When a solar water heater is used in mountain regions, the cold temperature may freeze water in pipes during night. What are the anti freezers and how can we use them?
Several chemicals are being used as antifreeze heat transfer fluid in SWH systems. The most common is propylene glycol (monopropylene glycol - 1,2 propanediol), which is widely used in many other applications. Propylene glycol is generally safe and nontoxic, and it is commonly used in domestic hot water systems due to its safety. On the other hand ethylene glycol (monoethylene glycol - 1,2 ethanediol), which is mainly used for engine cooling systems is considered toxic and not normally used for domestic SWH for that reason, although it has good thermal and physical properties.
Another alternative is bio-derived glycol (trimethylene glycol - 1,3 propanediol), which is produced from corn sugar using the process that produces 30% less greenhouse gases comparing to producing propylene glycol. In addition, it has better performance (lower viscosity) than propylene glycol at lower temperatures.
The most common glycol-to-water ratio is 50:50 or 60:40. Of course this ratio depends on local temperatures and should be tailored according to glycol mixture table (see for example: www.draughtquality.org/wp-content/uploads/2012/06/GlycolCharts.pdf).
In some instances (depending on the mineral content of the local water supply ) the mixture should be made with distilled water to avoid minerals or compounds that could corrode copper piping and other components of SWH system.
Of course, using antifreezes implies a heat exchanger (indirect or closed loop systems) that separates the potable water from the heat transfer fluid. In addition, since such mixture is more difficult to pump than water, a careful pump selection is required, especially for systems with long pipe runs and multiple collectors.
In our region in Switzerland we use water solution with 38%-propylene glycol .
How Milan said, you need then two separated circuits.
There is also the possibility to maintained a certain flow in the collectors to stay up to the freezing point using a small amount of the heat in the water storage; or you can also drain the collector when they are not in operation (drain back solar systems). So doing you can use only water. http://www.paradigma.de/drain-back or you can have a look to the "aqua solar system" produced by Paradigma.
Regards,
Serena
Don’t forget to add some corrosion inhibitors as is the case in commercially products, pure water-glycol mixtures are aggressive to most metals of the primary circuit
As far the cold condition maintain the proper the flow of water in solar water heater tank and their is drain the collector when they are not use in operation. The freezing point of the water is important parameter take into consideration.
Regards,
Shridhar
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