How Does a Solar-PV System Works

A Solar PV installation Solar Electric panels are commonly referred to as Solar PV panels. PV stands for ‘photovoltaic’ and is the process of converting light into electricity. Economies of scale, advances in technology and financial incentives have made Solar PV viable and useable within your home. So, you can generate your own electricity and earn income from it, giving a better return than current UK bank interest rates. See our article on earning tax-free income from Solar for more information about the financial benefits. HotSpot Solar install Solar PV panels in Gloucestershire and the surrounding counties. If you would like further information about installations or a no-obligation quote, please do not hesitate tocontact us and we will be happy to help. For a detailed explanation of how Solar PV works, read on… How Does a Solar-PV System Work? Edmond Becquerel first discovered the process of converting sunlight into electricity in 1839 and over the decades, the PV effect has been improved upon and developed for us to be able to build useable solar panels which generate enough electricity for us to harness and have a positive effect in contributing to our electricity consumption. A solar-PV panel consists of multiple solar PV cells. An individual solar PV cell is constructed and functions in the following manner: Electricity is the result of the flow of electrons. All matter at its lower level is made of atoms. Most atoms may be thought of as a nucleus containing positively charged particles with an equal number of negatively charged electrons orbiting round it.  Each element has a distinctive number of electrons which are arranged in concentric ‘shells’ around the nucleus, each shell representing a level of energy. A stable arrangement of electrons for the atom is 2 in the first shell, 8 in the second and 8 in the third. Silicon has 14 electrons, so only has 4 in its outer shell but in order to be stable, it has a ‘need’ for 4 extra electrons in its outer shell.  This ‘need’ causes neighbouring silicon atoms to get together to create a four-way electron sharing scheme (known as a crystal lattice). Each atom can then ‘pretend’ it has 8 electrons in its outer shell by sharing with 4 from its neighbours.  This bond is very strong, making it difficult for any electrons to wander off on their own. Phosphorus atoms have 5 electrons in their outer shell, but are capable of infiltrating a silicon lattice, and joining in on the ‘electron sharing scheme’ with 4 of its own electrons.  The main bond is still strong, but the ‘spare’ electron is not part of the lattice and is so relatively loosely joined just by the electrical attraction to its nucleus. It can wander off fairly easily, without affecting the structure very much, but when it does, the atom that remains is left with an excess positive charge. Silicon doped with a phosphorus atom creating an n-semiconductor Silicon doped with a boron atom creating a p-semiconductor When pure silicon is ‘doped’ with phosphorus in this way, it creates what is called an N-type semiconductor as it has ‘N’egatively charged electrons that are relatively free to wander off or be donated. Silicon can also be doped with (say) boron which has only 3 electrons in its outer shell. This causes a similar but opposite situation; the boron atom can still join in the electron sharing scheme with the silicon, but now there is an atomic vacancy for an electron to complete the outer shells of the atoms in the lattice, which we call a hole. This material is called a P-type or acceptor semiconductor. In one form of PV cell, wafer thin layers of both these materials are brought together and the point at which they meet is called a P-N junction. The ‘spare’ electrons in the N-type material only need a small amount of energy to escape from their arrangement. At normal room temperature most of these electrons are free and will have sufficient energy for some to diffuse across the junction. Any that do are readily captured by the atomic holes in the P-type material and they become bound into that lattice. However, the electrons carry with them their negative electric charge, and have left behind the equivalent positive charge.  As this diffusion continues an excess of negative charge builds up at the junction on the P side leaving an excess of positive charge on the N side.  This creates an electrical field of force across the junction which starts to push away any more of the free electrons back towards the N-side. A state of equilibrium is eventually reached with the boundary layer becoming depleted of mobile electrons, and a constant force existing across the junction. In the materials we are discussing this potential difference amounts to about 0.6 volts. The cell in action Sunlight is composed of varying bundles of energy called photons. If a photon has the right level of energy, it can impart it to an electron that is properly bound into the silicon lattice anywhere (doped or not). When this happens, the electron has too much energy to remain in its place, and escapes to become a free electron. It leaves behind the ‘vacancy’ in the atomic structure that is called a hole, and this hole is now associated with the positive charge that the electron has left behind. This is called the creation of an electron-hole pair. The electron has enough energy to wander off, and the positive ‘hole’ can also be said to wander by virtue of the fact that another electron, freed from a neighbouring structure, can fall into it leaving a new hole further away. The atoms themselves do not move, only the electrons. In fact no one electron actually moves very far in a material like silicon, the movement of the electrons (and holes) is the net effect of a lot of short hops from one structure to another. If there were no other forces, the electrons would just wander at random, with electron-hole pairs being created by the photons and re-combining at random. In the PV cell, however, when an electron-hole pair is created anywhere near the force field at the p-n junction (on either side), the force field pushes the electron towards the N-side and the resulting hole migrates towards the P-side. The N-side of the cell is then connected to the P-side with metal conductors and the force at the junction drives the electrons to flow as a current from the N-side to the P-side where they can fall back into the waiting holes, ready to be re-released and swept across to the N-side again. The free electrons carry the energy provided by the sunlight, and need to be less energetic in order to get back into a hole – hence, as they travel around the circuit, we can tap off this excess energy in the form of useable power (a load in the circuit such as a lightbulb) without disrupting the system. This energy would otherwise be lost as heat. Useable PV Cell Construction The field at the p-n junction causes freed electrons to drift to the n-region and resultant holes to the p-region The top surface of the cell is designed to allow the maximum amount of light onto the silicon. Hence, electrical contacts (electrodes) which carry the free electrons through the circuit are screen-printed onto the surface of the cell. This has the effect of minimising the losses on the uppermost surface of the cell. Rear electric contacts are applied across a larger area of the cell as there is no need for light penetration underneath. A single cell does not produce enough power on its own to be of any practical use. A cell typically produces around 0.6 volts in an ‘open circuit’ situation (no load) and 0.45 volts under load.  However, several cells can be linked together in series into a ‘module’ to collectively produce a useful amount of electricity. A module consists of about 36 cells and will produce a ‘nominal’ voltage of around 16 volts. Such a module could be (say) used to charge a 12v battery. The modules that form the solar-panel are more typically linked together in greater numbers to generate higher voltages; 48 or 60 cells linked will produce approximately 28 and 36 volts respectively in an open-circuit situation. A solar panel of this size is now generating enough power to be useful. By linking 10, 20 or more together, high voltages and useable current can now be generated. The output from arrays of panels is now sufficient to be connected into the mains electricity supply which supplements the householder’s power usage and contributes to the nation’s electricity generation. However, one further step is required to enable it to be connected to the National Grid utility network. The electricity generated is direct current (DC). This is unsuitable for use by the householder as mains electricity is alternating current (AC). In order to enable connectivity to the AC mains supply, an intermediary device is required called a grid-tie inverter. A Typical Solar PV Installation Layout The grid-tie inverter converts the solar-panel direct current to alternating current which matches the voltage and frequency of the National Grid. Typical inverters are powered by the solar panels themselves and work at around 94% efficiency. Therefore, some electricity generated by the panels is lost in the conversion from DC to AC. Most inverters attempt to maximize the output from the panels by performing a process call Maximum Power Point Tracking (MPPT) which optimizes the power drawn from the panels at any particular point in time. Solar-PV utilizes free high-grade energy which is silent in operation and very reliable. A typical installation layout in a property is as shown. Initial costs of such installations are high but with the introduction of the Feed-in-Tariffs (FITs) as introduced by the British Government, Solar PV is now an attractive investment giving good tax-free return on investment (ROI) over 20-25 years, as well as giving the consumer free electricity during the daylight hours when the Sun is shining.