Solar water heating systems consist of three main components:
- A Collector-designed to “trap” and transfer the sun’s radiation
- Heat Transfer Liquid-transfer the “trapped” heat from the collector to heat the water.
- Hot Water Cylinder/Geyser- for storage of the heated water.
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There are two main types of solar collectors:
Flat Plate Collectors
A flat-plate collector has the following features:
- A cover (glazing), usually made of toughened glass.
- An absorber plate, coated with a high absorption and selective coating.
- A tube system that removes the absorbed heat to the tank.
- A casing with insulation on the back and sides to assist in maintaining the absorbed heat.
Evacuated Tube Collectors
Vacuum tube collectors have the following characteristics:
The collector in this type of system is located in individual glass tubes. These tubes are rigged in parallel with one another to make up the collector.
When tubes are exposed to high temperatures, the air inside them is forced out or “evacuated”. This evacuation of the air creates a vacuum effect and it is this vacuum effect that makes the tubes good insulators. In plain English, this vacuum effect keeps the heat (hot water) inside the tubes, while leaving the outside cool. Heat pipes have a single layer of glass whereas evacuated tubes have a double layer of glass. Heat pipes and evacuated tube collectors both contain a special fluid which begins to vaporise at low temperatures. The steam rises in the individual tubes and warms up the water in the main pipe by means of a heat exchanger. The condensed liquid then flows back into the base of the heat pipe.
The DC pump will only function if there is enough sun light to power the photovoltaic panel, which will ensure that there is no heat loss through the collector. The AC pump is controlled with a differential controller which will monitor the temperature on the collector and the geyser. This differential controller switches on the pump when the correct temperature is detected at the collector (the controller will only allow water to be released once the water in the collector is higher than that stored in the storage vessel/ water tank at any given time).
The differential controller therefore also switches the pump off when the heat transfer liquid’s temperature is lower than the temperature of the water in the geyser.
In a split thermo siphon system the solar geyser and collector are separated with the geyser usually installed in the roof. The geyser must be above the collector with connecting pipes rising smoothly. The pipes should not level out or dip at any point, as this could cause poor circulation.
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