Discussion on heat dissipation technology of data center

The rapid growth of data center construction leads to more and more equipment in the computer room, which provides a constant temperature and humidity refrigeration environment for the data center. The power consumption of the data center will increase greatly, followed by the proportional increase of cooling system, power distribution system, ups and generator, which will bring major challenges to the energy consumption of the data center. At a time when the whole country is advocating energy conservation and emission reduction, if the data center blindly consumes social energy, it will inevitably attract the attention of the government and people. Not only is it not conducive to the future development of the data center, but also runs counter to social morality. Therefore, energy consumption has become the most concerned content in the construction of the data center. In order to develop the data center, it is necessary to continuously expand the scale and increase equipment. This cannot be reduced, but the utilization rate of equipment needs to be improved in use. Another big part of energy consumption is heat dissipation. The energy consumption of a data center air conditioning system accounts for almost more than one third of the energy consumption of the whole data center. If we can make more efforts on this, the energy-saving effect of the data center will be immediate. So, what are the heat dissipation technologies in the data center and what are the future development directions? The answer will be found in this article.

Air cooling system

Air cooling direct expansion system becomes air cooling system. In the air cooling system, half of the refrigerant circulation circuits are located in the air conditioner of the data center machine room, and the rest are located in the outdoor air cooling condenser. The heat inside the machine room is squeezed into the outdoor environment through the refrigerant circulating pipeline. The hot air transfers the heat to the evaporator coil and then to the refrigerant. The high-temperature and high-pressure refrigerant is sent to the outdoor condenser by the compressor and then radiates the heat to the outdoor atmosphere. The energy efficiency of air cooling system is relatively low, and the heat is dissipated directly by wind. From the perspective of cooling, the main energy consumption comes from the compressor, indoor fan and air-cooled outdoor condenser. Due to the centralized layout of outdoor units, when all outdoor units are turned on in summer, local heat accumulation is obvious, which will reduce the refrigeration efficiency and affect the use effect. Moreover, the noise of air-cooled outdoor unit has a great impact on the surrounding environment, which is easy to have an impact on the surrounding residents. Natural cooling cannot be adopted, and the energy saving is relatively low. Although the cooling efficiency of the air cooling system is not high and the energy consumption is still high, it is still the most widely used cooling method in the data center.

Liquid cooling system

Air cooling system has its inevitable disadvantages. Some data centers have begun to turn to liquid cooling, and the most common is water cooling system. The water cooling system removes the heat through the heat exchange plate, and the refrigeration is stable. Outdoor cooling tower or dry cooler is required to replace condenser for heat exchange. Water cooling cancels the air-cooled outdoor unit, solves the noise problem and has little impact on the environment. The water cooling system is complex, expensive and difficult to maintain, but it can meet the cooling and energy saving requirements of large data centers. In addition to water cooling, there is oil cooling. Compared with water cooling, oil cooling system can further reduce energy consumption. If the oil cooling system is adopted, the dust problem faced by the traditional air cooling no longer exists, and the energy consumption is much lower. Unlike water, oil is a non-polar substance, which will not affect the electronic integrated circuit and will not damage the internal hardware of the server. However, the liquid cooling system has always been thunder and rain in the market, and few data centers will adopt this method. Because the liquid cooling system, whether immersion or other methods, requires filtration of the liquid to avoid problems such as pollutant accumulation, excessive sediment and biological growth. For water-based systems, such as those liquid cooling systems with cooling tower or evaporation measures, sediment problems need to be treated with the removal of steam in a given volume, and they need to be separated and “discharged”, even if such treatment may cause environmental problems.

Evaporative or adiabatic cooling system

Evaporative cooling technology is a method of cooling air by using the decrease of temperature. When water meets the flowing hot air, it begins to vaporize and become gas. Evaporative heat dissipation is not suitable for refrigerants harmful to the environment, the installation cost is low, the traditional compressor is not needed, the energy consumption is low, and it has the advantages of energy saving, environmental protection, economy and improving indoor air quality. The evaporative cooler is a large fan that draws hot air onto the wet water pad. When the water in the wet pad evaporates, the air is cooled and pushed out. The temperature can be controlled by adjusting the air flow of the cooler. Adiabatic cooling means that in the process of adiabatic rise of air, the air pressure decreases with the increase of height, and the air block does work externally due to volume expansion, resulting in the decrease of air temperature. These cooling methods are still novel for the data center.

Closed cooling system

The radiator cap of the closed cooling system is sealed and an expansion tank is added. During operation, the coolant vapor enters the expansion tank and flows back to the radiator after cooling, which can prevent a large amount of evaporation loss of coolant and improve the boiling point temperature of coolant. The closed cooling system can ensure that the engine does not need cooling water for 1 ~ 2 years. In use, the sealing must be ensured in order to obtain the effect. The coolant in the expansion tank cannot be filled up, leaving room for expansion. After two years of use, discharge and filter, and continue to use after adjusting the composition and freezing point. It means that insufficient air flow is easy to cause local overheating. Closed cooling is often combined with water cooling or liquid cooling. The water cooling system can also be made into a closed system, which can dissipate heat more effectively and improve refrigeration efficiency.

In addition to the heat dissipation methods introduced above, there are many wonderful heat dissipation methods, some of which have even been applied in practice. For example, natural heat dissipation is adopted to build the data center in cold Nordic countries or to the seabed, and “extreme deep cold” is used to cool the equipment in the data center. Like Facebook’s data center in Iceland, Microsoft’s data center in the seabed. In addition, water cooling can not use standard water. Seawater, domestic wastewater and even hot water can be used to heat the data center. For example, Alibaba uses the water of Qiandao Lake for heat dissipation. Google has established a data center using seawater for heat dissipation in hamina, Finland. EBay has built its data center in the desert. The average outdoor temperature of the data center is about 46 degrees Celsius.

The above introduces the common technologies of data center heat dissipation, some of which are still in the process of continuous improvement and are still laboratory technologies. For the future cooling trend of data centers, in addition to high-performance computing centers and other Internet-based data centers, most data centers will move to places with lower prices and lower power costs. By adopting more advanced cooling technology, the operation and maintenance cost of data centers will be further reduced and energy efficiency will be improved.

Post time: Aug-02-2021