Exergy Assessment of a Solar-Assisted District Energy System

Abstract

Background:District Energy (DE) is a technology capable of using renewable energy (e.g., solar thermal systems) and waste heat as energy sources efficiently. DE technology nonetheless has potential for improvement. Thermal Energy Storage (TES) can enhance DE performance significantly.Objective:An exergy analysis of a DE system which includes a solar thermal energy system and TES is performed, so as to improve understanding of its performance.Method:A case study based on the Friedrichshafen DE system in Germany is used to assess thermodynamically the role of solar energy and TES in a DE system. The system performance is separated into three modes: (1) fossil fuel is the only source of energy, (2) a discharging TES and fossil fuel provide heat for the DE system, and (3) solar energy and fossil fuels are the energy supplies. Exergy analyses are conducted for each performance mode and the overall DE system.Results:The results quantify the benefits of incorporating solar energy and TES on the performance of the Friedrichshafen DE system, and demonstrate that the overall exergy efficiency of the DE system increases from 23% to 27% with assistance of solar thermal collectors and TES, while the total energy efficiency increases from 83% to 87%.Conclusion:An increase of exergy efficiency is observed when TES is added to a DE system, due to a reduction in solar thermal energy loss by the TES, which allows more solar energy to be converted to useful energy to satisfy the DE system thermal energy demand.