By Sandro Biccari of Marine & Refrigeration Engineering (MRE)
There has been a recent surge in misleading statements that carbon dioxide (CO2) is more efficient than ammonia (NH3) and Freon plants.The ‘marketing’ of transcritical CO2 refrigeration is portraying this as the ‘best thing since sliced bread’.
The excitement was initiated by the smaller commercial industries after having access to a ‘new’ natural refrigerant. CO₂ is far from new but has grown due to new transcritical applications and the need to replace Freons. For years, the commercial sector all used the synthetic Freons that were becoming increasingly difficult to work with due to Global Warming and Ozone Depletion. The newer Freon refrigerants are extremely expensive to purchase and also complex to use due to their ‘cocktail mixtures’. The new CO₂ growth in the commercial sector has created an environment for smaller commercial Freon companies to move into the industrial refrigeration sector. This is a normal progression as it was pushed really hard in developed countries that were previously Freon based or were anti-ammonia.
Unfortunately, the marketing that has been delivered with this new boom of transcritical CO₂ has been ‘stretching the truth a little bit’. We often hear from suppliers and clients that CO₂ transcritical systems are so much more efficient than ammonia and Freons. In reality, this is not the case.
To clear up the ongoing confusion caused by this marketing I would like to take the time to provide a comparison between the two most prominent natural refrigerants used today, ammonia and CO₂.
Each has distinct characteristics, advantages and drawbacks that make them suitable for different applications. The comparison must be made between ammonia and CO₂ refrigeration systems, considering aspects such as efficiency, safety, environmental impact and costs.
Chemical properties and thermodynamics
- Ammonia NH₃ (R-717): Ammonia is a naturally occurring compound with excellent thermodynamic properties. It has a high latent heat of vapourisation, which makes it highly efficient in transferring heat. Ammonia operates effectively in a wide range of temperatures and pressures commonly encountered in industrial refrigeration.
- CO₂ (R-744): Carbon dioxide is also a natural refrigerant. It has a lower critical temperature and requires higher pressures to operate effectively. CO₂ refrigeration systems work in transcritical cycles with acceptable efficiencies in cool climates. In warmer southern African climates, the efficiency drops but with sophisticated controls the transcritical is still able to work to provide the cooling requirement.
Energy efficiency
Ammonia systems are typically more energy- efficient than CO₂ systems, especially in large-scale industrial applications. This is due to ammonia’s superior thermodynamic properties, which allow for lower operating costs over time. As mentioned, CO₂ systems can be efficient in cooler climates, but they do require more energy in high southern African ambient temperatures due to the transcritical operation. The lower running costs of ammonia make it more suitable for large plants as electricity worldwide is no longer cheap.
Environmental impact
Ammonia has zero ozone depletion potential (ODP) and zero global warming potential (GWP). It is considered environmentally friendly, and leaks are minimised due to pungent odour. It is common knowledge that the leakage rate or top-up charge is the lowest for ammonia plants due to its pungent smell that assists in finding small leaks.
CO₂ also has zero ODP and a GWP of 1 (by definition). While CO₂ is a greenhouse gas, its use as a refrigerant does not add to atmospheric CO₂ levels, as it is recycled within the system. Both refrigerants are considered sustainable alternatives to synthetic refrigerants such as HFCs and HCFCs.
Safety considerations
Ammonia is toxic and can be hazardous to humans if inhaled in high concentrations. It is also flammable at certain unique concentrations. Strict safety protocols are designed and constructed in facilities using ammonia refrigeration, including leak detection and ventilation systems.
The strong odour of ammonia is one of its ‘safety features’ as small leaks are detected and rectified.
CO₂ is non-toxic at low concentrations and non-flammable. However, high concentrations can displace oxygen and cause asphyxiation as there is no odour safety feature. Therefore, leak detection and ventilation are critical to ensure safety. CO₂ systems operate at much higher pressures than ammonia systems, which can pose risks if not installed correctly and professionally managed.
All refrigeration systems fall within the legal requirements of SANS 10147 in South Africa. This design and construction standard is a legal part of the OSH Act and it is often confused by some people who believe SANS 10147 is only applicable to ammonia plants.
System complexity and maintenance
Ammonia systems are designed and built to a high ‘petrochemical’ standard. The nature of the equipment and construction materials allow for exceptionally long lifespans of plants. The majority of ammonia plants are still operating after 40–50 years and remain in incredibly good condition when well maintained. Technicians are specifically trained to work on ammonia plants.
CO₂ systems, while requiring high-pressure components, tend to be built as smaller ‘Freon rack’ systems based on commercial sizing and standards (for example, 20x small CO₂ compressors compared to 2x ammonia compressors). The transcritical cycle in CO₂ systems demands advanced control systems and technical expertise.
Cost considerations
Ammonia systems can have slightly higher initial costs due to the need for high-quality materials and design standards. However, the efficiency does result in lower operating costs.
CO₂ systems may have lower installation costs, especially in smaller applications, but high-pressure equipment can increase expenses for larger industrial installations and reduced longevity.
The yearly COP values of these two relatively small systems were measured in a warm climate and are shown in the table below. COP = refrigeration performance in kWR / absorbed power. The higher the value the better the system.
The saving is substantial when you consider the life ownership of a plant.
Applications
- Ammonia: Widely used in large- scale industrial refrigeration, such as food processing plants, cold storage warehouses and freezers
- CO₂: Increasingly used in commercial refrigeration such as supermarkets, convenience stores, transport refrigeration and smaller industrial applications
Summary table
Conclusion
The choice between ammonia and CO₂ refrigeration systems depends on the specific requirements of the application, including scale, safety, cost and environmental considerations. Ammonia remains the preferred choice for large- scale industrial refrigeration due to its high efficiency, while CO₂ is gaining popularity in commercial and transport sectors for its safety and sustainability.
There is a particularly good Danfoss Report that has recently been issued that covers detailed studies of energy consumption between ammonia, transcritical CO₂ systems and Freon R-507.
On a varying load profile CO₂ is 30–35% greater in energy consumption in a warmer climate such as South Africa.
Please email me directly (sandro@mre. co.za) if you would like a copy this Danfoss report. A copy will be available on our website as well: www.mre.co.za
Marketing is powerful tool to sell refrigeration systems, but the truth will prevail.