PV - How it works

How does a solar electric (pv) system work?

Solar panels are attached to an aluminium mounting system, which is secured to the roof (typically directly to the rafters). Solar cells can also be integrated directly into the roof tiles - this is more suitable for new builds than retrofits.

When the sun shines, the panels will generate direct current (DC) which flows into a box called an inverter to create an alternating current (AC) - AC is used by the appliances in your home. Note that the panels work best in sunlight; however they will still generate some energy when it is overcast using 'diffuse sunlight'.

The AC flows from the inverter into the fuse box. If you are using electricity - perhaps you are watching tv, or the washing machine is running - the electricity will flow straight into these appliances (topped up, if necessary by additional electricity from the national grid). If you do not need the electricity it can flow into the national grid so that someone else can use it - in effect your house has become a mini power station (hence the term 'home generation').

You will get paid for any surplus energy that you generate. In addition from April 2010 you will receive a feed in tariff in respect of all the energy that you generate (using a 'grid-connected system such as the above), whether or not you use it.

How does a solar cell work?

A solar cell consists of two thin layers of semi-conducting materials, usually silicon, that have been 'doped' with specific chemicals Sunlight shining on the solar cell knocks electrons from the orbits of the doped semi-conductor in sufficient numbers to generate a direct current (DC).

The cell is covered with a thin layer of anti-reflective coating (ARC) to minimize light reflection. The top semi-conducting layer, or 'n' type layer, is doped with tiny amounts of phosphorus so that almost every thousandth silicon atom is replaced by a phosphorus atom. This creates free moving negative charges called 'electrons'. The base semi-conducting layer, or 'p' type layer, is doped with miniscule amounts of boron so that almost every millionth silicon atom is replaced by a boron atom. This creates free moving positive charges called 'holes'. When the 'n' and 'p' type layers are placed close together, as they are in a solar cell, the positively charged 'holes' and the negatively charged 'electrons' are attracted to each other. As they move into their respective neighbouring layers they cross a boundary layer called the 'p-n junction'. This movement of negatively and positively charged particles generates a strong electrical field across the p-n junction. When sunlight strikes this field it causes the electron particles and the hole particles to separate, which in turn creates a voltage of around 0.5V. The voltage pushes the flow of electrons or 'DC current' to contacts at the front and back of the cell where it is conducted away along the wiring circuitry that connects the cells together.

Types of solar cell

Solar cells can be made from a number of semi-conducting materials. A semi-conducting material is one that has a limited capacity for conducting an electrical current and those used in solar cells are all uniquely suited to producing electricity from sunlight - the photovoltaic effect. By far the most commonly used material is silicon, which is the main component of quartz sand and, after oxygen, is the second most common element in the Earth's crust. The performance of a solar cell is measured in terms of its efficiency at turning solar radiation or 'sunlight' into electricity. A typical solar cell has an efficiency no greater than 13 - 15% as only a portion of the sunlight energy spectrum can be converted into electricity and much of the sunlight is reflected or absorbed by the materials that make up the cell. If this seems off putting bear in mind that a gas power station has an energy conversion efficiency of only 35% and that 70% of the electricity generated is lost during the long distance transmission to the consumers- you and I.

The solar panel

As an individual solar cell only generates a low voltage, approx 0.5V, a number of cells are wired together to form a solar panel or 'module' that can generate anything between 100-330 watts. Modules are then connected together to form a PV array that will be typically fitted onto a southerly facing roof at an angle of between 30 and 50 in order to receive maximum sunlight. South-easterly and south-westerly facing systems can be installed with only a 5% reduction in panel efficiency but panels placed on a northerly orientation do not receive adequate sunlight to generate sufficient electricity.