Transport refrigeration units as we know them will not disappear anytime soon, but alternatives to traditional refrigerants and diesel power systems are already proving themselves in some markets around the world.

Manufacturers are using low-impact, natural refrigerants such as carbon dioxide and liquid nitrogen to reduce the units’ global warming potential in the event of an unplanned release. And diesel power systems are being scaled to accommodate hybrid-electric systems with high-efficiency heaters and direct-drive compressors that are said to be more energy-efficient and quieter.

Last October, at a conference in Kigali, Rwanda, negotiators from nearly 200 nations hammered out a legally binding agreement that will scale back the worldwide use of hydrofluorocarbons, or HFCs, used in air conditioners and refrigeration units. The agreement is structured so the world’s hottest countries and emerging economies will come into compliance over a longer period with less stringent reductions. The richest countries, including the United States and the European Union, will freeze production and consumption of HFCs by 2018, eventually reducing them to about 15% of 2012 levels by 2036, the New York Times reports.

The Kigali agreement will put pressure on TRU producers to develop new refrigerants as well as the blowing agents used in the foam insulation between the trailer’s inner and outer walls.

Some manufacturers are already pursuing alternatives to the current HFC refrigerant, R-404A, including carbon dioxide.

If it sounds counterproductive to use CO2 as a refrigerant while trying to reduce the output of greenhouse gases, consider that CO2 has a “global warming potential” of 1, while R-404A’s GWP is 3,922 (both Thermo King and Carrier in Europe offer R-452A as an alternative with a GWP of 2,140 – 45% lower than R-404A. R-452A is awaiting approval from the U.S. Environmental Protection Agency for use in the United States.)

Last April in Great Britain, Carrier Transicold UK announced a three-year field trial of a modified road version of the company’s NaturaLine refrigeration system for ocean containers, which operates exclusively with CO2 refrigerant.        

The new prototype system uses technology from the NaturaLine system, assembled inside a traditional Carrier Vector unit chassis and powered by the same E-Drive all-electric technology as Carrier’s existing European Vector products. Carrier has not disclosed any details on the cost of such a system, or if or when it might come to North America.

Europe has at least one other set of regulations that the U.S. currently does not: noise emissions. Current EU standards require that transport reefer units not exceed a sound pressure level of 60 decibels at a 25-foot radius around the equipment. While equipment noise levels vary with the application, a study published in December 2013 by Rensselaer Polytechnic Institute found the average noise level of refrigerated trucks to be 69-74 dB.

Quieter hybrid-electric refrigeration systems are available, but they tend to be more expensive and don’t offer the return on investment expected by major for-hire fleets on their shorter trade cycles. Private carriers, however, tend to keep their equipment longer and often do nighttime deliveries in urban areas. They’re embracing the technology, HDT was told by one OEM official.

Last year we became aware of a trailer cooling system from Boreas Nitrogen Cooling Systems that uses cryogenic liquid nitrogen for cooling. Boreas has had several trailers in fleet testing with a grocery fleet in California and is reporting good results. Nitrogen makes up roughly 80% of the air we breathe and is, therefore, non-toxic and non-polluting, and it’s not a greenhouse gas. Nitrogen-rich environments, however, do not support human life, so special precautions must be taken to keep workers out of the trailer until normal oxygen levels have been restored.

Boreas says the system is cost-effective and offers significant environmental benefits, such as zero diesel emissions and zero global warming potential. Additionally, the system is said to be virtually silent, making it attractive to operators delivering in residential areas, and virtually maintenance-free.

While alternatives to current refrigeration technology exist, there’s little need to worry at this point about how your next reefer will work. Save for the Kigali agreement on GHG reduction, no one is demanding we comply with more stringent environmental or noise standards.

Yet.