Power Systems2

8/13/2019 Power Systems2

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Renewable Energy

I. Wind Energy:Wind power has been used for mechanical applications such as pumping water and as

mills for many centuries. However, the use of wind power for electrical generation

has accelerated rapidly in the last decade with increment in turbine sizes a year by

year as shown in the table below.

Wind power principles:

Betz's law was developed in 1919 by the German physicist Albert Betz. The theory estimates the maximum possible energy to be derived from a wind

turbine is about 59.3% of the kinetic energy in wind.

power coefficient (Cp) = power output from wind machine / power availablein wind.

Modern horizontal axis wind turbine can reach 65% to 75% of the theoreticalBetz limit.

70R


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Second: Vertical Axis Wind Turbine (VAWT)VAWTs become more popular with more new designs every year. Some of the most

common designs are discussed in the following sections.

Darrieus or Eggbeater wind turbines:

Developed by Georges Darrieus in 1931. Has moderate efficiency. Large torque ripple and cyclic stress on the

tower, which contributes to poor reliability. Starting torque is very low, thus, it requires

external startup power source.

Torque ripple is reduced (smooth operation)by using three or more blades which resultsin a higher solidity for the rotor.


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Helical blades wind turbines

It is a modified Darrieus wind turbine. It has three blades and a helical twist of 60

degrees. Torque is spreader evenly over the entire

revolution resulting in smoother rotation and

preventing destructive pulsations.

Giromill

It is like Darrieus turbine with straight blades. Simpler and cheaper but less efficient compared

to Darrieus turbine. Requires startup motor. Torque ripple is reduced (smooth operation) by

using three or more blades.Giromill (cyclo-turbine)

Each blade can rotate around its own vertical axis. The blade changes its angle of attack relative to the wind, resulting in smoother

torque. Torque remains near maximum for longer rotation angle producing more net torque. More efficient operation in turbulent winds with lower blade bending stress. Self starting.

Savonius

One of the simplest turbines. It is a drag-type devices with two or more scoops. High torque & low efficiency. Self-starting and can be installed at low heights. Low cost high reliability.


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Twisted Savonius

It is a modified savonius with long helicalscoops.

Low cost high reliability. Low efficiency. Produces smooth torque. Can be used for low heights low power

applications such as on roof or on boat wind

turbine.

Airborn wind turbines

Wind velocity increased at higher altedutessignificantly, thus, this tipe operates at very

high altitudes.

No tower required. Much lower building cost.

II. Biomass Energy:Biomass refers to any organic substance from plant materials or animal wastes used

as fuels. It includes for example, agricultural residues, urban wastes even sewage

sludge waste.

Three main biomass conversion processes:1. Direct combustion2. Biological conversion3. Thermochemical conversion: Pyrolysis


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GasificationDirect combustion

Main obstacles of biomass combustion: High moisture Low heating value High bulk and low density

Thus, main factors to achieve complete: combustion: Good air/fuel mixing Long combustion residence time

Biomass has to be pre-dried and converted into powder, pellets or chips first. Popular

combustors such as fluidized bed and cyclone.

Biological conversion

Biomass is converted into biogas by anaerobic digestion process in theabsence of air, either in landfill or modern biogas plans.

wet organic waste decomposing by bacteria into biogas, however, in landfilldigesters, the conversion takes long time (about month).

In modern biogas plans, CHP gas engines are used for electrical out put aswell as providing heat for the digesting tanks to accelerate the conversion

process.

Anaerobic digesters can be designed based on:

1. Process flow: Batch (simpler and cheaper design) or continues (complex andmore efficient)

2. Temperature: Mesophilic (20-40C) or thermophilic (50-70C) more stablewith faster production rate.

3. Solids content: High solids or low solids (liquid form).4. Complexity: Single stage (less reaction control) or multistage (different type

of bacteria in different stages to achieve maximum control and performance.


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Biogas

Biogas composition from biogas plant:50-75%CH4Methane25-50%CO2Carbon dioxide0-10%N2Nitrogen0-1%H2Hydrogen0-3%H2SHydrogen sulfide

Biogas has medium heating value


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Fast and flash pyrolysis

Fast and flash pyrolysis are the most studied types since they yield high amount of

bio-oil. Following conditions are required:

Very high heat transfer rates is required, thus, biomass has to be finelyground.

Carefully controlled reaction temperature. Low residence time of pyrolysis vapors in the reactor. Quenching (rapid cooling) of the pyrolysis vapors to give the bio-oil product.

Common systems used include: open-core fixed bed, ablative fast pyrolysis, cyclonic

and rotating core.

Bio-oilis a dark brown heavy oil of medium heating value (LHV 16-21MJ/kg).

Gas turbines and IC engines has to be modified before they can utilize bio-oil.Bio-oil can be used efficiently in boilers as attractive renewable alternative.Co-firing of bio-oil has been demonstrated in 350 MW gas fired power station in

Holland

Gasification

It is a thermochemical reaction of biomass material that occurs in limited presence of

oxygen with a higher temperature levels. The output product is a low heating value

gas fuel known as producer gas or syngas.

Oxidizer types:

1. Oxygen: gas heating value can go up to2. Steam: gas heating value3. Air: gas heating value (4-8Mj/m3)

Thermochemical Zones inside Gasifiers:

1. Drying zone: heat from combustion zone is used to dry biomass.2. Pyrolysis zone: oxygen doesnt reach to this zone, dry biomass is converted

into char coal , volatiles and tar at 200-350C.3. Oxidation or Combustion Zone: crated at the oxidizer inlet and its size

depends on oxidizer flow rate. Provides heat for other zones.

4. Reduction or Gasification Zone: produces the gas at 400-800C with limitedamount of oxidizer


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Bio-products From biomass fuels:

The petrochemical industry makes many products from fossil fuels such asplastics, chemicals, and other product. The same or similar products can, for

the most part, be made from biomass.

Bioproducts that can be made from sugars include antifreeze, plastics, glues,artificial sweeteners, and gel for toothpaste.

Bioproducts that can be made from carbon monoxide and hydrogen of syngasinclude plastics and acids, which can be used to make photographic films,

textiles, and synthetic fabrics.


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Bioproducts that can be made from phenol, one possible extraction frompyrolysis oil, include wood adhesives, molded plastic, and foam insulation.

III. Geothermal Energy:Core of earth can reach up to 4000C due to

the decay of radioactive materials. The

temperature at the base of crust is about

1000C and the average heat flow towards

the surface is about 0.063W/m2. Certain

regions on earth has the molten rocks

(magma) pushes towards the surface through

weak zones and cracks creating hot spots 2-

3km below surface. One of the main

advantages of geothermal is the availability

around the clock (constant all day long)

unlike solar or wind energy.Geothermal Natural Systems:

Main elements:

Heat source: can be high-temperature magmatic instruction (5-10km depth) orlow-temperature earths normal temperature graduation (2.5-3 C/100m).

Reservoir: is a volume of hot permeable rocks where fluid circulates. Fluid: pure water or mixed with other compositions such as CO2, H2S, etc.

Geothermal resources: Low enthalpy resources 150C.


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2. Bottoming cycle arrangement.Dry steam power plants

One of the first technologies in 1904,at the same Larderello dry steam

field.

Limited availability, depends on thewell properties and the extraction

depth.

Simple design since it doesntrequire water separator.

The largest dry steam field in theworld is the Geysers in USA.

Flash steam power plants

It is the most commontechnology.

Pressurized hot water >150Cremains in liquid form with.

This very hot water flows upthrough wells in the ground under

its own pressure. As it flows upward, the pressure

decreases and some of the hot

water boils into steam in a flash

tank and steam is separated from

the water.

Separated water and condensedsteam are injected back into the

reservoir.

Binary power plants Can operate on water at lower

temperatures


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The water and the working fluid are kept separated during the whole process, so thereare little or no air emissions.

A comparison between steam and hydrocarbon based power plants

HydrocarbonSteamOperates with wider range of temperaturesOperates with medium and high temp. wellsMuch higher pressure at similar temp.Low pressure cycleHigher overall eff.Lower overall eff.Dry gas expansionSaturated steam expansionLow enthalpy, simple single stage turbineHigh enthalpy, multi-stage turbine is requiredSmaller turbine and condenser is requiredHigher density / volume, larger system partsPositive pressure condenserLow pressure condenser, air can leak to system

Biphase rotary separator turbo-alternator

Can extract power from two-phase water/steam flow trough three maincomponents:

1.Two-phase nozzle: increasing kinetic energy of water/steam (pressure drop). 2.Rotary separator: separates them by centrifugal force. Steam is passed to steam

turbine.

3.Liquid turbine: power is generated from the pressurized water then re-injected to

the well.

It can be put before the steam turbine (topping cycle). It can be put after the steam turbine cycle (bottoming cycle).Biphase systems can be divided into two main designs based on the biphase locations:


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Topping cycle plant:Smaller biphase units can be used due to the lower specific volume.

Bottoming cycle plant:Larger biphase units is required due to the higher specific volume

Second: Geothermal heat pump (GHP)

Works in principle as the conventional air-con. Heat pump but takes theadvantage of the constant geothermal temp. (7-21C) through the year.

Works as cooler in summer and as heater in winter. Provides 25%-50% electrical savings depends on temperature range available. GHP piping can be drilled underground (soil) or submerged in water (lake or

well).


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Third: Geothermal for refrigeration and air conditioning

Two fluids are used such as Lithium bromide/water or Ammonia/water (one ofit has high absorption capacity for the other).

Main power input is thermal (hot water or saturated steam) with very low elec.Required for liquid pumps.

Low COP usually


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OTEC can be divided into two main designs: Closeand Opencycle turbines.

Open-cycle turbine: some of ocean water boils at low press./temp. condition and

saturated steam drives the turbine. Steam condenses to a desalinated water.


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V. Hydrogen Energy and Fuel CellFuel call

It is an electrochemical device that generates electricity directly from the chemicalenergy in fuel (mainly hydrogen) although some other hydrocarbons were alsostudied.

When hydrogen is used, reversed electrolysis process occurs. In electrolysis, water molecules split into hydrogen and oxygen molecules by

consuming electricity whereas in fuel cell reaction, hydrogen and oxygen molecules

combine to produce water and electricity.

Unlike conventional power generation systems, fuel cells do not involve intermediateconversion of chemical energy to thermal and mechanical energies. Consequently, of

all the existing energy conversion systems, fuel cells offer the highest efficiency

along with the lowest levels of pollutant emissions.


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First: point absorber

It is a floating devise fixed to

a generator at ocean floor by

cable. The vertical motion(up/down) of the float is used

to drive electromechanical or

hydraulic energy converters

to generate electricity.

Second: Overtopping

It is a reservoir with higher water level above sea level. The wave pressure and

kinetic energy is used to fill-up the reservoir.Electrical generator is fixed to a hydraulic turbine driven by water head pressure.


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Third: Attenuators

They are long multisegment floating structures oriented parallel to the direction of the

waves. The differing heights of waves along the length of the device causes flexing

where the segments connect, and this flexing is connected to hydraulic pumps or

other converters. One of the examples is the Pelamis which has four 30-m long by

3.5-m diameter floating cylindrical pontoons connected by three hinged joints (see the

figure below).

Fourth: Terminator

In terminators, wide wave area on or beside shore is accumulated in a conical-shape

barrier to rotate directly an axial or horizontal axis turbine. Another way is to use the

pressure of the collected water is by moving an oscillating water column that drives

directly a turbine or pushes compressed air column to drive air turbine.

VII. Tidal energyTides are periodic vertical rise and fall

of ocean water because of the

gravitational forces of sun and moon.Three common configurations are used:1.Single basin: single effect tidal

power scheme.


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2.Single basin: double effect tidalpower scheme.

3.Linked basin: double basin, singleeffect.

Main designs for the Tidal Power Plants are:

I. Single basin single effect tidal power schemeBasin is filled by keeping the

sluices open during flood tied.

Sluices are closed and water

flows back through the turbine

(axial of radial) inside the

power house. Power is

generated in one-way only.

II. Single basin Double effect tidal power schemeWater flows through the

power house during both flood

and ebb tied.

Thus, power generation takes

place at both ways of the

water flow, resulting in more

efficient power plant

III. Linked basinFlood tied fills the lowthen high basin. Power

house is located between

high and low basins.

Power is generated

during the fill-up process

of the higher basin and

also during the discharge

from high basin. Finally,

water flows to the seafrom the low basin

during ebb tide.


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VIII. Hydro EnergyHydro power depend on water potential energy (height difference) and water flow

rate. Commonly used hydro turbines are axial, combined axial/radial and Pelton

wheel.

Small scale micro and Pico hydro turbines can be installed directly on river without

the need for reservoir.


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IX. Solar Energy

First: Photovoltaics (PV):

It is the direct conversion of light into

electricity at the atomic level. Some

materials exhibit a property known as thephotoelectric effect that causes them to

absorb photons of light and release

electrons. When these free electrons are

captured, an electric current results that can

be used as electricity. photovoltaic cells,

also known as solar cells are made of

semiconductor materials, such as silicon,

used in the microelectronics industry. A

number of solar cells electrically connected

to each other and mounted in a support

structure or frame is called a photovoltaicmodule. Modules are designed to supply

electricity at a certain voltage, such as a

common 12 volts system.

Today's most common PV devices use a single junction, or interface, to create an

electric field within a semiconductor such as a PV cell. In a single-junction PV cell,

only photons whose energy is equal to or greater than the band gap of the cell

material can free an electron for an electric circuit. In other words, the photovoltaic

response of single-junction cells is limited to the portion of thesun's spectrum whose

energy is above the band gap of the absorbing material, and lower-energy photons are

not used.

One way to get around this limitation is to

use two (or more) different cells, with more
http://www.eren.doe.gov/pv/bandgaps.htmlhttp://www.eren.doe.gov/pv/bandgaps.htmlhttp://www.eren.doe.gov/pv/lightsun.htmlhttp://www.eren.doe.gov/pv/lightsun.htmlhttp://www.eren.doe.gov/pv/bandgaps.html


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than one band gap and more than one

junction, to generate a voltage. These are

referred to as multi junction or cascade cells.

Multi junction devices can achieve a higher

totalconversion efficiencybecause they can

convert more of the energy spectrum of light

to electricity. As shown in the figure, a multi

junction device is a stack of individual

single-junction cells in descending order of

band gap (Eg). The top cell captures the

high-energy photons and passes the rest of

the photons on to be absorbed by lower-

band-gap cells.

photovoltaic (PV) & concentrated photovoltaic (CPV):

PV cells converts light photons directly to electrical output in one-step conversion. Tremendous amount of materials and designs have been tested, however commercial

PV efficiency is only about 8% with higher eff. Up to 20% for under research cells. Overall system efficiency can be increased by concentrating light beams in CPV.

Second: Concentrated solar power (CSP)

It is one of the fastest growing technologies. Power capacity of the single unitincreases a year by year.

The systems use mirrors or lenses to concentrate a large area of solar thermal energy,onto a small area.

Electrical power is produced when the concentrated light is converted to heat anddrives an externally fired engine commonly steam although EFGT and stirlingengines have been tested.

Examples of some commonly used concentrators: parabolic trough, dish type,concentrating linear Fresnel reflector, Fresnel lenses and solar power tower.

CSP plants offers high amount of thermal power that is usually utilized as CHP plantfor different thermal uses.

CSP is the main candidate for solar refrigeration although low/medium temp. solarpanels are also used.
http://www.eren.doe.gov/pv/conveff.htmlhttp://www.eren.doe.gov/pv/conveff.html


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Third: Low and medium temp. thermal panels

Low and medium temperature collectors provides thermal power withoutconcentrating the sun radiation.

Low-temp. collectors are flat plates generally used to heat swimming pools andhouses. Medium-temperature collectors are also usually flat plates but for larger scale

water or air heating for commercial use with more effective radiation absorbent

materials.


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Renewable energy comparison

Renewable energy environmental impact

First: Impact of wind Energy: it has the advantage that it doesn't require cooling water nor water treatment, thus,

no water pollution is associated with it.


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It generates a considerable noise pollution and visual disturbance for thelandscape view that can affect the nearby community.

Construction hazards just like other high building construction. The effect on life forms especially birds where many bird killing accidents by the

blades were recorded.Second: Impact of biomass Energy:

For biomass fuels, growing plantation or forests for ethanol production or woodutilization presents a big challenge due to the huge amounts of water and land is

required for economical biomass utilization. The other challenge is the regular

removal and re-plantation causes a disturbance for other life forms such as birds

and small animals.

High potential for air pollution for single stage combustion and even in two-stageif combustion was not controlled properly.

The presence of CO2emissions although it is circulated in biomass growth cycle. High machinery maintenance and operation requirement. Solid waste (ash) disposal requirement. Ash contains valuable minerals content

that has to be returned to the plantation soil for further plantation growth. Non-controlled anaerobic digesters can cause an intensive smell pollution that can

affect the plant neighbors.

Third: Impact of PV & CSP solar Energy:

Very large land is required (especially PV) for a reasonable power output thatshould not be very far from population to reduce elec. trans. cost. Thus, the risk of

deforestation and disturbing other life forms is considerable. Very large quantity of row materials is required for the collectors and PV cells

fabrication with considerable amount of power consumption and emissions during

fabrication. For PV manufacturing, many non-recyclable and toxic materials can be used rising

the environmental concern of material disposal. Visual effect especially for CSP when it reflects light beams that can affect the

personal around the solar farm.

Fourth: Impact of Geothermal & OTPC Energy: Many pollutant gases are dissolved in the geothermal water such as: carbon

dioxide, methane, hydrogen sulfide, ammonia, nitrogen and hydrogen, that can be

either poisonous or contributes in global warming.

Flash steam plants have high risk of releasing high quantities of these gases if notbeen controlled.

Binary plants has much lower risk since water is re-injected directly to the wellafter the heat exchanger.

Ocean thermal power plants moves large water quantities (4m3/s per MW) causingthermal disturbance around the plant and affecting life forms.

Fifth: Impact of Hydro Energy:


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Large dams results in a submerge of large areas of land that converts into areservoir resulting in a large scale deforestation and loss of wildlife.

For large scale hydro reservoirs, large amount of greenhouse gas (methane) isemitted from the stagnant water. This issue is totally avoided in micro and Pico

hydro where turbines are placed in the river stream. For large scale hydro reservoirs, people relocation from reservoirs area presents a

serious problem.

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Power Systems2