If we review the Transcritical CO2 Booster system, many of the counterpoints stem from utilizing R-744 in hotter climate zones. The easiest solution would be to install the system in a colder climate where the outside temperature stays well below the critical point of R-744 for most of the year. Unfortunately, that is not possible for a large section of the U.S. A great solution to ensure R-744 stays below its critical point is apply it in a cascade arrangement where the CO2 is only serving low temperature loads and is always condensed in a cascade heat exchange below it’s critical temperature by using a high side (non-CO2) refrigeration system to cool it. Another solution is to simply pump and circulate it as a saturated fluid and use a high side (non-CO2) refrigeration system to maintain the temperature of the CO2 and condense any evaporated CO2 back into a liquid. In principle, both systems work and accomplish the same result. They work by having two sides, a high side and low side, running simultaneously. The high side looks familiar to many refrigeration systems found in stores today where you have compressors discharging hot gas to a condenser that condenses the gas to a liquid state. However, instead of the liquid flowing out to a case or coil in the store, it flows to a heat exchanger that condenses the CO2 on the low side. On the low side, R-744 is a great option as we get all the benefits previously mentioned in the previous system while also removing the efficiency concerns in higher ambient temperatures since it is condensed with a heat exchanger that is independent from outside temperatures. On the high side, we are faced with challenges on what refrigerant to use. Pending regulations for new remote systems limit high sides to 300 GWP starting January 1, 2027. This means many of the current HFC refrigerants being used today will not be a viable option long term in the industry. However, a low side CO2 system poses a unique opportunity for remodel applications where an existing HFC rack serving a store’s cases/coils can be converted to be the high side of a newly installed low side system. This is likely to be an intermediate step for many end-users today as it will reduce the amount of HFC refrigerant at a store, utilize R-744 under conditions it is more efficient, and allow for tactical remodels or new builds until all current and future refrigerant options are adopted by regulatory jurisdictions and vetted out by the industry.

All previously mentioned systems run into a variety of challenges that result in manufacturers and engineers having to add complexity to the designs when compared to the existing HFC systems that are running in many stores today. A standard direct expansion (DX) refrigeration system utilizing an A2L classified refrigerant has the potential to relieve some of these complexities for many end-users in the future. Utterly conventional in most ways, A2L classified refrigerant systems can be used in ways that are familiar in design to all refrigeration technicians making them easier to service while reaching regulatory GWP limits into the future. The major components of the system are the same four found in most grocery stores today: compressor rack, condenser, expansion valve and evaporator. These systems do not include the requirement for primary/secondary sides, booster system designs, nor electronic expansion valves with case controllers which can be seen as cost savings on equipment when compared to other leading solutions. Additionally, there is no significant efficiency penalty in higher ambient temperatures as seen with transcritical CO2 booster systems. Sounds like the clear answer for the system of the future, right? Not quite. The “catch” is A2L refrigerants are mildly flammable. When comparing an A2L classified refrigerant, R-454C, to an A3 classified refrigerant, R-290 (propane), it is seen that R-290 has a 4.5 times faster burning velocity and 7.5 times lower LFL (lower flammability limit in Lb/1000 ft^3) per ASHRAE 34. This means that an A2L refrigerant system charge can be significantly higher with proper safety mechanisms in place. These safety mechanisms include but are not limited to leak detection and means of shut offs throughout the system for leak mitigation, stricter circulation and ventilation requirements, additional pipe protection, and proper safety rated system components/equipment. Therefore, similar to the other previously mentioned systems, an A2L refrigerant cannot be used in a drop-in retrofit to an existing HFC system and will result in adding new compressors, condensers and cases/coils. However, a possible benefit of an A2L refrigerant system is some existing piping could be reused from a store’s system with correct verification and marking modifications. The challenges noted above for A2Ls are overall manageable especially when many end-users are confronted with the obstacles of the other leading design solutions. However, the biggest hurdle for widespread implementation of A2L refrigerants in U.S. commercial refrigeration is adoption by local and state jurisdictions. ASHRAE Standard 15 (safety code) was updated in 2022 to include requirements and larger changes for A2L refrigerants. However, ASHRAE does not equal building code at the state level and many jurisdictions have shown resistance to updating their legislation resulting in older standards being used. Older standards had little or no allowance for A2L refrigerants in direct systems so end-users are put in a vulnerable position where projects could be blocked by local jurisdictions and fire marshals. Possible regulations of PFAS, or better known as the “Forever Chemicals”, are also found in the conversation of A2L refrigerants in commercial refrigeration, but the extent of any regulation is still preliminary. The bright spot is trends show A2L adoption in commercial refrigeration will likely come as end-users, manufacturers and engineers all continue to lead the push but the question of when still remains. In the short term, A2L refrigerants in commercial refrigeration encounter several significant challenges for widespread adoption. However, in the long run, they could become an excellent solution, either as the primary side of a secondary system or in a standard DX refrigeration system, which we are all familiar with today.