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Why go green?
What’s the best way to reduce the electricity bill at data centers? Art Wittmann looks at some options
For most green technologies, putting a real financial value on the expected benefit isn’t too hard, but actually measuring the benefit across a data center—that’s a different story.
The challenge starts with the electric bill. Most data centers are just a room in a dual-use building. Separating out data center power usage often isn’t possible without retrofitting the room with sensors or installing a separate power meter. As a result, as an InformationWeek survey of 472 business technology pros confirms, almost no one in the IT organization is compensated based on saving energy, and only 22 percent of IT shops are responsible for managing power consumption. So while buying energy-efficient designs can reduce a system’s TCO, in most organizations IT doesn’t see the financial benefit.
Therefore, Job 1 is to get management to recognize and reward IT’s efforts to save energy. Once that’s done, a TCO calculation that includes power consumption is a lot more useful to IT. Beware, however, the dual-edged sword. Most organizations that charge back for utility usage simply divide the total utility bill by the square feet occupied. When properly burdened, IT could easily see its electric bill go up by an order of magnitude.
Changing best practices
The good news is that for most organizations the pressure to remodel or build new data centers can be alleviated through improved server and storage hygiene. But even as you get more out of existing data centers, new challenges threaten long-held best practices. As certain racks become more densely populated with 1U servers and blade systems, using perforated floor tiles on a raised floor no longer supplies enough cold air for the systems in the rack. For facilities built in the last decade, typical raised-floor cooling systems can exhaust 7 kilowatts per rack. Even today, most data centers won’t use that much power per rack, but in certain instances, they can use far more. For example, a fully loaded rack of blade servers can draw 30 kilowatts or more—only specialized, localized cooling systems can handle that sort of per-rack load.
In the past, the advice was to spread out the load. Put blade servers and other high-powered gear in with lower-consumption storage and networking systems, or simply leave the racks partially empty. While it’s still good advice for those who can pull it off, increasingly the geometry of the data center doesn’t allow it. Spreading out the load can push the average power draw per rack beyond what most data centers can deliver. The answer then is to pull those high-demand systems back together and use rack-based or row-based cooling systems to augment the room-based air-conditioning.
Rack-based cooling systems are available from a number of vendors. Two with very different approaches are IBM and HP. IBM’s eServer Rear Door Heat eXchanger replaces the back door of a standard IBM rack. The door uses a building’s chilled water supply to remove up to 55 percent of the heat generated by the racked systems.
The benefit to this approach is its simplicity and price, which is as low as $4,300. The system, introduced two years ago, removes heat before it enters the data center. By lowering the thermal footprint of the racked equipment, the IBM system can move the high-watermark from 7 kilowatts per rack to about 15 kilowatts, a nice gain for the price. The only downside is that the IBM solution requires water pressure of 60 PSI. Not all building systems can supply that much pressure, particularly if there will be a lot of these racks deployed.
HP’s solution is more comprehensive, takes more floor space, and costs considerably more. Introduced last year, its Modular Cooling System also uses the existing chilled water supply but adds self-enclosed fans and pumps. The result is a self-contained unit that can remove 30 kilowatts of heat with no impact on the room-based cooling system. Taking your hottest-running, most-power-hungry systems and segregating them into a rack that removes 100 percent of their generated heat goes a long way toward extending the life of a data center. The racks cost $30,000 a piece, but if it means not building new data centers, they’re worth it.
If you already own the racks and simply want a method for extracting large amounts of heat, Liebert makes systems that mount on or above racks. The company says that its XD systems remove up to 30 kilowatts per rack.
Finally, row-based systems such as Advanced Power Conversion’s Infrastruxure and Liebert’s XDH use half-rack-width heat exchangers between racks of equipment. The heat exchangers pull exhaust from the back (or hot-aisle side) of the racks and blow conditioned air out of the front. Because these systems substantially limit the ability for hot exhaust air to mix with cooled air—with APC’s product, you can put a roof and doors on the hot aisle for full containment—they can be much more efficient than typical computer room air conditioning (CRAC) units. Where CRAC units can draw as much as 60 percent of the power required by the systems they’re meant to cool, APC says its system can draw as little as 40 percent.
Any of these systems will go a long way toward extending the life of a data center. However, if the limiting factor is the capacity of the cooling towers on the building’s roof—that is, the ability of the building’s existing systems to produce chilled water—then deploying these rack and row solutions is practical only if you shut off some of your existing CRAC units. The good news is that, quite often, you can do just that.
Overcapacity in CRAC units is easy to determine. If you need to put on a sweater, or perhaps a parka, to go into your data center, you have more room-based cooling than you need. With proper planning, the ambient temperature of the data center can be as high as 78 degrees, says HP’s Paul Perez, VP for scalable data center infrastructure. Most data centers run at ambient temperatures well below 70 degrees. Perez says that for each degree of increased ambient temperature, figure at least a few percentage points in reduced energy consumption for cooling systems.
Where does the power go?
You’d think that most of the power consumed by data centers is by IT equipment. You’d think that, but you’d be wrong.
One of the biggest power hogs is chillers, the outdoor systems that cool water and other refrigerants. For large-scale systems, chiller manufacturers are working on bearingless designs, as the bulk of chiller inefficiency comes in energy lost through friction in the bearings. For smaller systems, technology such as Liebert’s Digital Scroll compressor offers substantial savings. The Digital Scroll rapidly turns the compressor on and off, letting it efficiently work from 10 percent to 100 percent of its capacity.
Another approach is IBM’s Cool Battery, which uses a chemical reaction to ‘store cold.’ Substantial savings can come by running chillers at off-peak hours and using the stored energy during peak load times. A deal with the local utility for lower-cost off-peak power goes a long way toward ensuring a good total cost of ownership for the Cool Battery.
The efficiency of uninterruptible power supplies also has changed substantially over the past few years. The inverter circuitry that turns the DC power from batteries into the AC power required by IT equipment draws a base level of power regardless of its load. New designs substantially reduce that base level load as well as the proportional loss under load, moving efficiency from the 80 percent range to about 95 percent.
Some UPS systems also have an economy mode which lets power from the grid feed IT gear directly. These systems can also monitor power consumption, which solves another data center challenge: knowing just how much power you’re using.
Regardless of the technology deployed, all of these systems, from chillers to UPS systems to humidifiers to CRAC units, operate most efficiently when they’re working at or very near their rated load. Modular data center designs that let systems either run at load or be turned off will be far more efficient than designs that load systems at some fraction of capacity.
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