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Doesn’t seem completely impossible with the estimated storage capacity, but like with all of these things, drastically changing the build up of concrete means changing the properties of the very thing used to hold your home together.
It might be cheap materials, but the effort involved also seem very high having to cut up after the hardening to create the super capacitors. Which again may affect structural integrity.
I use sync, but I think they still should be FOSS, or at least open source. Just because when anything gets left behind or never fixed, you can take it and patch it up yourself. That includes clients too, lots of good clients that get left behind because Dev stopped and whatever they were interfacing with changed.