The first phase of Apple's fuel cell project at the Maiden, NC data center began testing in October of 2012, and the company is already looking at doubling the capacity of the plant.
The plant is fueled by filtered landfill gas (AKA "directed biogas"), a fuel that is considered a renewable energy resource by North Carolina's green energy regulations. Apple filed paperwork in November with the North Carolina Utilities Commission to bring the plant from 4.8 megawatts of power to a total of 10 megawatts. Any excess energy generated by the fuel cells and the huge (covering 250 acres eventually) solar farm on the site will be sold to Duke Energy.
Apple is planning on using similar green power initiatives at all three of its newest data centers.