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Google has acquired a US company that generates power using turbines mounted on tethered kites or wings.

Makani Power will become part of Google X - the secretive research and development arm of the search giant.

The deal comes as Makani carries out the first fully autonomous flights of robot kites bearing its power-generating propellers.

Google has not said how much it paid to acquire Makani, but it has invested $15m (£9.9m) in the company before now.

, Makani said the deal would "provide Makani with the resources to accelerate our work to make wind energy cost competitive with fossil fuels".

Rather than use fixed turbines to generate power, Makani has been experimenting with "flying wings" adorned with several smaller turbines that act as propellers as the craft takes off.

Makani claims that mounting the turbines on a wing is more efficient as large numbers can be built with fewer materials than traditional tower-based turbines. In addition, the robot wings can land to avoid damage if wind speeds are too high or during bad weather.

It plans to operate the tethered wings in small groups of six with each one anchored at the points of a hexagon. The wings operate between 250m (820ft)and 600m above ground.

The firm has just successfully completed tests on a 30 kW prototype to see if its control system lets it launch, hover, generate power and land without human intervention. Fully working models will be larger and, Makani hopes, be able to generate 600 kW. It hopes the cost of the power generated will initially be competitive with off-shore wind farms.

Power is generated by the wing flying in a circular path, with electricity transmitted to the ground via the conducting core of the tether.

Google X is the "big ideas" arm of the search firm that is tasked with testing out ideas outside the normal remit of the larger company. The lab has given rise to Google's self-driving car initiative and did the early work on its augmented spectacles.