Abstract
Aqueous solutions of ethylene glycol and propylene glycol are used as heat transfer liquids in flat-plate solar collector systems. Both of the glycols are susceptible to thermal oxidative reactions, which produce organic acids with a resulting decrease in pH of the solutions. The effects of temperature, metals, common ions and oxidation conditions (aeration/deaeration) on the thermal stability of the glycol solutions were evaluated based on measuring changes in pH. Aerated heated glycol solutions produced acidic solutions within 3360 h (140 d) of testing, when in contact with either metallic aluminum or copper. Common ions (anions of the acid degradation products) were effective in suppressing decreases in pH, especially when aluminum was present.
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