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 There are two basic types of solar hot water heaters - active and passive. The difference between the two systems is simple. Active systems use a circulating pump to circulate water between the hot water storage tank and the solar collector where it is heated. Passive systems generally have no moving parts, the water either circulating using the normal household water pressure, or by a process called thermo siphoning. We will cover passive systems first, as these are the simplest and usually the least expensive to install. A very simple passive water heater can be constructed out of a plastic milk jug. Merely paint it black, or enclose it in a black plastic garbage bag and fill it with water. The black color will absorb the sun’s rays, transfer it to the water using a process called thermal conduction. Passive solar hot water heating systems are usually used to preheat water before it goes into a standard hot water. This can cut the cost of operating the water heater dramatically, since preheated water is entering the water heat and not as much energy is needed to heat the water. Passive solar hot water systems can be divided into two types - batch and thermo siphon. Batch heaters solar heaters can be as simple as the milk jug design outlined at the beginning of this discussion, or more complex. Some utilize tubes or pipes which heat the water as it moves in stages towards the outlet. The other type of passive solar hot water heater is the thermo-siphon. In this, the solar collector sits at a lower level than the storage tank. The sun heats the water in the collector and it rises, flowing through the piping to the storage tank. The water continuously circulates in the system. A variant of this uses antifreeze in the solar collector, which circulates back through the tank in a closed loop. The antifreeze solution heats the water in the tank and returns to the collector to be heated again. Passive solar hot water systems have the drawback of not being able to be used in cold climates because the water is exposed to the outdoors. Only the closed thermo-siphon system could be used in freezing weather, as only the antifreeze solution in the collector is exposed to the weather. Active solar hot water systems are a bit more complicated as the use circulating pumps to move the water around. Active system work basically the same as the thermo siphon passive hot water heater, but since a pump is used to move the water around in the system the storage tank can be located anywhere it is convenient to place it. There are both open and closed systems. In an open system, the water is pumped directly through the solar collector. In a closed loop system an antifreeze solution is pumped through the solar collector where it is heated, and then through the water storage tank, heating the water. Active closed loop solar heating systems are more expensive to install, but can be used in colder climates. This sums up the different types of solar hot water heating systems available. There should be a hot water heating system for just about every need. |