Evaluation of performance of air source heat pump water heaters using the surface fitting models: 3D mesh plots and 2D multi contour plots simulation

Modelling of the coefficient of performance (COP) of an air source heat pump water heater can lead to optimisation and prediction of its performance. The study focused on the utilisation of surface fitting models to predict the COPs of a 150 L split type ASHP water heater without an electric backup element and a 150 L integrated type ASHP water heater with an electric backup element. A robust and accurate data acquisition system was employed to measure the predictor parameters (electrical energy consumed (E) and product of ambient temperature and relative humidity (λ)) as well as the thermal properties to enable the computation of the COP during the vapour compression refrigeration cycles of the ASHP unit. It was observed that for both systems, the two predictors were primary factors. The surface fitting models for both systems showed that the COP increases with an increase in E by a rate of 0.30/kWh and 0.28/kWh for the split and integrated type systems, respectively. The models were simple and can be used to predict the COP of both systems with over 95% confidence level, and the determination coefficient of the split and integrated systems were 0.917 and 0.902, respectively. It was also depicted that the COP variation with the predictors in the controlled volume of hot water drawn off (50 L, 100 L and 150 L) under different ambient conditions can be accurately predicted with either the 3D mesh plots or the 2D multi contour plots simulation.