Types of Panels
While all photovoltaics use a similar structure of cells, panels and arrays, there are many different types of cells and panels that can be used. Photovoltaics vary in their basic materials, ability to produce electricity, and costs. Here we look at the types of panels available today.
Basic Panels
Monocrystalline Silicon Panels
Monocrystalline panels use crystalline silicon, a basic semiconductor material. Crystalline silicon is produced in large sheets that can be cut to a specific size and used as one large cell in a panel. Conducting metal strips are laid over the entire cell to collect electrons from the cell into an electrical current.
These panels are more expensive to produce than the polycrystalline panels that follow. However, they are highly efficient and are often more cost-effective in the long run as a result. Monocrystalline panels are typically 15-18% efficient, meaning that for every unit of solar energy that hits the cell, the panel can convert 15-18% of this energy into electricity.
Polycrystalline Silicon Panels
Polycrystalline, or multicrystalline, photovoltaics use a series of cells in place of the single large cell used in monocrystalline panels. Polycrystalline photovoltaics are the lease expensive form of photovoltaics available today, though the costs of producing individual cells can still be high. The drawback of these panels is that they have lower efficiency rates than monocrystalline panels, at 12-14% efficiency. There are several different types of polycrystalline panels:
Cast Polysilicon: In this process, molten silicon is first cast in a large block which becomes crystalline silicon once cooled. The cooled block can be sawn across its width to create thin wafers that can be used as photovoltaic cells. These cells are then assembled in a panel and conducting metal strips are laid over the cells to connect them to each other and form a continuous electrical current throughout the panel.
String Ribbon Silicon: String ribbon photovoltaics use the same molten silicon, but are produced by slowly drawing a thin strip of crystalline silicon out of molten silicon rather than casting the silicon in a block. These strips of photovoltaic material are then assembled in a panel with metal conductor strips connecting each strip to each other to form a current. String ribbon silicon is less costly than cast polysilicon panels because it eliminates the need to saw wafers off a block of silicon. Some string ribbon technologies also have higher efficiency levels than cast polysilicon.
Amorphous Silicon or Thin Film Panels
Thin-film panels are different from crystalline panels in their very makeup. Instead of molding, slicing, or drawing crystalline silicon to create a cell, amorphous silicon has no crystalline structure and can be applied as a thin semiconductor film on different materials. In addition to silicon, copper indium diselenide (CIS) and cadmium telluride (CdTe) can be used in amorphous or thin film panels. This film is then connected to metal conductor strips, but because the film is attached to another structural material it does not always require the same parts necessary for crystalline panels.
The primary advantage of thin-film panels lies in its low manufacturing costs and versatility. Because these panels are less time consuming and expensive to make, they can be produced much more efficiently. Because they can be applied in thin layers to different materials, it is also possible to make flexible, shaped, or unusually sized panels.
However, thin-film panels have several significant drawbacks. What they gain in cost savings and flexibility, they lose in efficiency, resulting in the lowest efficiency of any current photovoltaic technology at 5-6%. Thin-film technologies also use silicon with high levels of impurities. This can cause additional drops in efficiency when the panels begin to generate electricity.
Thin-film panels have the potential to grow in use, and already figure in some of the most interesting enhanced photovoltaic systems, like high-efficiency multijunction devices and building integrated photovoltaics (see enhanced panels below).
Group III-V Technologies
The name of these technologies comes from their basic materials, which are grouped on the Periodic Table as Group III and Group V elements. Group III-V technologies are highly effective but expensive photovoltaic technologies. They use a variety of materials with very high conversion efficiencies, typically around 25%. A typical material used in this technology is gallium arsenide, which can be combined with other materials to create semiconductors that can respond to different types of solar energy. Though these technologies are very effective, their current use is limited due to their cost. They are primarily used in aerospace applications.
Enhanced Panels
Building-Integrated Photovoltaics (BIPV)
BIPV technologies serve the dual purpose of producing electricity and acting as a building construction material. One common BIPV technology integrates semi-translucent layers of amorphous silicon into glass, which can then be used as window panes that allows sunlight into a building while producing electricity. Another common structure is the use of shingle-size PV panels as a roofing material. Currently, BIPV technologies tend to have very low efficiency levels but can be useful in replacing other construction materials and offering a wide variety of aesthetic choices.
Concentrator Systems
Concentrator systems use concentrating optics, or lenses that gather sunlight and concentrate its intensity, to increase the efficiency of solar photovoltaic cells. These systems reduce the amount of photovoltaics needed to produce electricity, and also reduce the amount of space needed for a photovoltaic installation. Their main disadvantage is that they depend solely on direct light to produce electricity, while stand-alone photovoltaic panels can use both direct and diffuse light. Many regions do not receive enough direct light throughout the year for these systems to be practical.
High-Efficiency Multijunction Devices
Multijunction devices use multiple layers of cells with each layer absorbing certain wavelengths of solar energy. In a typical device, the top photovoltaic layer reacts to solar energy traveling in short wavelengths and carrying high amounts of energy. As other solar waves pass through this layer, they are transformed into electricity by additional layers. Materials typically used in this device include gallium arsenide and amorphous silicon. Though some mutlijunction devices have been successfully built, these devices are still in the research and development stage.
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