Simply put, an active solar system is one that has moving parts.

Although a developing solar technology will probably embellish the state of the art considerably in coming years, the most common active device now used is the water or air-cooled flat plate collector. It is the simplest, and therefore least expensive, of the active solar systems now available to heat homes.

Sandy Kraemer explains the principles behind the active solar system in his book, Solar Law.

Collectors work on what is known as the "greenhouse effect," which traps solar energy that passes through the atmosphere and strike the earth's surface. s

Any kind of solar system is not very useful unless the house is tight and well-insulated.

An active system has three components -- collectors, storage, and distribution. Usually, the moving parts that distinguish an active system are involved in the distribution.

A major factor in an efficient active solar heating system is how well it encourages the sun's energy to enter the system, and how effective it is at preventing escape of energy.

Glass and plastic are good traps because they have a high transparency to incoming rays but hold in heat emitted from the surface of the absorber plate.

So, glass or plastic is usually used in the cover plate of the collector, through which the sun's radiant energy enters and is converted to heat on the absorbed plate.

The absorber plate usually is dark because darker shades absorb heat more quickly than lighter shades. Some absorbers are corrugated to enhance absorbtion. The position of active solar collectors affects their efficiency. There is a bit more flexibility with an active system than with a passive one, which cannot operate without good southern exposure for its windows. Still, active solar panels need to face south. The angle of their placement is another factor, which changes with the latitude of the site.

Because solar collectors work only during the day, and usually only at about 70 percent efficiency even then, storage of the energy they gather is necessary for night or at times when there is a heavy cloud cover.

The type of storage used is dictated by the type of collector -- air-cooled systems use rocks, and water-cooled systems heat water. Both at this time require large storage containers.

The distribution has to do with bringing heat from the collector to the storage area, and later throughout the house when the heat is needed. In an air-cooled system, fans draw heat away from the collectors and force the heated air through a bed of rocks or pebbles. the rocks heat during the collection periods, and air blown through them warms the living areas at night of during cool periods.