One of the crazier bits in the industry is that we have been seeing news and analysts still trumpet the benefits of 2-phase immersion cooling. There is one problem. Major hyper-scalers such as Microsoft and Meta have effectively halted two-phase immersion cooling research for a simple reason: the fluids used have been found to have such toxic side effects that companies like 3M will halt production of the chemicals in 2025. A major part of that is simply because the companies creating the immersion cooling liquids are facing many multi-billion dollar lawsuits.
What is Single-Phase Versus Two-Phase Immersion Cooling?
Immersion cooling operates on a fairly simple concept. IT equipment (for our purposes usually servers) is placed into a tank with non-conductive fluid. Since liquids generally conduct heat better than air, the idea with immersion cooling is to have heat transfer from hot components like CPUs, GPUs, AI accelerators, and more, to liquid circulating in the tank. The heat is then removed from the liquid to keep the servers running cool. What is awesome about immersion cooling versus having cold plates like one would have in consumer AIO liquid coolers, is that immersion cooling removes heat from all server components, so fans are not needed.
More efficiently removing heat means lower overall power consumption. In modern servers we review, fans can use somewhere in the 15-20% of the total system power consumption. Fans are also a potential source of failure, and companies like Alibaba have found that components are more reliable when submerged and not exposed to air.
On the subject of immersion cooling, there are generally two approaches. First is single-phase cooling. With single-phase cooling, the liquid in a tank is generally circulated using pumps between the tank with servers and a heat exchanger. There are a number of designs, but usually, this heat exchange happens with facility water. The key with single-phase cooling is that the fluid is designed to stay as a liquid. That is the single phase in the name.
Two-phase immersion cooling is similar in many ways, but there is one big difference. The two phases are liquid as well as gas. As the servers heat up fluid, it does not just warm liquid. Instead, it causes the liquid to boil and become gas. That helps circulate fluid without using a pump, a major plus. The gas is then cooled via a condenser and returned to liquid form. Usually, two-phase immersion tanks need to be sealed when operating to contain vapor. Increasing reliability is important with this because servicing equipment means, waiting for cooling, breaking the seal once the vapor has become liquid again, and then opening the tank. The industry was focused on this method not just in tanks.
We also saw individual supercomputer nodes with two-phase immersion cooling. Sometimes, like in the below Sugon supercomputer, this was simply called “phase change” referring to liquid-to-gas (and gas-to-liquid) phase changes. This approach allows for individual nodes to be serviced, without the challenge of having to shut down an entire rack. This was from SC19, but even then we were not allowed to get closer photos.
Two-phase immersion cooling showed promise because it generally could support higher-TDP parts. The challenge is that many of the fluids used are now being discontinued.
Better Cooling but More PFAS
PFAS stands for per- and polyfluoroalkyl substance. These are often referred to as “forever chemicals” because of their tendency not to break down naturally. PFAS also can cause cancer and other negative impacts on life. One of the biggest fluids used in two-phase immersion cooling but also things like semiconductor manufacturing was 3M Novec.
This class of substance was the center of 3M’s Belgium plant shutdown and Belgium working to get a PFAS ban in the EU. The US EPA has also been interested in these chemicals. These chemicals have gotten enough scrutiny, that 3M has a PFAS mini-site trying to convince folks that they are not all bad. Usually, when companies make mini-sites dedicated to saying something is not that bad, we assume it probably is.
In early June 2023, 3M announced nearing a $10B settlement with US towns over pollution from PFAS and there are other lawsuits for this class of chemicals still active. There are over 4000 lawsuits filed over PFAS, and this settlement is for a consolidated set of cases. Just this week, the trial was delayed as the settlement was near.
Companies like Meta, Microsoft, Google, and others have ESG goals, and have been looking at immersion cooling to help lower data center PUE to achieve those goals. Unfortunately using “forever chemical” substances like these is unlikely compatible with those goals. Still, we see many two-phase immersion cooling references still in the industry. For example, Dell’s Data Center Power and Cooling Solutions page still lists two-phase immersion cooling as an option despite the company’s ESG focus. Our sense is that Dell still lists that as a conversation starter. That is just one of many examples where the industry has been slow to update. Perhaps new fluids will be developed and tested for use in two-phase immersion cooling in the future.
Many will have noticed that we stopped covering two-phase immersion cooling last year as the US EPA and Belgium started taking a bigger interest in PFAS. Hopefully, the industry finds a solution, but this is generally a challenge with phase change approaches. With the phase transition to gas, opening tanks for maintenance is a bigger challenge. That gas also has the potential to be toxic depending on the fluid used, which is what makes two-phase immersion cooling that much more challenging for data center operations.
For now, it seems like we are in a single-phase versus direct-to-chip liquid cooling world as the big US hyper-scalers are cutting back on these projects and the primary fluid will cease production sometime in the next 18-19 months. If projects pick up with new safer fluids, we will cover those on STH again.