It's new and it's old. It's proven, but still mysterious. It's big, and a major energy saver, but it's gone largely unnoticed. It's the Kyoto wheel -- a tremendously improved design on an older idea that seems to have solved the challenges of air-side free cooling in the data center. Although it has been well proven since its introduction in 2007, the innovations that make the Kyoto wheel such an energy-efficient cooling device have not been well understood, and its potential for large savings in cooling costs have gone virtually unnoticed.
Free data center cooling
"Free cooling" is thought by many to be a very effective way to minimize data center cooling costs and electrical consumption. While nothing is ever truly free, the objective is to use outside air to remove heat rather than expensive and power-hungry mechanical refrigeration. There are two approaches to free cooling: air-side and water-side. These refer to whether heat is exchanged directly to the outside air (air-side) or if it is first transferred to a liquid (water-side).
Water-side cooling passes the heat to circulating water (or a water/glycol mix to avoid freezing in winter climates) and then from the liquid to the outside air. Water-side cooling has been the most popular form of free cooling in data centers.
Home attic or window fans are air-side cooling devices. They pull outside air through windows and into the house, where they pick up heat and goes out the attic vents or another window. The method works, but it also brings in dust, pollens and outside humidity. In theory, air-side cooling should be the most energy efficient, and it can be, but contaminant filters and humidity control can quickly offset the savings. The solution is to separate the inside and outside air through some heat exchange method. There's a way to do this, known as a "heat wheel."
Kyoto wheel: Heat wheel redesigned for data centers
There's nothing fundamentally new about heat wheels; they've been around for years. In its most simple form, the heat wheel is 50% inside the room and 50% outside, so it picks up inside heat as it turns and releases it into the cooler air outside. Cavities create maximum surface area to transfer as much heat as possible, but they have historically transferred a fair amount of air along with the heat. There have also been problems with dirt buildup on the wheel surfaces, which reduces heat transfer. In other words, they've been unacceptable for the data center -- until now.
Enter the Kyoto wheel. Named for the Kyoto Protocol (conceived in the hope of conserving the world's resources and reducing global warming), the Kyoto wheel is an improved form of the original heat wheel, tailored to the requirements of data centers. It almost completely separates the outside air from the inside air (less than 1% transfer) -- and it works! It's already in regular use in a number of data centers.
The wheel itself is a giant aluminum honeycomb, anywhere from 4 meters (about 13 feet) to 6 meters (about 20 feet) in diameter. It generally turns at three revolutions per minute, although it can be increased to as high as 10 rpm. When outside conditions are right, it picks up sensible heat (the heat you can feel) from the data center as it rotates through half its arc, then releases it into the cooler outside air as it completes the other half of its rotation.
One of the Kyoto wheel's innovations is the use of fans to evacuate the air from the honeycomb structure on each half of the rotation. This contributes substantially to the heat-transfer efficiency, but the fans also use the majority of the power consumed by the wheel. Even so, the wheel takes only 8-25% of the power of equivalent mechanical cooling systems. And the already-high efficiency is maintained by a sophisticated control system that, among other things, keeps the fans at no more than 80% of their maximum speed. This avoids the dramatic "cube law" increase in energy consumption associated with higher-speed fan operation. The fans also minimize the amount of outside air transfer, which eliminates most of the dirt and other contamination concerns of air-side free cooling. The honeycomb is also covered with a microscopically thin epoxy coating to protect the aluminum foil from corrosion.
The 6-meter wheels can dissipate as much as 600-850 kW of heat energy, depending on the setup. For larger data centers, they can be installed in multiples. Some data centers have been segmented internally, with a wheel for each section. This is great for incremental expansion, because areas of the floor can be built out only when they are actually needed.
Kyoto wheels and green data centers
Energy efficiency, however, is the name of the game, and the numbers are pretty startling. KyotoCooling data says you could dissipate 2.5 MW with less than 100 kW of energy in many parts of the U.S. But, you ask, under which environmental conditions? You'll be surprised! Obviously, you can get the most hours of the most days of the year out of any "free cooling" system in northern climates, where the air is cooler a higher percentage of the time. But calculations have shown you could use the Kyoto wheel effectively for 172 days of the year in Dallas, stay within the ASHRAE temperature envelope and use only 14.5% of your IT load energy for cooling. That's a whole lot better than the 60% that could be consumed by conventional mechanical cooling.
The first installation was in the Netherlands, where a 6-meter wheel dissipates 600 kW of heat using only 48 kW of energy. In that installation, the outside ambient temperature averages about 15 degrees Celsius (60 degrees Fahrenheit) and the air supplied back to the data center is 25 Celsius (77 degrees Fahrenheit). That's well within the ASHRAE allowable temperature envelope of 27 degrees Celsius (80.6 degrees Fahrenheit), and the wheel can fully cool the data center about 95% of the year.
In Chicago and New York, a group of wheels could theoretically supply total cooling to a 2.5 MW data center nearly 80% of the year, using only about 9% of IT power for cooling. Philadelphia isn't far behind at 75%, and San Jose, Calif., is way up at 87%. The rest of the days require either total mechanical cooling or a mixture of air-side and mechanical cooling, but the resulting energy savings can be significant. Again, KyotoCooling calculations predict a total energy savings (cooling plus IT) for a 2.5 MW data center in New York, Boston, Philadelphia or Toronto in the 30%+ range for annual power savings, based on $0.10/kWh, of more than $500,000. Those kinds of numbers can result in a pretty decent return on investment as well as the environmental benefits that come with it.
Possible drawbacks to the Kyoto wheel
So what are the drawbacks? Space, for one. The wheels are large, but their enclosures are even larger to provide the chambers where heat exchange occurs. A 6-meter wheel is enclosed in a chamber of about 8x12 meters, or roughly 1,000 square feet. Smaller wheels of 4 meters in diameter are over 4 meters high, 4 meters wide and 12 meters long, and dissipate 300 kW of heat load. Smaller wheels are available with dissipations of 200 kW or 150 kW. If several of these are needed (the 2.5 MW installation requires five large wheels), this can take quite a bit of space either next to the building or on the roof. Some substantial ducting is also required to direct the air flows where they are needed and to avoid hot and cold air mixing, which would quickly negate the energy advantage of the units. It's a good consideration for new data centers, but even for those, placement has to be fully considered. Existing facility retrofits could be challenging.
The wheels are also an additional capital cost in almost all locales. Since free cooling can be utilized only a percentage of the year, conventional mechanical cooling is still needed for those times when outside conditions are not conducive to using the wheel. And it is highly recommended that the wheel be run on uninterruptible power supply, or UPS, to avoid any cooling interruption. In the event of a power failure, even if outside conditions are above target temperatures, the Kyoto wheel is so efficient it can maintain the data center only 2 degrees Celsius warmer than the outside air. Even in the worst conditions, that will keep computer hardware operating until generators can start and mechanical cooling systems can be brought online.
In short, the Kyoto wheel undoubtedly deserves more consideration in data center projects than it has been given, particularly by those who are serious about energy efficiency and conservation.
ABOUT THE AUTHOR: Robert McFarlane has spent more than 30 years in communications consulting, with experience in every segment of the industry. McFarlane was a pioneer in the field of building cabling design and a leading expert on trading floor and data center design. He is currently president of the Interport Financial Division of New York-based Shen Milsom & Wilke Inc. and a data center power expert.
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