1. What are natural refrigerants?

Natural refrigerants are gases used in cooling and heating applications (and in foam-blowing and propellants) as an environmentally friendly alternative to laboratory-developed halogenated refrigerants, which use chlorine and/or fluorine. (Fluorine gases are often called f-gases.) Natural refrigerants are found in nature and consequently do not damage the environment upon escaping into the atmosphere. By contrast, halogenated refrigerants, upon escaping into the atmosphere, deplete the ozone layer (CFCs and HCFCs), contribute to global warming (CFCs, HCFCs and HFCs) or add to PFAS pollution (some HFOs and HFCs). Natural refrigerants have none of those effects.

Natural refrigerants are also have very favorable thermodynamic properties, making them extremely efficient gases, typically more efficient than f-gases. Because they are efficient and environmentally benign, natural refrigerants are not subject to phase downs or phase outs under government regulations; thus, they are considered future-proof. F-gases are patented while natural refrigerants are not; as a result of that and supply-demand issues, natural refrigerants are much less expensive than f-gases.

There are generally thought to be five natural refrigerants: carbon dioxide/CO2 (R744), hydrocarbons such as propane (R290) and isobutane (R600a), ammonia (R717), water (R718) and air (R729). The most common natural refrigerants are R744, R290, R600a and R717.

Natural refrigerants are employed in every cooling and heating application worldwide in a wide variety of industries. They are used in home refrigerators, supermarket refrigeration, industrial refrigeration, pharmaceutical and vaccine cooling, ice rinks, data centers, heat pumps, chillers, air conditioners, mobile air-conditioning, transport refrigeration, district heating and cooling, and more.

Refrigerant-grade CO2 (R744) is a byproduct of the industrial production of ammonia, alcohol and fertilizer. The gas is purified, compressed, and liquefied for use as a refrigerant. A greener way of producing CO2 is anaerobic digestion, where organic waste is naturally broken down by bacteria in an enclosed space and without oxygen. This process produces biogas, which typically consists of 60% biomethane and 40% CO2.

The traditional way of making ammonia (R717) is to strip hydrogen from natural gas using steam (producing CO2 as a by-product). The hydrogen is then combined with nitrogen from the air at high pressure and temperatures of hundreds of degrees Celsius. This procedure is called the Haber-Bosch process. A greener approach is to make hydrogen by splitting water with electricity sourced from renewables; the rest of the Haber-Bosch process remains the same.

Italian-based gas manufacturer GTS produces the hydrocarbons propane (R290), isobutane (R600a), n-butane (R600) and propylene (R1270) by extracting refrigerant-grade hydrocarbons from raw liquid petroleum gas obtained from companies that own oil refineries and extractive wells. GTS has successfully tested the performance of the 1MW solar photovoltaic (PV) plant at its hydrocarbon manufacturing facility.

2. What is the difference between synthetic and natural refrigerants?

Global warming potential (GWP) is a way to compare the global warming impacts of different greenhouse gases (GHGs) in the atmosphere, where they absorb thermal energy and slow the rate at which this energy escapes to space. Specifically, GWP measures how much energy the emissions of 1 metric ton of a gas will absorb over a given period of time (usually 100 or 20 years), relative to the emissions of 1 metric ton of CO2, which has a GWP of 1. The larger the GWP, the more that a gas warms the Earth compared to CO2 over that time period. While the 100-year GWP is the most common measure, the 20-year GWP is gaining increasing usage as a more relevant measure of climate impact and is the standard used by New York State. (The GWPs listed here are from the IPCC’s AR6 listing in 2021.)

F-gases have traditionally had extremely high GWPs relative to CO2. R12, the most popular CFC, has a 100-year GWP of 12,500 and a 20-year GWP of 12,700 in addition to depleting the ozone layer; thus, it is 12,700 times as powerful as CO2 in warming the Earth over 20 years. R22, the most popular substitute for R12, has a 100-year GWP of 1,960 and a 20-year GWP of 5,690 in addition to also depleting the ozone layer.

The replacements of ozone-depleting gases, HFCs, do not deplete the ozone layer (as required by the Montreal Protocol treaty enacted globally in 1987), but they still possess very high GWPs. Thus, HFCs R134a, R32, R404A and R410 have GWPs (100 year/20 year) of 1,530/4,140, 771/2,690, 4,728/7,208 and 2.255/4,715, respectively. As a result, HFCs are being phased down globally per the Kigali Amendment to the Montreal Protocol.

The latest iteration of f-gases, HFOs, have very low GWPs (both 100- and 20-year), such as 1 for the commonly used HFO-1234yf; however, HFO-1234yf is a PFAS and converts 100% in the atmosphere into trifluoroacetic acid (TFA), another PFAS, which descends in rain.

The GWPs (both 100- and 20-year) of natural refrigerants are very low: 1 for CO2, less than 1 for propane and isobutane, and zero for ammonia, water and air.

In the 1970s, scientists discovered the depletion of ozone in the stratosphere, which created a hole in the ozone layer around Earth’s polar regions. The main cause of ozone depletion and the ozone hole was found to be the escape into the atmosphere of halogenated refrigerants CFCs (such as R11 and R12) and HCFCs (notably R22). Once in the stratosphere, their chlorine atoms catalyze ozone (O3) into oxygen (O2). These are ozone-depleting substances (ODS).

The ozone layer limits the amount of ultraviolet light that can pass through the Earth’s atmosphere, thereby preventing skin cancer, sunburn, cataracts and other ailments. To protect the ozone layer, virtually all nations in 1987 passed the Montreal Protocol, which bans CFCs, HCFCs and other ozone-depleting chemicals. Since then the ozone hole has begun to close, though it has many decades to go.

The ozone-depletion potential (ODP) measures the amount of damage a substance does to the ozone layer, relative to that of CFC-11, which is given an ODP of 1. R22 has an ODP of 0.04, according to the WMO (World Meteorological Organization), as of 2011. Natural refrigerants have a zero ODP.

Per- and polyfluoroalkyl substances (PFAS) are a group of more than 14,000 synthetic chemicals that have been used in consumer products around the world since the 1950s. They are commonly used to prevent food from sticking to packaging or cookware, make clothes and carpets resistant to stains, and improve firefighting foam. Many f-gases such as HFC-134a and HFO-1234yf are also PFAS according to the widely accepted and scientifically endorsed definition published by the OECD (Organisation for Economic Co-operation and Development) in 2021. Under this definition, supported by more than 150 PFAS scientists, PFAS essentially includes “at least one fully fluorinated carbon atom.”

This definition also covers trifluoroacetic acid (TFA), which is formed by some f-gases when they leak into the atmosphere. Notably, HFO-1234yf, used in car air-conditioning and many refrigeration blends, breaks down 100% into TFA over a few weeks; the TFA comes down to Earth in rainfall, infiltrating waterways and land throughout the world.

PFAS molecules have a chain of linked carbon and fluorine atoms. Because the carbon-fluorine bond is one of the strongest, PFAS are extremely durable and have been dubbed “forever chemicals.” A key feature in PFAS is the number of carbon atoms it has. The first PFAS to be found, PFOA (perfluorooctanoic acid) and PFOS (perfluorooctanesulfonic acid), have eight carbon atoms.

PFOA and PFOS have been associated with several diseases, including, pregnancy-induced hypertension and preeclampsia, increases in cholesterol levels, changes in liver enzymes, lower antibody response to some vaccines, and kidney and testicular cancer (PFOA). Their replacements with six carbon atoms like so-called GenX have also been connected to cancer and damage to the kidney, liver, immune system and reproductive organs.

There is a growing consensus that ultrashort-chain two-carbon PFAS like TFA may also cause health problems. Germany, for example, has linked TFA to reproductive toxicity, and studies have shown that TFA may cause liver dysfunction in rats. TFA and PFOA belong to the same PFAS subgroup, perfluoroalkyl carboxylic acids (PFCAs).

Because PFAS are extremely persistent, people, animals, plants and water are repeatedly exposed to them, and blood levels of some PFAS can build up over time. A study of Indiana households found TFA in human blood, suggesting that it has bioaccumulative properties.

The prevalence of TFA in the environment was underscored by a recent study by the NGO Pesticides Action Network (PAN) Europe that found TFA in samples from 23 rivers and six aquifers across Europe at levels that in four of the rivers surpass the 2.2mcg/L TFA drinking water limit established in the Netherlands; the study attributed the TFA to pesticides and f-gases. Another NGO, BUND (German Federation for the Environment and Nature Conservation), identified TFA in drinking (tap) water and mineral water in German cities and Brussels, Belgium.

TFA cannot be removed from water by filters (such as activated carbon) or ozonation; it can only be removed by reverse osmosis, an expensive technology that requires more resources, leads to higher energy costs and raises the unresolved issue of disposing of the resulting concentrates.

TFA has also been found in plants and thus can be contained in plant-based food and drinks, including beer and tea.

Natural refrigerants are very efficient due to their thermodynamic properties.

Ammonia is well-known to be an extremely energy-efficient refrigerant. According to the IIAR (International Institute for All-Natural Refrigeration), ammonia is 3–10% more efficient than competitive refrigerants; as a result ammonia systems use less electricity.

Hydrocarbons like propane are also highly efficient. According to Carel, the latent heat of vaporization of propane is almost two times higher than that of the most common HFC refrigerants: this means a higher cooling/heating effect for the same refrigerant mass flow.

Transcritical CO2 booster systems run less efficiently than f-gas systems in warm climates; however, that issue is now resolved by the development of technologies that increase the efficiency of CO2 like adiabatic gas coolers, ejectors, parallel compression, subcooler systems, the pressure exchanger from Energy Recovery and Epta’s FTE, ETE and XTE technologies. Depending on a store’s geographic location, source of power (e.g., utility power cost) and degree of implementation of technological advances, a supermarket can save on average 5% to 18% on energy bills compared with a traditional commercial refrigeration systems, according to Hillphoenix. In addition, taking into account the robust heat reclaim offered by CO2, transcritical systems become highly efficient and save end users a considerable amount of energy.

3. What are the applications of natural refrigerants?

Isobutane was first used in residential fridges in the 1990s and is now present in billions of these units worldwide.

Commercial refrigeration is probably the application where natural refrigerants have been most widely employed. ATMOsphere research showed that, as of December 2023, 68,500 stores in Europe, 2,930 in North America and 8,385 in Japan were using transcritical CO2 refrigeration, while millions of self-contained propane units were also installed in stores worldwide. In Europe, transcritical CO2 has become the primary refrigeration system for new and remodeled stores, with some stores using only propane cases.

Ammonia has been the dominant refrigerant for industrial refrigeration (cold-storage and food processing) for more than 100 years, with low-charge central and packaged systems emerging in recent years to improve safety and reduce regulatory oversight. Over the past five years, transcritical CO2 refrigeration has become an increasingly viable and popular natural option for industrial applications that want to avoid ammonia. In Europe there were 3,360 industrial sites using low-charge ammonia systems and 3,300 using transcritical CO2 as of December 2023, according to ATMOsphere research. In North America, the numbers were 1,045 for low-charge ammonia and 498 for transcritical CO2. Another natural application is propane chillers.

CO2, ammonia and propane can all be used in heat pumps in a variety of applications. For example, CO2 is employed in many hot-water heat pumps, both residential (especially in Japan) and commercial. In Europe, CO2 heat pumps are capable of generating space heat and domestic hot water. CO2 is also seen in large heat pumps supporting district heating as well as in industrial process applications.

Ammonia is a well-known refrigerant used in large-capacity heat pump systems for industrial applications and district heating.

Propane is increasingly seen in outdoor monobloc residential heat pumps in Europe deployed for space heating and hot water generation. In the U.S., the charge limit for propane has to be raised for this application to become available.

Air-conditioning is an application with a relatively small percentage of natural refrigerant installations, but has great potential for growth. Chillers using CO2, propane or ammonia are being used for comfort cooling in commercial settings. And there is a small but growing market in Asia and Europe for split ACs using propane. As with heat pumps, in the U.S., the charge limit for propane has to be raised for this application to become available.

In Europe, a small but growing market for CO2-based heat pumps, which provide comfort cooling and heating, is developing for electric vehicles from manufacturers such as Volkswagen and Ford. Manufacturer ZF is offering an electric vehicle with a propane heat pump. Over the next five years, manufacturers of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) are said to be learning toward opting for natural refrigerants like CO2 or propane.

4. Types of Natural Refrigerants

R744 is the ASHRAE designation for refrigerant-grade carbon dioxide (CO2), which is 99.99% pure with only 0.01% impurities and non-condensable gas.

R744 is CO2 used as a refrigerant that meets the quality standards of a refrigerant, as opposed to any other uses of CO2 or CO2 found in nature.

R744 is a colorless, odorless gas. As an ASHRAE A1 refrigerant, it is rated nonflammable and non-toxic. However, like every other refrigerant, CO2 can cause suffocation in an enclosed space. It operates at high pressures compared to other refrigerants, ranging from 188psi (13 bar) in low-temperature cases to 1,495psi (103bar) in medium-temperature compressors; the higher pressures allow for higher volumetric capacity, which in turn enables more compact component and system designs. The critical point of CO2 is 87.8°F (31°C) and 1,070 psi (73.8bar). Above this point, CO2 operates in transcritical mode and exists as a supercritical fluid that can’t be condensed into a liquid. Its thermodynamic properties allow it to generate considerable heat of condensation that can be leveraged for other applications.

As a refrigerant, R744 has a GWP of 1, which means that it is hundreds if not thousands of times less powerful as a greenhouse gas than HFCs. It has an ODP of zero and does nothing to the ozone layer. It is not a PFAS, thus not contributing chemical pollution to the environment. This means that, although CO2 produced by fossil fuel combustion is the main contributor to climate change, in the context of refrigeration, air-conditioning and heat pumps, CO2 is a very benign, environmentally friendly, future-proof gas.

Hydrocarbons are a group of organic compounds consisting entirely of hydrogen and carbon.

Hydrocarbons commonly used as refrigerants include propane (R290), isobutane (R600a) and propylene (R1270). Blends containing ethane (R170), propane or butane (R600) are also used as refrigerants.

Hydrocarbons have excellent thermodynamic properties, and their efficiency is as good as or better than f-gases in most applications. They are environmentally friendly, with zero ODP and negligible (under 1) GWP. They are non-toxic (rated A3 by ASHRAE).

Hydrocarbons are highly flammable but, if used responsibly, can be employed in a variety of refrigeration, air-conditioning and heat pump applications. Isobutane is used in billions of home refrigerators worldwide, and propane is employed in millions of self-contained commercial (retail) display cases around the world. In Europe they are increasingly used in residential heat pumps and split ACs.

In order to ensure safety, hydrocarbon applications, including charge limits, are governed by various international, regional and national standards and regulation. The main standards are the IEC 60335-2-40, IEC 60335-2-89, ISO 5149, EN378, ASHRAE-15, UL 60335-2-40 and UL 60335-2-89. Domestic refrigerators, for example, are limited to 150g of isobutane, while commercial cabinets can contain up to 500g of propane (up to 300g for closed cabinets in the U.S.)

Hydrocarbons can only pose a flammability risk if the concentration is between the lower and upper flammability limits (LFL and UFL). Under safety standards the concentration of leaked refrigerant will not exceed LFL where ignition sources can ignite it. In general, ignition sources inside the application are avoided.

Ammonia (R717) has been used as a refrigerant in industrial refrigeration and food preservation for more than 100 years. It is also used in chillers (including comfort cooling) and heat pumps.

Ammonia is one of the most efficient refrigerants, delivering a wide range of temperatures. A flooded ammonia system is considered to be up to 20% more efficient than a DX system using R404A, says Danfoss. Ammonia can also be used in combination with CO2 in ammonia/CO2 cascade systems, which are extremely efficient for low- and very-low-temperature applications. In recent years, low-charge centralized and packaged systems have reduced the amount of ammonia used considerably, improving safety and easing regulatory responsibilities.

Ammonia’s superior heat transfer properties enable the use of equipment with a smaller heat transfer area, lowering plant construction cost and reducing operating costs.

In many countries the cost of ammonia (per kg) is considerably lower than that of f-gases.

Ammonia is an environmentally friendly refrigerant, with a GWP and ODP of zero. However, ammonia is rated by ASHRAE as a B2L refrigerant, making it toxic in large quantities and flammable at certain concentrations, though it is difficult to ignite.

Thus, all ammonia systems are designed with safety provisions, and thousands of ammonia systems are installed worldwide.

Unlike most refrigerants, ammonia has a characteristic odor that can be smelled even at very low concentrations, providing a warning for minor ammonia leakages.


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