Variable evaporating temperature control strategy for a VRF system based on continual estimation of cooling capacity demand of rooms

Abstract

A variable refrigerant flow system has multiple indoor units for cooling different rooms, and the cooling capacity supply of indoor units depends on evaporating temperature. Existing control strategies of VRF systems generally adopt a constant evaporating temperature which should be low enough to meet the large cooling capacity demand under full load condition, resulting in low energy efficiency. In order to improve energy efficiency, a variable evaporating temperature control strategy is developed, in which evaporating temperature is permitted to rise with the decrease of cooling capacity demand under part load conditions. During the control process, cooling capacity demand of rooms is continually estimated based on indoor thermal balance and room temperature response, and the target evaporating temperature is predicted based on the balance between the cooling capacity supply of indoor units and the cooling capacity demand of rooms. The proposed control strategy has been applied in a VRF system, and its energy efficiency and room temperature variation are tested and compared with those of a same type VRF system that adopts the constant evaporating temperature control strategy. It is shown that the proposed control strategy can improve energy efficiency by 12.6 % and control room temperature fluctuations within ± 0.5 °C.