"Picture book project for our heat supply"

18.11.2025

Source: E & M powernews

How Gothaer Stadtwerke Energie is advancing the heating transition with an iKWK system.

By combining three technologies, Gothaer Stadtwerke Energie is leading its city towards a climate-neutral heat supply: by linking a combined heat and power plant (CHP), air heat pump and power-to-heat. "With the commissioning of our iKWK plant, we are reliably supplying heat from renewable energy sources," explains Sven Anders, Managing Director of Gothaer Stadtwerke Energie. Ferdinand von Stryk, Head of Generation and Heating Network at the municipal utility, explains how the innovative system, which is intended to drive the green transition in Gotha, works as follows: "Heat pumps as base load units and gas engines whose peak load replaces gas boilers in winter."

The Thuringian municipality's ambitious goal is to generate 100% of district heating in Gotha in a climate-neutral way by 2045. The municipality was awarded funding for an innovative combined heat and power system by the Federal Network Agency in 2022. The system has recently been in operation. "The iKWK system is a picture-book project for our heat supply. The heat pump runs in summer and is very efficient during this time. When the heat load is high in winter and the electricity price is high, the CHP secures the heat supply," says von Stryk.

Centerpiece of the local heat transition

The new plant is located in the Siebleben district, one of the three district heating supply areas of the municipal utilities alongside "Gotha-West" and "Gotha-Mitte". The CHP module has an electrical output of 2 MW and a thermal output of 2.2 MW and was supplied by Innio Jenbacher. The air heat pump has a thermal output of 1.3 MW and is manufactured by Johnson Controls. The power-to-heat system is designed for 1.2 MW. The technology reduces annual CO2 emissions by around 230 tons. The construction time for the plant was 16 months. Gothaer Stadtwerke Energie estimates the investment volume at 5.2 million euros.

The air heat pump can supply around 400 households. "We operate it from an outside temperature of 10 degrees Celsius," says von Stryk. The COP (coefficient of performance) of the heat pump, which uses ammonia as a refrigerant, is then 2.5. When it is around 30 degrees outside, he assumes a performance factor of 3. "This is confirmed by our measurement results," reports von Stryk.

The fact that the heat pump has an output of 1.3 MW is a result of the funding criterion, according to which the green heat from the CHP plant must account for more than 35 percent of the reference heat. An upper limit of 3,500 full load hours per year applies to the funding of CHP units - this results in the reference heat quantity. To arrive at the required heat pump output, the planners looked at the number of hours per year when the temperature reaches the ten-degree limit.

"Then you add a safety buffer, for example 70 percent of these hours, and multiply them by the output to arrive at the required thermal energy," explains von Stryk. "With 1.3 MW, we can ensure that we can generate the 35 percent share with a very high probability at times when the outside temperature is above 10 degrees Celsius."

The municipal utility company controls which technology is used when with the support of Munich-based service provider VK Energie. "The latter maps our heating system technically and in terms of energy management in a mathematical model," explains von Stryk. An AI is used to create a daily district heating forecast based on weather data. This is used to calculate the schedule for the following day. The iKWK plant runs automatically. The employees at the power plant monitor the operation and intervene if necessary.

Sophisticated planning

The starting point for the Gotha iKWK project was the decision by Gothaer Stadtwerke Energie to merge two heating networks. With a pipeline between the city center network and the network in Siebleben, they opened up further areas for district heating supply and a plot of land that could be used for renewable energies. "From the very beginning, the municipal utility's aim was to actively help shape the path towards a climate-neutral heat supply and thus take responsibility for the region," emphasizes Managing Director Sven Anders.

What the "i" should stand for in the iKWK plant was initially an open question. Various options were examined. Geothermal energy was ruled out for cost reasons, solar thermal energy was not implemented due to higher costs and lower efficiency. The air-source heat pump won out due to its efficiency and investment costs.

Gothaer Stadtwerke Energie implemented the system itself. The threads came together with an in-house project manager. "It was important for us to have a central point of contact for the contractors and planners who could coordinate everything," says von Stryk. The company Jander Energietechnik in Tautenhain helped with the planning and assisted the municipal utilities with the tendering process. "We planned the system together with the engineering office and then awarded the contract for the execution in individual lots."

The CHP unit is operated in on-off mode, always at least two hours after it has been switched on. The unit is cascaded with the other CHP units of the municipal utilities - ten in total. The fuel is natural gas. Hydrogen is not yet economically and infrastructurally established. The hydrogen core network is scheduled to run along the A4 highway north of Gotha in 2028. But then it still needs a connection to the city. "In the long term, our transformation plan for our heating network envisages a gradual switch to hydrogen," explains von Stryk. The plan is to initially convert part of it - "up to 18 percent" - to hydrogen in 2040.

Gotha's heat transformation is progressing

Elsewhere, the transformation is still underway. The company has invested around 5 million euros in the remotorization of the West cogeneration plant. Two new engines with 10 percent more power have been procured. A third engine is due to follow in 2027. In addition, Stadtwerke will start building a heat storage facility at this location next year. This should be able to bridge "several days" in summer. During the transitional period in spring and fall, it should be sufficient to cover Gotha's heating requirements from Friday evening to Monday morning.

The district heating network of Gothaer Stadtwerke Energie measures 58 kilometers. By 2045, the supply is to be completely converted to green heat. The iKWK plant not only benefits climate protection, but also ensures price stability: according to the municipal utility, the flexible use of different energy sources makes it possible to make better use of renewable energies, reduce gas consumption and cushion price fluctuations on the energy markets.

Author: Manfred Fischer