So we all know we can pop some photovoltaic (PV) panels up on our roof and they will somehow convert the energy from the sun into useable electricity for our homes, but how do solar panels actually work?
Parts of a Solar PV System
- Solar panels: The individual panels are made up of groups of solar ‘cells’. The cells are made from a special material called a semiconductor and are grouped together to form a panel or module. The modules are then connected together in a group, known as a solar ‘array’.
- The inverter: The power that is generated by the solar array is direct current (DC). Homes work on the safer and more reliable alternating current (AC) system, so an inverter is used to convert the DC into useable AC for the home.
- The fuse box: From the inverter, current travels to the fuse box of the home in order to be distributed around the home and used by our appliances. This ensures the current is safe and our home is protected from any power surges or problems.
- The meter: Depending on the type of PV system installed, a range of meter options are available. For new installations, a meter will be installed to measure not only how much energy is produced for the home, but also how much excess energy is sent back into the grid, allowing the householder to claim additional payment under the Feed in Tariff (FiT) scheme. Some installations may also have nice display units to let you know what is being generated right now and other details of the system performance.
The Solar Cells
All solar cells are made from a semiconductor material, the most common being silicon. These materials have special chemical properties that are at the heart of the solar energy generation. An atom of silicon has 14 electrons arranged over three shells. The first two shells are full, with two and eight electrons each, but the final shell is only half full, with just 4 electrons. This means the atoms are inclined to ‘hold hands’ with neighbouring atoms, creating what is known as a ‘crystalline’ structure.
When energy is added to pure silicon, it causes some of these electrons to break their bonds and leave their atoms. Known as ‘free carriers’ these electrons wander around the crystalline structure, all charged up with energy, looking for a new place to bond. However, in pure silicon there are so few of these free carriers, it turns out to be a rather poor conductor of electricity.
To make a solar cell efficient, it must have an impurity added. Far from being a bad thing, these impurities are what turn a useless slab of silicon into a highly efficient electricity generating technology. Phosphorous is often used to dope silicon, mainly because its chemical composition gives it five orbiting electrons. This means it has four ‘hands’ to hold in a bond with the silicon atoms, but will still have one ‘spare hand’ that is not bonded to anything. This makes it a lot easier to knock one of the electrons out of its orbit, thereby making the silicon a much better conductor of energy. Because of the high number of free electrons in this type of panel, this is known as ‘N-type’ (negative type) silicon.
But, this is not where the science bit ends! On the other side of a solar cell, silicon is mixed with atoms of boron, an element that has only three electrons in its outer shell. When energy is applied to this type of silicon, there are ‘holes’ left in the structure where there are not enough electrons. This gives the panel a positive charge, creating what is known as ‘P-type’ silicon.
When you put these two, oppositely charged pieces of silicon together, the free carriers from the N side rush to fill the holes in the P side. This creates an electrical field and is the basis of PV panel technology. When light from the sun, carried as ‘photons’ hits the panel, the electron pairs are disrupted. The movement of electrons provides a current, and the electrical field of the cell provides the voltage; voltage plus current = power.
Types of Solar Panels
As previously discussed, the most common material for solar PV panels is silicon. However, this silicon can be presented in a number of different formats which will affect the performance, cost and efficiency of your solar system.
- Monocrystalline: Have slightly higher efficiency although polycrystalline types are catching up. Monocrystalline panels are more expensive than polycrystalline alternatives and whether the difference in performance (and look) is worth the extra money will depend on preference and the needs of the individual install. Often the quality and reliability of the manufacturer will be more important than the technology.
- Polycrystalline: The most common type of cells used, provides 14 – 16 per cent efficiency at a reasonable cost.
- Thin film / amorphous: Has the benefit of being able to be fitted to curved surfaces and a very thin finish but comes at a cost of efficiency. Typical efficiency is 5 – 7% but does work well in diffuse light conditions.
- Hybrid: Hybrid panels use a thin layer of amorphous film behind the monocrystalline cells, extracting additional energy from the sunlight and resulting in a more efficient system. These are, however, very much more expensive than any other type so generally not cost efficient for the majority of householders.
Without an inverter, all this power that is being generated by our solar cells would be useless. The inverter assumes the task of converting this DC power into useful and safe AC power for the home. The downside of this is that the inverter for your system will also require some power to operate, so choosing an efficient type of inverter is essential. Modern inverters will only consume around 4 – 8 per cent of the generated energy themselves, leaving a good 92 – 96 per cent of the energy for your use.
On and Off Grid Solar Systems
With any solar system, there is the question of whether you will connect into the grid or set up a standalone system.
- Grid connected systems are the typical type for most needs and make use of the existing mains grid to displace any excess energy. If the panels do not produce enough power, the grid will do the rest.
- Off grid systems are far less common but are used sometimes for agricultural or very rural properties where there is no grid connection present. With this type of system, the power generated by the panels will be stored in deep cycle rechargeable batteries for use later on.
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