Theoretical and experimental analysis of vertical, concentric-tube ground-coupled heat exchangers

An experimental and analytical project to study the design of vertical, concentric-tube ground-coupled heat exchangers for use in heat pump applications is described. A mathematical model was developed and converted into a computer program to simulate the operation of the ground-coupled heat exchanger. The experimental apparatus consisted of a concentric configuration of two 47.2-m (155-ft) polyvinyl chloride (PVC) pipes (sealed at both ends with connections so that hot or cold water could be pumped through the system) with instrumentation to measure heat transfer. This heat exchanger was placed in a 0.20-m (8-in.) inside diameter (ID) well and backfilled with sand to establish good thermal contact with the surrounding ground. Heat transfer was measured for heat exchanger operation under several sets of operating conditions. Data collected using the experimental apparatus were used to validate the computer program, and the computer model was then used to study the effects of variations in heat exchanger length and diameter, flow rate, and tube and ground thermal conductivities on the heat exchanger performance. Results are presented.