Applications-Oriented Review of Energy Storage


Hello! My name is Alexia Popescu, and I am a rising sophomore in Materials Science & Engineering at Northwestern University. This summer I’ve enjoyed working with Lynn Trahey and George Crabtree under the Joint Center for Energy Science Research (JCESR). In my project, I am compiling an applications-oriented review of specific shortcomings and outlooks in energy storage systems, helping accelerate the emergence of next-generation technologies.

Energy storage is essentially any device that converts and keeps energy in an accessible form. Most familiar are batteries, storing energy through chemical reactions, but there are also many thermal, mechanical, and hydrogen-based technologies. Their scope goes beyond portable power for a phone, as each plays a part in the complex system which consumers, manufacturers, businesses, and governments rely on for on-demand energy. Furthermore, they are pivotal in the push towards reducing green-house gas emissions (decarbonization), coupling with cleaner energy sources like wind and solar renewables to make them more reliable and competitive.

While energy storage has experienced considerable growth, the pace of discovery is nonetheless too incremental to meet the urgency of market demand or the pace of climate change. Part of the challenge is that distinct applications of energy storage have significantly different target performance standards, so no one technology can meet the needs of every application. Another concern is that energy researchers are often divorced from the lessons learned from engineers who integrate and deploy energy storage systems.

Thus, my work aims to be a step forward in bridging the practical with the theoretical through a literature review. It does not intend to be an exhaustive survey reaching through every corner of the energy sector, but rather a perspective piece, highlighting tangible energy storage needs in major application areas. Much of my research so far has been looking at transportation.

In transportation, innovation might seem to have stabilized around lithium-ion batteries. That is primarily the case for short-distance light-road transport like passenger electric vehicles (EVs) where there are more than 326,400 battery-based electrics on U.S. roads today compared to only about 7,600 hydrogen fuel cell cars (DOE Alternative Fuels Data Center). Batteries tend to have higher efficiency, lower cost, and more availability of charging infrastructure thus making them more favorable in passenger cars than hydrogen fuel cells. However, the faster charge time and longer range of hydrogen-based storage is promising in heavy-road transport like buses and trucks, as illustrated in Figure 1.

Figure 1: An illustration showing how short-distance road transport may be best electrified by batteries, while long-distance favors hydrogen and in between, there are a range of other feasible options [1].

Road, rail, maritime, and aviation electrification are each at a different phase of emergence, as technical and commercial requirements dictate which energy storage systems best fit each sector’s needs. Due to the chemistry happening at the molecular-level, a battery generally can output large amounts of energy —having high specific power and power density— while a fuel cell is optimized for outputting energy for a longer time —having high specific energy and energy density. These performance “metrics” are just a few of those considered when comparing different technologies. Figure 2 is another comparison of metrics, in this instance between two main battery types.

Figure 2: A filled “spider” plot comparing the different performance and cost priorities of a battery A) for an EV and B) for storing solar or wind for the electric grid. The closer an endpoint is to the edge of the plot, the higher its value. [2]

Next steps for my work include exploring other areas such as the electrical grid in a similar application-metrics lens. Part of the process will be understanding the current role of energy storage in the grid and then diving deeper into the outlook for future developments. Exciting applications are on the horizon, such as renewables-sourced “green” hydrogen decarbonizing heavy industry and cutting-edge batteries and fuel cells enabling electric flight in designs like Figure 3. The result of this project will hopefully lay a foundation for a review of energy storage on the forefront of innovation.

Figure 3: The Bartini prototype of an electric vertical take-off and landing (eVTOL) aircraft that runs on hydrogen fuel cells. [3]


1.         LePan, N. The Evolution of Hydrogen: From the Big Bang to Fuel Cells. 2019, April 13. [cited 2020, July 27]; Available from:

2.         Trahey, L., et al., Energy storage emerging: A perspective from the Joint Center for Energy Storage Research. Proceedings of the National Academy of Sciences, 2020. 117(23): p. 12550-12557.

3.         Bartini. The Future of Air Travel. 2020  [cited 2020, July 30]; Available from: