Abstract
CRIEPI is developing a methodology for evaluating the contribution of operational flexibility of thermal power plants to the stability of the electric power supply as the share of intermittent renewable energy sources increases. This paper calculates the thermal efficiency during flexible operation of a 700 MW class ultra-supercritical coal-fired power plant. Coal-fired power plants generally need a longer time to change the flow rate of pulverized coal when their output power changes, which is responsible for the time lag in the load change of the plants. In order to minimize the time lag, the throttle valve of a high-pressure steam turbine needs to be partially closed and rapidly controlled to provide speed governing and load frequency control operation for stabilizing the frequency of the power grid. In this operation, the thermal efficiency of the power plant decreases owing to the steam pressure drop of the throttle valve in the high-pressure steam turbine. The thermal efficiency of the coal-fired power plant during partial load and partially closed operation of the throttling valve in a high-pressure turbine is calculated using a material and heat balance calculation method. The decrease in the efficiency is responsible for the cost increase in backup operation for the intermittency of renewable energies.
