Out of the 3 mentioned there, I take the kinetic and thermoelectric most seriously.
With kinetic generation though, I would think you could use ballast diving to free-wheel your thrusters to generate electricity, in this case though the more ideal your hydrodynamic shape and thruster placement is, the better. Imagine the glider AUV we have already seen with a propeller on the back spinning up with the dive and turning a motor in reverse generating electricity.
Thermoelectric is simple enough, and provided you can find a large enough heat disparity could work. If maybe your pressure hull electronics put out appreciable heat and the cold side was to the ambient seawater that would be alright.
Another option I came up with however, is very common in the real submarine world: diesel-electric! Using a RC Airplane diesel engine coupled to a brushless electric motor and fed from a compressed air cylinder with matching volume of fuel. The brushless motor would serve as the starter motor for the engine and then as an alternator while running. The whole package should easily fit inside a PVC Pipe, the engine being no more than two and a half inches tall and the motor of whatever diameter you matched it to. I would say use compressed air to feed the engine instead of a snorkel because with the smaller hull I think it would be easier to foul the intake, and for exhaust, provided a check valve in the line the engine should put out enough pressure to exhaust directly into the water at the surface. They also make purpose built alternator disks for RC planes that could be stacked on a shaft to generate power possibly more efficiently.
I just wanted to pick up the conversation on the topic I saw, long dead on another forum someplace more lively. Im interested to hear any of your thoughts.
