Fundamentals of Large Heat Pumps

The illustration shows the application areas of large heat pumps with temperatures up to about 200°C and heating capacities of individual systems of up to 100 MW for district heating and industrial processes.

The operation of a heat pump involves transferring heat from the environment or waste heat from a source at a lower temperature level to a sink at a higher temperature level. This process requires additional technical or thermal energy. This information portal focuses on electrically driven compression heat pumps.

Applications of large heat pumps, © Fraunhofer IEG 2024

Compression heat pumps are available on the market in a variety of performance and temperature classes. Different heating classes can be defined to distinguish heat pumps based on technology and application. In the heat pump database, product series starting from a heating class of 500 kW are listed, as from this performance size, typically different compressor technologies are used compared to smaller performance classes. These are referred to as large heat pumps. Large heat pumps are primarily used to supply industrial consumers and provide heat for heating networks, whereas conventional heat pumps cover the heat demand of smaller consumers. In addition to closed large heat pumps, which can currently reach sink temperatures of up to 200 °C and performance classes per unit of up to 70 MWth, open heat pump systems (Mechanical Vapor Recompression, MVR) are also used in industrial contexts to provide process steam. MVR systems can reach temperatures of up to 280 °C and are available in various performance classes of up to 100 MWth.

The illustration shows the principle of a closed compression heat pump, including a depiction of the main components as well as common heat sources and sinks.
Principle of the closed compression heat pump, © Fraunhofer IEG 2024

Closed compression heat pumps absorb thermal energy at a low temperature level and release it along with the drive energy at a higher temperature level to a heat sink. Inside the heat pump, a refrigerant circulates, undergoing the following central process steps:

Initially, heat from the source is fed into the system at a low temperature and pressure level through a heat exchanger (evaporator), causing the refrigerant to evaporate and superheat. Subsequently, compression in the electrically driven compressor results in an increase in temperature. In another heat exchanger (condenser or gas cooler), the thermal energy is then released to the sink's circuit. The refrigerant is then expanded through an expansion valve or an expander and returned to the evaporator. This simple cycle is enhanced by additional components with various configurations to optimize efficiency and application range.

In the industrial application of high-temperature heat pumps, process steam generation plays an important role. In addition to conventional closed heat pump cycles, open heat pump systems (MVR) are also used. In these systems, the refrigerant (process steam) is compressed in a compressor without additional heat exchangers and raised to a temperature level required for the respective industrial process. Depending on the heat source, MVR systems can be combined with other technologies for steam generation.