Why do HFOs have lower GWPs than HFCs?

Internationally, the Kigali amendment to the Montreal protocol, and in the US, the AIM act, seek to reduce or phase-down the amount of Hydrofluorocarbon (HFC) refrigerants that are used in refrigeration and air conditioning systems. The two main alternatives today are Hydrofluoro-olefins (HFO) and nature-identical refrigerants1.  However, the question remains, why do HFOs have a lower global warming potential (GWP) than HFCs when they are both made up of the same basic molecular blocks: Hydrogen (H), Fluorine (F) and Carbon (C)?  The answer lies within the molecules’ structure. 

GWP is a comparative measure of how much a particular gas will cause atmospheric warming.  It considers the gas's ability to affect radiated heat from the earth's surface and for how long it can last. GWP measurements are normalized using CO2 as 1 as the comparison basis for all gases. For example, if a gas is assigned a GWP of 150, it means that one pound of that gas will have the same warming effect as 150 lbs of CO2 when released into the atmosphere.  It is important to note also that GWP is measured over a specific time period.  Most of the GWP values we see today in literature, as well as those used by the US-EPA, are based on a 100-year basis (GWP100).  However, some are beginning to use a 20-year period (GWP20), which will be different for the same gas than the value of GWP100. 

A gas with a short atmospheric life has the potential for a lower GWP than one with a long life since it will only affect the atmosphere for a shorter time.  This is precisely the difference between HFCs and HFOs when it comes to GWP.  HFCs are a stable saturated molecule with longer lifetimes.  HFOs have a double bond that is fragile and will break quickly when exposed to the right conditions in the atmosphere.  The lifetime of HFOs can be measured in days, while that of HFCs can typically be in the order of years. 

This does not mean that HFOs will degrade quickly as compared to HFCs when used in refrigeration systems.  HFOs break down under atmospheric conditions such as exposure to visible and UV wavelengths that are not present inside a system.  A properly maintained system can be expected to have the same life expectancy for the refrigerant regardless of whether it uses HFOs or HFCs. 

HFOs are a lower-GWP alternative to HFCs that should be considered for new equipment or retrofits. 

Please contact our experts for further information on how to take advantage of the benefits of HFOs in your application.