The price decrease of PV modules and system is opening new opportunities, in both grid-connected and off-grid applications.
The key components of a photovoltaic power system are various types of photovoltaic cells (some-times also called solar cells) interconnected and encapsulated to form a photovoltaic module (the commercial product), the mounting structure for the module or array, the inverter (essential for grid-connected systems and required for most off-grid systems), the storage battery and charge controller (for off-grid systems but also increasingly for grid connected ones).
Photovoltaic cells represent the smallest unit in a photovoltaic power producing device, typically available in 12,5 cm, 15 cm and up to 20 cm square sizes. In general, cells can be classified as either wafer-based crystalline (single crystal and multicrystalline silicon, compound semi-conductor) thin film or or-ganic. Currently, crystalline silicon technolo-gies account for about 80% of the overall cell production in the IEA PVPS countries. Single crystal silicon (sc-Si) PV cells are formed with the wafers manufactured using a single crys-tal growth method and have commercial effi-ciencies between 16% and 24%. Multicrystal-line silicon (mc-Si) cells, usually formed with multicrystalline wafers manufactured from a cast solidification process, are becoming in-creasingly popular as they are less expensive to produce but are marginally less efficient, with average conversion efficiency around 14-17%. Quasi-monocrystalline silicon PV cells, manufactured using similar processes as mul-ticrystalline silicon PV cells, have been gaining recent attention. III-V compound semicon-ductor PV cells are formed using materials such as GaAs on the Ge substrates and have high conversion efficiencies of 40% and more. Due to their high cost, they are typically used in concentrator PV systems with tracking sys-tems or space applications. Thin film cells are formed by depositing extremely thin layers of photovoltaic semi-conductor materials onto a backing material such as glass, stainless Steel
or plastic. Module conversion efficiencies reported for thin film PV currently range from 7% (a-Si) to 13% (CIGS) but they are potential-ly less expensive to manufacture than crystal-line cells. The disadvantage of low conversion efficiencies is that larger areas of photovoltaic arrays are required to produce the same amount of electricity. Thin film materials commercially used are amorphous and mi-cromorph silicon (a-Si), cadmium telluride (CdTe), and copper-indium-gallium-diselenide (CIGS). Organic thin film PV cells, using dye or organic semiconductors, have created inter-est and research, development and demon-stration activities are underway.
Further research and development is being carried out to improve the efficiency of all the basic types of cells with laboratory efficiency levels of 25% for single crystal cells, and 20% for thin film technologies being achieved.
Photovoltaic modules are typically rated be-tween 50 W and 300 W with specialized products for building integrated PV systems at even larger sizes. Crystalline silicon mod-ules consist of individual PV cells connected together and encapsulated between a trans-parent front, usually glass, and a backing ma-terial, usually plastic or glass. Thin film mod-ules encapsulate PV cells formed into a single substrate, in a flexible or fixed module, with transparent plastic or glass as the front mate-rial. Quality PV modules are typically guaran-teed for up to 25 years by manufacturers and are type approved to IEC 61215 Ed. 2, IEC 61646 Ed. 2.0 and IEC 61730 International Standards.
A PV array consists of a number of modules connected in series (strings), then coupled in parallel to produce the required output pow-er. A wide range of mounting structures has been developed especially for building inte-grated PV systems (BIPV), including PV fa-cades, sloped and flat roof mountings, inte-grated (opaque or semi-transparent) glass-glass modules and