12. Thermodynamic Assessment of Seasonal Stratified Thermal Storage

District energy (DE) and thermal energy storage (TES) are two energy technologies that can enhance the efficiency of energy systems. Also, DE and TES can help address global warming and other environmental problems. In this study, a stratified TES is assessed using exergy analysis, to improve understanding of the thermodynamic performance of the stratified TES, and to identify energy and exergy behavioural trends. The analysis is based on the Friedrichshafen DE system, which incorporates seasonal TES, and which uses solar energy and fossil fuel. The overall energy and exergy efficiencies for the Friedrichshafen TES are found to be 60 and 19 % respectively, when accounting for thermal stratification. It is also found that stratification does not improve the performance of the TES notably. Considering the TES as stratified and fully mixed does not significantly affect the overall performance of the Friedrichshafen TES because, for this particular case, temperatures are very close whether the TES is treated as stratified or fully mixed.