Reserve provision in the optimal planning of off-grid power systems: impact of storage and renewable energy

The increasing penetration of variable renewable energy sources is progressively eroding the ability of conventional power plants to ensure grid stability. Alternative assets, including storage and control systems, are required to fill the gap and maintain the system stable, but all contributions should be previously analysed and studied by means of energy models. However, the modelling of operational constraints such as reserve requirements leads to increased computational burden, especially when employing long-term planning methods with unit-commitment needs. Therefore, planning tools often approximate or neglect reserve needs, leading to major sizing errors. This paper presents a novel model and policy recommendations for integrating short-term aspects and power reserve requirements in the planning of off-grid microgrids. The developed framework includes the formulation of an accurate cost and unit commitment model for fuel-fired generators, the formalisation of power reserve requirements and the representation of the contribution of storage and non-dispatchable technologies to power reserve. The mathematical formulation of the different assets is explored, also investigating the mutual influence of the modeled phenomena. The approach is implemented on a Mediterranean non-interconnected island. The results show acceptable trade-off between computation time and accuracy in one-year hourly simulations. Evidence demonstrates that ignoring the reserve requirements may lead to a 30% misjudgment of costs and an underestimation of the necessary reserve requirements. Enabling storage to provide reserve significantly reduces overall system costs (up to −20%), decreases fuel consumption (−35%), and improves resilience, hence suggesting the pivotal role of storage in providing reserve.