Performance evaluation and strategies of subcooling control in residential heat pump systems

In most residential heat pump systems, the refrigerant flow has been controlled with focus on preventing liquid intake at the compressor suction. The use of electronic expansion valves allows new control methods to be employed. One of the methods proposed is to control subcooling at the condenser outlet keeping the system at maximum efficiency or capacity. The paper presents an investigation based on ideal cycle analysis and experimental data of a residential heat pump system on the performance and possible subcooling control strategies. Subcooling control was able to improve performance by 8.6%, 7.4% and 5.9% for outdoor ambient temperature of, 0C, -5C and - 10C, respectively. A simple linear regression of the subcooling and discharge superheat based on the condensation and evaporation temperature difference can be used to achieve maximum efficiency. The use of discharge superheat to indirectly control subcooling provides an accurate and stable method of maximizing heating performance factor, but requires correction based on the refrigerant charge of the system.