A novel district heating system – comprising two air-to-water heat pumps from Danish OEM Fenagy, a hot-water storage tank and an electrical hot-water boiler – has been installed this year in Denmark to expand, and often replace, the heating capacity of an existing biomass boiler, cutting heating costs and helping to balance the electrical grid.
The district heating facility, known as Sønder Felding Varmeværk, serves 750 households in the small village of Sønder Felding, located in the countryside in Jutland, Denmark.
Ebbe Nørgaard, Energy Sector Sales Director at Fenagy, outlined the system at the ATMOsphere (ATMO) Europe Summit 2023, which took place September 19–20 in Brussels, and was organized by ATMOsphere, publisher of R744.com.
“Fenagy’s heat pumps are balancing an electrical grid, integrating a big thermal storage tank, and expanding the existing biomass boiler capacity,” said Nørgaard, adding that this is reducing energy cost by 59% in every MWh of heat.
The installation comprises two CO2 heat pumps (model H1800-AW) with a total heating capacity of 3.5MW (995.2TR) and a COP of 2.96 at an ambient temperature of 0°C (32°F). The large thermal storage tank has a volume of 3,500m3 (123,601ft3) and can deliver 163MWh of heat energy, equivalent to 55MWh of electrical energy. The electrical boiler has a capacity of 10MW (2843.4TR).
In addition to this capacity, the district heating facility still has a biomass boiler. However, “after the expansion of this facility with Fenagy’s heat pumps, the owner of this facility has not operated the biomass boiler since March,” said Nørgaard.
The electric boiler can be used when electrical demand is minimal and the cost of electricity is almost free (or the facility is being paid to consume excess electricity). At other times, the more efficient heat pumps are used.
This CO2 heat pumps have a unique glycol defrosting method. This secures stable operation and production of heat even during defrosting. “The defrost method is unique and patent pending and cannot be compared with defrost methods from any other known application,” said Nørgaard.
Balancing the grid
Nørgaard highlighted that the importance of the large thermal storage, which allows the system to start or stop depending on electricity prices and help balance the electrical grid. “That is worth a lot of money” for the owners of the heat pump and for the transmission system operators (TSO), who transport electricity in the region, said Nørgaard.
“Having these larger capacities, the heat pumps and storage can store a lot of energy, which provides flexibility to run turn on power during low electricity price and shut down at high electricity prices,” said Nørgaard. There is a lot of energy in the grid, and it’s challenging to balance supply and demand in Denmark since wind power and solar power are being integrated into a grid; hence, flexibility of consumption is required, he added. Today, 50% of electrical power in Denmark is supplied by wind and solar.
In addition, Denmark is connected to Germany, the Netherlands and other countries to which it supplies and gets electricity.
Nørgaard showed an example for one day in January 2023 when electricity prices were almost nil and stressed that was the right time to run the heat pump and even make money having such a large storage capacity.
Nørgaard quoted the manager of the Sønder Felding Varmeværk district heating plant as saying, “I am no longer a heat planner – I am an electrical planner.”
This district heating infrastructure can provide continuous heating for 10 to 12 days in a single-charge storage tank during the summer. This duration narrows down to 30 to 40 hours per charge in cold winter due to increasing heat demand.
Nørgaard emphasized that by elevating the storage temperature from 70 to 80 °C (158 to 168°F), the system could enhance its energy storage capacity by 30%.
Economic Impact
A conventional biomass boiler cost €32 (US$34)/MWh. However, combining the electrical boiler with the heat pump has drastically reduced this cost to €13 (US$14)/MWh over the past year.
“Their efficient system even led to compensation from Denmark’s transmission system operator for absorbing excess electricity, accentuating their grid balancing contributions when there is very little consumption, but the load is peak in the grid,” said Nørgaard.
Nørgaard underscored that customers still face some dilemmas in implementing CO2 heat pumps because of a lack of awareness about CO2 as a refrigerant.
However, the customer easily adopted it when told about the benefits of CO2, with a GWP equal to one and some knowledge about TFA (trifluoroacetic acid), an atmospheric byproduct of HFO-1234yf, and PFAS (per- and polyfluoroalkyl substances), which include some f-gases and TFA, according to the Organisation for Economic Co-operation and Development (OECD); CO2 is TFA- and PFAS-free, he said.
“Fenagy’s heat pumps are balancing an electrical grid today, and natural refrigerants, big thermal storage tanks, are the future of district heating networks,” said Nørgaard.
“I am no longer a heat planner – I am an electrical planner.”
Manager of the Sønder Felding Varmeværk district heating plant
“Fenagy’s heat pumps are balancing an electrical grid today, and natural refrigerants, big thermal storage tanks, are the future of district heating networks.”
Energy Sector Sales Director at Fenagy
