[osolemio]

The solar panels are not expected to reach 200 C, on the contrary, they should ideally be kept at 75 C or lower.

In case there is a fault in the thermal transfer system, or anything else that causes the temperature to runaway, the system should survive without substantial damage.

The panels themselves can sustain the 200 C stagnation temperature, although the PV part of them (PV-T focused panels) will degrade faster, and yield less, the higher the temperature is.

It is an experiment into ways of dealing with solar which is a combination of existing and new technologies. Being an experiment, it must be protected against unforeseen circumstances, whether that be freezing, boiling, overpressure and so on.

Since I cannot use the drain back principle, due to the design of the heat absorbers, I need to find the optimum liquid which ideally has a freezing point below -30C, high boiling point (ideally 200 C or more), doesn't gel, doesn't cost a fortune, isn't toxic and doesn't evaporate. I will have to look at characteristics of specific heat capacity vs pump power required as well, though I realise I cannot get everything but have to compromise.

I wasn't sufficiently specific in the first post, sorry about that, but I hope you understand the requirements now.

The project is located in Scandinavia. Had it been in Hong Kong, I wouldn't have had to care about freezing, on the other hand, I wouldn't have a high heating requirement either.