18 May 2017
Heat pumps utilizing variable-speed compressors and other capacity control techniques offer significant seasonal efficiency improvements over traditional single speed systems by reducing system cycling and consequent system inefficiencies. Yet, the majority of residential heat pumps operated in the United States utilize single-speed compressors and offer few capacity control options. This situation presents an opportunity for existing single speed units to be modified or retrofitted with variable speed technology. The main focus of this research is to optimize the seasonal cooling performance of a 5 ton residential split-system single-speed heat pump that has been modified with a variable-speed drive. The variable-speed drive is an inverter drive coupled with a controller that allows the user to control the compressor and fan motor speeds. The purpose of the variable-speed drive is to improve the seasonal performance of a system by operating traditionally single-speed components in variable-speed mode. Initial tests of the inverter drive have achieved SEER improvements of up to 10% compared to the baseline heat pump. A method has been developed to optimize the SEER rating of the modified heat pump as a function of compressor speed, outdoor fan speed, and indoor airflow rate. Results of this optimization indicate that improvements in SEER of greater than 10% can still be achieved by modifying the single-speed heat pump with the variable-speed drive.