Solar panel

A solar panel or photovoltaic panel converts the sunlight directly into electrical energy. The module consists of solar cells which are connected in series or in parallel. Solar modules are available as flexible and rigid design. Rigid solar panels are usually made of silicon-based solar cells, which are mounted on an aluminum frame and covered by a glass plate. The solar cells are mechanically protected here by the module against environmental influences, such as hail, TCO corrosion. Flexible solar modules based on organic materials and are primarily used in mobile applications.

Solar panels themselves are individually connected photovoltaic systems or as groups. They provide either independent power supply consumers such as satellite or used for energy fed into the public power grid. The totality of all modules for a photovoltaic system called the solar generator.

A solar panel (for example, open circuit voltage and short circuit current) characterized by its electrical characteristics. The characteristic of the solar module is dependent on the interconnection of the solar cells. To obtain high efficiency, it is important that the connected solar cells are as equal as possible. These solar cells are classified.

• 7.1 Crystalline Solar Cells
• 7.2 Amorphous silicon solar cells
• 7.3 Stress induced degradation

Mechanical Requirements

The following mechanical requirements on solar panels for installation in a photovoltaic system:

• Transparent, radiation and weather-resistant cover
• Robust electrical connections
• Protection of the brittle solar cell against mechanical influences
• Protection of solar cells and electrical connections from moisture
• Adequate cooling of the solar cells
• Contact protection of the electrically conductive components
• Handling and mounting option

Typical structure

Next, the structure is explained on the basis of the module type most commonly used worldwide:

• A pane of glass, mostly so-called toughened safety glass (ESG) on the side facing the sunny side, which is used to protect against hail and pollution, among other
• A transparent plastic layer ( ethylene vinyl acetate ( EVA), or silicone rubber), in which the solar cells are embedded
• Monocrystalline or polycrystalline solar cells are connected electrically to each other through solder strips
• Rückseitenkaschierung with a weather- resistant plastic laminated film, for example, polyvinyl fluoride ( Tedlar ® ) and polyesters
• Junction box with protection diode or bypass diode ( see below) and connecting terminal, often already equipped from the factory with connection cables and connectors
• An aluminum profile frame to protect the glass during transport, handling and installation, for fixing and for the reinforcement of the composite
• Embedded unique serial number on the frame or in some brands unchangeable together with the solar cells

Production

The manufacture of a solar module is largely automated with the optically active side down. First, an appropriate glass is cleaned and placed ready. In this then comes a cut path EVA film. The solar cells are connected to individual strands (called strings) by means of solder wires and positioned on the disk with the EVA film. Then the interconnects that connect the individual strings with each other and lead to the site of the socket are positioned and soldered. Then everything sequentially cut with an EVA film and a polyvinyl fluoride film is covered as a back statements. The next production step, the laminating of the module in a vacuum bag at about 140 ° C or in an autoclave with pressure ( 10 bar) and also 140 ° C At the laminating stage of the hitherto milky EVA film, a clear, three-dimensional crosslinked and no longer be melted plastic layer in which the cells are now embedded and which is fixed to the glass plate and the back sheet. After lamination, the edges are trimmed, the connection can is pressurized and equipped with the free-wheeling diodes. Now the module is still framed, measured and classified according to their electrical values ​​and packaged.

Technical Features

The data of a solar module are the same as the data of a solar cell for standard test conditions ( STC: 1000 W / m², 25 ° C, AM 1.5 ) specified.

Common abbreviations for the names:

The characteristics of a solar module are:

• Open-circuit voltage
• Short-circuit current
• Voltage in the best operating point
• Power is in their best operating point
• Performance in the best possible operating point
• Fill factor
• Temperature coefficient (TC ) for the power change (negative)
• TK for the open-circuit voltage change ( negative)
• TK for the short-circuit current change ( slightly positive )
• Module efficiency
• Aperture efficiency
• Allowable reverse current or maximum string fuse
• Maximum system voltage

Since penetrating moisture can shorten the life of a module by Corrosion strong and electrically conductive compounds cause the current-carrying components of the solar module, the permanent enclosure is of particular importance. In the calculation of the performance and efficiency of a PV system usually also the aging a Vermindungerung of 1 % is being computed, for example, annually.

The freewheel or bypass diode

When several modules are operated in series, so must be connected in anti-parallel to each module, a freewheeling diode. The maximum current and the reverse voltage of the diode have the same at least the current and voltage values ​​of a module. Rectifier diodes with 3 Ampere/100 volts are common. The freewheeling diode is connected to the terminals of each module that they ( module delivers electricity) in the normal operating condition is reverse biased ( cathode or ring marking to the positive terminal of the module). When the module delivers by shading or by a defect no electricity, which is now reverse-biased photodiode would take a string that consists of several series-connected solar modules, out of service. If the voltage of the series- functional and irradiated solar modules the blocking voltage of the non-irradiated solar module, this can even lead to the destruction of the same. Since the other cells continue to supply electricity, forms at this point overheating, which can lead to a fire in the module. This effect is referred to as a hot spot. This is prevented by the free-wheeling diode, the current can flow through the freewheeling diode. A string can therefore continue - provide electrical power - albeit smaller.

With current PV modules (September 2011) this free-wheeling diodes are often already integrated into the outlets on the back of the module. For a module with 6 × 10 solar cells for example, 20 per solar cell with a diode to be bridged at shading, so that in case of partial shading is not equal to the whole module is disabled.

One problem is that a deficient contacted freewheeling diode during normal operation is not noticeable. So this was, for example, the cause of the fire of the photovoltaic system Buerstadt.

Electrical power

The specified (peak) rated output of a solar module ( in Watt peak = Wp) is only in laboratory conditions (STC = English: standard test conditions ) with a light irradiance of 1000 W / m², 25 ° C cell temperature and 90 ° angle of incidence and a light spectrum AM reaches 1.5. These optimal conditions, there is in practice for permanently installed modules by the changing position of the sun caused only a short time and weather and time of year due to just happen. Either it is darker, the sun falling at a different angle on the modules or the efficiency of the cells decreases due to an elevated temperature in the summer. Each module responds to the different light intensities and light colors differently, so that the effective, current performance and the annual yield two equally strong module types can be greatly different. Thus with the actual day or annual income to the nature and quality of the modules and high-quality modules can therefore provide more income.

As a guideline you can start at the following: Daily provides a non- shadowed average modulus between 0.5 ( turbid, short winter day ) and 7 (clear, long summer day ) Full load hours. That is, a 100-watt module takes between 50 Wh and 700 Wh daily yield. For locations in Southern Germany, Switzerland and Austria you can expect as a rule of thumb, with an annual yield of 1,000 Wh for each watt of power ( Wp). From modern facilities with high quality and well- matched components, this value is quite surpassed. The detailed location and adapted to its particular planning play an important role. In southern Europe, these values ​​are generally better and worse in the north. While there on a clear, sunny summer days, between North and South very little difference, the contrasts in the winter are more serious. This is because, in the north of the summer days much longer and the winter days are much shorter and the sun then there is hardly over the horizon. In a solar simulation one can determine the typical solar income from weather data, in particular the radiation data, and the geographical location for the site.

In the series circuit differently oriented modules, for example on curved surfaces or differences of shading usefully Maximum Power Point Tracker ( MPPT ) are installed in the modules themselves.

Other types

• Films backs modules
• Semi-flexible module consisting of monocrystalline cells between transparent plastic sheets.
• Laminated glass-glass modules Advantages of glass-glass modules are their robustness and a longer service life.
• Glass-glass modules in cast resin
• Glass-glass modules in laminated foil technology ( laminated safety glass) with PVB film The use of PVB, it is disadvantageous because it has a lower UV transmission values ​​. Therefore, as mentioned above, EVA very useful.
• Thin-film modules ( CdTe CIGSSe, CIS, a-Si, Si - mu.C ) behind a glass or a flexible coating, such as copper tape
• Concentrator modules ( CPV also: Concentrated PV), see also concentrator The sunlight is concentrated by means of an optical system to smaller solar cells. This saves precious semiconductor material by illuminating it focused by comparatively cheaper lenses. Concentrator systems are usually used in conjunction with III-V compound semiconductors. Since a certain sunlight (usually vertical) is necessary for the optics, concentrator systems always require a mechanical tracking by the sun.
• Fluorescence collector This particular form of solar panels converts the incident radiation into a plastic plate into an adapted particularly to the solar wavelength. The plastic is doped with fluorescent dyes to. The solar radiation is absorbed by the dye and excites it to glow. The emitted thereby, wave radiation leaving the disk mainly on a front side, on all other sides it is held by total reflection or mirroring largely in the material. The free end page is equipped with solar cells that are optimally suited for the radiation emitted by the dye wavelength. By stacking a plurality of different plastic sheets and solar cells, each optimized for a different wavelength range, the efficiency may be increased because a wider spectral range thereby the solar light can be utilized than is possible with a solar cell.

Degradation

The term degradation of the aging-related change in the parameters of semiconductor devices is understood - in this case, the decrease in the efficiency of solar cells in the course of their lives.

Usually one considers a period of up to 25 years. The loss in efficiency is approximately ranging from 10 % and 13% in the period of 20 or 25 years. Solar cells in space age much faster because they are exposed to a higher radiation.

But Declining efficiency or power yields for solar modules often have mundane causes: general scale pollution of the module glasses; Algae ( " Verpilzen " ) specifically by the module frame, starting with partial shading of the cells; growing trees and shrubs that cause partial shading and were significantly smaller in the installation; Yellowing of the polymeric embedding material, which accomplished the cell - glass contact.

Crystalline solar cells

With crystalline solar cells, the initial efficiency is approximately 15-19 %. Often the manufacturers guarantee after 20 years of operation, nor a power of 80 to 85% of rated power.

Responsible for the degradation are recombination- defective, the lower the carrier lifetime to about 10% of its initial value ( light-induced degradation) in substantially. Responsible for the light-induced degradation, the formation of boron-oxygen complexes in Czochralski Silicon: by the photo- reaction in which the boron loses its positively charged hole and converts to a negatively charged ion, the oxygen is attracted. The oxygen is deposited by one in the connection between the boron and silicon.

To minimize the effect of the loss of effectiveness, the silicon wafer can be a lower percentage of boron and the least possible amount of oxygen used (<15 ppm). When using less boron, the wafer, however, due to the lower doping and high-impedance, causing the efficiency of the cell decreases.

Studies have shown that cells with gallium instead of boron have no significant degradation in doping of the p-type crystal. The lower active power loss was also detected in gallium -doped silicon with high oxygen content.

Amorphous silicon solar cells

A particularly high degradation of up to 25 % may occur in amorphous silicon solar cells in the first year of operation. But not the performance is specified at the beginning of life, but the performance by the aging process in the data sheets and the sale of solar modules made ​​of this material. Solar modules made ​​of this material so have initially a higher power than that for which you have paid. The degradation, also Staebler -Wronski effect ( SWE) called, is done under light irradiation. In this case, the metastable amorphous hydrogen-containing silicon (a- Si: H) undergoes an increase in the defect density by about one order of magnitude, with a simultaneous decrease in the conductivity, and shift of the Fermi level in the mid band gap.

After 1000 hours, a-Si solar cells achieve a stable saturation value for the efficiency. The first modules were in the early 1980s produced industrially by the American company Chronar. The 6 " x 12" large modules delivered up to 12 W of power for systems with a voltage of 12 V. Small -grid systems with a 12 V lead-acid battery can thus be operated. Until 1989, built Chronar manufacturing facilities in the U.S., UK, France and Croatia. Even after the bankruptcy in 1990 have been made ​​by some of these factories to the present day modules of the first generation.

Are modules having a front 2 mm thick glass plate which bears the active solar cell. The back forms a second glass plate is glued air-and water-resistant with a UV - curing acrylic resin. A plastic or metal frame guaranteed the protection of the edges. A connector was integrated into the frame. The solar cells were formed by alternately depositing thin layers of material and subsequent separation into narrow strips, the actual cells using a laser on a XY-table. Was started with the vacuum deposition technology a transparent layer of tin oxide, which serves as a conductive electrode. By plasma -assisted CVD of silane and hydrogen under timed addition of doping elements, the layer sequence was generated pin a diode structure. The second laser cut is offset by a few 100 microns and put the front electrode again. Finally, a highly conductive layer of aluminum was sputtered as a connector for series connection of the cells in a vacuum process. A third offset laser cut separating the cells, but to secure the connection of the aluminum layer of the one cell to the front electrode of the adjacent one. Disturbing residual connections of the cells were burned by a strong current pulse. Finally, aluminum foil tapes were bonded by means of ultrasonic and these strips connected to the connector at the edge of cells.

Stress induced degradation

Stress induced degradation ( also potential-induced degradation; potential- induced degradation English; PID) is a stress- induced performance degradation in crystalline photovoltaic ( PV) modules, caused by so-called leakage currents. This adverse effect may cause power loss of up to 30 %.

Cause of the harmful leakage currents is adjacent to the structure of the solar cell, the voltage of the individual PV modules over the ground - in most ungrounded PV systems, the PV modules of a positive or negative voltage are exposed. PID occurs mostly at negative voltage with respect to ground potential (exception: certain crystalline high-performance modules ) and is caused by high system voltages, high temperatures and high humidity accelerated.

PID is known as an effect for several years. First publications on the topic in 2006 (Photon 4/2006, 6 /2006 and 4/2007) then related only to crystalline high -performance modules from SunPower. 2007 PID has also been registered in some solar panels from Evergreen Solar ( photon 1/2008 and 8 /2008). Meanwhile PID is also in ordinary crystalline modules, a problem (Photon 12/2010, lecture by solar energy company Solon SE at PVSEC in Valencia 2010): Statement of the solar module manufacturer Solon SE: " At 1000 V, a now quite common voltage for larger PV systems, it can be critical for each module technology ".

The PID negative effect can be completely prevented by an inverter is used with the possibility of grounding the positive or negative pole. Which Generatorpol must be grounded, is clarified with the solar module manufacturer.

Plug - finished solar modules

Lately generation plants were from different manufacturers on the market that are to be connected by means of Schuko plug to any electrical outlet, also often named as " small PV system ." Log in to the network operator is held by the manufacturers to be unnecessary.

From " E.ON TAB circle ", a position paper was published, which expresses that the connection of such generation systems is contrary to the generally accepted rules of technology and also that each feed end into the power generation plant is to register with the network operator.

With regard to generally accepted rules of technology for the connection of generating plants is argued that generating plants exclusively on the supply side of fuses ( that is, in a sub or main distribution / meter cabinet ) of the final circuits may be connected. A connection of a generating plant directly to a final circuit is specifically excluded.

This topic has been treated in various professional publications.

Recycling

Materials in a photovoltaic module can be recycled up to 95%. The world's first pilot plant for the recycling of crystalline silicon solar cell production opened in 2004 in Freiberg in operation. Today, employ a small number of specialized companies and non-profit organizations, such as PV CYCLE in the European Union, with back and recycling of end-of -life modules.

In one of the currently available recycling processes for silicon - based modules, the plastics contained in the module are incinerated at temperatures around 600 ° C. What remains are glass, metal, fillers, and the solar cell. The glass and the metal fraction is passed on to appropriate recycling companies.

Of the solar cell, the surface layer ( etching ) is dissolved by a chemical purification step. New solar cells can be produced from the silicon solar cell again. It is noteworthy that much less energy needs to be expended when you recycle the silicon from the old solar modules, as if preparing it again.

For a qualitatively equivalent wafers made ​​from recycled silicon you only need 30 % of the energy compared to a new wafer. Recycling is so ecologically meaningful, since the energy payback time is less, that is, a recycled module plays the energy expenditure one has used for the manufacture faster again as a solar module from non- recycled silicon. A 2012 study published by the German Fraunhofer Institute shows that recycling one ton of silicon -based PV modules can save up to 1200 tonnes of CO2 equivalent. Today there are recycling technologies for all PV technologies available on the market.

Since 2010, an annual conference brings manufacturers, recyclers and researchers together to look at the future of PV module recycling. In 2011 the event was held in Berlin.