Optimal gas turbine inlet temperature for cyclic operation

Historically, the development of gas turbine technologies is intrinsically linked with the increase in the initial temperature of the working fluid, which provides an increase in the thermal efficiency and thermodynamic efficiency of the gas turbine cycle. The increase in the gas turbine initial temperature leads to the unit costs reduction and the unit capacity is increased. However, higher turbine inlet parameters increase significantly thermal stresses in the metal, especially during a start-stop operation. In this study we compared the efficiency of three gas turbines with different values of the turbine inlet temperature during covering different parts of the daily electric load curve. The sum of fuel and depreciation costs is chosen as an optimization criterion. Service life parameters were determined for the most thermal stressed gas turbine element, namely the first stage gas turbine blades. To implement the low-cycle fatigue analysis, the numerical simulations were implemented using ANSYS®. The modelling was divided in two stages. Firstly, the fluid dynamics behaviour was analysed around the blade cascade with the aim to determine the thermal state during the whole startstop cycle. In the second stage of calculation, the mechanical loads were added caused by centrifugal force and the fluid flow forces from the working fluid side. To determine depreciation costs, the modified equivalent operating hours principle was used. Obtained results showed that for cyclic operation mode with a high number of start-ups and shutdowns, the costs associated with service life reduction exceed markedly the fuel economy, making turbines with lower turbine inlet temperature more profitable.