Solar Energy Perspectives - IEA

Chapter 8: Solar thermal electricity
Concentrating solar power plants
Solar thermal power plants are based on the technologies examined in Chapter 7 to capture
the sun’s energy as heat, notably troughs and towers, while Fresnel reflectors and dishes
appear outsiders.
Figure 8.1 Efficiencies as a function of temperature for various concentration ratios
Linear systems
Point-focus systems
η
η
1 1
0.9
α c = 100
α c = 80
0.9
0.8 η
α c = 60
capt α
0.8
c = 40
η
α capt
0.7 0.7
c = 2 000
α
η c = 1 000
0.6 Carnot 0.6 α c = 500
α c = 100
0.5 0.5
η Carnot
0.4 0.4
0.3 0.3
0.2 0.2
η
0.1 global
0.1
η global
0 0
20°C
166°C
312°C
460°C
605°C
752°C
900°C
θ x
20°C
166°C
312°C
460°C
605°C
752°C
900°C
θ x
Notes: α c = concentration ratio; θ capt = the efficiency of the collector; θ Carnot = the efficiency of the conversion of heat into electricity;
θ global = the global efficiency. Values are indicated for an ambient temperature of 20°C.
Source: Tardieu Alaphilippe, 2007.
Key point
There is an optimal working temperature for any given concentration ratio.
Parabolic troughs and linear Fresnel reflectors
All parabolic trough plants currently in commercial operation rely on a synthetic oil as heattransfer
fluid (HTF) from collector pipes to heat exchangers, where water is preheated,
evaporated and then superheated. The superheated steam runs a turbine, which drives
a generator to produce electricity. After being cooled and condensed, the water returns to the
heat exchangers. Parabolic troughs are the most mature of the CSP technologies and form the
bulk of current commercial plants. Investments and operating costs have been dramatically
reduced, and performance improved, since the first plants were built in the 1980s. For
example, special trucks have been developed to facilitate the regular cleaning of the mirrors,
which is necessary to keep performance high, using car-wash technology to save water
(Photo 8.2).
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© OECD/IEA, 2011