These things are engineering marvels as fuel-burning engines go. From what I know, there is actually not that much that can be done to make gas turbines more efficient if weight is no object. Fuel weight and cost are very important for commercial turbofans, and engine manufacturers have been doing a huge amount of R&D in efficiency and are using some very advanced manufacturing techniques for certain parts. Turbine inlet temperatures are well into the white-hot range. A mechanical engineer described it to me as "A hundred degrees hotter and they'd melt and spill their internals out the back" - that is with exotic metals and active cooling. The main problem with these turbines is that they are somewhat old and will probably run at less than maximum power for longevity reasons. Turbines are terrible at less than full power efficiency. They also won't have a steam turbine added to use the exhaust heat like with stationary engines.

According to this article, the best current commercial turbofans can reach over 50% thermodynamic efficiency. I remember thinking that 50% seem like an unreachable holy grail when first reading about thermodynamic efficiency of common engines. https://nap.nationalacademies.org/read/23490/chapter/6#36

The Carnot limit pins maximum theoretical efficiency of a thermal engine at 50%. It's a limit of thermodynamics.

...more or less. There have been outliers.

It does say they replace the power turbine part with one designed to extract shaft horsepower. Given that the hot end is the difficult and expensive part that probably has loads of useful life left long after it's aged out of being usable in an aircraft, sticking a new power turbine on isn't the worst idea you could have.

Of course "jet" aircraft are mostly using the engine to produce shaft power to run a ducted fan anyway.

True, the turbines they're retrofitting are cores from high bypass turbofans. As far as I've managed to figure, they output somewhere in the neighborhood of 75% of their power through the shaft when in a turbofan engine.

However, these cores are designed to spin very fast, not at grid speed, so these need some form of gearbox or electronic frequency conversion.

The PE6000 and the LM6000 are two shaft machines, where there is a low speed shaft on which sits the low pressure compressor and low pressure turbine (the hot and cold ends), and a high speed shaft on which sits the high pressure compressor and high pressure turbine. The two shafts are concentrically located, with the high speed shaft being on the outside. The low speed shaft is where the generator is coupled, and can be done on either the hot or cold end.

You're right that the core doesn't spin at synchronous speed but the LP shaft does. It's optimized for 3600rpm, but could run at other speeds... the machine just isn't designed for it. The LM6000 only uses a gearbox for 50Hz units while 60Hz don't need it.

> However, these cores are designed to spin very fast, not at grid speed, so these need some form of gearbox or electronic frequency conversion.

That's true of any gas turbine engine. About the only engines you can use to drive a genny directly are diesels at 1500rpm for 4-pole machines or petrols at 3000rpm for 2-pole machines (or if you're in one of the tiny handful of countries in the world that uses 60Hz, 1800rpm or 3600rpm).

You can of course run these on alternative fuels like waste veg oil for diesels or propane instead of petrol, and the latter used to be quite common on remote farms where you'd have a big propane tank for heating and cooking - may as well run the generator on it too, and not have to worry about exhaust fumes.

> That's true of any gas turbine engine.

I don't believe that's right. Near as I can tell, the large turbines built for power generation, like the GE 9HA spin at 3000 or 3600 RPM. That's how they get efficiencies north of 60%.

Yeah, I suppose you can design the power turbine for lower speeds, I hadn't thought of that.

geared turbofans are a thing, but fairly new.