The Use of Ground-Coupled Tanks in Solar-Assisted Heat-Pump Systems

A research program at Brookhaven National Laboratory (BNL) has studied ground coupling, i.e., the use of the earth as a heat source/sink or storage element, for solar-assisted heat-pump systems. As part of this research program, four buried tank experiments were operated between December 1978 and March 1981 in order to determine the feasibility of ground-coupled tanks in these systems. Heat was added to or removed from the tanks according to a weekly schedule derived from computer simulations of solar heat-pump systems in the local (New York) climate. Each tank was operated according to a different control strategy. This paper presents experimental results from these tank experiments for this period, and compares these results to those generated by a computer model, GROCS, developed at BNL. The model is found to be valid, for the most part, using undisturbed soil thermal properties which provide the best fit to the data most of the time. Its results are very sensitive to soil thermal conductivity during periods of large heat addition to the tanks. A ground coupled tank is found to be desirable in series solar-assisted heat-pump systems. However, no important carry-over of summer-collected heat to winter was observed.