Cold Seawater Powers District Heating in Esbjerg, Denmark With Large Electric Heat Pumps

Esbjerg, Denmark, is using large electric heat pumps to extract heat from the North Sea and supply its district-heating network. During a two-hour winter test, seawater entered the system at 3.3°C and left at 1.2°C. The two heat pumps consumed 19.9 MW of electricity and supplied 55.2 MW of heat at a district-heating water temperature of 90°C. The difference, 35.3 MW, came from the seawater. The project team’s May 2025 operating report recorded a coefficient of performance, or COP, of 2.8. The figures describe that test, not annual average performance.

A heat pump transfers thermal energy from a colder source to a warmer destination. Its refrigerant absorbs heat at a temperature below that of the seawater, and an electrically driven compressor raises the temperature so the heat can enter the district network. Esbjerg uses carbon dioxide as its refrigerant in a high-pressure cycle. For each unit of electricity used by the compressors during the test, the plant delivered 2.8 units of heat. Resistance heating would have required 2.8 times as much electricity for the same heat output under those conditions.

The installation has a nominal heating capacity of 70 MW and began delivering heat in late 2024, as part of Esbjerg’s replacement of its retired coal-fired combined heat and power plant. The operating report describes controls for variable conditions and protection against freezing as seawater temperatures fall. The recorded COP of 2.8 was lower than the more favourable performance figures used in the original project expectations, and full guarantee testing had not been completed when the results were published. The city’s heating supply also includes a wood-chip boiler and waste-incineration heat. Emissions reductions reported for the wider replacement programme cannot be attributed in full to the seawater heat pumps.

Electricity price and heating-network temperature have a direct effect on the cost. At an illustrative electricity price of €100/MWh, compressor power at COP 2.8 costs about €36 per MWh of useful heat. That excludes seawater pumping, equipment maintenance, capital recovery, financing and heat distribution. A network able to operate at lower water temperatures generally asks the compressor to make a smaller temperature increase, which can improve COP. Older networks supplying 90°C heat face a harder operating requirement than newer, lower-temperature networks.

Esbjerg builds on decades of seawater heating experience. Monaco began using seawater heat pumps in 1963, and Stockholm has used large seawater-sourced district-heating equipment since the 1980s. Creative Energy’s Horseshoe Bay development in West Vancouver takes a different approach. Submerged heat exchangers transfer heat through a closed loop, with no seawater intake or discharge to the marina. Denmark’s Aalborg project is developing 177 MW of seawater heat-pump capacity. These installations serve different loads and use different marine arrangements, so their costs cannot be assumed to match Esbjerg’s.

Marine heat systems require more than suitable water temperatures. Engineers must address corrosion, biological fouling, intake and discharge effects, pumping requirements, maintenance access and freezing protection. The environmental assessment has to consider the local marine ecosystem and the discharge of colder water. The distribution network can be the larger financial obstacle in areas without existing pipes or concentrated heat demand. Wastewater, industrial waste heat, ground-source systems or individual air-source heat pumps may provide lower-cost heat at some sites.

Hot-water storage can add value by allowing a utility to make some heat when electricity is inexpensive and deliver it later. The available flexibility depends on storage capacity, network temperatures and heat demand. Esbjerg’s test provides direct evidence that cold seawater can support high-temperature district heating. Coastal utilities can use that evidence when comparing local heat sources, network investments and the costs of replacing combustion-based heating.

My TFIE Strategy Briefing assessment examines Esbjerg’s operating results, experience in Stockholm and Monaco, the Horseshoe Bay system near Vancouver, and the cost and environmental factors that determine where seawater heating is suitable.