Technical
People generally consume both electricity and hot water. Photovoltaïc solar modules (PV) provide electricity; thermal solar panels provide hot water. The sun is free of charge, in abundance available and it’s the only energy source which silent, easy to install and not polluting our environment.
The advantages of solar energy are:
- It reduces drastically the need of fossil fuels or nuclear power
- A tremendous reduction of the harmful CO2 emission
- The supply is guaranteed for the following 4 billion year
- Can easily and nearly everywhere been installed
- The energy production is located in the immediate neighbourhood of the consumer (no transport losses)
- The long life time of the panels (25 to 30 years)
- Entirely soundproof and nearly total maintenance free (only cleaning if necessary)
Further on we explain more in detail the functioning of PV and thermal panels.
What’s the functioning of a PV solar module?
The process of converting sunlight into electricity is called: photovoltaïc conversion. Often referred to using the abbreviation: PV (originating from the English photovoltaic).
The most common photosensitive material is silicon. It is a semiconductor metal, in large quantities available on earth as the raw material for it, is sand. For PV panels’ extremely pure silicon is made of which two different layers are prepared.
When these layers are in contact with each other and when these are exposed to light, the electricity starts to flow between these two not identical semiconductors. (The photovoltaic current). Hereunder we explain the functioning of crystalline solar cells. The functioning of thin-film panels is very similar. A solar cell is made out of 4 elements: 2 layers of silicon: 1 layer to which very small quantity phosphorus was added (doped) and a second layer with a small quantity boron. At the upper side of this, very thin contact strips are added and at the bottom side there is a very thin layer metal (generally silver or nickel). When hit by the light (photons), electrons in the solar cell are detached from the first layer silicon and move to the 2nd layer, creating this way a tension difference: minus (at the bottom) and plus (at the top) of the cell. When upper and lower contacts are interconnected an electrical current starts through this connection. Remark: for this process direct sunlight is not necessary needed. Also during cloudy days (diffuse light) the solar cell will provide electricity.
By interconnection the solar cells with each other (series connection) useful quantities of electricity are obtained. A solar module contains mostly 6 strings, having 12 solar cells, each of them functioning as a small battery.
For thin film panels, the upper and lower glass are covered with a conductive layer. The cells between these 2 layers are small and have the length of the module. They are made by laying several thins layers over the full surface of the panel followed each time by a very precise laser scribing of very thin lines up till the under laying layer. These lines, once filled up with next layer’s material, serve as the abovementioned contacts strips.
The thin film process is fully automatized and results in a considerable lower consumption of photosensitive material, which makes the overall cost lower compared to crystalline modules. At the other hand the technical limitations (usable materials for the layers is mostly amorphous silicon) limits the output.
Solar PV-systems
A solar installation is made of several interconnected solar modules, which are built out of interconnected solar cells (comparable with batteries), providing direct current (DC). This DC current is converted into useful alternating current (AC) by means of an inverter and is mostly connected to grid.
We explain further on more into detail the components of a PV installation:
The solar cell
The current solar cells are mostly made of silicon. The very pure silicon which is needed for solar modules can be obtained in 2 different physical appearances: as a crystal form or amorphous form(not liquid, no crystals). Depending from this basic material three types of solar modules are made: 2 of them (crystalline modules) have a similar appearance; the third type (thin film) is different. The 2 types of crystalline modules are monocrystalline and polycrystalline.
- Mono-crystalline solar cells: A large dark blue candle-shaped monocrystal of silicon is produced in the first place. Afterwards this candle is sliced into thin silicon slices, of which the cells are produced. They are even coloured rectangular, with cutted corners.
- Poly-crystalline solar cells: The basic silicon is casted into a form instead sliced out of a monocrystal. The solar cells are rectangular; the colour is uneven with ‘marble effect’ and they have generally about 1% less output as the monocrystalline solar cells.
- Thin film cells: The basic material is amorphous silicon (A-Si). There is no solid block made and sliced, but the silicon is vapoured directly onto a glass plate. Very little silicon is used as the layer thickness is only some µm. A cell pattern is made by laser scribing. The production cost is lower compared to the crystalline modules and the panels have a better functioning by diffuse light (cloudy weather) but the output/m ² is lower due to used basic material. These panels are used in the first place for larger surfaces (industrial buildings). As these panels are made out of laminated glass and they can be made semi-transparent - they are also very suitable for BIPV (Build in Photovoltaics) such as facades, verandas, etc
| Cell type | Mono-crystalline panels | Poly-crystalline panels | Amorphous silicon panels |
|---|---|---|---|
| Abbreviations | mono-Cr | poly-Cr or multi-Cr | amorf-Si (thin film) A-Si |
| Production process | saw/silicon slices | casted | vapoured: thin silicon layer and automized process |
| Market share | 85% | 15% | |
| Efficiency | 15%-18% | 14%-16% | 6% |
| Colour | dark blue/anthracite | blue, marble | anthracite |
Solar radiation and output
The earth receives an enormous amount of solar energy. In one hour our globe gets enough energy to cover the annual world energy requirement. For Belgium it equals to 500 times our annual electricity consumption and almost 60 time of our total energy consumption. Sunlight has however different wavelengths. Unfortunately, at present, we can only convert a part of it (6 to18%) , as the current solar cells convert only 1 or 2 wavelengths.
The capacity of a solar module is mainly dependent on 3 factors: radiation of the sun, the size and the efficiency of it.
Output
The capacity of solar module is expressed in Watt peak (Wp). This is the output at a radiation of 1000W/m ² and at 25°C. . Most of solar modules have a peak capacity between 40 and 200 Wp. In practice the output is dependent on the quantity light that reaches the panel. The output in Spain, for instance, is a much higher than in Belgium because of the number of hours the sun lights the panel. Also the orientation is very important for this. For Belgium the ideal positioning is: orientated to the south under an angle of 36°.
PV solar systems
A crystalline solar module is mostly made of 72 interlinked solar cells. In the panel, these cells are protected against wind and rain by a glass plate at the top and a EVA film at the bottom. In case of a thin film panel the solar cells are laminated between 2 glass plates.
A complete solar system has several modules connected by cables, an inverter, a counter and a fuse.Grid connected solar Systems
The panels supply direct current (DC). To obtain the desired usable tension (230 volts AC) the direct current (DC) coming from the PV panels is converted by inverter. If you consume more electricity (e.g. when washing machine and/or the dish cleaner are switched on) then the PV system is producing, then the gap will be filled with ‘normal’ electricity coming from the grid. If you produce more electricity than you consume (e.g. summer period), then, the surplus is fed into the grid and the counter turns reverse. Sometimes the counter (older types) must replaced, but this happens mostly free of charge and on simple demand to the grid company.