The power output of a gas turbine without inlet air temperature cooling or conditioning suffers at high ambient temperature owing to the effect of lower air density, which in turn reduces the mass flow of air. The power output could drop by as much as 15-25% between the coldest and hottest temperatures. The exhaust gas flow, temperature, and gas analysis also vary with ambient temperature, which affects the HRSG performance. Table 1.3 shows the data for a typical LM 5000 gas turbine.

Naturally, the performance of an unfired HRSG behind the gas turbine would be affected by the changes in exhaust gas flow and temperature. Using the ‘‘HRSGS’’ program (see Chap. 2), one can evaluate the HRSG performance under varying ambient conditions; the results are shown in Fig. 1.11. One can see the large variation in the HRSG performance between summer and winter months. In order to minimize the effect of ambient temperature on power output, several methods are resorted to, such as the use of evaporative coolers, mechanical chillers, absorption chillers, and thermal storage systems as discussed above.