Let’s face it, we know the operations of our gas turbines can’t all be perfect, and we’ll run through calculations, feasibility studies and more to pinpoint the exact cause. But before all of that is accomplished, you should keep a list in the back of your mind of what might be causing your loss in performance, based on common factors that affect gas turbine efficiency and more.
Here’s the list.
1. Air temperature and site elevation
Since a gas turbine is an air-breathing engine, its performance is changed by anything that affects the density and/or mass flow of the air intake to the compressor. Ambient weather conditions are the most obvious changes from the pressure and temperature of reference, respectively 14.7psi/1.013bar and 59F/15C.
Since humid air is less dense than dry air this directly affects the performance of gas turbines. Therefore, for the same volume entering the gas turbine, reduced mass flow rate will be used, reducing the power output.
3. Inlet and exhaust losses
Inserting air filtration, silencing, evaporative coolers or chillers into the inlet or heat recovery devices in the exhaust causes pressure losses in the system.
Work from a gas turbine can be defined as the product of mass flow, heat energy in the combusted gas (Cp) and temperature differential across the turbine. The mass flow in this equation is the sum of compressor airflow and the injected fuel flow. The heat energy is a function of the elements in the fuel and the products of combustion. Therefore different fuels will hold different power outputs.
5. Fuel heating
Fuel heating will result in a slightly lower gas turbine output because of the incremental volume flow decrease. Since use of this energy in the gas turbine fuel heating system is thermodynamically advantageous, the combined cycle efficiency can be improved.
6. Air extraction
In some gas turbine applications, it may be desirable to extract air from the compressor (cooling for the turbine, pressurized air for the cabin in an airplane, etc.) Generally, up to 5% of the compressor airflow can be extracted from the compressor discharge casing without modification to casings or on-base piping. But as the air extraction increases, the power generated naturally decreases due to the limited mass flow rate going through the turbine.
7. Inlet cooling
Lowering the compressor inlet temperature can be accomplished by installing an evaporative cooler or inlet chiller in the inlet ducting downstream of the inlet filters. Careful application of these systems is necessary, as condensation or carryover of water can exacerbate compressor fouling and degrade performance.