The flexibility dilemma―managing cost and emissions in a fossil fuel-dominated power sector under increasing penetration of variable renewable energy sources

The limited flexibility of thermal power plants (TPP) is crucial when assessing the transition to higher penetration of variable renewable energy resources (VRES). This study develops an enhanced TIMES-based power-sector optimisation framework that integrates unit-level technical and operational constraints into a multi-stage capacity expansion model with hourly dispatch resolution. This approach endogenises TPP-specific dynamics—including minimum load, ramp rates, start-up/shut-down behaviour, minimum on/off duration, and part-load efficiency variation—features that are typically simplified or omitted in conventional long-term energy system models. The cost-minimising model is applied to India's power sector, beginning from 2020 until 2032, to assess the trade-off between system cost, CO₂ emissions, and flexibility under alternative coal retirement and retrofitting scenarios. The results show that disregarding these constraints leads to overestimating the potential of TPPs to supply peak demand and underestimating the role of energy storage. Extreme strategies, such as a complete phase-down of coal TPPs or not retiring old TPPs altogether, might not be either economically or environmentally beneficial. Achieving higher flexibility for the present Indian coal TPP fleet would offset the requirement for energy storage and reduce renewable energy curtailment, but would marginally increase CO₂ emissions. It further highlights that meeting India's existing NDC will require around USD 127 billion in annual spending. The proposed unified modelling framework will provide a replicable tool for integrating dispatch realism into long-term energy system planning, enabling system planners in other similar fossil-dominated regions to investigate future cost-effective and emission mitigation pathways for the power sector.