Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar heat
Photo 7.4 Mid-size industrial solar oven at Mont-Louis (French Pyrenees)
Source: Four Solaire Développement.
Key point
Mid-size solar ovens can achieve temperatures of several hundreds °C and be used by small industries.
Such ovens have two stages of reflection: the first, using one or many heliostats 1 , tracks the
sun; the second, using a very large parabola, concentrates its rays on a fixed target. Industrial
solar ovens have very high concentration levels, which can reach high temperatures with
significant power. (Figure 7.6)
The world’s largest solar ovens are in Odeillo (French Pyrenees) and in Parkent (Uzbekistan).
Due to excellent solar irradiance, the one in Odeillo is more powerful with 1 MW th
(Photo 7.5). With a concentration ratio of 10 000 “suns”, it brings the target to temperatures
up to 3 500°C – with no combustion residues. Industrial solar ovens are mostly used for
scientific and technical experiments, such as testing the resistance of new materials.
Why concentrate the sunlight
Discussion of solar ovens brought us progressively into the area of concentration. Before
going farther, it is useful to consider how concentrating solar rays allows collecting solar
energy at higher working temperatures. This higher temperature offers a better efficiency in
the conversion of heat to electricity, in case of a power plant (see Chapter 8), or may be
needed to run industrial processes (Chapter 5) or chemical reactions in manufacturing solar
fuels (Chapter 9).
However, reaching high temperatures per se is not enough – it must be done with good
efficiency at the collector level. If the heat is not removed, the temperature will increase to
the point where the thermal losses equal the solar inputs, called the stagnation temperature,
and no useful energy is made available. Indeed, the efficiency in collecting heat decreases
1. A heliostat is a device with a mirror that turns so as to keep reflecting sunlight toward a predetermined target, compensating for
the sun's apparent motion in the sky.
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© OECD/IEA, 2011