Photovoltaic Information (Solar Electric Cells)

PV Cells and Arrays

The combination of cells and the array configuration depends on the desired voltage and amperage output.

The PV array consists of a number of individual photovoltaic modules connected together to give a suitable current and voltage output. Common power modules have a rated power output of around 50 - 80 Watts (W) each. A typical domestic system of 1.5 - 2 kWp may therefore comprise some 20 - 30 modules covering an area of around 120 - 250 sq. ft., depending on the technology used and the orientation of the array with respect to the sun.

Most power modules deliver direct current (DC) electricity at 12 volts (V), whereas most common household appliances in the US run off alternating current (AC) at either 110V or 230 V. An inverter is used to convert the low voltage DC to higher voltage AC. Numerous types of inverter are available, but not all are suitable for use when feeding power back into the main electrical supply grid. Good suppliers and installers of grid-connect PV systems will be able to offer advice on suitability of commonly available models.

Types of PV Technology

Monocrystalline Silicon Cells:
These cells are made from very pure monocrystalline silicon. The silicon has a single and continuous crystal lattice structure with almost no defects or impurities. The principle advantage of monocrystalline cells are their high efficiencies, typically around 15%, although the manufacturing process required to produce monocrystalline silicon is complicated, resulting in slightly higher costs than other technologies.

Multicrystalline Silicon Cells:
Multicrystalline cells are produced using numerous grains of monocrystalline silicon. In the manufacturing process, molten polycrystalline silicon is cast into ingots, these ingots are then cut into very thin wafers and assembled into complete cells. Multicrystalline cells are cheaper to produce than monocrystalline ones, due to the simpler manufacturing process. However, they tend to be slightly less efficient, with average efficiencies of around 12%.

Amorphous Silicon:
Amorphous silicon cells are composed of silicon atoms in a thin homogenous layer rather than a crystal structure. Amorphous silicon absorbs light more effectively than crystalline silicon, so the cells can be thinner. For this reason, amorphous silicon is also known as a "thin film" PV technology. Amorphous silicon can be deposited on a wide range of substrates, both rigid and flexible, which makes it ideal for curved surfaces and "fold-away" modules. Amorphous cells are, however, less efficient than crystalline based cells, with typical efficiencies of around 6%, but they are easier and therefore cheaper to produce. Their low cost makes them ideally suited for many applications where high efficiency is not required and low cost is important.

Other Thin Films:
A number of other promising materials such as cadmium telluride (CdTe) and copper indium diselenide (CIS) are now being used for PV modules. The attraction of these technologies is that they can be manufactured by relatively inexpensive industrial processes, certainly in comparison to crystalline silicon technologies, yet they typically offer higher module efficiencies than amorphous silicon.