Arturo Hidalgo adjusts a model of a prototype of wind turbine without blades inside a homemade wind tunnel in Avila, Spain, June 2, 2015. The windmill works by spinning air whirlpools in the inverted cones, aided by magnets, to create patterns of oscillation that are converted into electrical power by a generator. The same phenomena is responsible for the 'singing' of suspended power lines in the wind. Picture taken on June 2, 2015. REUTERS/Sergio Perez