Energy efficiency of district cooling systems (2): Influence of power consumption to heat supply ratio on system energy efficiency

Based on the definition of power consumption to heat supply ratio, the effects of power consumption to heat supply ratios on the system energy efficiency are analysed for the district cooling system (DCS) using the typical subsystem power consumption to heat supply ratio limit combinations of the existing standards and project practice. This paper provides optimization methods to optimize the system energy efficiency, and clarifies the bottleneck subsystems that should be focused on to improve the system energy efficiency. Based on the dimensionless system energy efficiency model with the power consumption to heat supply ratio similarity criterion as a variable, the mathematical model of the influence of the power consumption to heat supply ratio of each subsystem on the system energy efficiency impact rate is obtained. With the increase of the power consumption to heat supply ratio, the system energy efficiency of the DCS (COPDCS)decreases in a nearly linear proportion, and the decreasing slope is the energy efficiency of the chiller (COPR)+1. These models can directly evaluate the influence of each subsystem’s power consumption to heat supply ratio on the system energy efficiency, and identify the bottleneck subsystem affecting system energy efficiency. The influence magnitude of the external pipeline network energy consumption of the DCS on the system energy efficiency is close to that of the subsystems, such as the cooling water system inside the chiller plant, and the external pipeline network energy consumption is not necessarily the main limiting factor of the system energy efficiency of the DCS. The power consumption to heat supply ratio of the ethylene glycol system is the bottleneck affecting the energy efficiency of the ice storage chiller system, and reducing each subsystem’s power consumption to heat supply ratio is the common path to improve the system energy efficiency of the DCS and the chiller plants of individual buildings. The mathematical model of the influence of the subsystem’s power consumption to heat supply ratio on the system energy efficiency proposed in this paper can be used as an optimization basis for the design and operation of the DCS or chiller plants of individual buildings.