Heat pump refrigerants, efficiencies and impacts

1 Introduction
2 How heat pumps work
3 Why are some heat pumps more efficient?
4 New breeds of heat pumps
5 Different refrigerants
6 Types of refrigerants
7 Related articles on Designing Buildings
8 External Links

[edit] Introduction

Heat pumps are increasingly seen as the most likely alternative to gas heating systems because, depending on their performance rating and conditions, they can produce between 3-5 units of heat for each unit of electricity. Unlike wood stoves and gas boilers, no combustion is involved, and the only emissions associated with the production of heat relate to the manufacture, disposal of the unit at the end of its useful life, and the source of electricity used to run it. However, unlike producing heat through combustion, which by its nature occurs at high temperatures, heat pumps often run at lower temperatures, and every degree of temperature rise relates to a number of factors.

[edit] How heat pumps work

Heat pumps extract heat from a source, such as the surrounding air, waste heat, geothermal energy in the ground, or sources of water, and amplify that heat. This amplification is based on three interrelated gas laws that relate the pressure, volume, and temperature of a gas, which in the case of a heat pump is called a gas refrigerant (though there are also liquid refrigerants). The effective result of these gas laws, most relevant to heat pumps, is that when the pressure of a gas increases, its temperature also increases. So a heat pump extracts what heat it can from its heat source and exchanges this heat with a refrigerant, which is then partially warmed and compressed to increase the temperature, the heat of which is then extracted for use. The refrigerant then cools and expands through an expansion valve. The cooled refrigerant then performs the same cycle as and when required, to meet the demand temperature.

[edit] Why are some heat pumps more efficient?

The electrical energy of a heat pump is used in the same cycles as the heat pump works to support the extraction of heat from the source, often with the use of fans or pumps, and the application of pressure to the gas refrigerant to increase its temperature and pump the heat via a medium through the system to where it is needed internally. The efficiencies of the system lie in the number of cycles, which in turn relates to the heat source temperatures, the expected resultant temperatures, and the efficiency of the fans, pumps, and refrigerant.

While the source temperature will vary, the expected output temperature can be controlled by system design, such as larger surface areas of heat emitters and increased efficiency in terms of fabric loss, insulation, airtightness, etc. Likewise, the efficiency of the fans and pumps can be maximised through different controls and designs. Finally, the efficiency of the refrigerant, that is, its ability to carry temperature and increase its temperature under pressure and then cool, can vary and can be denoted by its volumetric cooling capacity (VCC). Many different refrigerants have been used over time, and the efficiency of these needs to also be balanced with their environmental impact, which is why the refrigerants used have continually changed.

The terms coefficient of performance (COP) and seasonal coefficient of performance (SCOP) are standardised measurements of the efficiency of the system in heating overall, or in the case of seasonal, in relation to seasonal temperature changes. These describe how well a heat pump has the potential to work, depending on the other factors described. COP and SCOP are denoted by a factor; 1:3 for example expressing how many units in heat output can be achieved from the electrical units of input, in this case one unit of electricity produces three units of heat. Though, as mentioned it is important to consider the temperature of that heat and maximising its benefit.

The terms energy efficiency ratio (EER) and seasonal energy efficiency ratio (SEER) work on the same principle but are most commonly used to describe efficiencies of systems in cooling, such as air conditioning systems or heat pumps used for cooling in warmer climates. The same issues need to be considered as with systems that produce heat.

[edit] New breeds of heat pumps

Today, in general there are two main types of heat pump used; monobloc systems and split systems.

A monobloc heat pump system is made up of a single unit heat pump that is located outside of the home, on a wall or area that has free air and maintenance access. This outdoor unit is draws heat or coolth from the outside air to heat or cool a home. It consists of the compressor, the heat exchanger and water side of the refrigeration cycle. In this system the entire process happens in the same unit with pre-heating occurring outdoors before moving inside to directly heat the house or with top up electrical heating.

In the newer split systems, a fan and condenser draws heat or coolth from the outside air, collecting and transferring the now pre-heated (or cooled) gas to another unit which is located inside the home. This secondary unit then transfers the heat from the compressed gas refrigerant to the water systems in the house where it is required or for storage, such as in a hot water cylinder. The newer split unit systems in general have the advantage of smaller outside units, that run more quietly and are often more efficient, however they are more complex and often more expensive to install than monoblock systems.

In September 2023 the UK government Department for Energy Security and Net Zero, Boiler Upgrade Scheme increased the grants available for UK households to install new systems. Acting on behalf of property owners, installers can apply for between £5,000 and £7,500 of the cost and installation of an air source heat pump, ground source heat pump (including water source heat pumps) or a biomass boiler (the lower amount). As of 2024 the status of the scheme remained as being a total of £450 million in grant funding available over the three years from 2022 to 2025.

In response to feedback and in relation to the efficiency points that are described above eligibility for the grant is dependent of the receipt of an Energy Performance Certificate (EPC) and where improved fabric performance is a recommendation on the EPC, it must be carried out to a degree before the property is eligible to receive a BUS grant. Elements such as heat emitters with larger surface areas, to counter balance the often lower temperature outputs of heat pumps are not currently a stipulation in the government scheme but a consideration that lies with the system designer.

[edit] Different refrigerants

Natural refrigerants such as propane and ammonia have been used for many years, but as the need to then keep food safe and cool became more difficult with ice over longer transport distances, a variety of refrigerants emerged. Prior to around the 1900s many refrigerants were unsafe and dangerous. By the late 1920s Thomas Midgley Junior developed the first synthetic refrigerant chlorofluorocarbon (CFC) R-12, or Freon, then a safer, non-flammable, and non-toxic refrigerant. It was stable, and could be used under different operating conditions, and could be combined with oil for use in compressors. Its effect on the ozone layer was not at that time known, and its use was widespread. Iterations such as R-22 (a HCFC) and R-134a, are still found today in applications like refrigerators and vehicle air conditioners.

The range of potential refrigerants is broad such as chlorofluorocarbons, ammonia, hydrocarbons, carbon dioxide, chlorodifluoromethane, tetrafluoroethane, freon, CO2, natural refrigerants, dichlorodifluoromethane and water. As such many are now referred to by a type and with a prefixed R, which was introduced by DuPont which eventually owned the brand Freon. There are other classifications that may be used such as class 1, 2 and three denoting the way the refrigerant works by storing latent heat, sensible heat or the ability to carry liquefiable vapours.

[edit] Types of refrigerants

[edit] Carbon dioxide (R744)

Carbon dioxide is not a new refrigerant. The use of carbon dioxide as a refrigerant lasted for well over a century, but was abandoned in the mid-1950s, with the widespread use of the CFC refrigerants, which were more efficient, more stable and safer. It was ‘rediscovered’ in the early 1990s. Due to its low environmental impact, low toxicity and non-flammability. CO2 is now regaining popularity with refrigeration system designers whilst an alternative to fluorocarbons is being sought.

[edit] Propane (R290)

Propane (R290) has a boiling point of –42°C, making it an excellent alternative to R22 as it requires similar working pressures. An added advantage is that, other than added safety measures because of its flammability, virtually no design change is required in systems when switching from R22 to propane. The combination of its good thermodynamic and thermo-physical properties yields systems that are at least as energy efficient as those working with R22. The use of propane is increasing in countries where regulations allow it. R290 heat pumps have a higher coefficient of performance (COP) than R32 heat pumps at lower outdoor temperatures, making them a better choice for cold climates.

[edit] HFC-32 (R-32)

HFC-32 (R-32) is an organic hydrofluorocarbon compound with the formula CHF. The comparatively low GWP of R32 which is 675 and zero ODP means it might be considered a more environmentally-friendly choice than traditional refrigerants like R410A. It is a single-component refrigerant meaning it can be easier to recycle and indication is that such R32 systems requite 20% less refrigerant than say R410A models, so also more efficient and with a lower cost.

[edit] Propylene (R1270)

R1270 (Propylene) is refrigerant-grade propylene which can be used in low and medium temperature refrigeration and air- conditioning applications. R1270 (Propylene) is a hydrocarbon and although highly flammable is an efficient refrigerant with low Global Warming Potential (GWP) and can be used as an alternative to R22.

[edit] Ammonia (R171 / R717)

Ammonia has been continuously used throughout modern refrigeration history, but has numerous drawbacks. It is toxic and flammable in concentrations between 15.5% and 28% in air. It is not compatible with copper, thus requiring other materials for construction. But ammonia’s thermodynamic and thermo-physical properties also yield very efficient refrigeration systems. Because of its acute toxicity, stringent regulations apply for ammonia systems, which require close monitoring and highly-skilled engineers and technicians. It terms of industrial usage R717 is a suitable refrigerant for heat pumps as its efficiency is relatively high and because it is a 'natural' refrigerant it does not contribute to the greenhouse effect.

[edit] Isobutane (R600A)

Isobutane (R600A) is a hydrocarbon, and is flammable. Its thermodynamic properties are very similar to those of R134a. Isobutane presents other advantages, such as its compatibility with mineral oil and better energy efficiency and it is cheaper than R134a. The use of isobutane requires minimal design changes, such as the relocation of potential ignition sources outside the refrigerated compartment.

[edit] 1,1,1,2 - tetrafluoroethane (R134a)

1,1,1,2 - tetrafluoroethane (R134a) is a hydrofluorocarbon (HFC) used for domestic, commercial and industrial refrigerated applications, air conditioning, fluid cooling and in heat pumps. Popular in automotive and agricultural air-conditioning systems manufacturers, with a variation more likely to be used today named R-1234yf.

[edit] R-454b

R-454b is a blend that consists of nearly 70% R-32. Whereas R-32 is a single component refrigerant.

[edit] R-410A

R-410A a hydrofluorocarbon compound (HFC), was considered to be the most common alternative to R-22 after it was banned 2010. The Montreal Protocol, Kigali Amendment, then proposed to phase-down the use of all HFC refrigerants including R-410A.

[edit] R-22

R-22, a hydrochlorofluorocarbon (HCFC) is the standard refrigerant utilised in residential air conditioners from the 1930s. It was included in the 1987 Montreal Protocol list of substances noted for a step-by-step phase out of production.