Applicability of thermal energy storage in future district heating system - Design methodologies and performance evaluations

Licentiate thesis, 2021

District heating (DH) enables efficient and economical utilization of energy resources to satisfy the heat and hot water demands in buildings and is, thereby, well-established in Northern European countries. To achieve the future renewable energy system, the current DH systems are proved to undergo transitions towards the future DH systems, with major characteristics including renewable-based heat sources, low temperature networks, lower heating demands and smart controls. An important step is the coordination of heating and electricity sectors to achieve synergies and optimal solutions for the overall energy system, which is also known as the smart energy systems. Such goal could be achieved in a cost-effective manner by the flexibilities added from short-term thermal energy storage (TES) technologies. 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 future DH systems remains unknown. The proposed benefits of TES might deviate from expectation considering the future characteristics, such as the low storage temperature levels and short space-heating period. Furthermore, the current studies about the TES applications have mostly focused on specific case studies. The findings are of limited applicability because they cannot be easily generalized and extrapolated to other future conditions.

To explore the practical challenges and optimal applications of short-term TES units in the future, a systematic design framework that considers the diverse factors from top-level targets to bottom-level implementations is developed in this study. The top-level theoretical analysis method is developed to identify the load shifting potentials and associated storage capacities for the whole energy system, by comparing and matching energy supply and demand profiles. Compared to current bottom-up detailed system models, the proposed method requires only the energy profiles, which has resulted in much shorter analysis time. The method is further validated by complex system models, and because a good agreement has been achieved, it can be applied in various scenarios to efficiently pre-study the storage potentials. Then, the design of the practical TES capacity is derived from the theoretical result by considering performance indicators during realistic operations, such as power-to-heat conversion efficiency and heat loss efficiency.

On bottom-level implementations, four typical short-term TES technologies were investigated including central water tank (CWT), district heating network inertia (DHNI), domestic hot water tank (DHWT), and building thermal mass (BTM). For this purpose, an integrated bottom-level model to simulate the operation dynamics of the district heating systems and to optimize the use of the TES units is developed. Techno-economic analysis and comparisons of TES technologies were performed on a variety of scenarios, which are representatives of the main characteristics of the current middle-temperature district heating system and future low-temperature district heating system. The changes in the source side, transportation networks and end-use building demands are considered. As a result, a performance map of the TES technologies indicating the strong links between the system characteristics and optimal TES applications has been identified. Based on that, the optimal combinations of TES technologies were proposed for a LTDH system. Consequently, combining this with top-level methods, the overall potentials and roles of short-term TES were identified by a systematic design framework.