Energy storage (ES) systems are capable of providing high-quality accurate services such as frequency regulation, peak shaving, and stability enhancement in power systems. The federal energy regulatory commission orders 755 and 784 pave a road for ES systems in providing such services in a competitive manner. In developing ES solutions for grid applications, often single-technology solutions are considered. Under certain conditions, a hybrid ES system (HESS) comprising different technologies might best provide expected services at the least annualized capital cost; an individual technology seldom provides all the desired characteristics at the least cost. In this work, an HESS is optimally designed at grid-scale for a desired performance at the least annualized capital cost. The design is constrained by requirements of the system and characteristics of an individual ES technology. The proposed HESS combines storage units based on lithium-ion batteries, flywheels, and ultracapacitors. A synthetic data set and Ontario power grid data are considered as candidate case studies seeking ES solutions. The proposed methodology optimally sizes HESS providing minimum cost. It is undoubtedly shown that in certain situations, HESS provides the least annualized costs in comparison to systems constructed out of a single technology.
