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Solar Cell
Technology
and
Applications
Electrical
Technology
BTEC
National
Level 3
Solar Cell Technology and Applications
First generation solar technology utilizes silicon.
Silicon is a naturally occurring element that is utilized by numerous industries including
semiconductors and computer chips. With the new demand from solar panel
manufacturers, demand for silicon far exceeds supply, driving prices of this critical raw
material – and the price of silicon driven solar energy – well beyond a commercially
viable price point in a non-subsidized environment.
How the Solar Cell
works
This diagram shows a typical crystalline
silicon solar cell. The electrical current
generated in the semiconductor is
extracted by contacts to the front and
rear of the cell. The top contact structure
which must allow light to pass through
is made in the form of widely-spaced
thin metal strips (usually called fingers)
that supply current to a larger bus bar.
The cell is covered with a thin layer of dielectric material - the anti-reflection coating,
ARC - to minimise light reflection from the top surface.
The actual process of converting photons to electricity is explained in the next 5 images.
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Solar Cell Technology and Applications
Solar cells are essentially semiconductor
junctions under illumination. Light generates
electron-hole pairs on both sides of the
junction, in the n-type emitter and in the ptype base. The generated electrons (from the
base) and holes (from the emitter) then
diffuse to the junction and are swept away
by the electric field, thus producing electric
current across the device. Note how the
electric currents of the electrons and holes
reinforce each other since these particles
carry opposite charges. The p-n junction
therefore separates the carriers with opposite charge, and transforms the generation
current between the bands into an electric current across the p-n junction.
A more detailed consideration makes it possible to draw an equivalent circuit of a solar
cell in terms of a current generator and a diode. This equivalent circuit has a currentvoltage relationship.
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Solar Cell Technology and Applications
In solar cell applications this characteristic is usually drawn inverted about the voltage
axis, as shown below. The cell generates no power in short-circuit (when current Isc is
produced) or open-circuit (when cell generates voltage Voc). The cell delivers maximum
power Pmax when operating at a point on the characteristic where the product IV is
maximum. This is shown graphically below where the position of the maximum power
point represents the largest area of the rectangle shown.
The efficiency (n) of a solar cell
is defined as the power Pmax
supplied by the cell at the
maximum power point under
standard test conditions, divided
by the power of the radiation
incident upon it. Most frequent
conditions are: irradiance 100
mW/cm2 , standard reference
spectrum, and temperature 25 0
C. The use of this standard
irradiance value is particularly
convenient since the cell
efficiency in percent is then
numerically equal to the power
output from the cell in mW/cm2.
Crystalline silicon was the original materials technology used by the PV industry to
manufacture solar cells. First widely used in space satellites, conventional crystalline
silicon solar cells are fabricated in a step-and-repeat, batch process from small wafers of
single crystal or polycrystalline silicon semiconductor materials. Although, substantial
advances have been made in the development of this technology, the cost of crystalline
PV modules is still high because of materials costs and numerous processing steps that are
needed to manufacture the modules. Crystalline silicon solar modules are bulky, break
easily, and consume more energy in manufacturing.
Second generation amorphous solar technology
Generally eliminates silicon, or utilises amorphous silicon, and
leverages “thin films” as the surface area for energy capture. The
elimination of silicon as an expensive raw material provides thin
film technologies
with a low cost
advantage over
first generation technologies on a per
unit basis, albeit with lower efficiencies.
Thin film solar technologies, however,
require extremely sophisticated and
expensive production lines, creating an
extremely high barrier to entry and
limiting supply of thin film panels.
Amorton Film is an exceptionally thin,
light and flexible amorphous silicon
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Solar Cell Technology and Applications
solar cell fabricated on plastic film. In addition to these advantages, Amorton Film is also
resistant to cracking. Its standard configuration includes protective film covering the
amorphous silicon solar cell which measures about 0.4mm in overall thickness.
Third Generation solar technology utilises dye technology.
With efficiencies similar to thin film technologies and low price per watt, DSC and
3GSolar represent a dramatic advancement in solar energy development, and offer the
opportunity to bring reasonably priced, easily manufactured solar power to world markets.
How Dye Solar Cells Work:
Conventional solar cells convert light into electricity by exploiting the photovoltaic effect
that exists at semiconductor junctions. The semiconductor performs two processes
simultaneously: absorption of light, and the separation of the electric charges ("electrons"
and "holes") which are formed as a consequence of that absorption. However, to avoid the
premature recombination of electrons and holes, the semiconductors employed must be
highly pure and defect-free.
In contrast, dye solar cells (DSC, also referred to as dye sensitized solar cells) work on a
different principle whereby the processes of light absorption and charge separation are
differentiated. Due to their simple construction and non-vacuum manufacturing process,
dye cells offer significant reduction in the cost of solar electricity. Solar-to-electric energy
conversion efficiencies in excess of 10% have been obtained in research cells of this type.
The dye solar cell consists of
two electrodes in a sandwich
configuration (see diagram). On
one of these electrodes, the
photoanode, is a several micronthick layer of nanocrystalline
titania TiO2 that is deposited by
screen printing and oven
treatment in air. This compact
layer is porous with a very high
surface area, allowing
monolayer absorption of dye
molecules. The dye-coated
electrode is then put together
with another conducting
electrode, the counter electrode,
and the intervening space is
filled with electrolyte (a redox
electrolyte usually based on an organic medium containing iodine). The counter electrode
is catalysed with trace platinum or carbon, efficient electrocatalysts for the cathodic
reduction of triiodide to iodide in the redox electrolyte. After making adequate provisions
for current takeoff from the two electrodes, the assembly is sealed.
Respectable Efficiency, thanks to Nanostructure
A respectable photovoltaic efficiency is obtained by the use of the porous nanostructured
titania layer of very high surface roughness. When light penetrates the photosensitised,
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Solar Cell Technology and Applications
semiconductor "sponge", it crosses hundreds of adsorbed dye monolayers. The end result
is greater absorption of light and its efficient conversion into electricity. The dye is
usually based on an organic complex of ruthenium that sensitises the titania to visible
light. An incoming photon causes a dye molecule to enter an activated state, and an
electron is injected into the conduction band of the titania. The electron passes out of the
cell via the conducting coating on the photoanode, does work in the external circuit and
re-enters the cell via the counter electrode. There it reduces tri-iodide to iodide ion that
diffuses to the activated dye molecule on the photoanode and gives up its electron. Triiodide is thus regenerated and the dye returns to its preactivated state ready for the next
photon.
Applications of Solar Cells
Alongside a variety of consumer products - electronic watches, calculators, power for
leisure equipment and tourism - there is an extensive range of applications where solar
cells are already viewed as the best option for electricity supply. These applications are
usually stand-alone, and exploit the following advantages of photovoltaic electricity:
 There are no fuel costs or fuel supply problems
 The equipment can usually operate unattended
 Solar cells are very reliable and require little maintenance
At the other end of the scale are grid-connected systems that are now being seriously
considered to supplement the conventional power generation in many industrialised
countries. Although they have yet to become viable on economic grounds, the
participation of PV in large-scale power generation is viewed with increasing prominence
as a means of halting the adverse environmental effects of conventional energy sources.
Rural electrification
The provision of electricity to rural areas derives important social and economic benefits
to remote communities throughout the world. Power supply to remote houses or villages,
electrification of the health care facilities, irrigation and water supply and treatment are
just few examples of such applications.
The potential for PV powered rural applications is enormous. The UN estimates that two
million villages within 20 of the equator have neither grid electricity nor easy access to
fossil fuel. It is also estimated that 80% of all people worldwide do not have electricity,
with a large number of these people living in climates ideally suited to PV applications.
Even in Europe, several hundred thousand houses in permanent occupation (and yet more
holiday homes) do not have access to grid electricity.
The economics of PV systems compares favourably with the usual alternative forms of
rural electricity supply, grid extension and diesel generators. The extension and
subsequent maintenance of transmission lines over long distances of often a difficult
terrain is expensive, particularly if the loads are relatively small. Regular fuel supply to
diesel generators, on the other hand, often present problems in rural areas, in addition to
the maintenance of the generating equipment.
Water pumping
More than 10,000 PV powered water pumps are known to be successfully operating
throughout the world. Solar pumps are used principally for two applications: village water
supply (including livestock watering), and irrigation.
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Solar Cell Technology and Applications
Since villages need a steady supply of water, provision
has to be made for water storage for periods of low
insolation. In contrast, crops have variable water
requirements during the year that can often be met by
supplying water directly to produce without the need for
a storage tank.
Deep well solar pump in Arizona. Courtesy of Paul
Maycock PV Energy Systems
Domestic supply
Stand-alone PV domestic supply systems are commonly encountered in developing
countries and remote locations in industrialised countries. The size range varies from 50
Wp to 5 kWp depending on the existing standard of living. Typically larger systems are
used in remote locations or island communities of developed countries where household
appliances include refrigeration, washing machine, television and lighting. In developing
regions large systems (5 kWp) are typically found for
village supply while small systems (20-200 Wp) are
used for lighting, radio and television in individual
houses.
Solar-powered house in Main, USA. Courtesy of
Paul Maycock PV Energy Systems
Health care
Extensive vaccination programmes are in progress
throughout the developing world in the fight against
common diseases. To be effective, these programmes
must provide immunisation services to rural areas. All
vaccines have to be kept within a strict temperature
range throughout transportation and storage. The
provision of refrigeration for this aim is known as the
vaccine cold chain.
Mobile solar vaccination cooler.
Courtesy of Neste Advanced Power Systems
(NAPS)
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Solar Cell Technology and Applications
Lighting
In terms of the number of installations,
lighting is presently the biggest application
of photovoltaics, with tens of thousands of
units installed world-wide. They are mainly
used to provide lighting for domestic or
community buildings, such as schools or
health centres. PV is also being
increasingly used for lighting streets and
tunnels, and for security lighting.
Professional applications
For some time, photovoltaic modules have proved to be a good source of power for highreliability remote industrial use in
inaccessible locations, or where the small
amount of power required is more
economically met from a stand-alone PV
system than from mains electricity.
Examples of these applications include:
Ocean navigation aids: many lighthouses
and most buoys are now powered by solar
cells.
PV powered navigation aid in Saint
Lawrence river. Courtesy of Paul
Maycock PV Energy Systems
Telecommunication systems: radio transceivers
on mountain tops, or telephone boxes in the
country can often be solar powered.
Courtesy of Paul Maycock PV Energy Systems
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Solar Cell Technology and Applications
Remote monitoring and control: scientific research
stations, seismic recording, weather stations, etc. use
very little power which, in combination with a
dependable battery, is provided reliably by a small
PV module.
Solar-powered weather station. Photo courtesy of IT
Power Ltd
Cathodic protection: this is a
method for shielding metalwork
from corrosion, for example,
pipelines and other metal structures.
A PV system is well suited to this
application since a DC source of
power is required in remote
locations along the path of a pipeline. Photo courtesy of IT Power Ltd
Electric power generation in space
Photovoltaic solar generators have been and will remain
the best choice for providing electrical power to
satellites in an orbit around the Earth. Indeed, the use of
solar cells on the U.S. satellite Vanguard I in 1958
demonstrated beyond doubt the first practical
application of photovoltaics. Since then, the satellite
power requirements have evolved from few Watts to
several kiloWatts, with arrays approaching 100 kW
being planned for a future space station.
A space solar array must be extremely reliable in the
adverse conditions of space environment. Since it is
very expensive to lift every kilogram of weight into the
orbit, the space array should also have a high power-toweight ratio.
Image courtesy of Matra Marconi Space
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Solar Cell Technology and Applications
Grid-connected systems
Two types of grid-connected installations are usually distinguished, centralised PV power
stations, and distributed generation in units located directly at the customer's premises
(PV in buildings).
PV Power Stations
Hysperia PV power station.
Photo courtesy of Paul
Maycock PV Energy Systems
A PV power station feeds the
generated power instantaneously
into the utility distribution network
(the 'grid') by means of one or
more inverters and transformers. The first PV power station was built at Hysperia in
southern California in 1982 with nominal power specification 1 MW, using crystalline
silicon modules mounted on a 2 axis tracking system.
PV power stations may be approaching economic viability in locations where they assist
the local grid during periods of peak demand, and obviate the need to construct a new
power station. This is known as peak shaving. It can also be cheaper to place small PV
plants within the transmission system rather than to upgrade it ('embedded' generation).
PV in buildings
Scheidegger Building with photovoltaic
facade near Bern in Switzerland. Courtesy of
Atlantis Solar Systeme AG
PV arrays mounted on roof tops or facades
offer the possibility of large-scale power
generation in decentralised medium-sized
grid-connected units. Studies in Germany,
Switzerland and the UK have shown that the
roof and facade area technically suitable for
PV installations is large enough to supply the
country's electricity demand. The size
envisaged for each decentralised residential
PV system is typically 1- 5kW, with systems
up to a hundred kW or so suitable for
commercial and industrial buildings.
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Solar Cell Technology and Applications
Atlantis Solar Systeme AG have recently introduced "Sunslates"TM that can be fitted to
existing roofs easily and unobtrusively.
The main advantages of these distributed systems over large PV plants are as follows:



There is no cost in buying the land and preparing the site.
The transmission losses are much lower because the load is on the same site as the
supply.
The value of the PV electricity is also higher because it is equal to the selling price
of the grid electricity which has been replaced, rather that to the cost of generating
it.
However, it should also be noted that the price paid by utility companies for electricity
exported from a decentralised source is a fraction of the utility sale price. The optimum
economic benefit is, therefore derived by consuming all PV produced electricity, with
direct reduction of the energy imported from the utility. Thus grid connected PV systems
are ideal for loads that vary in proportion to the irradiation. Typical loads are airconditioning, refrigeration and pumping. Other significant loads can be timed to operate
when PV power is likely to be available. Examples include washing machines and clothes
dryers that can operate on timing clocks.
Produced by the Interactive Learning Centre
© University of Southampton, 1997
http://www.soton.ac.uk/~solar/intro/tech6.htm
Solar thermal power plants
Technology Fundamentals
published in Renewable Energy World 06/2003 pp. 109-113
Many people associate solar electricity generation directly with photovoltaics and not with solar
thermal power. Yet large, commercial, concentrating solar thermal power plants have been
generating electricity at reasonable costs for more than 15 years. Volker Quaschning describes
the basics of the most important types of solar thermal power plants.
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Solar Cell Technology and Applications
Most techniques for generating electricity from heat need high temperatures to achieve
reasonable efficiencies. The output temperatures of non-concentrating solar collectors are
limited to temperatures below 200°C. Therefore, concentrating systems must be used to
produce higher temperatures. Due to their high costs, lenses and burning glasses are not
usually used for large-scale power plants, and more cost-effective alternatives are used,
including reflecting concentrators.
The reflector, which concentrates the sunlight to a focal line or focal point, has a parabolic
shape; such a reflector must always be tracked. In general terms, a distinction can be
made between one-axis and two-axis tracking: one-axis tracking systems concentrate the
sunlight onto an absorber tube in the focal line, while two-axis tracking systems do so
onto a relatively small absorber surface near the focal point (see Figure 1).
FIGURE 1. Concentration of sunlight using (a) parabolic trough collector (b) linear
Fresnel collector (c) central receiver system with dish collector and (d) central
receiver system with distributed reflectors
The theoretical maximum concentration factor is 46,211. It is finite because the sun is not
really a point-shaped radiation source. The maximum concentration temperature that can
be achieved is equal to the sun’s surface temperature of 5500°C; if the concentration ratio
is lower, the maximum temperature decreases. However, real systems do not reach these
theoretical maxima. This is because, on the one hand, it is not possible to build an
absolutely exact system, and on the other, the technical systems which transport heat to
the user also reduce the receiver temperatures. If the heat transfer process stops, though,
the receiver can reach critical high temperatures.
Parabolic Trough Power Plants
Parabolic trough power plants are the only type of solar thermal power plant technology
with existing commercial operating systems. In capacity terms, 354 MWe of electrical
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Solar Cell Technology and Applications
power are installed in California, and some new plants are currently in the planning
process in other locations.
The parabolic trough collector consists of large curved mirrors, which concentrate the
sunlight by a factor of 80 or more to a focal line. Parallel collectors build up a 300�600
metre long collector row, and a multitude of parallel rows form the solar collector field.
The one-axis tracked collectors follow the sun.
The collector field can also be formed of very long rows of parallel Fresnel collectors. In
the focal line of these is a metal absorber tube, which is usually embedded into an
evacuated glass tube that reduces heat losses. A special high-temperature, resistive
selective coating additionally reduces radiation heat losses.
In the Californian systems, thermo oil flows through the absorber tube. This tube heats up
the oil to nearly 400°C, and a heat exchanger transfers the heat of the thermal oil to a
water steam cycle (also called Rankine cycle). A feedwater pump then puts the water
under pressure. Finally, an economizer, vaporizer and superheater together produce
superheated steam. This steam expands in a two-stage turbine; between the high-pressure
and low-pressure parts of this turbine is a reheater, which heats the steam again. The
turbine itself drives an electrical generator that converts the mechanical energy into
electrical energy; the condenser behind the turbine condenses the steam back to water,
which closes the cycle at the feedwater pump.
It is also possible to produce superheated steam directly using solar collectors. This makes
the thermo oil unnecessary, and also reduces costs because the relatively expensive
thermo oil and the heat exchangers are no longer needed. However, direct solar steam
generation is still in the prototype stage.
Guaranteed Capacity
In contrast to photovoltaic systems, solar thermal power plants can guarantee capacity
(see Figure 2). During periods of bad weather or during the night, a parallel, fossil fuel
burner can produce steam; this parallel burner can also be fired by climate-compatible
fuels such as biomass, or hydrogen produced by renewables. With thermal storage, the
solar thermal
power plant can
also generate
electricity even if
there is no solar
energy available.
FIGURE 2. Typical
output of a solar
thermal power
plant with twohour thermal
storage and
backup heater to
guarantee
capacity
A proven form of storage system operates with two tanks. The storage medium for hightemperature heat storage is molten salt. The excess heat of the solar collector field heats
up the molten salt, which is pumped from the cold to the hot tank. If the solar collector
field cannot produce enough heat to drive the turbine, the molten salt is pumped back
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Solar Cell Technology and Applications
from the hot to the cold tank, and heats up the heat transfer fluid. Figure 3 shows the
principle of the parabolic trough power plant with thermal storage.
FIGURE 3. Schematic of a concentrated solar thermal trough power plant with
thermal storage
Trough Power Plant Efficiencies
The efficiency of a solar thermal power plant is the product of the collector efficiency,
field efficiency and steam-cycle efficiency. The collector efficiency depends on the angle
of incidence of the sunlight and the temperature in the absorber tube, and can reach values
up to 75%. Field losses are usually below 10%. Altogether, solar thermal trough power
plants can reach annual efficiencies of about 15%; the steam-cycle efficiency of about
35% has the most significant influence. Central receiver systems such as solar thermal
tower plants can reach higher temperatures and therefore achieve higher efficiencies.
Solar Thermal Tower Power Plants
In solar thermal tower power plants, hundreds or even thousands of large two-axis tracked
mirrors are installed around a tower. These slightly curved mirrors are also called
heliostats; a computer calculates the ideal position for each of these, and a motor drive
moves them into the sun. The system must be very precise in order to ensure that sunlight
is really focused on the top of the tower. It is here that the absorber is located, and this is
heated up to temperatures of 1000°C or more. Hot air or molten salt then transports the
heat from the absorber to a steam generator; superheated water steam is produced there,
which drives a turbine and electrical generator, as described above for the parabolic
trough power plants. Only two types of solar tower concepts will be described here in
greater detail.
Open Volumetric Air Receiver Concept
The first type of solar tower is the open volumetric receiver concept (see Figure 4a). A
blower transports ambient air through the receiver, which is heated up by the reflected
sunlight. The receiver consists of wire mesh or ceramic or metallic materials in a
honeycomb structure, and air is drawn through this and heated up to temperatures between
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Solar Cell Technology and Applications
650°C and 850°C. On the front side, cold, incoming air cools down the receiver surface.
Therefore, the volumetric structure produces the highest temperatures inside the receiver
material, reducing the heat radiation losses on the receiver surface. Next, the air reaches
the heat boiler, where steam is produced. A duct burner and thermal storage can also
guarantee capacity with this type of solar thermal power plant.
Pressurized Air Receiver Concept
The volumetric pressurized receiver concept (see Figure 4b) offers totally new
opportunities for solar thermal tower plants. A compressor pressurizes air to about 15 bar;
a transparent glass dome covers the receiver and separates the absorber from the
environment. Inside the pressurized receiver, the air is heated to temperatures of up to
1100°C, and the hot air drives a gas turbine. This turbine is connected to the compressor
and a generator that produces electricity. The waste heat of the gas turbine goes to a heat
boiler and in addition to this drives a steam-cycle process. The combined gas and steam
turbine process
can reach
efficiencies of
over 50%,
whereas the
efficiency of a
simple steam
turbine cycle is
only 35%.
Therefore, solar
system
efficiencies of
over 20% are
possible.
FIGURE 4.
Schematic of
two types of
solar thermal
tower power
plant, showing
(a) an open
volumetric
receiver with
steam turbine
cycle and (b) a
pressurized
receiver with
combined gas
and steam
turbine cycle
Comparing Trough and Tower
In contrast to the parabolic trough power plants, no commercial tower power plant exists
at present. However, prototype systems - in Almería, Spain, in Barstow, California, US,
and in Rehovot, Israel - have proven the functionality of various tower power plant
concepts.
The minimum size of parabolic trough and solar tower power plants is in the range of 10
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Solar Cell Technology and Applications
MWe. Below this capacity, installation and O&M costs increase and the system efficiency
decreases so much that smaller systems cannot usually operate economically. In terms of
costs, the optimal system size is in the range of 50-200 MWe.
Dish-Stirling Systems
So-called Dish-Stirling systems can be used to generate electricity in the kilowatts range.
A parabolic concave mirror (the dish) concentrates sunlight; the two-axis tracked mirror
must follow the sun with a high degree of accuracy in order to achieve high efficiencies.
In the focus is a receiver that is heated up to 650°C. The absorbed heat drives a Stirling
motor, which converts the heat into motive energy and drives a generator to produce
electricity. If sufficient sunlight is not available, combustion heat from either fossil fuels
or biofuels can also drive the Stirling engine and generate electricity. The system
efficiency of Dish-Stirling systems can reach 20% or more. Some Dish-Stirling system
prototypes have been successfully tested in a number of countries. However, the
electricity generation costs of these systems are much higher than those for trough or
tower power plants, and only series production can achieve further significant cost
reductions for Dish -Stirling systems.
Dish-Stirling prototype systems in Spain
Solar Chimney Power Plants
All three technologies described above can only use direct normal irradiance. However,
another solar thermal power plant concept � the solar chimney power plant � converts
global irradiance into electricity. Since chimneys are often associated negatively with
exhaust gases, this concept is also known as the solar power tower plant, although it is
totally different from the tower concepts described above. A solar chimney power plant
has a high chimney (tower), with a height of up to 1000 metres, and this is surrounded by
a large collector roof, up to 130 metres in diameter, that consists of glass or resistive
plastic supported on a framework (see artist’s impression). Towards its centre, the roof
curves upwards to join the chimney, creating a funnel.
The sun heats up the ground and the air underneath the collector roof, and the heated air
follows the upward incline of the roof until it reaches the chimney. There, it flows at high
speed through the chimney and drives wind generators at its bottom. The ground under
the collector roof behaves as a storage medium, and can even heat up the air for a
significant time after sunset. The efficiency of the solar chimney power plant is below
2%, and depends mainly on the height of the tower, and so these power plants can only be
constructed on land that is very cheap or free. Such areas are usually situated in desert
regions.
However, the whole power plant is not without other uses, as the outer area under the
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Solar Cell Technology and Applications
collector roof can also be utilized as a greenhouse for agricultural purposes. As with
trough and tower plants, the minimum economical size of solar chimney power plants is
also in the multi-megawatt range.
The solar updraft tower is a proposed type of renewable-energy power plant. Air is
heated in a very large circular greenhouse-like structure, and the resulting convection
causes the air to rise and escape through a tall tower. The moving air drives turbines,
which produce electricity. A research prototype operated in Spain in the 1980s.
The generating ability of a solar updraft power plant depends primarily on two factors: the
size of the collector area and chimney height. With a larger collector area, more volume
of air is warmed up to flow up the chimney; collector areas as large as 7 km in diameter
have been considered. With a larger chimney height, the pressure difference increases the
stack effect; chimneys as tall as 1000 m have been considered. Further, a combined
increase of the collector area and the chimney height leads to massively larger
productivity of the power plant.
Heat can be stored inside the collector area greenhouse, to be used to warm the air later
on. Water, with its relatively high specific heat capacity, can be filled in tubes placed
under the collector increasing the energy storage as needed.
Turbines can be installed in a ring around the base of the tower, with a horizontal axis, as
planned for the Australian project and seen in the diagram above; or—as in the prototype
in Spain—a single vertical axis turbine can be installed inside the chimney.
Carbon dioxide is emitted only negligibly while operating, but is emitted more
significantly during manufacture of its construction materials, particularly cement. Net
energy payback is estimated to
be 2-3 years.
A solar updraft tower power
station would consume a
significant area of land if it
were designed to generate as
much electricity as is produced
by modern power stations using
conventional technology.
Construction would be most
likely in hot areas with large
amounts of very low-value
land, such as deserts, or
otherwise degraded land.
A small-scale solar updraft tower may be an attractive option for remote regions in
developing countries. The relatively low-tech approach could allow local resources and
labour to be used for its construction and maintenance.
The solar updraft tower does not convert all the incoming solar energy into electrical
energy. Many designs in the (high temperature) solar thermal group of collectors have
higher conversion rates. The low conversion rate of the Solar Tower is balanced to some
extent by the low investment cost per square metre of solar collection.
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Solar Cell Technology and Applications
According to model calculations, a simple updraft power plant with an output of 200 MW
would need a collector 7 kilometres in diameter (total area of about 38 km²) and a 1000metre-high chimney. One 200MW power station will provide enough electricity for
around 200,000 typical households and will abate over 900,000 tons of greenhouse
producing gases from entering the environment annually. The 38 km² collecting area is
expected to extract about 0.5 percent, or 5 W/m² of 1 kW/m², of the solar power that falls
upon it. Note that in comparison, concentrating thermal (CSP) or photovoltaic (CPV)
solar power plants have an efficiency ranging from 20-40%. Because no data is available
to test these models on a large-scale updraft tower there remains uncertainty about the
reliability of these calculations.
The performance of an updraft tower may be degraded by factors such as atmospheric
winds, by drag induced by bracings used for supporting the chimney, and by reflection off
the top of the greenhouse canopy.
Location is also a factor. A Solar updraft power plant located at high latitudes such as in
Canada, only if sloped towards the south, would produce up to 85 per cent of the output of
a similar plant located closer to the equator.
Artist�s impression of a 5 MW solar chimney power plant SCHLAICH BERGERMANN
SOLAR (SBS) GMBH, STUTTGART www.sbp.de
Electricity Generation Costs
Due to the poor part-load behaviour of solar thermal power, plants should be installed in
regions with a minimum of around 2000 full-load hours. This is the case in regions with a
direct normal irradiance of more than 2000 kWh/m2 or a global irradiance of more than
1800 kWh/m2. These irradiance values can be found in the earth’s sunbelt; however,
thermal storage can increase the number of full-load hours significantly.
The specific system costs are between €2000/kW and €5000/kW depending on the system
size, system concept and storage size. Hence, a 50 MWe solar thermal power plant will
cost €100-250 million. At very good sites, today’s solar thermal power plants can
generate electricity in the range of €0.15/kWh, and series production could soon bring
down these costs below €0.10/kWh.
The potentials for solar thermal power plants are enormous: for instance, about 1% of the
area of the Sahara desert covered with solar thermal power plants would theoretically be
sufficient to meet the entire global electricity demand. Therefore, solar thermal power
systems will hopefully play an important role in the world’s future electricity supply.
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