Solar design, better insulation, and more efficient appliances could reduce this demand by 60 to 80 percent. There are more than 250,000 passive solar homes in America, but there should be many more. Simple design features, such as properly orienting a house toward the south, putting most windows on the south side of the building, and allowing for cooling breezes in the summer, are inexpensive yet improve the comfort and efficiency of a home. Besides using design features to maximize use of the sun, some buildings have active systems to gather and store solar energy. Solar collectors sit on the rooftops of buildings to collect solar energy for space heating, water heating, and space cooling.
Most solar collector are large flat boxes, painted black on the inside, with glass covers.
In the most common design, pipes in the box carry liquids that take the heat from the box and bring it into the building. This heated liquid, usually a water-alcohol mixture to prevent freezing, is used to heat water in a tank or is put through radiators to heat the air. Oddly enough, solar heat can also drive a cooling system. In desiccant evaporators, heat from a solar collector is used to pull moisture out of the air. When the air becomes drier, it also becomes cooler.