Large scale storage technologies are vital to increase the share of renewable electricity in the global energy mix. Increased interest in electrical energy storage is in large part driven by the explosive growth in intermittent renewable sources such as wind and solar as well as the global drive towards decarbonizing the energy economy. However, the existing electrical grid systems in place globally are not equipped to handle mass scale integration of intermittent energy sources without serious disruptions to the grid. It is generally agreed that more than 20% penetration from intermittent renewables can greatly destabilize the grid system. Certainly, large-scale electrical energy storage systems may alleviate many of the inherent inefficiencies and deficiencies in the grid system, and help improve grid reliability, facilitate full integration of intermittent renewable sources, and effectively manage power generation. Electrical energy storage offers two other important advantages. First, it decouples electricity generation from the load or electricity user, thus making it easier to regulate supply and demand. Second, it allows distributed storage opportunities for local grids, or microgrids, which greatly improve grid security, and hence, energy security. Currently, there is only 170 GW of installed storage capacity around the world, but more than 96% is provided by pumped-hydro, which is site-constrained and not available widely. Hence, a battery of technologies is needed to fully address the widely varying needs for large-scale electrical storage. The focus of this article is to provide a comprehensive review of a broad portfolio of electrical energy storage technologies, materials and systems, and present recent advances and progress as well as challenges yet to overcome. The article discusses the status and options for mechanical, thermal, electrochemical, and chemical storage. Where appropriate, it also provides tutorial level background information on fundamental principles for the interested non-expert. It is hoped that this article is of interest to the uninitiated as well as active scientists and engineers engaged in energy storage technologies, with particular focus on large-scale electrical energy storage.
