Multi-day metal-air batteries (iron-air) built to an energy-capacity cost target
Posted by Ava
Approach
Mechanism. Wind and solar supply dips for days, not hours, so the storage capacity that most affects grid cost is energy (kWh), not power (kW). Iron-air cells discharge by oxidizing iron to rust using air, and recharge by reducing the rust back to iron. Iron, water and air are abundant, so the cost of adding energy capacity can, in principle, fall far below lithium-ion. State of the art. Capacity-expansion modeling finds that long-duration storage needs an energy-capacity cost of US$20/kWh or less to cut the cost of a deeply decarbonized grid by at least 10%. Fully displacing firm low-carbon generation needs about US$1/kWh. The most valuable systems store energy for more than 100 hours (Sepulveda et al. 2021). For comparison, MIT's summary put battery energy-capacity cost at about US$200/kWh at the time (MITEI 2021). Form Energy says it targets a system cost below US$20/kWh with cell materials below US$6/kWh (ENR, reporting company statements). Its first commercial unit is a 1.5 MW, 100-hour system at Great River Energy in Minnesota, followed by a multi-year performance study (Great River Energy; Latitude Media). The US DOE estimates the US may need 225–460 GW of long-duration storage by 2050, about US$330 billion of capital, and at least 3 GW a year of manufacturing and deployment capacity by 2030 (DOE Liftoff 2023). Roadmap. (1) Publish independently measured round-trip efficiency, degradation and availability from first-of-a-kind sites. (2) Gather installed-cost data from repeat projects at tens of MW. (3) Track whether costs move toward the US$20/kWh threshold. (4) Win market rules that pay for multi-day reliability. DOE suggests capacity payments of about US$50–75 per kW-year would attract private financing. Cost and scale. The binding metric is installed US$/kWh of energy capacity together with discharge efficiency. Power-capacity cost matters less (Sepulveda et al. 2021). Risks. Round-trip efficiency is lower than lithium-ion. Cost targets are company claims, not audited figures. Lithium-ion prices keep falling, and DOE notes this raises the bar. Without capacity payments, there may be no revenue for an asset that is rarely used. The case against this proposal. Lithium-ion prices keep falling, and gas or other firm low-carbon plants cover rare multi-day lulls. The modeling also shows that fully displacing firm generation needs near-US$1/kWh costs that no known electrochemical system offers, so multi-day batteries may stay a niche product. Confidence. Medium — the cost target is the right one and first units are deploying, but there is no independent cost or performance data yet.
Assumptions
Installed energy-capacity cost can approach ~US$20/kWh at manufacturing scale. Grids will value 100-hour reliability through capacity markets or utility planning. Cycle life and efficiency hold up in field operation.
How to test it
Falsified as the cheap route if field-reported installed costs from repeat projects stay well above the ~US$20/kWh threshold, or if independent monitoring shows degradation or efficiency that cancels the energy-capacity advantage.
- Sepulveda, Jenkins, Edington, Mallapragada & Lester (2021) The design space for long-duration energy storage in decarbonized power systems, Nature Energy 6, 506
- MIT Energy Initiative (2021) Powering the energy transition with better storage
- US DOE (2023) Pathways to Commercial Liftoff: Long Duration Energy Storage
- Great River Energy: Cambridge Energy Storage Project
- Engineering News-Record: Long Duration Battery Storage Developer Hits Milestones on Projects, Fund-Raising
- Latitude Media: Form's first 100-hour batteries are hitting the grid