Speaker
Description
Environmental awareness is becoming a fundamental subject of discussion for large research infrastructures and for the conception of new detectors. At the end of the third “long shut down” of the LHC (LS3) the ATLAS and CMS experiments will start operating the largest silicon-based detectors ever built, featuring a cumulated power dissipation in the order of 800 kW and the requirement of keeping several hundreds of m$^{2}$ of silicon surface at temperatures well below 0 °C. The thermal management of such detectors will be ensured by a cascade of a transcritical R744 refrigeration cycle (R744 is the name of CO$_2$ when used in refrigeration cycles) coupled to pumped loops circulating pure CO$_2$ in the detector evaporators. This environmentally sustainable approach does not come at all at the expense of performance: on the contrary, it introduces in the design of very complex detectors the coupling of the electronics thermal management with a highly effective and versatile cooling system, explicitly designed for optimal performance under the strict requirements of a large HEP detector. This marks a solid step towards the abandon of synthetic refrigerants and the transition to natural fluids for the cooling systems of future detectors.
In the frame of the newly formed DRD8 (https://drd8.web.cern.ch/) a dedicated project is dedicated to develop sustainable cooling solutions in both the warm and ultra-cold domain, extending the domain of application of cooling systems based on CO$_2$ boiling flows.
The first activity focuses on supercritical carbon dioxide (sCO$_2$). This is an electrically non-conductive fluid with much lower viscosity than water and allowing for higher heat transfer coefficients operating at high pressure and temperatures above 31.7 $^{o}$C. Its adoption as single-phase refrigerant for electronics operated in the range +35 $^{o}$C to +40 $^{o}$C would allow for lighter and smaller pipes - potentially including even multi-microchannel devices. This would have a largely positive impact in all situations where space or material budget limitations, along with risks of short-circuits in case of liquid losses, make problematic the use of water as refrigerant.
The second activity will look on the other hand to the field of Ultra-Low-Temperature (ULT) refrigeration, in the range of -90 $^{o}$C / -60 $^{o}$C. This is beyond the practical temperature range for CO$_2$, as CO$_2$ freezes at -56 $^{o}$C. Krypton (R784 when used as refrigerant) has been identified as a new evaporative cooling fluid. Calculations have demonstrated that it works well in this lower temperature range, and its fluid properties make it a promising candidate. However, while CO$_2$ can be made liquid at room temperature by pressurisation, krypton can only be liquefied by cooling. This has a severe impact on the system architecture: for krypton a transcritical cooldown in a compressor cycle is foreseen as a method to achieve controlled cooldown and avoid thermal shocks. This cycle is quite different from any other evaporative cooling system used at CERN and in industry.
After briefly reviewing the working principles of the large CO$_2$ systems presently under commissioning at ATLAS and CMS, the talk will illustrate how the adoption of sCO$_2$ and Kr would extend the field of application of non-flammable environmentally friendly natural refrigerants to future detectors and electronic equipment; and will present the very encouraging results obtained up to now with two dedicated test facility developed for these studies.
| Position | EP-DT-DC Section Leader |
|---|---|
| Affiliation | CERN - European Organization for Nuclear Research |
| Country | Switzerland |