Energy Modeling of Air-Cooled Data Centers: Part II—The Effect of Recirculation on the Energy Optimization of Open-Aisle, Air-Cooled Data Centers

The work presented in this paper describes a simplified thermodynamic model that can be used for exploring optimization possibilities in air-cooled data centers. The model has been used to identify optimal, energy-efficient designs, operating scenarios, and operating parameters such as flow rates and air supply temperature. The model is used to parametrically evaluate the total energy consumption of the data center cooling infrastructure, by considering changes in the server temperature rise. The results of this parametric analysis highlight the important features that need to be considered when optimizing the operation of air-cooled data centers, especially the trade-off between low air supply temperature and increased air flow rate. The analysis is used to elucidate the deleterious effect of temperature non-uniformity at the inlet of the racks on the data center cooling infrastructure power consumption. A recirculation non-uniformity metric, θ, is introduced, which is the ratio of the maximum recirculation of any server to the average recirculation of all servers. The analysis of open-aisle data centers shows that as the recirculation non-uniformity at the inlet of the racks increases, optimal operation tends toward lower recirculation and higher power consumption; stressing the importance of providing as uniform conditions to the racks as possible. Cooling infrastructure energy savings greater than 40% are possible for a data center with uniform recirculation (θ = 0) compared to a data center with a typical recirculation non-uniformity (θ = 4). It is also revealed that servers with a modest temperature rise (∼10°C) have a wider latitude for cooling optimization than those with a high temperature rise (≥20°C).