Secondary CO2

A Forgotten Solution – Secondary CO2


Long before my almost nine-year old daughter Lucy was born (who I hope follows in my footsteps – Lucy is learning the basics!), Secondary CO2 refrigeration systems were proving to be a promising future-proof technology for food retailer’s, eventually losing favour for lower-capital cost alternatives. My blog explores revisiting CO2 as a secondary cooling medium.

If someone had asked me if there was a place for Secondary CO2 in retail a month ago, I would have said “probably not”. In the past couple of weeks, I have revisited 10-year+ calculations, layering in the rising cost of energy and innovative enhancements in technology (I touch on these later) that were presented in an IOR evening paper authored by me and Brian Churchyard back in 2012 – An Evaluation of Alternative Refrigeration systems.

First things first, I am not an innovator or pioneer, the idea of revisiting this technology was not mine, a simple “what if” type request was asked a couple of weeks ago. Whilst not being an innovator, what I do well is crunch numbers and model energy performance and environmental impact of retail refrigeration systems – within 5% of actual/metered performance.

Why, GWP & Leakage

The main foundation of me sharing my findings in this blog surrounds the impact of rising energy costs. There should be a fine balance in refrigeration technology choice and energy performance whilst we also work towards Net Zero.

It is my opinion and has been for a long time that we should not be fixated on a refrigerants GWP. There is a trade-off between energy performance and environmental impact, and at this moment in time the former is the most critical (provocative but this is the reality of 2022). Keeping costs low for the consumer is a priority as many families and/or individuals will and are struggling with the rise in the cost of living. Simply put, the less energy consumed by refrigeration systems can help keep the cost of food lower.

Back to GWP, CO2 (specifically in a trans-critical/DX configuration) is the sure-fire way to achieve Net Zero in overcoming fugitive emissions, there are also heat recovery benefits, which can be difficult to control owing to system complexity (my blog doesn’t discuss this.) To get my view across, I do not discount or support one technology over another. I am a supporter of DX CO2, as I am of A2L refrigeration systems etc., in other words I take an agnostic view that promotes a blend of customer needs, innovation, value, reduced energy performance, environmental stewardship, skill set and safety.

When we consider Net Zero, specifically fugitive emissions, a historic failing has been leakage and keeping systems “gas-tight”. However, one large UK food retailer through focussing on leak reduction have proven that a well-defined strategy, long-term partnership with a single contractor reaps significant environmental benefits. It is possible (with effort, focus and dedication) to maintain leakage below 3% in new and well-maintained refrigeration systems. Yet in 2022 it is still commonplace to see leak rates exceeding 20%! This failing (and a failing is what it is) makes the lowest GWP refrigerant option extremely attractive, or at least it did before the cost of energy increased by over 50%.


Over the past couple of weeks, I have taken the time to evaluate energy performance (my primary focus), layering in innovations and enhancements across a range of refrigeration systems – including Secondary CO2.

It was back in 2007 when Secondary CO2 systems were developed for use in supermarkets. At the time components for use in systems operating on CO2 were hard to come by and in many instances “silver bullet” components were not available or developed. For instance, a circulation pump is a key component in Secondary CO2 systems, back in 2007 those available were for industrial applications and were often oversized. The regulation and control of CO2 into chilled display cases was carried out by either “needle-type” valves or by orifice “plates” (to ensure all cases had an equal pressure drop/to prevent over-feeding/or starvation to a display case coil.) Any shortcomings aside, Secondary CO2 systems were successfully deployed.


Fast forward to 2022, there has been many improvements in display case design and refrigerant metering valves and controls (the R&D work of display case OEMs, component suppliers and controls specialists has been excellent). My calculations and results for Secondary CO2 systems provide serious “food-for-thought.” In my calculations I applied R454A, a relatively new A2L with a GWP of 238 as the primary system refrigerant to cool the Secondary CO2. I also selected/modelled the low temperature fixtures to operate cascaded arrangement – applying CO2 as a direct expansion circuit that takes advantage of a constant and low condensing temperature provided from the medium temperature system.

My “probably not” paradigm has changed completely, in a world where there must be a serious focus on energy consumption, it is my opinion that Secondary CO2 could also have a place in the world of retail refrigeration. Until I revisited my Secondary CO2 calculations, my thoughts of applicable technology of choice for food retailers generally extended to Trans-critical CO2, A2L, Secondary (Mistral) Air and Water-Cooled Integrals. Secondary CO2 can now be added to this list.


Below are a sample of my results (A2L, Trans-critical CO2 and Secondary CO2) – based on 5% leak rate (direct emissions) and kWh energy consumption per linear meter (annum) and emissions (applying the TEWI methodology – TCO2, also measured per linear meter (annum):


  • R454A (A2L) direct expansion Medium Temperature (chilled cases) – 3,205 kWh-meter
  • R454A (A2L) direct expansion Low Temperature (frozen cases) – 3,356 kWh-meter
  • Trans-critical CO2 Medium Temperature – 4,792 kWh-meter
  • Trans-critical CO2 Low Temperature – 2,423 kWh-meter
  • Secondary CO2 Medium Temperature – 3,546 kWh-meter
  • Secondary CO2 Low Temperature – 2,423 kWh-meter


  • R454A (A2L) direct expansion Medium Temperature (chilled cases) – 0.74 TCO2-meter
  • R454A (A2L) direct expansion Low Temperature (frozen cases) – 0.78 TCO2-meter
  • Trans-critical CO2 Medium Temperature – 1.02 TCO2-meter
  • Trans-critical CO2 Low Temperature – 0.51 TCO2-meter
  • Secondary CO2 Medium Temperature – 0.95 TCO2-meter
  • Secondary CO2 Low Temperature – 0.51 TCO2-meter

With respect to emissions, I applied the latest CO2 emissions factor – as we decarbonise Trans-critical CO2 refrigeration system emissions will be negligible owing to a GWP of one. Secondary CO2 will likely be around 80% lower than A2L owing to a relatively small primary charge of refrigerant. Though where leakage can be maintained at a low level, fugitive A2L system emissions could be offset whilst taking advantage of their excellent energy performance characteristics.

Final Thoughts

Though I haven’t applied capital costs to provide a Total Cost of Ownership approach, a valid question can be asked from my analysis, “could the forgotten Secondary CO2 solution be another system type that could be added to a retailer’s toolbox of future-proof solutions?” Regarding capital costs, Secondary CO2 systems could also utilise existing assets as the primary cooling system solution – another possible benefit.

When all is said and done, all any end user of refrigeration wants is to remain compliant through environmental stewardship, receive value and maintain consistency. Irrespective of adopting naturally occurring or next generation synthetic refrigeration systems, the name of the game is to provide accurate and repeatable designs and installations with equipment that can be easily maintained. Some system types consume more energy, some have a lower GWP, others are more expensive, and there are those that are more difficult to maintain, but one thing is certain, owing to F-Gas, Kigali and now Net Zero it is a case of “take your pick” out of high-pressure, flammable and/or toxic refrigerants; these “take your picks” will be explored in my next blog!

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