Cooling remains a major target for energy-efficiency improvement in data centers, and ASHRAE’s new operating temperature guidelines for IT equipment create an even greater opportunity to exploit free cooling. In particular, the one of the professional organization’s allowable temperature ranges permits free cooling all year round in almost every location in the world.
Air-Side Economization Strategies
Even considering only the power used to operate servers and other IT equipment—and ignoring for the moment peripheral infrastructure like cooling, lighting and so on—data centers are energy hogs. But the need to remove waste heat from the facility just compounds the problem, particularly when air conditioners (or other cooling apparatus) are commissioned with the task of keeping things cool. A flagging economy, political tensions with key energy-producing nations and rising energy costs are all leading companies to look for ways to reduce the power consumption of their data centers. And cooling is a major target by way of air-side economization, which involves bypassing traditional cooling infrastructure to enable the use of outside air to keep operating temperatures down. (Water-side economization follows a similar strategy for its own particular infrastructure.)
An APC whitepaper (“Economizer Modes of Data Center Cooling Systems”) offers a good overview of the basic methods of air-side economization in data centers. Each approach uses outside air as a heat sink, but they differ mainly in the isolation of outside air and the type of heat exchanger used. One strategy, for instance, is to simply draw in cool outside air, mixing it with some warmer exhaust to maintain specific temperatures and humidity, and to supply that air to the cold aisles in the data center. The intake air may also be filtered to limit contamination. To avoid problems of contamination and humidity variation, another broad strategy is to use a heat exchanger, whether a fixed-plate form or a heat wheel.
Obviously, free cooling is not entirely free, since even air-side economization requires energy to move air, rotate a heat wheel, and so on. Nevertheless, this certainly beats the energy consumption of traditional, mechanical cooling methods like CRACs. But using outside air is more efficient when the temperature difference relative to the warm inside air is greater, and few locations would seem to offer sufficiently cool temperatures to enable year-round air-side economization. But the new guidelines from the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) actually make free cooling a year-round possibility almost everywhere.
ASHRAE’s Updated Temperature Guidelines
ASHRAE updated its recommended and allowable temperature and humidity guidelines for IT equipment in May 2011. In addition to the larger recommended range, ASHRAE specifies four “allowable” ranges for data centers—A1, A2, A3 and A4—which permit server inlet temperatures of up to 45°C, or 113°F, for short periods of time (in the case of A4; the other ranges allow progressively lower temperatures). Outside of the recommended range, companies must evaluate manufacturer and other requirements to determine which allowable ranges—if any—are permissible for their data centers. These recommended and allowable ranges are reviewed briefly in The Green Grid’s whitepaper entitled “Updated Air-Side Free Cooling Maps: The Impact of ASHRAE 2011 Allowable Ranges.”
The Green Grid whitepaper updates that organization’s free-cooling maps, which identify the availability of free cooling (in hours per year) throughout the world. For the A3 allowable range (less than 40°C, or 104°F), the map of North America shows that nearly the entire continent can use free cooling all year round, with the exception of a few isolated areas in which small portions of the year are too warm or humid. The maps of Europe and even Japan show similar free cooling availability. For lower maximum temperatures (such as 35°C, or 95°F), significant areas of North America and Japan are much more limited in their free-cooling potential, as indicated by The Green Grid’s cooling maps.
Of course, this year-round free cooling applies only to data centers for which IT equipment can withstand the maximum allowable temperature of the range for short periods of time—neither all facilities nor all manufacturers will permit such a range. Again, a company must carefully evaluate its own particular situation before selecting one of the ASHRAE allowable operating temperature/humidity ranges. The recommended range remains a default, and companies running data centers in this range will have to rely more heavily on traditional cooling methods, depending of course on their location.
What’s the Next Target After Cooling?
As companies maximize their savings by relying more heavily on free cooling, the question will arise as to the next major area of focus. Naturally, companies are pursuing every available method to reduce energy consumption and, thus, operating costs. But certain areas seem to have more focus than others: for instance, virtualization has been a hot trend for years as it seeks to maximize server usage and efficiency, thereby avoiding energy waste owing to idle time. Air-side economization and other methods of free cooling will help push data center PUEs (power usage effectiveness ratings) closer to 1.0, but even if that goal is achieved, data centers will still be power hogs.
Every system will have a certain level of inefficiency, but even in a nearly ideal data center, the IT equipment will still consume vast amounts of power. But although semiconductor process technologies continue to progress, enabling more computational power on smaller and less power-hungry chips, the end may well be in sight. Eventually, Moore’s Law will end, and traditional semiconductor technology will offer no more power improvements (at least by way of simply shrinking the manufacturing process). At this point, the pursuit of greater efficiency will be reduced to tiny gains here or there, leaving few options for significant improvements that offer good return on investment. At such a point, companies will have little choice but to scale energy consumption directly with increasing demand.
ASHRAE’s updated temperature and humidity guidelines enable companies that are capable of using the A3 or A4 allowable ranges to use year-round air-side economization virtually regardless of their location in North America, Europe and Japan. Reduced reliance on traditional cooling means lower energy consumption, higher PUE, reduced infrastructure and capital costs, and less environmental impact. Not all facilities will be able to use the A3 or A4 ranges, meaning that they will still have to rely on mechanical cooling. In particular, high-density implementations and those with particularly sensitive equipment (whether in reality or just because the manufacturer says so) won’t be able to operate in these ranges. But the greater availability for free cooling creates an opportunity for IT equipment manufacturers as well: by designing equipment to handle higher operating temperatures, they can gain marketing points by boasting lower peripheral operating expenses (i.e., lower cooling costs).
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