Iron flow battery tech shows promise for mid-duration energy storage

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Iron flow battery manufacturer ESS Inc. has been in the news lately, most recently for releasing an updated version of its product guarantee.

Munich RE, one of the world’s largest reinsurance companies, also updated its insurance policy for ESS to address customer concerns over technology risk. ESS told pv magazine USA that the new policy covers the battery modules and electrolyte management system, as well any needed repairs or replacement for the first 10 years of the system’s life. ESS manufactures these components itself.

A battery stack on the move.

Image: ESS

The product’s pump and motor drives have an expected lifetime of 19-22 years. The industrial computer, which was described as “inexpensive,” can be expected to be replaced every 5-7 years. ESS offers an O&M service contract after the 10-year mark.

But even more striking is the battery’s expected longevity. Much like the grid’s oldest assets, (some of which were first built in the 1890s), ESS batteries show the potential to soldier on for many decades. While the ESS battery systems have a prescribed design life of 25 years, the battery modules, electrolyte, plumbing, and other components may well last for decades longer with proper maintenance.

For one thing, the battery is expected to experience zero degradation over 20,000 cycles. By design, iron flow batteries circulate liquid electrolytes to charge and discharge electrons using a process called a redox reaction, which represents a gain of electrons (reduction), and a loss of electrons (oxidation). ESS uses the same electrolyte on both the negative and positive sides, eliminating possible cross-contamination and degradation. As a result, ESS chemistry remains stable for an almost unlimited number of deep-cycle charge and discharge cycles.

ESS battery spec sheet.

Another reason for the expected long operating life is that ESS batteries use a “proton pump” to balance and maintain electrolyte health over the life of the asset.

The pump connects to the tank and passively mixes unwanted hydrogen back into the solution. Doing so maintains the balance of pH and states of charge in the liquid electrolyte circulating through the system.

In human terms, the battery is the heart, the electrolyte is the blood, and the proton pump is the kidney, which keeps everything in balance.

The pump itself is expected to be a key commercial differentiator for ESS. Other flow chemistries need to come offline regularly to rebalance their electrolytes. Not so with the ESS design.

In the unlikely event that the battery is to be decommissioned, the system and all of its components can be substantially recycled.

Any non-ESS manufactured system components come with third-party warranties. The full warranty wasn’t available to be distributed via pv magazine USA.

The company currently offers two products, the single shipping container Energy Warehouse and a string of warehouse units in an Energy Center.

An ESS Energy Warehouse

One Energy Warehouse shipping container holds 400-600kWh of storage capacity and can be configured with variable power to provide storage durations of 4-12 hours. That makes the power rating configurable from 50-90 kW. The round-trip efficiency is 70-75%, DC-DC. Each battery weighs 16,000 kg dry, and as much as 38,000 kg after it’s filled with the electrolyte.

For larger volumes of energy storage, ESS will string together multiple batteries in what it calls an Energy Center. At this larger scale, ESS batteries take up some real estate. One acre of batteries holds up to 6 MW/90 MWh, providing up to 12 hours discharge at rated power. These large battery centers are expected to have fast, easy permitting due to the lack of hazmat concerns as well as non-flammable, non-explosive materials.

But the most important news of the day – product sales news – shows that ESS has been busy.

  • The company recently announced that it entered into a framework agreement with SB Energy, a unit of SoftBank Group Corp., to deploy 2 GWh through 2026.  As part of the agreement, the first ESS system has already been delivered to an SB Energy location in Davis, California, and will be commissioned in October.
  • An order with Enel Green Power España of 17 Energy Warehouse units with a combined capacity of 8.5 MWh will support solar farms in Spain as a part of a broader EU-wide engagement, providing resilience for the local power grid.

To support its growth, ESS announced that they were to become a publicly listed company through a SPAC merger with ACON S2 Acquisition Corp this past May. And in a great coup, the company will be listed on the NYSE under the ticker symbol “GWH.”

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