What heat transfer fluids are used in solar water-heating systems?
The following are some of the most commonly used heat-transfer fluids and their properties: • Air Air will not freeze or boil, and is non-corrosive. However, it has a very low heat capacity, and tends to leak out of collectors, ducts, and dampers. • Water Water is nontoxic and inexpensive. With a high specific heat, and a very low viscosity, it’s easy to pump. Unfortunately, water has a relatively low boiling point and a high freezing point. It can also be corrosive if the pH (acidity/alkalinity level) is not maintained at a neutral level. Water with a high mineral content (i.e., “hard” water) can cause mineral deposits to form in collector tubing and system plumbing. • Glycol/water mixtures Glycol/water mixtures have a 50/50 or 60/40 glycol-to-water ratio. Ethylene and propylene glycol are “antifreezes.” Ethylene glycol is extremely toxic and should only be used in a double-walled, closed-loop system. You can use food-grade propylene glycol/water mixtures in a single-walled heat excha
