Exergy analysis of the regulating measures of operational flexibility in supercritical coal-fired power plants during transient processes

Operational flexibility improvement of coal-fired power plants has become a priority objective with the high penetration of renewable energy sources. The regulating measures applied frequently in improving the operational flexibility indeed affect the exergy efficiency of coal-fired power units during transient processes. In this study, the exergy efficiency evaluation of operational flexibility measures during primary frequency control processes was focused on, and the exergy analysis methods of the thermal system were used on the basis of the second law of thermodynamics to analyze the mechanisms for improving the flexibility at four regulation measures, and the exergy storage conversion efficiency and exergy destruction accumulation were quantitatively compared. It turns out that: among the throttling extraction steam of high-pressure heaters, feedwater bypass of high-pressure heaters, and condensate water decrease of low-pressure heaters measures, the exergy storage conversion efficiency and the exergy destruction accumulation increase with the regulating time of primary frequency control. The measure of throttling extraction steam of high-pressure heaters is the most potential option for improving operational flexibility, with maximum value of the maximal output power increment and maximal exergy storage change rate of 71.11 MW and −89.84 MW, respectively. With respect to the economy, the measure of throttling extraction steam of low-pressure heaters is the best option due to its smallest exergy destruction accumulation among all the measures. This study provides a detailed guidance for the selection of the optimal regulating measures when considering the enhancement of exergy efficiency and flexibility comprehensively under different conditions and periods.