Efficient Thermal Management of Data Centers—Immediate and Long-Term Research Needs

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Abstract
Power dissipation within computer rooms or data centers has been steadily increasing over the past decade. With the spread of CMOS technology into microprocessors and memory in the 1980s and 1990s, water-cooled mainframe systems were largely supplanted by lower power air-cooled systems. These systems were typically stacked into 2-m-high racks for efficient use of expensive data center floor space. Data center environmental cooling infrastructures correspondingly evolved into designs that recirculate hot exhaust air from the computer systems into air-conditioning units. The air-conditioning units remove the heat and return the cool air back into the room in a closed-loop fashion. These air-cooled infrastructures are largely open, nonducted environments where hot and cold airstreams are free to mix. The evolution of microprocessor fabrication technology has enabled the construction of high-power processors. The push by business, academia, and consumers for greater processing speed has motivated the design of computer systems that enable the greatest number of processors, and, thus, the greatest processing power, per rack volume. This increase in microprocessor density places a great deal of strain on current computer room environmental control technology. Furthermore, the rate of increase in power density in the data center is outpacing that of HVAC technology improvements. Because of this, computer manufacturers are faced with the choice of either limiting system performance in favor of reduced power consumption or of providing customers with higher performance products that are impractical to deploy. In this paper, we will highlight some of the primary challenges with cooling high-power density data centers. We will demonstrate that existing environmental infrastructures have inherent inefficiencies that can be very costly, and we will explore alternatives. Additionally, the use of numerical modeling to diagnose problems with data center design and layout will be demonstrated, and limitations to its effective use will be discussed. Finally, the high-power densities involved have increased the need for a theoretical treatment of data center thermophysics. We will discuss this need in detail and will suggest ways in which it might be addressed. Throughout the paper, focus will be placed on future directions with the hope of instilling enthusiasm for further research and development by academia and industry in this particular area of HVAC&R that will soon reach a critical point in its continuing evolution.

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