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Solar energy is only available during light hours and this free energy can be used either for heating or cooling but generally it is available during only for a limited day period. By storing this free energy one can either produce heat or alternatively provide a cooling source.

Although conventional solar collectors may not generate enough energy to charge an water tank during wintermonths they could still generate low grade heat around 30~35oC which is more than enough to charge PCM modules during day-time. Once the sun goes down the stored energy keeps the occupied space warm and the outside cold energy freezes the PCM rather than penetrating into the occupied space. This way the space conditions can be stabilised over a 24 hour day and night cycle.

Using conventional solar collectors one can store hot water in simple water tanks but once the sun goes down and water is gradually drawn the tank can not maintain a steady output temperature due to strafication. However, if the latent heat capacity of PCM is added to the water tank not only can the capacity of the tank be increased by as much as 3~4 times but also having fixed phase change the water outlet temperature can be fixed until the PCM energy is fully depleted.

Using thermoformed plastic  PCM plates as illustrated in Figure 4.4.2 any existing hot water tank capacity can be increased 2~3 fold however, the high cost of these rubber balls restricts their wide usage and this technology is applied mainly to sites which have limited space available and the tank size can not be increased.