Impact of start-up mode on flexible power plant operation and system cost

Due to the increase of variable renewable energy sources (RES) such as wind and solar power, cycling of thermal power plants is gaining importance. The short-term costs of cycling are well understood and easily taken into account when making scheduling decisions. The long-term costs, which consist primarily of maintenance, however, are both difficult to accurately determine and account for at the scheduling stage. As these long-term costs are rising as a result of increasingly flexible operating regimes, operators should consider them in order to reduce total generation costs. Furthermore, the accrued damage and resulting maintenance costs due to cycling are greatly affected by the required flexibility in terms of the start-up mode (FAST or SLOW). In this paper, a unit commitment model that accounts for long-term maintenance costs is set up and used to assess the impact of slow and fast starts on the operational regime of combined-cycle gas turbines (CCGTs) and total system cost components in a case study. The simulation results show that the CCGT units adopt a high-cycling regime with increasing RES penetration in both the fast and the slow starting scenarios. Furthermore, the total system cost in the fast scenario is slightly higher, but the trend exhibits a turning point, which shows the interplay between flexibility and maintenance costs depending on the amount of RES.