SIMULATION AND EXPERIMENTAL VERIFICATION OF VERTICAL GROUND-COUPLED HEAT PUMP SYSTEMS (CLOSED LOOP)

Scope of Study. The purpose of this study was to compare the performance of piping arrangements buried vertically in the ground to serve as couplings between water-to-air heat pumps and the earth, which is the sink or source of heat. A second purpose is to develop and analyze methods of designing and simulation system performance. The study is limited to ground-coupling designs that can be installed in bore holes of six inch diameter or less. Two experimental systems were utilized. One consisted of six ground couplings of various designs linked to a heat pump. The second is a single coupling design to study heat transfer near the pipe wall. Simulation is performed using finite difference equations and by the line source equation.; Findings and Conclusions. U-tube designs perform well because of smaller thermal resistance, reduced "short circuit" heat transfer and pressure losses. However, single smaller diameter designs require additional lengths. Parallel piping arrangements may be needed to avoid excessive pressure losses. Concentric designs require provisions for heat transfer coefficient enhancement and "short circuit" precautions. Finite difference equations offer a high degree of accuracy and flexibility for ground coupling simulation but require significant computer time. The line source equation gives accuracy approaching that of numerical methods and results can be obtained with micro-computer or with a pocket calculator. It is recommended over finite difference equations because errors due to line source approximations are small when compared to errors in ground property determination.