Solar Energy Perspectives - IEA

Chapter 7: Solar heat
inert materials with very low costs, or even negative costs in case of specific waste people
pay to get rid of (e.g. vitrified asbestos wastes) can be used as well. Heat exchanges may be
difficult with solid materials, since they occur only through conduction. Convection of liquid
media is much more effective.
Table 7.1 Characteristics of some possible storage media
Material Temp..range Density
Specific
heat
Specific
heat
Total heat
°C kg/m 3 J/kg/ °K J/m 3 /°K MJ/m 3
Water (1 atm) 0-100 1 000 4 190 4.19 419
Molten salts 142-540 1 680 1 560 2.62 1 043
Liquid sodium 100-760 750 1 260 0.96 520
Cast iron < 1 100 7 200 540 3.89 2 138
Aluminium < 650 2 700 920 2.48 1 366
Rock ... 2 600 890 2.31 1 271
Sources: Welle, 2010.
Key point
Sensible storage heat can use a variety of media.
Another option is that of latent heat, i.e. the heat that is absorbed by a solid that melts or
a liquid that boils, and the heat freed by a gas that condenses or a liquid that solidifies. In
practice, phase change materials (PCMs) used for thermal storage change only from solid to
liquid states and vice versa. When heated, PCMs first rise in temperature, like sensible storage
media. When the temperature reaches the melting point, PCMs absorb large amounts of heat
at constant temperature, until entirely liquid. When it releases its stored latent heat, the PCMs
solidify. Many PCMs are available in a large temperature range (from -5°C up to 190°C). They
store 5 to 14 times more heat per unit volume than sensible storage materials.
Thermo-chemical storage is an indirect way to store heat. The heat is not stored directly as
sensible or latent heat but by way of a physicochemical process, such as adsorption or
absorption, that consumes heat in charging mode and releases heat in discharging mode. The
sorption material can be a porous solid (e.g. silica gel, zeolite) or salt-hydrate solutions with
a high affinity for water (the sorption material releases water vapour when heated and
releases heat when water vapour is adsorbed or absorbed). Many compounds result in
products that can be stored over long periods without significant energy loss, making longterm
heat storage possible.
Thermo-chemical storage offers high energy density, reducing storage volumes. Many of
these systems act as heat pumps, making cooling as well as heating possible. However, they
are more complex, use expensive compounds, and require relatively high temperatures.
Thermo-chemical storage systems can be divided into open and closed systems. Open
systems, such as sorption processes for desiccant systems based on the adsorption, release
water/steam into the environment. Closed systems isolate the working fluid from the
atmosphere.
139
© OECD/IEA, 2011