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You don't need solar cells to heat water to boiling, just glass or a metal like aluminum or silver.

With glass you can build a lens to concentrate light. With a metal you can build a concentrating mirror.

Good points. But lens building is a complex endeavour that requires tooling at the manufacturing level (especially computing power!) - no matter the choice of available material, yes? Doing it with high precision at a civilisation-powering scale seems like an impossible ask of a species that was still considering much of their Home planet to be “new” (Americas).

Galileo was making lenses hundreds of years before the time we are talking about. Also, eye and magnifying glasses existed then.

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