Direct-to-chip cooling is gaining favor in the world of data center thermal management as the cooling process of the future. But what makes up a significant part of an effective data center coolant?
Among other things ... glycol. Good, high-quality glycol goes a long way for your data center cooling loop.
That said, which form of glycol should you go with? There are different forms of glycol that can be used for data center heat transfer fluids, each with their own characteristics and specifications.
In this article, we'll review a few key points, including:
- The differences between propylene glycol and ethylene glycol (and, to a lesser extent, polyethylene glycol)
- The important characteristics glycol provides to better protect your system, namely a lower freeze point
- Industry guidelines out there already from organizations like ASHRAE and OCP
Why Use Glycol in Data Center Cooling?
While water has excellent heat transfer properties, with volumetric heat capacity of water being nearly 3,500 times higher than that of air (Laloui & Rotta Loria, 2020), it does not offer sufficient freeze protection.
That's where glycol comes in, affording freeze protection for temperatures below 0°C. 
In short, glycol-water blends serve three functions simultaneously:
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They depress the freezing point of the working fluid
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Elevate its boiling point
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When properly inhibited, they protect wetted metal surfaces against electrochemical corrosion
Working in concert, these characteristics form the basis of an effective coolant, particularly for data center direct-to-chip (DTC) cooling needs.
Propylene Glycol vs. Ethylene Glycol vs. polyethylene glycol
What about different glycol-water formulations? What are the differences between propylene glycol, ethylene glycol and polyethylene glycol?
A coolant — typically a mixture of water and propylene glycol, such as the industry-standard PG25 — is pumped through cold plates mounted directly onto GPUs and CPUs to absorb heat in direct-to-chip architectures.
With that said, there are differences between EG, PG, and PEG.
As OCP notes, "coolant properties directly affect thermal performance and the CDU's (or RPU) pump selection."
In other words, selecting a coolant for your data center heat transfer needs is about more than just the coolant itself: you have to consider the entire architecture of your cooling process.
Ethylene Glycol
Chemistry
Ethylene glycol (C₂H₆O₂), also known as 1,2-ethanediol, is the simplest member of the diol family. Between the two main glycols used in cooling, ethylene glycol (C₂H₆O₂) is the better heat transfer fluid, while propylene glycol (C₃H₈O₂) is much less toxic. EG's compact molecular structure — two hydroxyl groups on a two-carbon backbone — produces a low molecular weight (62 g/mol) and low viscosity, both critical to its thermal performance. It is a colorless, nearly odorless liquid, fully miscible with water at any ratio.
Thermal Performance
EG's defining advantage is thermal efficiency. Ethylene glycol has a higher thermal conductivity, a higher density, and a lower viscosity than propylene glycol. At 100°F in a 50% solution, the relative heat transfer coefficient of ethylene glycol (0.2537) exceeds that of propylene glycol (0.2105), indicating somewhat better heat transfer at equivalent conditions. Notably, propylene glycol does have the higher specific heat of the two at comparable concentrations.
The performance advantage of EG widens at lower temperatures, where PG's higher viscosity increasingly penalizes both pumping efficiency and heat transfer. EG mixed 50:50 by volume with water has a freeze point of −36.8°C and can be used at temperatures down to approximately −30°C.
System designers must account for the fact that any glycol solution reduces heat transport capacity relative to pure water. The specific heat of ethylene glycol-based water solutions is less than the specific heat of clean water, meaning that for a heat transfer system using ethylene glycol the circulated volume must be increased compared to a system using only water. The key operational takeaway: oversizing pumps and piping for the higher viscosity and lower specific heat of the chosen concentration is a design requirement, not an afterthought.
For lower volume percent glycol concentrations, the aqueous solution achieves better specific heat and lower kinematic viscosity, which reduces pump loading — particularly important for high-power computing applications where the coolant's specific heat significantly affects performance.
Toxicity and Regulatory Status
EG's primary liability is toxicity. Acute exposure of humans to ethylene glycol by ingesting large quantities causes three stages of health effects: CNS depression including vomiting, drowsiness, coma, respiratory failure, and convulsions, followed by cardiopulmonary effects and metabolic changes. The metabolic pathway is the root of this toxicity: EG is converted by the liver to a series of increasingly dangerous metabolites including oxalic acid, which precipitates in renal tubules, the EPA notes.
From a regulatory standpoint, the consequences of a spill are materially more complex than with PG. Ethylene glycol is designated a CERCLA hazardous substance pursuant to Section 112 of the Clean Air Act.
The peer-reviewed ATSDR Toxicological Profile for Ethylene Glycol, published by the U.S. Agency for Toxic Substances and Disease Registry, confirms this regulatory status and provides the definitive reference for occupational and environmental exposure limits.
Inhibitor Chemistry and Degradation
Uninhibited EG must never be used in a cooling loop. All glycols produce acids in the presence of air (oxidants). These acids reduce pH, which results in corrosion — when system pH drops below 7, rust will form on ferrous metals, and nonferrous metals begin to corrode. Proper inhibitor packages — typically organic acid technology (OAT), hybrid OAT (HOAT), or nitrite/silicate/phosphate formulations — neutralize these acids and passivate metal surfaces. A wide range of industry sources cite 20% by weight of glycol as a benchmark of what should be present in any water-glycol system to prevent biodegradation of the glycol by bacteria that feed on it.
Water quality for dilution is equally important. High-quality water will help maintain system efficiency and prolong glycol fluid life — makeup water should contain less than 25 ppm sulfate, and cation exchange to remove total hardness is highly recommended whenever possible.
Propylene Glycol
Chemistry
Propylene glycol (C₃H₈O₂), formally 1,2-propanediol, adds a methyl group to the EG backbone, increasing molecular weight to 76 g/mol and introducing steric bulk that raises viscosity. It is a colorless, nearly odorless liquid with a faintly sweet taste, fully miscible with water and most organic solvents.
PG is conventionally produced via hydration of propylene oxide, a petrochemical intermediate. However, a well-established bio-based route now converts glycerol — a byproduct of biodiesel production — into PG through catalytic hydrogenolysis. A peer-reviewed lifecycle analysis published in the Journal of Chemical Technology & Biotechnology (Nachtergaele et al.) found that a switch from petrochemical to renewables-based PG results in a reduction of climate change impact of between 40% and 60% kg CO₂ equivalent.
The U.S. Department of Energy, meanwhile, said substitution of renewable propylene glycol made from biomass can reduce greenhouse gases by 61% compared to using petroleum-based propylene glycol.
Thermal Performance
PG performs well but measurably below EG. PG mixed 50:50 by volume with water has a freeze point of −34.0°C and can be used at temperatures down to approximately −20°C. The viscosity of PG/water solutions increases much sooner than EG/water solutions, further limiting pumping and cooling capacity at low temperatures. This viscosity penalty has real system-level consequences for pump selection, pressure drop calculations, and energy consumption in cold climates.
At moderate data center operating temperatures (20–45°C), the performance gap between EG and PG narrows somewhat. For pure 100% glycol, specific heat capacity for ethylene glycol reaches 2.35 kJ/kg·K versus 2.48 kJ/kg·K for propylene glycol at 20°C — meaning PG absorbs slightly more heat per unit mass. (For 50:50 EG-water mix, specific heat capacity is about 3.3 kJ/kg•K.)
However, the lower thermal conductivity of propylene limits overall heat transfer rates in practical systems. The net result, as confirmed by the property data in the ASHRAE 2013 Handbook — Fundamentals, is that EG retains the advantage in overall heat transfer coefficient at equivalent concentration and flow conditions.
Safety and Regulatory Advantages
The defining advantage of PG is its toxicological profile. Propylene glycol is generally recognized as safe (GRAS) by the FDA and is widely used in commercial formulations of foods, drugs, and cosmetics. Propylene glycol induces remarkably fewer adverse effects in both humans and animals than does ethylene glycol. The ATSDR Toxicological Profile for Propylene Glycol, the authoritative federal reference, confirms this: oral exposure to the small amounts of propylene glycol found in foods and drugs is unlikely to cause toxic effects.
This regulatory contrast has direct operational implications. PG does not carry a CERCLA hazardous substance designation, so spill reporting obligations are substantially reduced compared to EG — a meaningful consideration for large deployments where a failed coupling or rack leak could release significant fluid volumes over expensive IT hardware. Often glycols derived from bio-renewable sources, recycled, or even industrial grade can contain significant amounts of impurities such as ethylene glycol or diethylene glycol, which can impact toxicity, impart strong odors, cause excessive foam, and reduce the life of equipment. To achieve the best results for both heat transfer and fluid life, high-purity glycol is a necessity.
Polyethylene Glycol
Meanwhile, because polyethylene glycol's role in data center thermal management is still emerging and not as established as it is for PG and EG, this section will be a bit shorter.
PEG is a polymer, a large molecule composed of repeating ethylene glycol subunits, whereas PG is a smaller, single molecule. This difference in size and complexity influences physical characteristics such as viscosity and solubility.
PEG has been extensively considered for its well-established chemical and thermal stability, non-corrosive properties, lack of toxicity, and low price. However, one of the main concerns is related to its relatively low thermal conductivity, around 0.2 W·m⁻¹·K⁻¹.
Ongoing research efforts have attempted to address this shortcoming by way of introducing nanoparticles to the PEG solution. Research is active in addressing this limitation: in order to improve the thermal conductivity and heat transfer behavior, nanoparticles are added to PEG, as in nanofluid or nanocomposite manufacturing. As a 2021 review published in the journal Nanomaterials noted, research in this area is still in its "pioneering phase."
With that said, nanoparticle-infused PEG is something to monitor in the years ahead as research progresses.
Industry standards for glycol in data centers
PG has emerged as the preferred fluid for server-side (TCS) cooling loops in major industry frameworks. The Open Compute Project (OCP) has published a dedicated whitepaper — Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Single-Phase Cold Plate-Based Liquid-Cooled Racks — specifying that fluids must contain propylene glycol as a freeze point depressant as well as an inhibitor package designed for the metals present in the TCS loop. The OCP's own ACS Cold Plate Requirements document states the rationale directly: propylene glycol is preferred since it is less toxic than ethylene glycol — in small quantities, propylene glycol is even used in the food industry as an additive.
ASHRAE's liquid cooling guidelines recommend that systems include monitoring of coolant quality and filtration, as these can have an adverse effect on a liquid system, leading to lower efficiency or higher energy consumption.
The OCP Liquid Cooling Integration and Logistics whitepaper further specifies that any concentration of ethylene or propylene glycol used in place of water treatment additives must be consistent from integration through deployment, following manufacturer guidelines on appropriate selection — as blends can vary by manufacturer.
Glycol Summary: Key Procurement and Design Considerations
Loop segregation is standard practice.
ASHRAE TC9.9 (in its 2024 technical bulletin Liquid Cooling: Resiliency Guidance for Cold Plate Deployments) and OCP both recommend separating facility water systems (FWS) from technology cooling systems (TCS) via a Coolant Distribution Unit (CDU). This layout keeps facility liquid separate from technology liquid, allowing different liquids to be used in each loop. Most hyperscale designs capitalize on this by running EG in the primary outdoor loop (for thermal performance) and PG in the secondary server loop (for leak-safety and regulatory simplicity).
Never use uninhibited glycol.
All glycols produce acids in the presence of air, and when system pH drops below 7, rust will form on ferrous metals and nonferrous metals will start to corrode. A corrosion and scale inhibitor must be added to any water-glycol solution.
Fluid monitoring is not optional.
ASHRAE recommends that systems monitor coolant quality and filtration, as deteriorating fluid chemistry can lead to lower efficiency or increased energy consumption. Routine testing parameters should include pH, reserve alkalinity, glycol concentration (by refractometer), and visual clarity.
Water quality for make-up matters.
High-quality water maintains system efficiency and prolongs fluid life. Cation exchange to remove total hardness is highly recommended, and sulfate should be kept below 25 ppm.
Sustainability trajectory favors bio-based PG.
For operators with Scope 3 reporting obligations or ESG procurement requirements, bio-based PG offers a verifiable decarbonization pathway. As noted earlier, multiple sources documented an approximately 40–60% reduction in climate change impact from switching from petroleum-based to renewables-based propylene glycol — with no change in chemical performance or system compatibility.
Purity grade selection affects long-term system health.
Glycols derived from recycled or industrial-grade sources can contain significant impurities such as diethylene glycol, which can impact toxicity, cause foaming, and reduce equipment life. High-purity glycol is a necessity for best results. For server-side TCS loops, USP-grade inhibited PG (PG-25 specification), like Dober's COOLWAVE™ DC-25 heat transfer fluid, has become the recognized industry standard through OCP adoption.
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