SOLAR INVERTER

Internal view of a solar inverter. Note the many large capacitors (blue cylinders), used to store power briefly and improve the output waveform.Asolar inverter, orPV inverter, converts the variabledirect current(DC) output of aphotovoltaic(PV)solar panelinto autility frequencyalternating current(AC) that can be fed into a commercial electricalgridor used by a local,off-gridelectrical network. It is a critical component in aphotovoltaic system, allowing the use of ordinary commercial appliances. Solar inverters have special functions adapted for use with photovoltaic arrays, includingmaximum power point trackingand anti-islandingprotection.Classification

Simplified schematics of a grid-connected residentialphotovoltaicpower systemSolar inverters may be classified into three broad types: Stand-alone inverters, used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays. Many stand-alone inverters also incorporate integralbattery chargersto replenish the battery from anACsource, when available. Normally these do not interface in any way with the utility grid, and as such, are not required to haveanti-islanding protection. Grid-tie inverters, which matchphasewith a utility-suppliedsine wave. Grid-tie inverters are designed to shut down automatically upon loss of utility supply, for safety reasons. They do not provide backup power during utility outages. Battery backup inverters, are special inverters which are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection.Maximum power point trackingSolar inverters usemaximum power point tracking(MPPT) to get the maximum possible power from the PV array.[2]Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a non-linear output efficiency known as theI-V curve. It is the purpose of the MPPT system to sample the output of the cells and determine a resistance (load) to obtain maximum power for any given environmental conditions. Thefill factor, more commonly known by its abbreviationFF, is a parameter which, in conjunction with the open circuit voltage and short circuit current of the panel, determines the maximum power from a solar cell. Fill factor is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. There are three main types ofMPPT algorithms: perturb-and-observe, incremental conductance and constant voltage.The first two methods are often referred to ashill climbingmethods; they rely on the curve of power plotted against voltage rising to the left of the maximum power point, and falling on the right. Anti-islanding protection

In the event of a power failure on the grid, it is generally required that anygrid-tie invertersattached to the grid turn off in a short period of time. This prevents the inverters from continuing to feed power into small sections of the grid, known as "islands". Powered islands present a risk to workers who may expect the area to be unpowered, but equally important is the issue that without a grid signal to synchronize to, the power output of the inverters may drift from the tolerances required by customer equipment connected within the island.Detecting the presence or lack of a grid source would appear to be simple, and in the case of a single inverter in any given possible physical island (between disconnects on the distribution lines for instance) the chance that an inverter would fail to notice the loss of the grid is effectively zero. However, if there are two inverters in a given island, things become considerably more complex. It is possible that the signal from one can be interpreted as a grid feed from the other, and vice versa, so both units continue operation. As they track each other's output, the two can drift away from the limits imposed by the grid connections, say in voltage or frequency.There are a wide variety of methodologies used to detect an islanding condition. None of these are considered fool-proof, and utility companies continue to impose limits on the number and total power of solar power systems connected in any given area. However, many in-field tests have failed to uncover any real-world islanding issues, and the issue remains contentious within the industry.Since 1999, the standard for anti-islanding protection in theUnited Stateshas been UL 1741, harmonized withIEEE 1547.Any inverter which is listed to the UL 1741 standard may be connected to a utility grid without the need for additional anti-islanding equipment, anywhere in the United States or other countries whereULstandards are accepted. Similar acceptance of theIEEE 1547inEuropeis also taking place, as most electrical utilities will be providing or requiring units with this capability. Solar micro-inverters

A solar micro-inverter in the process of being installed. The ground wire is attached to the lug and the panel's DC connections are attached to the cables on the lower right. The AC parallel trunk cable runs at the top (just visible).Main article:Solar micro-inverterSolar micro-inverter is an inverter integrated to each solar panel module. The inverter converts the output from each panel toalternating currentThey're designed to allow parallel connection of multiple units connected in parallel. Each integrated module provides AC output and are connected together in parallel. This arrangement provides easier installation, redundancy and more effective capture of energy when they're partially shaded. As of 2010, they're mainly used for single phase applications and most units in production relied exclusively on electrolytic capacitors for buffering and there is a concern of long term reliability of these capacitors in each module.[11]A 2011 study at Appalachian State University reports that individual integrated inverter setup yielded about 20% more power in unshaded conditions and 27% more power in shaded conditions compared to string connected setup using one inverter. Both setups used identical solar panels. Grid tied solar inverters

An industrial grid-tied solar inverter

A PV inverter installed in a porchSolar grid-tie inverters are designed to quickly disconnect from the grid if theutility gridgoes down. This is anNECrequirement that ensures that in the event of a blackout, the grid tie inverter will shut down to prevent the energy it produces from harming any line workers who are sent to fix thepower grid.Grid-tie invertersthat are available on the market today use a number of different technologies. The inverters may use the newer high-frequencytransformers, conventional low-frequencytransformers, or no transformer. Instead of converting direct current directly to 120 or 240volts AC, high-frequency transformers employ a computerized multi-step process that involves converting the power to high-frequency AC and then back to DC and then to the final AC output voltage.[13]Historically, there has been concerns about having transformerless electrical systems feed into the public utility grid. The concerns stem from the fact that there is a lack ofgalvanic isolationbetween the DC and AC circuits, which could allow the passage of dangerous DC faults to be transmitted to the AC side.[14]Since 2005, the NFPA's NEC allows transformerless (or non-galvanically) inverters. The VDE 0126-1-1 and IEC 6210 also have been amended to allow and define the safety mechanisms needed for such systems. Primarily, residual or ground current detection is used to detect possible fault conditions. Also isolation tests are performed to insure DC to AC separation.Many solar inverters are designed to be connected to a utility grid, and will not operate when they do not detect the presence of the grid. They contain special circuitry to precisely match the voltage andfrequencyof the grid. See theAnti-Islanding sectionfor more details.Solar charge controller

A typical solar charge controller kitA charge controller may be used to power DC equipment with solar panels. The charge controller provides a regulated DC output and stores excess energy in a battery as well as monitoring the battery voltage to prevent under/over charging. More expensive units will also perform maximum power point tracking. An inverter can be connected to the output of a charge controller to drive AC loads.Solar pumping invertersAdvanced solar pumping inverters convert DC voltage from the solar array into AC voltage to drivesubmersible pumpsdirectly without the need for batteries or other energy storage devices. By utilizing MPPT (maximum power point tracking), solar pumping inverters regulate output frequency to control speed of the pumps in order to save pump motor from damage.Inverter failureSolar inverters may fail due totransientsfrom the grid or the PV panel, component aging and operation beyond the designed limits. Following are some common reasons specific components of inverters age quickly or fail:Capacitor failure Electrolytic materialsage faster thanpolycarbonateand other drydielectricmaterials Voltage stress Continuous operation under maximum voltage conditions Frequent short-term voltagetransients Current stress High current increases the internal temperature Thermal stress on component terminals Improper Charge and discharge rates Not operating inambient temperatures Mechanical stress VibrationsInverter bridge failure Usage beyond its rated operating limit Overcurrentandovervoltage Other malfunctioning components Thermal shock Thermal overload Extremely coldoperating temperatureElectro-mechanical wear Component stress Contamination at contacts Extreme temperature conditions Ultrasonic vibration originating in (magnetic cores of) inductive componentsSolar panel

A solar array composed of a solar panel with 24 solar modules in rural Mongolia

A solar photovoltaic module is composed of individual PV cells. This crystalline-silicon module comprises 4 solar cells and has analuminiumframe and glass on the front.

Solar modules on theInternational Space Station

A half-built homemade solar module, made from individual cells soldered togetherAsolar panelis a set of solar photovoltaicmoduleselectrically connected and mounted on a supporting structure. A photovoltaic module is a packaged, connected assembly ofsolar cells. The solar panel can be used as a component of a larger photovoltaic system to generate and supplyelectricityin commercial and residential applications. Each module is rated by itsDCoutput power under standard test conditions (STC), and typically ranges from 100 to 320 watts. Theefficiencyof a module determines the area of a module given the same rated output - an 8% efficient 230 watt module will have twice the area of a 16% efficient 230 watt module. A single solar module can produce only a limited amount of power; most installations contain multiple modules. Aphotovoltaic systemtypically includes a panel or an array of solar modules, aninverter, and sometimes abatteryand/orsolar trackerand interconnection wiring.Theory and construction

PolycrystallinePV cellsconnected in a solar module.Solar modules use light energy (photons) from the sun to generate electricity through thephotovoltaic effect. The majority of modules usewafer-basedcrystalline siliconcells orthin-film cellsbased oncadmium tellurideorsilicon. The structural (load carrying) member of a module can either be the top layer or the back layer. Cells must also be protected from mechanical damage and moisture. Most solar modules are rigid, but semi-flexible ones are available, based on thin-film cells. These early solar modules were first used in space in 1958.Electrical connections are madein seriesto achieve a desired output voltage and/orin parallelto provide a desired current capability. The conducting wires that take the current off the modules may contain silver, copper or other non-magnetic conductivetransition metals. The cells must be connected electrically to one another and to the rest of the system. Externally, popular terrestrial usage photovoltaic modules useMC3(older) orMC4 connectorsto facilitate easy weatherproof connections to the rest of the system.Bypassdiodesmay be incorporated or used externally, in case of partial module shading, to maximize the output of module sections still illuminated.Some recent solar module designs includeconcentratorsin which light is focused bylensesor mirrors onto an array of smaller cells. This enables the use of cells with a high cost per unit area (such asgallium arsenide) in a cost-effective way.EfficienciesDepending on construction, photovoltaic modules can produce electricity from a range offrequencies of light, but usually cannot cover the entire solar range (specifically,ultraviolet,infraredand low or diffused light). Hence much of the incidentsunlightenergy is wasted by solar modules, and they can give far higher efficiencies if illuminated withmonochromaticlight. Therefore, another design concept is to split the light into different wavelength ranges and direct the beams onto different cells tuned to those ranges.[citation needed]This has been projected to be capable of raising efficiency by 50%.Currently the best achieved sunlight conversion rate (solar module efficiency) is around 21.5% in new commercial products. typically lower than the efficiencies of their cells in isolation. The most efficient mass-produced solar modules have energy density values of up to 175W/m2(16.22W/ft2). A research byImperial College, Londonhas shown that the efficiency of a solar panel can be improved by studding the light-receiving semiconductor surface withaluminumnanocylinders similar to theridgesonLego blocks. Thescatteredlightthen travels along a longer path in the semiconductor which meant that more photons could be absorbed and converted into current. Although these nanocylinders were used previously in which aluminum was preceded bygoldandsilver, the light scattering occurred in the near infrared region and visible light was absorbed strongly. Aluminum was found to have absorbed ultraviolet part of the spectrum and the visible and near infrared parts of the spectrum were found to be scattered by the aluminum surface. This, the research argued, could bring down the cost significantly and improve the efficiency as aluminum is more abundant and less costly than gold and silver. The research also noted that the increase in current makes thinner film solar panels technically feasible without "compromising power conversion efficiencies, thus reducing material consumption".[3]Micro-inverted solar panels are wired inparallelwhich produces more output than normal panels which are wired inserieswith the output of the series determined by the lowest performing panel (this is known as the "Christmas light effect"). Micro-inverters work independently so each panel contributes its maximum possible output given the available sunlight.[citation needed]Crystalline silicon modules

Most solar modules are currently produced fromsiliconphotovoltaic cells. These are typically categorized asmonocrystallineorpolycrystallinemodules.Thin-film modulesThird generation solar cells are advanced thin-film cells. They produce a relatively high-efficiency conversion for the low cost compared to other solar technologies.Rigid thin-film modules[edit]Inrigid thin film modules, the cell and the module are manufactured in the same production line.The cell is created on a glasssubstrateorsuperstrate, and the electrical connections are createdin situ, a so-called "monolithic integration". The substrate or superstrate is laminated with an encapsulant to a front or backsheet, usually another sheet of glass.The main cell technologies in this category areCdTe, ora-Si, ora-Si+uc-Si tandem, orCIGS(or variant).Amorphous siliconhas a sunlight conversion rate of 6-12%.Flexible thin-film modulesFlexible thin filmcells and modules are created on the same production line by depositing thephotoactive layerand other necessary layers on aflexible substrate.If the substrate is aninsulator(e.g.polyesterorpolyimidefilm) thenmonolithicintegration can be used.If it is a conductor then another technique for electrical connection must be used.The cells are assembled into modules bylaminatingthem to a transparent colourlessfluoropolymeron the front side (typicallyETFEorFEP) and a polymer suitable for bonding to the final substrate on the other side. The only commercially available (in MW quantities) flexible module usesamorphous silicontriple junction(fromUnisolar).So-calledinverted metamorphic(IMM)multijunction solar cellsmade oncompound-semiconductor technologyare just becoming commercialized in July 2008. TheUniversity of Michigan'ssolar carthat won theNorth American Solar Challengein July 2008 used IMM thin-film flexible solar cells.The requirements for residential and commercial are different in that the residential needs are simple and can be packaged so that as solar cell technology progresses, the other base line equipment such as the battery, inverter and voltage sensing transfer switch still need to be compacted and unitized for residential use. Commercial use, depending on the size of the service will be limited in the photovoltaic cell arena, and more complex parabolic reflectors andsolar concentratorsare becoming the dominant technology. Flexible thin-film panels are optimal for portable applications as they are much more resistant to breakage than regular crystalline cells, but can be broken by bending them into a sharp angle. They are also much lighter per square foot than standard rigid solar panels.The global flexible and thin-film photovoltaic (PV) market, despite caution in the overall PV industry, is expected to experience aCAGRof over 35% to 2019, surpassing 32 GW according to a major new study by IntertechPira. Smart solar modulesSeveral companies have begun embedding electronics into PV modules. This enables performingmaximum power point tracking(MPPT) for each module individually, and the measurement of performance data for monitoring and fault detection at module level. Some of these solutions make use ofpower optimizers, a DC-to-DC converter technology developed to maximize the power harvest from solar photovoltaic systems. As of about 2010, such electronics can also compensate for shading effects, wherein a shadow falling across a section of a module causes the electrical output of one or more strings of cells in the module to fall to zero, but not having the output of the entire module fall to zero.Module performance and agingModule performance is generally rated under standard test conditions (STC):irradianceof 1,000W/m, solarspectrumofAM1.5 and module temperature at 25C.Electrical characteristics include nominal power (PMAX, measured inW),open circuit voltage(VOC),short circuit current(ISC, measured inamperes),maximum power voltage(VMPP),maximum power current(IMPP), peak power, Wp, and module efficiency (%).Nominal voltage refers to the voltage of the battery that the module is best suited to charge; this is a leftover term from the days when solar modules were only used to charge batteries. The actual voltage output of the module changes as lighting, temperature and load conditions change, so there is never one specific voltage at which the module operates. Nominal voltage allows users, at a glance, to make sure the module is compatible with a given system.Open circuit voltage or VOCis the maximum voltage that the module can produce when not connected to an electrical circuit or system. VOCcan be measured with a meter directly on an illuminated module's terminals or on its disconnected cable.The peak power rating, Wp, is the maximum output under standard test conditions (not the maximum possible output). Typical modules, which could measure approximately 1x2 meters or 2x4 feet, will be rated from as low as 75 watts to as high as 350 watts, depending on their efficiency. At the time of testing, the test modules are binned according to their test results, and a typical manufacturer might rate their modules in 5 watt increments, and either rate them at +/- 3%, +/-5%, +3/-0% or +5/-0%. Solar modules must withstand rain,hail, and cycles of heat and cold for many years. Manycrystalline siliconmodule manufacturers offer awarrantythat guarantees electrical production for 10 years at 90% of rated power output and 25 years at 80%.RecyclingMost parts of a solar module can be recycled including up to 97% of certain semiconductor materials or the glass as well as large amounts of ferrous and non-ferrous metals.[10]Some private companies and non-profit organizations are currently engaged in take-back and recycling operations for end-of-life modules.[11]Recycling possibilities depend on the kind of technology used in the modules: Silicon based modules: aluminium frames and junction boxes are dismantled manually at the beginning of the process. The module is then crushed in a mill and the different fractions are separated - glass, plastics and metals.It is possible to recover more than 80% of the incoming weight. This process can be performed by flat glass recyclers since morphology and composition of a PV module is similar to those flat glasses used in the building and automotive industry. The recovered glass for example is readily accepted by the glass foam and glass insulation industry. Non-silicon based modules: they require specific recycling technologies such as the use of chemical baths in order to separate the different semiconductor materials. Forcadmium telluridemodules, the recycling process begins by crushing the module and subsequently separating the different fractions. This recycling process is designed to recover up to 90% of the glass and 95% of the semiconductor materials contained. Some commercial-scale recycling facilities have been created in recent years by private companies Since 2010, there is an annual European conference bringing together manufacturers, recyclers and researchers to look at the future of PV module recycling. Production

The "solar tree", a symbol ofGleisdorf,AustriaIn 2010, 15.9GWof solar PV system installations were completed, with solar PV pricing survey and market research company PVinsights reporting growth of 117.8% in solar PV installation on a year-on-year basis. With over 100% year-on-year growth in PV system installation, PV module makers dramatically increased their shipments of solar modules in 2010. They actively expanded their capacity and turned themselves into gigawattGWplayers. According to PVinsights, five of the top ten PV module companies in 2010 are GW players. Suntech, First Solar, Sharp, Yingli and Trina Solar are GW producers now, and most of them doubled their shipments in 2010.[19]Top ten producersThe top ten solar module producers (by MW shipments) in 2010 were:[19]1. Suntech2. First Solar3. Sharp Solar4. Yingli5. Trina Solar6. Canadian Solar7. Hanwha Solarone8. SunPower9. Renewable Energy Corporation10. SolarWorldPriceAverage pricing information divides in three pricing categories: those buying small quantities (modules of all sizes in the kilowatt range annually), mid-range buyers (typically up to 10MWpannually), and large quantity buyers (self-explanatoryand with access to the lowest prices). Over the long term there is clearly a systematic reduction in the price of cells and modules. For example in 2012 it was estimated that the quantity cost per watt was about $0.60, which was 250 times lower than the cost in 1970 of $150.[20][21]Real world prices depend a great deal on local weather conditions. In a cloudy country such as the United Kingdom, price per installed kW is higher than in sunnier countries like Spain.Following toRMI,Balance-of-System(BoS) elements, this is, non-module cost of non-microinvertersolar modules (as wiring, converters, racking systems and various components) make up about half of the total costs of installations.For merchant solar power stations, where the electricity is being sold into the electricity transmission network, the cost ofsolar energywill need to match the wholesale electricity price. This point is sometimes called 'wholesale grid parity' or 'busbar parity'.[22]Some photovoltaic systems, such as rooftop installations, can supply power directly to an electricity user. In these cases, the installation can be competitive when the output cost matches the price at which the user pays for his electricity consumption. This situation is sometimes called 'retail grid parity', 'socket parity' or 'dynamic grid parity'.Research carried out byUN-Energyin 2012 suggests areas of sunny countries with high electricity prices, such as Italy, Spain and Australia, and areas using diesel generators, have reached retail grid parity.

Mounting systemsTrackers

Solar modules mounted on solar trackersSolar trackersincrease the amount of energy produced per module at a cost of mechanical complexity and need for maintenance. They sense the direction of the Sun and tilt the modules as needed for maximum exposure to the light.Fixed racksFixed racks hold modules stationary as the sun moves across the sky. The fixed rack sets the angle at which the module is held. Tilt angles equivalent to an installation's latitude are common. Most of these fixed racks are set on poles above ground.Ground mountedGround mountedsolar powersystems consist of solar modules held in place by racks or frames that are attached to ground based mounting supports.Ground based mounting supports include: Pole mounts, which are driven directly into the ground or embedded in concrete. Foundation mounts, such as concrete slabs or poured footings Ballasted footing mounts, such as concrete or steel bases that use weight to secure the solar module system in position and do not require ground penetration. This type of mounting system is well suited for sites where excavation is not possible such as capped landfills and simplifies decommissioning or relocation of solar module systems.Roof mountingRoof-mountedsolar powersystems consist of solar modules held in place by racks or frames attached to roof-based mounting supports.Roof-based mounting supports include: Pole mounts, which are attached directly to the roof structure and may use additional rails for attaching the module racking or frames. Ballasted footing mounts, such as concrete or steel bases that use weight to secure the panel system in position and do not require through penetration. This mounting method allows for decommissioning or relocation of solar panel systems with no adverse effect on the roof structure. All wiring connecting adjacent solar modules to the energy harvesting equipment must be installed according to local electrical codes and should be run in a conduit appropriate for the climate conditions.