For many years now, predictive AI has been an integral part of many data centre control and monitoring systems. It helps data centre operators to increase energy efficiency and detect impending failures at an early stage. With the triumph of ChatGPT, the subject of generative AI is now also evolving into an international megatrend.
Drafts, text, images, videos – nowadays everything can be processed or changed using generative AI. And with Copilot, now Microsoft has even integrated its GPT-4 language model – which also forms the basis for ChatGPT – directly in Windows 11, its 365 Office products, and its Bing search engine. This development requires data and storage capacities that would have been unthinkable just a few years ago.
But also in science, medicine, and applications such as autonomous driving, the requirements for computing performance are growing more exacting all the time. Add to this the fact that servers used for AI applications multiply their performance capabilities and energy requirement many times over with each new generation. This creates huge power densities in the racks and pushes the air cooling systems commonly used in data centres to their limits. Therefore, liquid cooling is the obvious choice for cooling servers efficiently and reliably despite this additional load.
The most common method of cooling a data centre involves the traditional separation of the data centre into hot and cold aisles in a process known as containment. Then, cold air can be blown through the raised floor or directly into the cold aisles. The servers take the cold air in at the front, emit their heat into the air and blow it back out into the hot aisles at the back of the rack. There, the air is conveyed through ducts into the air conditioning units and cooled once more. Alternatively, server racks can also be supplied with cold air by a rack-based cooling system. Here, side coolers on the front of the rack emit cold air to the servers and take in the heated air at the back, to cool it once again. However, if an airflow is conveyed through IT equipment, it will generally not reach all components uniformly. This effect is especially pronounced in room air cooling, whereas rack-based side cooling systems such as the Stulz CyberRow have a much lower risk of hot spot formation. If only air cooling is used, the achievable power density per rack is roughly 50 kW in practice. This figure is more than adequate for many IT applications, but rapidly becomes a limiting factor where high-performance AI systems are concerned.
Liquid cooling: Energy efficient cooling also at high power densities
If liquid cooling is used, hot and cold aisles are not needed in some cases, because most of the heat transfer takes place in a closed system without an intermediate medium. Here, additional air cooling is only required for cooling certain components such as power supply units, for instance, and for the heat load generated by the tank itself if immersion cooling is used. Nevertheless, there needs to be sufficient space between the racks or tanks, to allow for maintenance work or to replace equipment. As it requires less room, liquid cooling is also ideal for edge locations with little space and frequently changing ambient temperatures. Overall, liquid can absorb more heat than air, which means that the power density can also be significantly increased: with liquid cooling, figures of 120 kW per rack can be achieved without problem. Even power densities of 250 kW are no rarity in industry. In practical use, this places additional demands on the electricity infrastructure and hydraulics. Where waste heat recovery is concerned, liquid cooling has advantages over pure air cooling, because a higher temperature level can be reached, making direct connection to a transfer heat exchanger easier.
Versions of Liquid Cooling
Currently, different versions of liquid cooling are available, which differ in design and efficiency. In one version, the parts to be cooled come directly into contact with the cooling liquid (immersion cooling); in the other, the components are equipped with a heat sink, through which the cooling liquid flows (direct-to-chip liquid cooling).
