The thermal capacity of fluid:
It is the ability of the fluid to carry or store heat.
Boiling Point and Freezing points of the fluid:
It is the temperature at which the fluid might change its state i.e from fluid to gas and from fluid to solid.
It is the minimum temperature at which a vapor can ignite in air.
This one most of us would know. It is the capacity of a liquid to withstand shear forces.
The coefficient of expansion:
It is the indicator of minuscule changes in the fluid that occur with the change in temperature.
This brings us to the options of fluid transfer liquids that can be used in a solar water heater:
Easily available, virtually free of cost, non-toxic. Has high specific heat and low viscosity. The only problem with water is that it has to be pH neutral for its application here. Also, it has a high freezing point and a low boiling point. So unless you have soft water handy, using water as a heat transfer fluid would not be recommended as it may cause scaling.
A mixture of water and ethylene glycol and propylene glycol is also used as a heat transfer fluid. This mixture is also called antifreeze. As the liquid, in this case, won’t freeze in extremely cold conditions. This mixture degrades over time, hence it is advisable to change it every 3 years.
3.Phase change liquids and Refrigerants:
These are used widely in air conditioning and heat transfer appliances. These fluids have the high thermal capacity and low boiling points. They are also highly thermally efficient. They are best suited to solar operation as they respond very nicely to solar energy.
Now coming to the point about the ideal heat exchange fluid. Well, the answer to this lies in the environment where the solar heater is to be used. If its to be used in a place with stable or predictable weather conditions then a clean water will also do. If you plan to use it in places with sub-zero temperature then antifreeze will work. Hence the choice of heat exchange fluid will vary with pertaining working conditions.