Solar module

A solar module or photovoltaic module converts the light from the sun directly into electrical energy . The module consists of solar cells that are connected in series or in parallel. Solar modules are available as flexible and rigid versions. Rigid solar modules usually consist of silicon-based solar cells that are mounted on an aluminum frame and covered by a glass plate. The solar cells are mechanically protected from environmental influences by the module, e.g. B. Hail , TCO - Corrosion . Flexible solar modules are based on organic materials and are preferably used in the mobile sector.

Polycrystalline solar modules

Solar modules themselves are connected individually or as groups in photovoltaic systems . The photovoltaic supplies either electricity- independent consumers such. B. satellites or is used to feed energy into the public power grid . The entirety of all modules for a photovoltaic system is called a solar generator .

A solar module is characterized by its electrical characteristics (e.g. open circuit voltage and short circuit current). The characteristic curve of the solar module depends on the wiring of the solar cells. To maintain high efficiency, it is important that the connected solar cells are as similar as possible. Solar cells are classified for this purpose.

Mechanical Requirements

The following mechanical requirements are placed on solar modules for installation in a photovoltaic system :

• transparent, radiation and weather resistant cover
• robust electrical connections
• Protection of the brittle solar cell from mechanical influences
• Protection of solar cells and electrical connections from moisture
• Sufficient cooling of the solar cells
• Protection against contact with the electrically conductive components
• Handling and fastening options

Typical structure

Solar module on a motorway bridge

In the following, the structure is explained using the module type most frequently used worldwide:

• a pane of glass , usually so-called toughened safety glass (ESG) on the side facing the sun, which, among other things, serves to protect against hail and dirt
• a transparent plastic layer ( ethylene vinyl acetate (EVA), polyolefin (PO) or silicone rubber) in which the solar cells are embedded
• monocrystalline or polycrystalline solar cells that are electrically connected to each other by soldering strips
• Another plastic film for gluing the rear encapsulation, similar to the embedding film on the front
• Back lamination with a weatherproof plastic composite film z. B. made of polyvinyl fluoride (Tedlar) and polyester or another pane of glass (so-called glass-glass modules)
• Junction box with free-wheeling diode or bypass diode ( see below ) and connection terminal, often factory-fitted with connection cables and plugs (mostly MC4 plug connections between 4 and 6 mm²)
• mostly an aluminum profile frame to protect the glass pane during transport, handling and assembly, for fastening and for stiffening the composite, frameless modules are mostly designed as glass-glass modules and are attached directly to a substructure with special clamps
• Individual serial number on the frame or, in the case of some makes, embedded in an unchangeable way with the solar cells

Manufacturing

The manufacture of a solar module is largely automated with the optically active side facing down. First, an appropriate glass is cleaned and placed ready. A cut length of EVA film is then placed on top of this. The solar cells are connected to individual strings (so-called strings ) by means of soldering strips and positioned on the pane with the EVA film. Now the cross-connectors that connect the individual strings and lead to the location of the junction box are positioned and soldered. Then everything is covered one after the other with a cut-to-size EVA film and a polyvinyl fluoride film as a backing. The next production step is the lamination of the module in a vacuum bag at approx. 140 ° C or in an autoclave with overpressure (around 10 bar) and also at 140 ° C. During lamination, the EVA film, which was previously milky, forms a clear, three-dimensionally cross-linked and no longer meltable plastic layer in which the cells are now embedded and which is firmly connected to the glass pane and the back sheet. After lamination, the edges are hemmed, the junction box is set and equipped with the free-wheeling diodes . Now the module is framed, measured, classified and packed according to its electrical values.

technical features

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

Characteristic curve (current / voltage) of a solar cell illuminated and not illuminated

Common abbreviations for the names are:

Short Circuit (SC)

Short circuit

Open Circuit (OC)

Neutral

Maximum Power Point (MPP)

Operating point of maximum performance

The characteristics of a solar module are:

• Open circuit voltage ${\displaystyle U_{OC}}$
• Short circuit current ${\displaystyle I_{SC}}$
• Voltage at the best possible operating point ${\displaystyle U_{MPP}}$
• Current at the best possible operating point ${\displaystyle I_{MPP}}$
• Performance at the best possible operating point ${\displaystyle P_{MPP}}$
• Fill factor ${\displaystyle FF}$
• Temperature coefficient (TK) for the change in power (negative)
• TK for the change in open circuit voltage (negative)
• TK for the short-circuit current change (slightly positive)
• Module efficiency ${\displaystyle \eta }$
• Aperture efficiency ${\displaystyle \eta }$
• permissible reverse current or maximum string fuse
• maximum system voltage

Since penetrating moisture can greatly shorten the service life of a module due to corrosion and cause electrically conductive connections between the components of the solar module through which current flows, permanent encapsulation is of particular importance. When calculating the performance data and the profitability of a PV system, aging is usually also taken into account, for example a reduction of 1% per year.

The freewheeling or bypass diode

Function of the free-wheeling diodes in a series connection of several solar modules

If several modules are operated in series , a free-wheeling diode must be connected in anti-parallel to each module as shown in the diagram on the right. The two solar modules PC ₁ and PC _{3 are} illuminated, the middle module PC ₂ is shaded. The resulting current flow in the circuit through the load resistor R _L through the freewheeling diode D ₂ and the active solar modules is highlighted in red. The maximum current and the reverse voltage of the diode must at least equal the current and voltage values ​​of a module. Rectifier diodes with 3  amps / 100  volts are common.

The free-wheeling diode is connected to the connection terminals of each module in such a way that it is polarized in the reverse direction in normal operating mode (module supplies current) ( cathode or ring marking on the positive pole of the module). If the module does not supply any current due to shading or a defect, the photodiodes, which are now operated in reverse direction, would take a string consisting of several solar modules connected in series out of operation. If the voltage of the functional and irradiated solar modules connected in series exceeds the blocking voltage of the non-irradiated solar module, this can even lead to its destruction. Since the other cells continue to supply electricity, overheating occurs at this point, which can even lead to a fire in the module . This effect is known as a hot spot . This is prevented by the freewheeling diode, the current can flow through the freewheeling diode. A string can therefore continue to deliver electrical power, albeit at a lower level.

With current PV modules (September 2011), these free-wheeling diodes are usually already integrated into the junction boxes on the back of the module. In the case of a module with 6 × 10 solar cells, for example, every 20 solar cells are bridged with a diode in the event of shading, so that the entire module is not deactivated immediately in the event of partial shading.

One problem is that an inadequately contacted freewheeling diode is not noticeable in normal operation. For example, this was the cause of the fire in the Bürstadt photovoltaic system .

Electrical power

The specified (peak) nominal power of a solar module (in watts peak = Wp) is only given under laboratory conditions ( STC = standard test conditions ) with a light irradiation of 1000 W / m², 25 ° C cell temperature and 90 ° irradiation angle and a light spectrum reached by AM 1.5. In practice, these optimal conditions exist with permanently installed modules due to the changing position of the sun only for a short time and only by chance depending on the weather and the season. Either it is darker, the sun falls on the modules at a different angle, or the efficiency of the cells decreases due to an increased temperature in summer. Each module reacts differently to the different light intensities and light colors, so that the effective, current output and the annual yield of two equally powerful module types can be very different. Thus, the actual daily or annual yield depends on the type and quality of the modules and high-quality modules can therefore deliver more yield.

The following can be used as a guide: an unshadowed average module delivers between 0.5 (cloudy, short winter day) and 7 (clear, long summer day) full load hours per day . In other words, a 100 watt module (depending on the quality, 0.7–1 m² required) produces between 50 Wh and 700 Wh daily yield. For locations in southern Germany, Switzerland and Austria, as a rule of thumb, you can count on an annual yield of 1000 Wh for each watt of nominal power (Wp). This value is surpassed by modern systems with high-quality and well-coordinated components. The detailed location and the planning tailored to it play an important role. In the south of Europe these values ​​are generally better and in the north they are worse. While there is little difference between north and south on clear, sunny summer days, the contrasts are all the more serious in winter. This is due to the fact that in the north the summer days are much longer and the winter days are considerably shorter and the sun hardly comes over the horizon there. In a solar simulation one can determine typical solar yields for the respective location from weather data, in particular the radiation data, and the geographic location.

When connecting differently oriented modules in series, for example on curved surfaces or with different shading, maximum power point trackers (MPPTs) are sensibly built into the modules themselves.

More types

Semi-flexible solar module on a car roof

Flexible solar module on a model car

• Foil back modules
• semi-flexible modules consisting of monocrystalline cells between transparent plastic plates.
• Laminated glass-glass modules The
  advantages of glass-glass modules are their robustness and increased service life.
• Glass-glass modules in cast resin technology
• Glass-glass modules in laminated safety film technology ( laminated safety glass ) with PVB film
  The use of PVB is disadvantageous because it has lower UV transmission values . Therefore, as mentioned above, EVA makes a lot of sense.
• Thin-film modules (CdTe, CIGSSe, CIS, a-Si, µc-Si) behind glass or as a flexible coating, e.g. B. on copper tape
• Concentrator modules (also CPV: Concentrated PV), see also concentrator cells
  The sunlight is concentrated on smaller solar cells with the help of optics . This saves precious semiconductor material by illuminating it using comparatively cheap lenses . Concentrator systems are mostly used in connection with III-V compound semiconductors . Since a certain incidence of sunlight (mostly vertical) is necessary for optics, concentrator systems always require mechanical tracking according to the position of the sun.
• Fluorescence collector
  This special form of solar module converts the incident radiation in a plastic plate into a wavelength specially adapted to the solar cells. The plastic is doped with fluorescent dyes . The solar radiation is absorbed by the dye and stimulates it to glow. The longer-wave radiation emitted in the process leaves the plate mainly on one face, while on all other sides it is largely held in the material by total reflection or mirroring . The free face is equipped with solar cells that are optimally suited for the wavelength emitted by the dye. By stacking several different plastic plates and solar cells, each of which is optimized for a different wavelength range, the efficiency can be increased, since this allows a wider spectral range of sunlight to be used than is possible with a solar cell.

Degradation

The term degradation is understood to mean the aging-related change in the parameters of semiconductor components - in this case the decrease in the efficiency of solar cells over the course of their life.

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

Decreasing efficiency or electricity yields in solar modules often have more trivial causes: general surface contamination of the module glass; Algae growth ("fungus") especially starting from the module frame, with partial shading of the cells; growing trees and bushes that cause partial shade and were significantly smaller when installed; Yellowing of the polymeric embedding material that makes the cell-glass contact.

Crystalline solar cells

Formation of the boron-oxygen complex in crystalline solar cells

In the case of crystalline solar cells, the initial efficiency is approx. 15–19%. Often the manufacturers guarantee an output of 80 to 85% of the nominal output after 20 years of operation.

Responsible for the degradation are essentially recombination-active defects, which reduce the charge carrier lifetime to approx. 10% of its initial value (light-induced degradation). The formation of boron-oxygen complexes in Czochralski silicon is responsible for the light-induced degradation: the oxygen is attracted by the photoreaction in which the boron loses its positively charged hole and changes to a negatively charged ion. The oxygen is stored in the connection between the boron and the silicon.

In order to minimize the effect of the loss of effectiveness, one can use silicon wafers with a lower proportion of boron and the lowest possible proportion of oxygen (<15  ppm ). If less boron is used, however, the wafer also has a higher resistance due to the lower doping, which reduces the efficiency of the cell.

Studies have shown that solar cells do not show any significant degradation when the p-crystal is doped with gallium instead of boron. The lower active power loss could also be demonstrated with gallium-doped silicon with a high oxygen content.

Amorphous silicon solar cells

A particularly high degradation of up to 25 percent can occur with solar cells made of amorphous silicon in the first year of operation. For solar modules made from this material, however, it is not the performance at the beginning of the service life, but the performance after aging that is specified in the data sheets and when sold. Solar modules made from this material therefore initially have a higher output than those for which you paid for. The degradation, also known as the Staebler-Wronski effect (SWE), takes place when exposed to light. The metastable amorphous hydrogen-containing silicon (a-Si: H) experiences an increase in the defect density of about an order of magnitude, with a simultaneous decrease in conductivity and a shift in the Fermi level to the middle of the band gap .

After around 1000 hours of sunshine, a-Si cells reach a stable saturation level for efficiency. The first modules were manufactured industrially by the American company Chronar in the early 1980s. The 6 ″ × 12 ″ modules provide up to 12 W of power for systems with a voltage of 12 V. Small, off-grid systems with a 12 V lead battery can be operated with them. By 1989 Chronar established manufacturing facilities in the USA, Great Britain, France and Croatia. Even after the bankruptcy in 1990, some of these factories continued to produce 1st generation modules up to the present day.

These are modules with a front, 2 mm thick glass plate, which carries the active solar cells. The back is made up of a second glass plate, which is glued on with a UV-curing acrylic resin so that it is airtight and watertight. A plastic or metal frame guaranteed the protection of the edges. A connector was integrated into the frame. The solar cells were created by alternately depositing thin layers of material and then separating them into narrow strips, the actual cells, with a laser on an XY table. It started with the vacuum-technical deposition of a transparent layer of tin oxide, which serves as a conductive electrode. The layer sequence pin of a diode structure was generated by means of plasma-assisted CVD of silane and hydrogen with the timed addition of doping elements. The second laser cut is offset by a few 100 µm and exposed the front electrode again. Finally, a highly conductive aluminum layer was sputtered as a connector for the series connection of the cells in a vacuum process. A third offset laser cut separated the cells, but secured the connection from the aluminum layer of one cell to the front electrode of the neighboring one. Disturbing remaining connections of the cells were burned out by a strong current pulse. Finally, aluminum foil strips were bonded to the edge cells using ultrasound and these strips were connected to the connector.

Stress-induced degradation

Voltage- induced degradation (also potential- induced degradation ; English potential induced degradation ; PID) is a voltage-related power degradation in crystalline photovoltaic (PV) modules, caused by so-called leakage currents . This negative effect can cause performance losses of up to 30%.

In addition to the structure of the solar cell, the cause of the harmful leakage currents is the voltage level of the individual PV modules in relation to the earth potential - in most ungrounded PV systems, the PV modules are exposed to a positive or negative voltage. PID usually occurs when there is a negative voltage with respect to earth potential (exception: certain crystalline high-performance modules) and is accelerated by high system voltages, high temperatures and high humidity.

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

The negative PID effect can be completely prevented by using an inverter with the possibility of earthing the positive or negative pole. Which generator pole must be earthed must be clarified with the solar module manufacturer.

variants

Intelligent module

An intelligent module has an integrated MPP tracker or the entire solar inverter for the module and can be connected via a DC link or directly to the grid.

Plug-in photovoltaic modules

Plug-in photovoltaic modules, also known under the brand name Plug and Save , are solar modules with an integrated microinverter. These modules can be set up on the terrace, in the garden, on a carport , garage, balcony or garden shed and connected to the electrical installation of your own apartment or house using a power plug .

Plug-in photovoltaic modules are fully assembled and packaged so that they can also be put into operation by laypeople. Such “balcony systems” can reduce the private electricity bill to a certain extent. However, they are not made to feed electrical energy into the public grid. There is also no possibility of reimbursement from the electricity supply company . An electricity meter must never run backwards.

The approval of such plug-in photovoltaic modules is handled differently in different countries. In Germany, operation on the power grid is subject to approval. A position paper has been published by the "E.ON-TAB group", according to which the connection of such generating plants contradicts the generally recognized rules of technology and, in addition, every generating plant that feeds into the network must be registered with the network operator. With regard to generally recognized rules of technology for the connection of generating plants, it is stated that generating plants may only be connected to the supply side of the fuses (i.e. in a sub or main distribution / meter cabinet) of the final circuits. A connection of a generating plant directly to a final circuit has been permitted in Germany using an energy socket (see pre-standard DIN VDE V 0628-1 ) since 2018. Legal operation is possible if it is fed into a separate circuit without a consumer, connected by a registered electrician and an electricity meter with a backstop is used.

In Austria, many network operators stipulate that generating systems must not be pluggable. In Switzerland and many other countries, on the other hand, plug-in photovoltaic modules can be connected and used normally, provided that it is ensured that a feed-in power of 600 watts is not exceeded under any circumstances. The same tolerance threshold exists in the Netherlands.

recycling

Materials in a photovoltaic module can be recycled up to 95%. The world's first test facility for recycling crystalline silicon solar cells went into operation in 2004 in Freiberg . Today, a small number of specialized companies and non-profit organizations, such as PV CYCLE in the European Union, deal with the take-back and recycling of end-of-life modules.

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

The surface layers are removed from the solar cell by a chemical cleaning step (etching). New solar cells can then be manufactured from the silicon of the solar cell. It is noteworthy that significantly less energy has to be used if you recycle the silicon from the old solar modules than if you manufacture it from scratch.

For a qualitatively equivalent wafer made of recycled silicon you only need 30% of the energy compared to a new wafer. Recycling is therefore ecologically sensible, as the energy payback time is shorter, i.e. a recycled module recovers the energy that was used for production more quickly than a solar module made from non-recycled silicon. A study by the German Fraunhofer Institute published in 2012 shows that recycling one tonne of silicon-based PV modules can save up to 1200 tons of CO ₂ equivalent . Today there are recycling technologies for all PV technologies available on the market.

Since 2010, an annual conference has brought manufacturers, recyclers and scientists together to look to the future of PV module recycling. In 2011 the event took place in Berlin.

literature

• Alan R. Hoffman, Ronald G. Ross: Environmental qualification testing of terrestrial solar cell modules. In: Proceedings of the 13th IEEE PV Specialists Conference. Washington, DC, USA, 1978, pp. 835-842.

• S. Pingel, O. Frank, M. Winkler, S. Daryan, T. Geipel, H. Hoehne, J. Berghold: Potential Induced Degradation of solar cells and panels. Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE, Valencia, Spain, 2010, ISBN 978-1-4244-5891-2 . (abstract)

Individual evidence

1. ↑ Solar generator ( Memento of the original from December 10, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Entry in the electricity lexicon on steckdose.de 
2. ↑ The cause of the fire in Bürstadt was inadequate solar laminates from BP Solar. on: photovoltaik-guide.de , August 24, 2009.
3. ↑ Fire on the roof. ( Memento from September 1, 2009 in the Internet Archive ) In: Financial Times Deutschland. July 3, 2009.
4. ↑ Power or yield of a solar module Section of the article PV modules / solar modules for power generation on oeko-energie.de, accessed on September 10, 2010.
5. ↑ Successful start for the thin glass photovoltaic module Thin glass photovoltaic module, accessed on November 21, 2013.
6. ↑ More detailed calculation here: http://www.rechner-photovoltaik.de/rechner/solardegradation
7. ↑ M. Sheoran, A. Upadhyaya, A. Rohatgi: A Comparison of Bulk Lifetime, Efficiency, and Light-Induced Degradation in Boron- and Gallium-Doped Cast mc-Si Solar Cells . In: Electron Devices, IEEE Transactions on . tape 53 , no. 11 , 2006, p. 2764-2772 , doi : 10.1109 / TED.2006.883675 .
8. ^ SW Glunz, S. Rein, J. Knobloch, W. Wettling, T. Abe: Comparison of boron- and gallium-doped p-type Czochralski silicon for photovoltaic application . In: Progress in Photovoltaics: Research and Applications . tape 7 , no. 6 , 1999, p. 463-469 , doi : 10.1002 / (SICI) 1099-159X (199911/12) 7: 6 <463 :: AID-PIP293> 3.0.CO; 2-H .
9. ↑ Fraunhofer CSP presents results on potential-induced degradation (PID) of solar modules. ( Memento from February 17, 2013 in the web archive archive.today ) Fraunhofer Center for Silicon Photovoltaics CSP, accessed on January 28, 2013.
10. ↑ Sven Ullrich: Solar balconies: Prohibited and risky?
11. ↑ Position paper of the E.ON-TAB group on generating systems on final circuits (socket systems). (PDF) (No longer available online.) E.ON-TAB group, January 15, 2013, formerly in the original ; accessed on March 13, 2014 .  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.    
12. ↑ VDE-AR-N 4105, Section 4
13. ↑ DIN VDE 0100-551 section 551.7.1 and DIN VDE 0100-712 section 712.413.1.1.1.1
14. ↑ VDE
15. ↑ Plug-in PV systems. August 23, 2018. Retrieved August 23, 2018 . 
16. ↑ The first plug-in module approved on the network. March 2, 2015, accessed August 25, 2015 . 
17. ↑ Plug - & - Play photovoltaic systems Federal Heavy Current Inspectorate ESTI 2014.
18. ^ Report of the WDR broadcast "Markt". June 15, 2015, accessed August 25, 2015 . 
19. ↑ Recycling of modules, solar companies are fighting for their green image. on: Spiegel Online. April 25, 2010.
20. ↑ Nicole Vormann: Study: Sustainability and Social Responsibility in the Photovoltaic Industry. (Study) January 2010, accessed March 4, 2010 . 
21. ^ Anja Müller, Karsten Wambach, Eric Aslema: Life Cycle Analysis of Solar Module Recycling Process. Life Cycle Analyzes Tools Symposium, MRS Meeting, 2005.
22. ^ First Breakthrough In Solar Photovoltaic Module Recycling, Experts Say. (No longer available online.) European Photovoltaic Industry Association , formerly the original ; accessed in October 2012 .  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.    
23. ↑ International Conference on PV Module Recycling ( Memento from February 10, 2013 in the web archive archive.today )