To help you in your selection of PV (Photovoltaic) modules, let me tell you a bit about the types of silicon cells that are on the market today and the pros and cons of each. Solar electric cells come in three basic varieties, all made from silicon.
The first commercial solar cell produced was the single crystal cell. These are made from a pure ingot of silicon — a long solid cylinder that was then sliced into thin wafers.
Unfortunately, producing pure silicon ingots required lots of energy and cutting the ingot into thin wafers produced lots of waste. The process was not only expensive; but wasteful; however, single crystal solar cells boast the highest efficiency of all three solar cells on the market today. They are about 15 percent efficient— that is, they convert about 15 percent of the sunlight energy striking them into electricity.
Single crystal cells are still manufactured today, but they’re not as commonly used in commercial PV production as their newer, less pure cousin, the polycrystalline cell (Figure 7-11). Polycrystalline cells are so named because they consist of numerous silicon crystals of varying size. They’re beautiful to behold, but slightly less efficient than single crystal models, about 12 percent, compared to 15 percent. However, because they require less energy to manufacture, they are cheaper. Lower production costs easily outweigh their lower efficiency. By and large, they’re the main type of solar cell in use today.
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Image caption: Fig. 7-11
Polycrystalline Solar Electric cells: (a) in production, (b) finished cell, and (c) modules in solar array.
Silicon solar cells can also be produced by depositing thin layers of silicon on a thin metal backing, producing long ribbons. This technology is known as amorphous silicon. It uses less energy, less silicon, and is less expensive than either the single crystal or polycrystalline cells.
Amorphous or thin film silicon technology was first used to create tiny solar cells for calculators and watches. Although this technique is less expensive than crystalline cell production techniques, the very first thin film materials were damaged by direct sunlight and had very low conversion efficiencies — only about five percent.
In the years that followed the introduction of thin film solar materials, however, manufacturers have found ways to layer thin film materials to boost efficiency to about eight percent and to make this material resistant to photodegradation. Today, thanks to this research, amorphous silicon is being used to produce solar cells and solar modules by a company called UniSolar. UniSolar produces amorphous silicon in long rolls with sticky backing that can be applied to metal roofing (Figure 7-12a). UniSolar is also using its thin-film process to create solar cells incorporated in roof shingles, as in 7-12b, although this product is not ready for prime time, according to Johnny Weiss, of Solar Energy International (SEI). When perfected, this amazing product will protect homes from the weather while generating electrical energy (Figure 7-12b).
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Solar Roofing. Amorphous silicon ribbons can be used to make (a) long rolls of material that can be applied to standing seam metal roofing and (b) solar shingles. Both are forms of building-integrated photovoltaics.
In recent years, thin coats of amorphous silicon have even been sprayed on glass, creating solar electric window glass that produces electricity from sunlight. This application is ideal for skylights and glass canopies, for example, over gas pumps at gas stations. ....read more
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