CO₂ heat pumps
- the perfect heating solution for greenhouses
Horticultural greenhouses operate with low return and high supply temperatures - conditions where CO₂ heat pump technology outperforms all other refrigerants. These temperatures match Danish district heating, where large-scale CO₂ heat pumps already deliver proven megawatt-scale results.
The case
Markhaven is a 32,000 m² horticultural greenhouse located in Denmark. Like many in the sector, the business has relied on natural gas for heating, but to reduce reliance on fossil fuels, Markhaven, in 2023, invested in an air-to-water CO₂ heat pump. The existing heating system already had an open accumulation (buffer) tank and manifold, which made it easy to integrate the new heat pump. “Many greenhouses are built like this,” explains
owner Klaus Søgaard. “We just added another unit to the manifold.”
CO₂ management is an essential factor in horticulture. During summer, when heat demand is lower, Markhaven prioritises CO₂ enrichment to stimulate plant growth - a standard practice in greenhouse horticulture. While this means the gas boilers are still used during certain periods, the heat pump reduces overall fossil fuel use and improves system flexibility. Over the year, the heat pump now covers around 50% of the heating demand, significantly lowering Markhaven’s carbon footprint without compromising crop performance.
Year: 2023
Model: H-1200 air-to-water heat pump
Capacity: 1.5 MW · 40°C/65°C return/flow (5°C/85% humidity)
Refrigerant: CO₂ (R744)
- natural, non-toxic, non-flammable
Defrost: Glycol
The heat pump
The heat pump is an H-1200 air-to-water, supplying 1.5 MW of heat and using CO₂ as a refrigerant. It is installed in a high-quality technical enclosure, which is a fully approved machine room with lighting, ventilation, CO₂ alarm and sound dampening panels. The enclosure was delivered with a concrete foundation, so only a levelled sand pad needed to be prepared on site for the installation.
Next to the heat pump and the existing accumulation tank, built in 1997, stand four evaporators that collect heat from the air. A glycol circuit, developed by Fenagy, ensures reliable defrosting through a unique control algorithm.
The heat pump has been in operation since January 2024 and supplies about 50% of the greenhouse heating demand, operating alongside the existing natural gas boilers (6.5 MW) and CO₂ fertilisation systems.
The accumulation tank - where heat can be stored when electricity is cheap - is a central part of the heating system. It is cycled daily - typically ranging from 15–20% to 80–85% full, and even more actively during winter when the load increases. “Even though the COP of the heat pump falls to around 2.75 when it’s cold outside, we make money with it in the winter,” says Klaus Søgaard.
Markhaven’s installation shows that even a relatively small project can deliver measurable energy savings and emissions reductions - and the same CO₂ heat pump technology can be scaled for greenhouses that are ten or twenty times larger.
“With the heat pump, we’ve chosen a solution that is both sustainable and reliable. Our production is now secure – even if the gas supply were to be interrupted. That gives us peace of mind in uncertain times.
Fenagy has delivered a solid product and provide excellent service. They have a lot of experience from similar projects – and I would choose them again without hesitation. For me, it’s also about taking responsibility and leading the way in the green transition.”
Klaus Søgaard · Markhaven Greenhouse