Passive Solar Heating
The conversion of sunlight to heat requires a collection device. The simplest solar collector is a south facing window behind which the structure and furnishings of the room serve as the opaque surfaces which convert light to heat. All collection devices should include: (1) "collecting" surface in position for the sun to shine on it; (2) a surface to convert a maximum amount of sunlight that strikes it to heat; (3) "mechanism" for transferring the heat from the collection surface to a desired location for use or storage; and (4) a way to reduce heat losses from the system.
Another example of a collection device is a “sunroom”. Sunrooms have a lot of glass and are heated most of the year just from the sun. On sunny days the heat entering the room can be distributed into the home by opening doors or windows to this room. The benefits of the heat and plentiful natural light can reduce heating and lighting expenses.
Energy from the sun travels to the earth by electromagnetic waves that range from infrared (longer and slower than visible light) to ultraviolet (shorter and faster than visible light). What happens when these waves strike a surface depends on the properties of that surface. If the surface is reflective, (i.e. polished metal), most of the sunlight will be reflected off it. If the surface is translucent (i.e. a window), much of the sunlight will pass through, although some will also be reflected. If the surface is opaque (i.e., not translucent) much of the sunlight will be converted to heat energy and absorbed by the material.
The amount of heat available from the sunlight striking the surface of earth is tremendous; approximately 200 BTUs per square foot per hour. But there are difficulties in storing this heat for use. In the first place, availability and demand are unbalanced: more solar heat is available in the summer when it is not needed than in the winter, when it is needed. None is available during the night and in stormy weather when demand may be high. Second, not all of the sunlight available can be collected and/or converted. Enough "useable" heat could still be collected to reduce the amount of energy purchased by the resident.