A study on an optimal approach temperature control strategy of condensing water temperature for energy saving

An optimal approach temperature (OAT) control strategy is proposed for resetting the condensing water temperature hourly, so to maximize the performance of the combined water chiller and cooling tower system. A system performance factor is introduced for evaluation of the system performance. A regression function is presented for the calculation of an optimal condensing water temperature at each hour of air-conditioning operation. The parameters in the regression function include the ambient wet bulb temperature, the chiller load ratio at the hour, and a dimensionless relative efficiency of chiller and cooling tower. The regression function has an R² close to 1 compared to the computed results. When applied to two cities in Taiwan, the OAT control strategy has a potential to save energy more than 4% on an annual basis. The OAT control strategy is most advantageous when applied to regions with large seasonal variation of wet bulb temperature.