Solar thermal fluid, also called heat transfer fluid or HTF, is a specially formulated liquid circulated through solar collectors to absorb and transport thermal energy efficiently. These fluids are engineered to withstand high temperatures, resist freezing, and transfer heat effectively across wide operating ranges. Common types include propylene glycol solutions, mineral oils, and specialized thermal oils.
Technical Details
Propylene glycol solutions typically contain 40 to 50 percent glycol mixed with water, providing freeze protection to negative 50 degrees Fahrenheit and boiling points above 350 degrees Fahrenheit. Thermal fluids must maintain viscosity between 20 and 60 centistokes at 104 degrees Fahrenheit for proper pump operation. Heat capacity of glycol solutions ranges from 0.6 to 0.8 BTU per pound per degree Fahrenheit, lower than water but acceptable for solar applications.
Applications and Practical Significance
Solar thermal fluids are mandatory in indirect circulation and drainback systems operating in freeze-prone climates. Fluid degradation occurs over 10 to 15 years from thermal stress and oxidation, requiring periodic testing and replacement. Proper fluid selection ensures system longevity, prevents corrosion of copper and aluminum components, and maintains thermal efficiency by resisting viscosity changes across operating temperature ranges.