After spending many years in the sub-zero wilderness, natural refrigerants are staging a comeback. Indeed, the transition away from synthetic coolants including hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), and hydrofluoro-olefins (HFOs) marks a significant shift in the world of cooling and refrigeration systems.

The history of natural refrigerants

Natural refrigerants – those existing in, or derived from, nature rather than those made by people – are by no means new. Their use dates back to the first steam compression refrigeration machine built in 1834, which employed ethyl ether as the refrigerant.

The first carbon dioxide (CO2) refrigeration system was patented in the 1867 by Thaddeus Lowe, while the first machine to operate with ammonia was developed just a few years later. Chloromethane and sulphurdioxide were also used in early mechanical cooling systems.

But all this changed in 1928 when American mechanical and chemical engineer Thomas Midgley Jr. helped develop the first synthetic refrigerant, known as chlorofluorocarbon (CFC) R12, or ‘Freon’.

Freon, however, had a lethal flaw – it contributes to the destruction of the ozone layer. In 1973, it was found that the emissions of chlorine atoms present in CFCs (and, to a lesser extent, their refrigerant gas successors – HCFCs) depleted the ozone layer.

Ozone depleting substances

Ozone depletion occurs when chlorine and bromine atoms meet ozone in the stratosphere, where they destroy the ozone molecules. Harmful health effects of this include increases skin cancer, eye cataracts, and immune deficiency disorders.

Excessive ultra-violet radiation (which can occur on the surface of the planet without the protection afforded by the ozone layer) also damages ecosystems by changing growth patterns, food chains, and biochemical lifecycles.

Types of refrigerant gases

To combat ozone depletion replacement for CFC and HCFC-based refrigerant gases were developed. These included HFCs and HFOs, which have zero ozone depletion potential (ODP) because they contain no chlorine.

These synthetic refrigerants have been widely used due especially to their non-flammable and non-toxic properties. However, HFCs are greenhouse gases with a high global warming potential (GWP). This is important because the greater the GWP of a refrigerant gas, the more it warms the planet.

Although HFOs do not contribute to global warming, they are not without their problems. While HFOs may be less harmful to the climate as a whole, they do break down in the atmosphere to create high levels of trifluoroacetic acid, a corrosive chemical and toxin that can damage human health and the environment.

So, what can be done to mitigate the environmental impact of synthetic coolants? The answer is a return to natural refrigerants. Indeed, the signs are that natural refrigerants are making a comeback as decarbonisation and net zero goals become top priorities.

What are the benefits natural refrigerants?

Natural refrigerants have a great deal to commend them. For example, they can be more sustainable, eco-friendly, and futureproof than synthetics and they are more environmentally friendly because they have a lower global warming potential than synthetic refrigerants.

Another ecological advantage of natural refrigerants for heat pump, air conditioning system, and refrigeration applications are that they are naturally occurring so producing them isn’t energy intensive.

In heat pump applications, natural refrigerants achieve a higher coefficient of performance (CoP).

Furthermore, natural refrigerants are more efficient than their chemical equivalents at very high temperatures.

Finally, natural refrigerants tend to be cheaper than their synthetic counterparts, largely because the latter demand more processing. And total cost of ownership is more competitive with natural refrigerants because they use less energy and maintenance costs are lower.

Types of natural refrigerants

There are a number of natural refrigerants to choose from including ammonia and propane. The former is used extensively in large scale refrigeration applications such as ice rink cooling as well as district cooling systems, supermarkets, and convenience stores. The latter is typically used in low-to-medium temperature refrigeration applications like freezers and refrigerated display cases.

Can propane be used as an air conditioner refrigerant?

Yes, propane can be used as an air conditioning (and chiller) refrigerant. Its properties are similar to the now-banned R22. Propane is commercially available and relatively inexpensive and can be stored and transported in steel containers.

Both propane and ammonia have problems, however. In the case of hydrocarbon refrigerants, these include high flammability and, in the case of ammonia refrigerants, their toxic nature.

That, arguably, makes the best natural refrigerant CO2. Using CO2 as a coolant for air conditioning systems can help save energy and reduce the carbon emissions that increase the likelihood of global warming.

CO2 has low viscosity, a large refrigerating capacity per unit of volume, physical stability, and high thermal conductivity.

On top of this, the ‘LETI Climate Emergency Design Guide – How new buildings can meet UK climate change targets’, says: “Besides having zero ozone depletion potential, CO2 also has an ultra-low GWP of 1 (unlike most currently used refrigerants, the most common of which typically have GWPs in the range of 1,400 to 2,100), is non-flammable (unlike almost all other natural refrigerants) and is non-toxic (unlike ammonia, which is often used as a refrigerant in large scale industrial applications).”

Since CO2 is a naturally occurring substance, it forms part of natural biogeochemical cycles and does not therefore create persistent waste in the atmosphere, water, or biosphere. Moreover, CO2 is neither toxic nor flammable, is relatively cheap, and is sustainable and future-proof. It is also far easier and cheaper to dispose of than manmade refrigerants.

Taken together, these advantages make for a forceful argument for CO2 as a green alternative to traditional refrigerant-based cooling media.

What heat pumps operate with natural refrigerant, CO2?

One example is the Enerblue HP90/HP90W heat pump. With a heating capacity of 14.5 to 133.2kW, this CO2 refrigerant gas-based range is designed for most commercial/industrial sectors and can heat water to temperatures up to 90°C with an external temperature of -20°C.

It boasts a host of features and benefits including semi-hermetic reciprocating compressors, axial fans, total cool recovery, and the ability to provide heating. This leads to high thermodynamic performance and zero impact on the environment for an excellent future-proof solution for both new and retrofit buildings.

Available in several configurations – including water to water, air to water, and air to water with cool recovery – the HP90/HP90W heat pump is flexible enough to suit most applications.

Meanwhile, the Engie range of heat pumps, available from 150 to 1,500kW, is water to water only and more suitable for industrial applications. Its high temperature heat pumps provide hot water temperatures of up to 110°C. Low heating return temperatures in the CO2 recycle process result in lower energy loss than conventional heat pumps with a subcritical cycle.

Heating processes such as drinking water preparation and process water heating allow low return temperatures and can therefore be implemented with higher-performance CO2 heat pumps.

Contact Klima-Therm for more information.