Today there are working motors on the scale of 1000 nm. Image a nanogenerator that uses the kinetic energy fo nitrogen gas as an energy source.
It could be a simple paddle with a transverse axis. As the paddle is wacked back and forth, it moves a magnet that causes a current to flow in a nanowire.

Nitrogen composes about 80% of our air so we will just concern ourselves with nitrogen gas here. A single nitrogen molecule has a mass of 4.68 x 10^{-26} kilograms.
In common air, at 68F, the average velocity is 506 m/s and it has 6.1 x 10^{-21} Joules of kinetic energy.

Roughly, 15,000 joules per kg of nitrogen gas, if we could get 1/100 of it per second we would have 150 watts of power. If we used 1000 atoms(of about the same weight
as nitrogen) per nanogenerator. Our 150 watt generator would weigh a ton. So, that is ok.

Assuming that we could make the nanogenerator, the obvious problem is making 10^{23} copies of it. I have heard of the
idea of nanobots making copies of themselves. If each nanobot could copy itself in one day, then we would have 2^{365}
in a year. So, if we can ever get self-replicating construction nanobots, game on.

Gas Ions

Put a big charge on a smoke particle and think of brownian motion. Charged particle cages whose
movement is choked and energy captured by electromagnetic interaction? There are over a billion transistors
and 14nm technology chips today. Again, if it is possible it will be about massive nano-copying.
But, it may power nano-devices today.

Radiation

Imagine an isolated system at 20C. There will blackbody radiation inside of the box. A blackbody absorbs
all radiation incident on its surface and emits radiation based on its temperature. Blackbodies derive their
name from the fact that, if they do not emit radiation in the visible range, they appear black due to the
complete absorption of all wavelengths.

On a hot day, have you ever noticed the difference in temperature between a white car and a black car.
I just searched and found results of one test and they report a difference of 65F. We could easily
run a stirling engine with temperature differential. So, is it possible to coat the walls of our isolated
system at 20C to have differing temperatures? Enough of a difference to run a stirling engine?

I learned about a photoswitch-based solar thermal fuel. Is it possible to make something similiar
for the radiation distribution at 20C? If so, that would be another single-heat motor.