Eos Energy Storage, the zinc-air battery startup targeting a super-low $160 per kilowatt-hour for grid-scale energy storage, has raised a $15 million Series B round from a syndicate of twenty-one strategic and financial investors, including Fisher Brothers and a big potential strategic partner, NRG Energy.

New York-based Eos has previously raised about $12 million from investors, though it hasn’t disclosed much about its investors before Tuesday’s funding announcement. The startup was originally bootstrapped by founders Michael Oster and Steven Amendola, and has spent the past five years bringing their technology through R&D and lab trials to more recent field tests.

Eos is working with hometown utility Consolidated Edison and five other unnamed partners to test small-scale versions of its battery chemistry and supporting grid interconnections. The company’s goal is a 1-megawatt, 6-megawatt-hour cargo container-sized battery called the Eos Aurora, set to be released in 2014 with characteristics that include long life, fairly high efficiency and a super-low cost of $140 per kilowatt-hour -- a fraction of the price of competing flow batteries, lithium-ion and advanced lead acid batteries on the market. 

Eos represents the first energy storage investment for NRG, but the Princeton, N.J.-based energy giant is deeply involved in wind and solar power, plug-in vehicle charging and other green endeavors. “Eos’ technology is of strategic interest to NRG as we seek to enhance the value of our generation assets and evaluate novel energy storage business opportunities,” Denise Wilson, NRG executive vice president and head of its New Businesses unit, said in Tuesday’s release.

Fisher Brothers is a privately owned New York City-based real estate firm which also owns Plaza Construction, a contractor with experience building urban power plants and renewable energy projects. It’s also a co-sponsor with Morgan Stanley of the City Investment Fund, as well as a founding partner of Convergent Energy + Power, an energy storage asset development company with a pipeline of projects in New York, California and elsewhere.

Here’s our previous coverage on Eos.

The company claims its batteries can achieve 75 percent round-trip efficiency, along with a 10,000-cycle, or 30-year, lifetime. That compares favorably to the lifespan of other batteries on the market, and matches the round-trip efficiencies of flow batteries now on the market, though not those of the latest lithium-ion battery chemistries now in deployment on the grid.

But it’s the low price Eos is offering that has utilities and competitors most interested. Eos is targeting a total system cost of $1,000 per kilowatt, or $160 per kilowatt-hour of energy storage for its six-hour system, according to Philippe Bouchard, Manager of Business Development at the firm. Today’s flow batteries are being priced anywhere from $400 to $600 per kilowatt-hour, but at lower efficiencies. Lithium-ion batteries, on the other hand, while offering certain power delivery strengths that Eos doesn’t match, cost $800 to $1,000 per kilowatt-hour and up, he said -- although some Chinese Li-ion manufacturers are targeting $500 per kilowatt-hour.

Part of what makes it so cheap is its use of zinc, Bouchard noted. Zinc is much cheaper than lithium, at approximately $2 a kilogram, with global reserves of 1.9 billion tons and 30 million tons a year in production. That’s made zinc air-based battery chemistries popular, and indeed, many are on the market today -- but almost all in non-rechargeable formats. Getting them to recharge is much tougher, for various complex technical reasons having to do with the way air affects the anodes and cathodes of the batteries.

Eos says it has fixed those cycling issues by using what’s essentially salt water as an aqueous electrolyte in its battery cells, with the ability to amend or refill the system with more liquid as needed, he said. That’s gotten it to 6,000 cycles in lab tests, way beyond other zinc-air-based rechargeable batteries, and on a par with lithium-ion batteries, giving the startup optimism that it will reach its 10,000-cycle goal for its commercial-scale units, he said.

Eos isn’t the only startup promising groundbreaking advances in batteries, of course. On the rechargeable zinc-air front, contenders include Revolt Technology and PowerAir, PowerGenix and Taiwan’s APET. On the more esoteric front, venture investors like Vinod Khosla believe that batteries for the grid will involve new chemistries like Pellion's magnesium ion. Kleiner Perkins is betting on sodium-ion batteries from Aquion, and Kleiner Perkins and Khosla Ventures have also invested in battery startup QuantumScape.

Eos is just in the early stages of real-world testing, Bouchard stressed. The ConEd project won’t actually get underway until 2014, and while Eos expects to unveil other utility partners in the coming months, those projects will also be running on similar timeframes.

That may seem to put it behind competing energy storage vendors like NGK, A123 or Xtreme Power, which collectively have installed hundreds of megawatts of grid batteries to date. At the same time, Eos is taking a slow and steady approach that befits an as-yet-unproven technology, Bouchard said.

“We’ve built this fully transparent relationship with major utilities that are themselves going to be trendsetters for the industry,” he said. “They know exactly what we’re doing -- we’ve given them an in-depth perspective on the technology and how it operates, so there are no surprises.”

At the same time, Eos is still perfecting various parts of its grid-ready storage system, including developing a new generation of its current battery management software (BMS), which monitors the state of each liquid-filled cell within the battery arrays that Eos stacks together for power and energy data, as well as safety, he said. But unlike lithium-ion, which must be carefully managed to avoid thermal runaway, Eos’ aqueous-based, electrolyte-filled cells power themselves down in high-temperature extremes, presenting no fire hazard, he said.

As for what ConEd plans to use Eos’ first battery for, the partners are still looking into various options, Bouchard said. But the utility certainly has a lot of challenges in keeping its mostly underground, highly stressed metropolitan grid balanced, both in terms of delivering power to certain congested corridors during peak demand times and in keeping power quality stable amidst new intermittent generation sources like rooftop solar, he said.

Indeed, ConEd is also working on a broad range of energy storage projects as part of its $181 million in smart grid stimulus grants, including an $18 million project with Brooklyn-based Green Charge Networks to install batteries at 7-Eleven stores and airport EV charging stations to help balance end-use power for grid purposes. With 2012 grid upgrade plans that added up to $1.2 billion (PDF), the utility certainly has a lot of opportunities to look for places where batteries could help it avoid replacing or augmenting the 86 percent of its 130,000 miles of power lines that lie underground, for instance.

The grant Eos and ConEd are using for their pilot is also just part of millions of dollars in energy storage grants given out by the New York State Energy and Research Development Authority (NYSERDA). The state agency has backed everything from flywheel maker Beacon Power’s 20-megawatt energy storage facility in upstate New York (the company went bankrupt, but the facility is still providing frequency regulation to the grid), to startups like Eos and Albany, N.Y.-based BESS Technologies in more recent grants.

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