Go with the flow when it comes to long-duration storage

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Lithium ion may dominate the commercial storage market now, but other types of batteries may actually be more cost-effective and reliable in applications that require longer duration storage. One of those alternatives is flow batteries.

Conventional flow batteries contain two electrolyte solutions in two separate tanks, circulated through two independent loops, separated by a membrane and energy is stored chemically in the electrolyte tank. Source: Primus Power

How flow batteries work
Most conventional flow batteries use two electrolyte liquids: one with a negatively charged cathode, and one with a positively charged anode. The cathode and anode are separated into two tanks by a membrane, because if they come into contact with each other the battery will short and require replacement. This is often what happens with lithium-ion batteries; the membrane degrades over time. But the exchange of negatively and positively charged fluids in flow batteries produces electrical current without degradation, providing a longer cycle life and quick response times.

“We think lithium ion is a great technology for certain applications,” said Paul Ferrera, business development manager at flow battery manufacturer Primus Power. “It’s energy-dense, high-power and lightweight. Flow batteries are never going to power your laptop, cell phone or electric car—it wouldn’t be cost effective. However, we do think that flow has the ability to quickly leapfrog lithium ion in applications that demand long-duration power.”

Flow batteries are able to store power for four hours or longer and don’t need much O&M. While lithium-ion batteries typically last five to ten years in commercial applications, flow batteries can last for decades before needing replacement. However, flow batteries are large units, ranging from 2-meter cubes to the size of a shipping container.

Primus has found its sweet spot in large renewable utility applications that are not space-constrained.

“Utilities who are used to upgrading substations and transmission lines and replacing gas peaker plants talk in decades,” Ferrera said. “You don’t say, ‘Oh yeah, you’re going to replace a major part ten years later’—these things need to last for decades. When you talk about an asset that can perform for 20, 30, 40 years, their ears perk up. Utilities are still a hard nut to crack, but that’s the market we’re going after.”

Primus also has systems installed at military bases and data centers including Microsoft.

“Microsoft has plans to be 100% renewable,” Ferrera said. “It’s looking to implement storage at much larger data centers where land is cheap and there’s plenty of room to add turbines or solar panels and have a net-zero data center.”

Source: Rocky Mountain Institute (click to enlarge)

Another flow battery manufacturer, ESS, also sees opportunities in markets with a high penetration of renewables, as well as off-grid and microgrid projects.

“Looking at off-grid applications with expensive diesel generators, installing solar+storage can really drive down the cost per kWh of electricity—primarily with the low cost of renewables, but also using storage to utilize those renewables efficiently,” said ESS CEO and founder Craig Evans. “In addition, adding storage to microgrids can help perform peak shaving, bulk shifting, demand response and energy arbitrage. These are the markets we feel flow batteries can really dominate.”

Though vanadium-based chemistries have been known to be used in flow batteries, there are many chemistries available.

When DOE-backed ESS was founded in 2011, the company used vanadium before eventually switching to iron, salt and water.

“They are abundant, low-cost materials—basically a water battery with salt,” Evans said. “The move from vanadium to iron really came down to cost, then secondly the environmental factor of it being widely available, non-toxic and easily disposable, so it’s cheaper and cleaner.”

Primus diagram

Primus Power’s design utilizes a single tank, a single flow loop design and no membrane and energy is stored in a plated metal on the surface of titanium electrodes. Source: Primus Power

Primus Power was formed in Silicon Valley in 2009 and also received DOE funding for its zinc bromine chemistry. Ferrera said that because most zinc bromine is from the Dead Sea in the Middle East, it partnered with Israel Chemicals (ICL)—the largest global producer of zinc bromine—to develop an additive that increases the density difference between the two electrolytes. This allows the liquids to self-separate in one tank, similar to oil and water. Energy is stored in a plated metal on the surface of titanium electrodes.

“The advantage of having one tank is that you eliminate the membrane, which is an expensive part of a traditional flow battery, lowering initial capital cost and extending cycle life,” Ferrera said. “Then there’s no need for two of everything—pumps, pipes, tanks. Cutting that in half makes it easier and cheaper from a manufacturing perspective, and from an O&M perspective the less parts you have, the less parts can break.”

ViZn Energy also offers a zinc-iron flow battery. Lockheed Martin is planning to bring a metal complex model to market in 2018. And scientists with the Joint Center for Energy Storage Research, a research consortium lead by Argonne National Laboratory (led by the DOE), have also developed an aqueous oxygen sulfur flow battery cell that could store energy for a month or more.

Safety and disposal
Though it depends on the chemistry, flow batteries tend to be less reactive and easy to dispose, with no fire risk.

Primus’s battery contains a fire suppressant—bromine is the same substance as the red powder that planes spew onto forest fires to put them out. Since it isn’t flammable, it doesn’t have the thermal runaway issues associated with lithium-ion batteries.

ess storage flow battery

ESS flow battery cut-out and installation.

“Our chemical is actually a natural fire retardant,” he said. There are also no fumes to inhale.

Fire suppression and HVAC requirements aren’t a concern with ESS’s battery either.

“The ESS flow battery doesn’t require advanced battery management systems to ensure safety and longevity, like with lithium ion,” Evans said. “With our chemistry, the system lasts for 25 years. And at the end of life, you can reuse it in another battery. Or you can just dispose of it since the iron we use is the same iron used in wastewater treatment facilities or fortifying cereal”

Evans said the upfront cost of ESS’s battery is similar to that of lithium-ion, except it’s going to last decades longer, store more power and require less maintenance, on top of the recyclability.

Ferrera said Primus’s electrolyte is also recyclable, able to be reused or sold, and that looking at the levelized cost of storage, or the costs holistically throughout the life of the project, flow batteries definitely come out on top in long-duration applications.

“Flow is really a growing market,” Evans said. “The dropping price of renewables is really putting a huge demand on batteries. We see an increasing demand for four-plus-hour, long-duration storage as the industry became more aware of its unique applications. And when you have a low-cost, non-toxic option, that’s really going to enable growth.”