The Danish city of Esbjerg has implemented a seawater-sourced district heating system that uses large electric heat pumps to extract thermal energy from the North Sea. During a winter test, the facility took seawater at 3.3°C and cooled it further to 1.2°C while raising district heating water to 90°C. The two heat pumps consumed 19.9 megawatts of electricity and supplied 55.2 megawatts of heat, with the remaining 35.3 megawatts derived from the seawater itself. Testing conducted in May 2025 recorded a coefficient of performance of 2.8, indicating that for each unit of electricity consumed, the system delivered 2.8 units of heat output.
The technology operates by using carbon dioxide as a refrigerant in a high-pressure cycle. The refrigerant absorbs thermal energy from seawater at below-seawater temperatures, then an electrically driven compressor raises the temperature sufficiently for the heat to enter the district network. The installation began operation in late 2024 with a nominal heating capacity of 70 megawatts, replacing a retired coal-fired combined heat and power plant. The recorded performance was lower than original project expectations, and full guarantee testing had not concluded at the time of reporting.
At an illustrative electricity price of €100 per megawatt-hour, the compressor power costs approximately €36 per megawatt-hour of useful heat, excluding seawater pumping, maintenance, capital costs and distribution expenses. Lower-temperature networks generally achieve better performance, as they require smaller temperature increases from the compressor. Esbjerg builds on decades of international experience with seawater heating, following systems in Monaco since 1963 and Stockholm since the 1980s. Other projects include a West Vancouver development using closed-loop heat exchangers and an Aalborg initiative developing 177 megawatts of seawater heat-pump capacity.
Implementing seawater heating systems requires addressing multiple technical and environmental challenges including corrosion, biological fouling, intake and discharge effects, pumping requirements, maintenance access and freezing protection. Environmental assessments must consider impacts on local marine ecosystems and effects of discharging colder water. The existing distribution network infrastructure can present significant financial obstacles in areas lacking pipes or concentrated heat demand. Alternative heat sources such as wastewater, industrial waste heat, ground-source systems or individual air-source heat pumps may offer lower costs at some locations, depending on local conditions and available resources.
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