A Nondimensional Analysis of Vertical Configuration, Ground-Coupled Heat Pump Startup

A nominal 10.6 kW (three ton), vertical configuration, ground-coupled heat pump was installed in Abilene, TX in Dec. 1989 and has been monitored since then using a remote data acquisition system. Monitored data include: temperature and relative humidity of return and supply air, water temperature entering and leaving the condenser, power consumption of the individual system components, cycling rate, on-time, and soil temperatures at various depths and radial locations. Water and air flow rates have been measured twice during the monitored period, and have remained constant. The measured quantities allow calculation of instantaneous capacity, power, coefficient of performance (COP), and ground-coil heat rejection. Data for operation in the cooling mode are discussed herein. Based on the experimental data, it was discovered that the water temperature entering the condenser (EWT) exhibited a prolonged minimum after startup due to cooling of the water during the off-cycle. The decreased levels of EWT early in the cycle increased capacity and decreased power, both acting to increase the COP. Two sets of nondimensional groups were developed based on the experimental data. The first nondimensional group allowed analysis of startup independent of changes in independent variables, while the second nondimensional group quantified the performance benefits due to cyclic operation. The results tended to indicate that an improvement in COP may be achieved in cyclic operation, due to the decreased values of EWT early in the cycle. This is in contrast to behavior for air source heat pumps, where cycling always degrades efficiency.