Theoretical and experimental investigation of startup and shutdown behavior of residential heat pumps

Standardized efficiency and capacity measurements of residential heat pumps deviate from real time performance due to several factors. While standardized measurements are performed at steady state operation, heat pumps in the field are usually capacity controlled using on-off cycling. In order to determine the difference between these operating modes, laboratory and field measurements were conducted to verify transient models of air-source and geothermal heat pumps. Heat pump performance was then simulated for several operating conditions typical for central European installations. It was shown that air-source heat pumps experience performance losses of only 1–2% for short cycling times. In the case of geothermal heat pumps, the recovery of the ground probe during the off time can lead to efficiency gains of up to 5%. Results of this study help to provide improved control strategies for heat pumps using on-off cycling as capacity control. They also support the conclusion that standardized measurements according to EN14511 accurately represent the efficiency of residential heat pumps both in continuous and in part load operation.