A flexible air source heat pump testing platform is being developed at the National Renewable Energy Centre that will have the advantages of both a field trial and a laboratory test. It may be considered as a “black box” testing method that will be able to produce performance data that reflects the actual operating performance of the heat pump. At the same time it will also enable more experimental investigations to be carried out to understand the parameters that influence a heat pump’s coefficient of performance. The testing platform is controlled by automated instruments programmed in NI Labview software that enables customised tests to be carried out. The heat pump is installed normally as it would in operating condition. The heat pump under investigation in this study is a transcritical CO2 inverter controlled heat pump. It is installed with a 220 litre buffer tank that provides combined hot water and space heating and is supported by an optional solar pre-heat system. The characteristics of two alternative heating circuits for controlling heating in the test platform are compared based on experimental and theoretical findings. The first heating circuit is based on standard radiator installation and it is entirely controlled by flow rate. The second heating circuit combines both water temperature and flow rate control. The first heating circuit required a complex multi-band variable step control strategy, partly to overcome resonances and slow valve traverse. The second heating circuit required a much simpler control algorithm and achieved more responsive heating control. Dynamic behaviour of the cooling tank in the test platform for heat transfer from the first heating circuit was investigated using vertical tank temperatures and theory. At low level heating of 3kW, long duration (12 minutes) temperature fluctuations were observed due to on/off cycling of chiller. Short duration (1-2 minutes) temperature fluctuations were also observed and attributed to natural convection. At high levels of heating of 7-8kW, only short duration (1-3 minutes) temperature fluctuations with larger amplitudes were seen with fluctuations due to chiller cycling masked by these fluctuations. According to theory and published experimental work on turbulence, the conclusion can be drawn that the temperature fluctuations are primarily due to laminar convection in the cooling tank. Daily and night-time COPs were obtained using the second heating circuit continuously for a month in the winter season. Daily COPs showed an unexpected positive linear correlation with average daily heating. The COP varied from 1.8 to 2.6 for heating between 3 and 7kW. Based on the theory of CO2 as a heat pump refrigerant and published experimental work, it is suggested that the rise in COP with higher heat output can be explained by the rise in refrigerant pressure as the heat pump tries to output more heat.
|Publication status||Accepted/In press - 2013|