Optimizing renewable energy integration pathways: Inter-regional coordination and storage in India's power sector

India's rapid economic growth and ambitious renewable energy (RE) targets require modeling frameworks capable of capturing the complexities of high RE integration. This study develops a capacity-expansion and dispatch model with a 15-min resolution for India's Western (WR) and Southern (SR) Regions, which together represent over half of the nation's electricity demand, covering 592 million people and 59 % of India's Gross Domestic Product. We examine multiple scenarios for RE integration, including the operationalization of India's policy for Renewable Purchase Obligations (RPO). The analysis considers capacity constraints on coal and hydro power, the role of battery storage, and two RPO compliance frameworks: one demanding each region individually meet its RE requirement (layered) and another allowing the WR + SR system to fulfill the target collectively (centralized). Results show that regional and inter-regional coordination reduces system costs and boosts RE penetration compared to purely state-based dispatch. Limiting coal or hydro expansions to pipeline projects increases reliance on battery storage, raising costs under current assumptions. When solar is paired with large-scale storage, RE shares often exceed mandated levels but with higher investment. Under layered compliance, each region expands RE more evenly but may incur larger costs. Centralized compliance concentrates capacity in areas with lower costs yet can introduce equity concerns in some regions. Overall, the findings underscore the importance of coordinated resource planning, robust transmission infrastructure, and adaptable policy mechanisms to meet India's climate objectives. By tailoring RPO strategies to regional resource endowments, India can move toward a cleaner and more reliable power system.