from Hacker News

Concrete spheres for energy storage; California plans a 9-meter diameter sphere

by joe_the_user on 5/10/25, 10:49 PM with 39 comments

  • by bell-cot on 5/10/25, 11:34 PM

    > In 2026, a sphere nine metres in diameter and weighing 400 tonnes will be submerged off the coast of California at a depth of 500 to 600 metres. It will have a storage capacity of 0.4 megawatt hours (400 kWh), enough energy to power an average household for several weeks.

    What is this going to cost? From a quick search, Tesla Megapacks are now about $250/KWh. With battery costs still falling steadily, those might be considerably cheaper by the time the first 9m sphere hits the water.

    And with all the recent anchor-dragging incidents, how many countries would be eager to have their energy storage located far off-shore?

  • by joe_the_user on 5/10/25, 11:15 PM

    This seems like a workable idea but just electric cranes lifting and lowering weights seems like a simpler approach and I think that has already been proposed.

    My guess is there are many ways to balance the grid and the biggest is the utilities not wanting pay.

    One thing to consider is the America power grid is in poor shape already and utilities are aiming to avoid modernizing. IE, adding storage to the grid would involve the double of cost of the actual balance equipment and the fixing the old equipment that needs fixing anyway. And utilities are looking avoid both cost.

  • by stubish on 5/11/25, 12:53 AM

    I'm curious why pumping the water out, creating a vacuum for water to rush in, is better than pumping air in for water to displace.

    I had assumed it would be cheaper to have large underwater balloon connected by a hose to a pontoon, and use air. Rather than install and maintain at depth the pumps and a giant concrete sphere able to withstand that sort of pressure.

    Have I got the economics wrong? Or is there an efficiency gain from dealing with a liquid rather than compressible gas?

  • by engineer_22 on 5/11/25, 1:54 AM

    Article claims pumping water out to a "relative vacuum"

    How do they do this without causing the pumps to cavitate?

    Edit: here's the paper https://www.sciencedirect.com/science/article/abs/pii/S23521...

  • by mikewarot on 5/11/25, 6:56 AM

    This sounds like an actually reasonable thing, as long is there's a large exclusion zone around it, and they don't put a second one anywhere near it.... a catastrophic failure of one would take them all out, possibly killing all the marine life nearby, and damaging any submarines in the area.

  • by manarth on 5/11/25, 8:08 AM

    The wikipedia article has a lot more detail on the proposal: https://en.wikipedia.org/wiki/Stored_Energy_at_Sea

    The target depth is 750m (compared to 100m for the pilot in Lake Constance).

  • by yummypaint on 5/11/25, 8:29 AM

    I wonder what the plan is to deal with biofouling, it's a notoriously challenging thing to deal with on surface ships with barnacles etc clogging intakes. Maybe it's less of a problem on the sea floor?

  • by mrDmrTmrJ on 5/11/25, 12:37 AM

    Is there any reason to need a concrete sphere? Couldn't a robust, durable flexible bag do the trick?

    My hydroflask, when compressed, will push water out :)

  • by cosmicgadget on 5/10/25, 11:03 PM

    20-year maintenance intervals for something pumping seawater? Impressive. Way better than a chemical battery.

  • by klysm on 5/11/25, 1:14 PM

    Cool idea. It’s like pumped water storage but backwards