Following the EU’s crackdown on fluorinated greenhouse gas (F-gas) use, CERN, the European Organisation for Nuclear Research, has moved towards carbon dioxide (CO2) refrigeration technology for its ATLAS and CMS tracking detector experiments at the Large Hadron Collider (LHC).

The adoption of CO2 as a refrigerant during high-powered collision experiments also provides significant advantages over existing refrigerant gases such as hydrofluorocarbons (HFCs).

And in an exclusive interview, Paolo Petagna, Project Leader at CERN, spoke to gasworld about the transition.

CERN's Large Hadron Collider (LHC)

CERN’s Large Hadron Collider (LHC)

How will CO2 be used?

During collider experiments at the LHC, charged particles crash into one another at close to the speed of light. Silicon detectors are used to reconstruct the paths taken by these particles.

Due to the high amount of radiation emitted following the collision, extremely low temperatures are required for the silicon detectors to continue functioning.

“Silicon detectors at the LHC require cold operational temperatures in order to survive the high radiation field produced by the proton collision,” explained Petagna. “The specific operational conditions of these highly sophisticated detectors require very accurate and stable temperature control.”

The use of CO2 as a coolant at CERN isn’t a first; prior to use on the LHC, temperature control via the use of CO2 was proposed and realised for a small silicon detector for LHCb – a special purpose experiment for the LHC. LHCb used a 2-phase Mechanically Pumped Loop (2PACL) which allowed cold liquid CO2 to cool the detector before partially evaporating in a network of small pipes.

Petagna said that this 2PACL system has been successfully operating since 2008. He added, “It’s proved to be highly reliable and delivers excellent performance, while using a ‘green’ and inexpensive refrigerant (CO2) for the thermal management of the silicon detectors.”

From this, CERN created a dedicated team in 2010 to develop the potential of this technology to be used with other detectors.

With the 2PACL system still requiring an external chiller to lower the temperature of the liquid CO2, this first cooling stage required standard off-the-shelf chillers that relied on synthetic refrigerants.

As revealed by Petagna, two larger experiments (ATLAS and CMS), in addition to LHC upgrades, are planned for 2025-2027. Due to new silicon detectors being installed in ATLAS and CMS, more thermal power will be dissipated (from tens to several hundreds of kilowatts). This increase in power will result in the need for much lower temperatures.

“CERN decided at this point to invest in a new, efficient and environmentally friendly technology, switching to a natural refrigerant – in particular, CO2, or ‘R744’ in refrigeration nomenclature,” said Petagna.

Compared to traditional refrigerating systems, Petagna said that the new method of cooling will further reduce the environmental footprint of CERN’s detector cooling systems.

Advantages of using CO2 and its systems

With a Global Warming Power (GWP) coefficient =1, CO2 is regarded as one of the refrigerants with the smallest environmental footprint. In addition, its extraction is cheaper and less harmful to the environment than the production of synthetic refrigerants.

Speaking about the high pressure of CO2, Petagna said, “This is particularly well suited for all cases where the refrigerant needs to travel long distances from the cold source to the heat sink – which is the case at the LHC.”

Other advantages include its well-suited nature for operation from room temperature down to -53°C and its relatively low cost.

The upgraded LHC, known as HL-LHC (‘high luminosity’), in operation from 2027, will see its silicon detectors cooled by CO2, as well as all detectors used in the ATLAS experiment and some special detectors in the CMS experiment.

“Altogether, several hundreds of square metres of silicon sensors will be kept cold by these systems and a dissipated power not far from one megawatt will be intercepted,” said Petagna.

Environmental footprint – the potential for CCUS

By minimising its use of synthetic refrigerants, CERN is already reducing its impact on the environment. Further efforts to recycle process-products are also being made via the re-use of high-pressure hot gases that occur as a by-product of the ‘refrigeration cascade’.

Petagna revealed that several concepts are being evaluated at CERN to make use of this high-value energy source, such as hot water production and as zero-emission HVAC (heating, ventilation, and air conditioning) of a data centre.

He added, “Globally speaking this is an example of how the challenging requirements that the engineers face at CERN push farther the application limits even of known technologies, potentially showing new ways for their application outside.”

Ahead of environmental summit COP26, many international organisations and companies are looking at methods to make use of or remove their carbon emissions through carbon capture, utilisation and storage (CCUS) methods. When asked about CERN’s activities in the area of CCUS, Petagna emphasised that it is a subject that needs to be pushed forward.

He continued, “It really makes no sense to burn on purpose natural gas to produce and store CO2, when in principle it can be harvested from the large CO2 emissions produced by industrial plants.”

With CCUS still in its infancy, Petagna said that the level of development is ‘too far’ from the needs of CERN in terms of CO2 consumption, at least when put in context the scope of its natural gas refrigeration requirements.

Despite this, CERN’s main academic partner in the area of CO2 refrigeration and thermal management, the Norwegian University of Science and Technology (NTNU), has a dedicated team of experts developing R&D centred around carbon capture and storage (CCS).

Prototypes of CERN’s CO2 cooling methods are currently being tested, with the whole chain of refrigeration in R744/CO2 due for full operation from 2027.

The full study undertaken by Petagna and his colleagues on the use of CO2 refrigeration for LHC cooling systems is available to view here.