Articles

In line with the 2030 energy strategy plans, Frederiksberg District Heating (DH) Company has taken a significant step toward fuel-free district heating production. Marking 120 years since the network’s inception, a new 6.5MW renewable heat pump system has been established – right where Denmark’s oldest district heating network began.

By Henrik Schmidt, Project Manager Heat Pump Project and Hans Martin Bayer Kühl, Manager of the Energy and Environment department, Frederiksberg Utilities

Published in Hot Cool, edition no. 1/2025 | ISSN 0904 9681 |

The heat pump plant has received technological and political approval based on a detailed project plan outlining its social, economic, and environmental benefits in compliance with national legislation.

Based on national guidelines, the heat pump plant is estimated to produce 4,700 MWh of heat per year and reduce CO2 emissions by nearly 5,000 tons over its 20-year lifespan.

In Denmark, DH emissions are closely monitored and published annually in environmental declarations. According to the 2023 report, CO2 emissions from the Greater Copenhagen DH system have steadily declined since 1992 and are now approximately 10 kg/GJ of CO2.

Figure 2: Historical development of average emissions from the entire Greater Copenhagen area DH system since 1990 (Ref. www.veks.dk).

Besides reducing CO2 emissions, the project is part of an overall strategy of further electrification of the DH network of Greater Copenhagen.

The Danish energy system has been undergoing major changes over the years, which entails a need to link the energy sectors further. Denmark’s electricity mix consists of mainly wind and solar energy – 63% in 2023. The Danish Energy Agency expects this share to increase to 90% in 2030.

Since fluctuating electricity is increasingly becoming the backbone of the Danish energy infrastructure, there is a growing need for flexibility in terms of consumption. DH can help the energy system by using electricity from fluctuating sources when there is plenty of it. However, DH only helps if the system can react to price signals from the electricity market.

Expanding heat pumps for a 100% renewable Copenhagen

Heat pumps and other electrical-driven heat production should be used when electricity prices are low, which generally means there is a high share of wind and solar energy. 85% of the heat supply to the Greater Copenhagen DH system is today produced from renewable heat sources. For the further increase of the use of renewable heat sources to reach 100%, a significant increase in electrically driven heat production as heat pumps need to be implemented in the coming years.

There are tentative plans to implement additional heat pumps in the Greater Copenhagen DH network, utilizing seawater, sewage wastewater, and drinking water sources, similar to the new 6.5 MW heat pump system presented here. These water-to-water heat pumps offer higher electrical efficiency than air-to-water heat pumps.

Drinking water as heat source for the new 6.5MW heat pump system

The Frederiksberg heat pump system makes use of drinking water as a heat input source from Frederiksberg’s newly established drinking water treatment and distribution plant.

Frederiksberg is situated atop an underground water source that, from 1847, initially served as a primary water source for the Danish Carlsberg brewery, and since 1869, has served as an underground water source for distribution at Frederiksberg.

During 2020-2022, Frederiksberg Utilities built a new drinking water treatment and distribution plant in a new building, which also hosts the heat pump system. Due to strict limitations in the available ground area at Frederiksberg, the water utility plant is installed in a tall building of 27m, with the heat pump system at the top floor 20m above ground level.

Optimizing heat pump performance with dynamic water flows

The utility’s water supply flow averages around 14,000 m3 per day (~ 580 m3/h). Approximately 50% of this water comes from underground water intake at Frederiksberg, where it is pumped from the underground at a constant rate of about 300m3/h to maintain a stable underground water level and prevent flooding in the densely populated areas of Frederiksberg and nearby parts of Copenhagen. The remaining 280m3/h of raw water is sourced from water utility plants outside the Greater Copenhagen area as an integrated part of the primary drinking water supply system to the entire Greater Copenhagen city area.

Water consumption in Frederiksberg community supply area varies throughout the day, ranging from approximately 250m3/h during nighttime to 1,200m3/h during morning and early evening peak hours. The heat pump system has been designed with a highly flexible system to cope with the fluctuation of the drinking water heat source. Due to the process conditions in the water production facilities, the water flow typically decreases by around 30% almost immediately after early evening peak hours. The heat pump system efficiently and automatically adjusts for these changes.

Ensuring safe and efficient heat transfer in drinking water systems

The new heat pump system is designed with an indirect circulation loop separating the utility water from the main heat pumps as a safeguard to prevent pollution of utility water in case of leakage from the high-pressure heat pumps. The heat is transferred from the utility water to the circulation loop in two water-parallel installed stainless-steel plates heat exchangers. The control system ensures that the system is always operated with a lower pressure on the drinking water side of the heat exchangers than the circulation loop pressure.

Figure 3: Schematic drawing of the 6.5MW drinking water heat pump system illustrating the separation of drinking and cooling water with the intermediate circuit.

The utility water is available for the heat pump system between 9ºC and 12º depending on the season of the year. Heat is extracted in the heat pump system to a drinking water discharge temperature of 4ºC to 7ºC.

Assessing the impact of heat extraction on drinking water temperature

Before the project’s technical, economic, and political approval, a detailed analysis of the drinking water temperature in the underground distribution piping network at Frederiksberg was performed to include the environmental and socioeconomic effects of reheating water on the consumers in the project approval process.

Due to the heating of the drinking water in the underground distribution pipes, the average drinking water temperature will only be lowered by approximately 1ºC for the average consumer at Frederiksberg. Approximately 1/3 of water for typical urban consumption is typically being reheated by the consumers, which in this project corresponds to approximately 5% of the heat production from the heat pump system in total.

District heat supply from the new 6.5MW district heating renewable heat pump system.

The new 6.5MW renewable heat pump system is connected directly to the DH system close to a nearby main transmission heat exchanger station, with a nominal heat transfer capacity of approximately. 25MW. The total capacity of Frederiksberg’s entire DH system is approximately 125MW.

The return temperature of the DH system at Frederiksberg is typically 43-54ºC, and the DH supply temperature is automatically controlled at the DH distribution station by an energy optimization system to between 70ºC and 85ºC, depending on the weather conditions.

Flexible and Efficient Ammonia Heat Pump System for Optimal Performance

The heat pump system comprises four parallel 1.75MW 2-stage reciprocating ammonia heat pumps. The four individual heat pumps facilitate high flexibility of the system, which has proven to be able to efficiently cope with both fluctuations of the heat input to the heat pump system as well as the demands for the DH supplied from the heat pump system.

The heat pump system is designed for automatic operation with heat output in the range of 2.5-6.5MW. The control system secures real-time optimization of the operational parameters to produce the maximum possible heat output with the heat source available at the optimum electrical COP efficiency.

Figure 4: Capacity diagram for the new highly flexible 6.5MW heat pump system

High electric efficiency is an important objective when selecting equipment for the project to ensure a competitive heat source.

The heat pumps have been tested at 100% load in the full DH supply temperature range and have proven electricity efficiencies in COP ranging from 3.6 to 3.9, which is considered highly satisfactory for an ammonia heat pump cycle at the prevailing conditions.

The DH system of the Greater Copenhagen area is fully integrated into one large grid, where it supplies heat to industrial buildings and approximately 1.0 million inhabitants. Heat load dispatch to the Greater Copenhagen DH grid is based on a liberalized heat market, where the different heat-producing plants offer a heat supply to the entire DH system on an hourly basis 24/7. The heat pump project includes a semi-automatic bidding system integrated with the automatic plant control system.

The new heat pump system in a historical perspective

This first Danish DH energy complex was established in 1903 by the city council of Frederiksberg to overcome increasing challenges of waste from a rapidly growing population in the very densely populated municipality of Frederiksberg and the need for heat and electricity to the nearby newly built Frederiksberg hospital.

Figure 5: Present boiler house and water utility plant with the new 6.5MW heat pump system next to the original and oldest waste-to-energy building in Denmark from 1903. The old boiler house has been renovated and now hosts a small café and facilities for public use.

This first waste-to-energy plant was replaced with a modern, more energy-efficient plant with a larger capacity based on rotary incinerators, commissioned in 1934. This plant operated for the next 41 years until 1975 when it was taken out of operation.

During the energy crisis in the 70ties, five municipalities joined forces in the Greater Copenhagen Area to produce a common district heating system, and in 1984, the Metropolitan Copenhagen Heating Transmission Company, CTR, was formed.

The new 6.5MW heat pump system means that Frederiksberg is now producing heat again.
So, 120 years apart, a new electrical-driven heat-producing technology is installed at Frederiksberg, which hopefully can inspire other DH supplies since we need to exploit all possible heat sources to succeed with further electrification of the district heating systems.