Startup EnerVault has hit a major milestone: a demonstration site for its flow battery made of iron and salt water.


Drive about a hundred miles east of San Francisco, and you’ll hit the heart of the Central Valley, Stanislaus County, where agriculture is king, trucks have four-wheel drive, and the temperature commonly creeps up into the nineties in the summer. The county’s larger towns were founded in the late 1800’s as stops on the railroad. The biggest city, Modesto, was forever immortalized in the movie American Graffiti as the backdrop for cruising teenagers, diners and late 1950’s Americana.

Twenty miles off of one of the county’s main highways, past miles of almond orchards, grape vineyards and dairy farms, sits a symbol of the future of energy technology, and a demonstration of how California is becoming ground zero for such energy innovations. It’s the world’s largest battery of its kind — made of iron and salt water — and it sits on a sprawling almond farm next to a bunch of solar panels. The battery bottles up the energy from the sun for use when it’s needed later on, like at night, and helps power an irrigation pump that waters about 300 acres of the almond farm.


The technology was developed by a five-year-old Silicon Valley startup called EnerVault, which is backed by venture capitalists and corporate energy investors and is one of a handful of startups tackling the newly emerging market for hooking up energy storage to the U.S. power grid. On Thursday I made the trek out to the official opening of EnerVault’s big new project, where a small but passionate group gathered to celebrate the milestone, including EnerVault employees, construction partners, policy makers and investors. An investor from 3M flew in from Munich to glimpse the site; a former NASA scientist who invented the basics of the battery tech back in the 1970’s attended just to see his theoretical dream become a reality.

A small step for energy storage

If you want to make sure you stand out in Turlock, the city which the battery project officially sits in, drive a bright red Mini Cooper emblazoned with a City CarShare logo through the back roads of the farm land. In the early afternoon, the time I arrived at the site, every other car on the road was a truck with a pickup compartment so large it could fit my entire car in the back.


Glimpsing EnerVault’s battery project for the first time, it doesn’t look like much. It’s made up of four large tan tanks, connected to a big box of industrial equipment, and surrounded by grid infrastructure including a transformer and a metering system. The battery project sits next to a dozen or more solar panels that rest on trackers that follow the sun throughout the day. The entire project sits behind a wire fence to keep out anyone that might mess with it.

If you were hoping for the equivalent of a Tesla car on the power grid, you’d be out of luck. Of course it doesn’t look sexy. It’s industrial power equipment. The real technology magic is what’s happening quietly inside the big box behind the tan tanks. It sound cooler than it looks.

Almond grove

EnerVault has built a “big ass battery,” as CEO Jim Pape jokingly called it at the event, using iron-chromium redox flow technology…say what?

A “flow battery” stores energy like the basic lithium ion battery in your laptop, but flow batteries have their electrolyte (the substance that acts as the medium for the charging and discharging of the battery) separated out of the battery cell in liquid-filled tanks. These are the four big tan tanks at EnerVault’s site. It’s as if you broke open a lithium ion battery, and separated out its parts — then you could watch it weirdly and slowly charge something.

EnerVault engineer Jeremy Meyers

EnerVault engineer Jeremy Meyers

While it might sound awkward, the benefit of a flow battery is that it can be cheaper than traditional lithium ion batteries (EnerVault says it can eventually get down to $250 per kWh), it can be more flexible (you can add more tanks and electrodes to that type of open system), it can last many more years (the electrolyte doesn’t degrade as fast) and it can provide a longer burst of sustained power (about four hours for EnerVault’s battery).

Perhaps most importantly, the chemistry of EnerVault’s is pretty safe. The electrolyte is made of 95 percent water, and it doesn’t use a inflammable material like lithium oxide. In a world where lithium ion batteries occasionally display significant fire hazards, the biggest safety hazard at the EnerVault site are just the basic issues at any grid connection spot, said Jeremy Meyers, an EnerVault electrical engineer who helped build the project and gave me a tour of it on Thursday.

Wrapping it all together, pumps flow the mostly-water electrolyte from the tan tanks over to the black box containing the “cascades,” and push it across the battery cells inside the cascades. Inside the cells is the proprietary secret sauce of the project. The company asked people on the tour not to take pictures of the details inside the fence, and mostly it’s about these cells. Namely they’re using a design in which multiple cells are arranged to allow the electrolyte solutions to flow from one cell cluster to another along the same path — along the way the electrolyte moves up in an electrical state, like the way a bicycle changes gears. Yeah, it’s a little complicated, but hey, we’re not all engineers.

A big step for the California power grid

The most important thing to know about the site is the application — it’s enabling a new type of long-lasting low-cost battery that can be coupled with clean power.


If you step back from the company’s hype, the EnerVault Turlock project is really quite small. It’s a demonstration-scale system that’s meant to help prove the characteristics of the technology and collect data about how it works. The California Energy Commission provided a $500,000 grant to help deliver those findings, as did the Department of Energy, with almost $5 million in support. The entire project cost about $9.5 million to install.

EnerVault Turlock provides just 1 MW-hour of energy from the 250 kW battery — the irrigation pump sucks in about 260 kW. But the core technology at the Turlock site is meant to prove that it can scale to hundreds of MWhs, many times bigger than this latest one. All eyes are on what EnerVault will be able to do with the tech going forward now that it’s reached this important step.


The project is happening in California because state regulators had the foresight (unusual for state bureaucrats) to incubate a market for energy storage there. Late last year, California regulators approved an ambitious plan to install a large amount of energy storage projects by 2020 to help the state meet its renewable energy mandate, which is 33 percent clean energy by 2020. Both the chairman of the CEC and the energy storage lead at the DOE were at the event, showing how dedicated the groups are to these early projects in California.

Clean power projects like wind and solar are variable, which means they only provide power during certain times of day (when the sun shines and the wind blows). But energy storage can capture wind and solar energy and keep it ready for when it’s needed during the dark and windless times. Thus these projects make clean power a lot more viable.


EnerVault as an exception

A handful of startups are trying to tackle grid energy storage — some include Ambri, Fluidic Energy, and Aquion Energy. But in the grand scheme of tech startups, they’re still pretty rare. EnerVault has raised just $30 million to get to this point, including funds from early backers like Oceanshore Ventures, but also later funds from oil company Total, Japanese giant Mitsui, conglomerate 3M and the investment arm of Tokyo Electron, among others. Still, that’s a tiny amount of money for a battery startup, particularly a grid battery startup.


EnerVault now needs to raise significantly more money to help it move from this demonstration phase to a larger commercial scale. The main job of EnerVault’s  30-or-so employees this year, besides fundraising, is to secure more and larger projects going forward and to show how the tech can scale into large commercial projects. Many of these deals have very specific needs about why, when and how they need energy storage — like an almond farm that has a solar system that needs backup storage.

If startups like EnerVault are able to get more storage projects like this on the grid, they can show how clean energy — paired with energy storage — is a lot more robust than critics want to admit. Storage can help make solar and wind into much more impactful energy resources, and can be deployed at a reasonable price. If EnerVault is able to succeed from a startup perspective, it would help show how clean tech startups can make money, given enough time, patience and the right customers.

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  1. Hmmm. ?? bigassbattery.com ??

    1. Katie Fehrenbacher B Friday, May 23, 2014

      buy it, we can start a battery blog

  2. Resourceguy Friday, May 23, 2014

    It might be helpful to at least mention the battery tech firms entering manufacturing stage and initial pilot client delivery for grid batteries with a target of $150 per kWh cost. Lack of comparisons in such areas as solar PV has caused a lot of policy mistakes and mis-allocation of taxpayer funds over the years.

    1. Katie Fehrenbacher Resourceguy Friday, May 23, 2014

      EnerVault says it’s heading toward $250 per kWh, but this is demo plant, so I’m assuming more expensive right now.

      1. Katie… The man above is talking abt energy (kwhr’s) while you’re talking abt power (kw’s). Apples & oranges. Dunno abt the costs quoted but you need to do a little research. The man above seems to know what he’s talking abt.

        1. Sorry. I apologize. You said kwhr.

  3. Alejandro Moreno Saldarriaga Friday, May 23, 2014

    How much water do these big ass batteries use? California is in a drought you know.
    Since this is just a demo model, how much water will a larger, full scale installation use?
    What happens to the water that is being stored in these cells as part of its electrolyte solution? Is that water held inside those cells indefinitely? Or is it discharged at any point in time?
    If it is discharged, what kind of effects does that water have on the environment, particularly on groundwater and the local watershed?

    1. It’s a closed loop system, the water in the electrolyte is not discharged or lost. The beauty of redox flow batteries is that the electrolyte is a perpetual asset, so at the end of the system life after 20+ years, you would transfer the electrolyte to the new system or sell back to EnerVault. The system is 99% reusable, 99.9% recycle/re-usable by weight.

      Like all chemicals except water, the electrolytes are required to have double containment to avoid spills. Even if there were a spill, it’s environmentally benign using elements that are not regulated in even California’s strict water controls…it’s iron and chromium, 2 elements every organism needs to live.

    2. The positive water-use benefits and drought management of implementing long duration energy storage is significant. Because thermal generation uses a lot of water, using energy storage to enable the more efficient generation means less fuel and less cooling water. If you avoid thermal generation altogether by storing renewable wind and PV energy, you save even more water. It also takes a lot of water to mine and process fossil fuels (in particular coal and fracking natural gas) so to the extent that the extraction is near the energy generation (for example, in Texas), you free up more water to other uses. For a great paper showing the drought management benefits of grid management (which storage enables), see: http://iopscience.iop.org/1748-9326/8/3/035029/

  4. How often does this project cycle from storage to discharge? How many cycles can it perform before the unit degrades? How do the engineers measure the degradation of the system?

  5. Skeeter Jenkins Friday, May 23, 2014

    How does this compare to storing all the energy by pumping water up into a reservoir during the day and taking it out via hydro-electric at night?

    1. Pumped Hydro Storage is a great technology that has been in use for nearly a hundred years. EnerVault’s redox flow battery is targeting the same applications currently performed by pumped hydro. RFB’s can do everything pumped hydro can do at a similar cost, but RFB’s are more responsive, easily move continuously from charging to discharging without a ‘dead zone’, can be sited where needed inside the urban grid instead of running transmission lines hundreds of miles to the mountains, are economical at a smaller scale so are more easily planned.

      For example, see the current Iowa Hill pumped hydro project in California. They applied for permits in 2002, final approval is expected in 2016, and if everything goes to plan, will be online in 2022. The planning for this started much earlier, with the land purchased in 1972. So more than 50 years to plan, permit, and build the project. Pumped hydro is a great technology but there are a lot of situations where the 20+ planning window and the huge risky up front cash outlays just aren’t practical.

  6. Wayne Lusvardi Saturday, May 24, 2014

    This is not economically feasible at $250 per kilowatt hour when the average price for electricity is hovering around ten to fifteen cents per kilowatt hour including transmission costs. To be feasible it would have to be able to store power at at least peak time prices which are what? maybe 25 to 50 cents per kilowatt hour?

    The article below states that battery storage costs need to come down to $100 per killowatt hour to be economically feasible according to the US Dept. of Energy.

    Re: “New Battery Material Could Help Wind and Solar Power Go Big,” Technology Review, Jan. 8, 2014

    Not even close

    1. if the battery lasts 20 years, let’s say that’s about 7000 cycles, so your talking about 3.5 to 4 cents per delivered kWh. EOS Energy’s project in NY might come in at half that. Power plants are generally priced in dollars per Watt. Consider that grid storage allows you to skip having to build extra capacity for the intermittency of renewables, and that 0.25 per Watt looks mighty cheap.

  7. “””inflammable material like lithium oxide”””

    Lithium oxide is not inflammable!!! It is nonflammable. One could not burn it, let alone set on fire. Where you got such ridiculous idea about lithium oxide flammability?

  8. Duncan H. Haynes Monday, May 26, 2014

    “Iron in seawater”? What state of the iron? Zero, +2 or +3? Complexed or at its natural solubility? Or is it colloidal?

    What are the electrodes made of? How many hours of use before they are fouled?

    The article says that the system is proprietary. That cannot be for long. A patent must have been at least filed and will have to be made public in one year.

    Duncan H. Haynes, Ph.D.
    (Linked In)

  9. Wolf Gehrisch Tuesday, July 22, 2014

    Evidently cost per kWh and per W are important and probably too high yet. But what is important too is the duration of electricity feed back into the applications it is meant for. Four (4) hours is not much when you consider that wind may not blow or the sun may not shine for several days in a row. On the other hand, I can see why the project was started in California, where weather conditions are just ideal.

    1. At 250$/kWh capacity combining battery storage with PV is already a no brainer for locations relying on diesel gensets. The flow batteries have the advantage that if you need higher storage hours you just add additional electrolyte tanks without the need to change the membrane or pumping units. Power and time are two decoupled variable in those systems, very much in contrast to most batteries, where the power and energy contained in the battery are coupled by design.

  10. Keep a eye out for a Australian company that has designed a revolutionary hydro generator, will make big inroads into power generation

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