Solar PV Technology
PV systems split into "Standalone" and "Grid-connected" systems. An example of a standalone system would be a parking metre, powered by its own solar panel. (The power only feeds that one system and hence it is connected to nothing else - i.e. "Stand-alone"). A grid-connected system is where the solar panels are connected via the correct infrastructure into the national grid.
Green Energy (eu) Ltd only install Grid-connected systems and this is all we will look at here.
Components of a grid-connected system
A grid-connected PV system essentially comprises the following components:
- 1 PV modules/array (multiple PV modules connected in series or parallel with mounting frame);
- 2 PV array combiner/junction box (with protective equipment);
- 3 direct current (DC) cabling;
- 4 DC main disconnect/isolator switch;
- 5 inverter;
- 6 AC cabling;
- 7 meter cupboard with power distribution system, supply and feed meter, and electricity connection.
The diagram opposite shows the typical layout of a grid-connected PV system.
Green Energy-Solar Ltd supplies this kind of installation for business, agricultural, industrial and land-based installations.
The components of the above that require further detailed explanation are the functioning of the panels (1) and the inverter (5).
Photovoltaics and how solar panels work
The term photovoltaics means the direct conversion of light into electrical energy using solar cells. Semiconductor materials such as silicon, gallium arsenide, cadmium telluride or copper indium diselenide are used in solar cells. The crystalline solar cell is the most commonly used variety. During 2006, these had a worldwide market share of 95 per cent.
How a solar cell works
The way in which solar cells work is shown here, using crystalline silicon cells as an example. Pure silicon with a high crystal quality is used to make solar cells. Each silicon atom has four bonding electrons. In the silicon crystal lattice, two electrons from one atom bond with two electrons from a neighbouring atom to form an electron pair bond. That electron bond can be broken by the action of light or heat, leaving effectively an electron "hole" in the crystal lattice. This is known as the "separation of charge".
Each silicon solar cell comprises two differently doped silicon layers. The layer that faces the sun's light is negatively doped with phosphorus. The layer below it is positively doped with boron. Where the two layers meet, called the "boundary layer", the separation of charge caused by the effect of sunlight on the silicon atoms produces an electrical field.
In order to be able to take power from the solar cell, metallic contacts are fitted on the front and back of the cell. On the back of the solar cell it is possible to apply a contact layer over the whole surface using an aluminium or silver paste. The front, by contrast, must let as much light through as possible. Here, the contacts are usually applied in the form of a thin grid or a tree structure. Placing a thin film of silicon nitride or titanium oxide onto the front face of the solar cell reduces light reflection off the surface of the cell.
Design and functioning of a crystalline silicon solar cell (diagram):
- 1 charge separation;
- 2 recombination;
- 3 unused photon energy (e.g. transmission);
- 4 reflection and shading caused by front contacts.
Source: V. Quaschning
The electrical current received from a solar panel is Direct Current or DC. This type of current is unsuitable for connection to the national grid which is Alternating Current (or AC).
An inverter unit is consequently required to convert the DC to the correct voltage and frequency required by the local mains AC. It is called an inverter, because the early AC/DC converters converted AC to DC (i.e they were "Rectifiers"). Consequently when the need to convert from DC to AC arose, the process was "Inverted", so giving rise to the name.
Specifically inverters used to convert DC to Grid AC are known as "Grid-tie" inverters or "Grid-interactive" inverters. The distinction is useful as they are specifically designed not to be used in standalone (i.e. non-grid-connected) installations.
Because of its grid-interactivity, an inverter requires two specific features to enable safe supply of current to the national grid:
- 1. The inverter has in-built circuitry to sense the current AC grid waveform and output a voltage to correspond with the grid.
- 2. The inverter has to be able to quickly disconnect from the grid if the grid itself goes down. This is to ensure that in the event of a blackout, the energy being produced from the solar panels will not harm any line workers who are sent to fix the power grid.