Heat absorption is typically, but not necessarily, done by evaporation (boiling of refrigerant) and heat release by condensation. Refrigerants normally are denoted by R followed by a combination of numbers, which is built from the chemical structure of the refrigerant.
The boiling points of refrigerants differ with the pressure; the higher the pressure, the higher boiling points. At atmospheric pressure, their boiling points can be lower than –40°C and therefore they can be used even with low temperature heat sources or to provide low temperature for refrigerated storage. Different refrigerants have different characteristics and these are used to match the requirements of the application in question.
One important characteristic of refrigerants is the environmental impact that is caused if they are released or leaked from the heat pump. To estimate this, indicators as ODP, GWP and TEWI are commonly used.
ODP, Ozone Depletion Potential, indicates the expected ozone layer depletion of a specific refrigerant in relation to that caused by R11 (trichloroflouromethane).
GWP, Global Warming Potential, is the amount of heat a greenhouse gas traps in the atmosphere compared to the heat trapped by the same amount of CO2, which is the reference gas with a GWP of 1. The degradation in the atmosphere of CO2 and most common refrigerants are driven by quite different processes. Hence most of the effect of CO2 remains after 50 years whereas there is virtually no effect left by the common refrigerant. Therefore the time frame is quite important, and must be specified, in stating a value of GWP (IIR gives values based on 100 years of integration time).
In the operation of heat pump systems, there are direct contributions of greenhouse gases from refrigerant leakage and indirect contributions resulting from the energy supply required to run the systems over their normal lives. The sum of the direct and indirect emissions of greenhouse gases represents the Total Equivalent Warming Impact (TEWI) of the technologies being compared. The calculated TEWI is sensitive to assumptions on the system lifetime, emission losses, and the integration time chosen to calculate the GWP values and, of course, the use of drive energy.
Traditionally used refrigerants – CFCs and HCFCs
CFCs (halocarbons) and HCFC (hydrohalocarbons) were the most commonly used refrigerants for a long period of time. Unfortunately research showed that these substances are harmful to the global environment due to their ozone depletion and global warming potentials. Therefore these refrigerants are listed on a phase-out schedule, which was agreed under “The Montreal Protocol on Substances that Deplete the Ozone Layer” and its amendments. In some countries, this was implemented already in the 1990s.
The result of this is that it is currently globally prohibited to sell, install and refill systems with CFCs. However, HCFCs were given a longer phase-out period due to their lower ozone depleting potential but in many countries they are already banned by national legislation.
Instead of CFCs and HCFCs, HFCs were developed (in some contexts they are referred to as F-gases or Flourinated gases). HFCs do not deplete the ozone layer but they still contribute to global warming by having high GWP values. Because of the ozone aspect, HFCs have already replaced CFCs and HCFCs in a wide range of products. Unsaturated HFCs, or hydroflouro-olefins (HFO) have been developed as alternatives to HFCs and can be used as single substances (e.g. HFC-1234yf in automotive air-conditioning systems) or in mixtures with HFCs, where they reduce the GWP of the blend.
The use of HFCs is regulated in some parts of the world, for example by the revised F-gas regulation which was adopted by the European Commission in 2015. A global HFC phase-down would deliver a significant contribution to the fight against climate change.
Refrigerants based on substances that exist natural in the environment are called natural refrigerants. These refrigerants generally have a low impact on the global environment, with both Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) at a zero or close-to-zero level. Therefore, these are viable options for long-term replacement of the synthetic CFCs, HCFCs and HFCs However, in some applications they are related to other risks or challenges that must be handled. Commonly used natural refrigerants today are ammonia (NH3), hydrocarbons (HCs), carbon dioxide (CO2), air and water.