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Solar hot water heating is an excellent, efficient way to utilize solar energy. Inexpensive flat plate solar collectors can be used to achieve the water temperatures needed for hot water in homes. What's more, hot water is needed year round, unlike space heating, which is needed only during the cold part of the year.
There are several types of choices for solar hot water systems. Plans are available for DIY solar water heaters and there are also many commercial suppliers/installers for solar hot water heating. Another choice to be made is whether to use passive solar water heating or an active solar system. Also, water may be heated directly in the solar collector(s) or it may be necessary to heat anti-freeze in the solar collectors and use the heated anti-freeze to heat water with a heat exchanger. More details about these options follow in the next few sections.
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Passive Solar Water Heaters
Passive solar water heaters use natural convection to drive the flow of heated water from the solar collector to the hot water tank, as shown in the figure at the left. Natural convection refers to the natural upward motion of heated fluid due to its decrease in density. This type of system, sometimes called a thermosyphon solar heating system, is simple, because there are no pumps required. The hot water tank must be located higher than the solar collector, so that natural convection will send the heated water from the collector to the tank. Hot water for use is drawn from the hot water tank as in a traditional hot water system. Due to the effects of nighttime and cloudy periods, a backup water heating system is almost always needed with any solar water heating system in order to have a continuous, reliable supply of hot water.
Image Credit: H. Bengtson, Passive Solar Heating Basics
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Active Solar Hot Water Systems
Active solar hot water systems use a pump to move heated water from the solar collector(s) to a hot water tank, and bring cold water to be heated into the solar collector(s), as illustrated in the diagram at the right. This diagram shows the typical controls and flow patterns for direct heating of water (without anti-freeze) with an active solar hot water heating system.
The diagram at the left shows an active solar water heating system with the use of anti-freeze and a heat exchange loop to heat water with the heated anti-freeze. This system is somewhat more complicated but is necessary where freezing temperatures occur, in order to prevent freezing in the solar collector overnight.
Image Credits (both images in this section): Arizona Solar Center, Solar Hot Water: a Primer
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Additional Solar Hot Water Heating Options
Direct heating of the water by solar water heaters is only suitable where the overnight temperature doesn't go below freezing, or at least very rarely goes below freezing, and then not by much. Some type of freeze protection is common in most solar hot water systems. The thermosyphon passive solar water heater shown above can be modified to use anti-freeze and a heat exchanger loop, as shown for the active solar hot water heating system in the previous section.
One more option is a solar batch heater. This is one of the earliest, and one of the simplest types of solar water heating system. It consists of a dark colored tank in an enclosure with glass on the sun-facing side. The water in the tank is heated by sunlight and circulated into the hot water tank for domestic use. More details on this option are given in reference #2.