A preliminary discussion on energy-saving measures for HVAC systems in the planning and design phase of data centers

In recent years, the number of data center projects in China has been increasing, with some data centers having a construction area of over 10,000 square meters. This change makes the traditional precision air conditioning cooling mode for computer rooms no longer suitable for the current development needs of China. PUE is a very important indicator in the process of energy efficiency evaluation. Considering various factors, relevant national departments require that the PUE indicator of data center projects in actual use should be at least above 2.0. This requirement means that large data center projects consume a lot of electricity during operation. Currently, to save energy consumption, chilled water systems are generally used for cooling to achieve energy saving. This article analyzes these issues in detail.

I. Reasonable Utilization of Outdoor Climate Conditions

　　First, data center projects need to operate continuously year-round due to actual usage requirements. This means that the air conditioning system also needs to operate year-round. However, for northern China, during winter, data center projects can utilize outdoor air as a cold source, reducing the actual operating time of refrigeration equipment and further reducing the overall system operating costs. This can reduce the average PUE value of the entire project.

　　Secondly, when using functional systems such as cooling towers, if the outdoor wet-bulb temperature has dropped to a certain level, when using water-cooled chillers, and the cooling water supply temperature is 2 degrees Celsius lower than the return water temperature, a portion of the natural cooling mode can be utilized, allowing the return cooling water from one side of the plate exchanger to the other side. After the cooling work is completed, it flows into the chiller. This can reduce the load rate of the chiller equipment to some extent, reduce the compressor power of the entire system, and further achieve energy saving. If the outdoor wet-bulb temperature continues to decrease due to various factors, then at this time, the return cooling water from one side of the plate exchanger, after the cooling work is completed, if the temperature meets the relevant supply water temperature standard, then it can enter a completely natural cooling state. At this time, the normal operation of the chiller can be stopped, and the cooling tower can bear all the load.

　　Finally, evaporative cooling systems and segmented natural cooling systems such as heat exchangers can generally utilize the outdoor dry-bulb temperature in a certain way. If the dry-bulb temperature is about 2 degrees Celsius lower than the design temperature of the indoor air supply due to various factors, the natural cooling function of the air end can be activated. For Northwest China, which is a dry and hot region, some appropriate sprinkler devices can be installed before the outdoor air enters the heat exchanger to maintain the heat exchange temperature of the outdoor air at a temperature close to the earth's temperature, thereby further extending the natural cooling time.

II. Variable Frequency Design of Equipment

　　First, in the actual use of the refrigeration and air conditioning system of data center projects, if the chiller is kept running, the energy consumption will be extremely large. By making full use of the variable frequency chiller, not only can the compressor power be reduced to a certain extent under partial load, but also the cooling water inlet temperature flowing into the chiller can be further reduced as the outdoor wet-bulb temperature decreases. This can increase the overall COP value of the chiller system to a certain extent. At the same time, when using the data center air conditioning system, the specific opening time of the chiller is closely related to the capacity of the cold storage tank. By making full use of the variable frequency chiller design, the opening time can be shortened to a certain extent, thereby further reducing the capacity of the cold storage tank.

　　Secondly, it is generally difficult to effectively control the pressure difference of the entire loop during the actual operation of the water pump. In this case, a variable frequency water pump can be reasonably selected. If it is found that the load at the end of the variable frequency water pump is very small during use, the specific frequency of the water pump will also decrease to a certain extent, thereby reducing energy consumption to a certain extent.

　　Finally, if the end air conditioner fan uses the EC model, then further combined with the return air temperature control fan frequency in the raised floor can be appropriately controlled. This can reduce the energy consumption of the fan during use.

III. Scientific Airflow Organization

　　In the actual use of data center projects, scientifically and reasonably planning and designing the airflow organization inside the main computer room is of great significance for energy saving. Through reasonable planning and design, the phenomenon of hot and cold air mixing can be avoided during the use of the data center. Once the heat density of the cabinet is less than 5KW due to various factors, the design principle of cold and hot aisles can be followed. If the heat density of a single cabinet is greater than 5KW due to various factors, then a closed cold and hot aisle design criterion can be selected. According to the current situation in China, there are relatively more closed cold aisles. With the continuous development of the times, the supply air temperature gradually increases during the formal process of indoor design. In this case, choosing a closed hot aisle mode is more conducive to the normal progress of energy-saving work. The specific reasons are as follows: first, the closed hot aisle mode can make people more comfortable in the building; second, the reasonable use of the closed hot aisle can substantially reduce the leakage rate of the cabinet itself during actual use.

IV. Design of Air Supply Parameters

　　In data center projects, designers refer to the standards of ASHRAE in the United States, stipulating that the supply air temperature of IT rooms should be controlled within the range of 18-27 degrees Celsius. The chilled water outlet temperature should be controlled above 15 degrees Celsius.

　　Conclusion: Based on the content described above, this article comprehensively analyzes the HVAC system in China's data center projects from the perspective of energy saving. To make it more energy-efficient during use, effective choices can be made in combination with actual design projects. Energy-saving technology makes the functions of the air conditioning system more superior during actual use.

Source: Data Center Operation and Maintenance

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