A startup’s quest to store electricity in the ocean
31 comments
·November 7, 2025edarchis
Relying on a salinity differential, even between salted and unsalted, seems like a terribly small amount of energy. There are projects to put large spheres at the feet of offshore windmills to pump water in and out. That has some pressure challenges but store a lot more.
The advantage I see for the salinity difference is that you can make them a lot larger than the pumped water ones. But is worth it, I'm skeptical.
alex_duf
What I don't understand is how the top reservoir is floating when filled with brine. Are the small floaters enough to hold it up?
Otherwise I love the fact that's simple. Simplicity scales. It's also salt water, so assuming they're not putting anything else than NaCl, it can break and it's no big deal
sixtyj
From the article:
[Company] has tested a small model of the reservoirs in wave tanks and off the coast of Reggio Calabria, Italy. It’s now deploying a pilot of the floating components in advance of a full demonstration plant. By 2026, it’s hoping to deploy several commercial projects at sites around the world.
At full size, the turbines would generate around 6 to 7 megawatts of electricity each, and there will be one for every 100 meters of pipe. Deeper sites would have more storage potential, and each commercial site would host multiple reservoirs. Sizable hopes to deliver energy storage for €20 per kilowatt-hour (about $23), about one-tenth what a grid-scale battery costs. —-
Testing in calm reservoire is different from potentially .wild offshore (ocean/sea)
What happens to 100-200 m long pipe in underwater waves when e.g. a hurricane or a storm comes?
curiousObject
What happens to 100-200 m long pipe in underwater waves when e.g. a hurricane or a storm comes?
That’s an excellent question, but it is also similar to asking what will happen to wind turbines in a storm.
Maybe some will break. Maybe that’s an acceptable outcome. Probably they can be improved to reduce that risk
alex_duf
It's anchored to the seafloor. Also surely we have the technology to hold a pipe in high sea, as this is what petrol platforms are doing.
closewith
> What happens to 100-200 m long pipe in underwater waves when e.g. a hurricane or a storm comes?
Nothing, to a rounding error. The effects of surface storms are only noticeable to ~2x wave amplitude.
There are plenty of other forces at work, especially tides, but storms will only affect the surface plant.
vkou
Even in a storm, just a few meters below the surface (half the wavelength), the sea will be calm.
The bigger issue with this idea is that it's a megastructure sitting in the ocean, and salt water turns everything it touches into shit. Oh, and there's very little energy storage potential from just a salt gradient. You need to move way more water, to get less energy, but your container costs are fixed.
Land-based pumped hydro has no shortage of engineering problems (and risks if, you know, you get a dam collapse), but this has colossal capex costs.
theoreticalmal
Wait a second $23/kWh? I pay ~ $0.15/kWh for power at my residence the majority of the year. Is this a proof of concept number? What am I not understanding such that the power this produces is 4 orders of magnitude more expensive than what’s in place currently?
NooneAtAll3
"An average lithium battery costs around $139 per kWh in 2024" - random result from first page of googling in ddg
https://www.renogy.com/blogs/buyers-guide/how-much-does-a-li...
Maxion
I think they mean kWh of storage capacity – your talking about your energy costs which in a battery is the round-trip cost.
Battery capacity and energy consumption are measured in the same unit.
alex_duf
kWh of capacity, as compared to a kWh of capacity on a battery, over the lifetime of the product.
On each of these kWh you'll have (hopefully) multiple orders of magnitude of charge cycles
Gabrys1
My understanding of this technology is that it's closed-circuit. No water is exchanged between the power plant and the ocean once filled with ocean water.
badestrand
To be honest, I find it a bit hard to understand even from the video. The top part doesn't look like it has any container at all.
jnovacho
How exactly are they pushing the brine against the ~50BAR pressure differential?
joha4270
They're not dealing with a pressure differential. Or at least I don't think so.
I don't think the Journalist who wrote the article understood the technical details, but from digging a little at their website I think what's going on is they're moving heavy brine up and down, all of it equalized with local pressure.
Despite them describing it as pumped hydro, I think its better framed as a cousin of the "chunk of concrete suspended over a mine shaft" style gravity battery. Replace the mineshaft with water and the concrete with salt.
CMYKninja
The maintenance on this will be a real killer and by the time you build the robotic infrastructure to maintain it you’re not a power company anymore kindof how Amazon isn’t a bookseller.
world2vec
Maybe I'm missing something but won't submerged structures like these get all covered in barnacles in a few months?
marcyb5st
I don't think it is a problem for the outside shell, or maybe just a minor one. For the interior of the reservoirs, I guess the hyper salty water will kill everything that tries to grow there.
klntsky
> Sizable’s reservoirs could connect to any grid that’s near waters that are at least 500 meters (1,640 feet) deep.
How many big cities are there on earth with that depth available nearby?
OtherShrezzing
It depends on the definition of "near", but there's a sizeable population within ~40km, which is a reasonable distance for an offshore wind-farm.
Almost the entire Mediterranean is >500m depth within just a few km of the shore, and that's half a billion people. All of the eastern seaboard of the North+South American continent is available at 100km distance (another 100-200mn people). Most of west Africa, all of Australia, and almost all of the western flank of the Pacific.
Maybe a quarter of all people live within 40-50km of a 500m deep sea. Definitely a large TAM.
tiarafawn
Could also be interesting in case the idea of under water data centers ever returns
https://en.wikipedia.org/wiki/Project_Natick
Or it could help offshore wind farms provide a more stable/predictable output.
worldsayshi
80% round-trip efficiency sounds very good. What am I missing?
marcyb5st
That to store enough energy with just haline gradient the reservoirs need to be enormous
jillesvangurp
And under water construction is expensive. And durable construction in a marine environment is challenging (and makes things more expensive).
That doesn't mean it's a bad idea but they are factors that add to the overall cost. 20$/kwh is very attractive of course. But that's also a number that e.g. CATL is chasing with sodium ion batteries. And they are going to be making those by the gwh/year from next month.
Mistletoe
Is the salt NaCl?
It sounds quiet inefficient to me. The energy differential comes from the different salt concentrations, so you have to move a lot of water to exploit a relatively low mass differential.
Mentions of efficiency are conspicuously absent from the article.
Another potential problem is marine ecology: pumping high-salt sea water to the top and releasing it en masse might lead to much larger fluctuations in salt concentration than what the ecosystem is used to.
That said, we need many different approaches to solve energy storage, and I hope to be wrong, and that they end up very successful.