The efficiency of solar panels depends on the kind of silicon used in the solar cells, the physical construction of the cells and on the way the solar cells deliver power. Standard, off-the-shelf solar panels using inexpensive technology have an efficiency of only about 10 to 12 percent, while commercially available high-efficiency solar panels using a voltage controller to keep the voltage at its maximum can operate at close to 20 percent. Such advanced cells are more expensive. The local cost of electricity is the main factor for deciding whether it makes sense to install the more efficient panels at a higher cost.
The photovoltaic effect was discovered by the French physicist Alexandre Edmond Becquerel in 1839, but it was the American space program in the 1950s that led to the development of solar cells that produced useful power. These early solar cells were used for satellites and had an efficiency of 2 percent. By the 1960s cells with 10 percent efficiency were commercially available. Research continues with multi-junction cells having several layers and nanotube cells having increased surface areas. Such cells have achieved 40 percent efficiency in laboratories.
Types of Silicon in Cells
Solar cells are made from silicon, and the silicon material is either monocrystalline, polycrystalline or amorphous. Monocrystalline cells are the most efficient and each cell is a slice of a single crystal of silicon. The single-crystal silicon solids are difficult to produce and the cells are the most expensive of the standard cells. Polycrystalline cells lie in the middle in terms of cost and efficiency while amorphous cells are the least expensive but the most common. They are made from silicon that has little crystallization and, while generally less efficient, they perform well under low light such as cloud cover.
Apart from the performance of the silicon layer of the solar cells, their efficiency is influenced by the amount of light that reaches the silicon layer. Silicon itself is naturally shiny and, when covered with glass, reflects even more light. Nonreflective coatings help but different light frequencies are still not used fully by standard cells. Multi-junction cells can achieve much higher efficiencies by using layers of silicon tuned to different light frequencies. While the efficiency is very high, approaching 40 percent, the high cost of these cells makes them unsuitable for commercial use.
Solar cells operate at their highest efficiency when the ambient temperature is cool. High-efficiency cells are especially sensitive to temperature and may lose 0.5 percent of their rating for each degree Fahrenheit over 80 degrees. For panels mounted in preferred, high-sunlight locations such as the southern U.S. deserts, temperatures of 120 degrees Fahrenheit are common. It is important for efficiency to mount the cells so that natural convection cooling can take place as the temperature rises.
Solar cells deliver their maximum power at their rated voltage. The most efficient arrangement for the solar panel circuits is with a constant voltage controller that keeps the voltage at the rated level. Such an arrangement extracts the most power from the solar cells, independent of their crystalline characteristics or temperature.
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