How does altitude affect the performance of a gas turbine in a combined cycle power plant, and why?

Altitude negatively affects the performance of a gas turbine in a combined cycle power plant primarily because the air density decreases with increasing altitude. Gas turbines rely on the mass flow rate of air through the compressor to generate power. At higher altitudes, the lower air density means that less air mass enters the compressor for the same volume, reducing the amount of oxygen available for combustion. This results in a lower power output from the gas turbine. The reduced air mass flow also affects the pressure ratio across the compressor, which can decrease turbine efficiency. Furthermore, the lower ambient temperature typically associated with higher altitudes can also reduce the density of the air entering the compressor, further exacerbating the power loss. The overall effect is a reduction in both the power output and efficiency of the gas turbine, which translates to a lower overall combined cycle power plant output. Therefore, gas turbine performance is significantly derated for higher altitude operation.