The Effect of Cyclic Operation of a Horizontal Ground Loop on Ground Coupled Heat Pump Performance

Abstract

When the required load for single-speed heat pump is a fraction of its capacity, the heat pump must cycle. Multispeed heat pumps are able to vary their speed, and so cycle much less than single-speed heat pumps. The effect of cyclic operation on the heat pump ground loop and the consequent effect on heat pump performance have been examined. To analyze the effect of cyclic operation on heat transfer within the ground, separate finite element models were written for the ground loop and the surrounding soil. The models were used to compare the operation of the ground loop for single-speed and multispeed systems. Results were used to explore the effects of the pipe length, and to examine parameters which alter the effects of cycling. Parameters included pipe size and percent capacity. Results show that cyclic operation will decrease the performance of the heat pump, based solely on the performance of the ground loop (i.e., the total load remains fixed). It is also shown that as the pipe radius is increased, the effect of cyclic operation decreases, again due to the fluid capacity, and that as percent capacity decreases the cycling penalty increases.