Looks like a
heat accumulator, which comes in many sizes (from a few hundred to tens of thousands of liters) and flavors (water, phase change materials).
Instead of heating the "pool" directly with mirrors, I think it may be better to superinsulate the tank and use solar collectors. Not as simple, but probably more efficient in the long run.
Have you been over to
EcoRenovator?
__________________
e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread