Phase change materials (PCMs) for thermal storage are available in a number of forms to suit various applications. In its simplest form variations in cooling load can be provided from the latent heat of melting of ice or a frozen eutectic. Ice can be formed by allowing it to build up on the outside of evaporator coils in a tank. Brines are more normally held in closed tanks or plates, again with evaporator coils inside, the outside of the tank forming the secondary heat exchange surface. Eutectics can be formulated according to the temperature required.
An example of alternative PCM is a paraffin-based material. The specific heat capacity of latent heat paraffins is about 2.1 kJ/(kgK). Their melt enthalpy lies between 120 and 160kJ/kg, which is very high for organic materials. The combination of these two values results in an excellent energy storage density. Consequently, latent heat paraffins/waxes offer four to five times higher heat capacity by volume or mass, than water at low operating temperature differences. They have a low heat conductivity, but for thermal storage this can be overcome by introducing sufficient surface.
A variation is to have a pumpable fluid such as one of the glycols, and to contain a PCM within capsules in a storage tank. The capsules are in the form of plastic balls and the PCM within may be formulated to suit any required thermal storage temperature.
Bound PCMs incorporate the PCM into an existing structure. When the PCM changes into a liquid within the supporting structure, the combined influence of capillary forces of the supporting material and the special cross-linking additives ensures that the PCM, even when in the liquid phase, remains within the structure. This means that one always works with a ‘dry’ material and liquid handling problems are eliminated. Free spaces within bound PCMs allow for volume expansion of the PCM to take place within the structure.
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