CO2 (R744)-based air source heat pumps are optimal for centralized heating applications up to 10MW (2,843TR), according to Kim Gardø Christensen, CEO at Fenagy, a Danish manufacturer of heat pumps and refrigeration systems.

Christensen delivered his remarks during a presentation in an HVAC and heat pumps session at the ATMOsphere (ATMO) Europe Summit on natural refrigerants. The conference took place November 15–16 in Brussels and was organized by ATMOsphere, publisher of

Christensen previously stated that CO2 heat pumps are “really good” for district heating applications and the best available technology in the 0.5–5MW (142–1,422TR) capacity range.

Most larger industrial heat pumps use ammonia.

Founded in 2020 and partly owned by Beijer Ref, Fenagy produces modular CO2 heat pump units that offer up to 2MW (569TR) in heating capacity. Where larger capacities are needed, multiple units can be combined to produce a larger system. For example, Fenagy has recently received an order for an 8MW (2,275TR) heat pump that will consist of four of its 2MW modules, explained Christensen.

While Fenagy focuses predominantly on CO2 systems, the company also manufactures integrated systems that utilize hydrocarbons in addition to R744.

According to Christensen, Fenagy manages projects from system design and development through to commissioning and remote monitoring. The company is currently working on 30 installations across Europe – mainly Norway, Denmark and England – with a total capacity of more than 50MW (14,217TR), he added.

Current projects include multiple district heating systems in Denmark, as well as some larger commercial and industrial heat pump systems.

District heating

Working with utilities on districting heating projects requires a slightly different approach when it comes to project design, said Christensen during his presentation.

“Utility companies invest in the life of a system, not first cost,” he explained. “But we have to design for a longer life – around 20 years – and a higher COP as the net present value needs to be higher than any other alternative.”

To ensure a higher COP, Fenagy has integrated a number of technologies into its heat pump systems, including complex PLC controls, a custom-built ejector and a specialized heat exchanger.

Precise defrosting design and control is also of “extreme importance” to ensuring a high COP. According to the company, inefficient defrosting can cost more than 25% of COP and heating capacity during defrost cycles. To achieve efficient defrost cycles, Fenagy uses warm glycol.

In addition to high efficiency, district heating providers also prioritize quick system start/stops “to follow the electric price signals in the market,” said Christensen.

“We are using these heat pumps to balance the electrical grid,” he explained. “We’re only operating the heat pump when the electricity prices are low so we store the heat in a large hot water vessel.”

As the contractor (as well as the manufacturer), Fenagy is measured on the system’s output temperature, capacity and COP and is penalized if targets are not met. For this reason, installation, commissioning and performance testing is highly important, said Christensen.

To enable higher capacity systems, Fenagy is looking at integrating larger components, including compressors.

Combined heating and cooling

According to Christensen, integrating cooling systems into heat pumps and vice versa is important to further improve the efficiency of these systems.

“We need to combine heating and cooling; otherwise we find ourselves in a situation where we’re producing a lot of waste cooling from our heat pumps,” he said.

As an example of such a system, Christensen highlighted a project Fenagy has done for IKEA in northern Denmark in which the system provides cooling for the site in summer and heating in winter.

However, he pointed out that the ability to combine the two elements depends on the location of the installation.

“Our projects are typically in smaller cities where there is no industry, so there’s no one that needs the cooling, but in bigger cities we can more easily combine heating and cooling.”