Developing More Environmentally Friendly Refrigerants

Interview conducted by Alessandro PiroliniJun 26 2015

Dr. Stephen Ritter, Senior Correspondent for Chemical & Engineering News - the weekly news-magazine published by the American Chemical Society - speaks to AZoCleantech about the various chemicals used as refrigerants and how recent attempts have been made to make them more environmentally friendly.

In the past what kinds of chemicals were used in refrigeration systems and what were the associated risks with these traditional methods?

I would like to start off saying that “environmentally friendly” is a label that needs some context. Many synthetic chemicals and materials that we use in our daily lives – including natural ones derived from biobased sources – can have a negative impact on the environment.

These are the chemicals that we use in everything from plastics to shampoo and household cleaning products to electronics to our medicines. Some are better than others when it comes to their environmental impact. It’s better to think about chemicals on a sliding scale. Some are “environmentally friendlier” than others.

When it comes to refrigerants, a host of chemicals have been used over the years. These include ammonia, methyl chloride, propane, sulfur dioxide, carbon dioxide, chlorofluorocarbons, and hydrofluoroolefins. What is important for selecting any chemical is that the compound functions in the way it is needed and it is readily available and affordable. In the past, being environmentally friendly was a bonus. But that way of thinking has changed.

For refrigerants, the performance need means the chemical must be able to cycle easily between being a liquid and a gas. When a liquid evaporates or a gas expands the process absorbs heat. We can take advantage of this property by creating a cyclic system using a motor to compress the refrigerant and then allowing it to expand. With the use of a heat exchanger, this chilling effect allows us to have air conditioners and refrigerators to cool what we want.

Over time, companies that use refrigerants have made choices on which ones to use based on their needs at the time. Ammonia, methyl chloride, and sulfur dioxide worked well but are reactive molecules that can be toxic to people if they leak out of the refrigerant system. Propane is not really toxic, but it’s flammable. So each of those early refrigerants had a risk associated with their use.

When Thomas Midgley Jr. developed his infamous Chlorofluorocarbon (CFC-12), the issue of toxic refrigeration techniques appeared to be solved. How and why are CFCs so bad for the environment?

When Thomas Midgley and his research team at the Frigidaire division of General Motors were charged with coming up with a new refrigerant in the late 1920s, they were given two criteria: The refrigerant had to be nontoxic and nonflammable.

Midgley’s first commercial refrigerant was the chlorofluorocarbon CCl₂F₂. This compound, named CFC-12, was produced by DuPont under the Freon brand.

It is perfect as a refrigerant gas, because it is nontoxic, nonflammable, and fairly inexpensive.

CFC-12 and a succession of related compounds such as CFC-11 (CCl₃F) and CFC-113 (CCl₂FCClF₂) became widely used as refrigerants in home and automotive applications and in other uses such as propellants in aerosol spray cans, foam blowing agents, and fire suppressants.

Looking back now, we realize that Midgley should have added one more item to his list of criteria: environmentally benign.

In the 1970s, CFCs became the center of one of history’s great unintended consequences, as scientists discovered the compounds participate in reactions in the atmosphere that destroys Earth’s protective ozone layer. The stratospheric ozone layer serves as a sunscreen to prevent the most harmful ultraviolet rays from the sun from reaching us on the surface.

If the Montreal Protocol on Substances That Deplete the Ozone Layer hadn’t have banned CFCs in 1987, how bad would the situation had been in 2015?

The Montreal Protocol is one of humanity’s great achievements because it represents unprecedented cooperation between scientists, national governments and their regulatory agencies, and the public to recognize and solve an environmental crisis.

The prediction in the early 1970s that the use of CFCs would cause an “ozone hole” over the Antarctic came true in 1985. But by eliminating the use of CFCs, ozone depletion was stopped and it actually started to reverse after 1993. The ozone layer is now healing and might be back to its former level by about 2050.

A recent study concluded that without the Montreal Protocol the Antarctic ozone hole would have grown by an additional 40% by now, an Arctic ozone hole would have become prevalent, and the ozone layer elsewhere around the globe would have thinned by about 15%.

What alternatives were developed for CFCs, and are these alternatives much safer?

Because CFCs worked so well as refrigerants, chemists had to be creative in finding replacements that worked as well, were also nontoxic and nonflammable, and would not harm the ozone layer. The first place chemists looked was at other types of halocarbons. The first round of replacement refrigerants included the hydrochlorofluorocarbon HCFC-22 (CHClF₂).

These compounds deplete stratospheric ozone as well, but to a much lesser extent than CFCs. Under the Montreal Protocol, these gases are already phased out or in the last stages of being phased out. The next round of replacements is hydrofluorocarbons, such as HFC-134a (CH₂FCF₃), in which hydrogen or fluorine has completely replaced chlorine. They are safe to use and don’t deplete the ozone layer.

But like the CFCs they have replaced, these compounds still have a problem in that they are potent greenhouse gases relative to CO₂. In fact, a secondary benefit of the Montreal Protocol is that eliminating the use of CFCs and HCFCs is helping reduce the threat of global warming.

The global warming impact of refrigerants is small compared with CO₂, because the amount of CO₂ released into the atmosphere by burning fossil fuels dwarfs that of CFCs and their replacements.

Because of the global warming potential, manufacturers are moving to yet another round of refrigerant replacements, such as the hydrofluoroolefin HFO-1234yf (CF₃CF=CH₂). This compound, which is just now starting to be used on a large scale, meets many of the criteria that are needed: It’s nontoxic, doesn’t deplete the ozone layer, has low global warming potential, and is relatively cheap.

It may be the most environmentally friendly refrigerant so far. HFO-1234yf’s only drawback is that it is slightly flammable. For that reason, some manufacturers are considering going back to ammonia and carbon dioxide. Although neither of those last two meets all the criteria for an ideal refrigerant, they don’t deplete ozone and are nonflammable.

In March 2015, the US Environmental Protection Agency (EPA) approved 5 new refrigerants, could you explain what these were and how they will be better than CFCs and current refrigerants?

Under the provisions of the Clean Air Act, EPA recently approved the use of five refrigerants that won’t harm the ozone layer and have lower global warming potentials than most other refrigerants. These include ethane, propane, isobutane, difluoromethane (HFC-32), and a blend of four alkanes called R-441A (ethane, propane, butane, and isobutane).

These refrigerants are already being used in some places around the world. It’s unclear whether these replacements will be better than CFCs and other refrigerants. Although they have low toxicity and won’t damage the ozone layer, they may be less efficient refrigerants. And in the case of the alkanes, they are flammable.

Keeping Cool without Killing the Planet - Speaking of Chemistry Ep. 21. Video Credit: CEN Online/YouTube

The important thing to remember here is that companies that manufacture products such as cars, refrigerators, and air conditioning systems for homes and commercial buildings will have to make choices as to the best fit for them.

They will be balancing the performance and cost of different refrigerants against the environmental benefits. Consumers will ultimately decide which type of product they want to buy based on their needs and pocketbooks, and weigh the safety risks and environmental benefits.

Can attempting to make refrigerants more environmentally friendly actually produce different problems?

There’s still a lot chemists don’t know about how the chemical products and processes we develop will impact the environment. No one imagined CFCs would be a problem, and yet they were. So there is always a risk that trying to build a better mousetrap will actually lead to unintended consequences.

Precaution is always warranted, but we can’t throw up our hands and live in fear and abandon useful chemicals and materials altogether. We have to realize that we will never be able to provide an absolute guarantee that any of the materials we make to live comfortable and happy lives are totally safe.

How do you believe the development of refrigerants will advance over the next decade?

Perhaps the hydrofluoroolefins or their mixtures with nonhalogenated compounds is as far as we can go in developing environmentally friendlier refrigerants. What would come next is to eliminate the need for a refrigerant altogether. That would require a fundamental new technology.

Scientists are already working on using thermoelectric, thermomagnetic, and thermoacoustic properties to develop refrigeration systems.

Some examples of these are already being used in niche applications, but they might be too expensive for large-scale uses.

Time will tell if any of these will work out technologically and economically,
and whether they will be environmentally friendlier.

About Dr. Stephen Ritter

Dr. Stephen Ritter is a Washington, D.C.,-based writer and editor covering areas of sustainability and green chemistry for Chemical & Engineering News, the weekly news-magazine published by the American Chemical Society.