Energy efficiency of district cooling systems (3): Design and operation optimization of cooling water systems

The temperature difference between the inlet and outlet cooling water of the condenser and the pipeline resistance (pressure drop) both affect the system energy efficiency, and there is a coupled correlation between the two effects, which can be expressed by the similarity criterion of the cooling water system’s power consumption to heat supply ratio. According to the system energy efficiency model expressed by dimensionless variables of power consumption to heat supply ratios, the model of the power consumption of cooling water systems affecting the system energy efficiency is further obtained. The calculation formula of the optimized temperature difference between the inlet and outlet cooling water of the condenser for the system energy efficiency is obtained under different inlet cooling water temperatures of the condenser, pipeline resistance coefficients and chiller load rates, and the optimized temperature difference considers the influence of the cooling water temperature on the energy efficiency of the chiller. The results show that the optimized temperature difference is linearly related to the chiller load rate. When the pipeline resistance at the design condition is large, the value of the optimized temperature difference is large, and the optimized temperature difference is quite different from the temperature difference under the operation of constant flow rate or constant temperature difference. The optimized system energy efficiency is higher than that of the constant flow rate and constant temperature difference operation, and the optimized system energy efficiency is a function of the pipeline resistance coefficient, the chiller load rate, the inlet cooling water temperature of the condenser, and the chiller energy efficiency. When the pipeline resistance coefficient is large, the inlet cooling water temperature of the condenser is low, the chiller load rate is low (less than 70%), or the chiller energy efficiency is high, the system energy efficiency can be significantly optimized and improved. This paper also analyses the calculation method of NPLV in the AHRI standard, and the results show that the comprehensive system energy efficiency can be nearly 20% higher than that of constant flow rate operation when the optimized temperature difference between the inlet and outlet cooling water of the condenser is adopted. Therefore, for the chillers with higher NPLV under the constant flow rate operation, the corresponding system energy efficiency is not necessarily higher. The comprehensive performance NPLV of the chiller and its equipment performance specifications based on the constant flow rate operation performance evaluation have certain limitations.