Propylene glycol has become a very common heat transfer fluid which is used in the solar heating systems as an anti-freeze. But the alternative of it is very toxic which is ethylene glycol, any many do not recommend it for household heating system. Many are unaware of its characteristics and it toxicity which poses a danger. You will have to look for propylene glycol supplier that can formulate glycol products which are compatible with solar heating suppliers. There are suppliers like alibaba to for this. I guess they sell at a reasonable price. I think propylene glycol is what should be used. The rest I guess depends on what kind of solar heating system is used. If the manufacturer recommends something else that might be suitable. I hope this is helpful for people who would have a similar doubt when they come to this thread.