Thank you for a post that actually talks about some thermodynamic principles. Tapping into "waste" heat does seem like an attractive idea to people who do not have an understanding of thermodynamics. My understanding is that if you try to simply strap on another heat engine like a thermocouple, you're working with a very low temperature differential which means low efficiency.
One question though. Isn't a gas turbine just another heat engine that that is governmed by the limits of any thermodynamic cycle? So would a "combined cycle" be two heat engines connected to each other? Unfortuantely my understanding of thermo is limited to one undergraduate class.