Can we cool our spaces without heating our planet?

Setting foot outside in Albuquerque any given day this summer has served as a constant reminder that each of the last 14 months has broken records as the hottest of its kind in history. Indeed, there is little doubt that 2016 will go down as the hottest year on record. However, perhaps the one positive piece of news associated with the atmosphere this year was the announcement that the hole in ozone layer over the Antarctic is finally beginning to heal. The Montreal Protocol worked.

Originally signed in 1987, the Montreal Protocol was the first global legislation to preserve the quality of Earth’s atmosphere. This agreement was signed by 197 countries in response to the discovery of the gaping hole created in the ozone layer caused primarily by chlorofluorocarbon (CFC) emissions. At the time, CFCs were the most widely used refrigerants with R-12 or Freon being the most well known one of these. The ozone layer is a most important part of the atmospheric construction because it reflects a great deal of UV-B radiation back into space. Without this protection, skin-cancer rates would soar, marine life would suffer, and crop yields would decrease. The protocol created a framework to phase out ozone-depleting chemicals beginning by banning CFCs.

Although the biggest culprit had been dealt with, CFCs are not the only halogen-containing refrigerants that contribute heavily to global warming. Hydrochlorofluorocarbons (HCFCs) were the cheapest alternative to CFCs, and had a lower ozone-depleting potential. The most common HCFC refrigerant is R-22, and although no new systems using R-22 have been manufactured in the US since 2010, it is still important enough for Wiley to include complete thermodynamic tables for the refrigerant in my Fundamentals of Engineering Thermodynamics textbook. R-22 will no longer be manufactured after 2020.

The hole in the ozone is healing and the most ozone-depleting chemicals have been banned and phased out, so why are the current refrigerants still under attack from environmentalists? The answer lies in the global warming potential (GWP) of these refrigerants. A GWP is assigned to a gas with carbon dioxide as the reference, and many commonly used refrigerants have GWPs in the triple and quadruple digits (i.e. If emitted, these gasses contribute to global warming hundreds to thousands times more intensely than carbon dioxide). For reference, R-22 has a GWP of 1810, R-134a has a global GWP of 1430, and highly used R-410A has a GWP of 2088. The latter two refrigerants are hydrofluorocarbons (HFCs), which are used in most commercial and residential air conditioning applications today.

Industry leaders, such as Emerson in a whitepaper published in 2014, claim that “one good option for air conditioning applications is to stay with HFC options such as R-410A until an economically viable alternative becomes available.”1 The justification for this is a metric called the Total Equivalent Warming Impact (TEWI) this takes into account not only direct emission of refrigerant, but the effects of system efficiency and source of electricity. Furthermore the total contribution of HFCs to global emissions is less than 3%.1 However, the bottom line is that it is more expensive and often more dangerous to create air conditioning systems using alternative refrigerants with similar efficiencies to those achieved with HFC refrigerants. It would appear that this excuse is losing effectiveness as the US is already proposing an amendment to phase down HFCs to 15% of baseline by the early 2030s. World climate leaders recently met in Vienna to discuss this next stage that could set the stage for a 2016 amendment to the Montreal Protocol.2

So where does the air conditioning industry proceed from here? Halogen-free refrigerants do exist. In fact, carbon dioxide (a.k.a. R-744) itself is one of the oldest refrigerants. A major hurdle in developing systems with CO2 as the refrigerant is the 30-50% efficiency hit incurred by using CO2 over HFCs in a simple thermodynamic cycle. However, CO2 does have favorable heat transfer coefficients and one effective way that systems can be designed with CO2 as the working fluid is to employ transcritical operation. CO2 does not condense at higher pressures like HFC refrigerants, so a gas cooler replaces the condenser in transcritical cycles. Initially as least, this thermodynamic cycle modification will require additional spending on R&D and a higher first cost of systems. Ammonia can also be used as a refrigerant and is attractive because it has no direct GWP. Unfortunately, ammonia’s toxicity makes it unsuitable for residential air conditioning applications. Finally, traditional hydrocarbons can be used as refrigerants, and at this point in history they are still cheap and abundant. However, their extreme flammability make them a tough sell to commercial operators.

Since 1987, the air conditioning industry has successfully continued to deal with regulations on their most common refrigerants while still improving efficiency. I commend the industry for innovating in system design rather than pushing back against regulations. It could be considered ironic, but it appears that the gas, which we have been pumping into the atmosphere since the industrial revolution thereby warming the planet, could soon be the standard working fluid coursing through thermodynamic cycles to provide cool air through our diffusers.

About the Author

                  Jeffrey Sward recently graduated from the University of New Mexico with a Bachelor of Science in Mechanical Engineering, and will continue his studies at Cornell University in the coming fall. As a New Mexico native, Jeffrey naturally enjoys the backpacking, fishing, snowboarding, skiing, running, and most other things outdoors, and he uses this as a justification for his interest in climate change and energy efficiency.

References

1 Refrigerants for Residential and Commercial Air Conditioning Applications. Tech. Emerson Climate Technologies, 2014. Electronic. http://www.emersonclimate.com/Documents/Resources/2007ECT-136.pdf

2 Ragendran, Rajan. Refrigerant and Energy Regulation Updates. Rep. Emerson Climate Technologies, 2015. Electronic. http://www.emersonclimate.com/en-us/About_Us/industry_stewardship/E360/Documents/Anaheim-Presentations/e360-anaheim-refrigerant-and-energy-regulations-update.pdf

3 Moniz, Ernest, and Gina McCarthy. "A "Cool" Way to Combat Climate Change under the Montreal Protocol." Energy.gov. US Department of Energy, 20 July 2016. Web. 21 July 2016.