Air Conditioning, Control, Domestic Heat Pump, Electronic Expansion Valve, Optimization, Subcooling
An electronic expansion valve can be used to improve efficiency and capacity in residential heat pump system by changing the focus of its control method on condenser subcooling and allowing slightly wet compressor suction to reduce discharge temperatures. This paper will present results of an experimental and theoretical analysis of the use of subcooling control. An investigation based on ideal cycle analysis shows potential improvements in HPF can be obtained if the subcooling is controlled by the system’s expansion valve, but higher specific heating capacity and pressure ratios may reduce the overall improvement as outdoor temperatures are further decreased. A 2-Ton (7 kW) off-the-shelf residential system was used to evaluate the effect of subcooling control the system’s performance characteristics under a range of external conditions for HSPF calculation and compared with the original system’s expansion control. HPF (Heating performance factor) was increased by up to 19.1% in low load conditions and up to 4.2% in high load conditions. Heating capacity was also improved by up to 18.1%, which penalizes low load conditions by requiring more often on/off cycling but could lead to even higher HPF increase if the compressor speed is lowered to match the load of the residence in higher load conditions and can also improve efficiency at conditions that require auxiliary heating. HSPF was calculated for both subcooling controlled and baseline system showing an improvement of 19.2% in HSPF with a negative effect only observed between 0C and 5.5C which suffer from higher cycling degradation. The control scheme was defined as a linear function of the refrigerant condensation and indoor air inlet temperature difference. The control curve showed good agreement with both experimental and model data for the system, with the charge compensator causing some deviation from the rest of the data. The use of an accumulator as a charge receiver may eliminate the requirement of a charge compensator simplifying the cycle architecture while still providing an increase in efficiency with subcooling control.