Active Solar Water Heating for the Home

An active solar hot water system, as opposed to its passive counterpart, uses electric fans or pumps to push heated water through the system and throughout the house. There are a few types of active solar water heaters, differentiated from each other by type of solar collector and/or type of circulation system.

Flat Plate vs. Evacuated Tube Collectors

Flat plate collectors are easily the most common water heater collector for residential applications. Upon first glance, they're simply large, flat metal boxes. The sides and bottom are insulated to prevent loss of solar heat. Inside that simple-looking box, however, is a copper or aluminum absorber plate, darkened and covered with glazing to absorb solar radiation. Beneath that, a series of pipes are connected, through which cool water flows to be heated within the box collector. The warmed water then runs back to the house, constantly circulated by an electric pump. In freeze-prone climates, a transfer fluid other than water is used to prevent freezing within the collector.

Evacuated tube collectors are the more complicated and expensive option, but also reach the highest temperatures. Evacuated tubes consist of flat, metal absorbers housed within glass tubes. At the time of assembly, all air surrounding the tubes within the collector is removed, forming a vacuum to prevent heat loss. Those metal absorbers, or "fins," absorb solar heat.

At the same time, a heat transfer fluid passes through the tubes, collecting the heat from the metal absorbers, which is then transferred by heat exchange to the water sent to the house. Evacuated tube collectors are the most complex of solar hot water collectors.

Direct vs. Indirect Systems

Active solar water heaters use either a direct circulation or indirect circulation system. The system you choose will depend on a number of factors, the most important being your local climate.

Direct circulation systems use pumps to circulate the water itself through solar collectors into the home. Direct systems are simpler to design because they do not require heat exchangers, heat transfer fluids or other additional components. On the other hand, because the water itself runs through the solar collectors, direct systems do not work well in climates that reach freezing temperatures during the winter time. Frozen water within the water heater can cause major damage to the system.

Indirect systems are the solar water heating answer to the problem of winter freezing. Instead of water, indirect systems use a heat transfer fluid - usually propylene glycol (anti-freeze) - to collect solar heat. This heat is then transferred to water within piping connected to the house by way of a heat exchanger (copper tubing, etc.). The transfer fluid and the water must never come in direct contact because of the toxicity of the fluid itself.

Passive Solar Water Heating / Active Solar Water Heating