Efficiency optimization of a variable-capacity/variable-blower-speed residential heat-pump system with ductwork

The purpose of this study was to evaluate the cooling operation of a variable-capacity/variable-fan-speed residential heat-pump connected to a typical single-zone duct system located outside the conditioned space in a hot-dry climate. This was accomplished through full-scale laboratory testing of prototype heat-pump equipment connected to a complete residential duct system within a climatic chamber wherein a range of summer conditions were simulated. Compressor speeds and evaporator airflows were manipulated over a range of practical combinations to characterize the efficiency of the entire system. The analysis includes the delivery effectiveness of the duct system (cooling delivered to the conditioned space over cooling delivered at the evaporator coil) and the efficiency of the HVAC equipment. Our results indicate that the effectiveness of the distribution system decreases as the compressor and evaporator-fan speeds are reduced, the effect being more pronounced at hotter duct-zone temperatures. Although the ducts perform more efficiently at higher evaporator fan speeds for a given compressor speed, the blower power usage increases, resulting in an optimum cooling Coefficient of Performance (COP) of the overall system at 80% airflow for the different equipment capacities tested. Finally, the delivery effectiveness at individual grilles of the duct system vary disproportionately as the compressor and fan speeds are changed, depending on the dimension and length of the ducts.