Articles

Seasonal thermal storage is a key element in decarbonising district heating networks. It can shift excess heat in summer from industrial waste heat, geothermal energy, heat pumps, and solar thermal energy to the winter and make an important contribution to covering the winter load and thus reducing the demand for fuels (see Figure 1).

By Ralf-Roman Schmidt, Senior Research Engineer, AIT Austrian Institue of Technology
Gerhard Totschnig, Research and Development Engineer, AIT Austrian Institute of Technology
Bernhard Mayr, Junior Research Engineer, AIT Austrian Institute of Technology

Published in Hot Cool, edition no. 5/2024 | ISSN 0904 9681 |

This contribution describes the possible role of seasonal thermal storage for decarbonizing district heating networks in Austria based on a concrete case study, where a decarbonization strategy has been developed for the local utility. The analyses show that seasonal storage can be economically feasible if their storage capacity is maximized using heat pumps, low-cost surplus heat is available (i.e., low electricity prices or surplus from geothermal energy), and the costs of biofuels are high. However, some challenges for their integration still need to be overcome, including high space requirements, technology availability, and cheap biofuels.

Figure 1: Top: Temporal mismatch between the demand for heat and the supply of waste heat and other renewable heat sources as well as heat pumps in summer, use of (fossil) fuels to cover the heat demand in winter; bottom: With the help of seasonal
thermal storage, surpluses from the summer are brought into the winter and contribute to covering the heat demand.

Where we started.

In 2022, an Austrian utility aimed to develop an optimized “no regrets” strategy for decarbonizing the heat supply in the whole federal state, including a long-term and holistic view. The Austrian Institute of Technology (AIT) and a German consultancy were asked to provide support related to strategy development and optimization [1] .

The district heating network considered is a medium-sized urban system that is currently supplied by

• 65% natural gas CHP,
• 16% industrial waste heat,
• 11% biomass CHP and
• 8% from peak load heating plants.
• A power-to-heat unit (direct electric) is mainly used to balance energy.

The district heating network has already been undergoing a decarbonization process for the last few years. A new biomass CHP plant is already under construction and will supply 12% when commissioned. By 2024, a share of 40% of renewables was reached in the overall district heating network.

A minimum share of 50% is aimed for 2030, and a 100% renewable heat supply should be reached from 2040 onwards (following the Austrian national targets).

Technologies in the focus.

The decarbonization strategy focused on choosing a suitable heat supply mix, including storage. Besides repowering the existing plants, the possible new plants include:

• a block gas-CHP,
• waste incineration [2] ,
• biomass heat-only boilers,
• one new waste heat source and the extension of an existing waste heat source,
• different large-scale heat pumps (air, water),
• direct electric heaters,
• solar thermal energy,
• deep geothermal energy and
• different kinds of storage (a steel tank and different sizes of PIT storage).

Techno-economic parameters were collected for each technology, focusing on the know-how of the utility since some feasibility studies for individual technologies have been performed. This approach resulted in a realistic data set, although some assumptions remain conservative to avoid overestimating the economic performance.

The world around us.

Together with the utility, important systemic boundary conditions and scenarios to be considered for the development of the transformation pathway were discussed and agreed:

• Energy price scenarios for electricity (including hourly variations), natural gas, biomass, biomethane, and hydrogen, as well as CO2 prices and the availability (i.e., maximum energy provided per year) for each renewable fuel.
• Regulatory framework conditions related to, e.g., the use of biofuels, subsidies, etc.
• A minimum and maximum heat demand expansion scenario with +40% and +100% in 2040 compared to the reference year.

A modelling deep dive.

One focus for developing the district heating transformation strategy was to set up and use an innovative model-based optimisation framework. This included a detailed model of the existing and possible future heat supply plants and storage facilities in the district heating network. The model used can simulate and optimize the district heating system with different network sections and limited transmission capacity in between (see Figure 2).

Figure 2: Modelling the district heat grid

Characteristics of the implemented unit commitment optimization:

• Complex and nested configurations can be modeled (see Figure 3)
• Multiple input and output energy flows per unit are modeled (diverse fuels, electricity, steam, high-pressure heat, low-pressure heat)
• Part load operation considered at all inputs and outputs
• Startup costs, minimum on and off times
• Different plant configurations (steam extraction, backpressure, e.g.)

Figure 3: Modelling of district heat generators with multiple inputs and outputs in a complex system configuration.

Optimizing the seasonal thermal storage integration.

The different heat supply options in the district heating network are considered available in the summertime, resulting in a potential heat surplus. The optimization model foresees different sizes of seasonal thermal storage. A simplified sensitivity analysis has shown that important parameters for their economic feasibility are the specific fuel costs in winter and the delta T between the maximum storage temperature when fully charged and the minimum storage temperature when fully discharged (see Figure 4).

Figure 4: Results of a simplified sensitivity analysis: impact of the delta T in the storage and the specific fuel costs in winter on the return on investment of a seasonal thermal storage.

Since the biomass price scenarios were optimistic, the analysis aimed to increase the delta T to improve the economic performance of the seasonal storage. The model used heat pumps to decrease the storage temperature to 5°C after the winter (see Figure 5). As a result, the used storage capacity is 2.5 times higher compared to using the storage only between the return temperature of 60°C and 95°C.

Figure 5: Mean temperature level in the seasonal district heating storage.

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.

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).

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, ralf-roman.schmidt@ait.ac.at