Models based simulation of the coefficient of performance of a domestic heat pump water heater

Abstract

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.

Sanitary hot water production make up to over 50 % of the monthly electrical energy consumption in South Africa residential sector. Employing an effective and efficient mechanism for sanitary hot water heating can lead to a substantial energy saving and demand reduction as well as creating a benign environment owing to the decrease of the carbon dioxide emission. Most of the renewable energy devices for sanitary hot water heating utilized the free and abundant solar energy. Domestic air source heat pump (ASHP) water heater is one of the renewable energy device used for sanitary hot water production. In this study, a data acquisition system (DAS) was constructed to measure the predictors (E, electrical energy consumed) and (  , average product of ambient temperature and relative humidity) and relevant parameters to compute the COP during the vapor compression refrigerant cycle (VCRC) of the ASHP unit. The coefficient of performance (COP) of an ASHP water heater under two different heating up cycle scenarios were critically examined. Modelling and simulation of the COP of the system provided a distinctive opportunity for optimization and prediction of its performance under different operational conditions. It was depicted that the mean COP in the both scenarios of the heating up cycles (firstly, where there was no successive hot water draw off and secondly, with simultaneous hot water draw off) was on average 2 and above. Finally, using the mathematical models in the both scenarios, it was revealed that increases in both predictors (E and  ) can result to decrease in the COP. The one way analysis of variance (ANOVA) test showed, the mean COP of 1.96 and 2.14 for the heating up scenario of simultaneous hot water drawn off and without a successive hot water drawn off. Predictors’ weight ranking demonstrated that the contribution from the input parameters was 100 times more during the heating up scenario whereby successive hot water drawn off occurred. The modelled equations were used in the mathematical blocks of the Simulink to design the simulation application of the COP of an ASHP water heater. We therefore concluded that the COP of an ASHP water heater during simultaneous hot water drawn off was higher than without successive hot water drawn off.