The Chouteau Power Plant has greater efficiency than a simple-cycle combustion turbine unit because it employs both a steam turbine and a combustion turbine to power the generator.

Chouteau features two heat-recovery steam generators (HRSGs), each measuring about 70 feet by 100 feet, that capture exhaust heat to power a steam turbine. In contrast, hot exhaust from the gas turbine is vented to the atmosphere on a simple-cycle plant.

At Chouteau, exhaust heat enters the HRSG, or boiler, at about 1,085 degrees Fahrenheit and moves through the structure, heating tubes of water to create steam to power the steam turbine, which turns the generator to produce electricity. Afterward, the exhaust is vented from the stack at about 200 degrees.

This heat-recovery system increases the efficiency of the unit to 58 percent, compared with 33 percent efficiency of a simple-cycle plant.

The second cycle receives 67% of the initial energy and has a hot temperature of 1287K and a cold temperature around the boiling point of water (as low as you can get if you use steam), that is, 212F = 400K. The theoretical maximum efficiency would allow it to extract 69% of this 67%, leaving only 21% of the initial energy and for a total efficiency of 79%. So, the achieved 58% efficiency is at most 73% of what is thermodynamically achievable.

Of course, the boiling-point stem that comes out of the power plant could be directed to some industrial use that only requires boiling water, or even to heat homes or heat water for homes, for reduced heat losses.

But the absolute maximum efficiency with an initial hot temperature of nearly 2000K and a final cold temperature of 300K, where the end result is water at room temperature, is about 85%. It's possible that with gas you cannot get any higher than that in any case, and 58% is 68% of that.

It's not about energy, it's about free energy (in fact, the Gibbs Free Energy as the inputs and outputs not only happen at room temperature but also at atmospheric pressure).

We have met the enemy, and he is us — Pogo

Are these percentages relative to the maximum thermodynamic efficiency of the processes we're talking about, or does 100% represent the unattainable situation in which all energy is converted to work with no heat loss?