What is solar panel manufacturer and Why Do We Use Them?

A new solar project was just installed in the US. 

Set a timer for 60 seconds and wait. Maybe take a step outside and soak up some sunlight. 

Zing!—another solar project was just installed. 

The sun is shining on the solar industry; the numbers are impressive. Today, there is enough solar capacity in the US to power the equivalent of 23 million homes, according to the Solar Energy Industries Association (SEIA). That’s 126 gigawatts (GW), coming from millions of solar systems across the country.  

The US solar industry was valued at $33 billion in 2021, employed more than 230,000 people, and continued to grow in power capacity at an average rate of 33 percent per year.  

Solar panels generated almost 4 percent of electricity in the US in 2021, up from less than 1 percent in 2015. In some places that number is much higher; for example, 17% of California’s electricity generation came from solar in 2021. Almost half of all new energy capacity added to the US grid in 2021 came from solar. Even more encouraging, by 2030, the solar industry aims to generate nearly a third of US electricity.  

With so many solar panels planned for the coming years, you might be wondering: what exactly are solar panels and how are they made? 

Meet your solar panel

There are two types of solar technology for electricity generation. The most common are photovoltaic (PV) panels or modules, which use the sun’s light to make electricity. Another technology, concentrating solar power (CSP), uses the sun’s heat instead. 

The most common type of PV panel is made using crystalline-silicon (c-SI). That technology accounts for 84% of US solar panels, according to the US Department of Energy. Other types include cadmium telluride, copper indium gallium (di)selenide panels, and thin-film amorphous silicon. Because c-SI panels compose most of the US and global market, I focus on them in this blog.

What’s in a solar panel? By weight, the typical crystalline silicon solar panel is made of about 76% glass, 10% plastic polymer, 8% aluminum, 5% silicon, 1% copper, and less than 0.1% silver and other metals, according to the Institute for Sustainable Futures. Graphic: UCS.

Building a crystalline silicon solar panel is a bit like building a sand castle, because silicon comes from sand! Beach sand is silicon dioxide, aka silica. (If beach patrol put that on a warning sign, I bet no one would step foot on the beach!). Silicon, in the form of silicon dioxide sand and gravel, is the second most abundant element on Earth, next to oxygen. 

Before it’s used in a solar panel, silicon dioxide must be turned into pure “metallurgical grade silicon” (MGS). This process uses a lot of energy: producing 1 kilogram of metallurgical grade silicon requires 14-16 kWh of power, which is roughly equivalent to using your home oven for seven hours. Still, over their lifetimes, solar panels emit 25 times less carbon dioxide equivalent per kilowatt hour than coal-powered electricity. 

Chemistry break! The recipe for cooking up metallurgical grade silicon is

Add 1 part silicon dioxide (gravel) and 2 parts carbon (sourced from coal, charcoal, or wood chips) to an electric arc furnace 

Crank up the heat to 2200 degrees Celsius (this is a third of the temperature of the sun!!) 

Ta-da! You’re left with 99% pure silicon and carbon monoxide (that’s from the carbon we added, bonded to the oxygen we removed from the silicon dioxide) 

But solar panels are perfectionists; they demand silicon to be close to 100% purity. To achieve that, we need to upgrade the silicon into an even more pure polysilicon metal using a process that involves hydrochloric acid and hydrogen gas. (Fun fact: about 12% of the world’s silicon production is currently processed into polysilicon for solar panels.)

Source: UCS

From sand to modules

After adding the acid and gas, we are left with chunks of polysilicon metal, which are typically melted down again in a roughly 5-meter-long cylindrical mold. Boron is added to give the metal a positive electric charge on one side. The hot, melty silicon cools and forms a single crystal (“monocrystalline”) structure as a cylindrical ingot. Ingots are any material cast into a rectangular shape, like bars of gold.

(Another process is used to make “polycrystalline” silicon wafers, in which multiple crystals form. This process tends to lead to less efficient panels but can reduce the cost of wafers.)

Next, a wire saw cuts the pure metal blocks of polysilicon into paper-thin, typically 7-inch by 7-inch flat slices called wafers.

Source: UCS

The wafers are heated in an oven and a thin layer of phosphorous is added, giving one side (the opposite of the positive boron side) a negative charge. Next, an anti-reflective coating is added to the wafers because without it these shiny disks reflect sunlight—and we want them to absorb it instead. At this stage, the wafers are now capable of absorbing the sun’s energy and converting it into electrons. Now we need to add silver metal conductors so those electrons can get turned into an electrical current that devices can use! 

Silver—the most conductive element in the world—intercepts the electrons in the silicon wafers and turns them into current. The silicon wafers now form a conductive solar cell. Each solar panel, usually containing 60 or 72 cells, uses about 20 grams of silver—a fraction of the panel’s weight but about 10% of its total cost.  

Copper metal conductors and wiring connect the solar cells together into one big solar panel, giving it the classic matrix appearance. Copper is a good electrical conductor and very malleable, making it a great material for forming the wiring that moves the current through the panel.

Workers install a completed solar panel. Photo credit: GRID Alternatives.

Zap! A solar panel has been made.

Now multiply it by about 60 million for the US alone, each year.  

And then speed it up because we need solar to play an ever-growing role in achieving our clean energy and climate goals.  

There’s a lot that goes into making solar happen beyond building panels, but responsibly accelerating solar panel manufacturing and installation is a critical step in the journey towards a just, sustainable renewable energy future.  

Want to learn more about the solar panel supply chain? Which countries are driving PV material mining and PV manufacturing? What does the solar panel repair and reuse industry look like today? How can we recycle solar panel materials and create a lower-waste circular supply chain? Click the links for answers!

Mining Raw Materials for Solar Panels: Problems and Solutions

Solar Panels Should Be Reused and Recycled. Here’s How

Assembly of photovoltaic cells used to generate electricity

Solar array mounted on a rooftop

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. The 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.

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. A photovoltaic system can be used to provide electricity for off-grid applications, such as remote homes or cabins, or to feed electricity into the grid and earn credits or payments from the utility company. This is called a grid-connected photovoltaic system.

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 for space and transportation applications.

History

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In 1839, 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 1873, 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 1876, describing the experiment they used to replicate Smith's results.[1][3]

In 1881, 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 1939, Russell Ohl created the solar cell design that is used in many modern solar panels. He patented his design in 1941.[5] In 1954, 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 2008 and 2013.[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 2013 many solar panel manufacturers announced and began shipping their smart module solutions.[9]

Theory and construction

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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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In solar modules, the cells themselves 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

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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 output, energy storage, and reduce power loss during power transmission, and convert from direct current to alternating current.

Smart solar modules

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Smart module

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

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Market-share of PV technologies since 1980

Most solar modules are currently produced from crystalline silicon (c-Si) solar cells made of polycrystalline or monocrystalline silicon. In 2021, crystalline silicon accounted for 95% of worldwide PV production,[16] 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).[17]

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

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In rigid thin-film modules, the cell and the module are manufactured on the same production line. The cell is created on a glass substrate or superstrate, and the electrical connections are created in situ, a so-called "monolithic integration". The substrate or superstrate is laminated with an encapsulant to a front or back sheet, usually another sheet of glass. The main cell technologies in this category are CdTe, a-Si, a-Si+uc-Si tandem, and CIGS. Amorphous silicon has a sunlight conversion rate of 6–12%.[citation needed]

Flexible thin film cells and modules are created on the same production line by depositing the photoactive layer and other necessary layers on a flexible substrate. If the substrate is an insulator (e.g. polyester or polyimide film) then monolithic integration can be used. If it is a conductor then another technique for electrical connection must be used. The cells are assembled into modules by laminating them to a transparent colourless fluoropolymer on the front side, typically ethylene tetrafluoroethylene (ETFE) or fluorinated ethylene propylene (FEP), and a polymer suitable for bonding to the final substrate on the other side.[citation needed]

Mounting and tracking

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Solar modules mounted on solar trackers Workers install residential rooftop solar panels

Ground

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Large utility-scale solar power plants usually use ground-mounted photovoltaic systems. Their solar modules are held in place by racks or frames that are attached to ground-based mounting supports.[18][19] 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.

Solar array ground mounting

Vertical bifacial solar array

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Agrivoltaic vertical bifacial solar panels Vertical Bifacial vs south facing solar array power output

 

 Vertical

Vertical Bifacial

 

 South facing solar array

Vertical bifacial solar cells 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.[20]

Vertical bifacial solar along highway

Roof

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Roof-mounted solar power systems consist of solar modules held in place by racks or frames attached to roof-based mounting supports.[21] 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 installed on canopies, which could be a parking lot canopy, carport, gazebo, Pergola, or patio cover.

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.

A 5V, 2A, 10W solar panel with USB port

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.[22][23]

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.[24]

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 provided more economically valuable power during morning and evening peak demands than north or south facing systems.[25]

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%.[26]

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.[27][28]

Power curve

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A typical voltage/current curve for individual unshadowed solar panels. Maximum power point tracking ensures that as much power as possible is collected.

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.

Power/Voltage-curve of a partially shaded PV module, with marked local and global MPP

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.[29][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.[30]

Module interconnection

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A connection example, a blocking diode is placed in series with each module string, whereas bypass diodes are placed in parallel with modules.

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 also include an auxiliary power outlet and/or USB adapter. Indoor panels (including solar pv glasses, thin films and windows) can integrate a microinverter (AC Solar panels).

Efficiency

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Reported timeline of champion solar module energy conversion efficiencies since 1988 (National Renewable Energy Laboratory)[31]

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.[32][33] Currently,[needs update] the best achieved sunlight conversion rate (solar module efficiency) is around 21.5% in new commercial products[34] 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).[35]

The current versus voltage curve of a module provides useful information about its electrical performance.[36] 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%.[37] Globally, capacity factor for utility-scale PV farms was 16.1% in 2019.[38][unreliable source?]

Overheating is the most important factor for the efficiency of the solar panel.[39]

Radiation-dependent efficiency

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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.[40]

Performance and degradation

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This chart illustrates the effect of clouds on solar energy production.

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.[41] 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.[42]

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.[43] 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%.[44][45][46]

Influence of temperature

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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.

- 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[47]

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.[48]

Potential-induced degradation (also called PID) is a potential-induced performance degradation in crystalline photovoltaic modules, caused by so-called stray currents.[49] This effect may cause power loss of up to 30%.[50]

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.[51]

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.[52] 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,[53] 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.[54] 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%.[55] However, other technologies, such as CIGS, show much lower degradation rates, even in those early years.

Maintenance

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General cleaning of ground-based solar panels at the Shanta Gold mine in Tanzania Deeper level of cleaning with pressure washing of the car port solar panels at Googleplex, Mountain View, California

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.[56] The average soiling loss in the world in 2018 is estimated to be at least 3% – 4%.[57]

Paying to have solar panels cleaned is a good investment in many regions, as of 2019.[57] However, in some regions, cleaning is not cost-effective. In California as of 2013 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.[58]

There are also occupational hazards with solar panel installation and maintenance. A 2015–2018 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.[59]

A 2021 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.[60] A 2021 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%).[61]

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 2014, 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".[62]

Waste and recycling

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There were 30 thousand tonnes of PV waste in 2021, and the annual amount was estimated by Bloomberg NEF to rise to more than 1 million tons by 2035 and more than 10 million by 2050.[63] For comparison 750 million tons of fly ash waste was produced by coal power in 2022.[64] In the United States, around 90% of decommissioned solar panels end up in landfills as of 2023.[65] 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.[66] Some private companies and non-profit organizations take-back and recycle end-of-life modules.[67] EU law requires manufacturers to ensure their solar panels are recycled properly. Similar legislation is underway in Japan, India, and Australia.[68] A 2023 Australian report said that there is a market for quality used panels and made recommendations for increasing reuse.[69]: 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.[70] It is possible to recover more than 80% of the incoming weight.[71] 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.[72] 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.[73] Some commercial-scale recycling facilities have been created in recent years by private companies.[74]

Since 2010, there is an annual European conference bringing together manufacturers, recyclers and researchers to look at the future of PV module recycling.[75][76]

Production

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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.[78]

With over 100% year-on-year growth in PV system installation, PV module makers dramatically increased their shipments of solar modules in 2019. They actively expanded their capacity and turned themselves into gigawatt GW players.[79] According to Pulse Solar, five of the top ten PV module companies in 2019 have experienced a rise in solar panel production by at least 25% compared to 2019.[80]

The basis of producing solar panels revolves around the use of silicon cells.[81] These silicon cells are typically 10–20% efficient[82] at converting sunlight into electricity, with newer production models now exceeding 22%.[83]

In 2018, the world's top five solar module producers in terms of shipped capacity during the calendar year of 2018 were Jinko Solar, JA Solar, Trina Solar, Longi solar, and Canadian Solar.[84]

Price

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Swanson's law–stating that solar module prices have dropped about 20% for each doubling of installed capacity—defines the "learning curve" of solar photovoltaics.[85][86]

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 2012, a phenomenon known as grid parity.[87] 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.[88] 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 2012 it was estimated that the quantity cost per watt was about US$0.60, which was 250 times lower than the cost in 1970 of US$150.[89][90] A 2015 study shows price/kWh dropping by 10% per year since 1980, and predicts that solar could contribute 20% of total electricity consumption by 2030, whereas the International Energy Agency predicts 16% by 2050.[91]

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.

Short term normalized cost comparisons demonstrating value of various electric generation technologies[92] Long term normalized cost comparisons demonstrating value of various electric generation technologies[92]

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'.[87]

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 their electricity consumption. This situation is sometimes called 'retail grid parity', 'socket parity' or 'dynamic grid parity'.[93] Research carried out by UN-Energy in 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.[87]

Standards

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Standards generally used in photovoltaic modules:

• IEC 61215 (crystalline silicon performance), 61646 (thin film performance) and 61730 (all modules, safety), 61853 (Photovoltaic module performance testing & energy rating)
• ISO 9488 Solar energy—Vocabulary.
• UL 1703 from Underwriters Laboratories
• UL 1741 from Underwriters Laboratories
• UL 2703 from Underwriters Laboratories
• CE mark
• Electrical Safety Tester (EST) Series (EST-460, EST-22V, EST-22H, EST-110).

Applications

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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:

Limitations

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Impact on electricity network

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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 2017. The duck curve appeared often for a lot of communities from 2015 onwards. An over-voltage issue may result as the electricity flows from PV households back to the network.[94] 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.[95]

Quality assurance

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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.[96] They should continually and reliably convey and deliver the power anticipated. modules presented to a wide exhibit of climate conditions alongside use in various temperatures. Solar modules can be tested through a combination of physical tests, laboratory studies, and numerical analyses.[97] 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" [98]

Stages of testing

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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

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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

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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/UL1703, IEC 17025, IEC 61215, IEC 61646, IEC 61701 and IEC 61730-1/-2.

See also

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References

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