What is a Thermosolar Plant?

It is well-known the existing problem derived from generation of electricity, its related environmental impact and sustainability of different technologies. The demand for electric power is continuously increasing due to several demographic subjects such as, for instance, the increase of world population, the rising industrialization or the incorporation into the consumer market by developing countries. In order to generate enough electric power that is necessary to satisfy the demand, more and more technologies are incorporated into the energy mix. Renewable energies have emerged with particular strength and they combine solutions for the two most important concerns, electric power generation but with a minimum harmful environmental impact since it is significant the reduction of many tons of CO₂ emitted into the environment as well as other pollutant compounds.

Focusing on renewable energies, we find an especially interesting way to generate electric power, solar thermal plants. This technology cannot be considered as mature as other ones and therefore it is very changing regarding its technical developments and implementations. Solar radiation is the energy source for solar thermal plants and it is concentrated by means of different type of components, such as heliostats or collectors. Solar beams are concentrated in a receiver through which a fluid named heat transfer fluid flows. Later heat transfer fluid is conducted up to a power block where thermal energy is converted into electrical one. Basically a solar thermal plant for power generation is as indicated above. Nevertheless its practical implementation involves some diversity. Solutions most used currently in industry are indicated below.

Considering the difference between several and most important systems of concentration employed, this lies in a concept named as concentration ratio which is defined as the rate between concentration area and radiant solar energy absorber area. Below the most important concentration systems in industrial plants are explained.

• Parabolic-trough collectors. The power generation plant has a field with parabolic collectors whose target is to concentrate solar beams towards a receiver tube located in the collectors focus. The heat transfer fluid flows through the receiver tube. Usually this heat transfer fluid is a synthetic oil and will be heated up to 400ºC. The parabolic-trough collectors have a system that allows them to reorient in function of solar beams incident direction. This device maximizes power generation of the plant. Thermal energy from heat transfer fluid is used, by means of a heat exchange system, to generate steam that will be expanded in the power cycle.

• Tower plant. In this type of plants flat mirrors named heliostats are used. Besides, there is a tower where the receiver is installed. These heliostats work concentrating solar radiation in the receiver which causes considerably higher temperatures than in previous technology. As in parabolic-trough collectors, and for the same reason, heliostats have a system that allows them to reorient according to the incident direction of the solar beams. The receiver installed in the tower could be developed using different types in function of peculiarities of the system and the fluid flowing through it. As indicated in previous technology, the fluid that works inside the receiver is used, by means of a steam generation system, to heat water that will be expanded in the turbine for generating electrical power.

Many solar thermal plants currently in service have a thermal storage system which involves really important benefits such as the improvement of the flexibility of the power plant what allows electric power generation to satisfy the demand. This target is achieved because of power plants with thermal storage system are able to discharge energy into the network during times of the day in the absence of solar radiation. Solar thermal plants that include thermal storage allow to absorb more energy from the solar beams that is necessary to operate the plant at nominal power. The excess of energy is stored for further use during low or no solar radiation times, mainly at night or adverse weather.

Nowadays, the most commonly implemented technique for storage of thermal energy in solar thermal plants is by means of molten salts. These salts are in liquid phase inside the system at all times. Molten salts flow through the system and, during times with low or no solar radiation, provide heat to the system by means of a heat exchange device (heat exchangers, mainly) which is used for heating water in a previous stage to the power cycle. When an excess of energy power capture occurs, molten salts perform the reverse route through the system, absorbing thermal energy in order to storage it and to use it to generate electric power later.

Molten salts storage tanks deserve a special mention due to their importance and criticity. In general, those tanks are used to storage molten salts between their trips to, and from, heat exchangers in order to exchange thermal power with heat transfer fluid.