Evaluation of a Heating System using a Heat Pump and Heat Store

Abstract

A commercial model reverse cycle heat pump of split system design having a nominal 8.35 kW heating capacity operating in an air-to-air mode, was modified by the author to operate in an air-to-water mode. The 240v, single phase unit was incorporated into a purpose designed and constructed hot water storage system. The water circuit comprised the condenser connected via a closed-loop copper pipe network to two direct contact sensible heat stores. The storage vessels, modified to satisfy the basic requirements of a research programme, could be used singly or in parallel. The system was instrumented throughout for monitoring individual circuit temperatures and pressures. A microcomputer, in conjunction with an Analogue-Digital (A-D) converter unit measured and recorded system conditions. Software used in the computerised control of the data collection was written by the Author. Performance of the evaporator, condenser and water store was evaluated individually, and collectively, and compared with design performances corrected for prevailing conditions. The refrigerant mass flow rate was calculated based on ASHRAE methods and the air volume through the evaporator was accepted as the manufacturers stated value. Manufacturers published performance data has been incorporated into the computer evaluation routine. Theoretical aspects and practical problems associated with sensible heat storage have been examined. Results of promoting thermal stratification in one vessel are compared with the results from a similar, non-stratified vessel. A mathematical relationship, based on emperical results, for predicting the development of stratification in a thermal store has been produced. The resultant computer model predicts the amount of energy capable of being stored in a sensible heat store, being supplied from a reverse cycle heat pump, over selected running periods. The temperature of the store can be predicted along with the instantaneous coefficient of performance (COP) of the overall system.