There are already a few people who are heating their houses with their mining equipment.
You can't really use mining equipment heat to power any turbines (to make more electricity) the heat is too low of grade. However, if you find a substance that boils at the CPU temperatures and recombines in air temperatures, you can use that to make all kinds of electricity. Just got to find it.
In the future, just about everyone will be mining bitcoins. And not on special devices, but on EVERY device. It will be like gold prospecting. People will be doing it so that they will have something that farmers will exchange for food.
Electric motors are not 100% efficient in the manner you said. There is loss in magnetics. Electric motors spin at 1750 rpms as opposed to 1800 rpm like the frequency provided. That loss is called slip. And some electricity disappears.
Of course you are correct that an electric heater is 100% efficient.
Of course, of course, the burning of the coal for heat is far better than burning it for heat -> electricity -> heat.
And far better renewable energy is on the horizon, as you say.
RE: Bitcoin is the Alpha; Bitcoin is the Omega.