Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar heat
into the liquid salt over a distance of several meters. The system absorbs light and stores
energy in the liquid salt, from which it is extracted and turned into electricity at will – thanks
to thermal storage (Forsberg, 2010).
Photo 7.8 Experimental beam-down solar tower in Abu Dhabi
Source: Yutaka Tamaura, Tokyo Institute of Technology.
Key point
Beam-down design avoids putting heavy and complex receivers on top of towers.
Storing the sun’s heat
Thermal storage is a critical component in achieving high penetration levels with solar energy
technologies. It needs to compensate for the variability of the solar resource and increase the
capacity factors or value of the solar systems. It has applications in all solar applications:
solar water heating, solar heating and cooling, solar process heat, solar thermal electricity,
and even the manufacturing of solar fuels (which constitute a specific sort of transportable
stored solar energy).
There are several ways to store the heat collected from the sun. One, “sensible heat,” works
by modifying the temperature of some medium. Another is “latent heat,” in which the phase
of some medium is changed – from solid to liquid, or from liquid to gaseous states – when
the heat is being stored and in the opposite direction when the heat is extracted. The storage
medium can be the final desired product, such as hot water for sanitary or other purposes, or
a specific medium introduced in a solar system for storage purpose. Table 7.1 shows the
relevant characteristics of various media for sensible heat.
Water has a high calorific value, as shown by its specific heat value in J/m 3 /°K. But above
100°C, it needs to be pressurised which, depending on the pressure, may significantly
complicate the storage system. Despite lower specific value and thanks to higher temperature
ranges, molten salts currently are the preferred option in generation of electricity. Two-tank
(hot and coled) storage systems are standard, as on Figure 8.2, while development on singletank
systems, as on Figure 9.2, are underway, with potential cost reductions. Stones and other
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© OECD/IEA, 2011