Green Data Centers: A Survey, Perspectives, and Future Directions <s> C. Open Questions and Research Directions <s> This paper discusses the preliminary design of a controller for the cooling air conditioning units (CRAC) fan speed based on server loads. The CRAC is paired with a rear door heat exchanger. The main purpose in implementing this controller is the reduce energy consumption and to have finer control in the provision of cooling within a data center. Tools that will be essential for developing this controller are CRACs with variable frequency drives (VFD), BCMS power monitoring, data archiving software (PI from OSISOFT) and MATLAB. For this experiment, the zone of interest in the data center consists of 10 racks of IBM Blade Centers with a maximum electrical load of approximately 24 kW each and a total of 3360 nodes. Results are compared with the conventional cooling method to quantify energy savings and corresponding chiller coefficient of performance (COP).Copyright © 2011 by ASME <s> BIB001 </s> Green Data Centers: A Survey, Perspectives, and Future Directions <s> C. Open Questions and Research Directions <s> Reliable operation of today's data centers requires a tremendous amount of electricity to power both the IT equipment and the supporting cooling facilities. As much as half of total data center electricity consumption can be attributed to the cooling systems required to maintain the thermal status of IT equipment. In order to lower the electricity usage of the cooling system and hence reduce the data center environmental footprint, alternative cooling resources, such as water and air-side economizers, are being exploited to supplement or replace the traditional chilled water based cooling schemes. The various cooling resource options, together with the mechanisms to distribute and deliver the cooling resource to IT equipment racks, constitute a cooling microgrid. In this paper, we present a holistic perspective for the optimization and control of the data center cooling microgrid. The holistic approach optimizes the sourcing and distribution of cooling resources from the site portfolio in response to real-time weather changes and site demand. The cooling microgrid optimization and control framework has been implemented in a research data center; we estimate that the framework cuts the yearly cooling costs by 30%. <s> BIB002 </s> Green Data Centers: A Survey, Perspectives, and Future Directions <s> C. Open Questions and Research Directions <s> Lately, for geo-distributed data centers, a workload management approach that routes user requests to locations with cheaper and cleaner electricity has been developed to reduce energy consumption and cost. We consider two key aspects that have not been explored in this approach. First, through empirical studies, we find that the energy efficiency of cooling systems depends critically on the ambient temperature, which exhibits significant geographical diversity. Temperature diversity can be used to reduce the cooling energy overhead. Second, energy consumption comes from not only interactive workloads driven by user requests, but also delay tolerant batch workloads that run at the back-end. The elastic nature of batch workloads can be exploited to further reduce the energy cost. In this paper, we propose to make workload management temperature aware . We formulate the problem as a joint optimization of request routing for interactive workloads and capacity allocation for batch workloads. We develop a distributed algorithm based on an $m$ -block alternating direction method of multipliers (ADMM) algorithm that extends the classical two-block algorithm. We prove the convergence and rate of convergence results under general assumptions. Through trace-driven simulations, we find that our approach consistently provides 15-20 percent cooling energy reduction, and 5-20 percent overall cost reduction over existing methods. <s> BIB003
The continued growth of data centers is resulting in sharply increasing levels of energy consumption required for both powering these data centers and their thermal management. Several methodologies have recently been introduced that offer promising approaches to reducing the escalating energy costs associated with meeting their cooling requirements. However, there are still several challenges and research issues that must be addressed regarding the temperature and thermal control of data centers: • Most existing studies have focused on heat dissipation and cooling load management within data centers. Chillers are used to cool the hot water that returns from the Computer Room Air Conditioners (CRACs) via mechanical refrigeration cycles, and the compressors for these chillers consume large amounts of energy BIB002 . Recently, various so-called free cooling technologies, such as air-side economizing and water-side economizing, have been adopted to reduce the dependence on mechanical chillers. Air-side economizing refers to directing cold outside air into the data center to cool the servers and ejecting the hot exhaust air back outside instead of cooling and recirculating it. Water-side economizing refers to cooling down the warmer chilled water from the computer room using water streams. However, these free cooling technologies heavily depend on air conditions and geographical locations. An interesting question is whether and how these technologies can be coupled to address power and cooling demands. A related study has already been performed BIB003 , but more thorough theoretical and practical studies are needed to explore this possibility. • As mentioned earlier, service providers may own geographically distributed data centers to better meet their QoS needs, such as delay guarantees. Many of these providers, such as Google  , have been relying upon fresh air for cooling to reduce the energy costs of operating their data centers. When renewable energy is considered, the problem becomes more interesting and sophisticated. For example, for systems that rely on airside economization, it is typically preferable to operate in a cold-temperature environment. However, the generation of renewable energy using solar panels requires hot, sunny days, which will adversely affect the cooling energy consumption. Addressing this issue will require holistic methods that simultaneously consider cooling energy costs, the availability of renewable energy, and performance requirements. • In the current thermal control approaches implemented in most data centers, the CRAC fans are operating at their peak speed. Lowering CRAC fan speeds allows energy consumption to be reduced. However, the challenge is to develop CRAC fan speed controllers that can supply a data center with only the necessary cooling demands. The existing work of BIB001 represents an initial effort to address the problem of fan speed management.