The other day, my college-age daughter asked me about some of the subjects I’ve covered for The NEWS, and the terms ODP and GWP came up. She hadn’t heard of them, so I offered a brief explanation. That wasn’t good enough for my little engineering major, as she wanted to know exactly how the numbers were calculated. I had absolutely no idea, but I said I would find out. So here goes (and remember, I’m not a scientist).

When I first started writing about the HVACR industry more than 25 years ago, the term used to describe the harmful effects of refrigerant emissions in the atmosphere was ODP. After scientists discovered the hole in the ozone layer, they determined that it was caused by the chlorine in chemical compounds like CFCs, which were widely used in aerosols and refrigerants. They needed a measurement to describe the amount of ozone depletion caused by various substances, thus the term ODP was born.

Specifically, ODP is the measure of the impact on ozone of a chemical compared to the impact of a similar amount of R-11, which is a CFC. With its three atoms of chlorine per molecule, R-11 was used as the baseline, because it was considered to be the most harmful to the ozone layer; therefore, it was given the highest designation of 1 ODP. Other CFCs and HCFCs were given ODPs ranging from 0.01 to 1, depending on the amount of chlorine they contained. So, for example, R-22 — an HCFC — has only one chlorine atom per molecule, so its ODP is 0.05, which means its potential to destroy ozone was calculated to be 5 percent that of R-11.

ODPs were a very important measurement in the Montreal Protocol, the international treaty that led to the complete phaseout of CFCs and other ozone-depleting chemicals. With CFCs and HCFCs being phased out, HFC refrigerants were developed to take their place in air conditioning and refrigeration equipment. HFCs have zero ODP because they do not contain chlorine. However, scientists started becoming concerned that HFC emissions might be contributing to global warming, so the new GWP measurement was created.

GWP measures the total amount of energy that a gas absorbs over a particular period of time (usually 100 years) when compared to CO₂. CO₂ is used as the baseline and has the lowest designation of 1 GWP. The higher the GWP, the more likely it is to contribute to global warming. So, for example, the commonly used HFC refrigerant R-410A has a GWP of 2,088, which means it can cause 2,088 times as much global warming as an equal amount of CO₂ over 100 years.

Calculating GWP is nowhere near as straightforward as ODP, and I must admit that I don’t entirely understand the methodology, as it involves concepts like “radiative forcing” and “pulse emissions,” combined with various “time horizons.” (For anyone interested in learning more about how GWP values were initially calculated, check out page 385 of the Intergovernmental Panel on Climate Change (IPCC) report, Climate Change 2001: The Scientific Basis.)

While the GWP values calculated by the IPCC are widely accepted in the HVACR industry and elsewhere, critics contend that they don’t mean much. They cite the 100-year metric as being an arbitrary value and point to the IPCC’s own statement in the aforementioned report that says, “Some of the GWPs have larger uncertainties than that of others, particularly for those gases where detailed laboratory data on lifetimes are not yet available … The direct GWPs for those species whose lifetimes are well characterized are estimated to be accurate within ± 35 percent, but the indirect GWPs are less certain.”

Critics of GWP offer an alternative method that can be used to assess the potential harmful effects of refrigerant emissions on the environment — the global temperature change potential (GTP), which the IPCC defines as being the change in global mean surface temperature at a chosen point in time in response to an emission pulse relative to that of CO₂. This metric has its own critics, who say that the uncertainty in absolute GTP is ± 90 percent at 100 years.

It seems like there is still a lot of uncertainty surrounding the best way to calculate a refrigerant’s potential harm to the environment. So I would like to make my own suggestion: Let’s spend less time arguing over these types of metrics and more time on designing and installing air conditioning and refrigeration systems that don’t leak in the first place.

Publication date: 9/3/2018