Propylene glycol is the non-toxic antifreeze mixed with water to protect closed solar thermal circuits from freezing. A 40% glycol mixture protects to about -21 °C and a 50% mixture to about -33 °C. Solar-grade fluids add corrosion inhibitors and must be checked every 2–3 years, because repeated stagnation above roughly 170 °C degrades glycol into corrosive acids.
Why do solar thermal systems need antifreeze?
Solar collectors sit outdoors and routinely drop below 0 °C — on clear nights, radiative cooling can pull an absorber several degrees below air temperature. Plain water would freeze, expand by roughly 9% and split absorber tubes and pipework. Mixing propylene glycol into the closed circuit lowers the freezing point below any realistic winter design temperature. Unlike the ethylene glycol used in car cooling systems, propylene glycol is a low-toxicity substance also used in food and cosmetics, which is why it is the standard fluid wherever a single heat-exchanger wall separates solar fluid from potable water. The main alternative is a drain-back system, which removes the fluid from the collectors instead of protecting it chemically.
What glycol concentration protects against frost?
A 40% propylene glycol mixture protects to about -21 °C and a 50% mixture to about -33 °C — between them, these two concentrations cover practically all of Europe. Solar fluids are supplied as ready mixes or dosed on site with demineralised water:
| Propylene glycol share | Freezing point | Typical climate |
|---|---|---|
| 30% | ≈ -13 °C | Mild coastal and Mediterranean |
| 40% | ≈ -21 °C | Most of Central Europe |
| 50% | ≈ -33 °C | Alpine, Nordic and continental |
Below its freezing point the mixture first forms a pumpable ice slush before it solidifies, so burst protection extends roughly 10 K further down than the listed values. Concentrations above about 55% bring no useful gain: frost protection plateaus while viscosity keeps climbing.
How does glycol change heat transfer and pump sizing?
Glycol carries less heat than water and resists flow more. A 40% mixture has a specific heat capacity of roughly 3.7 kJ/(kg·K) against 4.19 for pure water — about 12% less — and several times water's viscosity at low temperature. The circuit therefore needs a slightly higher flow rate for the same transported power, and the solar pump station must be selected with the extra pressure drop in mind. Always verify the installed concentration with a refractometer rather than trusting the label on the canister.
Why does stagnation age the fluid?
When the tank is full and the pump stops, collectors climb toward their stagnation temperature — often 170–200 °C in flat-plate collectors and higher in evacuated tubes. The fluid boils, and repeated high-temperature episodes crack glycol molecules into organic acids that push pH down, consume the corrosion inhibitors and turn the fluid brown. Collector manufacturers — Solimpeks included — publish the stagnation temperature of each model from ISO 9806 testing, and that is the number the fluid must survive. Solar-grade propylene glycol fluids are formulated with high-temperature stabilisers for exactly this duty, but no fluid is immune: a system that stagnates every summer afternoon needs fluid changes far sooner than one designed to avoid stagnation.
How often should solar fluid be checked and replaced?
Check the fluid every 2–3 years and after any summer with known stagnation events. Three tests tell the story: frost protection with a refractometer, pH with a test strip (fresh fluid is typically 8–9; replace below about 7), and a look-and-smell check — dark colour or a burnt odour means the fluid is spent. Typical service life is 5–10 years. Replace like-for-like with an inhibited solar-grade product, never top up with plain tap water beyond small corrections, and re-test concentration after any intervention.
Frequently asked questions
Is propylene glycol in solar systems toxic?
No — propylene glycol itself is a low-toxicity substance used in food and cosmetics and registered under EU REACH. Solar fluid is still not drinking water: it contains corrosion inhibitors, so cross-contamination into potable circuits must be prevented by design and spent fluid disposed of under local rules.
What is the difference between propylene glycol and ethylene glycol?
Ethylene glycol transfers heat slightly better and costs less, but it is toxic to people and animals. Drinking-water safety practice therefore requires non-toxic propylene glycol in solar systems where only a single heat-exchanger wall separates solar fluid from potable water.
Can I top up my solar system with water?
Only in small amounts, and only if you re-check frost protection with a refractometer afterwards. Larger losses should be replaced with the correct premix — diluted fluid loses both freeze protection and corrosion inhibition.
How do I know when solar fluid must be replaced?
Three signs: pH below about 7, frost protection weaker than the design value, and dark colour or a burnt smell. Any one of them justifies a fluid change — typically due every 5–10 years.