Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 141

hypocausts can only be charged in a regulated way, whereas the heat discharge is
not regulated.
As the heat capacity of rock is significantly lower than the capacity of liquids,
two to three times larger volumes are required for the same storage capacity. Ad-
ditionally, the inlet and outlet of heat, at low temperature differences requires
large heat transfer areas evenly distributed within the storage. This heat transfer is
not required in fills with a direct heat carrier flow and the hypocausts. The disad-
vantage of the need for more space is set off by the advantage of easier produc-
tion, as the rock storage is run without pressure. Furthermore there are only a few
requirements in terms of tightness; it can also be run at very high temperatures.
Latent heat store. Changing the state of aggregate (phase change) of a material is
performing at a constant temperature by charging and discharging energy. During
the melting or evaporation processes, heat has to be added; correspondingly, heat
is released during the solidification or condensation processes. Melting and solidi-
fication temperature, and evaporation and condensation temperature are the same
in that case. The heat accumulated or released by the material during this phase
change is called latent heat or heat of fusion. If the phase change takes place at
higher temperatures than the ambient temperature, the latent heat can be stored by
the material. In order to accumulate heat, heat has to be added accordingly in or-
der to increase the temperature to the level of the phase change temperature.
In the case of low-temperature heat storage, only the phase change from solid
to liquid is used, as the volume increase during the phase change from liquid to
gaseous under normal pressure conditions requires a lot of effort for expansion
devices in closed heat stores.
Latent heat stores are defined by a high level of energy density. Heat charging
and discharging can occur at almost constant temperature levels. The main disad-
vantages are the volume changes that occur during phase changes. Some materials
can also cool down too much during the release of heat. The varying heat conduc-
tivity in a solid and a liquid condition is also problematic. If inorganic salts are
used, corrosion is an additional problem.
A special type of latent heat store is the sorption heat store. Silica gel for ex-
ample can be used as a sorbent. During charging, water is extracted from the silica
gel through heating. This can start from temperatures of 60 °C onwards; thus the
heat provided by the solar collectors can be used effectively. The dried silica gel
is easy to store. In order to extract heat, it is brought in contact with steam, and
through an exothermal reaction water is absorbed. The resulting heat can be used.
Due to low absolute operating pressures (10 to 100 mbar), steam can be produced
by solar collectors during the winter. The energy densities range from 150 to
250 kWh/m 3 /4-7/. Unfortunately, the released heat has nearly the same amount of
energy as the steam production, although at a higher temperature. Therefore sorp-
tion heat stores can be seen as a kind of heat pump. So far, no latent heat stores
for solar installations are available on the market.