The industry standard for solar fluid is a mix of propylene glycol and water. It's non-toxic, biodegradable and relatively stable. However, you should always mix as much water as possible with your choice of glycol product, as simple water is the ideal solar fluid.
All glycol products used in solar fluid mixes come with a dilution chart, an example of which is shown below. It is vital that you consult the chart to avoid mixing too little or too much glycol into your system – too little will risk consequences in cold weather, too much will reduce the mix's efficiency. Dilution charts always list two figures: freeze and burst.
The freeze figure is the percentage mix required to protect against ice crystals forming in the mix at the outside temperature in the first column. This is important because propylene glycol mixtures get thicker as they cool, which makes them a lot harder to pump around the system. As they approach their freezing point, ice crystals form in the mix and the fluid becomes a kind of jelly – it won't move, but it won't burst your pipes.
The burst figure is the percentage mix required to protect the mix from freezing solid and bursting your pipes (assuming they're rigid pipes, which they usually are). This is obviously catastrophic for any solar thermal installation and must be avoided.
When you're choosing a glycol, bear in mind the following:
- Never use less than 20% glycol in the mix.
- Never use more than 60% glycol in the mix.
- Always mix in as little glycol as possible to keep optimum efficiency, but never risk burst pipes.
- Always pick a glycol that provides protection at 10°F lower (around 6°C) than the lowest temperature ever recorded in your location.
There's an obvious question to answer here: what happens to your solar thermal system between the freezing and bursting temperatures? The answer is that the solar fluid becomes too thick to pump, so the system stops working. You're still protected from burst pipes and once the temperature rises, the mix will start flowing again.
Regardless of the exact amount and type of glycol you use, it's important to bear in mind that the chemical is not as pump-friendly as water. Glycol is a viscose material that is slippery and harder to push around the pipes, so it is very important to bear your percentage mix in mind when you're calculating pump sizes.
The main downside of a glycol/water mix being essential – apart from reduced pumping efficiency – is that the mix deteriorates over time, eventually wearing out. As it deteriorates, it increases in acidity, and can damage your system components. It also becomes less resistant to cold temperatures and thus provides less protection against freezing and bursting.
In an ideal situation, the deterioration process takes between 15 and 20 years, but it is quicker in systems where the fluid lies stagnant for long periods or where extremely high temperatures are experienced.
You should check your solar fluid regularly, using litmus paper to verify the acidity and a refractometer