Advanced control of heat rejection airflow for improving the coefficient of performance of air-cooled chillers

Air-cooled chillers are generally the major electricity consumers in air-conditioned buildings in the subtropical climate. This paper presents how the coefficient of performance (COP) of these chillers can be improved by modulating heat rejection airflow via variable speed condenser fans. The thermodynamic model of an air-cooled screw chiller was developed using the simulation program TRNSYS and validated using the operating data and specifications of the chiller. The set point of condensing temperature was used to determine the number and speed of condenser fans staged in various operating conditions. It is found that adjusting the set point in response to the outdoor temperature alone cannot achieve maximum COP when using variable speed condenser fans. Advanced control is proposed for these fans to minimize the chiller electric demand. Compared to the traditional head pressure control with constant speed condenser fans, the use of variable speed condenser fans with the advanced control enables the chiller COP to increase by 4.0–127.5%, which corresponds to a decrease of 1.8–154.6 kW in the chiller electric demand. This provides important insights into how air-cooled chillers can operate more efficiently.