Solar Cells and Modules
Solar Cells
A solar cell or photovoltaic cell is a semiconductor device that converts light directly into electricity by the photovoltaic effect. The most common material in solar cell production is purified silicon that can be applied in different ways.
Monocrystalline Silicon PV Cells
Monocrystalline silicon PV cells are made from silicon wafers that are cut from cylindrical single-crystal silicon ingots. To form nearly quadratic cells, that can be easily integrated in one module, the rotund cells have to be cut. Thus, refined silicon is wasted in the cell production process.
Modules consisting of monocrystalline silicon PV cells reach commercial efficiencies between 15 % and 18 %. So far, they are the most efficient modules and have the largest market share.
Polycrystalline Silicon PV Cells
Polycrystalline or multicrystalline silicon PV cells are made from cast square ingots - large blocks of molten silicon, carefully cooled and solidified. They are less expensive to produce than monocrystalline silicon PV cells, but are marginally less efficient, with module conversion efficiencies between 13 and 16 %.
Thin Film PV Cells
The various thin film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency (an average 6 to 12 % module efficiency),
Thin film PV cells are constructed by depositing extremely thin layers of photovoltaic semi-conductor materials onto a backing material such as glass, stainless steel or plastic. Thin film materials commercially used are amorphous silicon (a-Si), cadmium telluride (CdTe), and copper-indium-(gallium)-diselenide (CI(G)S).
Commercially available thin film modules:
- Are potentially cheaper to manufacture than crystalline cells
- Have a wider customer appeal as design elements due to their homogeneous appearance
- Present disadvantages, such as low-conversion efficiencies and requiring larger areas of PV arrays and more material (cables, support structures) to produce the same amount of electricity
Partly taken from: IEA PVPS
Solar Modules
A solar or photovoltaic module or panel is a packaged interconnected assembly of solar cells.
In order to use solar cells in practical applications they must be:
- connected electrically to one another and to the rest of the system,
- protected from mechanical damage during manufacture, transport and installation and use (in particular against hail impact, wind, sand and snow loads). This is especially important for wafer-based silicon cells which are brittle.
- protected from moisture, which corrodes metal contacts and interconnects, (and for thin-film cells the transparent conductive oxide layer) thus decreasing performance and lifetime.
The peak power output of a solar module depends on the number of cells connected and their size. Module performance is generally rated under Standard Test Conditions (STC) : irradiance of 1,000 W/m², solar spectrum of AM 1.5 and module temperature at 25°C. Solar modules are rated in peak watts [Wp] according to their output under STC. Thus, a 50 Wp module can be expected to supply 50 W of power under optimal conditions. The performance is reduced by high temperatures.
Modules can be connected in series and/or in parallel depending on the system requirements. A serial connection increases the voltage, a parallel connection increases the current.
Comparison of Different Types of PV Modules
|
Cell material |
Module efficiency |
Surface area needed for 1 kWp |
|
Monocrystalline silicon |
15-18 % |
7-9 m² |
|
Polycrystalline silicon |
13-16 % |
8-9 m² |
|
Micromorph tandem (aµ-Si) |
6-9 % |
9-12 m² |
|
Thin film: Copper indium diselenide (CIS) |
10-12 % |
9-11 m² |
|
Thin film: Cadmium telluride (CdTe), |
9-11 % |
11-13 m² |
|
Amorphus silicon (a-Si) |
6-8 % |
13-20 m² |
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