On the face of it, using water in a solar thermal shouldn't be a problem, it's cheap and readily available – you could drain it down during winter when there is little sun anyway.
There are a few good reasons not to use water as the working fluid in solar thermal installations. Earlier in the year I visited a system designed to heat a swimming pool. The company charged with servicing the system filled it with water in the spring and drained it down early in the Autumn before the frosts. I was called out because the system would not hold pressure, a quick check revealed that some water had remained in the system during the winter, this water had frozen and split the flat plate collector. It was an expensive repair, and needless to say it was refilled with a glycol based solar fluid instead.
Protection from frost is only one consideration when selecting a working fluid. Look closely at the data sheet of any solar rated pump and you will see that the warranty is invalidated if water is used without 'inhibitor'. Inhibitor is a chemical additive that protects the moving parts of the pump from corrosion and should be present in all thermal systems renewable or not.
The mineral content of water varies across the country and can be described as 'hard' or 'soft'. Hard water can cause scale to build up (just like in a kettle) reducing the flow rate of the system, it can also build up in the heat exchanger of the hot water cylinder and reduce its efficiency.
The above fact is important if your solar thermal has been fitted with a 'top up valve' from the mains water. Each time you top up, you dilute the glycol fluid and introduce more mineral bearing water. Solar thermal systems should never include a 'top up' valve, though it was common practice at one time.
The characteristics of glycol based solar fluid are not perfect either, and like many things it's a bit of a trade off when compared to water. Firstly, although glycol solar fluid contains inhibitors it is also slightly 'thicker' than water. This means that the pump will have to work a little harder to move the fluid around the system – affecting the overall efficiency. The boiling point of solar fluid is lower than water and as a result needs to be held at a slightly higher pressure to prevent it boiling.
To conclude, it is always better to use the correct fluid and to check your system for pressure and flow at least once a year.
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