Just as an FYI:
I have a prototype sand battery I am testing for a hot-air heater.
Using a stovetop element and connecting directly to my test 250W panel, in early spring, it easily gets to 500C inside a couple of hours. I have had it over 750C in summer.
PEX will not handle such temps.
This week I am upgrading to a 44 Gal drum with a 3000W element, and hopefully, a pair of new ~400W panels. I've ordered three new k-type thermocouples so I can measure the data properly (safe up to 1250C).
The element is inside a sand-filled 60L steel drum, sitting on a couple of bricks inside the 44 Gal drum, also filled with sand. 50mm tubes go through the sand outside the 60L drum through which air flows and is collected into ducting and piped up to a grate in the floor above.
For water transfer, you really need a copper or steel coil. Copper is good up to 1050C, steel to 1700C. I would use an intermediate heat-exchanger rather than use the heated water directly. Something like a coil in the sand with mineral oil or waste oil, into a tank which is plumbed with PEX. That way, you are never exceeding 100C in water.
