40% of the energy consumed by data centers is used by cooling systems. It is recommended to maintain the temperature of the servers between 15°C and 32°C. Managing the consumption of cooling equipment is therefore crucial for improve the energy efficiency of a data center.
Different cooling techniques exist, at various levels of energy efficiency. Traditionally, servers are cooled by air, either using a conventional air conditioning system with refrigerant fluid, or using outside air (Free cooling). This technique, although easy to implement, reaches its limits when energy densities increase - and the latter could go from ten kW per rack to more than 30-40 kW per rack in the future for the development of AI.
To cool the ambient air, water can be used as a heat transfer fluid. The heat is then dissipated thanks to the evaporation of part of the water through a semi-open loop (adiabatic cooling, Evaporative cooling). These techniques are effective but can pose water consumption problems : in 2022, Microsoft saw its water consumption increase by 34% compared to the previous year.
To tackle the ever increasing powers and densities of servers, the liquid cooling is developing. This technique consists in bringing a fluid directly into contact with the components:
- The direct liquid cooling (Direct Liquid Cooling (DLC) or Direct to chip):
The processors are cooled directly through pipes integrated into the
waiter. The liquid, water or a refrigerant, is in contact with the components
electronics through cooling plates. So you have to anticipate.
the integration of pipes as early as the design of this equipment.
In addition, the DLC must still be combined with another technique of
cooling because it only evacuates approximately 70-80% of the heat dissipated
by the servers.
- The Immersion cooling (Immersion Cooling) : The servers are in a deep dive
in a sealed container containing a dielectric fluid that dissipates heat.
The fluid can be single phase or two phase. Immersion cooling
is a technology with the advantage of being silent, and allows a gain
of space and energy efficiency compared to air cooling.
However, this technology also includes numerous constraints,
such as a high initial investment cost or the absence of
norms and standards to regulate it, which makes interoperability difficult.
- The Rear door heat exchanger is an intermediate technique, sometimes classified as
liquid cooling but actually using air to cool the servers:
the traditional back door of server racks is being replaced by
fans that draw hot air to a liquid heat exchanger.
Source: https://www.us.jll.com/en/trends-and-insights/research/data-center-outlook
Far from being in the majority today, liquid cooling technologies are nevertheless of increasing interest to tech giants, who are confronted with growing energy needs of their data centers. The Xai Colosseus supercomputer has thus benefited from a direct liquid cooling technology developed by Supermicro. Data center operators are not left behind: Equinix is deploying the liquid cooling within its data centers, while OVH Cloud offers solutions forImmersion Cooling.
Although they make it possible to absorb high energy densities, liquid cooling technologies also bring their share of complexities. In particular, they are difficult to integrate into existing data centers. The circulation of liquid near sensitive electronic components requires reliable systems to avoid leaks and materials compatible with the nature of the liquid. The dielectric fluids used are often mineral or fluorinated oils, and some, based on PFAS, pose environmental and public health problems. The lack of standardization can also hinder adoption, which is now limited to massive data centers for AI, although most studies consider a liquid cooling market expansion.