Liquid Cooling in Data Centers

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In the era of big data explosion, especially with the development of AI technology, the amount of data has increased exponentially. The data center responsible for these data calculations and storage has also seen the development trend of high density and high power. One problem brought about by such development is heat dissipation. The traditional air cooling heat dissipation system is gradually becoming overwhelmed, while liquid cooling technology has emerged due to its higher efficiency and lower energy consumption.

The so-called liquid cooling does not simply refer to water. It refers to using a liquid with high specific heat capacity as a transmission medium to take away the heat generated by IT equipment or servers and cool it down. In fact, as early as the 1960s, IBM had begun using water cooling, the predecessor of liquid cooling technology, to cool its mainframe computers. Due to cost issues, current liquid cooling technology is mainly used in the field of high-performance computing.

Classification of liquid cooling

There are two main types of liquid cooling, one is called direct contact liquid cooling, and the other is called indirect contact liquid cooling. This distinction is mainly based on the contact between the refrigeration liquid and the equipment.

First, let’s talk about indirect contact liquid cooling. The typical example here is cold plate liquid cooling, which fixes the equipment that needs to dissipate heat on a cold plate, and the liquid takes away the heat of the equipment when passing through the cold plate to achieve the purpose of heat dissipation. However, components such as the hard drive and power supply inside the chassis cannot come into contact with liquid, so they still need to rely on fans to dissipate heat. Servers that use this type of cold plate liquid cooling are also known as air-liquid dual-channel servers. The advantages of this cold-plate liquid-cooled server include limited impact on the data center architecture, low noise, high energy efficiency and controllable costs. It is more suitable for heat dissipation in data centers with higher heat density. At present, Baidu, Alibaba and Tencent have all demonstrated their own cold plate liquid-cooled servers.

In addition, there is another method called immersion liquid cooling. The characteristic of this method is that IT equipment that needs to be cooled, such as servers, is completely immersed in the coolant. Relying on the circulating flow of liquid to remove heat. This is also called direct contact liquid cooling. Typically the server is placed in a specially designed container. The coolant will be vaporized after taking away the heat, and then cooled through other devices before being recycled. This method has higher heat dissipation efficiency due to the full contact between the coolant and the equipment. In addition, there is no fan, so the noise is lower than the cold plate liquid cooling. For example, Sugon’s Silicon Cube is a typical immersion liquid cooling.

What are the coolants of liquid cooling technology?

water

The most direct and cheapest coolant for liquid cooling is of course water. However, since water is not an insulator, it can only be used for indirect contact liquid cooling. In addition, security protection is very important. Once a leak occurs, the damage to IT equipment such as servers will be very fatal.

mineral oil

In addition to water, mineral oil is an inexpensive and high-quality coolant. Single-phase mineral oil is non-toxic, odorless and non-volatile. However, due to its high viscosity, it is easy to form residues on the surface of the equipment. In addition, although the ignition point is high, it is still possible to burn under certain conditions, so attention must be paid.

Fluorinated liquid

The biggest feature of this liquid is that it is insulating and non-flammable. It is the safest type of data center liquid cooling technology and is currently the most widely used. However, its high price has left many manufacturers interested in liquid cooling technology speechless.

In the past, liquid cooling technology was mainly used in the field of high-performance computing. However, as data centers now develop towards high density and high power, traditional air cooling technology can no longer fully meet their cooling needs. These are the main factors that promote the sinking of liquid cooling technology.

Although the innovation of air cooling systems has never stopped, there are endless methods emerging from traditional computer room air conditioners to AHUs with direct air side natural cooling and indirect air side natural cooling AHUs. However, due to the gap in heat carrying capacity, liquid cooling technology can significantly reduce the energy consumption of data centers compared to air cooling. For example, the PUE of many immersed liquid-cooled data centers can even reach an astonishing 1.03-1.05. However, the PUE value of an air-cooled system data center of the same size is still between 1.3-1.5.

Thanks to higher heat dissipation efficiency, lower dependence on buildings, and silent and safe features, data centers with higher requirements for space and power will gradually consider using liquid cooling technology. In addition to traditional large-scale data centers, with the advent of the edge computing era, about 20% of future edge computing data centers will also use liquid cooling technology.

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