The Open Fuels & Energy Science Journal
2018, 11 : 30-43Published online 2018 March 30. DOI: 10.2174/1876973X01811010030
Publisher ID: TOEFJ-11-30
RESEARCH ARTICLE
Exergy Assessment of a Solar-Assisted District Energy System
2 Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, ,
* Address correspondence to this author at the Department of Mechanical Engineering, College of Engineering, University of Idaho, Behnaz Rezaie, 875 Perimeter Dr., Moscow, USA; Tel: 208-885-1026; Email: Rezaie@uidaho.edu
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.