Single phase versus two phase


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Aug 08, 2023

Single phase versus two phase

With the continuous increase in the thermal design power (TDP) of chips, traditional air-cooling methods are struggling to meet the cooling requirements of modern hyperscale data centres. As a result,

With the continuous increase in the thermal design power (TDP) of chips, traditional air-cooling methods are struggling to meet the cooling requirements of modern hyperscale data centres.

As a result, liquid cooling has emerged as a promising solution due to the high specific heat capacity of liquids.

Liquid cooling can be categorized into two main types: single-phase liquid cooling and two-phase liquid cooling. The distinction between these two lies in whether a phase change occurs during the cooling process.

Two-phase cooling, while offering higher cooling capacity, also presents several challenges, such as higher costs and regulatory concerns.

Two-phase coolants are commonly used in both immersion cooling, where the entire electronic component is submerged in the coolant, and direct-to-chip cooling, where the coolant is brought in direct contact with the chip.

In single-phase cold plate cooling, a coolant such as water glycol is used, which is circulated inside the cold plate by coolant distribution units (CDUs).

The coolant absorbs heat through convection as it passes over the heat sources (e.g., GPUs). On the other hand, two-phase cold plates or evaporators utilize dielectric refrigerants with low boiling temperatures. In two-phase cooling, heat absorption primarily occurs through the latent heat during the phase change of the refrigerant.

Unlike single-phase cooling, two-phase cold plates do not rely on pumps and CDUs for circulation but instead use temperature-controlled self-regulation, resulting in easier maintenance due to the absence of moving components.

Additionally, most two-phase coolants are non-corrosive, allowing for a wider range of material selection for rack manifold and cold plate housing and reducing maintenance requirements.

However, it is important to note that some two-phase refrigerants contain organic fluoro components, which raises concerns about their global warming potential and environmental impact.

The discontinuation of certain polyfluoroalkyl substances (PFAS) manufacturing by companies like 3M by the end of 2025 will have implications for both two-phase cold plates and two-phase immersion cooling.

In the case of single-phase cold plates, the pressure drop is a critical parameter. The CDU pumps the coolant through manifolds and quick disconnects, and an uneven pressure drop can lead to varying flow rates among different cold plates, resulting in uneven cooling.

While the flow rate can be theoretically controlled manually to achieve infinite cooling capacity, in practice, the coolant is typically pumped at a temperature close to its boiling point, necessitating the use of around 20 per cent of the maximum pumping speed to prevent cavitation.

This imposes practical barriers to adoption, as achieving high cooling capacities would require a significant volume of water, such as 2.5 litres per minute for a cold plate with a 1000W cooling capacity.

More details about the single and two-phase cold plates, along with the material considerations for CDUs and manifolds, can be found in IDTechEx's latest research report Thermal Management for Data Centres 2023-2043.

Immersion cooling is known for its high cooling efficiency, offering a low partial power use effectiveness (pPUE) of 1.01, which is the lowest among all data centre cooling approaches.

However, due to PFAS regulations and environmental concerns, there is a trend toward transitioning to single-phase immersion cooling and using PFAS-free and low fluorine coolants.

These transitions, although promising, come with high costs and operational complexities. Additionally, ensuring compatibility between the liquid coolant and servers remains an ongoing challenge.

Nevertheless, there have been notable collaborations between immersion cooling vendors and server suppliers. Major companies have initiated pilot projects, indicating a growing interest and investment in immersion cooling technology.

IDTechEx forecasts that the annual revenue of immersion cooling hardware will exceed US$200 million, presenting a significant opportunity for stakeholders in the industry.

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IDTechEx technology analyst, Yulin Wang, examines the benefits of liquid cooling and determines whether single phase cooling is better than two phase.