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Comment by kolinko | Hacker News Reader

Carbon or tungsten I don't know, but sand as a thermal energy storage medium can be quite economical. Energy loss scales with the surface area (^2), energy stored with volume (^3).

With grid scale, above certain dimensions, you can store energy for months while maintaining economic viability.

Even for single days or weeks it makes sense. You need hot water in your home 24/7, but sun doesn't shine every day in most regions.

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Molten salt heat storage [1] is used relatively widely, with the advantage that the salt is liquid and can directly work as a heat transport fluid (usually to heat water or gas that actually turns an engine).

The same geometry that allows heat to be effectively trapped also prevents its fast extraction as radiation. A fluid that can relatively easily change its volume to surface ratio could have an advantage here, too.

[1]: https://en.wikipedia.org/wiki/Thermal_energy_storage#Molten_...

Is the sand flowing or stationary?

Flowing sand has issues with blockages and erosion.

Stationary sand has pretty low conductivity, so getting all the energy out of your 200 yard cube of hot sand might be a challenge.

Just have rods similar to nuclear (well the opposite)

You can progressively sink them in as the sand cools/ you want more energy out.

Stationary, this has already been done. It does require a heat exchanger aswell rather than getting a direct current out.

You don't really need to get the energy out fast. You typically have 8-12 hours every day where you want energy out, and similar amounts where energy goes in. Just put some pipes through the sand and you can get plenty of transfer.

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