8.3 Applications of phase change materials for sustainable energy  (Page 2/26)

A PCM is a substance with a high latent heat    (also called the heat of fusion if the phase change is from solid to liquid) which is capable of storing and releasing large amounts of energy at a certain temperature. A PCM stores heat in the form of latent heat of fusion which is about 100 times more than the sensible heat. For example, latent heat of fusion of water is about 334kJ/kg whereas sensible heat at 25° Celsius (77°F) is about 4.18kJ/kg. PCM will then release thermal energy at a freezing point during solidification process (Figure Phase Change of a PCM ). Two widely used PCMs by many of us are water and wax. Think how water requires significant amount of energy when it changes from solid phase to liquid phase at 0°C (32°F) or how wax extends the burning time of a candle. Moreover, the cycle of the melting and solidification can be repeated many times.

There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications in the development of the energy storage systems. Materials that have been studied during the last 40 years include hydrated salts , paraffin waxes , fatty acids and eutectics    of organic and non-organic compounds (Figure Energy Storage Systems ). Therefore, the selection of a PCM with a suitable phase transition temperature should be part of the design of a thermal storage system. It should be good at heat transfer and have high latent heat of transition. The melting temperature should lie in the range of the operation, be chemically stable, low in cost, non-corrosive and nontoxic.

Even though the list of the PCMs is quite long, only a limited number of the chemicals are possible candidates for energy applications due to the various limitations of the processes. Paraffins and hydrated salts are the two most promising PCMs. Generally, paraffins have lower fusion energy than salt hydrates but do not have the reversibility issue, i.e paraffin is only in physical changes and keeps its composition when heat is released or gained whereas hydrated salt is in chemical change when heat is released or gained. Therefore, a major problem with salt hydrates is incongruent melting, which reduces the reversibility of the phase change process. This also results in a reduction of the heat storage capacity of the salt hydrate. On the other hand, paraffins also have a major drawback compared to salt hydrates. The low thermal conductivity creates a major drawback which decreases the rates of heat stored and released during the melting and crystallization processes and hence results in limited applications. The thermal conductivity of paraffin used as PCM is slightly above 0.20 W/mK (compare with ice; k _ice =∼2 W/mK). Several methods such as finned tubes with different configurations and metal matrices    filled with PCM have been investigated to enhance the heat transfer rate of PCM. Novel composite materials of PCM, which have superior properties, have also been proposed for various applications. For example, when PCM is embedded inside a graphite matrix    , the heat conductivity can be considerably increased without much reduction in energy storage.