Performance improvement of heat pumps by optimizing refrigerant charge using novel variable liquid-line length system

Heat pumps are often operated with nonoptimal amounts of refrigerant charge under various operating conditions; this results in excessive energy consumption by the air-conditioning systems in buildings. This paper proposes a novel variable liquid-line length system (VLLS) to improve the performance of heat pumps by optimizing the amount of refrigerant charge under various operating conditions. The feasibility of the VLLS is experimentally verified by proving that changes in the condensing and evaporating pressures afford the same effects as those of varying the amount of refrigerant charge. The performance improvement in the heat pump using the VLLS was evaluated by varying the heat sink and source temperatures under a fixed electronic expansion valve (EEV) opening and fixed superheat. The results indicate that the VLLS with a fixed EEV opening enhances the maximum capacity and coefficient of performance (COP) of the heat pump by appropriately controlling superheat and subcooling under various operating conditions. Furthermore, the heat pump adopting the VLLS with a fixed superheat exhibits substantial improvements in the heating and cooling COPs by varying the optimal amount of refrigerant charge.