Large Temperature Lifts: A Major Challenge? The Future of Industrial Process Heat

Christoph Rau, SPH Sustainable Process Heat GmbH, Germany

The industry faces the challenge of providing process heat as efficiently and sustainably as possible. Industrial heat pumps that are able to utilize typical waste heat sources offer significant potential. However, the high temperatures required and the associated temperature lifts often pose a challenge for conventional heat pump technologies. Innovative high-temperature heat pumps extend the boundaries of conventional systems and enable efficient utilization of waste heat.  

Introduction

Every year, more than 400 terawatt-hours (TWh) of energy are used to generate process heat, accounting for about two-thirds of Germany’s annual industrial energy demand [1]. Decarbonizing these processes is essential for achieving European climate goals. However, the high temperatures required in many processes and the necessary temperature lifts of more than 80 Kelvin (K) present a technical challenge for conventional industrial heat pumps. Specialized systems are needed to combine future-proofing, efficiency, and economic viability.

Electric process heat generation is a promising approach: According to a study by the Niederrhein University of Applied Sciences, the German industry sector could save up to 21 billion Euros (23 billion USD) annually in energy costs for process heat generation [2]. The largest savings potential arises from electrification, heat recovery, and waste heat utilization—12.8 billion Euros (14 billion USD) could be saved through immediately implementable measures alone. Overall, this represents a potential reduction of up to 33% in the industry’s final energy demand, equivalent to approximately 226 TWh per year, with a significant portion amortized within 3 years.

At the EU level, the annual energy demand for process heat is estimated at 2,950 TWh, with a share of 25% in the 100-200 °C temperature range [3][4]. This makes high-temperature heat pumps such as the ThermBooster™, developed by SPH Sustainable Process Heat GmbH, located in Overath, Germany, a key technology for the European industrial sector. A McKinsey analysis projects annual market growth exceeding 15% for this sector by 2030, with global investments reaching approximately 12 billion US dollars [5].

Large Temperature Lifts: A Challenge for Heat Pump Systems?

Despite the potential of electrifying process heat generation, technical challenges remain: industrial waste heat sources typically range from 40 to 80 °C. For temperature requirements between 100 and 200 °C, this necessitates a temperature increase exceeding 100 K in some cases.

Industrial heat pumps face significant challenges when the temperature lift exceeds 80 K. The reason being that, for maximum efficiency, the system must transfer as much heat energy as possible from the heat source to the heat sink. This typically requires a high evaporation pressure and high suction density. At low waste-heat temperatures, which result in low evaporation pressures, high pressure ratios are required to achieve significant temperature lifts.

For example, in a piston-based heat pump compressor, residual gas remains in the cylinder after each compression cycle (dead volume). This residual gas expands during the subsequent intake phase (re-expansion), thereby reducing the fresh-gas intake. At a pressure ratio of 9.5 between the high- and low-pressure sides of the heat pump cycle and a dead volume of only 3% of the total volume, up to 28% of the actual suction volume is blocked by residual gases. The larger the dead volumes and the higher the pressure ratio, the lower the overall system efficiency.

A low evaporation pressure of only 2 bar and a suction density of around 10 kg/m³ result in low specific thermal performance, as the system can evaporate less refrigerant and thus transfer less heat per cycle. This low performance requires high displacement capacity, which can only be achieved with a larger, more powerful compressor or multiple compressors. However, this significantly increases the investment costs for such systems.

Refrigerants with specific performance characteristics, unfavourable pressure ratios, and limited volumetric efficiency—these obstacles ultimately lead to a fundamental problem: traditional single-stage refrigeration cycles often struggle to meet the requirements of industrial heat generation and efficient waste heat utilisation due to their design limitations.

Innovative Heat Pump Technologies Enable Temperature Lifts Beyond 100 Kelvin

Solutions such as the modular high-temperature heat pump ThermBooster™ offer various approaches to overcoming these challenges. One possible solution is two-stage cascading: The ThermBooster™ can use two separate refrigerant cycles in series to efficiently, reliably, and economically handle temperature lifts beyond 80 K. Unlike other compressor types, the specially developed piston compressor can achieve the required pressure ratio regardless of its speed, making it ideal for applications with high temperature lifts.

In the lower cycle, refrigerants such as R-515 B, propane, or R-1234ze can be used for temperature lift at lower temperatures, whereas the upper cycle can utilize R-1233zd, butane, or isobutane. For even higher temperatures, R1336mzz(Z) or pentane can be used as refrigerants. Each of the two stages provides a temperature lift exceeding 50 K, yielding a total temperature lift exceeding 100 K.

Combining the system with a steam compressor (mechanical vapour recompression) offers a second possible solution: By integrating a steam compressor, the ThermBooster™ can economically bridge even greater temperature differentials.

Achieving Nearly Any Temperature Lift with Cascading

Sterilization and drying processes, two key application areas for process heat in the food and chemical industries, benefit significantly from the efficiency of two-stage systems such as the ThermBooster™. A project example:

• Heat source: 50/36 °C (evaporation at 33 °C)
• Steam: 3.5 bar(a), 139 °C (condensation at 144 °C)
• Capacity: 646 kW

Figure 1 illustrates a cascaded high-temperature heat pump system that uses two refrigerants, arranged in two stages, to achieve a substantial overall temperature lift. In the first stage, the refrigerant evaporates at approximately 33 °C and condenses at approximately 85 °C, thereby raising the temperature to an intermediate level. In the second stage, the intermediate heat is further boosted, with evaporation occurring at approximately 80 °C and condensation at approximately 144 °C, enabling the system to supply high-temperature process heat or steam up to approximately 139 °C.

The use of highly efficient piston compressors in both stages, subcoolers for feedwater preheating, and parallel intermediate heat exchangers to minimize the pinch point between the hot and cold sides of the heat exchanger increased the COP in this project to 2.3. This means that 1 unit of electrical energy provides 2.3 units of heat energy, corresponding to a Carnot efficiency of 61% relative to the evaporation and condensation temperatures. Given the efficiency limitations imposed by heat transfer between the first and second stages, this value is high.

Use of Steam Compressors Expands Performance Ranges

The combination of one- or two-stage heat pump systems with steam compressors significantly extends the range of possible applications of this technology, particularly in industries with high temperature and pressure requirements. In an innovative project with the pharmaceutical company Takeda in Vienna and the AIT (Austrian Institute of Technology), high-temperature lift and high steam pressures have been applied [6]. Here, the ThermBooster™ is combined with a steam compressor to achieve higher pressures and temperatures. In this project, waste heat from a chiller used to provide cold is being reused in winter by a heat pump to generate hot water for building heating. In the remaining months, this potential had not yet been used. The innovative new system uses this hot water to generate high-quality steam. To achieve this, the efficient one-stage ThermBooster™ system uses two compressors in parallel, combined with two evaporators in series on the source side. This configuration maximizes thermodynamic performance and ensures optimum use of the available energy. The steam generated by the ThermBooster™ is compressed by a mechanical vapour recompressor (MVR) based on piston-compressor technology, which offers good part-load behaviour. Both existing and new systems use natural refrigerants:

• ThermBooster™:
  • Heat source: 70 °C/65 °C hot water
  • Heat sink: 1.7 bar(a) saturated steam
  • Refrigerant: Butane
  • COP: 4.3
    
    
• MVR Spilling Project Partner GmbH:
  • Heat sink: 11 bar(a) at 195 °C
    
    
• Total:
  • Heating output: 1,561 kW
  • Electrical power consumption: 672 kW
  • COP: 2.3

Figure 2 shows the cascaded high-temperature heat pump system in which a standard refrigerant cycle is combined with a steam compressor to reach very high process-steam temperatures. The heat pump first lifts the temperature from a heat source of about 65 to 70 °C to around 115 °C, corresponding to a heat-pump temperature lift of about 58 K. From this intermediate level, a mechanical vapour recompressor further compresses and superheats the steam from 115 °C up to about 195 °C, enabling the system to supply high-temperature process steam.

This configuration maximizes thermodynamic performance and optimizes the use of available energy, enabling efficient conversion of low-temperature waste heat into high-quality process heat. Thanks to their low GWP values, synthetic and natural refrigerants are possible options for significantly reducing CO₂ emissions. The system’s flexibility enables its use for steam production and for specific applications, such as sterilization or drying.

Unused Potential in Numerous Branches of Industry: Investments that Pay Off

In addition to drying and sterilization processes, two-stage systems also show their potential in other branches of industry:

• Textile industry (washing, dyeing, pressing)
• Paper industry (bleaching, drying)
• Metal industry (electroplating, phosphating)
• Food industry (pasteurization, evaporation)

Precise temperature control, reduced energy losses, and improved efficiency are key benefits for these sectors. Especially in energy-intensive industries with high process-heat requirements, the savings in operating costs and waste heat recovery from the efficient operation of the ThermBooster™ can more than offset the higher initial investment. Modular industrial heat pumps, such as the ThermBooster™, therefore position themselves as the first choice for processes that require high temperatures with the greatest possible precision and maximum efficiency.

ThermBooster™ Technology Offers a Number of Advantages:

1. Energy savings: The primary energy requirement for process heat could be reduced by over 50 % by 2040 compared to 2022 [2].
   
   
2. Cost-effectiveness: measures that can be implemented close to the market pay for themselves quickly.
   
   
3. Sustainability: Electrification of process heat generation minimizes CO₂ emissions.
   
   
4. Flexibility: the two-stage cascading and modular design can be adapted to different requirements.

Conclusions

The decarbonization of industrial process heat is a central building block on the path to climate neutrality. Two-stage high-temperature heat pumps overcome the technical hurdles of conventional heat pump systems and offer a powerful, efficient, and economically attractive solution today. Companies that rely on this technology are therefore not only making an important contribution to the sustainable transformation of industry but are also benefiting from increased efficiency and significant cost savings.

Author contact information

References

[1] Greenhouse gas emissions data for the year 2023 according to the Federal Climate Protection Act (2024): https://www.umweltbundesamt.de/sites/default/files/medien/361/dokumente/2024_03_13_em_entwicklung_in_d_ksg-sektoren_thg_v1.0.xlsx

[2] Meyer, J., Zaubitzer, L., Alsmeyer, F., and Madsen, M. “Short Study: Energy‑efficient and CO₂‑free Process Heat”. SWK E² – Institute for Energy Technology and Energy Management, Niederrhein University of Applied Sciences, Krefeld, Germany. 2024.

[3] Eurostat. “Energy Balances”. 2019.

[4] Fleiter T, Elsland R, Rehfeldt M, Steinbach J, Reiter U, Catenazzi G, et al. “Heat Roadmap Europe”. 2017.

[5] Bauer, H., Ehrmaier, J., Gigliotti, L., Liebach, F., Schleyer, T., and Simoncini, A., “Industrial heat pumps: Five considerations for future growth”. McKinsey & Company Insights. 2024.

[6] https://www.takeda.com/de-at/news/2025/ahead-opening/