Ice-Ex: Heat Exchanger Analyses for Ice Storages in Solar and Heat Pump Applications

The combination of solar thermal and heat pump systems with ice storages, the so-called solar-ice systems, are becoming popular in Switzerland for the provision of space heating and domestic hot water. These systems can have a high efficiency, in the range of ground source heat pumps (GSHP) or higher, without being submitted to regulations of ground water protections.
However, today's system cost are still higher than those from GSHP and research is needed to make solar-ice systems more efficient and cost competitive.

One approach to improve the efficiency and the investment cost of solar-ice systems is to optimize the heat exchanger area in the ice storage. Besides the aims of increasing the efficiency and reducing installation cost, an appropriate heat exchanger design can increase the ice fraction. The aim of the present project is to find suitable heat exchanger types and define the optimum heat exchanger area needed in the ice storage based on energetic and cost indicators.

This report is separated into six main chapters. Besides the first (Introduction) and the last (Conclusions) chapters, the report has four main chapters. Experimental results of different heat exchangers, i.e. capillary mats, flat plates and coils are presented in chapter 2. The mathematical description of an ice storage model able to cope with all experimentally analyzed heat exchangers is presented in chapter 3. In chapter 4, results from the validation of the model using the experimental data from chapter 2 are shown. The optimum heat exchanger area for the most promising heat exchangers was evaluated from a system perspective in terms of energetic efficiency and heat generation cost. The energetic simulations of the complete solar-ice system with varying heat exchanger type and area for different collectors areas and storage volumes is presented in chapter 5. In the same chapter, the cost effective heat exchanger area was evaluated and compared with state of the art ground source heat pump systems.

The main conclusions of this project can be summarized as follows. Very high ice fraction can be reached with all heat exchangers and the casing did not show any damage under high ice fraction conditions. This means that designing an ice storage able to reach an ice fraction up to 95 % or higher should be safe. Therefore, the size of the ice storage, and thus the cost of the system, have the potential to be reduced. From all the heat exchanger analyzed, the most promising designs for small ice storages (below 5 m³) are polypropylene capillary mats (CM) and flat plates (PF) of stainless steel. From these two heat exchangers, capillary mats showed the best performance considering energetic and cost indicators. Solar-ice systems using ice storages volumes from 0.3 to 0.5 m³ per MWh of yearly heating demand, and selective uncovered collectors with 1.5 to 2.5 m²/MWh in the city of Zurich were able to achieve system performance factors SPF ranging from 3.5 to 6 with both CM and FP in a single family house with 10 MWh of yearly demand. Considering the cost of the system, only simulations with CM were able to achieve lower heat generation cost than that of the GSHP with higher SPF (an SPF of 4 was assumed for GSHP). For example, a system with a collector area of 1.5 m²/MWh and an ice storage volume of 5 m³/MWh, can reach heat generation cost of 29 Rp./kWh, 0.5 Rp./kWh below the GSHP reference cost, with an increase of SPF of 20 % respect to the GSHP system. However, there targets can only be achieved using CM and with the appropriate heat exchanger area. The optimal heat exchanger area was found to be around 5 to 6 m² per m³ of ice storage for CM and around 10 to 12 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 %.