|
Español
- STIRLING DISH.
- SOLAR POWER TOWER TECHNOLOGY.
- PARABOLIC TROUGH TECHNOLOGY.
- FRESNEL POWER PLANTS.
- ISCCS
( Integrated Solar Combined Cycles System).
- SOLAR THERMAL POWER PLANTS INSTALLATION. BASIC REQUIREMENTS.
INTRODUCTION.
Solar
thermal power plants are based on a reflective surface mirror that
concentrates solar radiation onto a a smaller surface (receiver), so
temperature of concentrated surface increases noticeably. For this
reason the term CSP (Concentrating Solar Power) is used
to refer to them. Two concentration systems exit: Concentration onto a
point and concentration onto a line.
The first ones which focused onto a point, there are several
technologies. It is available in these technologies very important
concentration ratios ( more than 1000 times). It means that incident
radiation onto a large surface is concentrated onto another one
thousand times smaller. Currently, the two concentration onto a point
technologies are: Dish Stirling, it consists of a parabolic solar dish
concentrating the sun's rays on the heating element of a Stirling
engine. And Solar Power Tower, also known as ' central
tower' power plant or ' heliostat' power plant)captures and focuses the
sun's thermal energy with thousands of tracking mirrors (called
heliostat) which focus concentrated sunlight on a receiver which sits
on top of the concrete tower.
The second ones which focused onto a line, there are
two principal available technologies: Fresnel lenses system, it
consists of a series of mirrors to focus light onto one or more linear
receivers positioned above the mirrors with water flowing inside them.
And parabolic cylinder system, currently it is the most popular and whose technology is considered
the most mature. These last technologies are called PCC power plants (parabolic cylinder concentrator).
Besides these ones, there are different kinds of hybridization .In
these power plants coexist different technologies in order to keep the
plant in operation when no solar radiation happens. The most common are
called ISCC (Integrated Solar Combined Cycle) that integrate a standard
combined cycle with a CPC solar field. Other kind of hybridization
consist of biomass power plants based on burning solid combustible such
as vegetable, forest or agriculture wastes to keep the power plant
working when solar radiation is gone.
Let's deepen in each one of these technologies:
1. STIRLING DISH.
In this case the Stirling dish concentrator system consists of a solar concentrator in a
dish structure that supports an array of curved glass mirrors. The parabolic
dish tracks the sun throughout the day and concentrates the radiation onto the
heat absorption unit of a Stirling engine that is
coupled to an alternator. The focused solar thermal energy is
then converted to grid-quality electricity. The conversion process involves a
closed cycle, high-efficiency solar Stirling engine using an internal working
fluid that is recycled through the engine. The
working fluid is heated to a temperature around 750ºC and pressurized by the solar receiver, which in turn
powers the Stirling engine. For optimum
operation, the system should be provided by mechanisms needed in order to be
able to carry out tracking of the position of the sun in two axes.
Figure 1. Stirling Dish Concentrator.
2. CENTRAL TOWER TECHNOLOGY.
Central Tower technology is positioned as a medium maturity solar thermal technology.
Power towers capture
and focus the sun's thermal energy with thousands of tracking mirrors (called
heliostats). A tower resides in the centre of
the heliostat field. The heliostats focus concentrated sunlight increasing
up to 600 times on a receiver
which sits on top of the tower. Heat is transferred to a fluid which is pumped to a steam generator. The steam drives a
standard turbine to generate electricity.
Figure 2. Solar Power Tower operating diagram.
The tower technology operation is based on three characteristic elements:
the heliostats, the receiver and the tower.
1) Heliostats perform the function of focusing concentrated sunlight on a receiver which sits on top of the tower. Heliostats consist of
a
reflective surface, a structure that serves as a support, and
sun-tracking mechanisms to follow sun's movement. Currently, glass
mirrors are the most reflective surfaces used.
2) Receiver,
within it the concentrated sunlight heats a fluid such as water, molten
salts, etc. The heated fluid is responsible for transferring the heat
to the rest of the power plant. Then, it flows into a thermal storage
tank where it is stored, and eventually pumped to a steam generator.
The steam drives a standard turbine to generate electricity.
3) Tower serves
from support to the receiver, it should be placed some distance above the level of the
heliostats in order to avoid or at least reduce the shadows and blockades.
Figure 3. View of a Solar Tower System and its heliostats field.
In
the continued search to
improve the efficiency, it has been advanced mostly in two fronts: to
reach higher temperatures and hybridize and to improve the
storage.
1. High temperatures to improve the efficiency. High temperatures (above 1000º C) that can be reached with this technology
allow aspiring to high efficiency in electricity generation, even up to 25%
in the solar radiation transformation to electricity.
2. Storage is used in solar power tower systems. Heat storage allows a solar thermal plant to produce electricity at night and
on overcast days. Currently, the most used solution is to transfer the heat to a thermal storage medium in an insulated reservoir
during the day, and withdrawn for power generation at night. Thermal storage
media include pressurized steam, concrete, a variety of phase change materials,
and molten salts. It accumulates the energy to be distributed in another moment. That is
why the plant is be over measured.
3. Hybridization
is another improvement used in
the tower technology. It is based on using other energy sources, such
as biomass, to keep the plant working even with the lack of radiation.
Both of the systems, storage and hybridization look for an improvement
in the number of operation hours. Normally, it does not exceed 2.500 (a
year has 8760 hours).
3. PARABOLIC TROUGH TECHNOLOGY.
The
parabolic trough is a clean
technology, mature and with an extensive history that proves to be
ready for
the large-scale installation. This technology is being installed to
commercial
level from 80s with an exceptional behaviour. Since those days, it has
improved on costs and efficiency. Currently, there are more than 800
MWs
in operation, more than 2.000 MW under construction and about 6 GWs in
development worldwide in countries like Spain (the main solar
technology driving force) United States, Morocco, Algeria, Egypt,
Australia, South Africa, India, Mexico and Chile.
Parabolic
trough technology uses a curved, mirrored trough which reflects the
direct solar radiation onto a glass tube containing a fluid (also
called a receiver, absorber or collector) running the length of the
trough, positioned at the focal point of the reflectors. The trough is
parabolic along one axis and linear in the orthogonal axis. For change
of the daily position of the sun perpendicular to the receiver, the
trough tilts east to west so that the direct radiation remains focused
on the receiver. Inside it, heat transfer fluid (HTF), usually
synthetic organic fluid, runs through the tube to absorb the
concentrated sunlight. This increases the temperature of the fluid to
some 400°C. The heat transfer fluid
is then used to heat steam in a standard turbine generator. Parabolic
trough is the most developed technology among all types of solar thermal
power plants.
Figure 4. Operating diagram of a parabolic trough technology.
The
main components of parabolic trough technology are:
1) Parabolic trough reflector reflects and concentrates the direct solar radiation onto an absorbent tube. The mirrored surface is constructed as a long parabolic mirror, usually coated silver or aluminium. It is deposited on a support which gives sufficient
rigidity.
2) Absorbent tube consists
of two concentric tubes separated by a vacuum layer. The inside, where
the heat transfer fluid circulates, is made of metal and the outside is
made of glass. Heat transfer fluid that passes trough the receiver is
different depends on the technology: At low temperatures (< 200 ºC) demineralised water is often used with ethylene glycol. While for higher temperatures (200º C < T < 450 º C) common fluids are synthetic oil. The
latest technologies allow steam direct generation under high pressure
to the tubes and the use of salt as heat transfer fluid.
3) Solar tracking system.
The most common system is the parabolic trough along one axis that
changes of the daily position of the sun perpendicular to the receiver.
4) Metal structure serves
from support to the collector and it is which gives rigidity to all the components.
Figure 5. Parabolic Trough Collector.
Parabolic trough collector technology can incorporate methods for energy storage in order to be
able to produce electricity at night and
on overcast days, the most common one is
the storage with a mixture of silica sand (salts). This technology is based on using two tanks of
salts to store heat.
1)
During the full-load cycle, salts exchange heat with the fluid from the solar field
and stored in the tank hot.
2)
During the unloading cycle, the system simply operates on the contrary
to before. Salts heat the fluid to produce steam. Finally, this steam
is driven to the turbine to
produce electricity.
Figure 6. Functional diagram of molten salts storage.
Figure 7. Molten salts deposits.
4. FRESNEL POWER PLANTS.
One
of the new solar thermal energy exploitation systems are linear Fresnel
reflector power plants. It uses simple technology apart from minimizing
structural costs. This technology consist of a series of long, narrow,
shallow-curvature mirrors to focus light onto one or more linear
receivers positioned above the mirrors. Minimized structural costs are
attributed to the use of curved glass reflectors. The shape of the
parabolic trough technology makes them 15% more efficient than Fresnel
mirrors, but features that enhance the cost effectiveness of this
system compared to that of the parabolic trough technology include
minimized structural costs.
Figure 8. Fresnel Power Plant.
5. ISCCS (Integrated Solar Combined Cycle System).
ISCC technology combines all the benefits of the solar energy with a
combine cycle benefits. The solar resource partially replaces the use of fossil
fuel and in this way also reduces the emissions. The solar field is based on
cylinder-parabolic technology.
4.1 Convencional combined cycle.
A
conventional combined cycle plant consists of a gas turbine, a heat
exchanger
and a steam turbine. In the case to a solar hybrid plant ISCC, solar
power
is used as auxiliary energy that it will increase the cycle efficiency
and also it will decrease emissions. It means that the power plant
produces most of its energy in combined cycle, and the solar
field contributes between 2 and 5 % additional energy.
4.2 Solar combined cycle.
The operation of a hybrid solar combined cycle
plant is similar to a conventional combined cycle. Fuel is normally burnt in
the combustion gas turbine camera. The exhaust gases are directed to the heat
exchanger. Heat is added from the solar system. Steam generation capacity is
increased and consequently an increased production of electricity in steam
turbine.
Figure 9.Function
diagram plant ISCC.
6. BASIC REQUIREMENTS FOR THE INSTALLATION OF A SOLAR
THERMAL POWER PLANT.
For
the installation of solar thermal power plants, it should be followed the requirements:
1)
The climate. The economic viability of a solar power project depends directly
on the values of direct solar radiation that are anually registered in the area
concerned, so normally this kind of plants are installed in warm and sunny areas.
2)
The orography. A flat surface facilitates the design and construction
of the solar field, and also shadows are avoided.
3)
Availability of water.
4)
Availability of electrical connection to the network.
Figure 10. Solar Power Tower Installation.
The
construction of a solar thermal plant it is needed a large area to
install all the mirrors and to avoid the shadows. As shown in table 1,
the guidling areas for different kinds of plants, depend on the power
and configuration.(Radiation conditions around 2120kWh/m2 year).
Table 1. Installation power and occupied area comparison.
|
