In any hydronic closed-loop solar collector system, heat transfer fluid is the lifeline. It must be sealed and pressurized in the solar heating piping, much like freon fluid in a refrigeration system. To ensure that a solar heating system is reliable over a long period of time, the thermal fluid in the system must not leak, freeze or boil, and it must be able to withstand the high temperatures inside the solar collector without "cooking".
Propylene glycol (PG) has become the most common heat transfer fluid used in closed-loop solar heating systems containing antifreeze. It has a long track record of decades in this application and is widely available from many sources. This is not an automotive antifreeze, it is a different substance (ethylene glycol) that is much more toxic and should never be used in home solar heating equipment. When using PG, it is best to understand its properties, capabilities and limitations, which have a direct impact on the pumps, piping components and temperature control required for these systems.
Solar home heating systems are most often used to heat potable domestic hot water, and for this reason, in-tank heat exchanger coils have become very popular. When a single-wall heat exchanger fails, there is a risk that the heat transfer fluid in the coil will leak into the potable water. Because this (and other environmental leaks) are real, the ideal solar heat transfer fluid is biodegradable when released into the environment and non-toxic if consumed by humans or animals.
Pure PG scores highly in this regard, as evidenced by its use as a food and drug additive. Millions of people use pure PG daily as part of their diet, mixing it into their food, cosmetics, medications, and more recently, inhaling it while snorting. So, how pure is the PG used in solar heating systems? The answer is typically 95% pure before it is mixed with water. A typical PG heat transfer fluid contains additives to prevent corrosion and improve resistance to high temperature degradation. Additives make up approximately 5% of the PG fluid concentrate by weight. The concentrate is mixed with demineralized water before final use, so, for example, if it is mixed half and half with water, the final concentration of the additive is about 2.5%.
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