Digital twins pave the way for sustainable heating

Over 50% of final energy consumption in Germany is accounted for by heating for buildings and heat for industrial processes (1). A sustainable heating supply is thus essential in order to meet the national climate targets. District heating networks are an effective means of improving overall energy efficiency in urban areas and increasing the percentage of carbon-neutral energy in the heating supply.

The municipal utility company of Hennigsdorf in Brandenburg is continually expanding the district heating network for the town’s 26,000 residents. The network is notable for covering a large geographical area stretching from north to south. Both the town and its district heating network have grown continuously since the first boilerhouse was built in the 1960s. Over time, the original raw lignite fuel was replaced first by bituminous coal, then by heating oil and eventually by natural gas. Today, around 80% of the town is provided with district heating via seven central heating plants. The individual supply areas have been linked up to form an interconnected network. The district heating network operates with flow temperatures of between 85 °C and 108 °C and a return temperature of approximately 60 °C (2).

A whole-system strategy for a sustainable transformation

In 2015, Hennigsdorf town council adopted a framework climate strategy to provide the town with sustainable district heating. Taken long before the introduction of statutory municipal heat planning and district heating network transformation plans, this decision put Hennigsdorf and its municipal utility company at the forefront of the transition to sustainable heating. Together with Tetra Ingenieure GmbH, Ruppin Consult GmbH and the Steinbeis Research Center Solites, the Hennigsdorf municipal utility company went on to develop an integrated strategy that increased the percentage of carbon-neutral heat in Hennigsdorf’s district heating system from 50% to over 80% (2).

The strategy comprises several different elements:

• use of waste heat from the local steel mill,
• operation of an existing biomass cogeneration plant,
• renovation of a solar thermal array
• linking up the supply areas to form an interconnected heating network,
• expansion of new sections in the district heating network,
• reduction of flow temperature to a maximum of 95 °C,
• upgrading of existing district heating plants/construction of new ones,
• optimization of customer systems and building connection units in order to reduce return temperature and improve efficiency,
• intelligent overarching whole-system control, and
• integration of two new steel heat storage tanks with a water volume of 1,000 and 5,000 m³ respectively.

Now operating as an open, interconnected network, the district heating network must be able to handle high heat transfer rates.

The digital twin: simulations reduce the risks

The long-established heating supply structure in Hennigsdorf and the incorporation of new, carbon-neutral, decentralized heat sources into the existing system are a major challenge in terms of the planning and implementation of a strategy like this. Unlike district heating networks with a central, fossil fuel-powered and easily controllable heat source, several different components and the factors they depend on must be taken into account in the Hennigsdorf network.

The fact that the heat sources are decentralized, with some located on the outskirts of the town, means that heat must be transported to buildings from different directions than was previously the case. Moreover, heat sources such as industrial waste heat, large solar thermal arrays and a biomass cogeneration plant that also generates electricity at the same time (using the ORC process) all have their own specific characteristics and cannot always be controlled to meet demand whenever required. For instance, it is the industrial process that determines waste heat availability in terms of time, output and temperature, and this won’t necessarily correspond with demand from the district heating network.

It is in cases like this that the use of digital twins makes it possible to examine several different versions of the overall strategy before final planning and implementation. Thorough digital checking and testing of the whole system reduces the investment and operational risks and enables predictable operating performance and coordinated control. The digital twin replicates the district heating network and all the heat sources and storage tanks in either a single or multiple simulation programs.

The Steinbeis experts produced models of Hennigsdorf’s heat generation and storage in the dynamic system simulation program TRNSYS and carried out a number of simulation studies. Meanwhile, planning consultants Tetra Ingenieure GmbH modeled the district heating network’s hydraulics in Bentley sisHYD and simulated several different versions and loads. The network hydraulics simulations identified mass flow limitations in individual pipe sections, and this information was fed into Solites’ system simulations.

The digital twin helped to analyze different variables such as the amount of waste heat available with operating times, outputs and temperatures, different solar thermal array and heat storage tank sizes, seasonal operating times of the biomass cogeneration plant, future heat demand for the district heating network, etc. The TRNSYS simulations were performed with a time step of ten minutes over a simulation period of three years. Only the results for the third year were analyzed, since it takes that long before a realistic temperature distribution is reached in the heat storage tank. The simulation’s goals were as follows:

• at least 80% of the total heat supply should come from renewable heat with the lowest possible investment costs and optimal cost-effectiveness,
• optimal utilization of the variable industrial waste heat supply,
• optimization of the biomass cogeneration plant’s operating times and efficiency,
• ensure a reliable heat supply for all customers,
• development of the overarching control strategy, and
• determine optimal size of the heat storage tanks and possible additional renewable heat sources such as solar thermal arrays.

In order to determine the optimal size of the multifunctional heat storage tank, different storage capacities ranging from 500 to 150,000 m³ were simulated, in each case for three different waste heat profiles. Since the industrial waste heat is available year round, a large multifunctional heat storage tank was designed to enable distribution of the industrial waste heat – which undergoes short-term supply fluctuations – to the district heating network, as well as seasonal storage of the solar thermal output. Based on these requirements, the initial dynamic system simulations of different alternatives with multiple assumptions resulted in storage capacities of up to 150,000 m³ and a large solar thermal array.

From strategy to implementation

The measures set out in the strategy were progressively implemented from 2017 on while the district heating network continued to operate. Once the waste heat extractor had been installed at the steel mill in 2019, it was possible to include the first measurements and detailed analyses of the amount of waste heat generated and how it varied over time in the digital twin. However, this happened in 2020 and 2021 when, due to the pandemic, there were times when the steel mill was not operating because its personnel were on furlough. Consequently, in order to prepare for the next construction phases, the digital twin was used to simulate several alternatives that extrapolated the amount of waste heat expected to be generated during normal operation. The measured outputs, times and temperatures of waste heat that could be used in the district heating network provided invaluable data for validating and optimizing the size of the heat storage tank.

More detailed analyses of the geographical distribution of heat consumption and the renewable heat sources resulted in the construction of a steel heat storage tank with a water volume of 1,000 m³ next to the heating plant in the town center. The opportunity was also taken to optimize operation of the integrated biomass cogeneration plant and hydraulics.

However, it was decided not to install large solar thermal arrays, since there was not enough space for a seasonal heat storage tank and the priority was to utilize the waste heat from the steel mill. The aboveground multifunctional steel heat storage tank with a water volume of 5,000 m³ was completed in 2024 (see also Steinbeis TRANSFER magazine 03|24 p. 70).

Now that the planned measures have been implemented in the Hennigsdorf district heating network, the digital twin can still be used for operational optimization and to assess the risks of any future changes – for example if it is decided to move to 100% carbon-neutral heat sources or replace some of the existing heat generators.

[1] Umweltbundesamt. Energieverbrauch privater Haushalte. Online, retrieved on 29.09.2025. Available at: www.umwelt-bundesamt.de/daten/private-haushalte-kon-sum/wohnen/energieverbrauch-privater-haushalte#endenergieverbrauch-der-priva-ten-haushalte
[2] Stadtwerke Hennigsdorf. Online, retrieved on 29.09.2025. Available at: www.stadtwer-ke-hennigsdorf.de/fernwaerme