Federal and state regulations for fuel economy and emissions of passenger cars, light- and heavy-duty trucks, and buses have been grabbing headlines for decades. Meanwhile, a quieter revolution has been underway in the transport refrigeration sector.
The Montreal Protocol on Substances that Deplete the Ozone Layer was the root cause of the Clean Air Act, which required the U.S. Environmental Protection Agency to undertake actions to reduce the use of chlorinated refrigerants. The recent Kigali Amendment adds the most widely used replacements for chlorinated refrigerants to the list of substances to be phased down under the Montreal Protocol.
As legislation ramps up, the next generation of refrigerants and cooling technologies that do not use conventional compressor lubricants are shaping the future of refrigerated transportation by road vehicles, intermodal containers and ships.
The Montreal Protocol is a global agreement coordinated by the United Nations to phase out the production and consumption of chemicals that react with ozone in the Earth’s atmosphere. The protocol is a treaty or binding international agreement that went into force in 1989. To date, 197 countries—including the United States—have ratified the protocol, which requires parties to gradually reduce their use of ozone-depleting substances—also known as ODSs—by 80%-85% by the late 2040s.
After the U.S. signed the Montreal Protocol, its Congress amended the Clean Air Act to implement the protocol through the Environmental Protection Agency. That is, it mandated regulations on the use and production of ODSs, and Title VI of the CAA authorized the EPA to run programs to accomplish these goals. The EPA’s schedule calls for a 100% ban on remaining production and import of hydrochlorofluorocarbons by Jan. 1, 2030.
Ozone, or O3, is a relatively unstable molecule that is present in the stratosphere—the layer in the Earth’s atmosphere that extends from between 5 and 9 miles to 31 miles above the Earth’s surface. Because ozone molecules scatter ultraviolet radiation emitted by the sun, they act as a shield to protect the planet. However, ODSs, including chlorofluorocarbons and hydrochloroflurocarbons that are commonly used as refrigerants, solvents and blowing agents for foams, undergo degradation when exposed to sunlight in the upper atmosphere and release chlorine atoms, which attack and destroy ozone.
Section 608 of the CAA, Stationary Refrigeration and Air Conditioning, established the National Recycling and Emission Reduction Program. It prohibits individuals from deliberately releasing ODS refrigerants, including CFCs and HCFCs and most substitutes—like hydrofluorocarbons—in the process of maintaining, servicing, repairing or disposing of refrigeration or air conditioning equipment.
According to Section 609 of the CAA, Motor Vehicle Air Conditioning, the EPA is responsible for regulating the release of refrigerants during the servicing of motorized vehicle air conditioners in passenger cars, buses and truck cabs as well as “MVAC-like appliances” in cabs of agricultural and construction vehicles. Section 609 regulates the training and certification of service technicians, approval of refrigerant recovery equipment and certification of facilities.
The EPA reported that fluorinated gas emissions in the U.S. rose 86% from 1990 to 2019, largely due to a 275% increase in the release of HFCs used as substitutes for CFCs and HCFCs. Because HFC molecules contain fewer chlorine atoms than CFC and HCFC molecules, they are less destructive to ozone on a per molecule basis. However, HFCs can affect the atmosphere negatively.
EPA and UNEP describe HFCs as “powerful greenhouse gases.” Greenhouse gases in the Earth’s atmosphere—including water vapor, carbon dioxide, methane, nitrous oxide, ozone, sulfur hexafluoride and HFCs—absorb and emit radiant energy from the sun. These gases—and other factors, like clouds—in the atmosphere control the amounts of radiation that are absorbed by the Earth and reflected as light and heat. Changes in the composition of the Earth’s atmosphere shift the balance of these effects, which causes changes in the global climate.
From before 1750 to the present, the amounts of carbon dioxide, methane and nitrous oxide in the troposphere have increased by 47%, 150%-170% and 20%-21%, respectively.
Fluorinated gases were not present in the atmosphere prior to the Industrial Revolution. Although they are now detected at relatively low concentrations, fluorinated gases, including HFCs, have a particularly strong insulating effect that traps heat instead of reflecting it out of the Earth’s atmosphere. This greenhouse effect is a major cause of climate change.
According to the EPA, “In general, fluorinated gases are the most potent and longest lasting type of greenhouse gases emitted by human activities.” In 2019, 92% of fluorinated gas emissions in the U.S. were substitutes for ODSs, primarily refrigerants in vehicles and buildings. The main causes of these emissions are leaks as well as emissions that occur during servicing and equipment disposal.
In 2016, the United Nations adopted the Kigali Amendment to the Montreal Protocol, which added HFCs to the list of substances controlled under the protocol and mandated that they be phased down. At present, 122 nations have ratified the Kigali Amendment, but the U.S. is not among them.
The Ozone Secretariat of the United Nations Environment Program said: “By phasing out ozone-depleting substances that are powerful greenhouse gases, the protocol has avoided the equivalent of around 135 billion tons of CO2 emissions. The protocol’s Kigali Amendment builds on this achievement, avoiding up to 0.4 degrees Celsius warming by 2100, by phasing down the use of hydrofluorocarbons.”
Phase down of HFCs is underway in Western Europe, and it seems to be inevitable in the U.S., China and India.
Most conventional refrigeration systems use direct expansion vapor compression cycles, a closed system of tubing and devices, a mechanical compressor and a refrigerant that circulates through the system. In a single cycle, the compressor applies pressure and squeezes the gaseous refrigerant. Its temperature rises, and it travels through the tubes to the condenser, where it cools and liquefies.
Next, the refrigerant passes through an expansion valve—a tiny opening in the tubing—and quickly expands. Its pressure drops, and some of it evaporates to a gas and cools, drawing heat from the liquid portion that has not evaporated. The gas-liquid mixture flows to the evaporator, where the fluid evaporates and becomes even colder. A fan circulates air from inside the refrigerated unit over the evaporator, where it cools before it returns into the unit to cool the contents. The vaporized refrigerant returns to the compressor and begins a new cycle.
Refrigerant rarely leaks from stationary systems, like refrigerators and air conditioners in homes and supermarkets, because the compressor and its motor are integrated inside a welded container. Leaks of refrigerant and compressor lubricant are much more common from shaft seals of systems in which the compressor and motor are externally coupled, as in mobile systems and automotive air conditioning units.
The transport refrigeration (TR) sector includes three sub-sectors: road vehicles, including vans, large trucks and trailers; intermodal containers carried short distances on trucks and trains and long distances on ships; and ships. Perishable food and beverages are the primary cargo of TR systems, and pharmaceuticals are secondary. Unlike stationary refrigeration systems, TRs must be flexible to operate at medium temperatures for chilled products (from 0˚C to 8˚C) and low temperatures for frozen products (from -8˚C to -25˚C). They also operate under a wide range of ambient conditions (from -30˚C to 50˚C) and are prone to leakage.
In general, TR systems rely on dedicated diesel or electric motors to drive compressors that cool individual vans, trailers and containers. In small delivery trucks, the compressor is driven by the vehicle’s motor while operating. Leaks of HFC refrigerants are ubiquitous.
Refrigerants have evolved from substances like CFCs and HCFCs with high efficiency but high ozone depletion potential to substances like HFCs with much lower ozone depletion potential. But it became apparent that these replacements were GHGs with high global warming potential. Climate change is driving development of new alternative refrigerants, like hydrofluoroolefins, which are substances that do not deplete ozone and have relatively weak effects on global warming.
For the lubricant industry, this means adapting formulations for compressor lubricants for compatibility with new refrigerants and accompanying modifications in compressors. Flammability, commercial availability, cist, energy efficiency, use under severe ambient conditions, and retrofitting existing equipment are all considerations affecting decisions to implement new refrigerants. For example, the UNEP reported that the use of R-744 (CO2) as a refrigerant for large RVs and intermodal containers requires major changes to the design and components of TR systems.
Ozone Depletion Potential and Global Warming Potential of Refrigerants
|Refrigerant||Ozone Depletion Potential||Global Warming Potential|
However, a future without compressors—and compressor lubricants—for TR systems has already arrived. Liquid CO2 and N2 have been implemented as cryogenic refrigerants in TRs and as blowing agents to produce high-density foams used to insulate TR systems. Cryogenic systems inject very cold liquid carbon dioxide or nitrogen from on-board tanks through spray nozzles into refrigeration units, and these gases “boil” at -78.5˚C and -195.8˚. Advantages include lower operating costs, less noise, no odor and compliance with emissions mandates. Liquid CO2 and N2 are readily available, but on-board tank size is a limitation, and infrastructure for storage and distribution are needed.
|Sub-sector||Road Vehicles||Intermodal Containers||Ships|
|Refrigerant charge||1-8 kg (2-18 lb.)||4-8 kg (9-18 lb.)||20-1000 kg (44-2200 lb.)|
|Typical annual leakage rate||8%-20%||3%-12%||5%-30%|
|Refrigeration circuit||Each vehicle has a dedicated compressor connected to a diesel or electric motor.||Each IC has a dedicated compressor connected to an electric motor.||ICs with electric motors are connected to the ship’s electric system.|
Nevertheless, in 2007, Norway was the first country to begin implementing cryogenic refrigerants in commercial TR systems in trucks and trailer systems. By 2011, 16% of new TR systems were cryogenically cooled, and work was underway to put CO2 “filling stations” in place. The EPA reported that cryogenic TR systems were in use in Sweden, Denmark, Finland, France, the Netherlands and Germany. They were also piloted in the U.S.
This may point to a shift in opportunities for compressor lubricant suppliers from compressors in individual TRs to those used by manufacturers of compressed cryogenic gases.
Mary Moon, Ph.D., is a professional scientist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at email@example.com or 267-567-7234.