Long Term Refrigerants – Ammonia vs. CO2: Find your optimal refrigerant and balance your business needs for competitiveness, compliance and cooling efficiency

As legislation evolves to meet global environmental goals, refrigeration system owners and operators will need to be aware of how these changes affect their existing cooling equipment and what needs to be considered when making future investments. Growing restrictions around the use of environmentally damaging synthetic refrigerants may require changes to existing plant, including the retrofitting of alternative fluids or entire system replacement.

Against a backdrop of the Montreal Protocol and subsequent Kigali amendment where 146 countries, including the UK, committed to cutting the production and consumption of HFCs by more than 80 percent by 2047 to limit global warming, the EU has introduced proposals to accelerate the HFC phase-down through a review of the current F-gas Regulation (2014).

In April 2022, the European Commission proposed that the current EU F-gas Regulation (2014) be revised, with the aim of accelerating the planned HFC phase-down and further reducing the availability of synthetic refrigerants with higher Global Warming Potential (GWP). The proposal set targets of a 95% reduction in the amount of HFCs placed on the market by 2030 and a 97.5% reduction by the year 2050. These targets would replace the existing goal of a 79% HFC reduction by 2030, as measured against the baseline of average consumption of HFCs on the EU market during the years 2009 and 2012.

In March 2023, the European Parliament formally adopted its position on an HFC phase-down that was steeper still; now aiming for a full phase-out by 2050. The associated report proposed bans on HFCs and HFOs with higher GWPs in multiple applications, such as split heat pumps and stationary refrigeration equipment, from 1^st January 2028 and 1st January 2027 respectively.

The final shape of the revised F-gas Regulation will ultimately be determined through ‘trilogue’ negotiations between the European Commission, Parliament, and Council. These negotiations are anticipated to conclude by the summer of 2023.  Although the UK is not legally bound to adhere to the proposed update post-Brexit, the government is likely to follow much if not all of what the EU’s decides in order to ensure compliance with the Montreal Protocol and its own net zero emissions law.

Those looking to invest in new cooling systems can avoid the tightening phase-down timeline by opting for ‘natural’ refrigerants, such as ammonia and carbon dioxide; substances that can be found to occur naturally in the environment. These gases are not subject to the F-Gas regulation and when used as refrigerants, provide end users with low GWP solutions that safeguard against an ever-changing legislative landscape.

Selecting a refrigerant

An ideal refrigerant should have attractive thermal and physical properties, be stable over the system’s operating range and run efficiently to minimise energy consumption for the application conditions. This could be low temperature for cold storage and freezing, high temperature for heat pumps or anything in between. It should also be environmentally benign, non-corrosive, non-toxic, non-flammable, cost-effective and widely available, without being subject to quota restrictions and price hikes.

Unfortunately, no single refrigerant or blend meets all these requirements. Over the past thirty years, a number of fluids have been phased out of use due to the damaging effects they have on the environment. This started with ozone depleting substances such as chlorofluorocarbons (CFCs) and more recently the focus has switched to reducing our reliance on high global warming refrigerants.

So what choices do operators have for the future?

Synthetic and Natural Refrigerants

Refrigerants are often categorised into two groups; ‘synthetics’ such as hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) or ‘naturally occurring fluids’ including carbon dioxide (CO₂) and ammonia (NH₃).

Synthetic refrigerants

There have been four generations of synthetic refrigerants.

CFCs were developed in the 1930s but later found to contribute to the depletion of the ozone layer. This resulted in them being phased out under the Montreal Protocol. Hydroclorofluorocarbon refrigerants (HCFCs), such as R22, were implemented as alternatives to CFCs with lower ozone depletion potential, however these were also eventually banned due to their impact on the environment.

Many operators of refrigeration equipment are still using the third generation of synthetic refrigerants, HFCs, which don’t deplete the ozone layer but do still have high global warming potential. These greenhouse gases are often thousands of times more potent than carbon dioxide in contributing to climate change if released into the atmosphere.

What Does Global Warming Potential Mean?

Global Warming Potential is the contribution that one kilogram of a substance makes to global warming when released into the atmosphere, measured relative to one kilogram of CO₂. For example, the refrigerant R449A has a GWP of 1,397, which means that 1 kg has the same impact on climate change as 1,397 kg of carbon dioxide.

The graphic below compares the global warming effect from a release of one kilogram of five different refrigerant fluids, including three synthetics (R449A, R410A and R404A) and two naturally occurring substances (R717 ammonia and R744 carbon dioxide). It uses the analogy of the number of miles travelled in a family car to create the same amount of CO₂ and shows that releasing one kilogram of R404A is equivalent to driving the car 23,433 miles, or almost an entire trip around the planet.  Releasing one kg of CO₂ is equivalent to driving 6 miles, whereas ammonia has no effect on global warming. This highlights the essential role that HFC refrigerant phase-down will play in reducing global warming.

HFOs – The latest generation of synthetic refrigerants

In response to the F-gas Regulation, manufacturers have developed a fourth generation of synthetic refrigerants, HFOs. These are fluorinated hydrocarbons, similar to HFCs but less stable when released into the atmosphere. This means they break down faster, resulting in a lower GWP. They can be used in their pure form and have the same ‘mildly flammable’ classification as ammonia (2L). To mitigate this flammability, HFOs are often mixed with HFCs to create non-flammable blends at the expense of an increased global warming potential.

Scientists have also raised questions over the by-products of HFOs when they break down in the atmosphere, including production of trifluoroacetic acid (TFA) which is highly corrosive and can contaminate water supplies. There are also ongoing discussions in Europe around perfluoroalkyl substances (PFAS) which include these refrigerants. Recent studies have indicated risks to human health from PFA exposure, including cancer, liver damage, decreased fertility, and increased risk of asthma and thyroid disease. Some are calling for a phase out of PFAs, and the European Parliament has voted to link the F-gas Regulation to the European Chemical Agency (ECHA) PFAS restriction proposal that may regulate a number of F-gases as PFAS.

Refrigerant price volatility

Refrigerant price fluctuations are expected to continue as the new F-gas regulation revision proposes a steeper phase-down in quotas to speed up the decrease of greenhouse gases in the atmosphere.

The graph below maps the impact on pricing of a number of popular refrigerants against the current F-gas regulation phase-down timeline (represented as a dotted line).

There is an overall upward trajectory in refrigerant pricing, with notable price volatility surrounding the phase down steps under the current regulation. The potential for future cost increases for synthetic refrigerants is a distinct possibility based on recent history.

Natural Refrigerants

Naturally occurring fluids have been used as refrigerants throughout the history of mechanical refrigeration. Ammonia has been in continuous use in industrial applications over this time. Fluids such as carbon dioxide were displaced for a time by synthetic refrigerants but have experienced a significant resurgence over the last 20 years as a result of growing environmental awareness and advances in system design.

Ammonia (NH₃)

Ammonia is used in a variety of industrial applications, including temperature-controlled warehouses and food production. It is widely available at a relatively low cost due to the large volumes produced for agriculture and is a patent-free substance. A key environmental benefit is that it does not affect ozone depletion or global warming.

Ammonia is classified as both toxic and mildly flammable, however due to its long history of use, there are well developed mitigation measures in place to ensure its safe operation. These include regulatory and good practice guidance around gas detection, ventilation and safe handling.

Ammonia systems are typically manufactured using mild or stainless steel piping, heat exchangers and other equipment as it is corrosive to copper. This comes at a premium but ensures a robust installation and a typical life expectancy exceeding 20 years.

Ammonia refrigeration technology has advanced in the past few decades to enable its use as a replacement refrigerant for CFC, HCFCs and now HFCs. Reduction in refrigerant charge has been a major focus point and modern systems are available with less than 20% of the ammonia inventory of traditional system designs. These include modular, low pressure receiver systems and the use of secondary chillers in place of direct refrigerant installations.

Main ammonia system types

Central ammonia plant

A central system typically circulates ammonia from a plant room across an entire facility where it is used to cool rooms and processes. The direct use of ammonia removes a step of heat transfer and requires less pumping power than secondary systems using water or glycol. When integrated with a well-designed control system, this style of ammonia solution can deliver efficient cooling and is widely used for temperature controlled warehousing applications and in food factories. Charges can vary from hundreds of kilogrammes to tens of tonnes.

Low-charge ammonia packaged units

A low-charge ammonia system reduces refrigerant inventory but maintains equivalent (and often enhanced) system performance when compared to a central system. There are two main types of low-charge systems, either circulating refrigerant directly to the point of cooling or indirectly through water or a secondary fluid (e.g. glycol). A modular ‘plug and play’ approach to system design provides a simple, competitive solution for both types of systems, often eliminating the need for a machinery room and reducing pipe runs.

Low-charge ammonia chillers

Packaged low-charge ammonia chillers contain the refrigerant within the unit and circulate a secondary fluid to the building and/or process that requires cooling. Charge is reduced to as little as 0.18 kg/kW for air-cooled chillers and less than 0.1 kg/kW for water-cooled units.

These systems can be housed internally or externally and are easy to relocate as all the refrigerant equipment is located on a steel base frame. They can also use waste heat recovery to generate warm glycol for defrosting, eliminating the need for electrical elements in air coolers.

Low-charge ammonia freezers / condensing units

This direct system design can also be packaged and located externally, adjacent or on top of the building that requires cooling to best suit the layout of the site. The packaged design make it easy to relocate to another area of the site or a different facility altogether, creating a valuable, long term assets for any organisation.  Refrigerant charge is reduced from 2 kg/kW (typical for central systems) to 0.5 kg/kW of ammonia which is circulated into the building without the need for a refrigerant pump, reducing energy consumption. Keeping air coolers or process equipment near to the package helps minimise refrigerant charge and while some low charge designs have a limitation on distance between the package and evaporators, this isn’t the case for the Azanefreezer system shown below. This design incorporates reverse cycle defrost to deliver a fast, efficient defrost.

Azanefreezer 2.0 installation at Lineage Great Haddon

Carbon Dioxide (CO₂) Refrigeration Systems

Carbon dioxide or CO₂ (R744) was first proposed as a refrigerant in 1850, and its use gathered popularity until the early 1900s when synthetic gases were introduced. Since the phase down of HFCs under the F-gas regulation, there has been an exponential uptake in CO₂ systems, initially driven by the retail sector but also adopted for industrial applications.

CO₂ is a naturally occurring substance and, with a GWP of 1, it is the benchmark gas against which others are measured. It’s also patent free and available at a low cost compared to synthetic fluids.  It is not subject to the F-Gas Regulation , making it a reliable, future-proof solution.

CO₂ operates at higher pressures than other refrigerants and at positive pressures across its entire operating envelope. This is a particular advantage for low temperature freezers down to circa -50⁰C. Discharge temperatures are higher than most other fluids which results in good potential for heat recovery, particularly when operating transcritically.

CO₂ has an A1 refrigerant safety classification, which means it is non-flammable with low toxicity, similar to many of the synthetic refrigerants that have been phased out or are being phased down. It is colourless and odourless and, being heavier than air, can concentrate at low level and/or in confined spaces. Gas detection systems are installed where concentrations can exceed safe limits, along with mechanical ventilation if necessary.

CO₂ has a high volumetric efficiency, meaning that a large amount of heat can be transferred by a relatively low quantity of refrigerant, resulting in smaller components and pipe work than required for historical fluids.

CO₂ system types

All refrigerants have a critical point, described as the pressure above which the gas does not condense. The critical point for CO₂ is relatively low, at 31 degrees Celsius and this results in two main types of CO₂ system design; subcritical (when the system runs below the critical point) and transcritical (when the system runs both below and above the critical point).

Subcritical CO₂ systems

This system type operates in the same way as a traditional refrigeration system, as illustrated in the diagram below. Lower pressure CO₂ evaporates to provide cooling and is then compressed to a higher pressure in order to reject heat. For the system to operate subcritically however, the refrigerant CO₂ must remain below its critical point of 31^oC. This wouldn’t be possible for air-cooled systems during the hottest days of the year so subcritical systems typically reject heat to a secondary fluid as part of a cascade arrangement instead. They are also used as the low temperature stage of a two stage system.

Transcritical CO2 systems

Transcritical systems are CO₂ systems that reject heat to ambient air in the usual fashion. They operate subcritically (below the critical point) for the large majority of the year in UK climates but also have the ability to operate transcritically (above the critical point) when required. Transcritical operation is activated when the CO₂ within the system exceeds the critical point of 31^oC, typically at ambient temperatures in the low- to mid-twenties (^oC). When this happens, the condenser becomes a gas cooler and the high pressure CO₂ is cooled over a temperature range rather than condensing, with the system instead making use of control valves to generate liquid in the receiver.

Transcritical CO₂ systems provide excellent opportunities for heat recovery, allowing mains water to be heated from 10°C to +65°C or higher.

Combined CO₂ and Ammonia Systems

Subcritical CO₂ can be combined with ammonia in a cascade arrangement to provide a high-efficiency, fully natural solution. With this system configuration, the CO₂ operates subcritically at all times, condensing against the ammonia circuit in an intermediate heat exchanger. This ensures efficient, subcritical CO₂ operation all year round and can delivery efficiencies greater than a two-stage transcritical plant.

Investing For the Future

Growing environmental and economic pressures mean that choosing the right refrigerant for your business is crucial and should include consideration of both capital and ongoing operating costs. For operators using synthetic refrigerants, replacement with a more modern synthetic refrigerant system may be an attractive option commercially and for some situations, may be the only available solution. It may fit short term business needs but consideration should be given to longer term factors such as energy costs, refrigerant cost and refrigerant availability. Where long term refrigerant availability becomes an issue it can require costly system modifications or even system replacement in order to comply with regulations; something that should be built into any life cycle costing.

Thinking for the medium to long term, a move to natural refrigerants may represent the best option for a business, especially when considering the system will remain fully operational for the next 20-30 years and beyond. Capital outlay will typically be higher compared to synthetic chemical refrigerants but it provides greater certainty in terms of availability as well as opportunities for reducing energy and maintenance costs, as well as heat recovery.

Carbon dioxide will typically be more commercially competitive, particularly for small to medium capacities where synthetic refrigerants would historically have been used. CO₂ systems have the advantage of enabling the combination of low and higher temperature cooling requirements into a single packaged plant, resulting in cost savings. Also, in the case of larger capacities, the addition of extra compressors or the installation of multiple plants can enhance redundancy. There is of course  a practical limit to the number of compressors that can be used and for larger industrial systems a smaller quantity of ammonia compressors may be preferable over numerous CO₂ compressors. Technological developments in CO₂ system design mean that annual efficiencies are comparable and often better than synthetic systems. The use of adiabatic condensers can extend periods of subcritical operation for improved efficiency, particularly in UK climate conditions, and can provide a good return on investment.

When it comes to larger industrial systems, ammonia has been the refrigerant of choice for over 100 years. The equipment design, controls and materials of construction used for ammonia systems are typically of a high quality and are built to outlast other system types. The use of welded mild and stainless steel for constructing ammonia plant adds cost compared to the brazed copper systems outlined above but results in a robust plant construction with a long life expectancy of typically 20+ years. The extended life span of industrial ammonia systems should be considered as part of the project life cycle cost comparison, factoring in the need to potentially replace a more commercial solution within a shorter time frame.

As cooling requirements increase, plant running costs rise and become a larger part of the overall life cycle cost. A well designed and maintained ammonia plant should deliver energy savings of 20% or more for most applications compared to the other systems mentioned. This comes from inherent thermodynamic advantages, reduced defrost energy consumption (e.g. hot gas rather than electric or glycol), avoidance of transcritical operation and improved system controls. Capacities of ammonia installations are typically larger than those of CO₂ installations however they can be achieved by using fewer pieces of equipment. One ammonia screw compressor, for example, can deliver the capacity of more than a dozen CO₂ scrolls or reciprocating machines. Bespoke, PLC based control systems also offer improved opportunities for efficient plant control and off site monitoring.

In summary, although there is no one-size-fits-all solution, natural refrigerants CO₂ and ammonia offer the industry well-established and versatile options for a wide range of applications and capacities. Legislative and regulatory demands along with enforcement actions are likely to continue to intensify. This puts pressure on owners and operators of refrigeration equipment to make business decisions that are suitable for the long term in order to limit business exposure and avoid the impact of increasingly stringent legislation as the world moves closer to Net Zero.

This article has been CPD certified by the CPD certification service. To obtain your CPD diploma please email cpdcertificate@star-ref.co.uk.