Recycling Initiative: Solar thermal power plants

The following article was initially published in Renewable Energy World 06/2003 pp. 109-113.

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Most techniques for generating electricity from heat need high temperatures to achieve reasonable efficiencies. The output temperatures of non-concentrating solar collectors are limited to temperatures below 200°C. Therefore, concentrating systems must be used to produce higher temperatures. Due to their high costs, lenses and burning glasses are not usually used for large-scale power plants, and more cost-effective alternatives are used, including reflecting concentrators.

The reflector, which concentrates the sunlight to a focal line or focal point, has a parabolic shape; such a reflector must always be tracked. In general terms, a distinction can be made between one-axis and two-axis tracking: one-axis tracking systems concentrate the sunlight onto an absorber tube in the focal line, while two-axis tracking systems do so onto a relatively small absorber surface near the focal point (see Figure 1).



FIGURE 1. Concentration of sunlight using (a) parabolic trough collector (b) linear Fresnel collector (c) central receiver system with dish collector and (d) central receiver system with distributed reflectors

The theoretical maximum concentration factor is 46,211. It is finite because the sun is not really a point-shaped radiation source. The maximum concentration temperature that can be achieved is equal to the sun’s surface temperature of 5500°C; if the concentration ratio is lower, the maximum temperature decreases. However, real systems do not reach these theoretical maxima. This is because, on the one hand, it is not possible to build an absolutely exact system, and on the other, the technical systems which transport heat to the user also reduce the receiver temperatures. If the heat transfer process stops, though, the receiver can reach critical high temperatures.