SOLAR WATER HEATING

The sun can heat water for bathing and laundry. Most solar water-heating systems have two main parts: the solar collector and the storage tank. The collector heats the water, which then flows to indirectly heat water in the storage tank. The storage tank can be just a modified water heater, but ideally it should be a larger, well-insulated tank. The water stays in the storage tank until it is needed.

Flat-plate collectors

A common type of solar collector, usually mounted on the roof. This collector is a rectangular box with a transparent cover that faces the sun. Small tubes run through the box, carrying the water or other fluid such as antifreeze to be heated. The tubes are mounted on a metal absorber plate, which is painted black to absorb the sun's heat. The back and sides of the box are insulated to hold in the heat. Heat builds up in the collector, and as the fluid passes through the tubes, it heats up.

Solar water-heating systems can be either active or passive. The most common systems are active, which means they use pumps to move the heated fluid from the collector and into the storage tank.

Passive thermosyphon systems

These involve a water storage tank located above the height of the collectors, usually inside the roof space. Water heated by the collectors is pushed through them and enters the top of the water storage tank and the cold water at the bottom of the tank is simultaneously pushed out and into the bottom of the collector, creating a cyclical motion.

Evacuated-tube collectors

In an evacuated-tube collector, sunlight enters through the outer glass tube, strikes the absorber tube, and changes to heat. The heat is transferred to the liquid flowing through the absorber tube. The collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate in a flat-plate collector) covered with a selective coating. Evacuated-tube collectors are modular - tubes can be added or removed as hot-water needs change. Conductive and convective heat losses are eliminated because there is no air to conduct heat or to circulate and cause convective losses since, during manufacture, air is evacuated from the space between the outer tube and the absorber tube.