Thermal efficiency degredation

[StevenUcci]

Hello all,

I seem to have a problem understanding Thermal efficiency degradation.

My problem is that if you assume that you are dealing with an ideal simple brayton cycle that has a thermal efficiency of 30%.

however, if you change you assumptions for the turbine and compressor efficiencies from 100% to 75%, what would be the new thermal efficiency ?

will it be n < 20%

or it simply depends on the conditions of the cycle?

Your help will be much appriciated!

-Steven

 

[georgeverghese]

Assuming the gas turbine is a single shaft unit running on an open simple Brayton cycle, and if you are running at the design case speed, then decrease in overall thermal efficiency is usually related to decrease in compression efficiency and / or power turbine eff ( as you have indicated). Decrease in efficiency is always shown by high turbine exhaust temp. The max turbine fuel gas firing rate is high limited by this exhaust temperature by the GT manufacturer.

Other reasons for decrease in eff can be high inlet air temp, high air filter dp and high power turbine exit backpressure.

However, if this unit is running at off design case speed, then overall thermal eff will drop even when the GT is new ( and compressor and power turbine is not fouled and eff is high). The drop in eff will be less in a 2 shaft unit in comparison to a single shaft unit. Also, for a single shaft new unit running at fixed speed (as in power generation), thermal eff begins to drop off when required torque is less than 80% or so cf design case torque.

The higher the compression ratio, the higher will be the overall net thermal eff. Same effect/ trend for increasing power turbine inlet temp.

It is posssible to get an idea of the decrease in thermal eff if you can make the effort to run the calcs. The axial compressor / power turbine isentropic eff can both be assumed to be say 90%, with a compression ratio of say 10-15, and assuming air flow is adjusted to get to a final combustion temp of say 1000-1200degC, with a fuel gas LHV of 900btu/scf. Then run another case for decreased eff and see what the effect on net power output is.

Things you could do to recover efficiency are (a) cool the inlet air with an evaporative cooler if air RH is low (b)water wash the air compressor with atomised DI water (online or offline) (c)steam injection into combustion chamber (d)clean up the air filter or check if you have high PT exhaust backpressure. Else it may be time to call the GT area rep and arrange for a full rotor changeout.

Decrease in E for turbines as the turbine ages / fouls is to be expected, and adequate margins for this must be included when selecting the GT. Margin for fouling is usually in the 10-15% range. Other margins should also be included as necessary (for example off design speed operation, altitude, air temp higher than ISO etc)