Paula McMullan (asker) Jan 3, 2020:

More info has come out of the equipment lease I'm just finishing off this mammoth project and will post an update tomorrow. There was a lot more info in the equipment lease and I found the manufacturer's marketing blurb which explains a lot more.

Daryo Dec 29, 2019:

That's what the text says source of energy for the electricity generator:
- reciprocating engines, dual fuel
- possible addition of a steam turbine

They might have found some new ways of doing it, but I can't see what else could the ST mean.

But one way or another, to go back to the initial question, "moteurs alternatifs" are "reciprocating engines" - what else?

Tony M Dec 29, 2019:

@ Daryo Having done a great deal of work over many years with these industrial-scale generating plants, I can assure you that the situation is one whole lot more complicated than that — and if different drive motor technology is used, there are even greater complexities, like optimization of the speed control loops, for one!

Daryo Dec 29, 2019:

Just a variant of the most ordinary "clutch" would do the trick. One engine stays uncoupled to the generator and kicks in and gets coupled to the generator only when and if needed (while the other engine gets uncoupled). Nothing unfeasible.

Maybe no one has done it so far, but it seems to me quite a good solution.

These diesel powered generators are likely to be for "internal use" - for that they would make sense. In any electricity producing plant you need some "internal" source of power to start the big generators ... and when they are off.

Tony M Dec 29, 2019:

@ Daryo It is highly unusual, since there'd have to be a way of uncoupling the 'spare' motor to avoid an unaccepetably large mechancial load; but more importantly, the use of reciprocating engines is rare in power plants, for a number of perfectly valid technical reasons.
In any case, Asker has now confirmed this is for a 'combined cycle' plant, so I think that rules them out even more...

Daryo Dec 29, 2019:

@ Tony M the idea of having two engines hooked to the same generator of electricity might be unusual, but it doesn't sound strange to me at all.

If only the engine fails, and not the electricity generator, it's enough to have another back-up engine to keep the generator running, costs far les than having another whole set engine+generator as a back-up.

BTW, these generators powered by diesel engines might be only for the internal needs of the power plant (autoconsommation), not for the grid.

Daryo Dec 29, 2019:

"alternatifs" is qualifying "moteurs".

It's an unfortunate coincidence that in this ST this engine is dual fuel - that's nothing more than a fat big XXXL red herring.

the "alternating" here is about the "alternating movement" of a clip / (le va-et-vient d'un piston).

If the "alternating" of the clip is done by steam coming from outside the chamber it's "un moteur alternatif à combustion externe"

If the clip is pushed by explosions inside the chamber, it's "un moteur alternatif à combustion interne" (petrol or diesel engine)

"alternatifs" is used to distinguish this type of engines from those where the energy is supplied by a rotating part - turbines.

"Reciprocating engine" is perfectly correct. https://en.wikipedia.org/wiki/Reciprocating_engine

Paula McMullan (asker) Dec 29, 2019:

@mrrafe That's really helpful. Yes, it's a combined cycle facility.

Paula McMullan (asker) Dec 29, 2019:

@Tony it's not helped by the fact that the ST has errors in it which makes me wonder how accurate the terminology actually is! I'm submitting a draft to the client tomorrow with comments...!

Tony M Dec 29, 2019:

@ Asker That's exactly what I initially thought: a reiprocating internal combustion engine (usually diesel) can indeed be used to run a generator, though usually only smaller ones, not the size you get in a power plant.
But then as soon as we start talking about turbines, that idea goes out of the window, since only in the most bizarre scenario might we imagine both together! Your wider context might give some clues...
It's perfectly possible that an internal combustion engine might work on either HFO or gas, but it's not clear where the steam turbine might get involved? Though I'm not sure you could run a turbin on HFO (!!), so that might be being used in a boiler to produce the stem.
I note that there appears to be only one 'generating set', yet they talk about 'moteurs' in the plural. I hope these are all points that are obvious from your overall context, but remain a mystery to me taken in isolation.

mrrafe Dec 29, 2019:

alternatifs In USA I think the "set" is what energy litigators (not engineer here, sorry) back in the day would call a dual fuel or combined cycle generator, or combustion turbine (CT). The alternative motors are either the choice between with/without steam turbine (vapeur) depending on which fuel. The point of the CT is to have a "peaker" generating plant that can be fired up relatively fast with gas, for the sake of more spontaneity in response to demand fluctuations albeit at higher cost than a "baseload" generator which needs more startup time but is the cheapest alternative for prolonged operation because all other fuels are cheaper than gas.

Paula McMullan (asker) Dec 29, 2019:

Hi Tony, I found a translation of à moteurs alternatifs as "reciprocating engines", but that doesn't make a great deal of sense to me as I'm not familiar with "engines" being used in the context of a power plant.

Tony M Dec 29, 2019:

@ Asker 'set de production d'électricité' = 'generating set'

As for the 'moteurs alternatifs', do they simply mean that it can be driven by various different types of motor?