Selection of refrigerant for large heat pumps

In recent decades, refrigerants have changed significantly due to the growing environmental and climate protection awareness. Until the 1990s, chlorofluorocarbons (CFCs) were mainly used, but they damaged the ozone layer severely due to their high ozone depletion potential (ODP). Since 1995, Germany has imposed a zero ODP requirement, which led to the replacement of CFCs by partially halogenated hydrofluorocarbons (HFCs). However, these substitutes have a considerable global warming potential (GWP), which is why the F-Gas Regulation prescribes a gradual reduction of HFC refrigerants by 2030. Alternatively, hydrofluoroolefins (HFOs), which are synthetic refrigerants with much lower GWP, are being used more frequently. Another challenge is the longevity and environmental persistence of synthetic refrigerants. These usually contain per- and polyfluorinated alkyl substances (PFAS), which can accumulate in the environment and in humans and are associated with various diseases. Due to this problem, bans on PFAS substances are being discussed in the EU, which drives the development for the use of natural refrigerants. These are environmentally friendly and can cover a wide range of applications, while some properties of toxicity and flammability have to be taken into account.

To begin with, the thermodynamic properties of the refrigerant are crucial because they substantially affect both the efficiency and the scope of the heat pump's usage. The choice of refrigerant in large heat pumps depends strongly on the required source and sink temperatures.

For flow temperatures up to 80 °C, the synthetic refrigerants R410a and R134a are widely used. A natural alternative in this range is ammonia (R717), which is also suitable for temperatures up to 110 °C. Propane (R290) also represents an environmentally friendly option for temperatures up to 80 °C.

Refrigerants such as carbon dioxide (R744) are used in systems that achieve flow temperatures between 80 °C and 120 °C. CO₂ heat pumps operate particularly efficiently at large temperature differences between flow and return, by releasing heat through gas cooling.

For applications above 120 °C, hydrocarbons such as iso-butane (R600a), butane (R600) or pentane (R601) as well as synthetic hydrofluoroolefins (HFO) such as R1336mzz(Z), R1233zd(E) and R1234ze(Z) are often used. For high-temperature applications from 160 °C, such as for the provision of process heat and process steam, water (R718) is a preferred refrigerant.

Notably, some natural refrigerants exhibit the property of being flammable or toxic compared to synthetic alternatives. This primarily concerns hydrocarbons, which are flammable, as well as ammonia, which has high toxicity. In contrast, natural refrigerants like CO₂ and water are unproblematic regarding flammability and toxicity. Although the risks of flammability and toxicity can be managed through technical safety devices, they can still limit the applications in certain areas.

Ammonia (R717) is a well-established and extensively tested refrigerant, having been utilised for many decades and widely applied in practice. It is widely used in industrial applications, especially in district heating networks, due to its efficiency and high flow temperatures of up to 110 °C. However, the operation of ammonia systems requires certain safety precautions due to its flammability and toxicity, which have to be taken into account. Another factor is the high pressure level, which must be taken into account for efficient operation.

Carbon dioxide (R744) as a refrigerant offers numerous advantages and is characterized by its environmental friendliness. It is not flammable and not toxic, which makes it a safe choice for various applications. CO₂ exhibits a high volumetric cooling capacity and functions especially well across substantial temperature gradients, with sink temperatures reaching up to 120 °C.

A special feature of CO₂ is the transcritical operation: Under these conditions, no isothermal condensation occurs during the heat transfer to the sink, but the gas cools down in a gas cooler, which requires a special return control. In addition, source temperatures below 30 °C are necessary. The refrigerant is therefore well suited for low source temperatures and flexible use, especially in heat pumps that have to fulfill both heating and cooling functions.

In addition to propane (R290), which represents a natural alternative at sink temperatures up to 80 °C, hydrocarbons such as butane (R600), pentane (R601) and isobutane (R600a) are suitable as refrigerants for applications at high temperatures. This makes them particularly attractive for the provision of industrial process heat as well as for district heating applications. Due to their comparatively low pressure level, a transcritical operation is also conceivable, which can enable temperatures of 200 °C and beyond.

However, the use of hydrocarbons requires safety devices, as they are flammable. This property requires careful planning and monitoring in operation. The safe handling of these substances in industrial plants is part of the established state of the art.

Water as a refrigerant offers numerous advantages, including the possibility of achieving high temperatures of 200 °C and more. The refrigerant is environmentally friendly, widely available and extremely cost-effective. Since water is neither toxic nor flammable, no special safety concepts are required. Water can also be used directly as process steam in open heat pump systems (MVR). This makes this refrigerant well suited for use in energy-intensive areas such as the food, chemical, or paper industry based on, for example, industrial waste heat or deep geothermal energy as a heat source.

However, the source temperature must be high enough. At temperatures below 100 °C, operation under negative pressure would be required, which would lead to large facilities and high volume flows. A better solution would be a two-stage system, in which another refrigerant circuit is used in the lower temperature range. For operation at high temperatures, especially the compressors have to be designed for the high pressures and temperature peaks, which requires measures such as intercooling as well as adjustments of seals and oils.