A new open-access review published in the journal Carbon Neutral Systems argues that high-temperature heat pumps, machines that lift low-grade waste heat up to the temperatures factories actually need, are becoming a central technology for cutting those emissions. The paper, authored by Jiale Wu, Luwei Yang, and Chong Zhang and published on 29 December 2025, systematically analyzes the state of the technology from four angles: thermodynamic cycle configurations, refrigerants, compressors, and integration into energy-hungry industries such as paper manufacturing and textile dyeing.

The scale of the opportunity is large. The review notes that process heating accounts for 50 to 70 percent of total industrial energy consumption, and that across different industries waste heat resources generally constitute roughly 17 to 67 percent of the total heat supply, indicating substantial untapped potential. Heat demand below 150 degrees Celsius, they add, makes up more than 40 percent of China's total industrial thermal energy consumption, and the review states that high-temperature heat pumps are already capable of large-scale replacement of traditional coal and gas-fired boilers in the temperature range up to 150 degrees.

The review describes heat pumps as using high-grade energy, such as electricity, to transfer thermal energy from low-grade heat sources to high-grade heat demand sectors, and notes that in the context of end-use electrification and carbon-neutral energy system development they can improve the current primary energy-based heating paradigm through their highly efficient electricity-to-heat conversion principle. The review classifies machines by output temperature: medium-temperature units deliver 80 to 100 degrees, high-temperature heat pumps operate in the 100 to 160 degree range, and ultra-high-temperature machines exceed 160 degrees and can generate medium-pressure steam. Conventional vapor compression systems are derived from the reverse Carnot cycle, and the authors show that the choice of cycle configuration largely determines how big a temperature jump a machine can achieve.

Single-stage compression, the simplest configuration, manages temperature lifts of 30 to 60 degrees. The review’s theoretical comparisons show that at an evaporation temperature of 50 degrees and a 50 degree lift, the refrigerant R1233zd(E) reaches the highest coefficient of performance among the fluids listed, 5.04, but that when the lift grows to 60 degrees the COP of all five refrigerants examined drops to around 4.0 or lower. Quasi-two-stage cycles, which inject part of the working fluid into the compressor’s intermediate chamber, extend lifts to 60 to 90 degrees and boost heating capacity; one flash-type design using propane demonstrated an 18.57 percent COP improvement over a single-stage cycle at an optimal first-stage pressure ratio. Two-stage compression, constrained by the operating ranges of single-component refrigerants, typically achieves lifts between 50 and 80 degrees.

For the biggest jumps, the review points to cascade and coupled cycles. Cascade systems couple two independent compression circuits, each with its own refrigerant, and typically achieve lifts of 60 to 120 degrees or more; one numerical study of a large centrifugal cascade heat pump found that four refrigerant pairs all reached COPs of 3.0 or higher at evaporating and condensing temperatures of 20 and 90 degrees, with the R601/R245fa pair yielding the highest COP of 3.08. Coupled cycles, which combine vapor compression with absorption heat pumps or with water vapor compression, suit ultra-high-temperature duties requiring lifts beyond 90 degrees and supply temperatures above 160 degrees.

The review documents an accelerating transition away from high global warming potential hydrofluorocarbons such as R134a and R245fa toward low-GWP hydrofluoroolefins, hydrochlorofluoroolefins, and natural refrigerants. New fluids are closing the performance gap: thermodynamic evaluations found that R1233zd(E) and R1336mzz(Z) beat R245fa under 70 degree evaporation and 140 degree condensation conditions, with COP improvements of 8.32 and 11.68 percent respectively. Water itself, designated R718, stands out for ultra-high-temperature work, with a critical temperature of 374.15 degrees and a latent heat of vaporization 15 times greater than R134a; a prototype water vapor heat pump achieved 150 degree output with a 65 degree lift at a COP of 1.96. Ammonia-water systems have generated process steam exceeding 150 degrees, while hydrocarbons such as propane and butane show promise despite flammability risks and relatively high discharge temperatures.

Compressors are the other critical enabler. The review finds a direct correlation between system heating capacity and compressor type, and identifies large-capacity screw and centrifugal compressors as the key to industrial-scale deployment. Scroll compressors offer compact structure, smooth operation and typically higher energy efficiency ratios than reciprocating compressors of equivalent capacity, but their single-compression-chamber structure limits heating capacity, making them less viable for large-scale industrial applications; the Ochsner single-screw unit delivers 170 to 750 kilowatts of heat at 95 to 130 degrees, and the Combitherm HWW9583 twin-screw system scales from 0.3 megawatts in a single unit to 3.3 megawatts with three units, reaching 120 degrees with R1233zd(E). Centrifugal machines go further still: Gree’s CVP series delivers 120 degree heat output with 10 megawatts of heating capacity, and Mitsubishi Heavy’s ETW-S system uses a multistage centrifugal compressor running at up to 42,000 rpm to deliver heat at up to 130 degrees with a peak heating capacity of 627 kilowatts. For water vapor compression above 100 degrees, the authors conclude that centrifugal turbocompressors are among the optimal solutions thanks to their high efficiency and oil-free operation.

The most striking part of the review is its sector-by-sector analysis of six Chinese industries. In paper manufacturing, where thermal energy accounts for over 50 percent of total energy use and annual steam consumption in pulping and papermaking ranges between 400 and 450 million tonnes with 80 percent derived from fossil fuels, heat pump integration could unlock annual CO2 reduction potential approaching 100 million tonnes. In textile dyeing, recognized as the most energy-intensive segment of the textile chain, HTHPs could achieve over 90 percent reduction in thermal carbon emissions, with an estimated mitigation potential of more than 50 million tonnes; a cascading system that harvests high-grade and low-grade dyeing wastewater separately cuts fossil fuel-based steam usage by 45 percent. In electronics manufacturing, the review states that adopting efficient heat pumps and other low-carbon technologies to fully exploit ambient air energy and process waste heat as alternatives to conventional heating could achieve annual carbon reductions of up to 240 million tonnes, with medium-to-low and high-temperature processes contributing 160 million and 80 million tonnes respectively.

The remaining sectors show equally concrete pathways. Edible oil processing, which consumes more than 5 million tonnes of standard coal equivalent in thermal energy and emits roughly 15 million tonnes of CO2, could see carbon reductions exceeding 12 million tonnes by deploying large-temperature-lift heat pumps below 150 degrees, with evaporation-stage heat recovery reducing steam consumption by over 70 percent. Fermentation pharmaceuticals aims to replace over 70 percent of fossil-fueled steam with heat pump systems, a transition projected at 10 million tonnes of CO2 annually. Beer production, where saccharification and wort boiling alone consume 35 to 40 percent of a brewery’s steam, can save 42 percent of energy in the boiling stage by combining 105 degree heat pumps with plate heat exchangers, while also avoiding the 15 percent thermal loss typically incurred through steam condensate discharge.

Looking forward, the authors see high-temperature heat pumps evolving toward higher output temperatures above 160 degrees and larger capacities, paired with advanced working fluid design and digital control integration with renewable power and thermal storage. The IEA Heat Pumping Technologies Annex 58 reports that while units of 200 kilowatts to 10 megawatts capacity with supply temperatures up to 120 degrees saw limited commercial adoption in 2022 and 2023, they are projected to become the preferred industrial process heating technology by 2026; machines delivering 160 degrees or higher remain in prototype testing or demonstration. Taken together, the authors argue, high-temperature heat pumps constitute an essential electrification pathway for deep industrial decarbonization and carbon-neutral energy systems, cutting 15 to 22 percent of CO2 emissions when powered by low-carbon electricity.

Subject of Research: High-temperature heat pump technologies and their industrial applications for carbon-neutral process heating

Article Title: High-temperature heat pumps: key technologies and industrial applications toward carbon–neutral process heating

Article References: Wu, J., Yang, L., & Zhang, C. (2025). High-temperature heat pumps: key technologies and industrial applications toward carbon–neutral process heating. Carbon Neutral Systems, 1(1), Article 21. https://doi.org/10.1007/s44438-025-00021-z

Image Credits: AI Generated

DOI: 10.1007/s44438-025-00021-z

Keywords: high-temperature heat pumps, industrial decarbonization, waste heat recovery, process heating, refrigerants, compressors, cascade cycles, coupled cycles, carbon neutrality, renewable energy integration, thermal energy storage, China industry