Applicability of thermal energy storage in future low-temperature district heating systems – Case study using multi-scenario analysis

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With the flexibilities added from thermal energy storage (TES) technologies, low temperature district heating (LTDH) system can coordinate the heat and electricity sectors in a cost-effective manner. Such combinations have therefore become an important step to achieve a 100% renewable energy system. Despite the importance of TES has been demonstrated in previous studies, giving drastic changes compared to the current systems, the practical applicability of TES in the LTDH systems remains unknown. Furthermore, the proposed benefits of TES might deviate from the expectations considering the development of future characteristics, such as the low temperature levels and small space-heating demand. This study investigates the performances and benefits of four typical short-term TES technologies, including the use of central water tank (CWT), district heating network inertia, domestic hot water tank (DHWT), and building thermal mass, based on a case LTDH system in Roskilde, Denmark. Techno-economic analysis is conducted on a variety of scenarios, based on future changes in operation of the heat sources to the end-users. An integrated model is also developed to simulate the operation dynamics of the district heating system with regards to optimizing the use of the TES units. This study provides a performance map of the TES technologies in accordance with the transitions from current to future LTDH systems, indicating the relationships between the system characteristics and optimal TES applications. The CWT is found to be most preferable for integrating the variable renewable energy due to its ability to store heat for long periods. In the end-use side, with the improved building performances and reduced space heating demand in the future, there is less potential for the use of building inertia. In contrary, the benefit of the DHWT, which mainly comes from the reduction of bypass loss during the non-space-heating period, is increased in the future. Furthermore, raising the network temperatures for active storage is found to be infeasible under all future LTDH scenarios because this measure significantly influences the heat source efficiency.