Design of phase change thermal energy stores (PCM-TES) for residential heat pump applications

This paper will present experimental research on the performance of a novel residential heat pump coupled to a phase change material (RT21 paraffin) and water in an adjacent thermal energy store. The storage element is associated with low-grade waste heat recovered from domestic sources through a mechanical ventilation system for example in order to minimise energy losses. This heat is then used at some later time as either the evaporator heat source or it augments the performance of an air-source evaporator. The work presented here focuses upon the performance of the heat pump using the water and PCM store as a heat source. The configuration of the heat exchanger is a multi-tube system with small fins to enhance heat transfer but not inhibit convection. Fundamental heat exchanger design was underpinned by an extensive experimental programme which included 40 tests with different tube dimensions, spacing and temperature differences for both the charging and discharging process. The volume of the PCM TES was 60 litres and it is coupled on low pressure side of the heat pump. A Copeland scroll compressor ZH15K4E-PFJ was employed with refrigerant R-134a in the system. The results are presented along with suggestions for further improvements.