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I am a non-engineer and I am using a gas power plant data base. Some plants listed are of the type "combustion engine", some of the type "gas turbine", some "steam turbine" and some "combined cycle". I believe to understand that combined cycle is a combination of combustion engine and steam turbine. But what is the difference between combustion engine and gas turbine then? Or does it mean the same?

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    $\begingroup$ Not really an electronics question; Combustion engine probably internal combustion engine: wartsila.com/energy/learning-center/technical-comparisons/…. Not a turbine. CCGT has a gas turbine, then drives a steam turbine with heat from its exhaust. $\endgroup$ – pjc50 Jun 6 '17 at 12:35
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    $\begingroup$ OK thanks! Will move question to "engineering" forum I guess... $\endgroup$ – LenaH Jun 6 '17 at 12:39
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    $\begingroup$ I'm voting to close this question as off-topic because it is nothing to do with electronics. $\endgroup$ – Leon Heller Jun 6 '17 at 12:53
  • $\begingroup$ Did you try looking up the terms in WIkipedia? $\endgroup$ – Carl Witthoft Jun 6 '17 at 14:08
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Combustion engine - an internal combustion engine like a car motor, with pistons moved in cycles. It's inefficient but can be easily scaled down to almost arbitrarily small sizes; usually used as a backup.

Gas turbine - in power plants, these are very similar in construction to jet engines, where gas is the fuel - a multi-stage turbine compressor, a turbine on exhaust, high RPM; the torque produced is used to run the generator. It's not as efficient as steam or combined cycle, though more than combustion engine - but the power output can be rapidly tuned to needs, providing a response to changing demand (which is typical for the "more inert" types that use steam.)

Steam turbine - gas heats water in a boiler; overheated steam runs through turbines, then is cooled. This is the same principle as great most other thermal power plants (coal, nuclear, geothermal). It's usually a large installation and may take hours to get up to speed (so no rapid response to demand + waste of energy as demand rapidly vanishes) but it has a very good efficiency.

Combined cycle - exhaust from gas turbine (the "jet engine") is used to heat water into steam and run a steam turbine. Better efficiency than both above, and provides the much desired rapid response. Of course cost of construction is similar to sum of costs of construction of the two, and maintenance is more complex, but the operational costs are reduced.

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  • $\begingroup$ Great, thank you. Now I know for sure it's not an error. I have one follow-up question. Why is it that in this document (diw.de/documents/publikationen/73/diw_01.c.424566.de/…) the efficiency, start-up time and ramping gradient of a steam turbine are comparable to gas turbine? (You implied that efficiency should be higher while ramping is slower...?) $\endgroup$ – LenaH Jun 6 '17 at 16:42
  • $\begingroup$ @LenaH: Per turbine, yes. Per boiler, no. Starting the furnace and building pressure up can take hours, depending on the installation. Once you have the steam pressurized and open the valve, time to spin the turbine up is comparable. Same about shut-down, shut the valve and the turbine spins down rapidly... meanwhile hot steam accumulated in the boiler cools down, loses pressure all its energy is wasted uselessly. Not sure why efficiency is comparable - maybe gas turbines got better since last I learned about them. $\endgroup$ – SF. Jun 6 '17 at 17:04
  • $\begingroup$ Hmmm... For a steam turbine the cold start time might be comparable to a combined cycle plant (2-5 h)? $\endgroup$ – LenaH Jun 6 '17 at 18:00
  • $\begingroup$ @LenaH: Combined cycle plant reaches the full capacity of 'gas turbine' part rapidly, but may take more time for the steam part to reach full capacity on the steam side. Since the rapid demand changes are hardly ever of order as big as 50%, the variance provided by the gas turbine part is quite sufficient to satisfy them. Cold start is rare enough it's only an issue for backup power applications, but spikes in demand, like a factory starting up, require a rapid response to prevent brown-outs. $\endgroup$ – SF. Jun 6 '17 at 18:26
  • $\begingroup$ You also shouldn't make rapid changes in steam pressure (to be more precise: temperature) as it places stress on the boiler. At Fukushima they were trying to slow the cooldown of the reactor by cycling the isolation condenser on and off. When TWHTF, they did not know if they had left it "on" or "off", so they went outside and looked at the steam vent, and out of inexperience misread it. So they assumed they had 8 hours they did not have. But for that, they had a good plan, and would have saved the complex. $\endgroup$ – Harper - Reinstate Monica Jun 7 '17 at 19:46

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