Several years ago, CERN made it its mission to put the environment at the heart of its scientific research. This philosophy underpins various concrete measures to protect the climate and biodiversity that have been taken at all levels of the Laboratory. In the context of the Laboratory’s target to reduce its greenhouse gas emissions by 28% by the end of Run 3, the EN-CV and EP-DT groups, with the support of the whole Organization and its partners in science and industry, are preparing to renovate the cooling systems of the ATLAS and CMS detectors, which will help to drastically reduce direct emissions of these gases. The aim is to save the equivalent of 40 000 tonnes of CO₂ each year... by choosing a technology that’s actually based on CO₂ (commonly known as R744 in the field of refrigeration, air conditioning and heat pumps).

CERN uses considerable energy and resources to cool its scientific facilities. Alongside its cryogenic systems, which cool the LHC superconducting magnets to as close as possible to absolute zero, the Laboratory uses more conventional cooling systems to keep the detectors and its flagship collider at temperatures as low as -50°C. This invigorating temperature helps to protect the particle detection systems of ATLAS and CMS from ionising radiation during the operation of the machine. Until now, CERN’s cooling systems have not differed from those used in industry in that they are based on refrigerants with a very high global warming potential (GWP), namely perfluorocarbons.

In the context of today’s climate crisis, the use of CO₂, which has a GWP of 1, is an excellent alternative to the perfluorocarbons used in low-temperature applications, which have a GWP of several thousand, hence CERN’s decision in 2017 to invest heavily in the development of a CO₂-based cooling system. The EN et EP departments’ engineers have been working tirelessly ever since to push back the limits of the equipment and the standard cooling cycles, meticulously optimising every parameter in order to cool CO₂ to close to its -56.6°C limit for use.

In addition to high costs, the path towards environmental sustainability was strewn with obstacles, as Pierre Hanf, an engineer from the EN-CV/PJ section confirms: “CO₂-based cooling systems operate at higher pressure than commercially available systems and are known for their greater complexity. Although the operating principle has been validated for small “plug and play” systems, the large-scale production and distribution of cold, taking account of the constraints associated with a cavern 100 metres below ground, had never been achieved in industry. That’s why we needed to develop our own solutions, drawing on the expertise of partner institutes. Our collaboration with the Norwegian University of Science and Technology (NTNU), which is renowned throughout Europe for its expertise in refrigeration and its CO₂ applications, was of prime importance in this process.”

Six years after the project began, these efforts are bearing fruit: the concept has been validated, the development phase is over, and the industrial production of the new equipment is under way. Once deployed, the CO₂ will circulate in the primary cooling circuit, which is operated by the EN department upstream of the detectors, and in the secondary circuit, the domain of the EP department, at below -53°C. “Over and above environmental considerations, the choice of the new cooling system will equip the ATLAS and CMS detectors to cope with the increased ionising radiation associated with high luminosity”, explains Paolo Petagna, the project leader from the EP-DT section. “In this hostile environment, it’s crucial that we provide the collaborations with the lowest possible temperatures.”

There is still a long way to go before the new system is commissioned ready for the fourth accelerator run. CERN’s partners in industry are building more than thirty CO₂ pumps, which will be delivered over the next few years. According to Roberto Bozzi from the EN-CV/PJ section, “The development of these large-scale CO₂ cooling systems is a striking example of the transfer of CERN's own know-how to European industry. Partner companies will be able to reproduce this solution and disseminate it in cooling-intensive sectors such as the food and pharmaceutical industries, thereby contributing to the green transition of those industries.”

The list of potential beneficiaries also includes CERN detectors and installations other than ATLAS and CMS, which also have considerable cooling needs and might thus also switch to CO₂ in the longer term. That’s what the EP and EN teams working on the project are hoping, at any rate. They emphasise, above all, the collective nature of this long-term undertaking, which could not have succeeded without the invaluable collaboration of the groups responsible for the equipment and services within CERN’s AT (Accelerators and Technology) and RC (Research and Computing) sectors and the ATLAS and CMS collaborations’ technical and coordination teams. “With the CO₂ cooling system, the whole of CERN is making a concrete and constructive long-term contribution to the climate. It’s a recipe that requires many ingredients and we can’t wait to try the finished product”, Roberto Bozzi concludes.