Using solar power for daily energy needs is not a new idea. In the modern world, harnessing solar energy has reemerged with new technologies. Photovoltaic cells (commonly known as PV cells) function differently from solar heating collectors; the former harness the sun's energy directly and convert it into electricity while the latter transfers heat through warm fluids.
The word "photovoltaic" is a construction of the terms "photo" referring to light and "voltaic" meaning voltage that assists in electric flow. The effect was first observed by Alexandre-Edmond Becquerel in 1839 and the PV cell was properly developed by Bell Laboratories during the 1950s. NASA quickly capitalized on these cells to supply electricity in space: after many enhancements and many missions, the price of efficient PV cells became more affordable. Today, the technology is utilized in a myriad of conveniences such as watches, clocks, calculators, lamps, street signs and in homes as renewable-energy systems.
A home renewable-energy system uses standard PV products which anyone can accurately assess and compare. There are 4 primary product technologies currently used in the home:
- single-crystalline laminate (roof shingles)
- polycrystalline
- string ribbon cells
- thin films
Construction of a PV Cell
PV cells can be thought of like integrated circuits or transistors on steroids. Even the most modern microchip is just a piece of silicon wafer as big as your thumbnail with millions of transistors and electronic parts on it. PV cells have a similar history to chip circuits and were not publicly used until advances in technology reduced them to about 4 inches (10 cm) wide. Over time, they have become thin, polished wafers which are assembled with interconnecting electric wires.
A single wafer exposed to the sun's rays gives an electrical pressure of 0.6V DC on a meter. Remember that a higher voltage than your battery's rating is needed for charging, so a 12V DC battery will require a minimum input of 15V DC to cover electrical losses. This is why PV cell manufacturers connect 36 cells in series to create a 'module', which is then mounted on a frame.
36 cells in series x 0.6V DC per cell = 21. 6 volts (open circuit)
This "open circuit" voltage (or "VOC"), measured when there is no load connected, is more than the required 15V to allow for voltage reduction when the PV cells are below optimal load, such as when charging the battery. Heat also reduces PV cell voltage. If the PV cells are operating at their maximum, the voltage is still less than VOC rating. This is the "maximum load voltage" with a standard output of 17V DC on a 12V unit and twice the voltage for a 24V unit.
A complete electric circuit must have a source as well as current flow. The sun is a great source of energy, providing as much as 1,000W per square meter at noon on a clear day at sea level. To test PV panel efficiency, light sources of similar intensity were developed. These tests supply electrical ratings which prove helpful in assessing each type of PV panel's value.