Improving Grid Stability under High Renewable Penetration: Optimal Flexibility Reserve Provision Using Short- and Long-Duration Energy Storage Technologies

The integration of high shares of renewable energy sources introduces variability and uncertainty that challenge the reliability of the power system. This paper investigates the role of short- and long-duration energy storage technologies in providing flexibility and reserve services under high renewable penetration scenarios. A co-optimisation modelling framework is developed in PLEXOS for a 23-node benchmark network representing future Gulf Cooperation Council system characteristics. The model simultaneously optimises capacity investment and dispatch while co-optimising energy, regulation, spinning, and inertia reserves. Technologies assessed include lithium-ion batteries, flow batteries, flywheels, compressed air energy storage, and hydrogen-based long-duration storage using proton exchange membrane electrolysers coupled with gas turbines. The results demonstrate that coordinated deployment of complementary storage technologies can maintain reliability targets while enabling very high renewable penetration. The framework also provides practical guidance for modelling energy-constrained resources in market-based simulations, supporting planners and system operators in designing resilient, low-carbon power systems.