# Combined cycle



## Saad85 (Sep 19, 2017)

What are the advantages and disadvantages of the combined gas and steam cycle?


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## polaz (Sep 19, 2017)

Combined cycle with steam using gas turbine exhaust can reach over 60% efficiency but take 30 minutes or more so start up. These are about 1200$ per KW. Several articles talk about more than 250 minutes to a cold start of a combine cycle plant to 100%.


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## Sheshtawy (Sep 19, 2017)

I agree with polaz, and I'd like to add the following

Advantages:
1. Increased efficiency (less fuel costs)
2. Less required cooling water
3. Better adaptability to load fluctuations
4. Although they are slower to start up than stand-alone gas turbine cycles, they're actually much faster than steam turbine cycles

Disadvantages:
1. Higher capital cost in the majority of cases (sometimes it can be actually cheaper to use a combined cycle than to use a steam cycle for the same capacity)
2. Adds more complexity to the system operation
3. Trouble in the first plant (gas turbine part) causes the entire plant to shutdown


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## Saad85 (Sep 20, 2017)

Thanks all


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## Philscrimp (Oct 30, 2017)

Without going into any details, THANK YOU for posting this...


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## A Random Guy (Nov 9, 2017)

polaz said:


> Combined cycle with steam using gas turbine exhaust can reach over 60% efficiency but take 30 minutes or more so start up. These are about 1200$ per KW. Several articles talk about more than 250 minutes to a cold start of a combine cycle plant to 100%.


$1200 per kW? Not sure where you get that number from. Simple cycle gas turbine (and recips plants) are at that or slightly above. I did development and execution  for a large utility on new CCGTs and we were around $850/kW on the large units (1 GW) and around $1000/kW on units half that size.


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