PV SYSTEMS The main components of a PV system are an array of PV panels and an inverter. PV Panels A typical PV panel is about 1.0 x 1.6 metres in size, and composed of 60 PV cells arranged in a 6 x 10 grid. Each cell is about 15 cm square, and can generate a voltage of about 0.5 volts in bright sunshine. The cells are usually connected in series, sandwiched between a sheet of tempered glass (low-iron for maximum light transmission) and a backing of metal or plastic. (Or in special cases the backing may also be glass, in order to allow some light through the panel.) Several panels connected together form an array. (The panels are usually connected together in series, as a 'string'.) PV cells The PV panels fitted on roofs incorporate cells of crystalline silicon. Silicon is the second most abundant element in the Earth's crust it is part of the molecular structure of sand and clay. It is a 'semiconductor', that is to say its electrical conductivity is between that of a conductor, eg, copper, and an insulator, eg, rubber. It has long been widely used in the electronics industry for making diodes that conduct electricity in one direction only. (Such devices have also come to be called 'semiconductors'.) A PV cell is similar. When electrons in the wafer of silicon that form a cell are agitated by solar radiation, the electrons can flow only in one direction, creating an electric current. Manufacture of silicon cells: Monocrystalline cells A single large crystal of silicon with a minute amount of boron is formed as a cylinder with a diameter of about 15 cm. Four sides of the cylinder are cut off, leaving rounded corners. Then sections are sliced off to form wafers less than a quarter of a millimetre thick in a square shape with rounded corners. Phosphorus is diffused into one surface of a wafer ('doping'), and the phosphorus boron interface creates the vital p n junction. (p n: positive negative). Electrons can only flow across the junction from the negative side to the positive side many electrons moving together constitute an electric current. For collecting and carrying the current, thin silver strips are printed on the outer face of the wafer, with a metal foil on the under surface. The wafer is encapsulated to form a PV cell. When these cells are arranged into a grid in a glass-fronted panel, the missing corners show up in a regular pattern by which monocrystalline panels can usually be recognised. (See the photo and the schematic illustration below.) But some new monocrystalline panels look different. Some have hexagonal cells with missing corners; some have square cells without any missing corners. Polycrystalline cells A multitude of small silicon crystals are cast into a block. Again, the block is cut into thin wafers, and formed into square cells. PV SYSTEMS 1 OCTOBER 2016.
Panel with monocrystalline cells (The rounded corners of each cell allow the background to show through in a regular pattern.) Panel with polycrystalline cells (The panel has 60 cells in a 6x10 grid.) Manufacturing processes for monocrystalline and polycrystalline panels (Source: The Solar Blogger see Further Info.) Amorphous Silicon can be deposited by vapour deposition to form a layer, creating one type of 'thin film' cell. This is a relatively cheap process, but efficiencies are low, at most 8% good enough for solar garden lights, and the like, but not for a domestic PV system. (For thin film technologies, other materials can be more efficient than amorphous solar. Some 'perovskite' materials can have conversion efficiencies of 15% - 20%.) PV SYSTEMS 2 OCTOBER 2016.
Efficiencies Panels with monocrystalline cells are more efficient but more costly than polycrystalline panels. Nowadays, a typical monocrystalline panel might have an efficiency of 18% - 20%. Some panels have even higher efficiencies out in front are Sunpower who manufacture a panel with an efficiency of 21.5%. Efficiencies are measured under 'Standard Test Conditions' (STC), with the main condition being an irradiance of 1,000 watts/m 2. This roughly corresponds to the radiation on a bright summer's day see last month's article on Solar Power. Polycrystalline panels have lower efficiencies: typically 15% - 18%. On average, they are about 18% cheaper per Wp, and, in the UK, they are the more popular. (Wp: Watts-peak see below.) Note that the PV industry likes to use the word 'module' in place of 'panel', perhaps because of the confusion amongst the public about the meaning of 'solar panel'. Does the panel produce electricity or hot water? Anyway, I think 'PV panel' is clear enough. Watts-peak A measure of the theoretical output of a solar panel is given by its 'watts-peak' rating. A typical panel has a rating of 250 Wp under the STC conditions mentioned above, its output is 250 watts. One of the STC conditions is that the panel is at 25 C, whereas in use panels are likely to get hotter and then their output falls. For techies who want to go into the details, manufacturers of panels supply a 'temperature coefficient of power' rating. A typical figure is 0.5%. On a hot summer's day, such a panel may reach a temperature of 45 C, ie, 20 C greater than the STC temperature. So the panel's output would be reduced by 10% compared to its output at 25 C. (10 = 20 x 0.5.) Lifespan of panels Very gradually, PV panels become less efficient. Their output drops by about 0.5% per year though the efficiencies of the latest panels may decline more slowly. Their lifespan is expected to be at least 20 years. And it could be twice that. (A conundrum for future householders will be when to replace their ageing PV panels. When in the future will improved conversion efficiencies and lower costs make it worthwhile to replace panels that still work?) Panels are generally maintenance free, though those on shallow pitched roofs may occasionally need a clean. (Nowadays, this sort of work can be done by window cleaners standing on the ground, using water-fed poles and brushes.) The inverter The basic purpose of an inverter is to transform the direct current of uncertain voltage that comes from a PV array into an alternating current at mains voltage (nominally 230 V). This output is fed into the household circuits (for 'self-consumption'), and any surplus is exported to the local electricity network. A string inverter is usually fitted onto a wall in the loft, utility room, or the like. ('String'? the inverter is for use with several panels connected together in series.) A display shows the power output at any moment, and perhaps the output over time. It may show diagnostic information, too. Some models can connect with a home computer, smart phone, or tablet. PV SYSTEMS 3 OCTOBER 2016.
A 'grid-tied' inverter has another important function: to disconnect the grid in the event of a power cut. (If there is a power cut, the system must automatically stop exporting into the local network for the safety of any people working on supposedly dead power lines.) Most inverters incorporate what is called Maximum Power Point Tracking: MPPT. At any particular moment, the voltage and current generated by the array are controlled by the inverter so that the power (voltage x current) delivered by the array is a maximum. The output from some cheap inverters is a Modified Sine Wave, rather than a Pure Sine Wave like that of grid electricity. Most appliances will work with a Modified Sine Wave, but not all. Overall, most inverters are about 95% efficient. That 5% of inefficiency means that they get warm. They may hum, too. Some inverters allow more than one string of panels to be connected. This would be useful, for example, if you have separate arrays facing east and west. It is common practice to undersize an inverter, by say 20%. (Inverters can operate at above their rated power for short periods.) Some power may be lost on bright sunny days by the inverter temporarily deviating from MPPT. But this 'clipping' will be more than compensated for by better efficiencies on duller days. For domestic systems up to nearly 4kWp, the inverter should have G83/1 type approval see below and be suited to handling array voltages in the range 120 500 volts (direct current). The Microgeneration Certification Scheme (MCS) recommend that the inverter should be the type that uses an isolating transformer, so that the dc and ac wiring circuits are completely separate. The expected lifespan of a string inverter is about 10-12 years. The cost of replacing one is many hundreds of pounds, possibly as much as 1,000. Schematic PV system (Source: Photovoltaics in Buildings see Further Info.) PV SYSTEMS 4 OCTOBER 2016.
Micro inverters Instead of using a single string inverter, another approach is to use several micro inverters possibly a micro inverter for every panel. (Indeed, panel and micro inverter can be supplied as a single unit.) As mentioned last month, shading of panels usually results in a disproportionately large reduction of output from an array. However, if each panel has its own micro inverter, though the output from a shaded panel will be reduced, the output from the other panels will be unaffected. The expected life of micro inverters is about double that of an equivalent, stand-alone string inverter. But renewing micro-inverters would involve roof work and maybe scaffolding. PV systems with micro-inverters are more costly. A 4kWp system with a string inverter might cost, say 6,000, but with micro-inverters 7,000. If the array will be subject to appreciable shading, then the better output with micro inverters may make this the better option. Wiring Beware. A typical PV panel can generate about 30 volts dc ( 'direct current'). Just four such panels in series could generate 120V dc enough to give a person a severe shock, though not directly lethal. (Direct current is more dangerous than alternating current of a comparable voltage.) Most systems will generate a high voltage in open circuit, and require double insulation for the wiring, connectors, etc. (Special PV cable is available. Colours brown for positive, grey for negative.) It is not possible to stop the panels generating a voltage only to wait for darkness! (It is good practice to label the cabling with a notice such as 'High voltage dc live during daylight'.) There should be a dc switch by which the PV array can be isolated. (A direct current switch needs to be more robust than a comparable alternating current switch.) The switch may be integrated into the inverter, or adjacent to it. (See diagram above.) Similarly, there should be a switch to isolate the inverter from the mains supply. Connection to the grid The grid connection of a PV system of up to 16 amps should be in accordance with Engineering Recommendation G83. (The output of a 4kWp system is around this limit, and whether or not it is below 16 amps depends on the specification of the system's inverter as noted earlier, the inverter might be deliberately undersized.) Larger installations, over 3.68 kwp, need to be in accord with G59 (3,680 = 16 x 230.) Getting G59 approval from your local DNO will be slow, taking several weeks. (DNO: District Network Operator.) Your PV installer will deal with the paperwork. Your string inverter should be certificated by the manufacturer to comply with G83 or G59. MCS approval To qualify for Feed-In Tariff payments, your system must be approved by the Microgeneration Certification Scheme: MCS approved products must be used, and the system commissioned by an MCS approved installer. Next month: PV roofs for new build. PV SYSTEMS 5 OCTOBER 2016.
FURTHER INFO: Choosing Solar Electricity A guide to photovoltaic systems By Brain Goss. Published 2010 by the Centre for Alternative Electricity. 184 pages. 14. Code of Practice for Grid Connected Solar Photovoltaic Systems By Cotterell and Goss. Published 2015 by the Institution of Engineering and Technology. 208 pages. Price 69. Ensure that your installer will install your PV system in compliance with this Code. The SolarBlogger An insider's comments about the PV and solar heat technologies. www.solarblogger.net. Energy Saving Trust Their website has a useful section for consumers about solar panels. Free advice centre: tel 0300 123 1234. www.energysavingtrust.org.uk. Microgeneration Certification Scheme To qualify for FIT payments, both the installer and the products must be MCS certified. The MCS website for consumers lists about 2,000 PV installers. (Enter your postcode to find local installers.) It also lists products there are, for example, 10,000 entries for polycrystalline panels! www.microgenerationcertification.org. Words: 2078 words. Copyright article by Robert Matthews in SelfBuild & Design magazine, October, 2016. PV SYSTEMS 6 OCTOBER 2016.