it seems with CaCl2, the magic number is 43%. Above 43% CaCl2, below 57% H2O, crystals appear in the solution. Above 57% H2O, the salt is fully dissolved. At room temperature.
Calcium chloride has five hydrate states:
772 °C (anhydrous)
260 °C (monohydrate)
176 °C (dihydrate)
45.5 °C (tetrahydrate)
30 °C (hexahydrate)
At atmospheric pressure.
So, at 45.5 degC it naturally dehydrates to dihydrate. Which is well in the range of solar thermal.
Aha! found another chart! Relative humidity vs Temperature
