A. PV solar panels
C. Fuse box
D. National grid
On Grid systems
Grid-connected photovoltaic systems are the most common type as they make use of the existing mains electricity grid. They are simpler in design and easier to fit than off grid systems. The electricity produced during the daytime is either used by the property owner, or directed back into the electricity grid and purchased by a utility company, an arrangement called ‘net metering’. At night, or on dark days when the panels do not produce sufficient power, electricity will be supplied via normal utility company grid system.
Off Grid systems
Far less common is an “off grid” or ‘stand alone’ system, which produces and stores power independently from the utility grid. These systems are particularly suitable in remote locations especially those where the property is more than one-quarter mile from the nearest power lines. Often the installation of an off grid PV system proves more costeffective than extending the power lines. The electricity generated by the panels is stored in a bank of rechargeable batteries as DC but in order to power household appliances an inverter will be required to convert the stored DC to AC. These rechargeable batteries contain specialised parts and chemicals not found in disposable batteries and are therefore larger and more expensive to purchase and maintain.
Here’s a quick explanation of how a solar cell works. If you want to know more we recommend this link: How Solar Cells 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.
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.
Here is an overview as to the properties of the types of commercial solar cell available today.
|Type of Cell||Cell Efficiency||Durability||Comments|
|Monocrystalline||14% - 17%||> 25 years||Similar output per square metre to polycrystalline, but not as efficient as hybrid panels.|
|Polycrystalline||14% – 16%||> 25 years||Most commonly used type of cell as offers good efficiency at reasonable cost. Similar output to monocrystal.|
|Thin Film / Amorphous||5% – 7%||> 20 years||Low efficiency requires large surface area. Made from flexible material and so can be used on curved building surfaces. Works better in diffuse light than mono & polycrystalline.|
|Hybrid||20%||> 25 years||Highest efficiency leading to maximum power output per square metre.|
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 80–360 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.