Energy modeling and optimization of building condenser water systems with all-variable speed pumps and tower fans: A case study

The emergence of building condenser water systems with all-variable speed pumps and tower fans allows for increased efficiency and flexibility of chiller plants in partial load operation but also increases the control complexity of condenser water systems. This study aims to develop an integrated modeling technique for evaluating and optimizing the energy performance of such a condenser water system. The proposed system model is based on the semi-physical semi-empirical chiller, pump, and cooling tower models, with capabilities of fully considering the hydraulic and thermal interactions in the condenser water loop, being solved analytically and much faster than iterative solvers and supporting the explicit optimization of the pump and tower fan frequency. A mathematical approach, based on the system model and constrained optimization technique, is subsequently established to evaluate the energy performance of a typical dual setpoint-based variable speed strategy and find its energy-saving potential and most efficient operation by jointly optimizing pumps and tower fans. An all-variable speed chiller plant from Wuhan, China, is used for a case study to validate the system model’s accuracy and explore its applicability. The results showed that the system model can accurately simulate the condenser water system’s performance under various operating conditions. By optimizing the frequencies of pumps and tower fans, the total system energy consumption can be reduced by 12%–13% compared to the fixed dual setpoint-based strategy with range and approach setpoints of 4 °C and 2 °C. In contrast, the energy-saving potential of optimizing the cooling tower sequencing is insignificant. A simple joint speed control method for optimizing the pumps and tower fans emerged, i.e., the optimal pump and fan frequency are linearly correlated (if both are non-extremes) and depend on the chiller part load ratio only, irrespective of the ambient wet-bulb temperature and chilled water supply temperature. It was also found that the oversizing issue has further limited the energy-saving space of the studied system and results in the range and approach setpoints being inaccessible. The study’s findings can serve as references to the operation optimization of all-variable speed condenser water systems in the future.