But the problem is that cards will not work in high temperatures and there is no way of heating something up to a higher temperature than the source temp without wasting a lot of energy.
You'd be better off building you own coal power plant (or whatever fuel you can get cheaply)
And any heat engine is subject to the calculations I posted earlier. Do not forget that you will need to cool the "cold" side too, so you may end up with all electricity produced going to the fans that cool your engine.
For example, in 25C room temperature, 80C hot side temperature and 1kW heat input, you will get 81W of electricity, provided you can remove the 919W of heat from the cold side of the engine and make it as close to room temperature (25C) as possible, so within a few degrees, or you will get even less power.
Now, if you had a vacuum tube transmitter, with tubes that can be allowed to heat up to, say, 250C, you would get more power, about 245W from 1kW, again, this excludes the power needed to cool the cold side.