Energy efficiency indicators for combined cooling, heating and power systems

Combined cooling, heating and power system is an effective technology for energy conservation and environmental protection, especially for buildings with multiple demands on electricity, cooling and heating load. Various energy indicators have been proposed for evaluating and optimizing the energy efficiency of combined cooling, heating and power systems, but previous studies have not yet clarified how to use them in a reasonable way that takes their specific merits into account. This paper aims to identify the underlying effects of three common energy indicators, including the primary energy rate, primary energy saving rate, and annual average comprehensive energy utilization efficiency, based on the optimized performance of combined cooling, heating and power systems with different typical configurations. This paper proposes three combined cooling, heating and power system configurations for an office building in Kunming to conduct a comparative analysis of three energy indicators. Two common operation strategy (the following thermal load and following electricity load) and the genetic algorithm are selected to performed single-objective optimization with energy indicator as target. The economic and environmental analysis is also carried out to show the impact of energy indicators on other indicators. The results show that, the indicators primary energy rate and primary energy saving rate have the same optimization effect of energy consumption when the system configuration and operation strategy are identical, as the difference between the primary energy rate values is not more than 1%, and the difference between the primary energy saving rate values is not more than 0.5%. The advantage of primary energy rate is that the difference between the primary energy rate values of combined cooling, heating and power systems and reference system could show whether carbon dioxide emissions are reduced, and the advantage of primary energy saving rate is that its value directly indicates whether the system achieves energy-saving or not. Furthermore, the annual average comprehensive energy utilization rate is not suitable for the single-objective optimization of combined cooling, heating and power systems. This study distinguishes the differences between indicators and the conclusions are of general significance which could provide valuable guidance for designers to select optimal energy indicators applying to the optimization design of combined cooling, heating and power systems.