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- STIRLING DISH.
- TOWER TECHNOLOGY.
- PARABOLIC TROUGH TECHNOLOGY.
- ISCC
(Integration Solar Combined Cycles).
- BASIC REQUIREMENT FOR THE INSTALLATION OF SOLAR THERMAL POWER PLANT.
INTRODUCTION
Currently
there are three main types of Solar Thermal Power Plants: the dish
stirling, the tower and parabolic trough collector, from these three
types are being
developed production models such as combining Solar Thermal Power
Plants with Combined Cycle, to improve yields.
1. STIRLING DISH.
A
Stirling dish concentrator system comprises a un highly reflective
solar concentrator for a solar receiver cavity, and
a Stirling engine or a microturbine that is coupled to an
alternator. The operation consists of heating a fluid
located in the receiver to a temperature around 750º C. This
energy is used for power generation by the engine or microturbine. For
optimun 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. Disc Collector Stirling.
2. TOWER TECHNOLOGY.
The tower technology is positioned as a solar thermal technology with a maturity average.
In
tower systems, a field of heliostats or mirrors that focus moving to
the position of the sun, reflecting solar radiation, increasing up to
600 times on a receptor that sits on top of a tower. This heat is
transmitted to a fluid in order to generate steam that expands in a
turbine connected to a generator to produce electricity.
Figure 2. Operating diagram of the tower technology.
The operation of the tower technology is based on
three characteristic elements: the heliostats, the receiver and the tower.
1) The heliostats
are meant to capture solar radiation and direct it to the receiver.
They are composed of a reflective surface, a structure that serves
as a support, and
mechanisms to guide you to go follow the movement of the sun. The more
reflective surfaces employed currently are of mirrors of glass.
2) The receiver,
wich transfers the received heat a working fluid, wich can be water,
molten salts, etc. This
fluid is responsible for transmitting the heat to another part of the
solar thermal power plant, generally a water tank, obtaining steam to
high temperature for production of electricity by moving a
turbine.
3) The tower
serves from support to the receiver, to 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 tower and its heliostats field.
2.1 Improvements in the tower technology.
In
the continued search to obtain better yields has bee moved forward
mostly in two fronts, to achieve great temperatures and hybridize and
to improve the storage.
1. High temperatures,
good yields, the high temperatures (above 1000º C) that can be achieved
with this technology allow aspiring to high yields in the generation of
electricity, even above 25% in the transformation of solar radiation to
electricity.
2. Hybridization and storage,
in the tower technology can be incorporated the energy storage. From
this storage the system can provide power even in cloudy conditions or
at night. Currently the most utilised solution is the use of a storage
tank of water/steam or molten salt that accumulates the energy to be
distributed in another moment, wich is why the plant should be over
measured. Another application used in the tower technology is the
hybridization.
3. PARABOLIC TROUGH TECHNOLOGY.
The
parabolic trough technology is a clean technology, mature and with an
extensive history that proves to be ready for the large-scale facility.
This technology is being installed to commercial level from 80s with an
exceptional behaviour. It has since undergone major improvements at the
level of costs and yields. Currently there are 300 MWs in
operation, 400 in construction and about de 6 GWs in promotion to world
level.
The
parabolic trough technology bases its operation on solar tracking and
concentration of sunlight on tubes of high thermal efficiency
receptors located in the focal line of the cylinders. In this tubes, a
fluid trasmitter of heat such as a synthetic oil is heated
approximately to 400 ºC for the concentrated solar rays.
This oil is pumped through a series of heat exchangers to produce
superheat steam. The heat in the steam, is converted into electrical
energy in a conventional steam turbine. The parabolic trough technology
is the most developed CSP technology.
Figure 4. Operating diagram of the parabolic trough technology.
The main components of the solar field of parabolic trough technology are:
1) The parabolic cylinder reflector:
The mission of the receiver cylinder parabolic is to reflect and to
concentrate on the absorbent tube the direct solar radiation that it
incident on the area. The mirror surface is achieved through films of
silver or aluminium deposited on a support wich gives sufficient
rigidity. Currently the most widely use means of support are the sheet
metal, glass and plastic.
2) The absorber tube:
The absorber tube consists of two concentric tubes separated by a
vacuum layer. The inside, by wich circulates the heated fluid is
metallic and the outside is glass.The working fluid flowing through the
inner tube is different depending on the technology. At low
temperatures (< 200 ºC) demineralized water is often used with
ethylene glycol while for higher temperatures (200º
C < T < 450 º C) used synthetic oil. The latest technologies
allow direct generation of steam under high pressure to the tubes and
the use of salt as heat transfer fluid.
3) The tracking system of
the sun: The most common track system is a device that rotates the
cylinder parabolic reflectors of the collector abous an axis.
4) The metal stucture: The mission of the collector is to give rigidity to the assembly of component parts.
Figure 5. Parabolic trough collector.
3.1 Storage.
The
technology parabolic trough collectors can incorporate storage in order
to be able to produce electricity in hours of darkness, the most
widespread one is the storage with salts. This technology is based on
the use of two tanks of salts to store heat.
1) During the charge cycle, salts excange heat with the fluid from the solar field and stored in the tank hot.
2)
During the discharge cycle, the system simply operates in reverse to
the above, heating the heat transfer fluid to generate steam to drive
the turbine to produce electricity finally.
Figure 6. Functional diagram of molten salts storage.
Figure 7. Molten salts deposits.
4. ISCC (Integration Solar in Combined Cycles).
ISCC
technology combines all the benefits of solar energy with the benefits
of a combined cycle. The solar resource substitutes partially the use
of fossil fuel with the saving of issuances that it supposes. The solar
field is constructed from parabolic trough technology.
4.1 The conventional combined cycle.
A
conventional combined cycle plant is formed by a gas turbine, a heat
exchanger and a steam turbine. In the case to a solar hybrid plant
ISCC, the solar power is used as auxiliary energy that it will allow
increasing the cycle efficiency, and to decrease emissions.
4.2 The solar combined cycle.
The
operation of a plant hybrid-solar combined cycle is similar to that of
a conventional combined cycle plant. The fuel is burnt usually in the
chamber of combustion of the gas turbine. The exhaust gases are
directed to heat recovery are added heat from the solar field,
resulting in an increase in steam generation capacity and consequently
an increase in electricity production in the steam turbine.
Figure 8. Function diagram plant
ISCC
5. BASIC REQUIREMENTS FOR THE INSTALLATION OF A SOLAR THERMAL POWER PLANT.
For the installation of solar thermal technology plants, there are certain requeriments to function properly:
1)
The climate. The economic viability of a solar power project depends
directly on the values of direct solar radiation that are registered
anually in the area concerned to implement, so normally this type of
plants are installed in areas warm and sunny.
2)
The orography. A flat surface facilitates the work of design and
construction of the solar field, and shadows are avoided that could
lead to rolling terrain.
3) Availability of water.
4) Availability of electrical connection to the network.
5.1 Minimum surface for the construction of different types of solar thermal power plants.
Figure 9. Installation solar thermal of tower.
For
the construction of a solar thermal power plant requires a large amount
of surface area to install all the mirrors and avoid shadows, in
table 1 shows the indicatives surfaces for different power and
configuration for radiation conditions around a 2120
kWh/m 2 year.
Table 1. Comparison of power of the facility and occupied surface.
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