Figure 2: Modelling the district heat grid
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 CO 2 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). 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.)
1 For reasons of confidentiality no details related to the concrete district heating network and its strategy can be provided. 2 The waste from the city is currently moved to an industrial facility for waste incineration and generation of process heat.
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