Carbon dioxide cooling casts a more environmentally-friendly chill

Initially developed at Nikhef in the Netherlands for the Alpha Magnetic Spectrometer (AMS)* on the International Space Station, CO₂ cooling presents many advantages and has, over the last ten years, found its way into the heart of the LHCb, ATLAS and CMS detectors at CERN.

"CO₂ has attracted a lot of interest as a cooling fluid for particle physics detectors for various reasons," explains Bart Verlaat, CO₂ cooling project leader in the EP group. "The high operational pressure of CO₂ allows us to make compact cooling systems with smaller pipes, thus reducing the amount of material inside the detector, which improves performance. Furthermore, at CERN, we often work in radiation environments and CO₂ is a good radiation-resistant cooling fluid. And last but not least, CO₂ has a very low global warming potential (GWP) compared to the fluorocarbons (FC) used in detectors and to the hydrofluorocarbons (HFC) usually used for refrigeration, which makes it much more environmentally friendly."

That being said, CO₂ cooling systems still use HFC refrigerant gases to help cool the CO₂ in the first place. This problem is about to be solved through CoolCERN, a PhD project conducted by CERN in collaboration with the Norwegian University of Science and Technology (NTNU), with the aim to develop a primary CO₂ refrigerant system for the LHC's detectors, making high-GWP refrigerants unnecessary.

Unlike traditional refrigeration systems, in which heat is rejected by a phase change from gas to liquid, this new primary system rejects the heat in a super critical state. It also operates at higher pressures than standard refrigeration and cools more efficiently in CERN's climate. "Indeed, for the secondary CO₂ systems underground, the high pressure is an advantage, as with high pressure, pressure drop losses are less of an issue and do not translate into temperature gradients in the detectors," explains Stefanie Blust, the PhD student working on CoolCERN. "This guarantees very homogeneous temperatures in the detectors despite the small cooling tubes."

The first prototype is about to be tested at CERN by NTNU, starting in mid-March. If the tests are conclusive, this new system will be installed in 2023.

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*In November 2019, ESA astronaut Luca Parmitano and NASA astronaut Andrew Morgan performed a series of spacewalks to service the Alpha Magnetic Spectrometer (AMS-02). A key task for the astronauts was to replace the AMS-02 CO₂ cooling system.