The Chemistry of Solar Part I: Propylene Glycol

Propylene Glycol is used in a lot of solar thermal applications. But why? Simply put it’s because of its heat transfer properties. So what does that mean?

Well, let’s take water as a baseline. Water freezes at 32˚F (0˚C) and boils at 212˚F (100˚C). Using only water can be problematic as most solar systems are located outdoors. If a system is outdoors during a cold day and with no sun to heat system, the fluids inside the system could freeze. Of course if the water freezes, the system would no longer operate, but additionally, as the ice expands it could break the system causing leaks or other damage.

Since these systems are designed to produce heat, one might also wonder what happens when the water gets too hot. At a certain temperature the water will boil and create steam. For a closed system, this can become dangerous as steam can significantly increase the pressure in the system. Most closed systems are designed for this and can take extremely high temperatures before becoming a hazard.

So what happens if we use propylene glycol instead of water? Well, the freezing and boiling points of the system change significantly. With a 60% solution of propylene glycol to water, the freezing point drops to -55˚F and the boiling point increase to 225˚F. This makes solar applications possible in cold locations.

One negative about propylene glycol is that adding it will decrease the specific heat of the fluid. The specific heat is a measure of the fluid’s ability to store heat. In other words, water does a better job of storing heat and keeping your system warm for an extended period of time. But, depending on the application this may or may not be desirable.

Propylene glycol is a very safe chemical that poses no health hazards, is not reactive and takes extremely high temperatures to catch on fire. You can find it in pharmaceuticals, food coloring, sanitizers and many other commonly used products.

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