Solar fluid (solar heating systems)

The liquid circulating in the closed solar circuit of a solar heating system, mostly a mix of water and glykol, is often denoted as solar heat transfer fluid or – in short – solar fluid.

In principle pure water is a very appropiate medium for transporting heat between the Collector and Storage Tank. Since however in many regions in winter the risk of freezing exists, whereupon ice crystals in the pipes can damage the collector, the water in the circuit is augmented with a special anti-freezing agent. These anti-freezing agent have to stay chemical stable, even if at hot summer days the fluid in the collectors vaporizes to gaseous state (which can happen if the thermal capacity of the storage tank is exploited and the system shuts down). In Germany it is stated by the german industry standard DIN 4757 T1, that such anti-freezing agents must not be poisonous, caustic or otherwise environmentally hazardous. Mostly propylene glykol is used as agent, which is biodegradable.

The higher the concentration of glykol in the solar fluid, the more extreme temperatures the system will endure without damages, but also the inferior the quality of the fluid as an heat transfer medium.

The so-called Drain Back Systems (DBS) don't make use of any anti-freezing agents; in this systems the pipes in and to the collector will be completely and automatically drained at shut-down.

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