Transition pathways for future district heating and cooling systems with thermal energy storage

Doctoral thesis, 2023

Buildings’ heating and cooling account for more than 20% of the final energy use within the European countries and are dominated by non-renewable resources. Future district energy systems should enable efficient, fossil-free, and economical energy supply at operating temperatures that end users can directly utilize. This can be achieved by lowering the system temperatures and boosting them on the demand side to increase the overall system efficiency. Ultralow-temperature district heating (ULTDH) and bidirectional fifth-generation district heating and cooling (5GDHC) systems are the solutions. However, the transition of district heating and cooling (DHC) systems from current high-temperature configurations to the future solutions is subject to several uncertainties and challenges, such as energy prices, investment costs, thermal energy storage (TES) distribution, and demand profiles. The variations in these uncertainties were not considered in previous studies. Most of the earlier studies only discussed current perspectives, leaving the future applicability of the DHC system unknown.

Hence, a generalized methodological framework combining energy system optimization with stochastic simulations, uncertainty analysis, and sensitivity assessment is developed in this study to investigate the effects of these uncertainties. Based on a variety of stochastic cases, the index named cost-saving probability (CSP) is utilized to reflect the potential of being economic attractive when comparing the energy systems. The preferred future conditions for different DHC systems are summarized in the roadmaps via proposed key performance indicators (KPIs), indicating a future promising area for DHC design. Meanwhile, the applications and roles of TES in future DHC systems were investigated. Furthermore, combined with the geographical information system-based methodologies and data sources, the proposed KPIs for the entire European building stock were calculated at the hectare level to identify the potential areas of 5GDHC.

The results reveal considerable differences between the systems as different design and operation objectives on least cost and imported electricity are set. The most sensitive factors of the CSP are area demand density, overlapping heating and cooling demand, and linear demand density for the transition to ULTDHC, 5GDHC, and individual systems, respectively. The roadmap also shows the hindering factors for different transitions, as well as the impact of the objective on imported electricity. Besides, the sensitivity analysis results reveal TES’s limited role in integrating variable renewable energy (RE) in high-efficiency DHC systems. In addition, less than 0.1% of the current European building stock has sufficient overlapping heating and cooling demands to efficiently implement 5GDHC. These potential areas are primarily found in city centres involving cooling demands from commercial and industrial processes. While a better energy performance of buildings and warmer climate in the future may decrease the heating and increase the cooling demand, the overlapping part is only slightly increased by around 4%, leading to limited additional application potentials of 5GDHC.