Is two-phase cooling the new black?

Two-phase, direct-to-chip cooling uses dielectric fluids to cool the heat-generating components of servers. Because of the phase change, considerable amounts of heat can be removed, protecting the infrastructure with increased longevity. This technology is also compatible with standard racks and infrastructure, in addition to today’s service and support protocols. While some may say this strategy is the “new black” with humor, there is truth in its effectiveness.

Most, if not all, two-phase cooling solutions for data centers currently use refrigerants that fall under the category of per- and polyfluoroalkyl substances (PFAS). The sustainability-related concerns associated with PFAS have only been amplified by the European Union’s recent string of proposed legislative actions to regulate or ban PFAS — the so-called “forever chemicals.” It’s important to note that the EU reversed earlier plans to ban PFAS after industry groups argued the chemicals are needed for technologies that will help reach net-zero.

Are all PFAS forever chemicals?

The next logical question that many ask is, “Should prospective data center stakeholders be concerned about the sustainability of PFAS refrigerants in two-phase cooling systems?”

Let’s dig in.

PFAS chemicals are fairly prevalent in today’s manufacturing and global goods production supply chain. This includes medical devices, automotive interiors, waterproof clothing and outdoor gear, food packaging, nonstick cookware, batteries, semiconductors, paints, building materials, and more. But not all PFAS are created equal.

First, we must understand that the PFAS family of chemicals is diverse. PFAS can exist in vapor or liquid form (refrigerants) or in solid form (plastics or plastic coatings, such as polytetrafluorethylene [PTFE]). The distinguishing feature of PFAS is the presence of one or more fluorine atoms attached to the carbon backbone of the substance’s molecular structure. Adding fluorine to a molecule tends to offer performance benefits, including lower chemical reactivity, lower flammability, toxicity, and friction, so fluorinated chemicals have been deployed across various uses.

The “forever” nature of PFAS depends largely on their life cycles as they get distributed into the environment. After being produced in a factory and integrated into everyday items, such as coatings and fabrics, the solid PFAS are ultimately discarded once the items reach the end of their life cycle. These PFAS are solid and inherently long-lasting. They, therefore, tend to find their way into soil and groundwater and, eventually, into the water table as they break down into small particles. Once in the water table, the particles are ultimately consumed by organisms, such as fish and humans. The pervasive nature of PFAS causes them to reside in these organisms, since they do not break down chemically. 

Lower molecular weight liquid or gaseous PFAS (such as those used in HVAC, refrigeration, and two-phase cooling systems) have a different life cycle. In the event of a leak, the PFAS refrigerants vaporize and eventually enter the upper atmosphere. After a few days, they break down, and the remaining elements return to Earth as TFA, a naturally occurring chemical with no impact on living organisms. The total contribution to existing amounts of TFA in the oceans because of the continued use of HCFCs, HFCs, and HFOs up to 2050 is estimated to be a small fraction (<7.5%) of the ≈ 0.2 μg/L acid equivalents/L estimated to be present at the start of the millennium¹. It should also be noted that many liquid and gaseous PFAS are routinely recycled at the end of their life, eliminating any risk of environmental release or harm.

Informed decision-making

The recent and enthusiastic interest in two-phase, direct-to-chip cooling directly results from its dynamic ability to handle intense heat loads from high-power chips and its reliability compared to other liquid cooling methods. However, there is often a moment of pause when people consider using two-phase cooling solutions due to a lack of understanding about which PFAS are truly forever.

There are nuances in PFASs existing in our supply chain, and it’s important to communicate them clearly so that users can make informed choices about saving energy while protecting the environment. We must continue to monitor coolant regulations and technical development to ensure the industry is always at the forefront of delivering the most sustainable cooling solutions, especially when it comes to energy, water use, and environmental impact. 

Solomon K, et al., "...RISKS OF TRIFLUOROACETIC ACID... AND KYOTO PROTOCOLS", UNEP Panel and Journal of Toxicology and Environmental Health B, 2016