For locations that have a mild climate, the open loop thermosiphon solar water heater is the most widely used system. With freeze protection, thermosiphon systems can also be used in locations that experience minor freeze conditions. This can be provided by water dump valves, electric heating in the collector header, or tapered riser tubes to control ice growth in the riser so that a rigid and expanding ice plug is avoided (Xinian, et al., 1994). All these techniques have been used successfully by solar water heater manufacturers, and their suitability is proven in areas with mild freeze conditions. They are not suitable, though, in areas with hard freezing. In such cases, the only suitable design is the use of antifreeze collector loops with a heat exchanger between the collector and the tank and an antifreeze heat transfer fluid circulating in the collector and the heat exchanger. For horizontal tank configuration, the most widely adopted system is the mantle or annular heat exchanger concept, shown in Figure 5.4.
Mantle heat exchanger tanks are easy to construct and provide a large heat transfer area. Mantle heat exchangers are also used in vertical tanks and forced circulation systems, as can be seen in Section 5.2.2. Manufacturers of horizontal tanks usually use as large a mantle as possible, covering almost the full circumference and full length of the storage tank. The usual heat transfer fluid employed in these systems is a water-ethylene glycol solution.
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