Highly secretive long-duration battery start-up Form Energy — which says its unidentified technology will be able to replace baseload thermal power plants — has just closed a $70m round of investment, bringing its total funding to date to $120m.

Chief executive Mateo Jaramillo, who previously led the team that created Tesla’s iconic Powerwall battery, told the Energy Transition North America online conference on Friday that the identities of the investors will be revealed in two weeks.

Previous investors include Bill Gates’ Breakthrough Energy Ventures, Macquarie Capital and oil companies Eni and Saudi Aramco.

The new round of funding shows that investors — who have been given information about the technology that has not yet been revealed to the public — believe that Form can achieve its stated aim of storing variable renewable energy for days at a time, eliminating the need for high-capacity-factor natural-gas and coal power plants.

Jaramillo told the conference that the design of the system allows it to be scaled up to “hundreds or even thousands of megawatts and many hundreds of gigawatt hours into terawatt hours”.

But he did not reveal what its 100-hour-plus battery was made of or how it worked. However, he did tell the Reuters event that it used materials that are fundamentally abundant and cheap.

“That’s really the driver for their selection criteria — across the board really, not just for the active materials in the battery that perform the electrochemical function, but for the entire thing overall.

“We are attempting to design a device and ultimately a plant that is entitled to replace thermal generation. And natural gas, to pick one of the sources of energy, is fundamentally abundant, there’s massive amounts of it. So we have to be able to compete, more or less, on those same terms. So we’re not selecting things that are rare in any sense of the word.

“And the other part of it is they are safe. Things that are abundant generally tend to be safe to humans because we evolved in this environment, and so that also drives the design of the system. Mitigating for safety adds cost, so we cannot from the very beginning select elements which are unsafe or unstable and pose safety risks.”

As Recharge reported in May, Minnesota-based utility Great River Energy is planning to build a 1MW/150MWh grid-connected demonstration storage plant by the end of 2023 using Form Energy’s secret battery technology — providing 1MW of output for 150 hours straight — as it closes all but one of its fossil-fuel power plants.

Form’s only public explanation of its technology has been that it is an “ultra-low-cost aqueous air battery system”.

It has been speculated that its core ingredient was sulphur, after Form’s chief scientist Yet-Ming Chiang discussed the yellow element in an interview last year.

Whether sulphur meets the abundant, cheap and safe criteria is hard to say, as the element is essential to humans in small quantities, but toxic in large doses.

Jaramillo was asked if it worked by dissolving metals and chemicals into a water solution — much like Zinc8’s zinc-air battery, but he declined to answer the question directly, merely stating that it would be made of metres-long cells and look as boring as other batteries.

Form Energy has several competitors in the build-anywhere long-duration intermittent energy-storage (Baldies) space, including:

  • Canadian company Zinc8's zinc-air battery, which will have three commercial pilot projects up and running by the end of 2022;
  • Highview Power’s liquid-air storage system, known as CRYObattery, which has already been commercialised;
  • Siemens Gamesa's ETES hot-rock thermal energy storage technology, now being tested in Hamburg;
  • Stiesdal Storage Technologies' GridScale hot-rock thermal storage, which is still at the pilot stage;
  • Google X spin-off Malta’s molten-salt system, also still at the pilot stage;
  • And of course, electricity can also be stored indefinitely by converting it to green hydrogen via electrolysis, and back to electricity at a later date using a fuel cell.