Investigation of different cooling tower fan control strategies using COP of actual chillers and calibrated models of actual cooling towers and fans

• An energy model of condensing water systems is developed using an ε -NTU cooling tower model. • The optimal fan speed is developed using COP of actual chiller and calibrated models of cooling tower. • The operation with less connected chillers to cooling towers is considered. • The open-loop fan speed control strategy still can achieve the best control and energy performance. • The optimal fan speed significantly decreases as the number of connected chillers decreases. Open-loop fan speed control and closed-loop approach temperature control of cooling towers were recommended by different researchers. With the further consideration of the coefficient of performance (COP) change of chillers over 30 years and the potential turndown operation of cooling towers, the paper is to reinvestigate different cooling tower fan control strategies. First a total power model of condensing water systems is developed to calculate the total system power and obtain the optimal fan speed, leaving water temperature and approach temperature of cooling towers. Then a studied condensing water system with the cooling tower turndown ratio of 50% is selected and calibrated. Finally, the developed model is applied on the studied system to evaluate different optimal and conventional controls and give the recommendations. The evaluation reveals that the optimal fan speed is significantly impacted by the chiller load and the total system power is less sensitive to the fan speed near the optimal speed. Thus, the simplified optimal speed control by resetting the setpoint with the chiller load is recommended. When physical models are not available, the conventional fixed fan speed control with the setpoint equal to the cooling tower turndown ratio is recommended with the energy penalty of 1.1%.