A cruise-ship terminal on Copenhagen’s outer waterfront draws its heating and hot water from a pump a few piers away. That pump is named FlexHeat: an 800-kilowatt, two-stage ammonia heat pump that the public utility HOFOR built in 2018 in Copenhagen’s Nordhavn district, feeding a local district heating grid that, by one count, warms four cruise-ship terminals and a large warehouse nearby.
Ammonia is the refrigerant doing the actual work inside the machine, carrying heat from cold water up to a temperature district heating pipes can use. Heating from underground is not unique to Copenhagen: a Dutch pepper grower drilled for geothermal heat to stop burning gas, at the scale of a greenhouse rather than a harbor. According to a case study published by the International Energy Agency’s Heat Pumping Technologies programme, FlexHeat was built as a demonstrator inside EnergyLab Nordhavn, a research project that used the newly built harbor district as a testbed for integrated energy systems.
Nordhavn itself is a former industrial harbor that Copenhagen has spent the past decade rebuilding into housing, offices and the cruise terminals FlexHeat now warms, which is why the city and HOFOR picked it to test a heat pump built to do more than one job at once.
Turning groundwater into heat for a cruise terminal
The heat itself starts underground. HOFOR pumps brackish groundwater from a well roughly 150 meters below Nordhavn, water that arrives at the surface around 10.5 degrees Celsius. That water runs through a heat exchanger with ammonia as the refrigerant, and two compression stages lift the temperature high enough to feed the district heating network. Two electric boilers of 100 kilowatts each and a 100-cubic-meter hot water tank back up the heat pump, and HOFOR treats that tank as a kind of thermal battery, storing roughly four megawatt-hours of heat that the plant can draw down instead of firing the boilers.
That storage is what turns FlexHeat from a simple heater into a flexible one. Instead of running at a fixed output, the plant can shift when it draws power from the grid and when it leans on stored heat, a design choice that matters once a heat pump is asked to do more than just keep terminal buildings warm. Denmark’s grid leans heavily on wind, which means supply swings with the weather rather than with demand, and a heat pump that can pause or speed up on short notice gives the grid operator a tool it would not get from a boiler that only knows how to burn at a fixed rate.
Teaching the pump to answer the grid in 90 seconds
In December 2019, HOFOR’s engineers got that flexibility down to a sharp number. The heat pump’s response time to a grid signal dropped from around four minutes to 90 seconds, comfortably inside the 150-second threshold the Danish grid operator Energinet sets for providers of fast frequency reserve, according to an account of the project by the Danish outlet CT Watch. Tore Gad Kjeld, HOFOR’s energy planner on the project, has said the pump responds even faster than the grid actually requires.
The pump also runs efficiently once it is working: at a supply and return temperature of 80 and 45 degrees Celsius, its coefficient of performance reaches 3.28, meaning it delivers more than three units of heat for every unit of electricity it consumes. HOFOR has paired that efficiency with algorithm-based scheduling that already cut the cost of producing the heat by about 6 percent, with a further 7 percent in view once Energinet formally approves the plant for grid services, for roughly 13 percent in total.
What the flexibility is worth, according to Danfoss
Danfoss supplied components for the project and has published its own case story on FlexHeat. According to Danfoss, running the plant on electricity instead of an LPG boiler, the fossil-fuel alternative HOFOR compared it against, avoids about 315 tonnes of carbon dioxide a year. The company’s estimate climbs to roughly 430 tonnes once the electricity drawn by the pump comes entirely from renewable sources on the Danish grid, a threshold the plant has not yet reached but that tracks the pace at which Denmark has been adding wind and solar capacity.
Those savings sit on top of the fuel switch itself: a heat pump drawing on groundwater and grid electricity replaces a boiler that would otherwise burn gas to keep four cruise-ship terminals and a warehouse at temperature through a Danish winter.
A demonstrator that outlasted its own research project
FlexHeat was built inside EnergyLab Nordhavn, a twelve-partner consortium led by the Technical University of Denmark that ran from 2015 to 2019, with DTU, the City of Copenhagen, HOFOR, Danfoss and eight other organizations testing how electricity, heating, efficient buildings and electric transport could work as one system in a single harbor district. The consortium itself wound down its activities in 2021. The heat pump it produced did not: FlexHeat remains HOFOR’s own asset, built into the utility’s district heating grid rather than retired alongside the project that funded it.
That distinction matters for what Nordhavn actually shows. FlexHeat is not a template HOFOR has rolled out across Copenhagen, and nothing about an 800-kilowatt plant warming four terminals scales up automatically to the rest of a city’s district heating network. What it demonstrates, piers away from the cruise terminals it quietly warms each season, is narrower and more concrete: a heat pump built to answer the grid as fast as the grid can ask, still running years after the research project that built it has closed its books, in a harbor district that is still being built out around it.
Photo : stephan_deutsch, Pixabay Content License
Photo : wilhei, Pixabay Content License