A much cleaner system can be formed by installing a heat exchange coil inside the insulated water tank. The water in the evacuated tubes and the insulated water tank is then used for heat exchange only; see Fig. 11.16. The usable hot water runs only in the heat exchange coil and thus can be pressurized and will not be contaminated by the heat collecting system. The insert in Fig. 11.16 shows some detail of the heat exchange coil.
Figure 11.16 Solar water heater with pressurized heat exchange coils. The hot water only runs in the heat exchange coil inside the insulated tank. Therefore, the hot water can be pressurized and will not be contaminated by the heat-collecting system. Insert shows some detail of the heat exchange coil.
System with a Separate Heat-Exchange Tank
Using a separate water tank and a heat exchange coil inside the tank, the usable hot water is completely isolated from the heat collectors. The typical structure of such a system is shown in Fig. 11.17.
The solar heat collector is filled with a heat exchange fluid which can be water or an antifreeze fluid, for example, a mixture of water and glycerol. In a flat-panel solar collector, in regions with freezing temperature, antifreeze fluid is a necessity. In evacuated-tube collectors, because of the superb insulation, ordinary water can be used. The heat exchange fluid is circulated with a pump. The temperature of the water tank is sensed by a thermometer (2). The temperature of the outlet water can be preset using a controller, which controls the pump station.