Active and Passive solar water heating systems

The difference between active and passive solar water heating systems is that active systems have circulating pumps and controls, while passive systems do not. Although passive systems are usually not as expensive as active systems and can last longer, they are generally less eficient.

There are two types of active solar water heating systems:

• Direct circulation systems

  In this system, water circulates from the collectors directly into the home. This only works well in mild climates.

• Indirect circulation systems

  In this system, pumps circulate a non-freezing liquid through the collectors and through a heat exchanger, which then heats the water that flows into the home itself. Because of the use of non-freezing liquids, this system is popular in climates with freezing temperatures.

There are two basic types of passive systems:

• Integral collector-storage passive systems

  This is suitable only for mild climates. It is passive because it does not require the use of any pumps: water goes directly from cold water supply to the normal water heater. So it supplements and preheats the water only. One disadvantage is that freezing can be a problem because of the large water volume being held by the collector. If it gets too hot, this is also a problem as the collector can burst. Finally, a full collector's weight is significant and needs to be considered when placing the collector on the roof.

• Thermosyphon systems

  This system takes advantage of the fact that warm water will rise when placed in contact with cold water. In this system, the storage tank is placed above the collector, so the warm water automatically and passively rises into tank as it is heated by the collector.