GREEN SYSTEMS SOLAR SOLUTIONS
Propylene glycol: Solar heat transfer fluid
BY BRISTOL STICKNEY CONTRIBUTING WRITER
like the ‘Freon’ fluid in a refrigeration system. To insure that the solar heating system is reliable over a very long time, the heat transfer fluid in the system must not leak out, it must not freeze, it should not boil and it must tolerate high temperatures inside the solar collector without ‘cooking’. Propylene Glycol (PG) has become the most common
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heat transfer fluid used in closed loop solar heating systems that contain antifreeze. It has a long track record over many decades in this application and is widely available from a number of sources at a reasonable cost. This is not automotive antifreeze, which is a different substance (Ethylene Glycol), is much more toxic and should never be used in domestic solar heating equipment. When working with PG it is good to get to know its properties, capabilities and limitations which have a direct bearing on the pumping, piping components and temperature controls required by these systems.
Non-toxic Solar home heating systems are most often used to heat
potable domestic hot water, and single-wall in-tank heat exchanger coils have become very popular for this purpose. When a single wall heat exchanger fails, it is possible for the heat transfer fluid in the coil to leak into the potable water. Because this (and other environmental leakage) is a real possibility, the ideal solar heat transfer fluid would be biodegradable when released into the environment, and non-toxic if consumed by people or animals. Pure propylene glycol has a very high score in this
regard, as evidenced by its use as a food and drug additive. Millions of people consume pure PG as part of their diet every day, mixed into their food and medications. So, the question is, how pure is the PG used in solar heating systems? The answer is typically 95% pure before it is mixed with water. Typical PG heat transfer fluid contains additives to prevent corrosion and boost the resistance to high temperature degradation. The additives make up about 5%, by weight, of the concentrated PG fluid. The concentrated fluid is mixed with de-mineralized water before final usage, so for example, if mixed half and half with water, the final concentration of additives would be about 2.5%. These small concentrations of additives are apparently
nowhere near toxic levels. The makers of the PG heat transfer fluid provide Material Safety Data Sheets (MSDS) for the concentrated and the pre-mixed products. The MSDS language is very reassuring. For example,
“First-aid measures” listed on one of these sheets include the following entries: Skin Contact:Wash skin with plenty of water. Inhalation: Move person to fresh air; if effects occur,
consult a physician. Ingestion: No emergency medical treatment necessary. The MSDS listing under “Ecological Information”
n any hydronic closed-loop solar heat collector system the heat transfer fluid is the life-blood. It must be sealed and pressurized in the solar heat piping, much
seems equally benign: Persistence and Degradability — For the major component(s):
Material is readily biodegradable. ECOTOXICITY — For the major component(s):
Material is practically non-toxic to aquatic organisms on an acute basis.
Heat tolerance for some common brands
Look for PG manufacturers who
specifically formulate their glycol products for compatibility with solar heating systems. Those that do will say so very clearly in their product literature along with a high temperature rating that indicates compatibility with the normal operating temperatures of hot solar collectors. Pure PG will ‘cook’ at high temperatures and long exposure will cause it to change from a clean, transparent liquid to a brown substance resembling molasses that smells like alcohol. The MSDS listing for DowFrost, for example,
acknowledges this in the section under “Thermal Decomposition” which states: Decomposition depends upon temperature, air supply and the presence of other materials. Decomposition products can include and are not limited to: Aldehydes, Alcohols and Ethers. In other words, the heat transfer fluid will remain
thermally stable in a closed system at recommended temperatures and pressures. But if the high-limit temperatures are exceeded and/or oxygen is introduced into the closed system, the fluid will degrade. During decomposition, gases are generated that can cause extra pressure in closed systems as well. So, you can see that preventing the glycol from
overheating is a design consideration of primary importance. That is why solar heating design discussions (even in this column) so often focus on controlled overheat dissipation (heat dumping) to keep the solar collectors below the high-limit temperature of the glycol in question. When overheat controls are provided, they are often set to keep the collectors below 220F (104C) to extend the life of the glycol. Here is a short list of some common glycol brands and their temperature ratings as listed by the manufacturers. DowFrost and DowFrost HD DOWFROST inhibited glycol-based fluid has an
effective operating temperature range of -50°F to 250°F (-46°C to 121°C), while DOWFROST HD inhibited glycol-based fluid is effective from -50°F to 325°F (-46° to 163°C). Cryo-tek and Cryo-tek AG Use any Cryo-tek anti-freeze in hydronic closed loop
solar heating systems that require freeze protection. Operating Temperature Range for Closed System: Up to
e Turn to STICKNEY on p 92
phc august 2011
www.phcnews.com
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