History of One Innovation

• 11/6/2026
• 28-minute read

The Rediscovery of CO2 as a refrigerant

S.Girotto

The rediscovery of carbon dioxide (CO2) as a refrigerant began in the late 1980s. Following an initial research phase that lasted until the early 1990s and the subsequent development of prototypes, production of machinery finally commenced in the early 2000s. The research originated at the University of Trondheim (Norway), spearheaded by Professor Lorentzen (1915-1995). Initially, the research focused on developing small heat pumps for domestic hot water. These studies led the renowned Japanese company Denso to develop “Ecocute” heat pumps around year 2000, quickly followed by other Japanese manufacturers. The next phase of research concerned Mobile Air Conditioning (MAC) systems, though this did not lead to immediate industrialization. It is worth to mention, in this regard, the company Obrist as one of main actors in this developement. They produced several prototypes of Mobile air conditioning systems with CO2 for large car manufacturers, already in the year 2005 circa.

Starting in 1996, the development of refrigeration systems using CO2 began in Italy. This application had not been initially considered, as it was deemed complex and it seemed commercially unattractive. Having personally participated in this evolution as one of the key figures, I would like to use this blog to reconstruct the history of CO2 refrigeration system.

Acknowledgements. The persons listed below have contributed substantially to the events described in this history. A big “thank you“ to all of them.

Silvia Minetto - ITC-CNR (I)

Gerald Heinzmann – Kaeltering (CH)

Kurt Goetz - Kaeltering (CH)

Raphael Gerber – Frigoconsulting (CH)

Jonas Schoenenberger – Frigoconsulting (CH)

Massimo Lorenzi – Realtime srl (I)

Armin Hafner – Sintef (N)

Petter Neksa – Sintef (N)

Finally, a special “thank you” to eng. Mario Dorin 

1985-1995

1.    Introduction: The Era of Natural Fluids and the Advent of Synthetics

Mechanical refrigeration took its first steps around 1850. Until 1930, refrigeration systems exclusively used natural fluids, including ammonia, sulfur dioxide, and carbon dioxide. A major turning point occurred around 1930 with the synthesis of the first artificial refrigerants, such as R12 and R11. These “miracle” molecules quickly replaced most of the natural fluids in use, with the exception of ammonia, which remained the standard in large industrial plants. It was only fifty years later that the truth emerged: those synthetic substances that made refrigeration systems so simple and efficient were damaging the environment.

2.    The Montreal Protocol: An Industrial Earthquake

The modern cooling industry grew up relying on synthetic refrigerants: CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons). Both contained chlorine, one substance harmful for atmospheric ozone. In the decade between 1990 and 2000, I lived through this change on the front lines as a technical manager of a large commercial refrigeration company. It was a time when established certainties were overturned by scientific evidence. Studies by Crutzen, Rowland and Molina (Nobel Prize winners in 1995) proved that chlorine contained in CFCs and HCFCs was responsible for the destruction of the ozone layer, the so-called “ozone hole". The Montreal Protocol (1987) mandated the phase-out of the fluids containing elements harmful to atmospheric ozone, among which CFCs and HCFCs; now, in 2026 and after several years from ban of harmful substances and with ozone hole recovering, we can say that Montreal Protocol was an environmental success, but at that time for the industry it was a kind of earthquake.

3.    The Ozone Crisis and the Role of R22

The sector’s first response was pragmatic: falling back on R22 (an HCFC), which had a significantly lower Ozone Depletion Potential (ODP) than CFCs (0.05 versus 1.0). However, R22 was not suitable for all applications. Consequently, the chemical industry developed and began proposing HFCs (hydrofluorocarbons), fluids with zero ODP because they did not contain chlorine. The market was searching for the “ultimate refrigerant,” but a new obstacle soon appeared: HFCs, much like CFCs and HCFCs, contributed heavily to the greenhouse effect.

4.    From the Ozone Problem to Global Warming (GWP)

While efforts were being made to save the ozone, the issue of the greenhouse effect and global warming emerged with force. To measure the impact of these fluids, a GWP index (Global Warming Potential) was defined. New refrigerants like R404A or R134a, despite being “Ozone Friendly”, turned out to be extremely powerful greenhouse gases. To put it in perspective: just one kilogram of R404A released into the atmosphere has an impact equivalent to nearly 4 tons of CO2.During those years, we witnessed a chaotic proliferation of new refrigerants and blends. Complexity increased drastically.

• For manufacturers: Every new fluid required long time consuming and expensive compatibility and reliability tests.
• For service technicians: Managing several different cylinders of refrigerants and oils became an unsustainable burden.

Above all, there was no clear direction—a total lack of ideas on how to solve this situation at its root.

5. Indirect Systems

To reduce the charge of synthetic gases, some companies tried the path of indirect refrigeration systems: a reduced primary circuit cooled a secondary fluid (e.g. glycol water) to be pumped to the end users.

For medium temperature systems technically it was quite simple. The problem was instead low temperature systems, as it was not possible to use calcium chloride, an “old” and well-known brine for low temperature, due to corrosion with copper.

Some of the secondary fluids proposed are difficult to believe today:

• for low temperature new brines were proposed, for example some based on potassium acetate or potassium formiate
• for MT the so-called Flo-ice was proposed, a mixture of ice and brine
• Somebody mentioned for the first time CO2 as a phase changing brine, but the pressure appeared too high for the standard of that period. So one of the many ideas was to use CO2 mixed with Acetone (a flammable fluid!) so as to limit saturation pressure.

No one of the above solutions or ideas was successful.

Field analysis revealed insurmountable physical and economic limits of indirect systems, that is high cost and low energy efficiency. Intermediate heat exchangers and pumps for viscous liquids increased electricity consumption, making not sustainable, especially for low-temperature applications, an indirect system.

Other drawbacks were:

• Low reliability: The use of aggressive secondary fluids (such as potassium acetate) caused frequent corrosion.
• Costs: Mechanical complexity and additional components made the systems much more expensive and prone to failure.

Conclusion: Toward the CO2 Revolution

By the late 1990s, it was clear that adding layers of complexity or seeking new synthetic molecules was not the final solution. While Southern Europe remained anchored to old patterns, news filtered in from Northern Europe regarding the use of carbon dioxide (CO2 or R744) as a phase-changing secondary fluid for low temperature systems, so as very low temperature (e.g.-30°C or lower) could allow to limit design pressure to 25 bar. Using CO2 not merely as a secondary fluid, but directly as the primary fluid—an idea that seemed obvious at that point—would not just be an evolution, but a revolution. In 1995, however, this meant embarking on a massive industrial effort: components had to be designed from scratch because they simply did not yet exist on the market.

1995-1996

The Return to basics – Why Physics Beat Chemistry

In the 1990s, while the chemical industry was hunting for a new “magic molecule” thermodynamics suggested the opposite direction: instead of searching for a fluid suited to the standard refrigeration cycle, it was necessary to modify the system design to allow the use efficiently a natural, non-toxic, and non-flammable fluid, namely CO2. Choosing carbon dioxide (R744) meant clashing with a physical limit that had been considered insurmountable for decades: its low critical temperature (31.1°C).

1. The End of Synthetic Alternatives

Around 1995, research by NIST (conducted by McLinden and Domanski) reached a definitive conclusion: the atomic combinations for creating new refrigerants that were simultaneously safe, efficient, and stable had been exhausted. There were no more new synthetic molecules capable of replacing old gases without reintroducing environmental (GWP) or safety issues. The “chemical solution” was stalling; in its place, the “thermodynamic solution” began to take shape.

2. The Transcritical Cycle: Lorentzen’s Revolution

The work of Professor Gustav Lorentzen in Trondheim between the late 1980s and 1995 was the turning point. Lorentzen theorized that CO2 should not be used like traditional fluids. Above 31.1°C, the critical temperature, the fluid no longer condenses like a normal refrigeration fluid in use, but it enters a “transcritical” state (or “dense gas”). Although practical experiences dated back to the 1920s—primarily in the maritime sector—it was necessary to carefully evaluate the advantages of using CO2 and resolve the drawbacks:

• The Advantage: High energy density and optimal heat exchange properties.
• The Challenge: Very high operating pressures—up to 120 bar, compared to the 15-20 bar of traditional systems.

3. 1995: From Theory to Reality – The First Compressor for CO2

At the time, using CO2 in refrigeration was a beautiful theory without real machines. Quite simply, there were no components, in refrigeration, capable of withstanding those pressures, let alone compressors and valves suited for the purpose. The practical breakthrough came from a collaboration launched in 1995 between Costan (the company I worked for at the time) and Officine Mario Dorin, after a meeting in Trondheim (myself and Peer Samuelsen for Costan and Petter Neksa and Jostein Pettersen from Sintef). In summer 1996 the world’s first prototype of a semi-hermetic compressor for transcritical CO2 was born. The testing phase had an almost heroic, artisanal feel:

• Tests were conducted outdoors, as equipped test rooms for managing such pressures did not yet exist.
• The gas cooler and evaporator were “built by hand”
• Valves were “borrowed” from the hydraulic sector
• Lubrication issues were solved directly in the field through continuous adjustments.

The results however were clear: the compressor performed excellently, and the machine “produced cold”. Above all, we proved that managing high pressures was not an impossible task.

4.    The Technical Lesson

Even though in 1996 the only existing component was the compressor, we had proven that using CO2 was not a problem of principle, but “only” a technological problem of pressure containment and control. Once the high pressure was managed, the road toward natural cooling was finally open.

At this point we need to remember that a great man and a great scientist, prof. Gustav Lorentzen, passed away in august 1995.

1996-2004

From Prototype to Market

Moving from theory to commercial practice required a phased strategy. At the end of the 1990s, the market wasn’t ready for a direct leap into pure transcritical systems; we had to “tame” the technology through intermediate steps, learning lessons that no refrigeration manual could have taught us.

1.    The Transition Phase: Subcritical

In 1997, to accelerate the project’s maturity, we introduced the first “subcritical” system with CO2 as a refrigerant. The idea was to use CO2 in direct expansion only for low temperature (LT, evaporating from -35 to -40°C), with a compressor in cascade with a conventional primary fluid, used for medium temperature users and chilled by brine chiller.

• The Conegliano Success: At the “Bingo” supermarket in Conegliano (TV), we installed the world’s first subcritical low-temperature system in cascade with an indirect system (R134a chiller). This milestone drew interest from technicians across Europe. Other installations followed, including one in the UK using Propane for the medium-temperature chiller.
• The Technical Lesson: Despite the enthusiasm, limits emerged: the dual circuit, circulation pumps, and additional heat exchange reduced efficiency and increased complexity and cost. It was a compromise that only solved maybe 30% of the environmental problem—in a supermarket context, only the low-temperature portion

2.    Year 2000: The World’s First Transcritical System

The first subritical systems in retail sector were the spark for the introduction of CO2 cascade also in industrial refrigeration, using ammonia for the brine chiller, for example.  In the meantime we had, at Costan, the possibility to build a proper circuit for testing transcritical compressors, after gaining one project financed partly by Italian Ministery of Research. The true “evolutionary leap” happened near Pordenone, always in Italy, in the year 2000. Here, we built the first transcritical system of a new type in the world: a single-stage, CO2-only, system. We literally had to invent what was missing:

• We created a self-regulating mechanical system for high pressure control, as nothing existed on the market. The system used a differential valve, with DP circa 20 bar, maintaining such differential pressure between liquid surface in the receiver and high pressure, so approximating the optimal high pressure in a wide range of external temperatures.
• We designed and built coaxial tube-in tube CO2/water heat exchangers sized for a CO2 pressure of 120 bar, building it by ourselves. The heat was discharged through a dry cooler
• Chemistry: We field-tested which lubricating oils were actually suitable and compatible.

We were writing the rules of the game while we were playing it.

3.    The “Baptism of Fire” (and Ice)

The first “full-scale” plant, multicompressor type, arrived in 2002 in the province of Treviso, featuring three machines with compressors in parallel: single-stage for MT (medium temperature) and two-stage compressors for LT (low temperature). However, radical innovation always collides with the unexpected. Two extreme events put the system to the test.

The Oil Emergency (Winter 2002-2003): With temperatures dropping much below 0°C at night, we discovered that—since the machines were outdoors—the CO2 in idle compressors would condense, causing the oil and CO2 to separate. Upon restart, because the oil selected was lighter than liquid CO2, the oil pump sucked up the latter instead of the lubricant, destroying compressors in few minutes.

The Record Summer (2003): One of the hottest summers since then, up to almost 40°C, for several months. The system sustained the pressure, but human factor “fragility” emerged. Maintenance errors on one occasion led to the safety valves blowing at 160 bar. The noise was so loud that neighbors called the Fire Department.The plant survived, but we realized that the technology demanded a new, much higher level of professionalism in maintenance and service. The results from that plant were presented at the IIR conference in Washington in 2003, sparking massive interest from technicians, particularly from Scandinavia.

4.    2004: Foundation of Enex

After twenty years as an employee—growing from being a laboratory technician to technical director—I felt that the structure of a large and prestigious company with a well-known brand was not suited to take the risks required for massively introducing CO2 in the market. On March 1st, 2004, I founded Enex. The name was a synthesis of EN-ergy and EX-ergy (the part of thermal energy that is actually usable). The concept of Exergy, a pillar of the second law of thermodynamics, embodied our mission: the highest possible efficiency. Enex became fully operational by the end of 2004. Enex was the first company focused only on CO2 refrigeration. It was followed some years later by 2 other companies.

2004-2008

1.    Technical Evolution – Booster, Economizer, and Double Stage

Following the completion of the first “full-scale” plant, with Costan we implemented numerous systems across Scandinavia, in particular in Sweden, and a few in Italy. Between 2003 and early 2004, about twenty systems were built. These machines were derived from the experience gained with the Treviso plant: units with compressors in parallel, single-stage type for MT (medium temperature) and internal compound two-stage for LT (low temperature), featuring an “open flash tank” type receiver. Meanwhile, one leader in commercial refrigeration, Linde Carrier, completed in 2004 its first plant with CO2 as a refrigerant in Wettingen, Switzerland. Although it was a relatively simple system—single-stage for MT and cascade for LT—it was of significant size and quite interesting. There were, in the Wettingen plant, some interesting steps forward: first of all the first Bitzer transcritical compressors, then the first electric valve - from industry - for controlling high pressure in gas cooler and the first gas cooler CO2-air from Luve. Another characteristic of this plant was the pressure control in the liquid receiver.

The company Bock had already available a very small trascritical “open type” compressor for CO2 at prototype stage, designed for testing air conditioning for bus for a German company. It was clear at this point that, even if many components were still at “field test” or prototype stage, there were more players exploring the new technology, however still at an early stage.

The High Pressure control valve and Economizer: Optimizing Flash Gas

In 2006 Danfoss released some field test valves for high pressure control, that is the model ICMTS, derived from a valve from industrial refrigeration, together with a proper controller, EKC326. The new devices made it possible to apply a proper control of pressure in liquid receiver, different from the first systems with a floating intermediate pressure. Enex started experiments of an innovative concept for CO2 systems: the “economizer”.                                                                     

This device was designed to significantly improve machine efficiency when outdoor temperatures rise. In such conditions, flash vapor production increases; by using a two-stage expansion scheme and having an auxiliary compressor for suction of the flash vapor from intermediate receiver, we managed to drastically increase cycle performance. The first large-scale real-world application for a system with economizer followed in 2008, in Freiburg. The control of new design proved to be quite difficult, due to huge variation of flash vapor mass flow.

Booster Systems and Double Stage: Efficiency in Retail

In 2006-2007 two new companies entered the market of CO2 “transcritical systems”. One of them was based in Denmark - Advansor - and the other in Sweden - Green&Cool; I knew both founders very well as we had contact in the past years for the plants we built, with Costan, in Scandinavia. Between 2007 and 2008, one of the most pressing challenges for supermarkets was reached: managing both “fresh” (medium temperature) and “frozen” (low temperature) products with a single centralized unit. This led to the “Booster system”. Enex and Advansor developed circa at the same time the new circuit, a design with integrated MT and LT that was considerably more efficient than a cascade system. By eliminating the intermediate heat exchanger, the associated temperature difference and the resulting thermodynamic loss between LT and MT was removed. Enex also refined the use of machines with “two-stage compressors”, produced at that time exclusively by Dorin—the only ones in the market who believed in this technology since the beginning.-

The Market Challenge: Skepticism vs. Innovation

Despite the clear technical data, the initial market response was still characterized by strong skepticism. Many operators feared the high pressures and the apparent complexity of the new designs. The market was substantially limited to Switzerland, Scandinavia and UK. There were, in that period, several attempts to design large heat pumps for sanitary hot water, similar to what was happening in Japan after the success of small “Ecocute”. All that was important also for refrigeration, as the company Swep developed - for the promising market of heat pumps - a high pressure plate heat exchanger which could withstand 120 bar, the model B16DW. This heat exchanger could also be used as a water cooled gas cooler and as a heat recovery heat exchanger in refrigeration units for retail. Moreover, Dorin compressors were improved, thanks to an extended test campaign which lasted several months, in order to bring at highest level their reliability. For Enex a commercial turning point came from Switzerland in 2007. Thanks to the foresight of partners like Frigo Consulting and Kaeltering, several cutting-edge systems were installed. The main markets for Advansor were initially Denmark and UK, and in particular one large retailer. In Scandinavia Green&Cool made quite well. There was a systematic lack of components, as valves, for example, but at this point it was possible to assemble refrigeration units which were reliable enough. The request from the market was however very limited.

2008-2013

The consolidation of the business and the ejector

1.    The consolidation of business

Between 2009 and 2013, transcritical CO2 technology was still considered a niche, nevertheless it started to attract large companies. The first consequence was the acquisition, or the partnership, of large companies with the 3 companies specialized in CO2 and founded between 2004 and 2007. In 2011 circa Dover group took over Advansor, Carrier took over Green&Cool and Huurre, a Finnish group, acquired a minority stake in Enex. It was the first signal of the interest of the market, even before a real significant increase in sales.

2.    The Ejector Revolution: An Unexpected Discovery

The ejector was an almost “mysterious” object in refrigeration. Invented in France around 1850 for other purposes, it remained a theoretical concept in refrigeration until it was revisited by SINTEF in Trondheim around 2010. It is a static device that recovers expansion energy to reduce the work required by compressors.

The real breakthrough happened at Enex, from an intuition born during a test gone wrong.

• The Unexpected: While testing an ejector on a heat pump, we noticed it wasn’t performing according to the calculations, but it was behaving as an excellent “liquid pump.”
• The Intuition: “What if we use it to recirculate refrigerant in the evaporators?” Thus, the concept of the “overfed (or semi-flooded) evaporator” was born.
• The Result: The first real plant, installed by Enex in Lahti, Finland, showed an average efficiency increase of 20-25%. This technological leap, protected by a series of patents, changed the rules of cooling in 2010.

3. Switzerland: A Permanent Laboratory

Switzerland was a source of excellence. Enex, together with partners like Frigoconsulting and Kaeltering,  perfected the integration of the ejector and the economizer circuit between 2011 and 2012. After the first experiments in Finland made by Enex with Huurre , some very interesting field tests were done. First of all one installation in Bulle, near Gruyere, then one installation near Luzern (Schwitz) followed by several others. Initially Enex produced directly the ejectors based on a design made by Sintef and specialists in fluid dynamic from Poland. More alternatives became soon available. Starting 2015 Danfoss developed the “multiejector”, an industrial breakthrough. Carrier and Carel developed the step-motor controlling mechanism for their ejector. The configurations developed  solved a limitation: they allowed the ejector to operate continuously and stably in all environmental conditions, without the need for complex control devices. In Switzerland the cooperation between Frigoconsulting, Kaeltering and Enex allowed to make preliminary tests with other devices, like for example with an expander. However the ejector, theoretically less efficient, was superior due to intrinsic simplicity and reliability.

2013: Expansion of CO2 to Eastern countries.

Thanks to the prestige earned through Swiss and Scandinavian installations, the “Eastern Europe” market unexpectedly opened up. In reality the business came from French and German large retailers who decided to use CO2 in Romania and Poland for some hypermarkets. Advansor, Enex and Green&Cool (Carrier) were initially the competing suppliers for these installations.

Beyond the CO2 Equator – Southern Europe and Total Integration

For years, the refrigeration sector—historically very conservative—fueled strong skepticism against carbon dioxide. In the beginning, “experts” were saying that CO2 was simply not suitable for refrigeration at all. A few years later, faced with the evidence of the first successful plants, the criticism shifted: it was said to be a solution limited to the cold climates of Northern Europe. Detractors even spoke of a “CO2 Equator,” an imaginary latitudinal boundary below which the technology would never be cost-effective. The challenge in that period was to tear down these prejudices, proving that with the right technical architecture (optimized cycles and ejectors), R744 was competitive even under the Mediterranean sun.

1.    Conquering Southern and Eastern Europe

The turning point for the expansion in the southern Europe was 2013.The success of these sites finally opened doors to markets where skepticism had previously reigned (France, Spain, and Italy itself). Shortly after, the wave of innovation reached Eastern Europe, with the first plants in Poland, Croatia, and Romania. Different manufacturing techniques were studied by the manufacturers. For example some of them used welded carbon steel for the piping, while others adopted stainless steel. It was an important element, much different from previous refrigeration installations with HFC, where brazed copper was used. Advansor (Dover) mainly was expanding its market thanks to a strong effort in standardization and industrialization, while Enex was more specialized in development and R&D, thanks to the connections with Sintef and CNR of Padova. A good help for spreading CO2 technology came from Shecco, with its informative activity. More suppliers introduced in their range components designed for CO2- compressors, valves and heat exchangers - so competition and improvement at all level started. A “new” alloy (K65, an alloy of Copper with Iron) was introduced for the piping by Wieland, which for small plants allowed to use brazing process, instead of the more demanding welding. In substance the “technology” for a CO2 system started to be “state of the art”. Also the rules to follow for having systems compatible with European standard related to safety were refined to limit unnecessary extra cost.

In 2014 the new F-gas regulation was in preparation. It entered in force 1st of January 2015. It introduced clear limitations to the use of syntetic refrigerants, finally convincing most of the users that natural fluids were the right choice for the future. Of course CO2, being no toxic and non flammable, was the candidate for retail refrigeration and light industrial refrigeration.

2014-2019

Italy and the “All-in-One” System

Another decisive chapter opened in 2014 in Trentino-Alto Adige. Collaborating with Realtime, a trusted local installer for a major regional retailer, Enex began implementing revolutionary combined systems. It was no longer just about “making things cold”: the central unit was now capable of simultaneously producing refrigeration, space heating and cooling. Summer air conditioning was integrated into the system, transforming the refrigeration rack into the true energy heart of the building.

The Chiller with Flooded Evaporator

In 2016, Enex introduced a radically new chiller design based on the principle of the gravity-flooded evaporator, new for CO2, but well known in industrial ammonia systems. Installations for skating rinks were done in Scandinavia by Green&Cool and Huurre, and by all the companies in general in light-industrial plants. A new sector for CO2 chillers was the north-Italian wineries, introducing environmental advantages in this demanding sector. Shortly after it was introduced a variant of gravity-fed chillers featuring the ejector—another Enex invention. This adjustment allowed for an efficiency improvement of approximately 10% specifically during peak thermal conditions, where CO2 refrigerant chillers typically struggle the most.

Zero-Cost Heat: Heat Pumps and High-Temperature Recovery

One of the most interesting thermodynamic advantages of CO2 in the transcritical cycle is the gas temperature at the compressor discharge. While in traditional systems the discharged heat is at relatively low temperature and it is difficult to reuse, with CO2 the refrigerant temperature after compression can exceed 90-100°C.

Hot Water and Heating: since 2008 solutions were developed to recover part of the heat available, to produce domestic hot water or space heating, practically at zero cost.

Total Integration: In modern supermarkets, the new units eliminated the need for gas boilers or separate systems. Essentially, the heat “extracted” from the refrigerated display cases to preserve food becomes the resource used to heat the store and the sanitary water. A perfect example of the circular economy applied to engineering.

The Future is Natural – The Market Opens to the World

Starting from 2016 more and more companies introduced gradually CO2 components and units to their range. After more than twenty years of technical challenges, doubts and successes, the reliability of CO2 systems was now a proven fact.

Basically the reasons for which the advantage of CO2 systems appeared evident were:

-        Direct thermal exchange, substantially improving efficiency compared to secondary systems. No additional temperature difference and no pumps.

-        No toxicity

-        No flammability

-        Long term solution

-        Low cost of refrigerant

Between 2018 and 2019, demand surged further: the market no longer asked only for small systems, but for increasingly larger machines. The industry responded by exceeding the 1 MW cooling capacity threshold, paving the way for sectors that, until then, were dominated by traditional systems.

1.    The New Frontiers of CO2

Today, carbon dioxide competes on equal footing in large industrial plants, thanks to three fundamental applications:

• Logistics Distribution Centers: CO2 avoids the toxicity risks associated with ammonia and reduces costs by distributing the refrigerant directly to the evaporators, often integrating heat recovery. In 2018 a large plant – about 3 MW refrigeration – designed and manufactured by Enex an installed by Scar (a large contractor in Milano) included, for the first time, hot gas defrost, using low pressure/high temperature fluid.
• Industrial Heat Pumps: A true revolution in heating. Thanks to specialized hydronic circuits, it is possible to produce high-temperature heat (60-65°C) with zero environmental impact and with a special design of water storage cylinder.
• Integrated Systems (Energy Hubs)

 The refrigeration plant of the future does more than just preserve products; it is a “beating heart” that provides climate control for environments and heats upmwater, recovering every single watt that was once dissipated into the atmosphere.

Conclusions: A Forward-Looking Vision

Thirty years after the first steps, the idea of reintroducing CO2 has proven to be a winning one. While the ozone hole is recovering and European regulations (F-Gas) signal the end of synthetic gases, by 2026 -CO2 has become the standard solution in retail and a top choice in industrial refrigeration. It is quite evident that the limitations of synthetic refrigerants introduced by F-Gas directive in 2015 was possible thanks to the availability of natural alternatives, among which CO2 technology. It is true also the opposite: the limitations introduced by F-Gas reinforced the use of natural refrigerants. Anyway the CO2 technology is now ripe. Several tens of companies are now manufacturing CO2 systems, and more than 100.000 plants are now in operation, while more and more are being installed also in industrial sector. This technology, developed in Europe, is now spreading in North and south America and it is popular in Asia, Australia and New Zealand.

Lessons for the Future.

The journey of reintroducing CO2 leaves us with three fundamental reflections:

1. The Uncertainty of Synthetics: New shadows are appearing on the horizon, such as global PFAS contamination, while some synthetic refrigerants are PFAS. History teaches us that the use of synthetic chemical compounds (which can be for example Teflon or CFCs or thousand of synthetic substances) must be evaluated with extreme caution, given the possible long-term impacts.
2. The Inertia of Change: Introducing a radically new technology requires a long time due to the inertia of the sector. Likely, in the case of CO2 technlogy, a more structured financial support and a more active communication could have accelerated this transition.
3. The Value of the Natural Choice: Investing in natural solutions is the only choice that proves truly rewarding in the medium and long term. Looking back, the choice made thirty years ago was not just a technical experiment: it was the correct direction for a truly sustainable cooling industry.

Efficiency improvement with CO2 refrigeration systems

Since the beginning of the use of CO2, in 1920s, many studies have been done to improve cycle efficiency under conditions of high temperature sink side. Low critical temperature penalizes this refrigerant in high temperature ambient conditions, when ambient air is the fluid used for heat removal high pressure side. This argument is interesting now due to high summer temperatures in Europe. The modified cycles analyzed were, briefly, of this type:

a) 2-stage compression, for evaporation temperatures below -20°C

b) Voorhees

c) Plank (post-compression and cooling)

d) Auxiliary compression (flash gas recompression)

e) Energy recovery from expansion with turbine or expander

f) Post-gas cooler subcooling

g) Evaporative cooling of the gas cooler

h) ejectors for semi-flooded feeding of evaporators

i)  flash gas pre-compression with ejector

If one has to decide where to invest to reduce losses not only it is important the amount of reduction, but also for how many hours during the year the losses are significant. For example it is possible to reduce the throttling losses with an ejector, but in some areas the hours during which the practical effect is negligeable are simply too much. Economically it is better to invest to eliminate first the losses due to temperature difference at the evaporator (source) and high pressure heat exchanger (sink) as they are present all the year.

It is mentioned above the expander, e). Today is not available yet a version fully proven. One expander must work with a 2-phase fluid, with obvious lubrication problems. Some trials were made but the result is not yet completely satisfactory.

Moreover there is already available a simple (mechanically) expander device without moving parts – that is the ejector - with isentropic efficiency in the range of 0,5-0,6 circa in expansion process, so this is the reference to consider. As the ejector is a very reliable device the additional gain which could be obtained with an expander is probably not so significant.

Separate considerations must be made for the Voorhees method, as I will say later.

 To increase efficiency in the immediate future, all what remains is to work on apparently secondary aspects but whose benefits are obtained at low cost and, above all, produce effects all year round:- more precise capacity control

• to avoid or limit losses that are normally neglected, such as leakage and pressure drops
• to limit electrical losses (consumption of electrical panel and control equipment)
• to limit the consumption of “auxiliary” components, such as oil heaters, fans and more
• to implement some “clever” control strategies

Another source of saving is obviously heat recovery.

Voorhees refrigeration cycle

G.T. Voorhees was an American inventor who had proposed a modified reciprocating compressor for CO2 systems. A “suction port” placed in the cylinder near the lower dead center of piston allowed a certain flow rate of vapor to be sucked in by drawing from an intermediate pressure, generally the receiver/separator.

A significant increase in flow rate, and therefore in cooling capacity, could be obtained, as well as a slight increase in efficiency, at least according to the texts of the time (e.g. prof. Ostertag).

I had the opportunity to test a compressor of this type, a prototype derived from the modification of a production compressor for CO2, some years ago and we noticed that the performance was higher than theoretical estimate even if the design was far from optimal. We measured about 10-15% higher EER (Energy Efficiency Ratio) than the standard cycle, as well as about 20% higher capacity.

The disadvantage was rather the difficulty of connecting two or more compressors of this type in parallel, because there was generally a pressure leakage through auxiliary port and crankcase when one of the compressors was stopped. However, such a system could be cost-effective with a single-compressor system. I think it is not easy, but the “rediscovery” of CO2 gives hope for a revisiting of this technology and a future industrial use of this method.