It’s no exaggeration to say that modern life hinges on the existence of refrigerants; their use allows the efficient transfer of heat in thousands of applications for both industry and consumers. They impact everything from chillers and heat exchange systems to food and drug production in industries such as food and drug manufacturing, processing plants and cold storage. However, despite their importance, they have their downfalls, notably their potential to harm the environment. 

In the late 19th and early 20th centuries, the first generation of refrigerants, including Carbon Dioxide (CO2/R744) and other natural refrigerants like ammonia and propane, emerged with a priority on practicality and availability. As chemistry advanced and allowed the creation of synthetic, efficient, and easily scalable refrigerants, the second generation of refrigerants came into fruition. These refrigerants were generally safer for human health since they reduced flammability and toxicity concerns. After decades of use, however, it was discovered that second generation refrigerants compromised the ozone layer, leading to the Montreal Protocol and the eventual phase-out of refrigerants with ozone-depletion-potential (ODP).

Currently, we find ourselves in the midst of another major refrigerant transition as third generation refrigerants are being phased down in favor of refrigerants with lower global warming potential (GWP). This fourth generation of refrigerants can present challenges in certain applications since they may require the use of slightly flammable refrigerants, typically classified as A2L refrigerants by The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).  While their chemistry poses challenges in scaling and carries a high price tag, there's an ongoing arms race among various application types and manufacturers to procure them. Interestingly, as this evolution continues, we are witnessing a resurgence of interest in CO2 as a refrigerant of choice.

As the global population and number of refrigeration facilities continue to expand, the need to balance access to safe, healthy food and environmental wellbeing becomes crucial. This has led to the advancement of modern refrigeration systems that aim to facilitate the production of food, drugs and other critical goods while minimizing environmental impact. When designing commercial and industrial refrigeration facilities, choosing the right refrigerant is critical. CO2 offers an affordable and eco-friendly alternative to modern synthetic refrigerants, and it’s easy to see why.

Striving for a Sustainability 

Given that CO2 can be diverted from existing fossil fuel fired power plants, its use as a refrigerant may offer an avenue to lower the impact of fossil fuel consumption while energy grids seek to decarbonize. It could also serve as a strong alternative to advance outcomes for refrigeration systems. With the introduction of new industry-specific regulations via the U.S. Environmental Protection Agency (EPA) Significant New Alternatives Policy (SNAP) Rules, organizations are being forced to reconsider climate-friendly alternatives in air conditioning, refrigeration and heat pump systems. In the cases of aging facility infrastructure requiring a system change, it might be wise to consider transitioning the facility’s refrigeration system to CO2. In the long-term, the industry would be wise to balance the incentive to capture CO2 from fossil-fuel energy sources with the need to de-incentivize the use of those fuels. The refrigeration industry can do so by seeking to source CO2 refrigerant from Direct Air Capture (DAC) projects, though that may provide another significant cost-constraint for the adoption of CO2 as a refrigerant.

System standardization is key when retrofitting or upgrading existing equipment and facilities, especially for companies that prioritize the ability to perform their own maintenance. By planning a facility with CO2 refrigeration, organizations champion responsible stewardship, reduce their carbon footprint, and comply with SNAP rules to minimize the use of substances with high GWP as outlined by the EPA. It becomes crucial to hire experts with a well-versed understanding of EPA regulations and their potential impacts on a facility design.

Serving as the base reference point when measuring the Global Warming Potential of substances when released into the atmosphere, CO2 has a GWP of 1. Additionally, it has an Ozone Depletion Potential of 0, meaning that it does not break down the ozone layer.  Given its existence in the atmosphere and the remaining widespread use of fossil fuels, the availability of CO2 is abundant. Consider a refrigeration system that carries 10,000 pounds of a refrigerant. In the event of a catastrophic leak, a system utilizing CO2 would, of course, carry a GWP impact of 10,000 pounds CO2 equivalent (CO2eq) However, for a system using a recently phased-out refrigerant such as R-507, this same 10,000-pound system would result in a GWP impact of 39.9 million pounds CO2eq, or the equivalent annual carbon footprint of more than 1,000 average U.S. citizens.

Social and Safety Influences

In the commercial and industrial world, safety is more than just a compliance requirement; it’s integral to the successful execution of projects, the reputation of a company and the wellbeing of employees. CO2 happens to be non-flammable and non-toxic, therefore it can reduce safety risks. Designing or retrofitting a facility with safety top of mind has many benefits including reducing potential hazards or liability risks, increasing efficiency and safe failure. 

Embracing the use of a natural refrigerant like CO2 can have far-reaching, positive impacts on an organization’s reputation for sustainability, creating a ripple effect to other organizations. When these sustainable practices are promoted and advertised, it not only strengthens the organization's brand image from a corporate social responsibility perspective, but also paves the way for fostering long-term growth. Additionally, by showcasing facilities that utilize this naturally occurring substance, organizations are positioned as forward thinking and innovative in the eyes of their competitors. 

It’s also important to underscore the fact that CO2 operates under a higher pressure compared to other refrigerants, requiring thicker piping, sturdier components, and overall, more robust equipment. This underlines the importance of partnering with a design and engineering team that is familiar with the specific design and system requirements for CO2 systems. 

Given its status as a natural refrigerant, CO2 can bring about significant cost savings overall when compared to fourth generation synthetic alternatives.  CO2 is also so simple to harvest from natural gas or coal-fired power plants. 

Although the initial investment to implement CO2 as a refrigerant in an existing or new system can come with a high cost, early adopters can gain from economies of scale. Over time and as regulations continue to update, the initial investment can pay off in the long run. In addition, CO2 has unique thermodynamic properties which may lead to an overall efficiency increase versus newer, fourth generation refrigerants. It has a low kinematic viscosity in its liquid phase, even at low temperatures, reducing the energy required to drive the system. It also has high thermal conductivity and energy content, meaning that a given volume of CO2 refrigerant can provide significantly more heat transfer per unit volume than traditional synthetic refrigerants, or even ammonia. This means that while the refrigeration system utilizing CO2 needs to be designed for significantly higher pressures (up to 2,000 pounds per square inch depending on the system type), the overall size of the system, including piping, compressor displacement and valves, can be reduced.

Current and Future Applications 

Due to system complexity and first-costs, current CO2 applications are best suited to low temperature environments, such as retail refrigeration, industrial refrigeration and cold storage. High compressor discharge temperatures allow for heat reclamation, such as hot water heating, freezer slab heating and building HVAC heating, similar to ammonia-based systems.

Looking at the future of planning facilities with CO2 refrigeration systems, it’s important to invest in trainings and education for design engineers tasked with designing the systems. And, given the unique architecture needs and equipment requirements that differ from traditional refrigeration systems, it’s crucial to partner with an engineering and design firm well-versed in this industry and its application. After all, while modern refrigerants remain a viable option, CO2 holds the potential to significantly influence the future trajectory of the refrigeration industry as it continually evolves.