|Fig. 1: Range of costs for various grid-scale battery technologies. Data from .|
While most people are familiar with batteries as devices that power their portable computers, cell phones, and flashlights, there a variety of applications for batteries on a much larger scale in electrical power grids. Grid-scale energy storage is necessary for balancing the production and consumption of power; when there is a short-term surge in demand from customers on the grid, having a buffer of stored energy is critical to prevent blackouts, and when there is a dip in demand, it makes sense to store energy that's being produced to use later rather than letting it go to waste.  In addition, as more renewable energy sources continue to be integrated into the grid, the need for grid-scale energy storage will only increase due to the intermittent nature of power production from renewable resources like solar and wind. [1,2] The grid-scale energy storage business generated $1.5B in revenue in 2010. 
This report focuses on battery technologies that have been identified as promising for grid applications. However, not all grid-scale energy storage comes from batteries; in fact, the vast majority (around 99%) of the world's energy storage currently comes from water pumped uphill into hydroelectric dams.  A group of entrepreneurs who recently got funded by Bill Gates are even looking at using ski lifts as a mechanism for storing energy.  Another technology for grid-scale energy storage that has attracted investor interest is compressed air energy storage (CAES), with LightSail Energy raising $37.3M in November of 2012. 
There are a number of battery technologies that are currently important to grid-scale applications, including lead acid, lithium ion, redox flow, and sodium sulfur. In lead acid batteries, which are traditionally used in transportation applications, electricity is generated when lead ions travel between a lead electrode and a lead sulfate electrode.  One strong limitation for traditional lead acid batteries is their relatively low cyclability, as many grid applications require constant charging and discharging.  The lithium ion battery, where lithium is shuttled between a lithiated anode (most often graphite) and an oxidized lithium alloy cathode such as LiMnO2, is currently a leading technology in consumer electronic devices like laptops and cellphones and is starting to see grid applications. In fact, in the aftermath of the bankruptcy of A123 (lithium ion battery manufacturer for the electric vehicles market), it has become apparent that their grid-scale storage business is actually one of their most valuable assets, and market research suggests that the lithium ion grid-scale battery market will grow to $1.1B by 2018.  Lithium ion batteries have high energy densities compared to other batteries, which explains their traction in the consumer electronics and EV markets; however, these considerations are less important in grid applications, where the size and weight of the battery is not necessarily the main constraint. In a redox flow battery, the electrolyte (rather than the electrodes) contains the "electro-active materials," and separated electrolyte tanks connected by a permeable membrane are used to separate the active species.  One particularly appealing aspect of flow batteries is that the solid electrodes are not involved in the electrochemical process, which may lead to enhanced reliability of these storage systems.  Finally, sodium sulfur batteries, where sodium ions travels from molten sodium through a high temperature solid beta-alumina electrolyte to a sulfur electrode, are already seeing commercial use, with as many as 200 storage sites already running globally.  However, there are a variety of safety concerns with these high-temperature sodium sulfur batteries, which have been have be exacerbated by a fairly large fire in Japan in 2011. 
Past these reasonably well-developed technologies and a few others not described here, there are university researchers and and startup companies all over the world looking to innovate and build new technologies for electrochemical energy storage. For example, at Stanford, Professor Yi Cui's lab recently produced a paper describing a copper hexacyanoferrate battery electrode that can cycle 40,000 times with only a 17% loss in storage capacity.  This combination of high cyclability and relatively low-cost materials makes this technology potentially appealing for grid applications.
For grid-scale energy storage to be economically viable, many point to a target cost of roughly $100/kWh, where the unit of $/kWh describes the cost per storage capacity of an energy storage system. [7,9] Pumped hydro, currently the lowest- cost option for grid-scale energy storage, can have costs below $100/kWh, although it carries enormous upfront capital costs and is only a viable option in certain geographic locations.  Batteries have a massive advantage over pumped hydro in that they are modular and can be put almost anyplace on earth. Fig. 1 shows a range of costs for a variety of different grid-scale battery technologies in use today.  As is evident from the figure, none of these technologies are close to the $100/kWh mark, and there is a great deal of variability in costs due to the fact that many of these technologies are in the pilot phase and have not matured and scaled.
There has not yet been a significant disruption in the grid-scale battery technologies; the playing field is wide open for researchers and entrepreneurs to innovate and create solutions to tap into the growing market for grid-scale energy storage. While we can expect the costs of existing technologies to decrease over time as industry optimizes manufacturing processes and scales the size of plants, it's unclear whether any of the existing developed battery technologies will be able to compete on the grid-scale and reach the $100/kWh target cost. Some people are even considering using re-purposed EV lithium ion batteries in grid-applications as an option to cut costs. However, resource constraints on lithium will likely become a bottleneck on the EV market in the future, which is encouraging some researchers to move toward other battery chemistries, such as ones involving magnesium instead of lithium. 
Batteries are not yet economically viable at the grid-scale, which explains why the majority of grid-scale energy storage still is in the form of pumped hydro. However, this is a huge market opportunity; research by Pike Research, an industry group, expects revenues from grid-scale energy storage to increase from $1.5B to $25.3B between 2010 and 2020.  With that kind of economic driving force, it's inevitable that battery researchers and material scientists globally will strive to innovate and compete in this market. Since the grid-scale battery industry is still at such an early stage, fundamental materials science research from the university will likely play a major role in driving innovation in grid-scale battery technologies over the next decades.
© John Melas-Kyriazi. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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