Image: RESOL GmbHThere are several factors to keep in mind when thinking about a heat transfer fluid - the coefficient of expansion, viscosity, thermal capacity, freezing, boiling points, and flash point. Each of these is important in its own way, and the importance level of each depends on your climate.
In a warm tropical area you won't need to worry about the freezing point of the fluid in your solar water heating system, but the type of solar fluid you choose will make a difference in colder climates. Also, viscosity determines how much energy is needed to move the solar fluid, and thus how efficient the heating system is.
Heat transfer fluids and their properties
While you'll want to contact a local expert in solar fluid for solar water heating, you don't need to go in unprepared. Here are some common heat transfer fluids and their pros and cons.
Water is a cheap and plentiful heat transfer fluid, but it can freeze easily in colder climates. It can also come with many problems, such as corrosive pH or mineral buildup. Mixtures of water and glycol anti-freezes can help get around that problem, but be forewarned that a combination solar fluid is more expensive. A water-glycol mixture is the most common solution in the UK area.
Hydrocarbon oils can be more effective for your solar fluid heating system, but they are more viscous than water. This means they take more energy to pump, which runs the risk of costing more money than the solar heating system saves. Some of them are also potentially toxic and require a more dedicated system of enclosures to protect against leaks. Refrigerants are very effective at transferring heat and more effective on cloudy days than other solar fluid types, but many of them are dangerous for anything outside of commercial use.
In the end you should contact a local specialist about which solar fluid is right for your heating system. They will help you choose the most appropriate solar heating components.