Advanced Battery Technologies for Stationary Energy Storage Applications: Page 5 of 5

Zinc-bromine hybrid redox. Exxon developed the zinc-bromine hybrid redox flow battery in the early 1970s. During a charge, zinc is plated as a solid metal on the positive electrode in the battery cell stack; upon discharge, the zinc metal releases two electrons and oxidizes to form zinc2+. This chemistry provides not only a high cell voltage, but also a very high energy density. Since 2012, Australia-based Redflow has deployed zinc-bromine hybrid redox flow batteries in a variety of interactive applications, from the grid scale down to the residential scale. In the US, California-based Primus Power recently released the EnergyPod 2, a second-generation zinc-bromine flow battery that is scalable in 25 kW/125 kWh increments up to 25 MW. The architecture that Primus Power uses is unique in that it features a single tank, a single pump and a single flow loop, eliminating components and costs relative to other flow batteries. The resulting battery has a 5-hour discharge duration and a small footprint.

Zinc-iron hybrid redox. Lockheed Martin pioneered the alkaline-based zinc-iron hybrid redox flow battery in the 1980s. In this battery, the catholyte is food-grade iron salt dissolved in an alkaline solution, and the anolyte is battery-grade zinc oxide suspended in an alkaline solution. During charging, zinc plates out of the anolyte onto the anode. While these electrolytes are caustic, they are nontoxic and do not require any exotic materials. Because the electrolyte is not acid based, the battery does not require complex, corrosion-resistant materials for the subcomponents. The resulting battery is less energy dense than an acid-based hybrid redox flow battery, but is also safer and costs less.

The highest-profile zinc-iron flow battery supplier is Austin, Texas–based ViZn Energy Systems. Founded in 2009, the company started deploying its batteries in pilot projects 6 years later. The company has since developed a suite of containerized solutions for commercial, industrial and utility applications that scale up from 100 kW to more than 100 MW and has delivered solutions to customers in the US, Canada, Central America, Europe and India. In March 2017, the company announced that it was supplying a 200 kW/800 kWh battery to a microgrid project at a luxury resort in Nicaragua that will include diesel backup and an 800 kW solar array.

OTHER TECHNOLOGIES
Though Li-ion and flow batteries account for the majority of the market in stationary applications, other technologies have some track record in the field or are beginning to come to market. Sodium-based chemistries, for example, had some market traction in the emerging grid-scale battery sector before the ascendance of Li-ion. In recent years, zinc-air batteries with aqueous electrolyte have made progress toward commercialization.

Sodium-sulfur batteries. The sodium-sulfur (NaS) battery uses molten sodium as the negative electrode and molten sulfur as the cathode. During discharge, the sodium donates an electron. Advantages of the NaS battery include high energy density, excellent cycle life, affordable materials, high efficiencies and low self-discharge. Disadvantages are that NaS batteries require high internal temperatures to keep electrolytes in a molten state and are not well suited for power applications. This technology is best for applications that require a long (>6 hour) discharge duration.

Tokyo-based NGK Insulators is a century-old Japanese ceramics company known for its insulators and NaS batteries. The company began developing molten-salt NaS batteries in 1984 and successfully commercialized large-scale products by 2002. NGK has deployed its NaS batteries at nearly 200 locations globally to provide a cumulative installation base of 530 MW/3,700 MWh for load leveling, renewables integration, transmission and distribution network management, and microgrid and ancillary services.

Zinc-air batteries. Nonrechargeable zinc-air batteries have a long commercial history, dating back to the 1930s. In the 1970s, companies started building small button-type zinc-air batteries to power hearing aids and other medical devices. Because atmospheric air serves as one of the reactants, these batteries offer excellent performance in terms of energy density and specific energy. Miro Zoric, a Slovenian inventor, produced the first rechargeable zinc-air battery in 1996 and began mass production in Singapore for traction applications the following year. Within the last 5 years, rechargeable zinc-air batteries have begun to make inroads in stationary grid applications. While zinc-air batteries cannot match Li-ion batteries for power delivery, they have the potential to be cost competitive and may be able to match the lifespan of flow batteries.

The companies pioneering grid applications for zinc-air batteries are generally small start-ups. For example, Eos, an Edison, New Jersey–based supplier founded in 2008, has developed a 1 MW/4 MWh zinc-air battery system for grid applications. The Eos Aurora 1000|4000 uses a zinc-hybrid cathode battery with an aqueous electrolyte. The company designed the product to provide 5,000 100% discharge cycles, which equates to a 15-year calendar life, and claims that it can undercut the per-kWh cost of Li-ion by as much as 50%. Scottsdale, Arizona–based Fluidic Energy, meanwhile, has been deploying commercial-scale zinc-air batteries in long-duration applications in the developing world as an alternative to lead-acid batteries. Bloomberg New Energy Finance selected Fluidic Energy as one of its ten 2017 New Energy Pioneers.

Market Maturation

The parallels between the nascent energy storage market and the early days of the solar market are striking. Promising and potentially disruptive technologies are moving from research and development to pilot projects and commercial applications. Venture capital–backed start-ups championing new technologies are going toe-to-toe with deep-pocketed multinational corporations heavily invested in somewhat more proven technologies. In spite of steep cost declines, viable business cases are few and far between. Though fielding projects is capital intensive, it is difficult to prove bankability and find financing. A few forward-looking states and utilities are developing and implementing goals, policies and incentives intended to jump-start the market. California and a handful of other states with high energy prices are leading the way in terms of field deployments.

For solar industry veterans, it is déjà vu all over again. The energy storage industry is changing so quickly that technologies and vendors in the ascendance last year could be out of the game next year. As was true in the Wild West days of solar, some pioneers of storage will take arrows and many will fail in order for a few to succeed. The stakes are high, however, as those settlers who succeed in staking a claim will transform the energy industry for decades to come.

David Brearley / SolarPro / Ashland, OR / solarprofessional.com

RESOURCES

Fitzgerald, Garret, et al, The Economics of Battery Energy Storage: How Multi-Use, Customer-Sited Batteries Deliver the Most Services and Value to Customers and the Grid, Rocky Mountain Institute, October 2015

GTM Research and the Energy Storage Association, US Energy Storage Monitor: Q3 2017, September 2017

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