Design and performance of an integrated heat pump-latent heat store water heating system

Abstract

Thermal energy storage devices may be combined advantageously with heat pumps as heating systems for domestic and commercial premises. A condenser-side heat store, which offers substantial energy cost savings and reduced on/off switching of the heat pump, was developed and its performance investigated experimentally and theoretically in this study. Calortherm 58, a phase change material (melting point 56"C) based on sodium acetate trihydrate was selected as the storage medium. The store consists of 1000 polypropylene tubes containing the PCM, each 25 mm in diameter and 1m long, giving a nominal storage capacity of 0.159 GJ (44.4 kWh) over a temperature range of 20 degC. An air-to-water heat pump, capable of delivering 7.5 kW at a circulating water temperature of 55"C for an ambient temperature of 70C, and a microcomputer to perform the control and datalogging functions complete the test facility.

Testing of the store according to a modified form of the ASHRAE standard procedure 94-77 showed the effect of flow direction, inlet temperature and flow rate on store performance. A finite difference model was developed to determine the movement of the phase change front radially and longitudinally through the tubes. The store charge/discharge rates predicted from the temperature profiles were in good agreement with those found experimentally. Performance of the heat pump was confirmed experimentally to be dependent on the ambient air and desired hot water temperatures. Parameters describing the deviation of the experimental performance from the ideal Rankine cycle were successfully employed by a diagram model to simulate the heat pump performance.

Experimental results are presented which show the performance of the Integrated system for typical climatic and load conditions. An elementary model of the Integrated system was developed and results presented which show the potential savings, in comparison to a heat pump alone, over a range of economic and operating parameters. A detailed model, which combined the heat store finite difference model with correlations expressing heat pump performance, was successfully employed to simulate operation of the experimental system.