This little glider can power itself for months on end, well, at least its movement could be powered by itself. It still needs battery power for all the electronics inside that make it useful, but it can use the oceans warm upper layer and cool lower layer to move.
Basically how it works is that there is a chamber inside full of air, that’s what makes this torpedo buoyant, but there is also an oil or wax or something like that which decreases the size of this chamber in warm water, causing the torpedo to fall in the water, and the cold water, at the bottom of the ocean/sea, causes the chamber to expand and rise again. All the while that is happening the fins on the side cause it to move in a forward motion as it rises and falls meaning the only thing it has to move with the battery inside is the rear fin that steers the device, the forward propulsion is taken care of.
That means this would be great for use in doing oceanic research over extended periods of time. The singular downside is that the only slice of the ocean it will ever see is the middle, hitting the bottom or the top would be the end of its journey for the most part, although I believe there is a small electric propeller on the back that can be use for navigation around obstacles, and that could presumably be used to get shots or info from the upper or lower layers, but would shorten the length of time it could be out on its own.