Scientists from the University of Waikato in New Zealand, Germany’s Paderborn University and the German Aerospace Center have conducted a large-scale study evaluating the potential of carbon dioxide as a refrigerant for high-temperature heat pumps. These units are being considered as an alternative to gas and coal boilers in processes such as food drying, paper production and car painting. Unlike boilers, which generate heat by burning fuel, a heat pump does not produce heat directly, but transfers it from a low-temperature level to a high-temperature one. This principle is especially important in the context of industrial electrification. While direct electric heating (for instance, with electric boilers or heating coils) converts one kilowatt-hour of electricity into roughly one kilowatt-hour of heat, a heat pump uses electricity differently: it uses it to pump the existing thermal energy. This makes it possible to generate much more useful heat from one kilowatt-hour of electricity.
However, in order to replace conventional boilers, which heat air or coolant in a wide temperature range (approximately from 20°C to 200°C), one needs specialized heat pumps that can operate efficiently in a wide temperature range.
The researchers focused on a unique property of supercritical CO₂. In these conditions, it releases heat not at a fixed temperature, like boiling water, but by cooling gradually. This makes it possible to achieve much higher precision when matching heat exchange to real industrial processes, where flows are heated smoothly, from cold to very hot. After analyzing various industries, the scientists identified 16 industrial processes where this property could yield the greatest benefits. They paid particular attention to drying units.
Worldwide, the drying of milk powder, instant coffee and starch consumes hundreds of petajoules of energy annually, which is equivalent to millions of tons of conventional fuel. These processes are ideal for heat pumps: firstly, they require hot air that is heated to 100–200°C; secondly, their warm and moist exhaust gases can be used as a low-grade heat source.
To evaluate their efficiency, the scientists compared three CO₂ heat pump designs: a classic design with an expander, which makes it possible to recover some energy when pressure drops; a design with split refrigerant flow for better temperature matching; and a cascade design, where two CO₂ circuits increase the temperature sequentially.
Modeling showed that there is no universal solution. At temperatures up to 150°C, the expander system provides the best results, while at temperatures up to 200°C and above, the flow splitting and cascade options are more efficient, especially under pressure limitations. In these modes, efficiency reaches 2.6 with the use of ambient air and rises to 4 with exhaust heat from the process itself.
These results look even more impressive from an environmental viewpoint. Even with an average European grid, the replacement of gas burners with CO₂ heat pumps reduces carbon dioxide emissions by 35% on average and by up to 94% in some cases. In low-carbon economies like Norway and New Zealand, emission reductions are approaching the highest level possible.
According to the researchers’ calculations, the implementation of these technologies, at least in the milk powder, whey, coffee and starch drying segment, could reduce emissions by millions of tons of CO₂ per year.
