) CLASSIFICATION OF WIND ENERGY SYSTEMS:
1. Based on the axis of rotation:
a. Horizontal axis machine.
b. Vertical axis machine.
2. Based on the power output:
a. Small scale - upto to 2kW.
b. Medium scale- upto 100kW.
c. Large scale -100kW and up.
3. Based on the output power type:
a. DC output: -DC generator –Alternator rectifier.
b. AC output: -variable frequency – constant frequency.
4. Based on the Rotational speed:
a. Constant speed with variable pitch blade.
b. Nearly constant speed with fixed pitch blade.
c. Variable speed with fixed pitch blade.
5. Utilization of output:
a. Battery storage.
b. Direct connection to converter.
c. Connection with the grid.
d) ADAVANTAGES OF WECS:
a) Renewable form of energy.
b) Clean energy resources.
c) Economical for small to medium production.
e) DISADAVANTAGES OF WECS:
a) Dilute and intermittent in nature.
b) Extra storage required as fluctuating.
c) Very noisy.
d) Bulky setup and high maintenance cost.
e) Large area required.
Thus a brief study within the scope of this paper is made for wind energy conversion and utilization.
8. BIO MASS ENERGY:
Biomass is organic matter produced by plants and their derivatives. Int is traditional solid mass such as wood and agricultural residue and non traditional waste like liquid fuel. These biomass is either directly burned to produce energy or indirectly burned to produce ethanol or methanol liquid fuels for energy or the third type where is processed an anaerobically to produce biogas. Approximate calorific values for various agricultural products is shown in the below figure and calorific value of fuels from biomass is shown in the next figure.
Figure14: Approximate heat content in terms of calorific value for agricultural wastes.
Figure 15: Calorific value of fuels produced by bio mass.
a) PRINCIPLE OF BIOMASS CONVERSION:
Figure 16: principle of biomass conversion.
Bio conversion can be done by one of the following methods:
a. Direct burning: Agricultural wastes wood waste , sugarcane refuge, cow dung etc., can be directly burned producing heat energy which is further used for aiding agricultural processes.
b. Thermo chemical conversion: these take two forms:
a. Gasification: In this process the biomass is heated at high temperature and pressure under low oxygen content producing medium heat value gas.
b. Liquefaction: the gas from gasification may be directly used for burning or may be further used in liquefaction process to produce methanol or ethanol.
c. Biochemical Conversion: this is of two forms :
a. Anaerobic digestion:
Here the biomass is digested using a microorganism in the absence of oxygen producing methane. This takes place at a temperature of 65oC at 80% moisture content. The carbon dioxide is a byproduct. The methane is directly used or used in the production of synthetic gas.
b. Fermentation: this is one of the widely used technologies. Here the biomass is broke down under the influence of ferments such as yeast, enzymes, bacteria etc.,
These concepts are explained in detail in the following sections.
b) ANAEROBIC DIGESTION :
An Anaerobic digester is shown below.
Anaerobic digestion (AD) is a biological process in which biodegradable organic matters are broken-down by bacteria into biogas, which consists of methane (CH4), carbon dioxide (CO2), and other trace amount of gases. The biogas can be used to generate heat and electricity. Oxygen-free is the primary requirement of AD to occur. Other important factors, such as temperature, moisture and nutrient contents, and pH are also critical for the success of AD. AD can be best occurred at two range of temperatures, mesophilic (30-40°C) and thermophilic (50-60°C). In general, AD at mesophilic temperature is more common even though digestion at
thermophilic temperature has the advantages of reducing reaction time, which corresponding to the reduction of digester volume. Moisture contents in greater than 85% or higher are suitable for AD.
Figure 17: Anaerobic digester.
The types of anaerobic digesters include Covered Lagoon, Batch Digester, Plug-Flow Digester, Completely Stirred Tank Reactor (CSTR), Upflow Anaerobic Sludge Blanket (UASB), and Anaerobic Sequencing Batch Reactor (ASBR), and others. The complete-mix, plug-flow, and the covered anaerobic lagoon are three types of the digesters that are recognized by the USDA's Natural Resource Conservation Service (NRCS) in the form of "National Guidance provided to States."
The complete-mix digester is a large, vertical poured concrete or steel circular container. Today's complete-mix digester can handle organic wastes with total solid concentration of 3% to 10%. Complete-mix digesters can be operated at either the mesophilic or thermophilic temperature range with a hydraulic retention time (HRT) as brief as 10-20 days.
The basic plug-flow digester design is a long linear through, often built below ground level, with an air-tight expandable cover. Organic wastes is collected daily and added to one end of the trough. Each day a new "plug" of organic wastes is added, slowly pushing the other manure down the trough. Plug-flow digesters are usually operated with a total solid concentration of 11%-13% at the mesophilic temperature range, with a HRT from 20-30 days.
A cover lagoon is an earthen lagoon fitted with a floating, impermeable cover that collects biogas as it is produced from the organic wastes. The cover is constructed of an industrial fabric that rests on solid floats laid on the surface of the lagoon. The cover can be placed over the entire lagoon or over the part that produces the most methane. An anaerobic lagoon is best suited for organic wastes with a total solid concentration of 0.5%-3%. Cover lagoons are not heated.
Covered lagoon digester O&M is simple and straightforward compared to complete-mix and plug-flow digesters. The capital cost for covered lagoon can be less than those required for the complete-mix and plug-flow types of conventional digesters. However, a key issue for covered lagoon is that digestion is dependent on temperature; therefore biogas production varies seasonally if the lagoon is not externally heated. This means that methane production is greater in summer than in winter. In general, a daily biogas production in summer could be averaged 35% higher than in winter. This may make end-use applications more problematic than plug flow and completed mix digesters. Another concern is that it can take an anaerobic lagoon as long as 1-2 years to achieve its "steady state" biogas production potential.
Production of renewable energy, improvement on environmental pollution in air and water, reduction of agricultural wastes, and utilization of byproducts as fertilizers from anaerobic digestion (AD), has increased the attractiveness of the application of AD. AD technology is well developed worldwide. Of the estimated 5300-6300 MW worldwide anaerobic digestion capacity, Asia accounts for over 95% or 5000-6000 MW. Traditional, small, farm-based digesters have been used in China, India and elsewhere for centuries. The number of digesters of this type and scale is estimated to exceed 6 million. European (EU) companies are world leaders in development of the AD technology. Currently, EU has a total generating capacity of 307 MW from AD technology. The countries in EU with the largest development figures are Germany (150 MW), Denmark (40 MW), Italy (30 MW), Austria and Sweden (both 20 MW). Germany led the small on-farm digesters for odor control. Italy developed a series of farm AD systems. Larger, centralized anaerobic digestion plants, which utilize animal manure and industry waste in a single facility, are a newer development and most prevalent in Denmark where there are 18 plants (worldwide there are 50 or so, all within Europe).
Municipal solid waste digestion is the newest area for anaerobic digestion. The most recent is for source-separated feed stocks, for which there are estimated to be over 150 commercial-scale plants. These plants have a combined capacity in excess of 6 million tons per year and the number of plants planned is increasing rapidly.
SOURCE : THE CALIFORNIA ENERGY COMMISION.
FERMENTATION:
The fermentation process is explained as a part of ethanol production from biomass in this section as described by the figure.
Bio ethanol Production:
Key Reactions: Two reactions are key to understanding how biomass is converted to bio ethanol they are:
Hydrolysis is the chemical reaction that converts the complex polysaccharides in the raw feedstock to simple sugars. In the biomass-to-bioethanol process, acids and enzymes are used to catalyze this reaction.
Fermentation is a series of chemical reactions that convert sugars to ethanol. The fermentation reaction is caused by yeast or bacteria, which feed on the sugars. Ethanol and carbon dioxide are produced as the sugar is consumed. The simplified fermentation reaction equation for the 6-carbon sugar, glucose, is:
C6H12O6---------- 2 CH3CH2OH + 2 CO2
glucose ethanol carbon dioxide
Process Description: The basic processes for converting sugar and starch crops are well-known and used commercially today. While these types of plants generally have a greater value as food sources than as fuel sources there are some exceptions to this. For example, Brazil uses its huge crops of sugar cane to produce fuel for its transportation needs. The current U.S. fuel ethanol industry is based primarily on the starch in the kernels of feed corn, America's largest agricultural crop.
Figure 18: Bio ethanol production using fermentation.
Biomass Handling: Biomass goes through a size-reduction step to make it easier to handle and to make the ethanol production process more efficient. For example, agricultural residues go through a grinding process and wood goes through a chipping process to achieve a uniform particle size.
Biomass Pretreatment: In this step, the hemicellulose fraction of the biomass is broken down into simple sugars. A chemical reaction called hydrolysis occurs when dilute sulfuric acid is mixed with the biomass feedstock. In this hydrolysis reaction, the complex chains of sugars that make up the hemicellulose are broken, releasing simple sugars. The complex hemicellulose sugars are converted to a mix of soluble five-carbon sugars, xylose and arabinose, and soluble six-carbon sugars, mannose and galactose. A small portion of the cellulose is also converted to glucose in this step.
Enzyme Production:The cellulase enzymes that are used to hydrolyze the cellulose fraction of the biomass are grown in this step. Alternatively the enzymes might be purchased from commercial enzyme companies.
Cellulose Hydrolysis: In this step, the remaining cellulose is hydrolyzed to glucose. In this enzymatic hydrolysis reaction, cellulase enzymes are used to break the chains of sugars that make up the cellulose, releasing glucose.
Cellulose hydrolysis is also called cellulose saccharification because it produces sugars.
Glucose Fermentation: The glucose is converted to ethanol, through a process called fermentation. Fermentation is a series of chemical reactions that convert sugars to ethanol. The fermentation reaction is caused by yeast or bacteria, which feed on the sugars. As the sugars are consumed, ethanol and carbon dioxide are produced.
Pentose Fermentation: The hemicellulose fraction of biomass is rich in five-carbon sugars, which are also called pentoses. Xylose is the most prevalent pentose released by the hemicellulose hydrolysis reaction. In this step, xylose is fermented using Zymomonas mobilis or other genetically engineered bacteria.
Ethanol Recovery: The fermentation product from the glucose and pentose fermentation is called ethanol broth. In this step the ethanol is separated from the other components in the broth. A final dehydration step removes any remaining water from the ethanol.
Lignin Utilization: Lignin and other byproducts of the biomass-to-ethanol process can be used to produce the electricity required for the ethanol production process. Burning lignin actually creates more energy than needed and selling electricity may help the process economics.
Converting cellulosic biomass to ethanol is currently too expensive to be used on a commercial scale. So researchers are working to improve the efficiency and economics of the ethanol production process by focusing their efforts on the two most challenging steps:
Cellulose hydrolysis: The crystalline structure of cellulose makes it difficult to hydrolyze to simple sugars, ready for fermentation. Researchers are developing enzymes that work together to efficiently break down cellulose. Read more about Enzymatic Hydrolysis.
Pentose fermentation: While there are a variety of yeast and bacteria that will ferment six-carbon sugars, most cannot easily ferment five-carbon sugars, which limits ethanol production from cellulosic biomass. Researchers are using genetic engineering to design microorganisms that can efficiently ferment both five- and six-carbon sugars to ethanol at the same time.
FACTORS AFFECTING BIODIGESTION:
1. Ph Value of the solution.
2. Feed material.
3. Temperature.
4. Seeding.
5. Input feed rate.
6. Uniformity of mixture.
7. Carbon to Nitrogen ratio.
8. Digester diameter.
9. Nutrients.
10.Retention time.
11.Pressure.
12.Toxicity due to end product.
13.Mixing/ Stirring.
c) CLASIFICATION OF BIOGAS PLANTS:
Here only the classification is made. No explanation for each is provided.
1. Continuous and batch type.
2. The dome type.
3. The drum type.
Thus biogas energy provides an easy and sustainable method of utilization of bio wastes and proving means of small source of electricity to rural area. This concludes the short study of bio energy conversion as a part of non conventional energy study.
9. GEO THERMAL ENERGY:
The energy trapped in the earth crust in the form of used to heat up a heat exchanger and this heat is used to produce energy to do useful work. The following figure shows a schematic diagram of the process.
A geothermal power plant uses its geothermal activity to generate power. This type of natural energy production is extremely environmentally friendly and used in many geothermal hot spots around the globe.
To harness the energy, deep holes are drilled into the earth (much like when drilling for oil) until a significant geothermal hot spot is found. When the heat source has been discovered, a pipe is attached deep down inside the hole which allows hot steam from deep within the earths crust to rise up to the surface.
The pressurized steam is then channeled into a turbine which begins to turn under the large force of the steam. This turbine is linked to the generator and so the generator also begins to turn, generating electricity. Then cold water is pumped down a new pipe which is heated by the earth and then sent back up the first pipe to repeat the process.
Figure 19: Geothermal energy extraction process.
a) PRINCIPLE OF GEO THERMAL ENERGY PRODUCTION:
The main problems with geothermal energy is that firstly, one cannot pump too much cold water into the earth, as this could cool the rocks too much,
resulting in your geothermal heat source cooling down. Secondly, geothermal power plants must be careful of escaping gases from deep within the earth as some may be highly combustible and also poisonous.
b) GEO THERMAL ENERGY SOURCES:
a. Hydro-thermal convective systems.
These are sub divided into:
Vapor-dominated dry steam fields.
Liquid-dominated wet steam fields.
Hot-water fields.
b. Geo pressure resources.
c. Petro-thermal or hot dry rocks (HDR).
d. Volcanoes.
c) ADVANTAGES OF GEO THERMAL PLANT:
Advantage of using geothermal heat to power a power station is that, unlike most power stations, a geothermal system does not create any pollution. It may once in a while release some gases from deep down inside the earth, that may be slightly harmful, but these can be contained quite easily.
The cost of the land to build a geothermal power plant on, is usually less expensive than if you were planning to construct an oil, gas, coal, or nuclear power plant. The main reason for this is land space, as geothermal plants take up very little room, so you don't need to purchase a larger area of land.
Another factor that comes into this is that because geothermal energy is very clean, you may receive tax cuts, and/or no environmental bills or quotas to comply with the countries carbon emission scheme.
No fuel is used to generate the power, which in return, means the running costs for the plants are very low as there are no costs for purchasing, transporting, or cleaning up of fuels.
d) DISADVANTAGES OF GEO THERMAL PLANT:
Geothermal heat is extracted from deep within the earths surface, and this is the main disadvantage concerning finding a suitable build location.
Building a geothermal energy plant mainly lie in the exploration stage. During exploration, researchers will do a land survey which may take several years to complete.
The big disadvantage of geothermal energy extraction, is that in many cases, a site that has successfully been extracting steam and turning it into power for many years, may suddenly stop producing steam. This can happen and last for around 10 years in some cases. So not very reliable.
e) ENERGY FROM THE OCEANS:
The energy from the ocean is of two main forms: Ocean thermal energy conversion (OTEC) and the tidal energy.
OTEC:
The conversion of solar energy stored as heat in the ocean into electrical energy using a heat engine by utilizing the temperature difference between the warm top layer and cooler bottom layer is called Ocean thermal energy conversion (OTEC).
There are two types of cycle in this according to the system used:
a) Open cycle system OTEC:
A typical open cycle system is shown below in figure. The open cycle consists of the following steps: (i) flash evaporation of a fraction of the warm seawater by reduction of pressure below the saturation value corresponding to its temperature (ii) expansion of the vapor through a turbine to generate power; (iii) heat transfer to the cold seawater thermal sink resulting in condensation of the working fluid; and (iv) compression of the non-condensable gases (air released from the seawater streams at the low operating pressure) to pressures required to discharge them from the system. These steps are depicted in Figure . In the case of a surface condenser the condensate (desalinated water) must be compressed to pressures required to discharge it from the power generating system.
Figure19:Open-Cycle OTEC Flow Diagram.
The evaporator, turbine, and condenser operate in partial vacuum ranging from 3 percent to 1 percent atmospheric pressure. This poses a number of practical concerns that must be addressed. First, the system must be carefully sealed to prevent in-leakage of atmospheric air that can severely degrade or shut down operation. Second, the specific volume of the low-pressure steam is very large compared to that of the pressurized working fluid used in closed cycle OTEC. This means that components must have large flow areas to ensure that steam velocities do not attain excessively high values. Finally, gases such as oxygen, nitrogen and carbon dioxide that are dissolved in seawater (essentially air) come out of solution in a vacuum. These gases are uncondensable and must be exhausted from the system. In spite of the aforementioned complications, the Claude cycle enjoys certain benefits from the selection of water as the working fluid. Water, unlike ammonia, is non-toxic and environmentally benign. Moreover, since the evaporator produces desalinated steam, the condenser can be designed to yield fresh water. In many potential sites in the tropics, potable water is a highly desired commodity that can be marketed to offset the price of OTEC-generated electricity.
Flash evaporation is a distinguishing feature of open cycle OTEC. Flash evaporation involves complex heat and mass transfer processes. In the
configuration tested by a team lead by the author, warm seawater was pumped into a chamber through spouts designed to maximize the heat-and-mass-transfer surface area by producing a spray of the liquid. The pressure in the chamber (2.6 percent of atmospheric) was less than the saturation pressure of the warm seawater. Exposed to this low-pressure environment, water in the spray began to boil. As in thermal desalination plants, the vapor produced was relatively pure steam. As steam is generated, it carries away with it its heat of vaporization. This energy comes from the liquid phase and results in a lowering of the liquid temperature and the cessation of boiling. Thus, as mentioned above, flash evaporation may be seen as a transfer of thermal energy from the bulk of the warm seawater to the small fraction of mass that is vaporized to become the working fluid. Approximately 0.5 percent of the mass of warm seawater entering the evaporator is converted into steam.
A large turbine is required to accommodate the huge volumetric flow rates of low-pressure steam needed to generate any practical amount of electrical power. Although the last stages of turbines used in conventional steam power plants can be adapted to OC- OTEC operating conditions, existing technology limits the power that can be generated by a single turbine module, comprising a pair of rotors, to about 2.5 MW. Unless significant effort is invested to develop new, specialized turbines (which may employ fiber-reinforced plastic blades in rotors having diameters in excess of 100 m), increasing the gross power generating capacity of a Claude cycle plant above 2.5 MW will require multiple modules and incur an associated equipment cost penalty. Condensation of the low-pressure working fluid leaving the turbine occurs by heat transfer to the cold seawater. This heat transfer may occur in a DCC, in which the seawater is sprayed directly over the vapor, or in a surface condenser that does not allow contact between the coolant and the condensate. DCCs are relatively inexpensive and have good heat transfer characteristics due to the lack of a solid thermal boundary between the warm and cool fluids. Although surface condensers for OTEC applications are relatively expensive to fabricate they permit the production of desalinated water. Desalinated water production with a DCC requires the use of fresh water as the coolant. In such an arrangement, the cold seawater sink is used to chill the fresh water coolant supply using a liquid-to-liquid heat exchanger.
Effluent from the low-pressure condenser must be returned to the environment. Liquid can be pressurized to ambient conditions at the point of discharge by means of a pump or, if the elevation of the condenser is suitably high, it can be compressed hydrostatically. Non-condensable gases, which include any residual water vapor, dissolved gases that have come out of solution, and air that may have leaked into the system, must be pressurized with a compressor. Although the primary role of the compressor is to discharge exhaust gases, it usually is perceived as the means to reduce pressure in the system below atmospheric. For a system that includes both the OC-OTEC heat engine and its environment, the cycle is closed and parallels the Rankine cycle. Here, the condensate discharge pump and the non-condensable gas compressor assume the role of the Rankine cycle pump.
The analysis of the open cycle :
Heat (added) absorbed from seawater (J/s) qw= m*ww Cp (Twwi - Twwo)
Steam generation rate (kg/s) m*s = qw/hfg
Turbine work (J/s) wT = m*s nT(h3 - h5)
= m*snT (h3 - h 5s)
Heat (rejected) into seawater (J/s) qc = m*cwCp (Tcwo - Tcwi)
where,
m*ww is the mass flow rate of warm water;
Cp the specific heat; Twwi and Twwo the seawater temperature at the inlet and outlet of the heat exchanger;
hfg the heat of evaporation; and the enthalpies at the indicated points are given by h, with the subscript s referring to constant entropy.
The turbine isentropic efficiency is given by nT. The subscript ‘cw’ refers to the cold water.
b) Closed cycle system OTEC:
The operation of a closed-cycle OTEC plant, using anhydrous ammonia as the working fluid, is modeled with the saturated Rankine cycle. Figure shows a simplified flow diagram of the CC-OTEC cycle. The analysis of the cycle is straightforward as shown by the figure.
Figure 20:Closed -Cycle OTEC Flow Diagram.
Based on a unit mass flow rate of ammonia vapor (kg s-1) in the saturated cycle:
Where h is the enthalpy at the indicated state point. It follows that the heat-added plus the pump-work is equal to the heat-rejected plus the turbine-work.
Thus at an OTEC site cold water is raised from the lower levels of the sea and used for the cooler side of a heat engine. The hotter side is supplied by the surface water.
The products of the whole process are:
1. Energy from the heat engine. This will usually be used to produce electricity. In tropical areas the output will be fairly constant and will continue night and day. That is, unlike direct solar converters it does not slow
down at night. This means that, like a nuclear power station, OTEC can be used as base load.
2. Cooler water at the ocean surface. This does not mean there is less heat stored in the water. The effect is to produce a thicker layer of warm water at a cooler temperature. Some of this water can be used on land for cooling purposes.
4. Nutrient rich water (because the bottom water has more nutrients than the surface). This can be used to promote water plant growth and fish.
10.TIDAL ENERGY:
Tidal energy is one of the oldest forms of energy used by humans. Indeed, tide mills, in use on the Spanish, French and British coasts, date back to 787 A.D.. Tide mills consisted of a storage pond, filled by the incoming (flood) tide through a sluice and emptied during the outgoing (ebb) tide through a water wheel. The tides turned waterwheels, producing mechanical power to mill grain. We even have one remaining in New York- which worked well into the 20th century.
Tidal power is non-polluting, reliable and predictable.Tidal barrages, undersea tidal turbines - like wind turbines but driven by the sea - and a variety of machines harnessing undersea currents are under development. Unlike wind and waves, tidal currents are entirely predictable.
Tidal energy can be exploited in two ways:
* By building semi-permeable barrages across estuaries with a high tidal range.
* By harnessing offshore tidal streams.
Barrages allow tidal waters to fill an estuary via sluices and to empty through turbines. Tidal streams can be harnessed using offshore underwater devices similar to wind turbines.
Thus energy form the ocean forms a non conventional energy that can be harnessed for useful of mankind, with the increase of energy demand OTEC
and tidal energy has found to be a important area to venture.
11. CONCLUSIONS:
In this survey we have done a brief study on the non conventional energy resources. A study on solar energy and its applications are made. A mention on wind energy conversion and utilization is performed. The biomass utilization and methods of producing energy is reviewed. OTEC and tidal energy are examined. Geothermal energy is analyzed. For all of the above technology the advantages and disadvantages are mention. Moving in this direction with the increasing demand of energy the non-conventional energy in the power grid may be the answer to future energy scarcity and meeting of additional energy demand.
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33. Figures drawn here are from sources: wikipiedia.com, googleimages.com.
