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

fusion and relatively low cost [88-91]. Such form-stable systems have many
advantages over conventional phase change storage systems. For example,
conventional phase change storage systems for steam applications would be larger,
less efficient, and would require an intermediate working fluid since most
encapsulation materials corrode in steam environment. Due to low thermal
resistance, the direct contact form-stable system is also particularly suited for low
grade waste energy recovery such as from industrial processes where the flow
gases are between 100 and 200 °C [88].
Commercially available polymers such as high density polyethylene (HDPE) have to
be usually cross-linked to make them form-stable with the help of a catalyst or by
more sophisticated method. The cross-linked storage units can use the granular
form that is usually available, or can utilize other shapes such as rods or other
geometries [88]. High density polyethylene offers definite advantages as potential
thermal energy storage material if it is rendered form-stable by cross-linking [101].
Kamamito et al. [6] concluded that cross-linked HDPE is an excellent thermal energy
storage material and can be used in direct thermal contact with ethylene glycol and
silicon oil. Sharma et al. [98] reported that their operating temperatures of 110 to
140°C are too high for some applications such as space and water heating, and
according to his knowledge no company has commercially developed cross-linked
polyethylene for heat storage applications at this time.
2.5.4 Solid-Liquid-Vapour Phase Change Storages
The use of a complete solid-liquid-vapor phase change cycle will further increase the
storage density, but large volume changes cause problems regarding the
containment of such materials, and then rule out their potential application in thermal
storage systems. Indeed, such systems are technically feasible, but quite more
complicated than the simple (and passive) solid-liquid-solid cycle [103]. Furthermore,
changes of phase from liquid to gas involve large changes in volume of the material
and so are difficult to make practical in a closed compact solar seawater desalination
and hot water storage systems. Indeed, no single attempt to use solid-liquid-gas
phase change for PCM storage has been found in the literature.
2.5.5 Solid-Liquid Phase Change Storages
Solid-liquid transformations are most commonly employed due to small changes in
volume and high heats of fusion. It is also possible to choose a melting temperature
in a wide range from a large amount of solid-liquid phase change materials. A wide
variety of solid-liquid phase change materials is commercially available with
temperature ranges from – 21 °C (sodium chloride solution) to more than + 200 °C
(salts and eutectic salt mixtures). Phase change materials can therefore be used as
a thermal storage medium for both heating and cooling. Phase change materials
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