Past and future merge in Lemgo's pioneering district heating project

5 Feb 2024

The town of Lemgo in northwestern Germany has a city center with half-timbered houses and attractive stone buildings. Nevertheless, officials plan to generate up to 80 percent of the district heating needed for the city from renewable sources – a bold plan made even more difficult by the limitations of Lemgo's historic building stock.

With over 100 years of experience in engineering, including mechanical and plant engineering, GEA knows that the only way to succeed in the fight against CO₂ emissions is to innovate, solve problems and work with customers to overcome barriers to progress.

According to the International Renewable Energy Agency (IRENA), cities are responsible for more than two-thirds of global energy consumption and three-quarters of carbon emissions attributable to human activity. With the rapid development of urbanization, district heating in cities has become one of the most important factors in making cities more carbon friendly. 

District heating is increasingly the preferred solution for new housing developments and public buildings. Heat pumps are often used to extract heat from renewable sources and boost it for use in entire communities, both in residential and public buildings. District heating is most effective in areas with high population density. It offers an environmentally friendly alternative to heating with fossil fuels.

Ambitious plans for the old town of Lemgo

The installation of district heating is most economical if the connection density in existing network areas can be increased. This is not an absolute prerequisite for installation, because if an area is newly developed or if the infrastructure changes, it is also possible to develop these areas with efficient technology. However, it is more difficult to introduce district heating in neighborhoods with historic building stock.

The town of Lemgo in northwestern Germany has a city center with half-timbered houses and attractive stone buildings. Nevertheless, officials plan to generate up to 80 percent of the district heating needed for the city from renewable sources – a bold plan made even more difficult by the limitations of Lemgo's historic building stock. With the help of GEA's innovative technology, the city has been able to significantly reduce its carbon footprint and bring its utilities into the 21st century for good.

The district heating network in Lemgo dates back to the 1960s and now stretches more than 80 km through the city. The original heating network was designed for a flow temperature of around 120⁰C to compensate for the high heat losses in the buildings at the time and the poor design of the radiators in the individual buildings. Since then, heat transfer technology has developed considerably. A unique system for controlling the temperatures in the district heating network makes it possible to reduce the flow temperature to 100⁰C in winter and still heat the entire network sufficiently, even the old historic buildings in Lemgo's city center. 

Important goals for modernization and preservation

Many of Lemgo's buildings are listed and must be preserved for future generations. Interventions in the building fabric can only be carried out to a very limited extent. Lemgo has set itself the goal of reconciling the following objectives for the town center:

• Achieving the goal of CO₂ neutrality (decarbonization).
• Safeguarding the historic building fabric (renovation)
• Preservation of the appearance and attractiveness of the old town for visitors
• Maintaining/increasing the attractiveness of the historic old town as a residential environment (activation of vacancies/vacancy register)

The main strategies were, on the one hand, to make the heat supply renewable through district heating, and, on the other, to put vacant properties in need of renovation to new uses. In addition, climate impact adaptations in the city center will follow the district heating project as part of ongoing urban development projects. 

Doubly efficient energy procurement

Following GEA’s successful heat pump projects for district heating systems at several established sites – including at a sewage treatment plant in Malmö and above a subway station in London – the city of Lemgo approached GEA to supply two heat pumps as part of a system to heat the old town in a modern way. 

One heat pump extracts heat from the municipal sewage treatment plant, and a second heat pump uses otherwise unusable waste heat from a large CHP plant as a heat source. The heat generated by the large heat pumps is sufficient to supply the entire historic city center of Lemgo with low-carbon heat. That is around 18 million kilowatt hours of heat per year, which saves 3,200 tons of CO2 emissions per year. 

Highly efficient heat conversion

District heating technology continues to be a proven solution for numerous locations around the world. In the case of Lemgo, the biggest gains come through the new operating strategy, which automatically controls and regulates the system. The strategy ensures that the heat pump can operate constantly for up to 7,500 hours per year to maximize benefit. Heat is extracted from the wastewater after the treatment stages in the sewage treatment plant. In this way, contamination of the heat exchangers is avoided. The wastewater treatment process is also not impaired by the heat extraction. 

To achieve the global goal of complete decarbonization by 2050, it is important to find ways to generate heat without burning fossil fuels. Lemgo is a good example of not only decarbonizing heat but also doing so with the best possible technology to minimize the amount of electricity needed.

Once again, technology for a better world

GEA's modern, efficient ammonia heat pumps live up to their claim of "engineering for a better world" and bring the district heating concept to its best possible conclusion. Combined with a small supply of renewable electricity, the possibilities are enormous, as the technology is able to generate heat from a variety of sources, including the air, the ground, a factory, a subway network, a water treatment plant and even old mining tunnels. This source heat can then be boosted to the desired level to provide heating and hot water directly to neighborhoods and public facilities in a cost-effective and completely clean way. 

By replacing fossil fuel boilers with heat pumps, GEA can help cities become more resilient by reusing different waste heat sources for different heating purposes, while at the same time significantly reducing CO₂ emissions. Operating heat pumps with natural refrigerants such as ammonia further ensures an ozone- and climate-friendly future.