Solar thermal power systems
Conversion of solar to mechanical and electrical energy has been the objective of experiments for more than a century, starting from 1872 when Mouchot exhibited a steam-powered printing press at the Paris Exposition. The idea is to use concentrating collectors to produce and supply steam to heat engines. A historical review of this and other experiments is given in Section 1. Much of the early attention to solar thermal-mechanical systems was for small scale applications (up to 100 kW) and most of them were designed for water pumping. Since 1975 there have been several large-scale power systems constructed and operated. Commercial plants of 30 and 80 MW electric (peak) generating capacity are nowadays in operation for more than a decade.
The process of conversion of solar to mechanical and electrical energy by thermal means is fundamentally similar to the traditional thermal processes. These systems differ from the ones considered so far as these operate at much higher temperatures.
This section is concerned with generation of mechanical and electrical energy from solar energy by processes based mainly on concentrating collectors and heat engines. There are also another three kinds of power systems, which are not covered in this paper. These are the photovoltaic cells for the direct conversion of solar to electrical energy by solid state devices, solar-biological processes that produce fuels for operation of conventional engines or power plants and solar ponds.
The basic process for conversion of solar to mechanical energy is shown schematically in Fig. 42. Energy is collected by concentrating collectors, stored (if appropriate), and used to operate a heat engine. The main problem of these systems is that the efficiency of the collector is reduced as its operating temperature increases, whereas the efficiency of the heat engine increases as its operating
temperature increases. The maximum operating temperature of stationary collectors is low relative to desirable input temperatures of heat engines, therefore concentrating collectors are used exclusively for such applications.
Identifying the best available sites for the erection of solar thermal power plants is a basic issue of project development. Recently the planning tool STEPS was developed by the German Aerospace Centre (DLR) , which uses satellite and Geographic Information System (GIS) data in order to select a suitable site. The factors taken into account are the slope of the terrain, land use (forest, desert, etc.), geomorphological features, hydrographical features, the proximity to infrastructure (power lines, roads, etc.) and of course solar irradiation of the area.
Three system architectures have been used for such applications, the PTC system, the power tower system, and the dish system. These are described in this section.