Progress in renewable power exploitation: reversible solid oxide cells-flywheel hybrid storage systems to enhance flexibility in micro-grids management

In energy assets governed by decentralized generation from renewables and variable power sources, energy storage (ES) plays a crucial role to address the need for additional capacity and flexibility, as well as to increase arbitrage. Renewables management introduces two main missions for energy storage: a long-term storage to extend storage capacity and a short-term storage to deal with power peaks. Therefore, multi-operation modes are required, shedding light on solutions with both features. Hybrid storage architectures combine the points of strength of single base technologies, while solving some shortcomings. To add extra capacity, hydrogen is a reliable storage medium with very low self-discharge rates and it can be integrated into micro-grids thanks to reversible Solid Oxide Cells (rSOCs), which perform both electrolysis and hydrogen re-electrification in the same device. Yet, for their operating features, rSOCs show a poor load-following capability and require a hybridization with a technology able to provide regulation in short time intervals. Therefore, the present study proposes to hybridize rSOCs with flywheels, to enhance power quality and improve fast-ramping and peak-shaving capabilities. A comprehensive methodology to measure the potentialities and the performances of hybrid storage systems is introduced and applied to the analysis of a case-study. Considering a micro-grid equipped with photovoltaic generators, where back-up supply is still provided by the power grid, the proposed HES system raises renewable self-consumption efficiency up to 58.04%, with an increase of +11.5% (versus +4.7% achieved with non-hybridized ES) with regard to the system without storage. The rSOC + flywheel HES reduces substantially the impact on the power grid, smoothing severe power peaks. The results of the proposed HES architecture are deeply analysed in a case-study, represented by a micro-grid located in a temperate climate zone. This choice is significant to take into account for environmental factors inducing a seasonal performances variability.