15 September 2011
On/off cycling is the most commonly used mode for capacity control in heat pumps. The effects of startup and shutdown on the coefficient of performance are described in this
paper. Air to water and brine to water (geothermal) heat pumps were simulated using a physical model, which was validated using data from field and laboratory measurements. Subsequently, several parametric studies were carried out in order to evaluate the effects of cycle time on the performance of a heat pump system. It was shown, that on/off cycling of air-source heat pumps leads to a performance penalty of 2-5% for very short cycle times. The performance reduction is insignificant if the heat pump is run for 15 minutes or longer at
a time. Geothermal heat pumps, on the other hand, profit from short cycle times. The benefit comes from the borehole regenerating during the off-time of the heat pump. Performance
improvements in the area of 5% can be achieved using an intelligent control of the cycle time.