Solar Water Heating:
Solar water heating has been used since the late 19th century and the technology of many of the components is mature. Many of the system designs are still quite primitive, but we can expect that competition in the market will deliver systems with higher solar efficiencies than ever before. Most systems now use back-up heating to insure hot water all the time.
Most solar hot water systems don't circulate the water through the collector directly. This can lead to calcium carbonate deposits in the collector tubes, corrosion of the tube, or damage to the collector during freezing weather. Instead, a heat exchanger is used and this can be a coil in the storage tank or a heat exchanger outside of the tank in which one fluid loop goes to the collector and one goes to the tank. Typically, these fluid loops have circulation pumps operated by a controller.
Most variations in solar hot water systems occur in the tank configurations.
In a batch passive water heating the tank is also the collector. This can be used for mild-sunny climates where freezes are not too severe. Solar heating fraction is 50-65%. The advantages are: simplicity, long life, and no power needed. The disadvantage is the weight. This type of water heating weighs 500 lbs., so reinforcing of the roof may be necessary.
Another system is the thermosiphon system, where the buoyancy of heated water operates the system rather than a pump. There are several variations on this approach. Variation 1: A percolation system using an alcohol water loop to drive hot water down to a separate tank. Variation 2: Integral collection-tank configuration with back-up heater. The solar heating fraction is 60-70%. The advantages are long life and no power is needed. The disadvantage is the complexity of freeze protection.
A one tank water heater solar fraction is 60-70%. The advantage is that the tank is on the building's interior where it is protected from the elements and is not a visual factor. The disadvantage with a one tank system with a pump is that heat tends to migrate down quite rapidly, so more back-up heat than necessary is used. The two-tank system is the most common and you have a separate solar heating system from back-up system. The solar heating fraction is 60-70%. The disadvantage is that solar heat only goes to the back-up tank when hot water is used, so if one goes on vacation one might as well not have a solar water heater.
Improvement is possible in the tank system configurations. A more optimized configuration combining the best of all active systems is the use of a double set of tanks. In this setup one tank is mounted above the other and hot and cold water is exchanged between them tanks whenever the solar tank is hotter than the back-up tank and during period of less use such as a vacation when no back-up heat is needed. This set of chimneys act as a thermal diode where heat can flow up but not down. Solar heating fraction is 80-90%.
Water consumption varies greatly from day to day, although most systems assume that it is always the same. To account for the consumption variability the total volume of tanks should be 40% larger than the average daily hot water consumption.
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