Exhaust-air heat-pump performance with unsteady-state operation

Abstract

The energy performance of two residential exhaust-air heat pumps (EAHP) with different condenser designs was studied experimentally in a laboratory with a focus on transient heat-pump performance associated with time varying requirements for water and space heating. Experimental variables included the total daily volume of hot water required, the schedule of hot-water demand, the temperature of water entering the hot-water tank, hot-water delivery temperature, and the temperature and flow rate of air entering the auxiliary refrigerant-heated fan coil supplied with one of the EAHP units to permit space heating. Based on the data, for a wide range of operating conditions, we derived linear correlations between the heat-pumps' time-average coefficient of performance (COP) and appropriate spatial and temporal average temperatures in the hot-water tanks. With the refrigerant-heated fan coil, the COP varied non-linearly with air flow rate. COPs ranged from 2.0 to 4.2. The control system of the EAHP with two condensers (one is the fan coil) gives priority to water heating. Based on the data, results from our previous hourly modeling of EAHP performance, data from field studies in Sweden, and new calculations, we propose a new control system that usually places priority on space heating and, thus, takes better advantage of the capacity to store heat in the water tank. We estimate that this proposed control system may increase annual energy recovery by approximately 1000 kWh if the EAHP is used in a Portland, Oregon house. Total annual energy savings due to EAHP operation in an all-electric house (compared to the same house with electric resistance space and water heating) is estimated to be approximately 6000–7000 kWh.