Many air conditioning units on the side of a large apartment building.
Technical Summary

Alternative Refrigerants

Project Drawdown defines alternative refrigerants as the gradual replacement of hydrofluorocarbons (HFCs) used in a variety of applications by alternative refrigerants with significantly lower global warming potential (GWP), including ammonia, carbon dioxide, propane, and isobutane. 

Refrigerants are used as working fluid in commercial refrigeration systems; in household appliances such as air conditioners and refrigerators; in refrigerated containers used for carrying perishable goods; as air conditioning systems onboard cars, trains, aircrafts, and ships; and in industrial cooling systems, etc. There are various classes of refrigerants.[1] Chlorofluorocarbons (CFCs) are ozone-depleting substances and have been phased out under the Montreal Protocol; hydrochlorofluorocarbons (HCFCs) are also being phased out. HFCs, which do not deplete ozone, emerged as an alternative to HCFCs and have grown to extensive use. All refrigerants have a high GWP, and their release into the environment contributes to global warming. Considering the large impact that the release of refrigerants has on global warming, world leaders have agreed to phase out HFCs and replace them with natural refrigerants with much less warming potential under the Kigali Accord of October 2016. Refrigerants are emitted into the environment during the production process, from refrigerant banks[2] due to leakages, and during end-of-life disposal of appliances.


Measuring the alternative refrigerants solution requires: generating a total addressable market forecast of HFC refrigerant gas emissions for the period 2020–2050 (in kilotons of carbon dioxide-equivalent emissions per year), forecasting an adoption of replacement refrigerants and the resulting decreased emissions from reducing the amount of HFC produces, used, and released into the atmosphere for each year from 2020 to 2050, and then comparing the difference in emissions from the total market and the adoption scenarios to arrive at the results. 

Emissions from refrigerants can be reduced in five main ways:

(i)         lower the demand/use of appliances and thereby production of refrigerants

(ii)        replace refrigerants with low-warming HFCs/new cooling agents/non-HFC substances

(iii)       Increase the refrigeration efficiency in appliances, thereby lowering the use of refrigerants

(iv)       control leakages of refrigerants from existing appliances by good management practices

(v)        ensure recovery, reclaiming/recycling, and destruction of refrigerants at end of life.

This Project Drawdown solution models the second option.

Total Addressable Market

The model uses available projections for global HFC emissions across various subsectors, such as commercial, industrial, domestic, and stationary (as opposed to mobile) AC systems. Velders (2015, 2017, 2019) provides refrigerant emissions (measured in kton refrigerant – HFCs). Velders provides an upper scenario and a lower scenario. A mean scenario is also calculated. These forecasts are pre–Kigali Accord and show a steadily increasing HFC emissions profile through 2050. These scenarios become the total addressable market for the alternative refrigerants model. 

Adoption of alternative refrigerants is modeled using forecasts from the literature.

Adoption Scenarios

Velders and others estimate the adoption of alternative refrigerants correlated with two different shared socioeconomic pathways (SSPs) developed by the International Institute for Applied Systems Analysis (IIASA). The literature presents a low scenario and a high scenario. A mean scenario is also calculated. These three scenarios—low, mean and high—determine the PDS 1, PDS 2, and PDS 3 adoption scenarios. These were assessed in comparison to a Reference scenario, in which the solution’s market share was fixed at the current levels.

Emissions Model

In the reference case, HFC emissions are prognosticated according to the year-over-year data provided in the literature.  Whereas the exact mix of alternative refrigerants is not determined, the average GWP of 10 is used. This is much lower than the dynamic GWP of HFCs, which nears an average GWP of 2000.


The total carbon dioxide-equivalent reductions that can be achieved from 2020 to 2050 in the Scenario 1 are 43.5 gigatons. Scenario 2 shows a larger reduction of 50.5 gigatons from 2020 to 2050.


Indirect emissions from powering refrigeration appliances is a significant component of grid emissions and is expected to increase in a warming world. Adoption of low-GWP HFC replacements with new appliances brings the potential for increases in energy efficiency. Some have estimated that the potential impact on indirect emissions may be half as much or as great as direct emissions reduction. At this time this model and report do not include a forecast for indirect emissions reduction.

In order to achieve a global phase-down of HFCs, replacement fluids must be identified and a global refrigerant transition must be implemented. Currently, there is no single replacement fluid, and all replacements include trade-offs. The theoretically ideal refrigerant is the one that has zero ozone depletion potential (ODP) and low GWP, is nontoxic and nonflammable, has appropriate thermodynamic and heat transfer properties, and is compatible with any type of lubricating oil (El-Sayed, El Morsi, and Mahmoud 2018). Hydrocarbons (HCs) satisfy all the requirements except that they are highly flammable oil (El-Sayed, El Morsi, and Mahmoud 2018).

Climate-friendly alternatives to HFCs are increasingly available for most uses and fall into two basic categories: “natural” or nonfluorinated substances with low GWPs, and fluorinated substances with low to mid-range GWPs (Borgford-Parnell et al., 2018). Currently there is an emphasis on "natural refrigerants" that are gradually being used as a replacement for refrigerants from the hydrofluorocarbon group (Zanchi, Boban, and Soldo 2019). Commercially available  natural refrigerants include: ammonia (R-717), with a GWP of near zero; hydrocarbons (HC )(e.g., propane [R-290] and isobutene [R-600a]), with GWPs of less than 4; and carbon dioxide (R-744)  (with a GWP of 1). The use of hydrocarbons (e.g., isobutane and propane ammonia and carbon dioxide).

Commercially available fluorinated substances include O2, the low-GWP HFCs, also known as hydrofluoro-olefins (HFOs), some with GWPs of less than 1, according to the IPCC. Other alternatives include, HFC-32, with a GWP of 677, and HFC-152a, with a GWP of 138, according to the IPCC. There are also “not-in-kind” alternatives that do not involve refrigerants, such as district cooling

[1] Such as chlorofluorocarbons (CFCs) such as R-11, R-12, and R-502; hydrochlorofluorocarbons (HCFCs) such as R-22; hydrofluorocarbons (HFCs) such as R404A, R134a, and R 407; and natural refrigerants such as CO2 and NH3.

[2] The total quantity of refrigerant gases in existing equipment.