Solar Power at Data Center Scale

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The installation planned for Phoenix ONE will dwarf all previous efforts to integrate solar power into a working data center. Its output will be nearly three times the 1.6 megawatts produced by the solar panels covering the roof of the Googleplex.

The first phase of 5,000 solar panels in Phoenix is scheduled to be operational in January, and will generate 500 kilowatt-peak (kWp), the company says. The array will be expanded in four additional phases during 2010 to reach a total capacity of 4.5 megawatts-peak.

That's just a fraction of the 80 megawatts of power capacity that the 538,000 square foot Phoenix ONE data center will need upon completion. The solar power is also expensive, costing about 18 cents per kilowatt hour to generate in a market where grid power is 7 cents.  

Time-Shifted Cooling
But Phoenix ONE plan capitalizes on another wrinkle in power pricing: the differential between the daytime and overnight rates. The solar plant will be combined with an on-site thermal storage facility to create a time-shifted energy efficiency system.

i/o Data Centers will run its chillers at night when power is cheap, and then tap the thermal storage "battery" to provide much of the facility's cooling during the day, reducing its power usage when electricity is most expensive. The solar power panels will further lower Phoenix ONE's reliance on utility power during peak hours.         

"If we can generate 3 megawatts during the day, combined with our thermal storage, we can shave our power costs by about 50 percent," said George Slessman, the CEO of i/o Data Centers. "Anything I can do to move my power consumption to off-peak hours is going to save a lot of money. Solar is the renewable approach that works best during peak daytime power pricing."

The Challenges of Solar
Solar power 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. 

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. Intel and Sun Microsystems have tested solar power systems generating 10 kilowatts to partially power data center containers.

PV solar is also much more expensive than grid power. Microsoft has discussed plans to install solar panels at its San Antonio data center, but said such a system wouldn't be economically feasible in the near future. "As solar technology advances, we anticipate that solar may become a more viable option within a few years," the company said.

The Key: Thermal Storage 
How has i/o Data Centers made solar viable? The key ingredient is a thermal storage system that will be able to cool the data center for as long as six hours per day, according to Slessman. 

Here's how it works: The facility's chillers cool a solution of water and 28 percent glycol. The thermal storage tank contains Cryogel ice balls, 4-inch polyethylene spheres filled with water. The balls freeze when the system is charging at night,  and then cool the glycol solution during the day. The glycol solution is then pumped through a heat exchanger, which chills water in a separate loop used in the data center.

The first phase of the thermal storage system will come online in October and provide 12,000 ton hours of thermal storage. A second phase will eventually boost capacity to 24,000 ton hours.

Meanwhile, the solar installation atop Phoenix ONE will also be growing. The roof will eventually be covered with more than 300,000 square feet of photovoltaic solar panels.    

"The vast majority of data centers don't have 4.5 megawatts of power, period, much less 4.5 megawatts of solar power," said Slessman. "About 5 percent of our power consumption will be offset by on-site solar.

A Sustainable Efficiency Model
This approach works particularly well in Arizona, one of the best locations in the U.S. for solar power. But i/o Data Centers is looking to expand, and is including renewable power in the equation.    

"We want to create a sustainable model for energy efficiency that we can repeat in any market around the U.S.," Slessman said. "We want to go into markets where solar is plausible, and are looking at states that are power exporters."

i/o Data Centers President Anthony Wanger says the focus on energy efficiency will likely increase in coming years. "Solar, nuclear, hydro and natural gas are going to be increasingly favored," said Wanger. "As they say in hockey, we're skating where the puck is going."