Solar Knowledge: PV Modules
Various types of solar module are available for the use in solar home system. Based on structure of silicon used in individual cell PV modules can be classified into three types as amorphous, polycrystalline and monocrystalline silicon. These modules may consist of from few cells to dozens of cell combined in series and parallel to generate desired module voltage and current. Overview of commercially available solar panels used in standalone solar home system is described in following section.
Amorphous silicon PV modulesAmorphous-silicon modules are one of the several types of thin film technologies. PV manufacturers began manufacturing thin-films to reduce the production cost. They are usually made by applying doped silicon material to the back of a plate of glass. The cells appear dark brown on sun-facing side and silver on the conductive side. When produced as a solar panel, it will appear to have several thin parallel lines running across its surface. These thin lines are actually breaks in the N and P layers of the silicon substrate and they create the boundaries of individual cells in the panel.
Amorphous cells are often seen in small solar panels such as those in calculators or garden lamps. The stabilized efficiency of a-Si module range from 5%-7%, with lab efficiency of 13%. Lower efficiencies means that more a-Si panels are needed for same power output and therefore more space is taken up. Amorphous-silicon module has some advantages over crystalline-silicon modules. All PV modules perform better under clear sky and colder temperatures. But at elevated operating temperatures, the output of a-Si module does not decrease as much as that of crystalline-silicon modules. This can be critical when PV arrays are to be deployed in hot climates. Further, at reduced irradiance levels or when partially shaded, the output of a-Si module decreases less compared to crystalline-silicon modules.
To increase their convenience, they can be made flexible which allows them to be perfect for camping, charging laptops and other portable appliances. They are often lightweight and can be folded to fit into suitcase and backpacks.
Figure: Amorphous PV Module
Monocrystalline silicon PV modules:Monocrystalline (single crystal) solar cells are cut from a silicon boule that is grown from a single crystal - this means the crystal has been grown in only one plane or direction. Typically, the cells are a few inches across, and a number of cells are laid out in a grid to create a panel. This is the most efficient of the photovoltaic technologies meaning that more electricity can be obtained from given area of panel.
The principle advantage of monocrystalline cells are their high efficiencies, typically around 14-17%, with lab efficiency of 24%, although the manufacturing process required to produce monocrystalline silicon is complicated resulting in slightly higher cost than other technologies. Production methods have improved though, and prices have fallen a great deal over the years, partly driven by competition as other types of panel have been produced.
Figure: Monocrystalline silicon module
These are generally considered the workhorses of the solar power market, and most people will choose monocrystalline panels due to their ready availability. They are usually used for large scale solar applications like business and domestic solar installation. This is useful if you only have a limited area for mounting your panels, or want to keep the installation small for aesthetic reasons. The lifespan of a monocrystalline cell is a minimum of twenty-five years and can be more than fifty, making them a worthwhile investment for long term use.
Polycrystalline silicon PV modulesUnlike monocrystalline cells, Polycrystalline solar cells are made up of multiple silicon crystals and are more efficient than amorphous panels. They are made from cells cut from an ingot of melted and recrystallized silicon. In the manufacturing process, molten silicon is cast into ingots of polycrystalline silicon, these ingots are then saw-cut into very thin wafers and assembled into complete cells. Polycrystalline cells are cheaper to produce than monocrystalline ones, due to the simpler manufacturing process.
In general, polycrystalline solar panels are among the cheapest and most widely found panels on the market today. They have a speckled crystal reflective appearance. They are slightly less efficient than monocrystalline cells and have efficiency of 12-15%, with lab efficiency of 18%. Polycrystalline cells provide a great alternative to mono crystalline cells, because they offer a slightly better cost-per-watt efficiency. Thus, many people prefer this type of solar technology today. They are designed to work best at relatively cooler temperatures.
Figure: Polycrystalline silicon module