**4. Summary**

Within this study, energy storage for sustainable transport applications was investigated with respect to service life. The theoretical background of di fferent energy storage systems, as well as di fferent use-cases were described in detail. For exemplary energy storage comparison and benchmarking, the use-case of a fully electric transit bus operating in urban public transportation was selected, whereas onboard energy storage and bu ffer energy storage inside the fast-charging station were considered. From a grea<sup>t</sup> variety of options, three established and feasible energy storage systems were chosen for a more detailed analysis. For energy storage inside the fast-charging station, it was shown that high demand on cycle life and other requirements, such as short storage time, high power and long targeted service life clearly favor flywheel energy storage systems (FESS) over supercapcitors or batteries. However, fewer load cycles and long-time storage onboard the transit bus calls out for state of the art Li-Ion batteries rather than supercaps or FESS. Hence, which energy storage technology is most suitable strongly depends on the envisioned use-case and consequently the actual duty cycle.

A representative FESS module (5 kWh, 100 kW peak) was analyzed for the proposed use-cases and measures to improve/maximize service life was suggested. For the same use-case, a comparable battery system was analyzed, showing that battery size, and therefore, DOD (depth of discharge) is crucial for battery life. Beyond that, the battery solution requires accurate thermal conditioning and monitoring as calendar and cycle life are strongly a ffected when operated outside a narrow temperature band.

In this context, the grea<sup>t</sup> potential of FESS was shown. It is to be expected that in future some of the major advantages of FESS will be exploited, e.g., nearly unlimited cycle and calendar life, easy state of charge determination, independence from limited resources, etc. FESS-specific drawbacks, such as self-discharge, weight and cost may be detrimental to mobile (onboard) applications, but can be mitigated or neglected in some stationary applications, such as EV charging stations. Hence, FESS represent a valuable contribution to the energy revolution by increasing grid stability and facilitating the integration of renewables into the grid.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2071-1050/11/23/6731/s1, Video S1: passive magnetic weight compensation demonstrator, Video S2: Basic principle of flywheel energy storage.

**Author Contributions:** Conceptualization, P.H., A.B. and H.W.; Writing—Original Draft Preparation P.H.; Methodology, P.H., H.W. and B.S.; Software, Validation, Formal Analysis and Investigation, P.H. and B.S.; Data Curation, M.B.; Visualization, P.H., M.B. and A.B.; Project Administration, A.B.; Supervision, Funding Acquisition and Resources, H.W.

**Funding:** This research was conducted within the Project FlyGrid, funded by the Austrian Research Promotion Agency (FFG) within the Electric Mobility Flagship Projects, 9th call, gran<sup>t</sup> number 865447. Open Access Funding by Graz University of Technology.

**Acknowledgments:** The authors would like to thank Myonic GmbH for their support regarding the service life of rolling element bearings and their lubrication, as well as Holding Graz for their input regarding urban bus fleet operation.

**Conflicts of Interest:** The authors declare no conflict of interest.
