Indirect Solar System: It is designed to accommodate climates where freezing weather occurs more frequently and a drain back system cannot be installed. This type of system is also called a Closed Loop system.The solar storage tank and the solar collectors do not differ from those utilized in the direct solar system.
The only difference lies in the way that the water is processed through the solar collector and back to the storage tank. Your potable water is heated indirectly through a heat exchange coil. The solar circulation pump sends a mixture of water and food grade propylene glycol through the solar collector and back to the heat exchange coil.
Heat is transferred to your potable water via the exchange coil which is wrapped around the inner collection tank. The Guardian IP system is freeze proof because the solution in the solar loop is a 2:1 (in colder climates) ratio of water and food grade propylene glycol. The Indirect solar system is only recommended in climates that experience frequent freezing conditions, and where a Drainback system cannot be installed so that it will drain.
Back in the 1990s a manufacturer of Evacuated Tubes used these to make a Solar Thermal Power Plant. The panels created steam that in turn drove a turbine generating electricity for an entire porduction facility. While this was probably not cost effective it did prove that it can be done. To cover all the ways Solar Thermal would take a book. These are the most widely used applications and systems involved.
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Tags: downside, hot water, tropical climates, water heaters, glass tube, substrate, solar collector