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(c) RENOVETEC 2010. All right reserved

Types of Solar Thermal Power Plants


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  1. STIRLING DISH.
  2. SOLAR POWER TOWER TECHNOLOGY.
  3. PARABOLIC TROUGH TECHNOLOGY.
  4. FRESNEL POWER PLANTS.
  5. ISCCS ( Integrated Solar Combined Cycles System).
  6. 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.

picture of solar thermal power plant dish stirling

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.

picture of solar thermal power plant tower

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.

molten salt storage

Figure 6. Functional diagram of molten salts storage.

 

molten salt deposits

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.


http://www.novatec-biosol.com/files/090217_schaubild_1.jpg
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.

SOLAR COMBINED CYCLE

 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. 

SOLAR POWER TOWER

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 SOLAR POWER PLANTS

Table 1. Installation power  and occupied area comparison.





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