Cost-Energetic Analyses of Ice Storage Heat Exchangers in Solar-Ice Systems
The aim of the present paper is to analyze ﬂat plates (FP) and capillary mats (CM) heat exchangers for solar-ice systems. The optimum heat exchanger area needed in the ice storage is based on energetic transient system simulations and on cost indicators. Solar-ice systems using ice storage volumes from 2 to 5 m3 and selective uncovered collectors with 15 to 25 m2 in the city of Zurich were able to achieve system performance factors SPFSHP+ ranging from 3.5 to 6 with both CM and FP in a single family house with 10 MWh yearly heating demand. Considering the cost of the system, only simulations with CM were able to achieve lower heat generation cost than that of a ground source heat pump (GSHP) with even higher SPFSHP+ (an SPFSHP+ of 4 was assumed for GSHP). For example a system with a collector area of 15 m2 and 5 m3 of ice storage volume can reach heat generation cost of 29 Rp./kWh, 0.5 Rp./kWh below the GSHP average reference cost, with an increase of SPFSHP+ of 20 % respect to the GSHP system. However, the targets can only be achieved using an appropriate heat exchanger area. The optimal heat exchanger area was found to be around 4 to 5 m2 per m3 of ice storage for CM and around 10 to 14 m2/m3 for FP. These heat exchanger ratios correspond to a distance between heat exchangers of around 12 - 17 cm for both CM and FP. These results were obtained assuming a conservative maximum ice fraction of 80 %.