We had a thread recently about a NZ-built ac generator which used the waste heat from the engine for space heating. It had a natural or propane gas fuelled Stirling hot-air engine, I think, and fed excess power to the Grid. I believe it fitted into the airing cupboard.
Now, provided the engine has a nice 'clean' burn, so as not to waste gas, and the exhaust gasses heat can be captured efficiently and not lost up the flue, the actual thermodynamics of the engine are irrelevant [within limits]. The overall efficiency has to be better than most power-stations since there, the waste heat is mostly lost to the atmosphere in the cooling towers. It all depends though on the price the poco pay you for your excess juice.
So, there may just be free lunch after all!
Alan
