While efforts are on to built power saving devices, a team at Woods Hole Oceanographic Institute has built a glider that requires no in-built power source. The glider is powered by the heat differential of the warm upper layers and the cold bottom layers of oceans to propel itself. The glider will however have limited application as it’s capable of flying only above oceans. It might also be difficult for the glider to pick up much height or else it might lose its ocean heat power source.

Source: Engadget178726