Comment by slow_typist
2 hours ago
Wouldn’t a Carnot engine with 33% efficiency produce 2 J of heat per 1 J of mechanical work? With 40% efficiency 1.5 J of heat? Efficiency of the generator should be better than 90 % so I think the 3x estimate is a bit off.
The heat doesn't disappear. Useful work is heat, it's just that some of it is intercepted before final dissipation.
If you look at real-world numbers, you're generally going to see 3x the thermal output as electrical output from any thermal energy plant. Two-thirds of that thermal output is wasted, and you'd see your 2x figure there. But the useful electrical output eventually ends up as heat as well, whether in direct thermal applications, from mechanical applications, lighting, refrigeration, audio equipment, or electronics.
The (admittedly theoretical) numbers in this example show that:
<https://energyeducation.ca/encyclopedia/Megawatts_thermal>
Incidentally: this all-but-inevitably leads to an online hand-wringing about the inefficiency of energy systems when an energy flow chart (Sankey diagram) is released, showing a 2:1 "rejected energy" ratio. It turns out that that's not a measured quantity but a modeled quantity, if you read the fine print. The 2/3 loss is just physics, thermdynamics and Carnot as noted previously.
See for example the LLNL (Lawrence Livermore National Labs) energy flow chart diagrams, here for 2023: <https://flowcharts.llnl.gov/sites/flowcharts/files/2024-12/e...>
Some of those qualifications are more legible in the fine print of the PDF: <https://flowcharts.llnl.gov/sites/flowcharts/files/2024-10/e...> (PDF).