Solar panels that harness the sun’s power to generate electricity provide clean power for homes, communities and businesses, and help cut global carbon emissions.
Solar photovoltaic (pv) modules generate electricity from sunlight, which can be fed into the mains electricity supply of a building or sold to the public electricity grid. Reducing the need for fossil fuel generation, the growing grid-connected solar PV sector across the globe is helping create jobs, enabling families and businesses to save money, and cut greenhouse emissions.
How grid-connected PV systems work
PV modules use semiconductor materials to generate dc electricity from sunlight. A large area is needed to collect as much sunlight as possible, so the semiconductor is either made into thin, flat, crystalline cells, or deposited as a very thin continuous layer onto a support material. The cells are wired together and sealed into a weatherproof module, with electrical connectors added. Modern modules for grid connection usually have between 48 and 72 cells and produce dc voltages of typically 25 to 40 volts, with a rated output (see box) of between 150 and 250 Wp.
In order to supply electricity into a mains electricity system, the dc output from the module must be converted to ac at the correct voltage and frequency. An electronic inverter is used to do this. Generally a number of PV modules are connected in series to provide a higher dc voltage to the inverter input, and sometimes several of these ‘series strings’ are connected in parallel, so that a single inverter can be used for 50 or more modules. Modern inverters are very efficient (typically 97%), and use electronic control systems to ensure that the PV array keeps working at its optimum voltage. They also incorporate safety systems as required in the country of use.
PV modules are specified by their ‘watt-peak’ (Wp) rating, which is the power generated at a solar radiation level of 1000 W/m2, equivalent to bright sun in the tropics. They still work fine with less solar radiation. The voltage produced by a PV module is largely determined by the semiconductor material and the number of cells, and varies only slightly with the amount of solar radiation. The electrical current and the power generated are proportional to the amount of solar radiation.
Many grid connected PV systems are installed on frames which are mounted on the roof or walls of a building. Used in this way the PV does not take up land that could be used for other purposes. Ideally the PV faces towards the equator (i.e. South in the northern hemisphere) but the exact direction is not critical. However, it is important to make sure that there is minimal shading of the PV. The inverter is housed inside the building and connected to the mains electrical supply, usually with a meter to measure the kWh generated. If the PV electricity production exceeds building demand then the excess can be exported to the grid, and vice versa.
What are the benefits of grid-connected PV systems?
By reducing the need for fossil-fuel generation, grid-connected PV cuts greenhouse gas emissions (and other air pollution), because no emissions are produced during PV operation.
In the past there has been concern about the greenhouse gases emitted (‘embodied’) in the manufacture of PV systems, particularly in the production of ultra-pure semiconductors. With current production techniques, these embodied greenhouse gases are saved within two to four years of use of grid-connected operation, depending on the amount of sunlight.
PV is the easiest renewable electricity source to incorporate into buildings. The electricity is supplied at the point of use, thus avoiding the losses which occur in electricity distribution (these average 7% in the UK). It can be used at any scale – from less than a kWp on an individual home up to MWp scale systems on large public buildings – and is simple and reliable. Because of this, it is a valuable way to raise awareness of electricity supply and use, and helps highlight the potential for renewable energy. Several schools that have won Ashden Awards like Cassop Primary School and Ringmer Collegehave installed PV, to supply part of their electricity and as an education aid.