Publish Time: 2023-07-25 Origin: Site
With the rapid development of computing-intensive applications such as artificial intelligence, Internet of Things, cryptocurrency, AR/VR, etc., the growing computing demand makes data centers gradually develop towards "high performance, high density, and high energy consumption". The energy consumption of the data center is roughly composed of communication and network equipment, power supply and distribution system, lighting and auxiliary equipment, and cooling system. The energy consumption of the cooling part accounts for about 40% of the total energy consumption of the data center. Improving the efficiency of data center cooling systems and reducing energy consumption are crucial to achieving the "double carbon" goal.
Common liquid cooling methods include cold plate, spray and immersion. Among them, immersion liquid cooling has the highest heat transfer efficiency and can avoid local hot spots. It is currently the most likely technical means to solve various problems faced by cooling systems in high-performance computing environments.
As the engine to boost the new development of the next-generation big data center, the advantages of immersion liquid cooling technology are mainly reflected in the following aspects.
Immersion liquid cooling uses coolant as the heat transfer medium. Liquids have higher thermal conductivity and specific heat capacity, so they conduct heat faster and absorb heat more efficiently. At the same time, data centers using immersion liquid cooling technology have lower PUE due to the reduced use of fans and air conditioners.
Immersion liquid cooling can greatly increase the server density per unit space of the data center, thereby better supporting high-density computing. Traditional data centers use air-cooled systems, and the rackable power densities that can be cooled are typically 10kW to 15kW. The immersion liquid cooling can increase the power of a single rack to 100kW or even more than 200kW. Therefore, it can fully meet the heat dissipation requirements of high-density computing scenarios.
Immersion liquid cooling keeps IT equipment at the right temperature. The immersion environment effectively avoids the adverse effects of humidity (water in the air will cause corrosion of components, and cooling fluid can protect the equipment), dust, etc. on the equipment. In addition, the noise and vibration problems are effectively solved because the servers and computer rooms no longer need fans.
The excellent heat dissipation performance of the immersion liquid cooling enables the servers to be arranged closely without separation. At the same time, there is no need to configure fans, and there is no need for air conditioners and refrigeration units in the machine room. There is no need to install hot and cold aisle containment facilities and raised floors, so immersion liquid cooling has higher space utilization than traditional cooling solutions.
Huge water consumption not only increases operating costs, but also faces regulatory pressure in areas with limited water usage. Traditional air cooling technology usually requires the use of a large amount of water for evaporative cooling. The coolant of immersion liquid cooling technology can work at higher temperature (up to 45°C). Even in hotter climates, free cooling can be used efficiently, reducing the need for active heat removal and therefore saving water.
Immersion liquid cooling immerses IT equipment directly in a coolant, relying on the coolant to absorb the heat generated by the equipment. According to whether the cooling liquid undergoes phase change in the process of circulating heat dissipation, it can be divided into single-phase immersion liquid cooling and two-phase immersion liquid cooling.
The cooling liquid of the single-phase immersion liquid cooling usually has a relatively high boiling point. After the cooling liquid absorbs heat, there will be no phase change and it will always remain in a liquid state. It circulates the coolant through natural convection or pump driven. The circulation heat dissipation process driven by natural convection takes advantage of the characteristic that the volume expansion density of the liquid decreases after being heated. The hotter coolant naturally floats up and is cooled by a heat exchanger connected to an external cooling circuit. The cooled liquid sinks naturally under the action of gravity to complete the circulation and heat dissipation.
Compared with natural convection, using a pump to drive circulating coolant can improve cooling capacity more effectively. The device consisting of a pump, heat exchanger, sensor, and filter is called a Coolant Distribution Unit (CDU, Coolant Distribution Unit). The temperature and flow rate of the coolant can be controlled more precisely by using the CDU. Cooler coolant is pumped through the heating element, removing heat. The heated coolant enters the heat exchanger to be cooled under the drive of the pump, and then continues to circulate under the action of the pump. The heat exchanger generally uses water as the cooling medium, and the heat is finally discharged through the circulating cooling water system.
The working principle of single-phase immersion liquid cooling is shown in the figure.
The advantages of single-phase immersion liquid cooling are reflected in two aspects. One is that the coolant is cheaper and cheaper to deploy. The other is that the coolant has no phase change. There is no need to worry about the health risks of coolant evaporative overflow or personnel inhalation, which is more conducive to maintenance.
In the two-phase immersion liquid cooling, the cooling liquid continuously undergoes a phase change process from liquid to gas and then back to liquid during the circulation and heat dissipation process. The IT equipment is completely submerged in an airtight tank filled with low-boiling coolant, which absorbs the heat emitted by the equipment. After the coolant absorbs heat, the temperature rises and starts to boil after reaching the boiling point. From liquid phase to gaseous state, a large amount of steam is produced at the same time. The vapor rises from the liquid and escapes above the liquid surface, forming a gas phase region in the liquid-cooled tank. The coolant vapor in the gas phase area is in contact with the water-cooled condenser, and the heat is absorbed by the condenser. The coolant condenses to a liquid that falls back into the container in droplets for recirculation. The heated cooling water in the condenser is exhausted through the circulating cooling water system.
The working principle of two-phase immersion liquid cooling is shown in Figure 3.
The coolant used for two-phase immersion liquid cooling should not only have good thermophysical properties, chemical and thermal stability, and non-corrosiveness, but also need a suitable boiling point, a relatively narrow boiling range, and a high latent heat of vaporization. Silicates, aromatic substances, silicones, aliphatic compounds, and fluorocarbons have all been tried to be used in two-phase immersion liquid cooling. Among them, fluorocarbon compounds have the best comprehensive performance, so they are more commonly used.
The two-phase immersion liquid cooling makes full use of the latent heat of evaporation of the cooling liquid, which can meet the extreme requirements of high-power heating elements for heat dissipation. It keeps IT equipment running at full power. However, the existence of phase change also makes the two-phase immersion liquid cooling system must be kept airtight to prevent steam from escaping. What’s more, the change in air pressure caused by the phase change process and the health risk of maintenance personnel inhaling gas during system maintenance must be considered.
At this stage, there are still many obstacles and challenges in the process of promoting data centers to quickly embrace immersion liquid cooling technology. This includes application scenario limitations, equipment vendor support and deployment and retrofit costs.
Whether and in what way the above problems can be solved will be the key to determining the rapid and large-scale deployment of immersion liquid cooling technology in the future.
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