Abstract

TANGWE, S.  and  KUSAKANA, K.. Evaluation of the coefficient of performance of an air source heat pump unit and an air to water heat pump. J. energy South. Afr. [online]. 2021, vol.32, n.1, pp.27-40. ISSN 2413-3051.  http://dx.doi.org/10.17159/2413-3051/2021/v32i1a7935.

Air source heat pump (ASHP) water heaters are efficient devices for sanitary hot water heating. The coefficient of performance (COP) of the air to water heat pump (AWHP) is constantly lower than that of the corresponding ASHP unit. The study focused on determining the COP of both the ASHP unit and the AWHP. This was achieved by the implementation of both experimental and simulation methods, with the help of a data acquisition system and the REFPROP software. The system comprised of a 1.2 kW split type ASHP unit and a 150 L high pressure geyser. A power meter, flow meters, temperature sensors, pressure sensors, ambient temperature and relative humidity sensor were installed at precise locations on the split type AWHP. Controlled volumes of 150, 50 and 100 L were drawn off from the AWHP during the morning, afternoon and evening for a year. The average COP for the summer and winter, in terms ofthe input electrical and output thermal energies ofthe AWHP were 3.02 and 2.30. The COPs of the ASHP unit, in terms of the change in the enthalpies ofthe refrigerant at the inlet and the outlet ofthe condenser and the evaporator, were 3.52 and 2.65 respectively. The study showed that the difference between the COP of the ASHP unit and that of the AWHP could be ascribed to the electrical energy consumed by the fan and the water circulation pump during the vapour compression refrigeration cycles. The work provides an energy optimisation opportunity to the manufacturers of this technology, helping to enhance the efficiency and COP of ASHP water heaters.Highlights 1. The COPt ofthe ASHP unit was higher than the COPe ofthe AWHP. 2. The COPe ofthe AWHP was the ratio ofthe input electrical energy consumed and the output thermal energy gained by the stored water. 3. The COPt of the ASHP unit was enthalpies-dependent and a function of inlet and outlet enthalpies of the evaporator and condenser. 4. The inlet and outlet refrigerant temperatures profiles ofthe condenser confirmed thermal energy dissipation.

Keywords : vapour compression refrigeration cycle; coefficient of performance; energy optimisation; data acquisition system.

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