Solar harvesting

Energy from light, whether it is natural or artificial, can be captured and used as an energy source. Earlier in this chapter, we discussed photodiodes and their relationship to sensing light. The same diode can be used in greater quantities to build a traditional solar array. The capacity of energy generation is a function of the area of the solar array. In practice, indoor solar generation is not as efficient as direct sunlight. Panels are rated by their maximum power output in the form of watts.  

Solar harvesting is only as effective as how much the sun shines, which varies seasonally and geographically. A region such as the Southwestern US can reclaim considerable energy from direct photovoltaic sources. The Photovoltaic Solar Resource of the United States map was created by the National Renewable Energy Laboratory for the U.S. Department of Energy www.nrel.gov shown as follows:

Solar energy map of United States in energy density of kWh/m2 1998-2009.

In the United States, Southwest regions fare particularly well with sun intensity, generally lack cloud light barriers, and have good atmospheric conditions. Whereas, Alaska has the weakest energy density. Solar photovoltaics are not typically efficient. One can expect an 8% to 20% efficiency, with 12% being typical. Regardless, a 25 cm² solar array could produce 300 mW at peak power. Another factor is the incidence of light. For a solar collector to achieve such efficiency, the light source must be perpendicular to the array. If the angle of incidence changes as the sun moves, the efficiency drops further. A 12% efficiency collector when the sun is perpendicular will be roughly 9.6% efficient when the sun is 30 degrees from being perpendicular.  

The most basic solar collector is the solar cell that is a simple p-n semiconductor, and similar to the photoelectric sensors discussed earlier. As explained earlier, an electric potential is generated between the p and n material when a photon is captured.