Alan Greenshields discusses the technology behind iron-flow batteries
As the world transitions to renewable energy, long-duration energy storage (LDES) is the last piece of the clean energy puzzle to finally enable the retirement of fossil energy generation.
Renewable energy sources such as wind and solar are inherently intermittent. Although this does not pose an issue for the grid in small quantities, when renewable energy penetration exceeds 25%, energy storage becomes critical to manage intermittency and maintain grid stability. LDES systems provide storage durations beyond the typical four-hour Lithium-ion (Li-ion) battery in use today, providing control, reliability and capacity to enable renewables to provide dispatchable baseload energy.
Today, power generation accounts for roughly one third of total emissions worldwide. Although renewable energy sources have grown in recent years, the grid still requires fossil fuel generators to provide power when the sun does not shine and the wind does not blow. Conversely, during periods of excess wind and solar generation, these resources are curtailed, wasting valuable clean energy. The deployment of LDES could eliminate between 1.5 to 2.3 Gt of CO2 annually by utilising clean energy that would otherwise go to waste, and its deployment could enable fully renewable energy-based grids by 2040.
Explaining LDES systems
Long Duration Energy Storage Systems is not a new concept, but it has yet to be widely adopted. The first LDES technology, pumped hydro, has been in use for over a century but requires suitable geography to be effective. Pumped hydro systems move water between reservoirs at different elevations to store and produce energy, pumping water uphill when energy is plentiful and using gravity to generate energy when needed.
More recently, Li-ion battery systems have been deployed to provide grid energy storage, but these are best suited to shorter durations. At longer durations, Li-ion technology quickly becomes cost-prohibitive.
Fortunately, there are a number of emerging technologies that can provide LDES cost-effectively and without geographical constraints. As these technologies become commercially available, the sector is expected to grow rapidly; Bloomberg NEF predicts that energy storage installations are on course to reach a cumulative 411GWh by the end of 2030 compared to the 27GWh deployed at the end of 2021. The LDES Council, an industry trade association, estimates that the world will need to deploy 1TWh of capacity globally by 2025 to stay on track to achieve net zero energy sector emissions by 2040.
Flow batteries are an emerging technology that can provide cost-effective LDES without the cost, safety or geographical constraints associated with pumped hydro and Li-ion. Although the fundamental technology has existed since the 1970s, technical and economic challenges have limited their deployment to date.
A new type of flow battery, using an iron-based electrolyte, has gained prominence in recent years as it overcomes both the cost and technical issues associated with prior flow battery technologies. Iron flow batteries do not require critical minerals, relying instead upon earth-abundant materials, are safe and nontoxic and can provide 6-12 hours of energy storage. Breakthrough Energy Ventures, the clean energy fund that is backed by Bill Gates, has backed iron-based battery companies including the US-headquartered ESS.
Iron-flow battery technology
Flow batteries store chemical energy in liquid electrolytes that circulate through the system separated by specialised membranes in the battery modules.
The initial discharged state of an iron flow battery consists of dissolved Fe2+ in an aqueous solution. To charge the battery, current is applied, and two reactions. When it discharges, the reverse reactions take place. Electrolyte continuously circulates through the power modules, enabling rapid shifts from charge to discharge.
Iron flow battery technology was first developed by NASA in the 1970s, but early prototypes experienced rapid capacity degradation due to side reactions in the electrolyte. Over many cycles, the electrolytes in early iron flow batteries became unstable due to a build-up of Fe(OH)3 on the separator and electrode. At the time, they had to be regularly treated with acid to remove the Fe(OH)3. This maintenance requirement made them impractical for many commercial applications.
The founders of ESS recognised the potential of iron flow technology to provide lower-cost LDES compared with competing technologies. The R&D team realised that the fundamentals of iron flow stability lie in understanding how the equilibrium potential of electrochemical reactions changes against pH.
During charge of the iron flow battery, the reaction at the negative electrode that forms iron metal competes with a reaction that will form hydrogen. This sees protons in the electrolyte accepting electrons to form hydrogen gas, H2. In addition, the plated iron metal may corrode in an acidic electrolyte to produce ferrous ion, Fe2+, and also generate H2.
These side reactions cause electrolyte charge imbalance and pH imbalance as the electrolyte becomes increasingly comprised of Fe3+ and the loss of protons in the form of hydrogen gas leads to an increasingly alkaline solution. The excess Fe3+ ions combine with the OH ions and form Fe(OH)3, which forms a precipitate. Over a series of cycles, these reactions reduce the battery’s performance.
Researchers at ESS developed the company’s proprietary Electrolyte Health Management System (EHMS) to manage these side reactions, enabling iron flow batteries to deliver unlimited cycling without degradation. A key component of the EHMS is the “proton pump”, which manages pH balance of the electrolytes, ensuring consistent performance over thousands of cycles.
The EHMS stabilises the battery by reacting the unbalanced Fe3+ ions from the positive electrolyte with the hydrogen gas by-product from the negative side. This is achieved by converting hydrogen gas back into protons and pumping them into the positive electrolyte. This proton pump has the effect of stabilising the pH across the battery and ensuring that the positive electrolyte returns fully reloaded with Fe3+ ions to match the state of charge of the negative electrode. The EHMS is a closed-loop system, and the proton pump is inherently self-limiting, reducing OPEX costs and enabling unlimited cycling over the battery’s 25-year design life.
Iron-flow batteries in action
LDES technologies that complement intermittent renewable energy sources can enable renewable energy to be baseload energy. ESS’ iron-flow battery technology can deliver 6-12 hours of flexible energy storage capacity with a cost-effective, safe and nontoxic alternative to competing LDES technologies.
ESS technology is already being deployed to support the global energy transition; the company most recently signed an agreement with Sacramento Municipal Utility District (SMUD), one of the largest municipal utilities in the USA, which includes up to 200MW/2GWh of LDES to support SMUD’s ambitious 2030 Clean Energy Vision.
Alan Greenshields is European Director at ESS