Types of Collectors
Solar water heaters fall into 3 basic categories. Thermosyphon systems take advantage of the fact that warm water rises. These systems have so pumps and are mechanically the simplest of the three types. The water storage tank is above the collector coils, so if the system is on the roof it is essential to check whether your roof structure is adequate. Because the fluid being circulated is water, precautions must be made to prevent freezing or overheating. These systems work well in mild climates.
The second basic category of collector types is the drainback. Water is pumped up through the collector when there is enough sunlight to heat the coils. When conditions are not right for heating water, the pumps turn off and the water drains back to a reservoir, this takes care of freezing and overheating. A variation of this type uses heat transfer fluid that drains back to a reservoir on the back of the flat plate collector. The proximity of this reservoir to the coils reduces the amount of energy that the pump consumes.
The third type of collector places the dark coils inside evacuated tubes. These systems also rely on pumps to move the fluid. The advantage is reduced heat loss from the collector to the surrounding air. This would be important in a clear shy climate with cold daytime temperatures where solar heated water is being used for space heating. Evacuated tube systems are generally more expensive that drainback flat plate collectors.
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