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How Photovoltaic Panels work |
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The sun is
the source of all life and energy on the Earth! Plants, it's estimated, are
able to use roughly 83 percent of the sun's light to make simple sugars
which they need to survive.
Photovoltaic panels or "PV" panels collect the sunlight and a silicon
crystal converts the light energy from the sun into electrical energy by
ejecting an electron. The electrons are collected by a transparent carbon
film or a tiny wire grid, which is connected to the electrical system. |
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| The sun covers the Earth with about 1,000 watts of light per square meter, all day, every day. With enough solar panels, we could produce enough electricity to supply the world with energy without the need for fossil fuels! | ||||
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Photovoltaic or "PV" Cells Photovoltaic cells, like the one pictured to the right, are a special type of silicon that converts light directly into electrical energy. When photovoltaic cells, also know as solar cells, are grouped together in a panel, they are often referred to as "arrays" or "modules". PV cells are generally made from a special type of silicon, similar to what is used in microprocessors, but with a few major differences. |
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In general, silicon is
manufactured to be as pure as possible. In a PV cell, however, the
manufacturers intentionally add impurities. The addition of impurities
allows electrons to move freely from atom to atom in the PV cell. These
electrons are collected by an wire grid commonly made of a high purity
metal, or by a transparent carbon film which coats the surface of the PV
cell. |
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| An important thing to mention is the process used to make a PV cell function. "Doping" is the process of intentionally adding impurities to a silicon wafer. The anode side of the PV cell is doped with boron and the cathode side is doped with phosphorus. |
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The animation to the left demonstrates how a PV cell converts the suns energy into electricity! The sunlight excites the silicon atoms and the impurities in the cell, thus setting electrons in motion, which produces electricity! | |||
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Unlike plants, solar cells absorb only 15% of the available energy from the sun, which is far less than the 83% plants are able to use. Many things contribute the PV cell's low efficiency. The sun produces light over a broad spectrum (the visible spectrum is shown below) which includes visible light, x-ray, gamma, radio waves, microwaves and others. A PV cell converts the visible spectrum of the sun's light into electricity, but it does not convert all of the visible spectrum.
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PV cells convert the
light or "photons" into electricity by knocking loose electrons from the
silicon or other materials they are made of. The amount of energy required
to knock an electron loose in a PV cell is known as the "band gap
energy". If a photon has too much energy, it will pass right through the
photocell as if it were transparent. If a photon doesn't have enough energy,
it will simply bounce off or be absorbed and radiated as heat. These loses
alone account for about 70% of the energy the sun radiates!
So why not build a PV cell with a low band gap energy and collect more of the sun's energy? Many factors make this unrealistic with the present materials, but the most evident reason is the voltage drop from a PV cell with a low band gap energy. The lower the band gap energy, the lower the voltage the cell puts out. Since wattage is the product of the voltage and current, the wattage will drop also, making the cell less efficient. If the wattage was maintained, but the voltage dropped, the current would have to increase, and this would burn off the collection wires from the heat! With the present materials, the ideal band gap energy is around 1.4 eV. The abbreviation eV stands for "electron volts" and is a measurement of the amount of energy an electron acquires while falling through a potential charge difference of 1 volt between both ends of a vacuum chamber, which is roughly equal to 1.602 × 10 -19 joules. (Did you get all that?) What this means is that until our materials improve, 15% is all we can expect from a PV cell. Keep in mind that 15% is actually quite a bit, because at 15% a PV array can produce 150 watts of electrical energy for every square meter of panel! To put that into perspective, the entire US could be powered by a PV array measuring 62.58 miles per side! |
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