Using renewable energy to fuel its data centers is one of the ways Google is fulfilling a four-year-old commitment to reduce its carbon footprint to zero. But electricity generated from the sun or wind isn't the best match for the data centers operated by the $29 billion Internet search engine provider. Wind isn't constant and sunshine is limited, while data centers have to have power around the clock. Nor can the energy be stored for later use to deal with the differences in the timing of supply and demand.

So Google's 2010 signing of a 20-year contract to buy 114 megawatts of electricity a year from a wind farm in Iowa being developed by NextEra Energy posed a challenge, one that Google solved with a series of financial transactions involving the electricity grid and the power markets.

The electricity from the wind farm comes with a Renewable Energy Certificate (REC) attached that differentiates it from electricity produced using coal or oil. Google buys the electricity from the wind farm at an interconnect, strips the certificate and resells the electricity into the market at that same interconnect. The company then buys power for its data center, either at the interconnect closest to the center or from a utility, and attaches the certificate to that power.