Experimental and Numerical Analysis of a PCM-Supported ...

according to the demand at night as well as cloudy hours and rainy days. How to
store energy is one of the most important issues in a solar energy system. The
general characteristics of energy storages may be summarized in three aspects [85].
The first is the time period during which energy can be stored. The second aspect is
the volumetric energy capacity for the same amount of energy, the smaller the
volume of energy storage, the better the storage is. The last aspect is that energy
can conveniently be added and withdrawn from the storage system. For instance, a
large heat transfer surface area should be required for storages. Basically, solar
energy could be stored in three forms; sensible heat, latent heat, and thermochemical
heat or a combination of them.
2.3 Thermo-Chemical Energy Storage
Thermo-chemical storage offers an order of magnitude larger heat storage capacity
over sensible storage. This type of energy storage, rely on the energy absorbed and
released in breaking and reforming molecular bonds in a completely reversible
endothermic chemical reaction, such as it can be reversed upon demand to release
back the heat. In this case, the heat stored depends on the amount of storage
material, the endothermic heat of reaction, and the extent of conversion. Thermochemical
energy storages also have the advantage of a long-term storage with low
losses. However, the chemical reactions employed must be completely reversible.
One concept is using a salt, such as sodium sulphide and water. The salt can be
dried using for instance solar heat. This will accumulate thermal energy, and this
energy can be recovered by adding water vapor to the salt. This concept works “on
paper” and in the lab, but there are problems with corrosion and air tightness, since
the dry salt must be stored in an evacuated (airless) environment [103]. Reactions
like these are combined with a heat pumping effect. Energy at a low temperature
level has to be provided in order to discharge the storage, for instance vaporisation
of water. At the charging process energy is withdrawn from the system for instance
by condensing water.
Another form of latent heat storage is the physical adsorption of water vapor from the
atmosphere at the surface of a highly porous solid like zeolite. When dry zeolite
material adsorbs water vapor, the heat of condensation is released in the adsorption
process, while the porous media is saturated with water. Then the water can be
driven off (desorbed) again by heating the porous media to more than 100 °C and
thereby storing the thermal energy. While desorbing water, the saturation of the
porous media decreases again, which means regeneration of the zeolite.
The adsorption/desorption processes can be repeated (almost) indefinitely without
any significant deterioration of the zeolite material [26]. This process is being used in
a heating/cooling plant in some buildings in Munich. Drying of the zeolite material is
done by cheap, off-peak heat from the district heating system [104].
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