In a solar heating system the energy gained is during the day is stored in large tanks for use at night.Why is neither pure water or pure ethylene glycol used? O_O please help me.
Solar thermal systems use a blend of a glycol and water to take advantage of favorable features. Glycol is included because it lowers the freezing temperature of the blend, to prevent formation of ice. Water is included because of its excellent thermal conductivity and specific heat capacity, highest among just about all fluids. The glycol is more often propylene glycol than ethylene glycol. It is surprising, but true, what the difference of two hydrogens and a carbon make in terms of toxicity...but propolyne glycol is relatively non-toxic, unlike its ethylene glycol counterpart. So, if the fluid blend is to interact with domestic use water on opposite streams of the heat exchanger, it is best to choose propolyene glycol in order not to make it a poison. If it isn't, and only will be used for heating, then ethylene glycol can be used.
