Exhaust from a gas turbine for a pipeline compression station is around 600C at about 95kg/second. That flow contains about 100MW of heat energy, which can be captured with Organic Rankine Cycle (like steam cycle, but using organic fluids like cyclopentane as a working fluid to better match the thermodynamics) about 20% of that can be converted to electricity. This is off-the-shelf tech.
The problem isn't the technology, it is the economics.
The parent was pointing out that the economics are also a fundamental limit of the technology/physics.
If you can do a lot of work with expensive machines to get 20MW from that plant, but you can also do less work and spend less money getting an extra 20MW by burning some cheap primary fuel (with higher quality heat/aka a bigger delta), then they’re just going to burn more primary fuel. It’s a bit silly to do it any other way (barring legislation or market pressures or whatever) if you care about the amount of energy you are getting for the money you are paying.
And that is a fundamental issue, as reclaiming energy from secondary heat is always going to be less ‘nice’ than from the primary fuel.
If you can do it easily enough that it doesn’t add a lot of extra cost, then chances are the primary turbine/power system could be built efficiently enough to not throw that waste heat out the back in the first place. They do so because the math doesn’t check out generally.
This effect goes back to the steam engine era. Steam engines were sometimes built with multiple cylinders, each successive cylinder larger, and running at lower pressure, than the previous cylinder. "Triple expansion" seemed to be the upper limit of cost-effectiveness. Some quadruple expansion engines were used successfully, but not many. At least one quintuple expansion engine was built, and used in a Norwegian fishing boat.[1]
> > gas turbine for a pipeline compression station
> primary turbine/power system could be built efficiently enough to not throw that waste heat out the back in the first place
It sounds like this might be a situation where maybe an older turbine already exists for generating mechanical power (pumping), and the add-on kit is supposed to capture more energy without having to replace the existing systems?
But it doesn’t pay for itself is the problem - and I’m pointing out that the prior poster was pointing to a fundamental reason why that is not likely to change if the input fuel is cheap.
If the input is expensive, then the economics change and it’s more worthwhile to pay more in equipment to get more out of the input.
I bet someone has a really detailed set of calculations that would tell you exactly when that line is crossed. But I doubt they are hanging out here.
In that context think of this this way, if cost of converting methane into heat is 1 currency unit per joule of heat produced, fusion is hundred times as expensive but produces a million times more heat so the cost per joule is one ten thousandth of the cost from methane.
The problem isn't the technology, it is the economics.