Capacity Control of Residential Heat Pump Heating Systems

Doctoral thesis, 2007

Heat pumps used for space and sanitary hot water heating of residential buildings are increasingly popular in northern Europe. As they compete with other heating equipment and in order to limit the environmental impact of their use, it is very important that heat pumps are energy efficient. The scope of this thesis is to investigate the potential for increased energy efficiency of heat pumps by applying variable-speed capacity control to compressor, pumps and fans, as well as an overall optimisation strategy for on-line optimisation of the operation. Focus has been on ground-source heat pumps connected to hydronic heating systems but the same principles apply also to other types of heat pumps. For air-source heat pumps the defrost function adds one extra parameter to consider and this is partly investigated by the evaluation of a proposal for optimised defrost initiation. Two prototype heat pumps with variable-speed compressors were designed and evaluated by laboratory measurements. The results from these measurements were used as input to models for calculating the energy efficiency of different heat pump systems. The models were used to compare the annual performance of the two prototype heat pumps to the performance of a state-of-the-art intermittently operated ground-source heat pump. One major result is that for the comparison between variable-speed and intermittent operation it is important to consider the transient behaviour of the heating system. In a separate study the possible benefits of using variable-speed controlled circulators were investigated. Results show that the use of efficient circulators is of primary importance, especially for heat pumps with a variable-speed capacity controlled compressor. Of secondary importance comes the variable-speed operation and optimisation of circulator and compressor capacities. Most important in this respect is to control the capacity of the circulator for the bore hole system. Capacity control of the circulators can be implemented by a simple strategy with results very close to an optimised control. Comparing the Seasonal Performance Factor (SPF) of an all variable-speed controlled heat pump (variable-speed pumps and compressor) with optimised pump and compressor capacities to the state-of-the-art intermittently controlled heat pump shows that the SPF could be increased by approximately 30 %. However, this requires improved compressor and system design compared to the two prototype heat pumps.