Energy and environmental footprints of flywheels for utility-scale energy storage applications

Abstract

Flywheel energy storage systems are feasible for short-duration applications, which are crucial for the reliability of an electrical grid with large renewable energy penetration. Flywheel energy storage system use is increasing, which has encouraged research in design improvement, performance optimization, and cost analysis. However, the system's environmental impacts for utility applications have not been widely studied. Evaluating the life cycle environmental performance of a flywheel energy storage system helps to identify the hotspots to make informed decisions in improving its sustainability; to make reasonable comparisons with other energy storage technologies, such as pumped hydro, compressed air, electro-chemical batteries, and thermal; and to formulate environmental policy in the energy sector. In this study, an engineering principles-based model was developed to size the components and to determine the net energy ratio and life cycle greenhouse gas emissions of two configurations of flywheel energy storage: steel rotor flywheel and composite rotor flywheel. The net energy ratio is a ratio of total energy output to the total non-renewable energy input over the life cycle of a system. Steel rotor and composite rotor flywheel energy storage systems were assessed for a capacity of 20 MW for short-duration utility applications. A consistent system boundary was considered for both systems with the life cycle stages of material production, operation, transportation, and end-of-life. Electricity from solar and wind was considered separately in the operation phase. The net energy ratios of the steel rotor and composite rotor flywheel energy storage systems are 2.5–3.5 and 2.7–3.8, respectively. The corresponding life cycle greenhouse gas emissions are 75.2–121.4 kg-CO2eq/MWh and 48.9–95.0 kg-CO2eq/MWh, depending on the electricity source. The net energy ratio and greenhouse gas emissions are highly influenced by the operation phase, which includes charging and standby modes. Uncertainty analysis shows that the life cycle greenhouse gas emissions are most sensitive to the solar, wind, and grid electricity mix emission factors. The results of this study will help to understand the carbon footprints of the various utility-scale energy storage systems.