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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
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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.
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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.