Christian Heerup, Danish Technology Institute, at ATMO Europe Summit 2022, presenting about an AHU with CO2 heat pump.
Christian Heerup, Danish Technology Institute, at ATMO Europe Summit 2022, presenting about an AHU with CO2 heat pump.

ATMO Europe: Unorthodox AHU with Reversible CO2 Heat Pump Under Development for European Offices

The Danish Technology Institute is partnering with Denmark-based manufacturer Unic Air to create a prototype of the HVAC unit for testing.

Danish manufacturer Unic Air has partnered with the Danish Technology Institute (DTI) and other stakeholders to develop an air handling unit (AHU) with a reversible CO(R744) heat pump, according to Christian Heerup, Senior Consultant at DTI.

A prototype of the unorthodox system, dubbed HVACO2, is currently being developed by the manufacturer, with a plan to install the unit at a Danish office building by the end of 2023 for real-world testing and demonstration, he explained.

To date, the concept has been lab tested for performance, efficiency and reliability.

Heerup introduced the HVACO2 during his presentation in a heat pump session at the ATMOsphere (ATMO) Europe Summit on natural refrigerants. The conference took place last November in Brussels and was organized by ATMOsphere, publisher of

The goal of this project is to produce an “HFC-free alternative for conditioning of fresh air to the European market,” said Heerup. “We also aim to achieve a 25% reduction in energy consumption compared to traditional systems with discrete components.”

Typically, an AHU would use one system for heating and a second system for cooling, but by using a reversible heat pump, DTI and Unic Air are creating a self-contained unit, he explained.

AHUs for office buildings

With several years’ experience manufacturing CO2-based cooling equipment and HFC-based reversible heat pumps, Unic Air is working to develop new products, including the reversible R744 heat pump to replace its existing HFC designs.

The plug-and-play HVACO2 unit will require connection to a power source and air ducts only, meaning that installation costs will be cheaper than traditional units, said Heerup.

It has been designed primarily for office buildings, due to their high market share of AHUs, he added.

Once developed, the new HVACO2 will be available in two models: one designed for Nordic climates and another for southern European climates.

‘Promising technology’

To model system operation and performance, the team developed calculations software.

One scenario DTI modeled was a 9,600m3 (339,000ft3) space, which is big enough to accommodate 200 people, said Heerup. Calculations were based on a five-day work week with the AHU system operating from 6 am to 6 pm.

Through the calculations, DTI expect to achieve a COP of around 7 in the winter and 4–5 in the summer.

“Overall, this CO2 system is quite promising technology for air-conditioning purposes,” he concluded.

“Overall, this CO2 system is quite promising technology for air-conditioning purposes.”

Christian Heerup, DTI

Cross-continental collaboration

Italian manufacturer Dorin is supplying the CO2 compressors for the unit, with one of its four-cylinder compressors being used in the prototype. Dorin has conducted significant tests on the compressor to ensure optimal performance, particularly when running in partial load, explained Heerup.

Carel Nordic will be supplying the CO2 valves, ejectors and controls, and British manufacturer 3T is supplying the unit’s heat exchangers.

Heerup noted that component supply chain issues might affect the delivery of the test unit, which is planned for the end of this year.

DTI is assisting with research and technology, as well as acting as project manager, while property management company DEAS Group is facilitating and hosting the demonstration, and the Danish Energy Agency is providing funding through its Energy Technology Development and Demonstration Program. ATMOsphere, publisher of, is also acting as market accelerator for the project.

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