energies Review Critical Review of Flywheel Energy Storage System Abdul Ghani Olabi 1,2,3, *, Tabbi Wilberforce 2, *, Mohammad Ali Abdelkareem 1,3,4 and Mohamad Ramadan 5   Citation: Olabi, A.G.; Wilberforce, T.; Abdelkareem, M.A.; Ramadan, M. Critical Review of Flywheel Energy Storage System. Energies 2021, 14, 2159. https://doi.org/10.3390/ en14082159 Academic Editor: Lorenzo Ferrari Received: 4 February 2021 Accepted: 30 March 2021 Published: 13 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Sustainable and Renewable Energy Engineering, University of Sharjah, Sharjah P.O. Box 27272, United Arab Emirates; mabdulkareem@sharjah.ac.ae 2 Mechanical Engineering and Design, School of Engineering and Applied Science, Aston University, Aston Triangle, Birmingham B4 7ET, UK 3 Centre for Advanced Materials Research, University of Sharjah, Sharjah P.O. Box 27272, United Arab Emirates 4 Chemical Engineering Department, Faculty of Engineering, Minia University, Minya 615193, Egypt 5 Department of Mechanical Engineering, International University of Beirut, Beirut P.O. Box 146404, Lebanon; mohamad.ramadan@liu.edu.lb * Correspondence: aolabi@sharjah.ac.ae (A.G.O.); awotwet@aston.ac.uk (T.W.) Abstract: This review presents a detailed summary of the latest technologies used in flywheel energy storage systems (FESS). This paper covers the types of technologies and systems employed within FESS, the range of materials used in the production of FESS, and the reasons for the use of these materials. Furthermore, this paper provides an overview of the types of uses of FESS, covering vehicles and the transport industry, grid leveling and power storage for domestic and industrial electricity providers, their use in motorsport, and applications for space, satellites, and spacecraft. Different types of machines for flywheel energy storage systems are also discussed. This serves to analyse which implementations reduce the cost of permanent magnet synchronous machines. As well as this, further investigations need to be carried out to determine the ideal temperature range of operation. Induction machines are currently stoutly designed with lower manufacturing cost, making them unsuitable for high-speed operations. Brushless direct current machines, the Homolar machines, and permanent magnet synchronous machines should also be considered for future research activities to improve their performance in a flywheel energy storage system. An active magnetic bearing can also be used alongside mechanical bearings to reduce the control systems’ complications, thereby making the entire system cost-effective. Keywords: flywheel energy storage systems (FESS); spacecraft; renewable energy; transport indus- try; electricity 1. Introduction The severe environmental impact of fossil fuels, used in all aspects of our lives, is a serious threat, as is clear from the resulting health problems and climate change [1,2]. To reduce the severe problems caused by the different fossil fuels, scientists have proposed different solutions, such as waste heat recycling [3,4], developing efficient energy conver- sion systems that have low or no environmental impact [5–7], transitioning from fossil fuel resources to renewable energy resources [8–10], and, finally, CO 2 capture [11–14]. In the last decade, the renewable energy sources’ capacity was exponentially increased, resulting in a critical need for energy conversion/storage systems that can effectively use/store such an increase in energy. Regarding energy conversion devices, fuel cells are efficient devices [15–17] that are fueled by renewable fuels such as biohydrogen [18], biogas [19], or other biomass resources [20,21]; they have high potential to replace conventional energy conversion sysetms in several applications, such as water desalination [22,23], transporta- tion [24–26], aviation [27], and portable applications [28]. The energy storage systems are divided into four categories, i.e., electrical, electrochemical, thermal, and mechanical. Mechanical ones are suitable for large-scale capacities with low environmental impacts compared to the other types. Among the different mechanical energy storage systems, the Energies 2021, 14, 2159. https://doi.org/10.3390/en14082159 https://www.mdpi.com/journal/energies