Cloud service providers operate multiple geographically distributed data centers. These data centers consume huge amounts of energy, which translate into high operating costs. Interestingly, the geographical distribution of the data centers provides many opportunities for cost savings. For example, the electricity prices and outside temperatures may differ widely across the data centers. This diversity suggests that intelligently placing load may lead to large cost savings. However, aggressively directing load to the cheapest data center may render its cooling infrastructure unable to adjust in time to prevent server overheating. In this paper, we study the impact of load placement policies on cooling and maximum data center temperatures. Based on this study, we propose dynamic load distribution policies that consider all electricity-related costs as well as transient cooling effects. Our evaluation studies the ability of different cooling strategies to handle load spikes, compares the behaviors of our dynamic cost-aware policies to cost-unaware and static policies, and explores the effects of many parameter settings. Among other interesting results, we demonstrate that (1) our policies can provide large cost savings, (2) load migration enables savings in many scenarios, and (3) all electricity-related costs must be considered at the same time for higher and consistent cost savings.
A numerical study has been carried out to characterize the metalorganic chemical vapor deposition (MOCVD) growth of Gallium Nitride (GaN) in a rotating-disk reactor. The major objective of this work is to examine the dependence of the growth rate and thin film uniformity on the primary parameters. First of all, for a rotating-disk system, the governing equations involved are obtained. Then, with the effect of thermal buoyancy included and based on the detailed mathematical model and chemical reaction mechanisms, the 3D simulation study is conducted for a rotating reactor. A comparison between the predicted growth rate and experimental data is presented. In addition, the effect of various primary operating and design parameters on the growth rate of GaN and thin-film uniformity is also examined. This provides further insight into the reactor performance and the characteristics of the entire process. The results obtained can also form the basis for the future design and optimization of this system.
A detailed mathematical model for the growth of gallium nitride in a vertical impinging metalorganic chemical vapor deposition (MOCVD) reactor is developed first, and the complete chemical mechanisms are introduced. Then, one validation study is conducted to ensure its accuracy. After that, the flow, temperature and concentration profiles are predicted by numerical modeling. The dependence of the growth rate and uniformity of the deposited layers on operating conditions, such as reactor operating pressure, susceptor temperature, inlet velocity and concentration ratio of the precursors, is investigated to gain greater insight into the reactor performance and characteristics. Based on the simulation results, discussion is presented in this paper to offer the possibility of better control of the GaN film growth process and to ultimately lead to an optimization of the process, with respect to production rate and film quality.
A numerical study has been carried out on the metalorganic chemical vapor deposition (MOCVD) process for the fabrication of gallium nitride (GaN) thin films, which range from a few nanometers to micrometers in thickness. The numerical study is also coupled with an experimental study on the flow and thermal transport processes in the system. Of particular interest in this study is the dependence of the growth rate of GaN and of the uniformity of the film on the flow, resulting from the choice of various design and operating parameters involved in the MOCVD process. Based on an impingement type rotating-disk reactor, three-dimensional simulations have been preformed to indicate the deposition rate increases with reactor pressure, inlet velocity, and wafer rotating speed, while decreases with the precursor concentration ratio. Additionally, a better film uniformity is caused by reducing the reactor pressure, inlet velocity and wafer rotating speed, and increasing precursor concentration ratio. With the impact of wafer temperature included in this study as well, these results are expected to provide a quantitative basis for the prediction, design, and optimization of the process for the fabrication of GaN devices. The flow and the associated transport processes are discussed in detail on the basis of the results obtained to suggest approaches to improve the uniformity of thin film, minimize fluid loss, and reduce flow recirculation that could affect growth rate and uniformity.
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