Energy efficiency of district cooling systems (4): Design and operation optimization of ice storage brine systems

Based on the dimensionless system energy efficiency model developed by the authors, the impact model of brine transport on the system energy efficiency is further developed, and the optimal brine inlet and outlet temperature difference calculation formula under different evaporator inlet brine temperatures and design condition pipeline resistances is obtained. The calculation results show that, taking ethylene glycol brine as an example, under the design conditions of the ice-making mode, for a system with a pipeline resistance of 40 m under conventional design conditions, when the design inlet ethylene glycol temperature is -1.80 ℃, the optimized inlet and outlet temperature difference is 5.76 ℃, which is 1.96 ℃ higher than the conventional design inlet and outlet temperature difference (3.80 ℃). By reducing the ethylene glycol flow rate and increasing the inlet and outlet ethylene glycol temperature difference, the optimized system COP is increased to 3.34, which is 7.1% higher than that of the conventional design system. For a system with a pipeline resistance of 20 m under design conditions, the effect of optimized temperature difference design on energy efficiency improvement is not significant. During the ice-making operation process, when the refrigeration temperature of the refrigerator drops from 5.5 ℃ to -5.6 ℃, it is necessary to continuously adjust the ethylene glycol flow rate to achieve optimized operation, which is especially obvious for the systems with a large pipeline resistance coefficient.