Google has bought a firm that uses robotic kites to generate power using miniature wind turbines in the sky.

Makani Power originally started with the assistance of ARPA-E, which isn’t the Defense Department’s gadget factory (that’s DARPA) but rather an agency of the Department of Energy exploring alternative power sources. Google had later come along and invested around $15 million before deciding to buy the firm outright.

The logic behind Makani’s work is that it’s more efficient to have small turbines moving around in the sky than to use turbines mounted on poles in the ground, which require more materials.

The robots carrying the turbines aren’t completely untethered like planes. Instead they are attached to the ground by cables and can fly between 250 and 600 meters high. Those are heights were winds are not just stronger than near the ground, but are more reliable.

The cables then carry the power generated by the turbines to the ground. The plan is to operate the devices in batches of six, each tethered to one corner of a hexagon on the ground.

The company will now become part of Google’s research unit Google X which work on a variety of projects involving physical objects rather than simply data. It specializes in “skunk works” — projects where staff have more room to experiment rather than have to stick to fixed protocol designed solely to maximize the chances of a profitable product.

Makani reports having successfully tested prototypes that can generate power at 30kW without problems, the long term plan being to generate 600kW with larger models. A “standard” fixed wind turbine can generate 3,000 kilowatts so a fleet of six robots would slightly outperform it if everything goes to plan.

Although the prototypes have all landed safely, Bloomberg BusinessWeek (which revealed the Makani purchase), notes that Google CEO Larry Page insisted that the purchase go ahead only if there was scope to crash at least five prototypes to see what would happen.