Natural refrigerants back in vogue

Whilst this year’s fashion is bringing back the styles of the 1950s, the trend in refrigerants is reminiscent of the 1920s with natural refrigerants returning to popularity. Alexander Cohr Pachai, Senior Engineer at York, a Johnson Controls company, looks at the benefits that are bringing these refrigerants back in vogue.
The first refrigeration systems, developed in the late 1800s, were based on absorption technology using ammonia (NH3) and water. Hydrocarbons (HC) and carbon dioxide (CO2), along with NH3, then gained favour in vapour compression systems, with York building its first CO2 compressor in 1897. These early natural refrigerants were widely used until 1929 when the first synthetic refrigerants were invented. These new so-called ‘safety refrigerants’ became the preferred refrigerants in the market due to the ease of use compared to the alternatives. NH3 had an unpleasant smell and was flammable and toxic, HC refrigerants were extremely flammable and CO2 only worked at high pressures.

It is ironic that these same refrigerants are now coming back into fashion to become viable alternatives to the ozone depleting and global warming synthetics that have been used in refrigeration and air conditioning systems for the best part of a century. It is easy to see why, however, when you consider that the synthetic fluorinated gases (F-Gases) currently in use can cause nearly 3000 times more global warming than a similar quantity of CO2.

The introduction of the F-Gas Regulations, which will stipulate stringent and costly maintenance regimes for cooling systems containing F-Gases can only increase the popularity and use of natural refrigerants further. Natural refrigerants present no threat to either the ozone layer or the earth’s climate and are not subjected to the same regulations as synthetic refrigerants in terms of supervision, reporting and periodic inspections.

However, there is no such thing as a perfect refrigerant and all options (including HFCs) must be considered in selecting the ideal refrigerant for each application.

Ammonia (NH3)

Opinions about the use of natural refrigerants vary widely. Some experts focus on the fact that they are lethal and/or and flammable; others on the fact that they have the advantage of being safe and environmentally friendly. Both are true as far as NH3 is concerned. Many industrial users have already changed from synthetic refrigerants to NH3, particularly on the continent where NH3 has been the preferred refrigerant since the 1920s. Customers not familiar with NH3 are initially wary of this refrigerant, but once they have tried it, they become more relaxed, finding it easier in use.

At room temperature, NH3 is a transparent gas that is lighter than air and extremely soluble in water. It is, however, toxic and becomes flammable at high concentrations and a temperature of 651ºC. These characteristics have made NH3 unwanted in some market segments such as public buildings. The good thing about ammonia is its very distinctive smell that can be easily detected at tiny, completely risk-free concentrations. The number of new applications is on the increase and modern chillers with NH3 are being used in centralised systems for air conditioning and computer cooling. The charge of NH3 in a system can be reduced by using CO2 or brine as the distribution medium. A fully indirect expansion system with a small amount of NH3 provides high efficiency without the environmental impact.

York’s experience shows that NH3 is an energy-efficient, safe and environmentally friendly refrigerant when risk evaluation, design, installation and maintenance are carried out by competent, certified professionals. The overwhelming benefits of the refrigerant were recognised by the design team for London Heathrow Terminal 5 where York has installed four 6.6mW NH3 chillers that will provide chilled water to meet the air conditioning needs of the terminal buildings.

Hydrocarbons (HC)

HCs started to become more popular in the 1990s and have been used in a wide range of commercial and semi-industrial applications for years. More than 90% of domestic refrigerators in Europe use an HC as the refrigerant. HCs such as propane and butane are flammable and explosive at a concentration in air of 2-11% but special national standards for combustible refrigerants have been established to solve the problem of its safety.

The need for safety measures and professional handling should not be underestimated but nor should the risks be exaggerated. Considerably greater volumes of HCs are used by campers at almost any campsite, for example, than used in any of York’s range of HC chillers.

As refrigerants, HCs are excellent, especially in reciprocating compressors. The efficiency and reliability is good and has been proven over years of use. Modern liquid-cooled and air-cooled equipment contain very small amounts of refrigerant and are ideal for air conditioning installations in all types of building.

Carbon Dioxide (CO2)

As a refrigerant, CO2 has no impact on the ozone layer. Safe, reliable CO2 installations are used today in secondary systems as well as in cascade systems with CO2 in the low temperature stage. Compared with indirect systems using glycols or other brines, the cost and power consumption associated with a secondary CO2 system are always lower. Because CO2 transports more cooling per unit of volume than glycol, it is possible to use small diameter pipes, which contributes to keeping costs down.

The disadvantage of CO2 is that the operating pressure is higher than many other common refrigerants, which places greater demands on system components. York is unrivalled as the biggest supplier and installer of CO2 refrigeration systems in the world and our experience shows that, as with NH3, CO2 can be used safely and with superior energy efficiency results when installed and maintained appropriately, especially in low temperature applications.

Trends

Several criteria are taken into account when selecting new refrigeration plant today. Energy efficiency has top priority and our customers tell us that they are now prepared to pay a premium for the more energy efficient system. This change is triggered by unprecedented energy price increases over the last twelve months. Also, concerns about changes in legislation and public opinion regarding use of fluorinated refrigerants are part of the discussions. This makes it easier to choose a system using natural refrigerants because they are energy efficient and future proof, as legislators do not plan to control these substances. Most natural refrigerants are also considerably cheaper than their synthetic cousins and their prices are very stable.

For these reasons, York sees a trend towards cooling systems using natural refrigerants and there is widespread use of NH3 in many applications. Where an NH3 charge has to be reduced, or if the application requires increased capacity by lowering the temperature, we clearly see a trend towards CO2. We have used CO2 in applications ranging from small supermarkets through to industrial blast freezers but we also see that customers are interested in air conditioning equipment using the possibilities that this refrigerant offers.

Since 1996, York has produced a large number of HC based chillers installed in many types of applications, including air conditioning in hospitals, offices and universities and heat pumps in schools, freeze driers for museums, cooling of CO2 and glycol in supermarkets and many other places.

In the future, there will be more focus on energy consumption and sustainability than ever, driven by the end user. These demands on the product side will bring the refrigeration plants to a higher standard than previously seen and the use of natural refrigerants will remain at the height of fashion as part of the solution, together with a continued use of HFCs for the foreseeable future.