Junk Ideas On Electrical Generation With Thermal to Mechanical Energy

In reading a book on the physics of temperature I developed a couple of ideas on the topic of converting thermal to mechanical energy, or rather of exploiting that principle. Of course these are simply junk ideas since I am not a physicists however I like to write them down anyway instead of letting them just disappear back into neuron space.

Mechanical energy can be converted 100% into thermal energy yet no vice versa. Well,the sunshine is free for use in heating trapped gases that expand with heat energy input so wouldn't it be possible to expand gas in a bladder with sunshine indirectly and use the increased lift to pull on a tether attached to a generator-the gas bladder would be under the water so getting the surface heat to it would require some additional engineering.

The principle of using thermal energy to store mechanical energy should be more broadly used to generate electrical power too-even some sort of gas bags expanding and compressing some mechanical items geared over toward generating power should work.

Maybe there are many more low-tech ideas for generating electrical power converting thermal into mechanical energy that could be used.