Natural refrigerants impact component design and selection

Refrigeration/cooling system manufacturers are replacing the old classes of chemical refrigerants that emit potent greenhouse gases, but it can be a challenge to determine its impact on component design and selection.

By William Bentley, Sensata Technologies April 12, 2018
In order to protect the environment and meet new U.S. government regulations, refrigeration/cooling system manufacturers are replacing the old classes of chemical refrigerants that emit potent greenhouse gases, such as hydrofluorocarbons (HFCs) hydrochlorofluorocarbons (HCFCs), and chlorofluorocarbons (CFCs), with “green” refrigerants that do not deplete the ozone layer and have a lower impact on global warming. 
These green refrigerants are naturally occurring, non-synthetic substances that can be used as cooling agents in refrigerators and air conditioners and include hydrocarbons (propane, butane, and cyclopentane), Carbon dioxide (CO2), ammonia, water, and air. 
Over the last five years, there has been significant global progress in phasing out the use of environmentally harmful synthetic refrigerants. The U.S. has already banned R22, an ozone-depleting refrigerant, from being used for air conditioning and has moved to the next generation of R410A. While R410A is still not a natural refrigerant, it is an important first step towards a cleaner, greener direction. 
There are two important criteria in measuring whether or not a refrigerant is environmentally acceptable:
Ozone depletion potential 
Ozone depletion potential (ODP) is a measure of the relative amount of damage a substance can cause to the ozone layer. UV radiation from sunlight causes the release of chlorine in CFCs and HCFCs into the atmosphere, which then damages the ozone. In comparison, natural refrigerants have an ODP of zero, meaning that they will not have any depleting effect on the ozone layer if they escape the system. 
Global warming potential 
Global warming potential (GWP) is a relative measure of how much heat a greenhouse gas traps in the atmosphere. The lower the GWP, the better a substance is for the environment. GWP compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A specific GWP is calculated over a time interval, typically 20, 100, or 500 years. 
Many of the refrigerants being used today have a global warming potential ranging from 1,400 to nearly 4,000. 
In Figure 1, one can compare varying GWP values as they relate to types of refrigerants. Current HFC Refrigerants R404a and R134a have extremely high GWP values—3,922 and 1,430 respectively—that are harmful to the environment. By comparison, the hydrocarbon (HC) propane (R290), a newer, natural refrigerant version of R404A, has an extremely low GWP of only three. At this time, most of the industry has transitioned from R404A to R134A, but there are even greater benefits of transitioning to R290 in the future. 
Hydrocarbons (HCs)
Many of the new generation of natural refrigerants are HC-based as opposed to hydrofluorocarbon (HFC) based. HC-based refrigerants include propane (R290), isobutane (R600a)—typically for use in small capacities like appliances—and R32, a blend of R290 and another refrigerant. These naturally occurring chemicals operate in mid-pressure ranges of 10-50 bar. 
R290’s thermodynamic properties are better than the older HFC refrigerants R134a and R404a. Its heat capacity is approximately 90% greater than R134a and 140% greater than R404a. These characteristics enable R290 to absorb more heat at an accelerated rate, resulting in higher device efficiency, with faster temperature recovery and lower energy consumption.
HCs have no ozone depleting properties and low GWP. However, it is not all good. This new generation of HC refrigerants is highly flammable and therefore require different and safer technologies for refrigeration and cooling systems and components.
Carbon Dioxide 
Carbon dioxide has a GWP of only one whereas HCs have three times more global warming potential. However, CO2 operates at nearly twice the pressure of HCs in a typical cooling system, making it much more difficult to manage. 
Ammonia is another natural refrigerant now being used, although not as frequently as HCs. It measures zero for both ODP and GWP and breaks down rapidly. The challenge with ammonia is that its alkalinity is extremely corrosive, so components used in applications require careful consideration of material compatibility.
In order to stay at the forefront of the green cooling trend, industry leaders are developing specialized components that are compatible with the latest common refrigerants: HCs, CO2, and ammonia. New products created for this market include a variety of pressure switches and sensors to meet different application requirements. 
Solving the high-pressure challenge
While CO2 is not combustible like HCs, it does pose a different set of problems because operates at almost twice the typical pressure level. Specialized components such as pressure switches are used to ensure reliable operation in applications where CO2 is used, which in turn affects product manufacturing. 
Because of the higher pressure running through the entire system, cooling units need a pressure switch incorporated for safety, and if they cannot be retrofitted, they need to be replaced with new units.  In addition to the safety challenges, CO2 also operates less efficiently compared to HCs, requiring more electricity to achieve the same heating or cooling power. 
For example, pressure switches are required in CO2 refrigeration systems to protect against high pressure burst or over pressure situations that could cause the coils to rupture. When the pressure builds up to a certain point, the switch opens the electrical contact and turns off the system compressor. When the pressure drops back down to normal levels, it automatically switches the compressor back on. But in today’s market, even more safety is needed. 
Since switches are an electro-mechanical device, they inherently can spark when the contacts make or break. To prevent a spark from accidently causing ignition of HC refrigerants, a sealed design can be used to seal the spark inside by isolating the specific pressure media (R290) from the electrical switch assembly. 
Each switch is manufactured with a hermetic seal around the gas path as well as a sealant around the electrical connections. The electrical switch connections signal back to the system whether the switch is open or closed. This safety design eliminates the potential for explosion by stopping the gas from entering the electrical switch compartment where the arcs can be generated during contact make or break operations. 
Testing and certifications
When analyzing the effects of natural refrigerants on cooling solutions, OEMs need to know if the pressure switch has been tested with the specific refrigerant that they are planning to use. For example, for use with R290, they need to know what the maximum current level of the switch is that prevents the arc from igniting the refrigerant, and whether it meets the required pressure actuation and switch points. For C02 systems, operating with twice the pressure of conventional systems, the big concern is burst pressure. What kind of high pressure can it withstand? How long will it reliably operate under sustained high-pressure environments?

To address these concerns, and to also ensure that switches can withstand flammable refrigerant applications, pressure switches specially designed for HCs can be submitted for rigorous testing by external agencies. It is clear that natural refrigerants are important to the future landscape of industrial and manufacturing plants. As OEMs research their choices, component manufacturers must meet the design challenges posed by these non-synthetic refrigerants in order to optimize safety and efficiency.

A critical part of this evolution is the knowledge of what kinds of hardware and systems must be specified for the safe implementation of these new and more technically challenging materials. 
William Bentley is an engineering director at Sensata Technologies, which develops pressure transducers for the global industrial, HVAC, and refrigeration markets.