Demonstration of residential air source heat pump water heaters performance in South Africa : systems monitoring and modelling

The purpose of the research was to evaluate the coefficient of performance (COP) of both 150 L split and integrated type air source heat pump (ASHP) water heaters via experimental analysis, statistical tests and mathematical modelling. The ASHP water heaters are used as a potential replacement of inefficient geyser for the production of sanitary hot water due to the excellent efficiency of COP ranging between 2 and 4 and also the capability of reducing the electrical energy consumption by 50-70%. Both types of ASHP water heaters together with a 150 L geyser that served as the control experiment were set up such that distinctive real-time simulated volumes of hot water (100, 50 and 150 L) were drawn off from each of the storage tanks per day over a full year. A data acquisition system (DAS) was designed and built comprising of power meters, flow meter, temperature sensors, ambient temperature and relative humidity sensors in order to monitor the electrical, thermo-physical and environmental contributions of the various hot water heating devices. The hot water set point temperature on each of the technologies was 55oC and the volume drawn off corresponded to the demand during the morning, afternoon and evening, respectively. This mimic the profile of a typical middle or highincome family (3-4 adults) in South Africa. The results depicted that the average annual COP, load factor, and energy saving of the split and integrated type systems was 2.95 and 2.45; 10.2 and 16.7% and 2.770 and 2.499 MWh while the simple payback period was 3.9 and 5.2 years, respectively. The reliefF test revealed that the predictors (ambient temperature and relative humidity) were secondary factors while the electrical energy consumed, the difference in the temperature of the refrigerant at the inlet and outlet of the compressor and condenser were the primary factors to the COP. The derived multiple linear regression models exhibited an excellent determination coefficient of over 90% between the calculated and modelled COP of both types of ASHP water heaters. Finally, the 2D multi-contour plots simulation was accurately used to show the variation of each predictors to the COP. Also, a simulation application to simultaneously compare the COP of both types of ASHP water heaters was developed in the Simulink environment utilising the derived mathematical models. Heat pump manufacturers and energy service companies can employ both the 2D multi-contour plots simulation and the simulation application to show the variation of the specific predictors with the COP and to predict the COP of both types of ASHP water heaters. Conclusively, the research provides substantial evidence for both policy makers and home owners to justify the techno-economic and social benefits of retrofitting a geyser with an ASHP water heater.