区域供冷系统能效分析(3):冷却水系统设计与运行优化
Energy efficiency of district cooling systems (3): Design and operation optimization of cooling water systems
摘要：
冷凝器进出口冷却水温差与管路阻力(压降)均影响系统能效,且两者的影响存在关联,可以用冷却水系统功耗冷热比的相似准则来表示两者的耦合影响。根据功耗冷热比量纲一变量表达的系统能效模型,进一步得到冷却水系统输送能耗影响系统能效的模型,并得到不同冷凝器进口冷却水温度、管路阻力系数和制冷机制冷负荷率时的系统能效最优化冷凝器进出口冷却水温差的计算式,该优化温差考虑了冷却水温度对制冷机能效的影响。结果表明,优化温差与制冷机制冷负荷率呈线性关系,参考设计工况管路阻力较大时,优化温差数值较大,优化温差与定流量或定温差运行时的温差有较大区别。优化后的系统能效均高于定流量运行和定温差运行系统能效,优化系统能效是管路阻力系数、制冷机制冷负荷率、冷凝器进口冷却水温度、制冷机能效等的函数,当管路阻力系数较大、冷凝器进口冷却水温度较低、制冷机制冷负荷率较小(如70%以下)、制冷机能效较高时,系统能效优化提升明显。本文对AHRI标准中NPLV的计算方法进行了分析,结果表明,采用优化冷凝器进出口冷却水温差运行时,其综合系统能效可以高于定流量运行时综合系统能效近20%,因此,对于定流量运行NPLV较高的制冷机,其对应的系统能效不一定较高,采用定流量运行性能评估的制冷机综合性能NPLV及其设备性能选型有一定局限性。
Abstract：
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.
Keywords:cooling water; temperature difference between inlet and outlet cooling water of condenser; optimization; constant flow rate; constant temperature difference; system energy efficiency; chiller load rate