Exergy-environment assessment for energy system: Distinguish the internal and total exergy loss, and modify the contribution of utility

Improving the efficiency of energy system is a way to reduce its environmental impact. However, high efficiency always means high system complexity and high expense, and will again increase the environmental impact. This work proposed an exergy-environment assessment method for energy system. With the exergy balance calculation, the method converted the internal and total exergy loss flows, as well as the life-cycle consumed material (including utility), to the values of environmental impact points. This work also proposed indexes to assess whether the energy transformation (exergy loss) or the life-cycle consumption dominates the environmental impact. Then, this work adopted a sludge digestion system to show the application of the proposed method. The exergy analysis and environmental impact points of the system were calculated. The entire system has a positive exergy output of digestion gas (4.18 GJ h-1). The objective exergy efficiency of system is 27.11%, and the general exergy efficiency of digester unit (80.10%) is over two times that of heating unit (35.00%). Heating unit also has a much higher environmental impact than digester unit (about 1.00: 0.15). Results show that the exergyenvironment assessment does not breach the classical thermodynamic analysis and provides the understanding of environmental impact. Compared to previous studies, this work resolves the issue of unreasonable data for the exergy-environment impact factor in methodology by amending its value from <0 or approximate to 0 to 0.40, clarifying the significance and rationality of exergy-environment assessment. The case study result shows the feasibility and advantage of the proposed method, which can be flexibly selected by decision maker. Impact factor based on the internal exergy loss is relatively preferred from the perspective of sustainability.

Automated summary

This study proposed a new exergy-environment assessment method for energy systems, applied to a sludge digestion system as a case study, demonstrating the feasibility and advantages of the proposed method.