Cooperative control strategy of variable evaporation temperature and variable superheat degree for a VRF system to improve temperature stability in multiple rooms

Abstract

The energy consumption of variable refrigerant flow (VRF) systems can be reduced by adopting the variable evaporation temperature and constant superheat degree (VECS) control strategy instead of constant evaporation temperature and constant superheat degree (CECS) control strategy. However, the constant superheat degree control strategy may weaken the adjustment ability of cooling capacity of indoor units, and result in obvious room temperature fluctuations. In order to decrease room temperature fluctuations and reduce energy consumption simultaneously, a cooperative control strategy of variable evaporation temperature and variable superheat degree (VEVS) is proposed, i.e. one indoor unit is chosen to be controlled by variable evaporation temperature and the rest of the indoor units are controlled by variable superheat degrees. In this control strategy, the target value of evaporation temperature is the lowest value among the upper limits of the evaporation temperatures of all indoor units, and the target values of the superheat degrees of indoor units are predicted according to the cooling demands of rooms. Comparative experiments on room temperature fluctuation and energy consumption among the control strategies of CECS, VECS and VEVS are done. It is shown that both the control strategies of VECS and VEVS achieve smaller room temperature fluctuation and lower energy consumption than those of CECS; compared with the VECS control strategy, the average room temperature fluctuation of the VEVS control strategy is decreased from 1.1 °C to 0.5 °C due to variable superheat degree, and the energy consumption of the VEVS control strategy is reduced by 4.4 %.