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Combined Cycle Plant Efficiency

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vtpwringeniero

Electrical
Sep 5, 2001
6
Hello all,

I'm trying to get some idea of the electrical efficiencies that one might realistically expect to achieve with combined cycle plants consisting of two GT's and one steam turbine and, having an overall capacity rating of 60 MW or less.

Has anyone had any practical operating experience with such a system who could help me out?


Best Regards,
vtpwringeniero
 
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You have several questions in one.
Electrical efficiencies are given by the mfg. of the equipment.
However if you mean plant efficiencies that is totally different.
Which do you want?
 
Well normally you'ld get a 50% increase in the total output of the plant. Like if you have two units of 10 MW each, 50% of their combined output comes to 10 MW. So if you install a steam power turbine as a combined cycle plant on these two gas turbines your out put would increase from 20 MW to 30MW. I hope this helps.
 

I'm looking for efficiencies based on electrical output and heat input and, a measure of efficiency based on electrical output + heat for process as compared to heat input.

One formula that I've come across is:

% eff = [{(Rated kW)(3412BTU/kWh) + BTU/hr recovered for process)}/Heat input in BTU/hr] x 100%.

We have been told of electrical efficiencies only, reaching 60% and efficiencies based on electricity and supplimental heat available for process reaching 90%. Intuitively, since a gas turbine operating on simple cycle has an efficiency of only 25% or so, these values seem excessive. However, I've had no experience with combined cycle plants to know for sure.
 
Max thermal efficiency of combined cycle plant natural gas fired could reach 55 % at base-loaded. Conventional fossil plant efficiency reaches an average of 35%. If the plant operates as a cogeneration with a host consumer of steam the overall efficiency could substantially improved.

Low ambient temperature in winter help to increase the efficiency. Operation altitude significantly above sea level and less than full gas turbine output produces lower efficiency.

Efficiency also could be reduced over time, around 3% over time requiring maintenance or overhaul periodically. There are small none recovered efficiency.




 
Combined cycle plant efficiencies are best shown in heat balance diagrams, which indicate the losses, fuel heat rates, temperatures etc
 

Thanks to all who replied. You've helped.

Best Regards,
vtpwringeniero
 
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