My last post discussed how energy is created in the sun and arrive on earth via packets of energy called photons; it also discussed the amount of sunlight power that falls on the earth, also called irradiance. This post will discuss one of the ways in which we can convert the sunlight into usable power.
One of the ways it can be converted to usable power is via the photovoltaic effect. It was noted that semiconductor materials like selenium release small amounts of electricity when exposed to sunlight. When photons strike these materials, their energy is absorbed and transferred to electrons within the materials. With this energy, the electrons are able to break free from their atoms and flow as in an electric current. Further research into material science resulted in using silicon doped with impurities such as phosphorous and boron.
Adding these impurities to silicon required less energy from the photon for the electron to break free. Silicon doped with phosphorous forms a negative or n-type semiconductor material and silicon doped with boron forms a positive or p-type material. Placing an n-type material in close proximity to a p-type material creates an electric-field at their junction. If free electrons and holes happen to get close to the electric field, the field will send the electrons from the n side to the p side and positive charges or holes from the p side to the n side.
If we add leads to either sides of the semiconductor material and connect them to a load, a circuit is created and electrons can flow through the circuit. The electron flow provides the current, and the cell's electric field results in a potential difference or voltage between the two material types. PV-cells develop direct-current or dc voltage.
If we add leads to either sides of the semiconductor material and connect them to a load, a circuit is created and electrons can flow through the circuit. The electron flow provides the current, and the cell's electric field results in a potential difference or voltage between the two material types. PV-cells develop direct-current or dc voltage.
The original PV-cells were made from materials such as selenium. These cells were only 1% efficient, meaning only 1% of the sunlight that fell on the material was converted into power. Research using doped silicon yielded cells with efficiency as high as 6%. Current manufactured cells have efficiencies in the range of 14 – 18% and laboratory tests on new cell-designs have shown efficiencies as high as 42%.
A standard solar panel is made from interconnected cells housed in a metal frame and protected with anti-reflective glass; they are typically 1200mm by 600 mm in size. A standard condition for testing solar panel rated power is under an irradiance of 1000W/m2, at 25 ºC and a sunlight attenuation factor of AM1.5. This represents the typical conditions in the 48 contiguous United States at noon in the spring and autumn with the solar cell aimed directly at the sun. A panel rated at 14% efficiency is rated to deliver approximately 100W under the standard test conditions.
Currently, the most prevalent material used to make solar cells is crystalline silicon. Silicon is a well-understood semiconductor material with good stability, physical, electrical and chemical properties. Furthermore, silicon cells have benefited from the enormous economies of scale achieved from the semiconductor and microelectronics industry. However, the manufacturing process used to make silicon wafers is complex, which makes them expensive in solar power applications. Furthermore, demand for silicon from the microelectronics and semiconductor industry has driven up material costs.
There is considerable research and development into so called thin-film PV-cells, which use less silicon material. Some of these new types of thin-film PV-cells include CdTe (cadmium telluride), a-Si (amorphous silicon) and CIGS (copper indium gallium selenium). Solar panels formed from these materials have efficiencies in the 11 – 12.2% range, which are lower than that of silicon based panels; however, are much cheaper to manufacture and have higher manufacturing yields. Efficiency advances through continued investment in R&D and in manufacturing techniques could get efficiencies that are comparable with silicon based technologies over time.
Solar Energy is poised for high growth especially with issues related to climate change, a strong desire to diversify sources of energy and from national security concerns. In my next post, I'll continue discussing photovoltaic power and applications for it.
A standard solar panel is made from interconnected cells housed in a metal frame and protected with anti-reflective glass; they are typically 1200mm by 600 mm in size. A standard condition for testing solar panel rated power is under an irradiance of 1000W/m2, at 25 ºC and a sunlight attenuation factor of AM1.5. This represents the typical conditions in the 48 contiguous United States at noon in the spring and autumn with the solar cell aimed directly at the sun. A panel rated at 14% efficiency is rated to deliver approximately 100W under the standard test conditions.
Currently, the most prevalent material used to make solar cells is crystalline silicon. Silicon is a well-understood semiconductor material with good stability, physical, electrical and chemical properties. Furthermore, silicon cells have benefited from the enormous economies of scale achieved from the semiconductor and microelectronics industry. However, the manufacturing process used to make silicon wafers is complex, which makes them expensive in solar power applications. Furthermore, demand for silicon from the microelectronics and semiconductor industry has driven up material costs.
There is considerable research and development into so called thin-film PV-cells, which use less silicon material. Some of these new types of thin-film PV-cells include CdTe (cadmium telluride), a-Si (amorphous silicon) and CIGS (copper indium gallium selenium). Solar panels formed from these materials have efficiencies in the 11 – 12.2% range, which are lower than that of silicon based panels; however, are much cheaper to manufacture and have higher manufacturing yields. Efficiency advances through continued investment in R&D and in manufacturing techniques could get efficiencies that are comparable with silicon based technologies over time.
Solar Energy is poised for high growth especially with issues related to climate change, a strong desire to diversify sources of energy and from national security concerns. In my next post, I'll continue discussing photovoltaic power and applications for it.
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