One of the biggest challenges for building an electricity system based entirely on renewable has been the problem of what to do to protect the system in the rare situation where a big transmission line goes down or a big source of generation is suddenly taken off-line or in shifting energy from season to season.  Smoothing out the daily mismatches between supply and demand seems now to be on the way to a workable solution with some combination of storage lasting up to 4 hours and more extensive load management.  Providing reliable 100-hour energy storage has been more of a problem.  The current solution is to keep an inventory of conventional fossil-fuel fired plants at the ready, perhaps with some on-site stored liquid renewable fuel to offset the use of fossil-fuels as much as possible.  That solution seems inherently unsatisfying since it can continue the burning of fossil natural gas. 

Now two companies have stepped forward with some interesting alternatives—one based on rust and one based on compressing carbon dioxide.  The question is whether these are going to be able to meet the claims being made as reasonable solutions.

 Form Energy

Form Energy is well-along in developing its iron-air battery system which works by cycling iron from a metallic form to an oxidized form (better known as rust).   It has raised $818 million from investors, with $450 million in its Series E in October.  That’s a whopping amount considering they are only a 12-person company and haven’t yet done a commercial demo at scale, with one planned in 2023 with Great River Energy in Massachusetts (1 MW discharge capacity and 150 MWh of total stored energy).  Bill Gates and Jeff Bezos are big fans and investors.  With all that investor enthusiasm, is this the big breakthrough everyone is hoping for?

Iron-air batteries are not a new idea.  NASA was interested in them in the 1960s.  They certainly used cheap materials, but cycle life was a problem, with only 20-50 cycles being achieved.  Form Energy says it has conquered that problem with a system that should be good for 5000 cycles.  Their primary innovation is a “proton pump” which efficiently replenishes protons (hydrogen ions) back into the electrolyte.  They also have relatively low power and energy density, which means no one should expect to see them in cars.  But for long-duration storage at a fixed site, they could be very attractive.  They are aiming for an 85% round-trip efficiency, but apparently are not there yet.  If you want more technical detail, delivered in a fun way, go to Matt Ferrell’s site “Undecided” when he covered Form EnergyYou can read a critical look at the system in this article.

Form Energy claims their solution will be 1/10th the cost of lithium ion storage in terms of $/kWh.  Their target is $20/kWh of capacity but of course lithium-ion costs are still coming down as well.  They readily admit this will only occur “at scale” and scale is what they are trying to achieve.  They have done only relatively small-scale tests at this point.  The $818 million is going to optimizing a manufacturing process and doing stress testing to assure the long-life of systems.  Lots of finger crossing going on.  It feels a bit like a “Hail Mary” with everything riding on the 2023 demo.

The second is a system developed by Italian company Energy Dome that cycles carbon dioxide from a liquid to a gas and back again, using the pressure of the vaporized carbon dioxide to spin a turbine-generator.  There are no technology advances involved.  It is an application of existing technology.  The idea is like Compressed Air Energy Storage (CAES) systems, but with a twist.  

To get the most out of CAES requires liquefying air to get the best energy density.  However, that requires cryogenic temperatures which require a lot of energy to reach.  Carbon dioxide liquefies at room temperature with the application of about 1000 psi in pressure.  That’s not all that much—nowhere near the 5,000-10,000 psi planned for use in gaseous hydrogen storage, for example.

Energy Dome works by storing liquefied carbon dioxide in a series of small high-pressure tanks and then extracting the energy by heating and expanding the gas through a turbine that powers a generator.  The liquid expands about 600-fold in volume.  The expanded gas goes into a big “bag” that looks like a blimp sitting on the ground.  The carbon dioxide is at atmospheric pressure, so the blimp doesn’t have to be very strong.  To recharge the system, the carbon dioxide is taken into the turbine-generator working in reverse—the generator becomes a motor and the turbine becomes a compressor.  

There are some important details.  Compressing the gas heats it up.  The gas needs to be cooled back to room temperature to get the carbon dioxide to liquefy.  In reverse, heat is needed to get the liquid to evaporate quickly.  Energy Dome stores hot and cold fluid to reduce the use of on-site energy to do these two functions.  Still the system is more of an energy user than the Form Energy battery, reflected in the 75-80% round-trip efficiency that Energy Dome is targeting.

The primary question with Energy Dome is about cost, not technology.  The system requires a lot plumbing, tanks, valves, turbines, etc.  While all those are commodity products with known costs, it is the total cost of the system as installed that is important.  Energy Dome has partnered with a major European wind farm company, Ørsted.  The plan is to build a 20 MW system with 10 hours of storage and have it operational in 2024.  Expanding to 100 hours duration is just a matter of adding more pressurized tanks and more “blimps” to the existing compression/expansion equipment.  The 2024 demo will be key to seeing whether cost can be contained enough to make this a reasonable solution.

Form Energy is certainly winning the enthusiasm from clean tech investors.  Still it has a lot to prove about its technology and about its costs.  Energy Dome needs to contain its capital costs.  Maybe both will work out.  We’ll come back to this next year.

Thomas Hall


Gary Simon is the Chair of CleanStart’s Board. A seasoned energy executive and entrepreneur with 45 years of experience in business, government, and non-profits.

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