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 monit ored since then using a remote data acquisition system. Monitored data include: temperature and relative humidity of return and supply air w ater temperature entering and leaving the condenser power consumption of the individual system components, cycling rate, on-time, and soil t emperatures at various depths and radial locations. Water and airflow rates have been measured twice during the monitored period, and have r emained constant. The measured quantities allow calculation of instant aneous capacity, power coefficient of performance (COP), and ground-co il heat rejection. Data for operation in the cooling mode are discusse d herein. Based on the experimental data, it was discovered that the w ater temperature entering the condenser (EWT) exhibited a prolonged mi nimum 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 nondimens ional groups were developed based on the experimental data. The first nondimensional group allowed analysis of startup independent of change s in independent variables, while the second nondimensional group quan tified the performance benefits due to cyclic operation. The results t ended to indicate that an improvement in COP may be achieved in cyclic operation, due to the decreased values of EWT early in the cycle. Thi s is in contrast to behavior for air source heat pumps, where cycling always degrades efficiency.