I've seen very simple DIY versions of this (stove coil directly wired to a panel) for home heating. At current panel pricing, I wonder if there is a config with a reasonable ROI.
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For an iterated version of this design, there's also this project posted a few weeks ago.
That's a very poorly thought out design: https://news.ycombinator.com/item?id=45789850
No worries, iteration is good and more iteration is better! I look forward to seeing how this new build performs. If I can link to a better set of (preferably tested) instructions I'd be happy to.
if you just want to store day's energy for night's heating a big ole tank of water is far easier solution to integrate with whatever house heating system you have (tho it performs vastly better on underfloor heating just because of the lower temperatures needed).
Just pump, mixing valve, and some heating coils. You could even just heat the water directly on the solar heat panels but frankly using electric panels is a bit more flexible as it is less plumbing, and power for the rest of the house.
Block of sand have more sense if you need to store it for longer and/or have industrial source of over 100C heat that can be used to fill it up.
Solar panels can pretty easily produce way over 100°, like 1000°. All you need is a nichrome wire. 1000° sand has about 3× as much energy as 100° water by mass, about 7× by volume. Water is cheaper but harder to contain.