Heat from Wasser
Water heat pumps: How rivers, lakes and seas are driving the heat transition
While significant progress has already been made in the use of renewable energy for electricity generation, the heating sector is still lagging behind. The majority of buildings in Europe are still heated with fossil fuels such as natural gas or heating oil. Without a heating transition, there can be no successful climate transition. To achieve climate goals, climate-neutral heat supply is becoming increasingly important — it should be achieved no later than 2045.
In addition to political determination, innovative technological solutions are needed that are both ecologically sound and economically viable. Among these solutions are large-scale heat pumps. A study by the consulting firm McKinsey estimates the total market for large-scale heat pumps to exceed € 43 billion by 2030. Heat pumps are considered large when they reach a capacity of over 100 kW. In particular, large heat pumps for district heating networks offer new opportunities for the sustainable heat supply of urban areas and industrial infrastructures.
Heat pumps primarily use environmental heat from the ground, air or groundwater to heat buildings. However, the heat source of surface water has so far been underutilized. Large heat pumps that use seawater or river water as a heat source could become a promising component of the heating transition.
Harnessing thermal energy from surface waters
Heat pumps that use natural bodies of water as a heat source operate on the same physical principle as a refrigerator — only on an industrial scale. Sea, lake, or river water serves as a low-temperature environmental heat source.
Compared to ambient air as a heat source, water has about four times higher specific heat capacity. With the same mass flow and cooling, a river water heat pump can thus achieve four times higher heating output. Even in winter, the more stable temperatures of water sources allow for more efficient and steady heat extraction. Even at water temperatures of just 2–15 °C, heat pumps can efficiently extract heat from water.
This process uses a closed refrigerant circuit, a compressor, and a heat exchanger. Water is directed through the heat exchanger, where its heat is transferred to a refrigerant, which then evaporates and is compressed in the heat pump. The resulting higher temperature is fed into the heating system, causing the refrigerant to cool down and restart the cycle.
European projects for water source heat pumps
Many European port and coastal cities have well-developed district heating networks, into which large-scale heat pumps can be efficiently integrated.
Port and Coastal Cities:
- Oslo: Large heat pump plant in the Oslofjord, over 100 MW thermal capacity.
- Stockholm: Värtan heat pump with over 180 MW thermal output from seawater.
- Marseille: “Thassalia” project — marine geothermal plant for heating and cooling from the Mediterranean.
- Copenhagen: Integration of seawater heat pumps by 2025 as part of the climate neutrality strategy.
- Esbjerg (Denmark): Largest seawater heat pump in the world, two systems with 70 MW heating capacity for 25,000 households, saving around 120,000 t CO₂ annually and replacing a coal-fired power plant.
Rivers and Lakes:
- Zurich: River water from the Limmat is used by the Zurich municipal utility (ewz) to supply entire city districts with heat.
- Basel: Rhine water heat pump of the Industrial Works Basel (IWB) for residential areas.
- Lyon: “Énergie des Fleuves” project uses Rhône water for public and private buildings.
- Vienna: Study on the use of the Danube for future heating strategies.
- Mannheim: Integration of a large-scale heat pump into the district heating network by MVV Energie AG.
- Cologne: Planned largest river water heat pump in Europe for 50,000 households, saving 100,000 t CO₂ annually, with a delivery temperature of 110 °C and an efficiency of 1 kWh of electricity → 3 kWh of heating.
Wastewater Projects:
- Hamburg: Planned wastewater heat pump with 60 MW capacity for 39,000 households, using wastewater temperatures of 12–22 °C.
Challenges for water source heat pumps
Despite their many advantages, there are several challenges to consider when using water source heat pumps.
Technical Requirements:
Seawater is particularly corrosive due to its salt content. Therefore, all materials and system components used must be appropriately protected.
High Investment Costs:
The development of water sources, the construction of heat exchangers and pipelines, and the operation of these plants are cost-intensive. Economic profitability is typically achieved only over long operating periods.
Approval Processes and Environmental Regulations:
Especially in urban areas, obtaining permits is often complex and lengthy. Environmental requirements, such as the protection of flora and fauna, present additional challenges.
Operational Maintenance:
Heat exchangers must be cleaned regularly depending on the water quality to maintain the system’s efficiency.
Temperature Management:
The returned water must not negatively impact the temperature balance of the body of water and needs to be properly regulated.
Potential of water source heat pump technology
Water source heat pumps extract around two-thirds of the required energy directly from the environment. Only one-third of the generated heat needs to be supplied through electricity — ideally from renewable sources. This makes climate-neutral heat supply a local reality. Studies show that, theoretically, up to 25 percent of Europe’s heat demand could be met by large-scale heat pumps.
Water source heat pumps play a particularly central role as they operate independently of the time of day and weather conditions and ensure a high level of supply security. Sea, lake, and river water heat pumps make it possible to integrate locally available and often untapped heat sources into urban energy systems.
They stand out for their operation, which is independent of weather and time of day, and their high supply reliability. In combination with existing district heating networks, storage solutions, and renewable electricity from wind and solar, intelligent, climate-friendly heating solutions can be created.
Water source heat pumps can contribute not only to climate neutrality but also to some extent promote ecological resilience. In times of increasing heatwaves, which reduce oxygen levels in rivers and threaten fish populations, targeted heat extraction from water could have a stabilizing effect on ecosystems.
Conclusion: Water Source heat pumps as a key technology for the heating transition
It’s a topic that surprises many: how can seemingly “cold” water, with perhaps only 10 °C, still provide heat that’s useful for a district heating network? The answer lies in heat pump technology and the principle of relative temperature differences.
Lake and river water heat pumps are no longer a theoretical vision. They are proven in practice and environmentally friendly. Given growing cities, rising energy prices, and ambitious climate goals, they are an essential building block for the heating transition.
Doris Höflich, Market Intelligence Senior Expert
Sources:
- Bundesverband Wärmepumpe (BWP)
- ingenieur.de
- VDI
- Utopia
