How Solar Hot Water Heaters Work

Simplest Solar Heater

• The most basic form of solar water heater is a simple black container filled with water. The container can be anything from a bucket or drum painted black to a re-purposed car inner tube. Black absorbs light very well and heats up very quickly. This form of heater is placed on a rooftop or platform where it can soak up the sun's rays. A line with a valve attachment runs down from the heater. This is the type of water heater used by portable camp showers. Though inexpensive and easy to set up, this type isn't very efficient and will not provide much hot water on cloudy days or at night.

Thermosiphon Components

• A thermosiphon system is much more efficient than a simple solar heater. These systems have solar collector panels in addition to storage tanks for the water. The collector itself consists of a flat box with a glass or Plexiglas top. It contains black pipes that connect the water line at the bottom to the line at the top. The tank is set on the roof or a raised platform. The panel is placed at a lower height than the tank. One water line runs from the bottom of the tank to the bottom of the collector. Another water line runs from the top of the tank to the top of the collector.

Thermosiphon Heating

• Both glass and Plexiglas let light pass through easily, where the black pipes absorb it. This causes the pipes and the water they carry to heat up. However, the glass and Plexiglas block heat, which remains trapped in the collector. The collector is, in essence, a small greenhouse.

Thermosiphon Circulation

• When water heats up, it expands. The only room inside the pipes is back up toward the storage tank. There the heat pushes cold water out of the bottom and back into the collector to begin the cycle again. Gravity and heat keep the water circulating until all the water is heated up. Because the tank is separate from the collector, it can be insulated and hold onto its heat even at night.

Cold Climate Thermosiphons

• Basic thermosiphon systems don't work very well in cold climates, where the water in the lines tends to freeze. To fix this problem there's a slightly more complex form made for cold climates. These heating systems carry antifreeze in their pipes instead of water. This antifreeze doesn't empty into the storage tank but instead travels through a heat exchanger set inside the tank, where it heats up the water.

Batch Systems

• Batch system combine the water storage and the solar collector into one unit. These heaters consist of a box of glass or Plexiglas holding a water storage tank, usually painted black. Lines run in and out of the box to bring water to and from the house. These systems don't work well in cold climates, as the external lines freeze.

Evacuated Tube Systems

• Evacuated tube systems are the most complex and expensive. However, they're much more efficient, produce more hot water, and take up less space. In these, one or more "heat pipes" are set alongside the water storage tank. These heat pipes have had all the air vacuumed out and contain a few drops of a special chemical. When the sun heats the pipes, the chemical expands and rises through the tube until it's next to the water tank, which it then heats up. When it transfers its heat up to the water tank, the chemical falls back to the bottom of the tube to begin the cycle again.