ATMO World Summit: CO₂ is ‘Close to the Ideal Refrigerant’

 “CO₂ is as close to the ideal refrigerant as you can come,” said Dorin Sales Manager Giacomo Pisano, citing CO₂ [R744] refrigeration pioneer Professor Gustav Lorentzen, who developed the modern transcritical CO₂ cycle.

His observation came at the end of a presentation about industrial refrigeration during the ATMO World Summit conference on March 30. The online, 24-hour conference was organized by ATMOsphere (formerly shecco), publisher of R744.com

Pisano talked about the use of ammonia/NH₃ (R717) and CO₂ in industrial applications, listing their benefits and drawbacks. While recognizing that ammonia is a “great refrigerant, seen from a thermodynamic standpoint,” Pisano still feels the scales tip in favor of CO₂ when you include cost, efficiency developments and maintenance.

Advantages to CO₂ refrigeration

There are many reasons Pisano believes CO₂ to be close to the ideal refrigerant. A big advantage that CO₂ has over ammonia is that it is not toxic, meaning it has fewer regulatory restrictions. And while CAPEX is difficult to compare one-to-one between ammonia and CO₂ systems, as they depend on a number of different factors, “a lower cost of between 30% and 50% is doable when talking about CO₂ compared to ammonia,” Pisano said, stressing that these figures are for branched CO₂ systems. For chillers, the cost difference is smaller, he noted.

Looking at efficiency, Pisano shared the results of a study comparing three different ammonia systems, an ammonia-CO₂ cascade, and seven different CO₂ systems with a number of different efficiency improving technologies like parallel compression, ejectors and adiabatic cooling. Without additional  efficiency-improving measures, CO₂ cannot compete with ammonia in very warm climates, but with the additional features, it comes close to being as efficient as the study’s baseline system, which was the ammonia-CO₂ cascade system.

CO₂ racks are often slimmer than similar-capacity ammonia systems, with a smaller footprint, leading to cost savings when it comes to building expenses, according to Pisano. As an example, he mentioned a typical 1.5MW (426,518TR) system, where the CO₂ system would require approximately 100m² (1,075ft²) of space but the ammonia system would require 350m² (3,767ft²).

In addition, he said, “CO₂ has larger specific capacity, and this brings in much smaller pipe diameters, and also with regards to the refrigerant charge, you normally have a much larger allowance in kg per square meter [for CO₂].”

There are also practical advantages when working with CO₂ systems, one of them involving oil. “CO₂ works with miscible lubricants, so oil return is definitely easier,” Pisano explained.

Commissioning CO₂ refrigeration systems is often quicker, with semi-hermetic compressors needing less maintenance and having no leakages, Pisano noted. 

The operation of CO₂ systems using adiabatic gas coolers can also be less water-intensive due to the transcritical operation. “This means that the surface of the gas cooler has to be wet in a lower ratio.” Pisano explained. “When working with ammonia, or HFC refrigerants, you can basically do adiabatic cooling, but you need the entire surface of the condenser to be wet. For CO₂ you only need to apply water at the very last section of the gas cooler.” 

In other words, adiabatic cooling with transcritical CO₂ works “extremely well” and can give a significant COP boost, according to Pisano. As proof of this he detailed a Chinese case study in a very warm locale, where adding adiabatic cooling raised the COP from 1.23 to 2.32. 

CO₂ also has a smaller pressure drop, meaning higher fluid speed is possible, giving a “very nice” heat transfer coefficient, as Pisano put it. 

Last, but not least, a really big practical and energy-saving benefit to CO₂ refrigeration is the opportunity for large amounts of heat recovery, which can be used to create hot water up to 90°C (194°F). That water can be used for any number of purposes like de-icing and underfloor heating.

In terms of maintenance, the approach to CO₂ systems is very different from that of ammonia equipment, noted Pisano. Ammonia compressors, for example, require service on a predictive basis. This is expensive, but also enables the compressors to last a long time, up to 20 years. With CO₂ compressors there is no predictive maintenance needed, according to Pisano, just oil and functional checks at regular intervals until their end-of-life replacement after 10 to12 years. In other words, “when it comes to maintenance costs, CO₂ can bring a significant cost saving, from 30% up to 50%,” he said.

As one of the reasons for why CO₂ is close to the ideal refrigerant, Pisano gave several examples where CO₂ refrigeration has been successfully used in industrial applications. An unusual one is CERN, the nuclear physics research center in Switzerland, which has adopted CO₂ refrigeration, using Dorin compressors, for some of its systems.

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