Tuesday, 15 November 2011

Mono- or poly-crystalline PV: Which is best?

By Robin Whitlock, Freelance Environmental Journalist

The three most recognised forms of solar PV are monocrystalline, polycrystalline and thin-film (amorphous). Of these three the first two form 93% of solar PV sales worldwide. Undoubtedly the most efficient is the first form, monocrystalline. It is called such because the crystals in the solar module are cut from a single continuous piece of silica crystal derived from molten sand. This is delivered in the form of a long cylinder shape which is then cut into circular wafers which can either be maintained as circular cells or cut into other shapes. Usually the cells are cut into a hexagonal shape in order to make them fit into the module in a tight formation thus allowing for maximum coverage and eliminating unnecessary space. The cells have a uniform colour, dark blue, because they are cut from the same piece of crystal.

Polycrystalline PV differs from monocrystalline in that instead of being cut from a single piece of crystal the silicon is melted and poured into a mould. Very often the silicon used is the remnants from manufacture of monocrystalline. The silicon from the mould takes the form of a square block which is again cut into wafers. Polycrystalline is cheap to produce and therefore cost-effective, but it is less efficient than monocrystalline PV. Poly panels therefore tend to be larger than mono for the same power output. An important consideration therefore is space. You can use poly effectively if you have lots of space on your roof or if you wish to use them as part of a ground-based array. It should be mentioned, however, that poly is generally more resistant to the long-term effects of heat than mono, so polycrystalline panels are often used in hotter countries.

Monocrystalline PV tends to be the most expensive of the two forms, but it is also the most efficient at between 13 to 17%, although some panels now can exceed 21%. The reason why PV is so expensive is precisely because it has to be refined from sand in order to achieve PV-grade silica. It is also a material that is high in demand because it is used for other electronic applications as well, notably the base material for computer chips and so on. Monocrystalline panels usually come with a 25 year power output guarantee. Monocrystalline PV panels are fast becoming the most used type of panel in the world.



RB Grant

Air and Water

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