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Unless you’re a hard core energy geek, chances are you haven’t spent a whole lot of time thinking about “flow batteries,” which are basically large liquid-filled thanks that are used to store energy for the power grid. The idea behind the tech is to create a lower-cost battery option than, say, lithium-ion batteries.
But that promise remains just that, given that batteries of any kind are still mostly being tested in pilot projects by power producers and utilities. There’s has been some investment and innovation in the space in recent years: a flow battery startup called EnerVault announced this week that it’s raised a $15.5 million B round to help it install its first demonstration project in California’s central valley.
Flow battery isn’t as well known as lithium-ion battery partly because we don’t use it in our daily lives like we do with lithium-ion batteries, which run our mobile phones and computers. So what makes up a flow battery?
How do flow batteries work?: In contrast to a lithium-ion battery, where the energy-storing materials and electrolyte are enclosed in a cell, a flow battery’s electrolyte is stored in two tanks that are separate from the cell itself. The flow battery generates electricity when the liquid electrolytes, which are mixed with energy-storing materials, flow through the two-half cells and react with the electrodes in each side of the cell. Flow battery developers are experimenting with different types of energy storing materials, such as iron, vanadium, zinc and bromine.
EnerVault’s design uses iron in one tank and chromium in the second tank. Commonly the two electrolyte tanks hold different energy storing materials. But for vanadium flow batteries (an increasingly important and advanced type of flow battery) vanadium is used in both electrolyte solutions. The Electric Power Research Institute says vanadium flow batteries are a more mature technology than zinc- and iron-based flow batteries, which are mostly in R&D or field demonstration stages.
Flow battery makers like to point out that the use of external storage tanks means a flow battery system can be scaled up and down easily. Flow batteries are also rechargeable; the electrolytes can last a really long time, and typically use abundant materials — therefore can be a more affordable option. At the same time, flow batteries tend to be bulky, are not easily moved and the components have to be assembled on site.
The history of flow batteries: Flow batteries may not be common, but the idea has been around since the 19th century. Many companies, along with NASA, have investigated various types of flow battery technologies in recent decades. NASA encountered a host of technical difficulties, such as significant losses of energy during charge and discharge. Creating a good separator for the ion exchange membrane also is a challenge.
What is the market for flow batteries like? Flow battery developers are targeting some of the same customers that competing battery technology developers — from lithium-ion to lead acid makers — are aiming for: power companies and utilities. Utilities are sought-after customers, because they are facing a growing need to manage the variable supply of wind and solar electricity, and they can use batteries to store solar and wind energy and release it when demand peaks.
Businesses and consumers also are potential customers for battery companies. Batteries can store electricity from, say, a rooftop solar system, and then draw energy from the batteries after the sun goes down. Battery owners can even sell the stored energy to their utilities when electricity pricing is high if they are located in states that allow such a practice. China is shaping up to be a large market for batteries of all types, and flow battery makers such as ZBB Energy have developed joint ventures with Chinese companies. Last week, ZBB said it made the first shipment of its new flow battery technology to its Chinese partner, Meineng Energy.
The players: Aside from ZBB and EnerVault, other flow battery makers include Prudent Energy, RedFlow, Primus Power and Deeya Energy.