Figure 6: top: decarbonized district heating system with an optimal storage size, bottom: decarbonized district heating system with a limited storage size (0,5 days of average annual heat demand)
We love optimizing For developing a robust and sound strategy, the model- based optimisation framework was used for the investment optimisation based on mixed integer investment and unit commitment optimization considering simplified plant operational optimisation. The optimization runs included the years 2026, 2030, 2035, and 2040. To improve the optimisation performance, the number of weeks per analysed year and the time resolution within the simulation weeks were carefully selected. The object function was to minimize the total discounted costs. Various scenarios and strategy variants were analysed, focussing on different energy price scenarios and restrictions on the generation side. Coming to the results The importance of seasonal thermal storage for the decarbonization of district heating systems was analyzed by allowing the optimization model to choose the optimal storage size and comparing it with a case where only a limited storage was allowed. Figure 6, top, shows the hourly district heat generation in a completely decarbonized system in 2040 without storage size limitations. In winter, the CHP operates at times with high electricity prices and high heat demand. In the summer, the heat pump and the geothermal heat supply more heat than is needed. This heat is stored in the seasonal heat storage. In winter, the storage is discharged, and the gray areas are the heat supplied from heat storage. The storage size chosen by the optimizer is in the range of a small soccer stadium, which equals a storage capacity of about 26 days (calculated with supply and return temperatures) of the annual average heat demand. In Figure 6, bottom, the storage size is limited to 0.5 days of annual average heat demand. Here, the overall costs were about 2% higher, 21% less geothermal energy could be used, the heat pump capacity decreased by 60%, and its heat generation by 84%. A large amount of biomass is used mainly in the winter (+400%), with questionable availability.
Overcoming challenges Although the utility has recognized the potential of seasonal thermal storage, there are a couple of barriers to its implementation. Seasonal thermal storages have high space requirements, which is particularly difficult to realize in urban areas, together with possible acceptance problems. Further, additional heating network infrastructure may be necessary, boosting the investment costs. Another challenge is the availability of technology, i.e., the largest storage so far has 0.2 million m3 and lower temperature levels than required in many urban district heating networks. Biofuels represent the main competitor since the availability, especially of solid biomass in Austria, is relatively good, and projections for its prices and availability are optimistic. Geothermal energy could be an important source of energy for charging storage. However, the exploitation risk is considered a major challenge. Alternatively, air-source heat pumps are particularly interesting when combined with seasonal storages and benefit from high source temperatures together with low electricity prices in the summer. However, they face challenges related to the district heating network temperature levels and the required electrical network capacities, especially for large-scale installations. What’s next? The implementation process of the strategy is still ongoing, and some implementation steps require an additional detailed analysis. Also, updates to the strategy are foreseen, especially considering changes in the global energy market. However, changes in the management or the political framework can delay further steps. Also, the utility tends to be conservative related to the heat supply, i.e., focusing on supply security, including market-ready and well-proven technologies, particularly those already applied in similar district heating systems. On the positive side, Seasonal thermal storage is an ongoing object of interest in Austria, i.e., a recently started research project focuses on Cavern Thermal Energy Storage in Crystalline Rocks.
For further information please contact: ralf-roman.schmidt@ait.ac.at
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