The results of this analysis are best summarized in Fig.1 below. This figure shows the specific storage capacity [kWh/m3], as well as the specific storage cost [Euro/kWh stored heat], as a function of the ice packing factor (that is, the volume fraction of PCM in the storage). Zero percent IPF is thus equivalent to a conventional stratified water storage. This particular case is assuming a AT for storage of 25 °C.

As shown, when considering the capital (first) cost only, the cost of the PCM-storage is always higher than for a water storage (see IPF=0) although the difference is not very large. However, if the cost of “space requirement” is important, such as in a house, the PCM — solution quickly becomes cost

effective as compared to the hot water storage. Assuming a cost of space of 300 Euro/m3, the specific storage cost levels off at just below 48 Euro/kWh regardless of IPF such that the cost of a PCM storage is the same as that of a conventional stratified water storage. Then the advantage of the PCM storage is clear — approximately one third the space requirement as compared to a water storage.

Fig. 1. also shows that one important attribute affecting the specific storage capacity is of course the Ice Packing Factor so that designing a PCM storage with as high an IPF as possible is good for the technical competitiveness of this technology option for storage. As the IPF increases, the cost- effectiveness of the PCM-storage is also likely to be enhanced. This finding was taken into account when designing the storage prototype presented below.