Optimizing flexibility retrofits of China's coal fleet in a renewables-dominant future

Coal-fired power plants play a pivotal role in providing operational flexibility to power systems; however, as China transitions toward a high-renewable electricity mix, their limited adaptability increasingly constrains system reliability. Here, we develop a multi-objective optimization model to design retrofit strategies for coal-fired units under varying system minimum load capability targets, where flexibility is defined as the improvement in system minimum load capability, while simultaneously considering trade-offs in retrofit scale, investment cost, ramping capability, and coal consumption. The results show that, under regionally constrained planning, grid-level retrofitting can meet the short-term target of 30 GW additional minimum load reduction capacity with a total investment of 17.3 billion CNY, involving 193 units. This pathway improves system-wide ramping capability by 18.2% while increasing coal consumption by only 1.7%. As system requirements increase from 30 GW to approximately 270 GW, marginal retrofit costs rise from 554 to 648 CNY per kW (79–93 USD/kW), reflecting the transition from low-cost, widely applicable upgrades to more advanced retrofit configurations. Even at higher demand levels, retrofit costs remain substantially lower than those of alternative flexibility options such as battery storage (400–4000 USD/kW). These results indicate that retrofit-based flexibility can provide a cost-effective source of minimum load reduction and ramping support during periods of rapid renewable expansion. Overall, the findings provide a quantitative basis for designing coordinated retrofit strategies under different spatial planning structures, supporting a secure and economically efficient transition toward a low-carbon power system.