Microsoft plans to install solar panels on the roof of its new data center in San Antonio, and will use photovoltaic power to supplement the 50 megawatts of capacity it has provisioned from local utility CPS Energy. The solar panels are just one example of the many steps Microsoft is taking to incorporate green technologies into its new data centers. While providing a visible illustration of the company’s commitment to environmentally-friendly technology, the solar panels may not make much of a dent in the energy bills for the $550 million San Antonio data center.

Solar energy hasn’t been widely used in data centers because of the large amounts of energy required to power the servers and cooling equipment in modern mission-critical facilities. It requires a very large installation of photovoltaic (PV) solar panels to produce even a fraction of the energy required by most data centers.

UPDATE: While Microsoft discussed plans to install solar panels at the San Antonio data center during Monday’s media event, the system won’t be operational in the near future. “While it is indeed sunny quite a bit of the time in San Antonio, the economics for solar are not yet a good fit for this facility,” said Mike Manos, general manager of Global Foundation Services for Microsoft. ”As solar technology advances, we anticipate that solar may become a more viable option within a few years. As a result, we have enabled our building to accept the technology and weight of solar panels when the technology matures.”

The only data center currently powered entirely by PV solar power is AISO (Affordable Internet Services Online), which operates a 1,500 square foot facility in Romoland, California. AISO powers its data center with 120 solar panels that generate DC power, which is then run through an inverter and stored in batteries.

The disparity between PV solar energy output and the power needs of data centers is best illustrated by existing solar power projects installed by Microsoft and Google in Silicon Valley. In April 2006 Microsoft built a solar panel array at its Silicon Valley Campus in Mountain View, Calif. consisting of 2,288 tiles with a peak capacity of 480 kilowatts. Four months later Google unveiled an even larger solar project on the rooftops of the Googleplex up the road in Mountain View. Google’s system featured 9,212 solar panels with a peak generating capacity of 1.6 megawatts.

By some estimates it takes up to 100,000 square feet of solar panels to generate 1 megawatt of power. Microsoft’s San Antonio data center is 477,000 square feet, which means that if the company covers a substantial section of the rooftop with solar panels it could ultimately generate several megawatts of power. That’s still a fraction of the 50 megawatts of utility power allocated for the massive facility.

Scalability isn’t the only issue hindering the use of PV solar power in data centers. In a recent presentation on renewable energy, Google energy guru Bill Weihl said PV solar is far more expensive than every other renewable energy alternative, costing 25 cents a kilowatt hour and more.

That doesn’t mean solar power has no future for data centers. Google has made several investments in solar thermal power, which used the sun’s heat – rather than its light – to generate energy. Solar thermal is cheaper to generate than PV solar (although still more expensive than coal) and has been used in “utility scale” installations in the Mojave Desert with capacity of up to 500 megawatts
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