Chimney Effect in Data Centers : On the possibility to achieve natural draft through servers

Sammanfattning: Data centers has in recent years experienced a rapid growth in numbers and size. Essentially, data centers are facilities housing IT equipment such as servers, switches and other network devices which use large amounts of electrical energy. Electrical energy is converted to heat by the data processing units in servers and therefore demand cooling. In large scale data centers, air is traditionally used as cooling medium where heat from the server components are transferred to the air as it flows through the server. Free cooling utilizes the cold outside air to avoid the use of chillers as cold source and therefore reduce energy consumption significantly. However, some facility energy and ventilation fan energy is still required to move the air. In addition, each server is commonly equipped with several small fans to move air through the server to keep CPU temperature at a safe operational level. The purpose of this paper was to study if fan energy consumption can be decreased due to an innovative server room layout. The idea was to connect a chimney to the backside of a server rack and route the air through the chimney and out to the ambient. The purpose of the chimney was to allow the heated exhaust air to rise upwards, due to buoyancy forces, and through this obtain natural draft through the servers. The induced air flow could potentially replace internal fans for moving the air through the servers and thus, reduce energy consumption. A mathematical model was developed and implemented in Simulink where simulations was performed on the system. The effect on temperatures and air flow in the system was studied by several simulations as different input parameters such as chimney dimension, server power and outdoor temperature was varied. Separate parts of the model were firstly validated against results presented in literature where good agreement was found. The complete model was assessed to be able to provide estimations on temperature and air flow to fulfill the general goal of the paper. The results showed reasonable values in most of the simulated cases despite lack of research to compare with. It could be concluded that low outdoor temperatures provided better air flow rates and hence, also better cooling abilities. A chimney height of 20 m and radius of 0.4 m was estimated to cover the cooling need for 160 servers where each server was assumed to consume 150 W. The induced flow from the chimney would be sufficient to replace all the internal fans based on climate data from Luleå, Sweden. The simulations provided interesting and promising results on the studied system. To further strengthen the findings, experimental measurements could be performed on small scale with real server hardware. The model could then be tested and compared to experimental values as the innovative configuration implied limited research similar to the system studied in the present paper.