Thermal solar power systems use solar radiation to produce heat in place of tradi­tional fossil fuels. To get heat at a temperature higher than 250°C, it is necessary to concentrate the solar radiation. To achieve this concentration, an appropriate opti­cal system (the concentrator) is used. This device gathers and delivers direct solar radiation to another device (the receiver) where it is transformed into high tem­perature heat [50]. The heat produced in this way can then be applied to different industrial processes (such as the desalination of seawater and production of hydrogen using thermal chemical processes) or to electrical energy production.

Currently, electrical energy production is the main purpose for which CSP sys­tems are used. In this case, solar heat is used in traditional thermodynamic cycles such as the Rankine, Stirling and Brayton cycles. Until now, systems that are able to convert about 30% of the solar radiation received on the Earth’s soil into electri­cal energy have been used. The range of power obtainable varies from 10 kW to a few hundreds of megawatts, including more than one modular system [50, 51].

As regards the concentration, solar systems can apply to different technologies; however, it is possible to point out the following processes in every one of these technologies [50]:

• gathering and concentration of solar radiation;

• conversion of solar radiation into thermal energy;

• transport and possible storing of thermal energy;

• thermal energy utilization.

The gathering and concentration of radiation, whose power density is very low by nature, is one of the principal problems of solar systems. As already stated,

this process occurs thanks to a concentrator which is composed of appropriately shaped panels with reflecting surfaces. During the day, the concentrator follows the Sun so as to gather the direct component of its radiation and concentrates it inside the receiver. This latter device transforms solar energy into thermal energy, which is then given to a fluid that flows inside (thermal vector fluid). As we will observe from the analysis of tower systems with a central receiver (par. 4.4.2), the use of a melted salts mixture as the thermal vector fluid lets the system have a system of thermal energy accumulation before its utilization in the production process. This storing is realized by collecting the thermal vector fluid that goes out from the receiver in appropriately insulated storage tanks. In this way, solar energy, which is very variable in nature, can become a thermal energy source that is always available to users whenever required [45, 50].

On the basis of geometry and the concentrator’s position, we can have three kinds of CSP systems [12, 45, 50-53]:

• linear parabolic collector systems;

• tower systems with a central receiver;

• parabolic dish collector systems.