Thermodynamic performance analysis of steam power plants during deep peak shaving processes: Integrating a novel top turbine system in ultra-low loads

High-penetration of renewable energy with intermittent nature poses great challenges to safety and stability of the power system. Steam power plants (SPPs), as the main regulation resource for operational flexibility, are frequently required to operate at ultra-low loads (lower than 30 % load) to meet grid requirements, which results in thermal efficiency reduction, higher generation costs, and increased greenhouse gas emissions. To address these problems, a novel top turbine system integrated into SPPs was proposed, and corresponding models were developed to evaluate system performance at all loads, especially for the ultra-low loads. The results reveal that integrating the top turbine system can enhance power generation efficiency at ultra-low loads, and the lower the design flow rate of the top turbine, the more pronounced the efficiency improvement. For a 350 MW supercritical SPP, to enhance efficiency at 20 % load, the optimized high-pressure top turbine (HPTT) design steam flow rate, inlet, and outlet pressures are 90 kg s−1, 24.2 MPa, and 9.79 MPa, respectively. Following these optimizations, power generation efficiency increases from 30.6 % to 35.2 %, accompanied by rises in live steam pressure of 10.28 MPa, reheat steam pressure of 1.74 MPa, and feedwater temperature of 78.2 °C.