Most energy sources are the indirect result of solar energy conversion processes. Wood burned gives back, in the form of heat, solar energy absorbed by the tree as it grew. Coal, oil and gas are also created from converted solar energy. If you remember from science class, energy isn’t ever destroyed, it’s just converted to another form. In our case, solar radiation energy converted to heat energy. A good comparison, one square meter of a dark surface exposed to sunlight will absorb about one horsepower of (746 watts) of energy.
Visable light that our eyes can see is only about 25% of the energy in solar radiation. Much of the solar energy is absorbed by the ozone and atmosphere so only 45% will reach the earths surface.
Passive and Active Solar Energy Systems
There are two types of solar systems, passive and active. Passive solar systems depend on the solar radiation striking directly on the surface or area to be heated. A good example of passive solar is a greenhouse. In a typical greenhouse the energy flows through the glass into the area where the plants grow. Passive solar energy collectors usually require no pumps or blowers to distribute the energy collected.
A house with south facing windows designed to allow sun in winter months would be considered a passive system.
In an active solar system the energy is absorbed into a collector, stored and then redistributed as needed by mechanical means.
An active solar energy system will have the following parts.
- Collector for solar radiation.
- Method of moving the heat from the collector to storage and point of use.
- Method of storage.
Collectors
Collectors trap solar radiation. The color black is produced when all visible radiation is trapped, so the most efficient collector will appear to be black because it has absorbed all the light. Collector surfaces should be finished with a high temperature black finish or one of the engineered selective finishes. The finish should not be shiney , reflective or smooth. The maximum temperature a black object can reach in full sunlight is 253F. If a glass is placed over the black collector temperatures of 625F are possible.
Glass is the most widely used cover for collectors. If the frame is metal care must be taken sealing the glass to the frame since the materials expand and contract at different rates. The glass will break if not allowed to expand at its own rate. Some manufacturers place their collectors in vacumn sealed glass tubes.
Installed Angle of Collectors
The surface of the collector is normally perpendicular to the rays of the sun. Since the angle of the sun changes an average is usually used. In midwinter the sun angle is 66.55 degrees minus the site latitude. The angle in summer is 113.45 degrees. If the collector is used to heat a building and since winter requires the most heat an angle of 45 to 55 degrees to horizontal would be used in the mid-section of the United States. Hot water only would be installed 35 to 45 degrees to horizontal. Collectors should face south or up to 30 degrees east or west from due south.
Storage
The heat from the collector is normally stored in a preheat tank about half the size of the powered hot water heater. The preheated water is fed into the powerd heater for any final heating from 105 to 120 degrees
