and active. A passive house relies on properly oriented windows that face the south, and has almost no windows on the north side. As sunlight passes through the windows it is absorbed into the walls and floors. The heat is stored by using dense interior materials that retain heat well. These materials absorb heat and radiate it back into the room after dark when it is needed most. Some passive homes have walls up to one foot thick. A passive solar home can get 50 to 80 percent of the heat it needs from the sun. A greenhouse is a good example of a passive solar space heating system.

Active solar homes use collectors that harvest and absorb solar radiation combined with electric fans or pumps to transfer and distribute the solar heat. They also have an energy storage system that uses either liquid or air as a heat transfer medium in their solar energy collectors. These systems both collect and absorb solar radiation and then transfer the solar heat to the interior space or to a storage system, from which the heat is then distributed.

Most solar water heating systems consist of a solar collector (usually a flat-plate collector), and a storage tank. Solar water heaters use the sun to heat water or a heat-transfer fluid in the collector. Heated water is then stored in a tank, with a conventional system providing additional heat when necessary. A well-maintained system can last 15-20 years-considerably longer than a conventional water heater. Solar water heating systems can also be passive or active, with the latter being the most common.

Active solar water heaters use electric pumps and controllers to circulate water or other heat-transfer fluids through the collectors. Passive systems rely on gravity and the tendency of water to circulate naturally as it’s heated. Because they contain no electrical parts, passive systems are typically more reliable and easier to maintain.

Solar water heaters can also be used to heat swimming pools. With this system, the pool’s filtration system pumps pool water through a solar collector, and the collected heat is transferred directly to the pool water. Solar pool-heating collectors operate just slightly warmer than the surrounding air temperature and often use inexpensive collectors made from specially formulated plastic materials.

Some smaller systems are operated with timers or even manually, while larger systems are operated with electronic sensors and controls. Solar pool-heating systems are generally used to lengthen the swimming season and allow the pool owner to use the pool in cold weather.