Best Practices for Data Centers : Lessons Learned from Benchmarking 22 Data Centers

Over the past few years, the authors benchmarked 22 data center buildings. From this effort, we have determined that data centers can be over 40 times as energy intensive as conventional office buildings. Studying the more efficient of these facilities enabled us to compile a set of “best-practice” technologies for energy efficiency. These best practices include: improved air management, emphasizing control and isolation of hot and cold air streams; rightsizing central plants and ventilation systems to operate efficiently both at inception and as the data center load increases over time; optimized central chiller plants, designed and controlled to maximize overall cooling plant efficiency, central air-handling units, in lieu of distributed units; “free cooling” from either air-side or water-side economizers; alternative humidity control, including elimination of control conflicts and the use of direct evaporative cooling; improved uninterruptible power supplies; high-efficiency computer power supplies; on-site generation combined with special chillers for cooling using the waste heat; direct liquid cooling of racks or computers; and lowering the standby losses of standby generation systems. Other benchmarking findings include power densities from 5 to nearly 100 Watts per square foot; though lower than originally predicted, these densities are growing. A 5:1 variation in cooling effectiveness index (ratio of cooling power to computer power) was found, as well as large variations in power distribution efficiency and overall center performance (ratio of computer power to total building power). These observed variations indicate the potential of energy savings achievable through the implementation of best practices in the design and operation of data centers. The Data Center Challenge The energy used by a typical rack of state-of-the art servers, drawing 20 kilowatts of power at 10 cents per kWh, uses more that $17,000 per year in electricity. Given that data centers can hold hundreds of such racks, they constitute a very energy-intensive building type. Clearly, efforts to improve energy efficiency in data centers can pay big dividends. But, where to start? “You can’t manage what you don’t measure” is a mantra often recited by corporate leaders (Tschudi et al 2005). Similarly, the high-availability needs of data center facilities can be efficiently met using this philosophy. This paper presents a selected distillation of the extensive benchmarking in 22 data centers which formed the basis for the development of 10 best-practice guides for design and operation that summarized how better energy performance was obtained in these actual facilities (LBNL 2006). With typical annual energy costs per square foot 15 times (and in some cases over 40 times) that of typical office buildings, data centers are an important target for energy savings. They operate continuously, which means that their electricity demand is always contributing to 3-76 © 2006 ACEEE Summer Study on Energy Efficiency in Buildings peak utility system demands, an important fact given that utility pricing increasingly reflects time-dependent tariffs. Energy-efficiency best practices can realize significant energy and peakpower savings while maintaining or improving reliability, and yield other non-energy benefits. Data centers ideally are buildings built primarily for computer equipment: to house it and provide power and cooling to it. They can also be embedded in other buildings including highrise office buildings. In any event, a useful metric to help gauge the energy efficiency of the data center is the computer power consumption index, or the fraction of the total data center power (including computers, power conditioning, HVAC, lighting, and miscellaneous) to that used by the computers (higher is better; a realistic maximum is in the 0.8 to 0.9 range, depending on climate). As shown in Figure 1, this number varies by more than a factor of 2 from the worst centers to the best we’ve benchmarked to date. Even the best have room for improvement, suggesting that many centers have tremendous opportunities for greater energy efficiency. Figure 1. Computer Power Consumption Index Computer Power Consumption Index