German energy utility company N-ERGIE is expanding its district heating network with a 15 to 20MW (4,265 to 5,686TR) isobutane (R600a) heat pump that will supply heat to roughly 5,000 homes in Nuremberg using heat from treated wastewater.
In 2024, the Nuremberg Department of Sewage and Environmental Analysis (SUN) treated 71 million cubic meters (18.7 billion gallons/70.7 billion liters) of wastewater at the city’s treatment plant on Adolf Braun Street. The treated water is discharged into the Pegnitz River, with a consistent temperature between 13 and 20°C (55.4 and 68°F). With the treated wastewater as a heat source, the heat pump will produce up to 120°C (248°F) hot water for the district heating network.
“The wastewater treatment plant is one of the city’s largest energy consumers,” said Britta Walthelm, City Councillor for Environment and Health and First Plant Manager of SUN. “By utilizing the excess heat, we are laying a central building block toward our goal of a climate-neutral city and wastewater management.”
The project, initiated in 2021, was approved by the Nuremberg city council in April, with engineering consultant Sweco commissioned to provide planning services for system components and infrastructure. N-ERGIE expects construction on the project to begin in 2026, with system startup in mid-2028.
Currently, the Nuremberg district heating network serves 56,000 households, with only 30% of the heat generated from non-fossil fuel sources, according to Mayor Marcus König. In the coming years, N-ERGIE and the city administration seek to expand the heating network in a “gradual carbon-neutral manner” to supply heat to 100,000 households.
The details: The hydrocarbon heat pump project, with an estimated cost of €40 million ($46.3 million), aims to supply 3‒4% of the overall district heating in Nuremberg.
- With a continuous electric consumption of around 7MWh, the R600a heat pump will feed 150GWh of heat into the district network annually with a COP range between 2.1 and 2.9, according to N-ERGIE.
- The heat pump will be housed in a 22 × 21 × 13m (72 × 69 × 43ft) building to be constructed directly behind the final clarification stage of the wastewater treatment plant.
- The project requires laying pipe and electrical lines over roughly 1km (0.6mi) to connect the wastewater treatment plant to the Muggenhof heating plant where the hot water will enter the district heating network.
Another benefit: The heat pump will also reduce the temperature of the water released to the Pegnitz by 3°C (5.4°F).
- Over the past 20 years, a Nuremberg water protection group has reported a steady increase in the Pegnitz’s water temperature, especially in the summer months, according to Walthelm.
A new trend: An increasing number of European cities are turning to hydrocarbon heat pumps for district heating.
- Solihull, a town of 126,000 in England has installed six Refra hydrocarbon heat pumps to provide 1.6MW (455TR) of district heating and hot water to several public buildings.
- Six Fenagy isobutane/propane (R290) heat pumps will supply more than 30MW (8,530TR) of heat to a Finnish city’s district heating network using waste heat from a local data center, with commissioning set for fall 2025.
Quotable: “Heat pumps connected to wastewater treatment plants with river discharge represent a win-win situation for people and nature,” said Klaus Strobl, Project Manager for Energy Plants at Sweco. “On the one hand, the cooled discharge water prevents river warming from the wastewater treatment process, which benefits the flora and fauna. On the other hand, people reuse the recovered waste heat.”
“Heat pumps connected to wastewater treatment plants with river discharge represent a win-win situation for people and nature.”
Klaus Strobl, Project Manager for Energy Plants at Sweco
