Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar thermal electricity
the Carnot theorem. The ideal Carnot efficiency is defined by the ratio of the difference in
temperatures of the hot and the cold source, divided by the absolute temperature (in
Kelvin) of the hot source. Receiver efficiencies, Carnot efficiencies of the conversion into
electricity, and total solar to electric efficiencies, are shown on Figure 8.1 in the function
of the working temperature for various concentration ratios or “suns”. On the left diagram,
ratios of 40 to 100 suns are representative of linear concentration systems. On the right
diagram, ratios of 100 to 2 000 suns are representative of point-focus systems. The
efficiency of the receiver depends on the state of the technology, while the Carnot
efficiency represents a physical law and expresses the maximum possible efficiency of the
conversion. The global efficiency is the product of the efficiency of the collector by the
Carnot efficiency and a fixed coefficient, set at 0.7, expressing the imperfection of the
thermodynamic engine.
Photo 8.1 Shuman’s concentrating solar thermal plant in Maadi, Egypt, 1913
Source: Ruiz Hernandez, 2010.
Key point
Parabolic troughs have been around for more than a century.
As Figure 8.1 shows, point-focus systems convert into electricity a larger fraction of the
energy that falls on the receiver than linear systems. For each concentration level, there is
an optimal temperature level – although this may change as receiver technology develops.
The economic optimum might be significantly different from the efficiency-maximising
value. In the case of concentration of 1 000 suns or above, moving from a temperature of
about 800°C to about 1 000°C does not bring a considerable improvement in conversion
efficiency but is likely to entail significantly higher costs for manufacturing the receivers and
the thermodynamic engine.
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