Solar panel

Assembly of photovoltaic cells used to generate electricity For solar thermal panels, see solar thermal collector and solar thermal energy.

A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct current (DC) electricity, which can be used to power various devices or be stored in batteries. Solar panels are also known as solar cell panels, solar electric panels, or PV modules.

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Solar panels are usually arranged in groups called arrays or systems. A photovoltaic system consists of one or more solar panels, an inverter that converts DC electricity to alternating current (AC) electricity, and sometimes other components such as controllers, meters, and trackers. Most panels are in solar farms or rooftop solar panels which supply the electricity grid.

Some advantages of solar panels are that they use a renewable and clean source of energy, reduce greenhouse gas emissions, and lower electricity bills. Some disadvantages are that they depend on the availability and intensity of sunlight, require cleaning, and have high initial costs. Solar panels are widely used for residential, commercial, and industrial purposes, as well as in space, often together with batteries.

History

[edit]

In , the ability of some materials to create an electrical charge from light exposure was first observed by the French physicist Edmond Becquerel.[1] Though these initial solar panels were too inefficient for even simple electric devices, they were used as an instrument to measure light.[2]

The observation by Becquerel was not replicated again until , when the English electrical engineer Willoughby Smith discovered that the charge could be caused by light hitting selenium. After this discovery, William Grylls Adams and Richard Evans Day published "The action of light on selenium" in , describing the experiment they used to replicate Smith's results.[1][3]

In , the American inventor Charles Fritts created the first commercial solar panel, which was reported by Fritts as "continuous, constant and of considerable force not only by exposure to sunlight but also to dim, diffused daylight".[4] However, these solar panels were very inefficient, especially compared to coal-fired power plants.

In , Russell Ohl created the solar cell design that is used in many modern solar panels. He patented his design in .[5] In , this design was first used by Bell Labs to create the first commercially viable silicon solar cell.[1]

Solar panel installers saw significant growth between and .[6] Due to that growth many installers had projects that were not "ideal" solar roof tops to work with and had to find solutions to shaded roofs and orientation difficulties.[7] This challenge was initially addressed by the re-popularization of micro-inverters and later the invention of power optimizers.

Solar panel manufacturers partnered with micro-inverter companies to create AC modules and power optimizer companies partnered with module manufacturers to create smart modules.[8] In many solar panel manufacturers announced and began shipping their smart module solutions.[9]

Theory and construction

[edit] See also: Solar cell

Photovoltaic modules consist of a large number of solar cells and use light energy (photons) from the Sun to generate electricity through the photovoltaic effect. Most modules use wafer-based crystalline silicon cells or thin-film cells. The structural (load carrying) member of a module can be either the top layer or the back layer. Cells must be protected from mechanical damage and moisture. Most modules are rigid, but semi-flexible ones based on thin-film cells are also available. The cells are usually connected electrically in series, one to another to the desired voltage, and then in parallel to increase current. The power (in watts) of the module is the voltage (in volts) multiplied by the current (in amperes), and depends both on the amount of light and on the electrical load connected to the module. The manufacturing specifications on solar panels are obtained under standard conditions, which are usually not the true operating conditions the solar panels are exposed to on the installation site.[10]

A PV junction box is attached to the back of the solar panel and functions as its output interface. External connections for most photovoltaic modules use MC4 connectors to facilitate easy weatherproof connections to the rest of the system. A USB power interface can also be used.[11] Solar panels also use metal frames consisting of racking components, brackets, reflector shapes, and troughs to better support the panel structure.[citation needed]

Cell connection techniques

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Solar modular cells need to be connected together to form the module, with front electrodes blocking the solar cell front optical surface area slightly. To maximize frontal surface area available for sunlight and improve solar cell efficiency, manufacturers use varying rear electrode solar cell connection techniques:

• Passivated emitter rear contact (PERC) adds a polymer film to capture light
• Tunnel oxide passivated contact (TOPCon) adds an oxidation layer to the PERC film to capture more light[12]
• Interdigitated back contact (IBC)[13]

Arrays of PV modules

[edit]

A single solar module can produce only a limited amount of power; most installations contain multiple modules adding their voltages or currents. A photovoltaic system typically includes an array of photovoltaic modules, an inverter, a battery pack for energy storage, a charge controller, interconnection wiring, circuit breakers, fuses, disconnect switches, voltage meters, and optionally a solar tracking mechanism. Equipment is carefully selected to optimize energy output and storage, reduce power transmission losses, and convert from direct current to alternating current.

Smart solar modules

[edit]

Smart modules are different from traditional solar panels because the power electronics embedded in the module offers enhanced functionality such as panel-level maximum power point tracking, monitoring, and enhanced safety.[citation needed] Power electronics attached to the frame of a solar module, or connected to the photovoltaic circuit through a connector, are not properly considered smart modules.[14]

Several companies have begun incorporating into each PV module various embedded power electronics such as:

• Maximum power point tracking (MPPT) power optimizers, a DC-to-DC converter technology developed to maximize the power harvest from solar photovoltaic systems by compensating for shading effects, wherein a shadow falling on a section of a module causes the electrical output of one or more strings of cells in the module to fall to near zero, but not having the output of the entire module fall to zero.[15]
• Solar performance monitors for data and fault detection

Technology

[edit] Main articles: Crystalline silicon and Thin-film solar cell

Most solar modules are currently produced from crystalline silicon (c-Si) solar cells made of polycrystalline or monocrystalline silicon. In , crystalline silicon accounted for 95% of worldwide PV production,[16][17] while the rest of the overall market is made up of thin-film technologies using cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (a-Si).[18]

Emerging, third-generation solar technologies use advanced thin-film cells. They produce a relatively high-efficiency conversion for a lower cost compared with other solar technologies. Also, high-cost, high-efficiency, and close-packed rectangular multi-junction (MJ) cells are usually used in solar panels on spacecraft, as they offer the highest ratio of generated power per kilogram lifted into space. MJ-cells are compound semiconductors and made of gallium arsenide (GaAs) and other semiconductor materials. Another emerging PV technology using MJ-cells is concentrator photovoltaics (CPV).

Thin film

[edit]

Mounting and tracking

[edit] Main articles: Photovoltaic mounting system and Solar tracker

Ground

[edit]

Large utility-scale solar power plants frequently use ground-mounted photovoltaic systems. Their solar modules are held in place by racks or frames that are attached to ground-based mounting supports.[22][23] 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.

Vertical bifacial solar array

[edit]

Vertical bifacial solar cells are oriented towards east and west to catch the sun's irradiance more efficiently in the morning and evening. Applications include agrivoltaics, solar fencing, highway and railroad noise dampeners and barricades.[24]

Roof

[edit] Main article: Rooftop solar power

Roof-mounted solar power systems consist of solar modules held in place by racks or frames attached to roof-based mounting supports.[25] Roof-based mounting supports include:

• Rail 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

Solar Canopy

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Solar canopies are solar arrays which are installed on top of a traditional canopy. These canopies could be a parking lot canopy, carport, gazebo, Pergola, or patio cover.

There are many benefits, which include maximizing the space available in urban areas while also providing shade for cars. The energy produced can be used to create electric vehicle (EV) charging stations.[26]

Portable

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Portable solar panels can ensure electric current, enough to charge devices (mobile, radio, ...) via USB-port or to charge a powerbank f.e.

Special features of the panels include high flexibility, high durability & waterproof characteristics. They are good for travel or camping.

Tracking

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Solar trackers increase the energy produced per module at the cost of mechanical complexity and increased need for maintenance. They sense the direction of the Sun and tilt or rotate the modules as needed for maximum exposure to the light.[27][28]

Alternatively, fixed racks can hold modules stationary throughout the day at a given tilt (zenith angle) and facing a given direction (azimuth angle). Tilt angles equivalent to an installation's latitude are common. Some systems may also adjust the tilt angle based on the time of year.[29]

On the other hand, east- and west-facing arrays (covering an east'west facing roof, for example) are commonly deployed. Even though such installations will not produce the maximum possible average power from the individual solar panels, the cost of the panels is now usually cheaper than the tracking mechanism and they can provide more economically valuable power during morning and evening peak demands than north or south facing systems.[30]

Concentrator

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Some special solar PV modules include concentrators in which light is focused by lenses or mirrors onto smaller cells. This enables the cost-effective use of highly efficient, but expensive cells (such as gallium arsenide) with the trade-off of using a higher solar exposure area.[citation needed] Concentrating the sunlight can also raise the efficiency to around 45%.[31]

Light capture

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The amount of light absorbed by a solar cell depends on the angle of incidence of whatever direct sunlight hits it. This is partly because the amount falling on the panel is proportional to the cosine of the angle of incidence, and partly because at high angle of incidence more light is reflected. To maximize total energy output, modules are often oriented to face south (in the Northern Hemisphere) or north (in the Southern Hemisphere) and tilted to allow for the latitude. Solar tracking can be used to keep the angle of incidence small.

Solar panels are often coated with an anti-reflective coating, which is one or more thin layers of substances with refractive indices intermediate between that of silicon and that of air. This causes destructive interference in the reflected light, diminishing the amount. Photovoltaic manufacturers have been working to decrease reflectance with improved anti-reflective coatings or with textured glass.[32][33]

Power curve

[edit] Main article: Solar inverter

In general with individual solar panels, if not enough current is taken, then power isn't maximised. If too much current is taken then the voltage collapses. The optimum current draw is roughly proportional to the amount of sunlight striking the panel. Solar panel capacity is specified by the MPP (maximum power point) value of solar panels in full sunlight.

Inverters

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Solar inverters convert the DC power provided by panels to AC power.

MPP (Maximum power point) of the solar panel consists of MPP voltage (Vmpp) and MPP current (Impp). Performing maximum power point tracking (MPPT), a solar inverter samples the output (I-V curve) from the solar cell and applies the proper electrical load to obtain maximum power.

An AC (alternating current) solar panel has a small DC to AC microinverter on the back and produces AC power with no external DC connector. AC modules are defined by Underwriters Laboratories as the smallest and most complete system for harvesting solar energy.[34][need quotation to verify]

Micro-inverters work independently to enable each panel to contribute its maximum possible output for a given amount of sunlight, but can be more expensive.[35]

Module interconnection

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Module electrical connections are made with conducting wires that take the current off the modules and are sized according to the current rating and fault conditions, and sometimes include in-line fuses.

Panels are typically connected in series of one or more panels to form strings to achieve a desired output voltage, and strings can be connected in parallel to provide the desired current capability (amperes) of the PV system.

In string connections the voltages of the modules add, but the current is determined by the lowest performing panel. This is known as the "Christmas light effect". In parallel connections the voltages will be the same, but the currents add. Arrays are connected up to meet the voltage requirements of the inverters and to not greatly exceed the current limits.

Blocking and bypass diodes may be incorporated within the module or used externally to deal with partial array shading, in order to maximize output. For series connections, bypass diodes are placed in parallel with modules to allow current to bypass shaded modules which would otherwise severely limit the current. For paralleled connections, a blocking diode may be placed in series with each module's string to prevent current flowing backwards through shaded strings thus short-circuiting other strings.

Connectors

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Outdoor solar panels usually include MC4 connectors, automotive solar panels may include an auxiliary power outlet and/or USB adapter and indoor panels may have a microinverter.

Efficiency

[edit] See also: Solar cell efficiency

Each module is rated by its DC output power under standard test conditions (STC) and hence the on field output power might vary. Power typically ranges from 100 to 365 Watts (W). The efficiency of a module determines the area of a module given the same rated output ' an 8% efficient 230 W module will have twice the area of a 16% efficient 230 W module. Some commercially available solar modules exceed 24% efficiency.[37][38] Currently,[needs update] the best achieved sunlight conversion rate (solar module efficiency) is around 21.5% in new commercial products[39] typically lower than the efficiencies of their cells in isolation. The most efficient mass-produced solar modules have power density values of up to 175 W/m2 (16.22 W/ft2).[40]

The current versus voltage curve of a module provides useful information about its electrical performance.[41] Manufacturing processes often cause differences in the electrical parameters of different modules photovoltaic, even in cells of the same type. Therefore, only the experimental measurement of the I'V curve allows us to accurately establish the electrical parameters of a photovoltaic device. This measurement provides highly relevant information for the design, installation and maintenance of photovoltaic systems. Generally, the electrical parameters of photovoltaic modules are measured by indoor tests. However, outdoor testing has important advantages such as no expensive artificial light source required, no sample size limitation, and more homogeneous sample illumination.

Capacity factor of solar panels is limited primarily by geographic latitude and varies significantly depending on cloud cover, dust, day length and other factors. In the United Kingdom, seasonal capacity factor ranges from 2% (December) to 20% (July), with average annual capacity factor of 10'11%, while in Spain the value reaches 18%.[42] Globally, capacity factor for utility-scale PV farms was 16.1% in .[43][unreliable source?]

Overheating is the most important factor for the efficiency of the solar panel.[44]

Radiation-dependent efficiency

[edit]

Depending on construction, photovoltaic modules can produce electricity from a range of frequencies of light, but usually cannot cover the entire solar radiation range (specifically, ultraviolet, infrared and low or diffused light). Hence, much of the incident sunlight energy is wasted by solar modules, and they can give far higher efficiencies if illuminated with monochromatic light. Therefore, another design concept is to split the light into six to eight different wavelength ranges that will produce a different color of light, and direct the beams onto different cells tuned to those ranges.[45]

Performance and degradation

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Module performance is generally rated under standard test conditions (STC): irradiance of 1,000 W/m2, solar spectrum of AM 1.5 and module temperature at 25 °C.[46] The actual voltage and current output of the module changes as lighting, temperature and load conditions change, so there is never one specific voltage at which the module operates. Performance varies depending on geographic location, time of day, the day of the year, amount of solar irradiance, direction and tilt of modules, cloud cover, shading, soiling, state of charge, and temperature. Performance of a module or panel can be measured at different time intervals with a DC clamp meter or shunt and logged, graphed, or charted with a chart recorder or data logger.

For optimum performance, a solar panel needs to be made of similar modules oriented in the same direction perpendicular to direct sunlight. Bypass diodes are used to circumvent broken or shaded panels and optimize output. These bypass diodes are usually placed along groups of solar cells to create a continuous flow.[47]

Electrical characteristics include nominal power (PMAX, measured in W), open-circuit voltage (VOC), short-circuit current (ISC, measured in amperes), maximum power voltage (VMPP), maximum power current (IMPP), peak power, (watt-peak, Wp), and module efficiency (%).

Open-circuit voltage or VOC is the maximum voltage the module can produce when not connected to an electrical circuit or system.[48] VOC can be measured with a voltmeter 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 1 by 2 metres (3 ft × 7 ft), will be rated from as low as 75 W to as high as 600 W, 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 W increments, and either rate them at +/- 3%, +/-5%, +3/-0% or +5/-0%.[49][50][51]

Influence of temperature

[edit]

The performance of a photovoltaic (PV) module depends on the environmental conditions, mainly on the global incident irradiance G in the plane of the module. However, the temperature T of the p'n junction also influences the main electrical parameters: the short circuit current ISC, the open circuit voltage VOC and the maximum power Pmax. In general, it is known that VOC shows a significant inverse correlation with T, while for ISC this correlation is direct, but weaker, so that this increase does not compensate for the decrease in VOC. As a consequence, Pmax decreases when T increases. This correlation between the power output of a solar cell and the working temperature of its junction depends on the semiconductor material, and is due to the influence of T on the concentration, lifetime, and mobility of the intrinsic carriers, i.e., electrons and gaps. inside the photovoltaic cell.

Temperature sensitivity is usually described by temperature coefficients, each of which expresses the derivative of the parameter to which it refers with respect to the junction temperature. The values of these parameters can be found in any data sheet of the photovoltaic module; are the following:

- β: VOC variation coefficient with respect to T, given by 'VOC/'T.

- α: Coefficient of variation of ISC with respect to T, given by 'ISC/'T.

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- δ: Coefficient of variation of Pmax with respect to T, given by 'Pmax/'T.

Techniques for estimating these coefficients from experimental data can be found in the literature[52]

Degradation

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The ability of solar modules to withstand damage by rain, hail, heavy snow load, and cycles of heat and cold varies by manufacturer, although most solar panels on the U.S. market are UL listed, meaning they have gone through testing to withstand hail.[53]

Potential-induced degradation (also called PID) is a potential-induced performance degradation in crystalline photovoltaic modules, caused by so-called stray currents.[54] This effect may cause power loss of up to 30%.[55]

Advancements in photovoltaic technologies have brought about the process of "doping" the silicon substrate to lower the activation energy thereby making the panel more efficient in converting photons to retrievable electrons.[56]

Chemicals such as boron (p-type) are applied into the semiconductor crystal in order to create donor and acceptor energy levels substantially closer to the valence and conductor bands.[57] In doing so, the addition of boron impurity allows the activation energy to decrease twenty-fold from 1.12 eV to 0.05 eV. Since the potential difference (EB) is so low, the boron is able to thermally ionize at room temperatures. This allows for free energy carriers in the conduction and valence bands thereby allowing greater conversion of photons to electrons.

The power output of a photovoltaic (PV) device decreases over time. This decrease is due to its exposure to solar radiation as well as other external conditions. The degradation index, which is defined as the annual percentage of output power loss, is a key factor in determining the long-term production of a photovoltaic plant. To estimate this degradation, the percentage of decrease associated with each of the electrical parameters. The individual degradation of a photovoltaic module can significantly influence the performance of a complete string. Furthermore, not all modules in the same installation decrease their performance at exactly the same rate. Given a set of modules exposed to long-term outdoor conditions, the individual degradation of the main electrical parameters and the increase in their dispersion must be considered. As each module tends to degrade differently, the behavior of the modules will be increasingly different over time, negatively affecting the overall performance of the plant.[citation needed]

There are several studies dealing with the power degradation analysis of modules based on different photovoltaic technologies available in the literature. According to a recent study,[58] the degradation of crystalline silicon modules is very regular, oscillating between 0.8% and 1.0% per year.

On the other hand, if we analyze the performance of thin-film photovoltaic modules, an initial period of strong degradation is observed (which can last several months and even up to 2 years), followed by a later stage in which the degradation stabilizes, being then comparable to that of crystalline silicon.[59] Strong seasonal variations are also observed in such thin-film technologies because the influence of the solar spectrum is much greater. For example, for modules of amorphous silicon, micromorphic silicon or cadmium telluride, we are talking about annual degradation rates for the first years of between 3% and 4%.[60] However, other technologies, such as CIGS, show much lower degradation rates, even in those early years.

Maintenance

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Solar panel conversion efficiency, typically in the 20% range, is reduced by the accumulation of dust, grime, pollen, and other particulates on the solar panels, collectively referred to as soiling. "A dirty solar panel can reduce its power capabilities by up to 30% in high dust/pollen or desert areas", says Seamus Curran, associate professor of physics at the University of Houston and director of the Institute for NanoEnergy, which specializes in the design, engineering, and assembly of nanostructures.[61] The average soiling loss in the world in is estimated to be at least 3% ' 4%.[62]

Paying to have solar panels cleaned is a good investment in many regions, as of .[62] However, in some regions, cleaning is not cost-effective. In California as of soiling-induced financial losses were rarely enough to warrant the cost of washing the panels. On average, panels in California lost a little less than 0.05% of their overall efficiency per day.[63]

There are also occupational hazards with solar panel installation and maintenance. A ' study in the UK investigated 80 PV-related incidents of fire, with over 20 "serious fires" directly caused by PV installation, including 37 domestic buildings and 6 solar farms. In 1'3 of the incidents a root cause was not established and in a majority of others was caused by poor installation, faulty product or design issues. The most frequent single element causing fires was the DC isolators.[64]

A study by kWh Analytics determined median annual degradation of PV systems at 1.09% for residential and 0.8% for non-residential ones, almost twice that previously assumed.[65] A module reliability study found an increasing trend in solar module failure rates with 30% of manufacturers experiencing safety failures related to junction boxes (growth from 20%) and 26% bill-of-materials failures (growth from 20%).[66]

Cleaning methods for solar panels can be divided into 5 groups: manual tools, mechanized tools (such as tractor mounted brushes), installed hydraulic systems (such as sprinklers), installed robotic systems, and deployable robots. Manual cleaning tools are by far the most prevalent method of cleaning, most likely because of the low purchase cost. However, in a Saudi Arabian study done in , it was found that "installed robotic systems, mechanized systems, and installed hydraulic systems are likely the three most promising technologies for use in cleaning solar panels".[67]

Waste and recycling

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There were 30 thousand tonnes of PV waste in , and the annual amount was estimated by Bloomberg NEF to rise to more than 1 million tons by and more than 10 million by .[68] For comparison, 750 million tons of fly ash waste was produced by coal power in .[69] In the United States, around 90% of decommissioned solar panels end up in landfills as of .[70] Most parts of a solar module can be recycled including up to 95% of certain semiconductor materials or the glass as well as large amounts of ferrous and non-ferrous metals.[71] Some private companies and non-profit organizations take-back and recycle end-of-life modules.[72] EU law requires manufacturers to ensure their solar panels are recycled properly. Similar legislation is underway in Japan, India, and Australia.[73] A Australian report said that there is a market for quality used panels and made recommendations for increasing reuse.[74]:'33'

Recycling possibilities depend on the kind of technology used in the modules:

• Silicon based modules: aluminum 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.[75] It is possible to recover more than 80% of the incoming weight.[76] This process can be performed by flat glass recyclers, since the shape and composition of a PV module is similar to flat glass 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.[77] For cadmium telluride modules, 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.[78] Some commercial-scale recycling facilities have been created in recent years by private companies.[79]

Since , there is an annual European conference bringing together manufacturers, recyclers and researchers to look at the future of PV module recycling.[80][81]

Production

[edit] See also: List of photovoltaics companies Top producers of PV systems, by shipped capacity in gigawatts Module producer Shipments
in
(GW)[82] Jinko Solar 14.2 JA Solar 10.3 Trina Solar 9.7 LONGi Solar 9.0 Canadian Solar 8.5 Hanwha Q Cells 7.3 Risen Energy 7.0 First Solar 5.5 GCL System 4.8 Shunfeng Photovoltaic 4.0

The production of PV systems has followed a classic learning curve effect, with significant cost reduction occurring alongside large rises in efficiency and production output.[83]

With over 100% year-on-year growth in PV system installation, PV module makers dramatically increased their shipments of solar modules in . They actively expanded their capacity and turned themselves into gigawatt GW players.[84] According to Pulse Solar, five of the top ten PV module companies in have experienced a rise in solar panel production by at least 25% compared to .[85]

The basis of producing most solar panels is mostly on the use of silicon cells. These silicon cells are typically 10'20% efficient[86] at converting sunlight into electricity, with newer production models exceeding 22%.[87]

In , the world's top five solar module producers in terms of shipped capacity during the calendar year of were Jinko Solar, JA Solar, Trina Solar, Longi solar, and Canadian Solar.[88]

Price

[edit] See also: Grid parity

The price of solar electrical power has continued to fall so that in many countries it has become cheaper than fossil fuel electricity from the electricity grid since , a phenomenon known as grid parity.[91] With the rise of global awareness, institutions such as the IRS have adopted a tax credit format, refunding a portion of any solar panel array for private use.[92] The price of a solar array only continues to fall.

Average 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 10 MWp annually), and large quantity buyers (self-explanatory'and 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 it was estimated that the quantity cost per watt was about US$0.60, which was 250 times lower than the cost in of US$150.[93][94] A study shows price/kWh dropping by 10% per year since , and predicts that solar could contribute 20% of total electricity consumption by , whereas the International Energy Agency predicts 16% by .[95]

Real-world energy production costs depend a great deal on local weather conditions. In a cloudy country such as the United Kingdom, the cost per produced kWh is higher than in sunnier countries like Spain.

Following to RMI, Balance-of-System (BoS) elements, this is, non-module cost of non-microinverter solar 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 of solar energy will need to match the wholesale electricity price. This point is sometimes called 'wholesale grid parity' or 'busbar parity'.[91]

Standards

[edit]

Standards generally used in photovoltaic modules:

• IEC (crystalline silicon performance), (thin film performance) and (all modules, safety), (Photovoltaic module performance testing & energy rating)
• ISO Solar energy'Vocabulary.
• UL from Underwriters Laboratories
• UL from Underwriters Laboratories
• UL from Underwriters Laboratories
• CE mark
• Electrical Safety Tester (EST) Series (EST-460, EST-22V, EST-22H, EST-110).

Applications

[edit] Main article: Applications of photovoltaics See also: List of solar-powered products

There are many practical applications for the use of solar panels or photovoltaics. It can first be used in agriculture as a power source for irrigation. In health care solar panels can be used to refrigerate medical supplies. It can also be used for infrastructure. PV modules are used in photovoltaic systems and include a large variety of electric devices:

• Agrivoltaics
• Solar canals
• Photovoltaic power stations
• Rooftop solar PV systems
• Standalone PV systems
• Solar hybrid power systems
• Concentrated photovoltaics
• Floating solar; water-borne solar panels
• Solar planes
• Solar-powered water purification
• Solar-pumped lasers
• Solar vehicles
• Solar water heating
• Solar panels on spacecraft and space stations
• Solar landfill

Limitations

[edit]

Impact on electricity network

[edit]

With the increasing levels of rooftop photovoltaic systems, the energy flow becomes 2-way. When there is more local generation than consumption, electricity is exported to the grid. However, an electricity network traditionally is not designed to deal with the 2- way energy transfer. Therefore, some technical issues may occur. For example, in Queensland Australia, more than 30% of households used rooftop PV by the end of . The duck curve appeared often for a lot of communities from onwards. An over-voltage issue may result as the electricity flows from PV households back to the network.[97] There are solutions to manage the over voltage issue, such as regulating PV inverter power factor, new voltage and energy control equipment at the electricity distributor level, re-conducting the electricity wires, demand side management, etc. There are often limitations and costs related to these solutions.

For rooftop solar to be able to provide enough backup power during a power cut a battery is often also required.[98]

Quality assurance

[edit]

Solar module quality assurance involves testing and evaluating solar cells and Solar Panels to ensure the quality requirements of them are met. Solar modules (or panels) are expected to have a long service life between 20 and 40 years.[99] They should continually and reliably convey and deliver the power anticipated. Solar modules can be tested through a combination of physical tests, laboratory studies, and numerical analyses.[100] Furthermore, solar modules need to be assessed throughout the different stages of their life cycle. Various companies such as Southern Research Energy & Environment, SGS Consumer Testing Services, TÜV Rheinland, Sinovoltaics, Clean Energy Associates (CEA), CSA Solar International and Enertis provide services in solar module quality assurance."The implementation of consistent traceable and stable manufacturing processes becomes mandatory to safeguard and ensure the quality of the PV Modules" [101]

Stages of testing

[edit] See also: Photovoltaic module analysis techniques

The lifecycle stages of testing solar modules can include: the conceptual phase, manufacturing phase, transportation and installation, commissioning phase, and the in-service phase. Depending on the test phase, different test principles may apply.

Conceptual phase

[edit]

The first stage can involve design verification where the expected output of the module is tested through computer simulation. Further, the modules ability to withstand natural environment conditions such as temperature, rain, hail, snow, corrosion, dust, lightning, horizon and near-shadow effects is tested. The layout for design and construction of the module and the quality of components and installation can also be tested at this stage.

Manufacturing phase

[edit]

Inspecting manufacturers of components is carried through visitation. The inspection can include assembly checks, material testing supervision and Non Destructive Testing (NDT). Certification is carried out according to ANSI/UL, IEC , IEC , IEC , IEC and IEC -1/-2.

See also

[edit]

• Renewable energy portal
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The goal of the United States Department of Energy is to reach a levelized cost of energy for solar PV of $0.03 per kilowatt hour at utility scale by . This objective will strengthen the U.S. economy, help the country reposition in the international energy market2,3, and reduce CO2 gas emissions4,5,6. Solar energy represents a 1% share of the energy share in the U.S and is set to expand its share to as much as 30% by . Potential land competition between energy and food production8,9 necessitates a deeper understanding of the available solar resource and the overlapping agricultural or ecosystem land use services10. The global expansion of solar energy will require that both the most sustainable energy infrastructure developments10 as well as the locations of these developments are identified. The aim of this study is to augment the scientific grounds for this discussion by ranking land cover classes according to their solar energy production potential.

Solar PV potential fundamentally depends on the incoming solar radiation, which is strongly dependent on geographic location, but it is also well-known that the system's efficiency depends on the temperature of the solar cells, and the temperature of the solar cells is a function of the local microclimate. Each potential location has an associated microclimate; therefore, the influence of local climatology on PV conversion efficiency must be addressed. The thermal processes that connect a solar panel to its surroundings are modulated by four primary environmental variables: insolation, air temperature, wind speed and relative humidity. A first order description of the influence of these factors can be cast in a simple energy balance model of the PV panel where wind speed and air temperature influence convective heating or cooling of the panel, water vapor alters the long wave radiation budget, and solar radiation is the primary energy source. Here, this new microclimate-informed PV efficiency model is validated using field data11 from a 1.5'MW solar array located at Oregon State University in Corvallis, Oregon12. The first order model is used to map global solar power potential in order to assess the overlap between solar potential and underlying land use.

Modeled PV efficiency as a function of air temperature, wind speed and relative humidity are consistent with measured values in the Corvallis solar array (Fig. 1). A full description of the field measurements and the reduced-order model is provided in the material section. Solar PV efficiency diminishes as a function of air temperature at a rate of approximately 0.5% per 10'°C. This is consistent with literature observations of decreased efficiency with increasing ambient temperature13,14. Light winds lead to increased energy efficiency relative to quiescent conditions with a 0.5% increase in efficiency from 0.5'm/s to 1.5'm/s. This result is consistent with Dupré et al.8, who show that small changes in the convective heat transfer coefficient can lead to significant changes in the solar PV efficiency. Increased vapor pressure is associated with a reduction in median efficiency that is not fully captured with the reduced order model.

We apply the reduced order model to obtain a global maps of solar PV efficiency and annual mean solar power potential (Fig. 2a,b), using data sets for the solar radiation, air temperature, wind speed and humidity, obtained at a global scale from re-analysis products11,12. The reported solar efficiency is the ratio of the solar power generated to the solar irradiance incident on the PV panel.

The most efficient continental locations include western America, southern Africa, and the Middle East. This pattern is generally consistent with prior assessments of solar power's potential which emphasize other factors15,16,17 including transmission and economic potential which are not considered in the present study18,19,20. The solar power potential associated seventeen underlying land cover types identified with NASA's Moderate Resolution Imaging Spectrometer (MODIS) data21 is ranked by its median value (Fig. 3). Here, we find that croplands, grasslands, and wetlands were the top three land classes. Barren terrains, traditionally prioritized for solar PV system installation22, were ranked fifth.

The top three land covers associated with greatest solar PV power potential are croplands, grasslands and wetlands. Solar panels are most productive with plentiful insolation, light winds, moderate temperatures and low humidity. These are the same conditions that are best for agricultural crops, and vegetation has been shown to be most efficient at using available water under mesic conditions where atmospheric evaporative demand is balanced by precipitation supply23. Estimates of cropland expansion since suggest that much contemporary cropland was previously savannas/grasslands/steppes and forest/woodlands, thus similarity in the power potential of croplands with grasslands and mixed forests (Fig. 3) is likely driven by the conversion to agriculture of land with similar climates. Further, one could think of agriculture as a form of solar harvesting where the sun's energy is stored in the chemical bonds of the plant matter, and agricultural activities already occupy those places on earth most amenable to solar harvesting.

Our rankings of solar power potential by land cover type (Fig. 3) may be interpreted to forecast increased land competition between dedicated food production and dedicated energy production. It could also be interpreted to forecast a significant increase in the adoption of agrivoltaic systems. Agrivoltaic systems leverage the superposition of energy and food production for mutual benefit12. Crops are grown in the intermittent shade cast by the PV panels in agrivoltaic systems. The shade does not necessarily diminish agricultural yield.

Researchers have successfully grown aloe vera25, tomatoes26, biogas maize27, pasture grass12, and lettuce28 in agrivoltaic experiments. Some varieties of lettuce produce greater yields in shade than under full sunlight; other varieties produce essentially the same yield under an open sky and under PV panels29. Semi-transparent PV panels open additional opportunities for colocation and greenhouse production30. The reduced order model was re-evaluated to assess the potential for agrivoltaic globally, and the global energy demand31 (21 PWh) could be offset by solar production if <1% of agricultural land at the median power potential of 28'W/m2 were suitable candidates for agrivoltaic systems and converted to dual use. Lack of energy storage and the temporal variance in the availability of solar energy will restrict this expansion.

Data sources

Field data used in this study were collected during a two-year study on a six acre agrivoltaic solar farm and sheep pasture at Oregon State University Campus (Corvallis, Oregon, US.)11,12. Climatic variables (temperature, relative humidity, wind speed and incoming short-wave radiation were collected at a height of two meters (as the solar panel height) and one-minute intervals over two years. Wind speed was measured with a DS-2 acoustic anemometer (Meter Group, WA); relative humidity and air temperature were recorded with a VP-3 hygrometer (Meter Group, WA), and incoming solar radiation was measured by a PYR sensor (meter Group, WA) which integrated the solar spectrum between 300 and 'nm. The arithmetic means of all data were calculated on 15-minute intervals that coincided with the energy production data at the solar array (provided by Solar City).

PV efficiency model definition

The low-order solar PV efficiency model is a simple energy balance of the solar PV module.

The incoming energy is the sum of the shortwave radiation from the sun and the incoming longwave radiation from the atmosphere and ground. The outgoing energy is composed of a reflected shortwave component, the black body radiation from the PV panel itself, the convective cooling of the panel, and the electrical energy output. The imbalance between the incoming and outgoing heat fluxes results in a gain or loss of stored thermal energy expressed through a change of the panel's temperature.

A schematic of the control volume and the associated energy fluxes is presented in Fig. 4. Steady state is assumed, and the atmosphere is modeled under a neutral stratification as a first order approximation. The consequence is that the energy storage term is neglected and that the ground temperature is equal to the air temperature. The resultant energy balance of the panel is expressed as:

$$(1-\alpha -\varepsilon ){R}_{{\downarrow }}^{sun}+{L}_{{\downarrow }}^{sky}+{L}_{{\uparrow }}^{g}-2{L}^{p}-2{q}_{conv}=0,$$ (1)

where ε is the efficiency of the solar panel, α'='0.2 is the PV panel surface albedo, Rsun is the measured incoming shortwave radiation from the sun, and expressions for the remaining individual terms are presented below. The integral longwave radiation reaching the solar module from the sky (assuming clear sky conditions) is modeled according to Brutsaert ()32.

$${L}_{{\downarrow }}^{sky}=1.24\,\sigma {(\frac{{e}_{a}}{{T}_{a}})}^{\frac{1}{7}}{{T}_{a}}^{4},$$ (2)

where ea is the measured vapor pressure of water (hPa), Ta, is the measured air temperature (°K) and σ'='5.'×'10'8 kg s'1 K'4 is the Stephan-Boltzmann constant. The incoming long wave radiation from the ground is modeled as a simple black body:

$${L}_{{\uparrow }}^{g}={{T}_{g}}^{4},$$ (3)

where Tg is the ground surface temperature. The PV panel is modeled as a black body for longwave emission:

$${L}^{p}={\rm{\sigma }}{{T}_{P}}^{4},$$ (4)

where Tp is the panel temperature. The convective cooling of the panel is modeled with the bulk transfer equation:

$${q}_{conv}=h({T}_{p}-{T}_{a}),$$ (5)

where h is the convective heat transfer coefficient which has been estimated as33:

$$h=0.036\frac{{k}_{air}}{{l}_{panel}}\,{(\frac{u{l}_{panel}}{\upsilon })}^{4/5}\,{{P}_{r}}^{1/3},$$ (6)

where \({k}_{air}=0.026\frac{W}{mK}\) is the thermal conductivity of dry air, υ'='1.57e-5 m2s'1 is the kinematic viscosity of air, Pr'='0.707 is the Prandtl number of dry air, and u is the measured wind speed at the panel height. PV panels are typically arranged in rows that span a distance much greater the size of an individual panel. Heat transfer is maximal when the flow is perpendicular to the row. In this case, the relevant scale is the length of an individual panel, lpanel'='1.5'm. The efficiency of the solar panel is modeled based on a linear relationship with panel temperature, according to34:

$$\varepsilon ={\varepsilon }_{ref}[1-A({T}_{p}-{T}_{ref})],$$ (7)

where εref'='0.135, is the reference efficiency of the panel at a reference temperature, Tref'='298'K, and A'='0./°K is the change in panel efficiency associated with a change in panel temperature34. This linear relationship is assumed valid when \(|{T}_{p}-{T}_{ref}|\le 20\,^\circ K\)34.

Substitution of Equations 2'7 into Equation 1 yields an equilibrium expression for the PV panel efficiency. This expression is a quartic polynomial with only one unknown: the PV panel efficiency, ε, and four input variables: \({R}_{{\downarrow }}^{sun}\), Ta, u, and ea. This equation also has only one real root which can be obtained numerically with any root finding algorithm. The field data described above were used as inputs to generate the model outputs plotted in Fig. 1. Night time periods and times of low sun angles ('15°) were excluded from the analysis. In the global scale analysis, the input environmental data were provided for each 0.5°'×'0.5° pixel. Monthly reanalysis datasets were used to compute monthly maps which were arithmetically averaged to produce Fig. 2a,b.

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