Heat pumps require a working fluid. Traditionally, the most common working fluids for heat pumps have been:

• CFC-12 Low- and medium temperature (max. 80C);
• CFC-114 High temperature (max. 120C); R-500
• Medium temperature (max. 80C); R-502 Low- medium temperature (max. 55C);
• HCFC-22 Virtually all reversible and low temperature heat pumps (max. 55C).
• R-500 and R-502 are mixtures of CFC-12/HFC-152a
•  HCFC-22/CFC-115, respectively.
   

Due to their chlorine content and chemical stability, the CFCs (ChloroFluoroCarbons) are harmful for the global environment. They contribute to both the depletion of the earth's ozone layer and the greenhouse effect, and are now controlled under the Montreal Protocol, and in the last revision the Parties agreed to establish a complete phase-out of CFCs by the end of 1995 (Appendix C).

As a general requirement, heat pumps using alternative working fluids should have at least the same reliability and cost effectiveness as (H)CFC systems. Moreover, the energy efficiency of the systems should be maintained or be even higher, in order to make heat pumps an interesting energy  saving alternative. In addition to finding new and  environmentally acceptable working fluids, it is also important to modify or redesign the heat pumps.

Generally speaking, the energy efficiency of a heat pump system depends more on the heat pump and system design than on the working fluid.

HCFC working fluids also contain chlorine, but they have much lower ODP (ozone depletion potential) than CFCs, typically 2-5% of CFC-12, due to a lower atmospheric chemical stability. The GWP (global warming potential) is typically 20% of that of CFC-12. However, in the last revision of the Montreal Protocol, the HCFCs were also included on the list of controlled substances, and it was decided to virtually phase them out by the year 2020, with complete phase-out by the year 2030 (see appendix C). Germany has accelerated the process and decided to phase-out HCFC-22 by January 1, 2000. The EU is considering an accelerated phase out as well.

HCFCs are generally accepted as a part of the solution for a rapid CFC phase-out (medium-term solution). HCFC-22 can be used to replace CFC-12, R-500, R-502 in various heat pump applications, while possible HCFC working  fluids for replacing CFC-114 in high-temperature  industrial applications are HCFC-124, HCFC-123, and HCFC-141b.

HFCs

All the HFC working fluids are chlorine-free and have no ODP. However, some of the HFCs have a relatively large GWP, and may for this reason be regulated later. The most interesting HFC working fluids for heat pump applications are HFC-134a, HFC-152a, HFC-32, HFC-125 and HFC-143a.

HFC-134a is quite similar to CFC-12 regarding thermo- physical properties, and is therefore regarded as an important working fluid, both for retrofitting existing heat pumps and for new installations. The COP of a heat pump with HFC-134a will practically be the same as for CFC-12. At low evaporating temperatures  (below -10C) and/or high temperature lifts the COP will be slightly lower.HFC-134a should only be used together with recommended polyol ester lubricants. When retrofitting from CFC-12 to HFC-134a, proper cleaning of the  systems is required before recharging with HFC-134a. HFC-152a has mainly been used as a part of R-500, but it has also been successfully applied in a number of small heat pump systems and domestic refrigerators. HFC-152a is currently applied as a component in blends. Because of its flammability, it should only be used as a pure working fluid in small systems with low working  fluid charge (see also Hydrocarbons). HFC-32 is moderately flammable and has a GWP close to zero. It is considered as a suitable long-term replacement for HCFC-22 in space-conditioning, heat pump and industrial refrigeration applications. Due to its flammability, HFC-32 is mainly applied as a main component in non-flammable mixtures replacing R-502 and HCFC-22. HFC-125 and HFC-143a have properties fairly similar to R-502 and HCFC-22. They are mainly applied as components in ternary mixtures replacing R-502 and HCFC-22. The GWPs are, however, about three times as high as that of HFC-134a. Other Alternatives. HFC-227 is an alternative to CFC-114, while HFC-245ca and HFC-356 are identified as possible long-term replacements for CFC-11 in chillers, and CFC-114 in high temperature heat pumps. In the United States, a number of partially fluorated propane's plus two- and three-carbon ethers have been synthesized. Eleven of these compounds show potential as substitutes, and their properties suggest they could be applied for most refrigeration and heat pump applications.

Blends

Blends or mixtures represent an important possibility for replacement of CFCs, both for retrofit and new applications. A blend consists of two or more pure working fluids, and can be zeotropic, azeotropic or near-azeotropic. Azeotropic  mixtures evaporate and condense at a constant temperature, the others over a certain temperature range (temperature glide). The temperature glide can be utilized to enhance performance, but this  requires equipment modification. The advantage of blends is that they can be custom-made to fit  particular needs. A possible problem might arise with leakages since it is difficult to know the quantity and composition of the leaked and remaining fractions. Blends for replacement of CFC-12 and R-502 are already commercially available. They all contain HCFC-22 and/or other HCFC working fluids, such as HCFC-124 and HCFC-142b, and are therefore considered as transitional or medium-term working fluids. Suitable high temperature blends with low temperature glide are still under investigation and development. The new generation of blends for replacement of R-502 and HCFC-22 are chlorine-free, and will mainly be made from HFCs (HFC-32, HFC-125, HFC-134a, HFC143a) and hydrocarbons (eg, propane).

Natural Working Fluids

Natural working fluids are substances, naturally existing in the biosphere. They have in general negligible global environmental drawbacks (zero or near-zero ODP and GWP). They are therefore long-term alternatives to the CFCs. Examples of natural working fluids are ammonia (NH3), hydrocarbons (eg. propane), carbon dioxide (CO2), air and water. Some of the natural working are flammable or toxic. The safety implications of the use of such fluids may require specific system  design and suitable operating and maintenance routines.

Ammonia (NH3) is in many countries the leading working fluid in medium- and large refrigeration and cold storage plants. Codes, regulations and legislation have been developed mainly to deal with the toxic and to some extent, the flammable characteristics of ammonia. Thermodynamically and economically ammonia is an excellent alternative to CFCs and HCFC-22 in new heat pump equipment. It has so far only been used in large heat pump systems, and newly developed high pressure compressors have raised the maximum achievable condensing temperature from 58C to 78C. Ammonia can also be considered in small systems, the largest part of the heat pump market. In small systems the safety aspects can be handled by using equipment with low working fluid charge and measures such as indirect distribution systems (brine systems), gas-tight rooms or casing, and fail-safe ventilation. Copper is not compatible with ammonia, so that all components must be made of steel. Ammonia is not yet used in high-temperature industrial heat pumps because there are currently no suitable high pressure compressors available (40 bar maximum). If efficient high pressure compressors are developed, ammonia will be an excellent high-temperature working fluid. Hydrocarbons (HCs) are well known flammable working fluids with
favorable thermodynamic properties and material compatibility. They include propane, butane and their mixtures. They are widely used in the petroleum industry, sporadically applied in transport refrigeration, domestic refrigerators/freezers and recently heat pumps (notably in Germany). Due to the high flammability, hydrocarbons should only be retrofitted and applied in systems with low working fluid charge. To ensure necessary safety during operation and service, precautions should be taken such as proper placing  and/or enclosure of the heat pump, fail-safe ventilation systems, addition of tracer gas to the working fluid, use of gas detectors etc.

Water is an excellent working fluid for high temperature industrial heat pumps due to its favor able thermodynamic properties and the fact that it is neither flammable nor toxic. Water has mainly been used as a working fluid in open and semi-open MVR systems, but there are also a few closed-cycle compression heat pumps with water as working fluid. Typical operating temperatures are in the range from 80C to 150C. 300C is achieved in a test plant in Japan, and there is a growing interest for utilizing water as a working fluid, especially for high temperature  applications. The major disadvantage with water as a working fluid is that the low volumetric heat capacity (kJ/m3) of water. This requires large and expensive compressors, especially at low temperatures.

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