An Energy-Efficiency Evaluation Method for Energy-Utilization Systems Based on Unutilized-Energy Decomposition

To address the possible limitations of conventional energy-efficiency indicators in terms of restricted evaluation boundaries and the insufficient characterization of energy coupling and reuse processes in complex energy-utilization systems, this paper proposes a modified energy-efficiency-evaluation method based on unutilized-energy decomposition. First, a unified representation of basic energy-utilization units is established based on the concept of minimum functional units. Typical structural mappings, including series, feedback, delay, and auxiliary structures, are then introduced to describe complex energy interaction paths in a unified manner. Second, unutilized energy is distinguished from actual loss and is further decomposed into inevitable unutilized energy and potentially recoverable unutilized energy, so as to reveal the internal differences and reuse potential of energy that does not form useful output. On this basis, modified energy-efficiency indicators for downstream-utilization, feedback-recirculation, and auxiliary structures are developed, together with supporting indicators such as recovery efficiency gain, auxiliary efficiency gain, and the actual loss rate, thereby forming a comprehensive evaluation framework for complex energy-utilization systems. Finally, a gas engine combined heat and power (CHP) system is used as a case study. The results show that conventional power-generation efficiency cannot distinguish system performance differences under different heat-utilization conditions, whereas the proposed modified energy efficiency and actual loss rate can effectively reveal the effects of waste-heat utilization, thermal-load matching, and thermal-storage shifting on overall system performance. This study can provide a reference for the comprehensive energy-efficiency evaluation of combined heat and power systems, as well as other chain-type and multi-energy coupled energy-utilization systems.