Compressor technologies in large heat pumps

The choice of compressor technology is of central importance for large heat pumps, as it influences the temperature and power range as well as the efficiency of the system. Since compressors are the most expensive single component of a heat pump, their selection is crucial for the economic viability and the application range of the plant. Different compressor types offer specific advantages depending on the application.

This illustration compares the four common compressor technologies for large heat pumps in terms of heating capacity, temperature lift, and operational flexibility.
Heating capacity, temperature lift, and flexibility depending on the compressor technology, © Fraunhofer IEG 2024

Scroll compressors are widely used and are mainly employed in smaller power classes below 500 kW. They impress with their compact design, reliability and high efficiency in partial load operation. Due to their limited performance, they are especially suitable for applications with smaller volume flows.

Reciprocating compressors are used in large heat pumps that require high pressure ratios and temperature lifts. They offer flexibility and efficiency, especially in partial load operation. While these compressors produce more noise and vibrations compared to others, this can be managed through thoughtful planning of the installation location.

Screw compressors are ideal for applications with high power requirements up to the megawatt range. They are robust, low-maintenance and offer high efficiency over a wide power range. Due to their design, they are suitable for applications in wide temperature ranges, including high-temperature processes.

Turbo compressors offer the highest volume flows and can achieve heating capacities of up to 70 MW, making them particularly attractive for large industrial applications as well as for district heating provision. This compressor technology can already be used oil-free by means of innovative magnetic bearing. However, due to the low pressure increase per stage and the high energy demand, they are less common in smaller power classes. Their controllability over wide power ranges is limited, which makes them better suited for specialized applications with constant high power requirements.

The illustration shows the maximum sink temperature for commercially available large heat pumps (both closed and open systems) as a function of the maximum heating capacity, with the compressor technology color-coded. It illustrates that scroll compressors are primarily used in the low power range, while piston compressors are more frequently employed at high sink temperatures. Screw compressors are considered all-rounders and are used in the medium power range between 1 and 10 MW, while turbo compressors dominate at large heating capacities above 10 MW.
Marketoverview of employed compressor technologies, © Fraunhofer IEG 2024

Further development and adaptation of compressor technology

There is still a need for development of compressors to adapt them to new requirements such as more flexible operating modes, new refrigerants and higher sink temperatures. This requires the redesign of cooling, lubrication and mechanical components such as bearings and seals. Innovative approaches, including oil-free compressors and heat-resistant materials, are being explored. In addition, the optimization of mechanical steam compressors for process steam generation and the adaptation of compressors to flexible, heat- or power-led operating modes are becoming increasingly important.

Besides the established compressors, alternative types such as rotary compressors are also being tested. In addition, new concepts such as rotary compression or compression as part of a Stirling cycle process are emerging. These innovative technologies offer specific advantages and could play a larger role for large heat pumps in the future.