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Solar PV energy

How solar power works

Photovoltaic energy is the resulting process from the transfer of solar energy (photons) to electrical energy (Direct Current) through a solar cell.

Panels in winterThe energy of a photon is transferred to an electron in an atom of a semiconductor material within the solar cell. With the addition of this energy the electron moves from its normal position to become part of an electric current.

Semiconductor materials such as silicon, gallium arsenide or cadmium telluride are used in these solar cells. The crystalline solar cell is the most common and accounts for nearly all of the present solar market.

The PV cells generally consist of two thin regions, one above the other, each with specially added impurities called dopants. The result is that one region is of "type n", with an excess of electrons, while the other is of "type p", with an excess of positive holes. This two-region structure, called a p-n junction, produces an internal electric field.

When the photons create free electrons and holes in proximity to the p-n junction, the internal electric field makes them move in opposite directions (the electrons move towards the side n and the holes move towards the side p). An electromotive force is generated between the p and n regions, with p positive and n negative. Strips of wire are then connected across the p and n sides of the cells. Cells are then connected in series to form a module.

PV systems

There are two main types of PV systems: the stand-alone system and the grid-connected system.

Power cablesStand-alone PV systems are not connected to the National Grid and are generally smaller systems as they are often located in rural areas or integrated into street furniture or telecom applications. Usually, all stand-alone systems will have storage batteries and a charge controller but some systems may also include an inverter where AC power is required.

Grid-connected PV systems generally comprise the following components:

  • PV modules (PV panels)
  • Direct Current (DC) cables
  • DC main isolating switch
  • Inverter
  • Solar energy meter
  • AC cables from the inverter to main electrical switchboard

PV modules (panels) come under various classifications depending upon the module's specific qualities and can be classified according to cell type (ie mono-crystalline, polycrystalline or thin film), encapsulation material (ie teflon or resin) or even frame structure (ie aluminium frame or frameless).

Standard modules are manufactured with the aim of achieving maximum energy yield per square metre at as little cost as possible. These are mostly glass-film laminates encapsulated in ethylene vinyl acetate. Typical standard modules consist of 36 to 216 cells and having power outputs in the range from 100Wp to 300Wp. There are also special modules manufactured for other applications, such as solar roof tiles.