When one thinks of green, the first thought that comes to mind probably is not a chilly warehouse packed sky-high with servers. Yet in the energy-efficiency world, that’s exactly what where an increasingly amount of focus is going to — data centers.

Data centers are used by companies to store Web servers and associated computer equipment. And the behemoth buildings, which typically take up hundreds of thousands of square feet, are huge energy hogs, requiring large amounts to power the equipment, as well as air conditioning to keep the servers from overheating. In 2006, the government reported that data centers consumed 1.5 percent of  all electricity in the U.S., costing $4.5 billion, and projected the energy use could nearly double by 2011, costing $7.4 billion a year.

It’s enough of an issue that the EPA’s Energy Star program has launched a program to certify data center equipment as energy-efficient, as well as underscore best practices.

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And investors are taking note of the opportunities. Kevin Skillern, who heads up GE’s energy venture capital investments, thinks the official numbers may be behind the curve. He put the current consumption rate at three to four percent of U.S. energy output a year. That’s one reason why his company has backed green data center startups like SynapSense and JouleX.

Companies like HP, Dell, Yahoo and Google have increasingly turned to greening their data centers. More and more pressure has fallen upon big companies to follow suit, as evidenced by the Facebook imbroglio this fall with Greenpeace over a data center in Oregon that will run on coal-burning power sources. (Facebook recently announced that it would place a green data center in North Carolina.)

The area is already rife for innovation because of inherent inefficiencies. When data centers were first being built, energy was cheap, and not much thought was given to optimizing space or server efficiency, says Simon Mingay, analyst for technology research company Gartner. But “those days are over,” Mingay says.

The lack-of-forethought point is underscored in approach of Nlyte, which makes a data center management tool that can tell operators the best place to put a new set of servers that avoids overheating issues, or predict how much capacity a company will need in the future — right now, most companies deal with that by over-provisioning, which also wastes resources and money, says Nlyte CEO Jon Temple.

“Telling a customer what they’ve got, what’s being used and what isn’t being used — that in itself is 10 percent savings on energy bill,” Temple said. The rather shoot-in-the-dark approach Temple says organizations have taken in the past to data centers means that, on average, 15 percent of the assets in a data center are powered up and consuming energy without actually having anything to do — “other than pushing up your energy bill.”

Other startups like Viridity, Power Assure and Sentilla also take a software-based approach — Earth2Tech notes that Sentilla could have an edge in its ability to identify underutilized servers as well as older equipment that wastes more power. Viridity makes a package called EnergyCenter that models and predicts energy usage for $500 per server rack.

These days, a number of companies have emerged to compete in the greener data center space. Solutions range from the unusual — like GRC’s method of dunking servers in a bath of oil to cool them down — to architectural approaches to what’s called “freecooling,” like Yahoo’s chicken coop-inspired, 155,000-square-foot Computing Coop and HP’s 200,000-square-foot Wynard Center. Both companies located the centers in cool climates, then structured the buildings in a way that brought in chilly winds in to cool down servers, rather than fire up the air conditioning.

Sensors also look like they’ll play an important part in data center efficiency. GreenBeat 2010’s innovation competition winner Redwood Systems uses sensors in networked lighting systems that can reduce data center energy usage — “We can tell if a door has been left open”  just by looking at its data from its sensors, the company’s VP of marketing Jeremy Stieglitz told me, which in turn can be used to generate additional energy savings.

SynapSense uses wireless sensors that feed into a system for data center operators to monitor and adjust heat and load. And JouleX monitors, analyzes and automatically adjusts energy usage of devices in a network to the tune of 30 to 60 percent in energy savings.

Some startups, like Calxeda (formerly Smooth-Stone), aim to tackle the problem at its roots. The company makes chips that will result in servers that suck less energy and take up less space.

Internet and computer titans have taken to investing or partnering with some of these companies. HP, for example, has a technical alliance with Nlyte, and also makes server management software that can cut energy use at peak times. Google has an entire website dedicated to green data center best practices, and in particular emphasizes using recycled water as part of the cooling process. The company notes that two gallons of water is consumed for every kilowatt-hour of electricity produced in the U.S., and has pledged to make recycled water 80 percent of its total data center water consumption by this year.

The varied nature of all the competing solutions in the green data center space says something about the nature of the problem itself — that it’s complicated, a conclusion echoed by Nlyte’s Temple.

“It isn’t going to be one vendor that provides a single silver bullet,” he said. “IT is a complex environment that requires a lot of complexity to manage.”

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