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

network by surface tension and capillary forces. Heat storage occurs as latent heat
of the PCM and as sensible heat of the basic ceramic material and PCM. Therefore,
the use of salt/ceramic materials represents not a pure latent heat but a
latent/sensible hybrid storage concept [101]. A novel energy storage system consists
of n-octadecane retained by capillary forces in a porous silica support [83]. This
configuration eliminates the expensive heat exchange surface, provides higher
energy density, and gives higher rates of energy storage and release [98].
2.6.5 Multi Grade Phase Change Materials
Sometimes the isothermal storage level might be disadvantageous such as the
following cases [112]:
1) If the energy to be stored is available at much higher temperature level than
the melting point of the heat storage material, the storage occurs at lower
temperature level than that of the source.
2) A worse case may occur if the melting point of the PCM is higher than the
temperature level of the energy source. Then there is no latent heat storage
and a large amount of heat may be lost.
These cases may occur during solar energy storage when the temperature of the
heat transfer medium from the solar collector is higher than the melting point of the
PCM applied (the storage is realized at not sufficiently high temperature) or the
temperature of the transporting medium is lower than the melting temperature of the
PCM (there is no storage). There are three possibilities to overcome these problems
[112]:
1) Application of materials with high specific heat. The average specific heat of
solid material is 3 (kJ.kg -1 .K -1 ), the specific heat of water is 4.18 (kJ.kg -1 . K -1 ),
therefore, a large volume must be involved in the storage process
2) Application of two phase change materials; one with a lower melting point,
and other with a higher melting point, then the storage volume can be
reduced but two separate storage tanks are necessary
3) Application of crystal hydrates which crystallizes in two different hydrate
forms and the two forms have different melting points which are called two
grade phase change materials
Calcium chloride may be a promising material to accomplish the third way mentioned
above. Calcium chloride forms different crystal hydrates (CaCl 2 .6H 2 O, CaCl 2 .4H 2 O,
CaCl 2 .2H 2 O) and they have different melting points of 29.9, 43.3, and 130 °C
respectively [112]. If a PCM of low phase change temperature is selected the
process of phase change will take place even at low solar gain. Thus the number of
days, when the latent heat storage can be used will be high during the year.
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