Photovoltaics

Sustainable energy generation

Solar modules

Solar modules are the most obvious components of a solar system. They convert sunlight into electrical energy. This occurs via the photoelectric effect, i.e. the “knocking out” and “removal” of electrons from the solar cell material by the incoming light quanta.

The modules are not damaged, as the electrons are immediately “replenished” from the environment. In fact, the modules are very durable and some come with a guarantee of over 25 years on the specified output. However, there is a whole range of differences between the module types, particularly in the cell material, the contacting and the construction of the modules.

Cell material – monocrystalline vs. polycrystalline, perovskite

The cell material is an important factor in the selection of solar modules and influences their performance and efficiency. Monocrystalline and polycrystalline silicon solar cells are the two most common types of solar cells on the market.
Monocrystalline cells are characterized by their high efficiency and performance, as they are made from a single silicon crystal, which results in a uniform structure. Efficiencies of over 20% and module outputs of over 400Wp (Wp = watt peak, i.e. the maximum output possible under ideal conditions) are already the standard here.
Polycrystalline cells, on the other hand, consist of several silicon crystals. They are generally cheaper, but are also less efficient than monocrystalline cells. This is why they are practically no longer found in modern solar systems where high yields are important. The fact that they have a very specific appearance in marbled blue, while monocrystalline cells are usually manufactured in elegant black, also contributes to this. As we all know, you can wear black with almost anything.
A cell technology that is not yet ready for the market but has a promising future is based on the mineral perovskite, which is very common in the Earth’s mantle. Perovskite cells have the potential to be produced more cost-effectively than conventional silicon cells and at the same time offer higher efficiencies. However, they are currently still too susceptible to moisture and have a shorter service life than silicon cells. A lot of research work is therefore still needed before the first perovskite modules are launched on the market.

 

Structure and contacting – PERC, TopCon, n-Type

The structure of solar cells has a significant impact on efficiency. Constant optimization and new developments are taking place here, which are pushing the potential yield ever further. For example, PERC (Passivated Emitter Rear Cell) technology has dominated for years. This technology has a reflective layer on the back of the cells that transports the long-wave light, which cannot be used in conventional cells, back into the cell and makes it usable. As a result, they also exhibit better low light and diffuse light behavior.
TOPCon (Tunnel Oxide Passivating Contact) cells are a modification of PERC technology. Instead of individual contacts on the rear, a full-surface passivating rear contact with a 1 nanometer thin layer of tunnel oxide is used, which further increases the efficiency.
Another area of innovation is doping. While doping silicon wafers with boron (b-type) was the standard until recently, the newer and increasingly popular doping with phosphorus (n-type) offers a larger number of free electrons that can be used to generate electricity, which also increases efficiency.

Construction – full cell, half cell, shingle technology, bifacial, glass-foil, glass-glass, plastic.

In addition to the cell material and structure, the interconnection of the cells is also important for efficiency. While only full-cell modules were sold until a few years ago, half-cell modules are now the standard. With these, the square solar cells are divided into two halves to reduce the resistance per cell. Half-cell modules are also divided into two independent halves (usually visible through a “separating strip” in the middle of the module). This makes them less susceptible to power losses due to shading.
The shingle technology, in which solar cells are arranged in shingle-like overlapping strips to further increase efficiency and improve the appearance of the modules, is new. They are usually manufactured in a completely black look and are the most elegant solar modules on the market.
Solar modules also differ in the “packaging” of the cells. The classic modules for rooftop solar systems are glass-foil modules, which have solar glass on the front and a plastic film on the back. They are the cheapest module variant and are very widespread.
Glass-glass modules, on the other hand, also have a glass layer at the back, which makes them heavier overall. Thanks to the transparent surfaces between the solar cells, they can also be used for particularly attractive roofs and have a particularly high fire resistance.
A new trend is bifacial glass-glass modules, which can also generate electricity on the back. These are particularly suitable for installation on light-colored and reflective backgrounds or for vertical, free-standing installation, for example as a solar fence. Under certain circumstances, they can increase the yield by up to 30%.
Where glass modules should not be used for structural reasons, plastic solar modules have been available for some years now. The panels made of glass fiber composite or ETFE are in no way inferior to classic solar modules in terms of efficiency, but cost a little more per watt peak and have slightly shorter warranty periods. On the other hand, they often weigh only a quarter of their glass counterparts and can be attached using the same mounting solutions as glass modules, provided they are fitted with appropriate frames.

Conclusion

Solar modules are available in many variants. When choosing, you should make sure that they suit your needs, installation location and budget. If you pay attention to these points, they will reliably supply electricity from the sun for many years. And that’s what matters in the end.

Assembly solutions

Although the roof of a house is the most common place to install a PV system, it is by no means the only one. Wherever you want your system to go, it must be installed securely. The installation method must not only be suitable for the installation situation itself, but must also be able to withstand wind and snow loads, UV radiation, rain, frost and other environmental influences for many years.

Fortunately, there is a large selection of solutions for all types of roofs, for installation on flat/garage roofs, for façades and also for balconies, terraces and gardens.

Roof shapes

Photovoltaic systems on roofs are the most common case. Experience shows that a number of points need to be taken into account to ensure that the project is successful, the functionality of the solar panels and the integrity of the roof are maintained. There are a few basic steps to follow.

The first step is to take a close look at the structure and load-bearing capacity of the roof. Not every roof is equally suitable for supporting the photovoltaic modules and at the same time ensuring sufficient solar radiation for a sensible system size. The load-bearing capacity is determined by a structural engineer or a specialized roofer/solar installer. The general solar irradiation can be calculated using free tools such as PVGIS and the possible yields for a specific roof orientation can be calculated using many different online tools. If there are sources of shade, such as roof structures (chimneys, bay windows, dormers, etc.) or trees and houses in the neighborhood, then you are well advised to hire a specialist to calculate the appropriate size and placement of the system.

The second step is to select the right mounting systems. A distinction is made here between roof shapes.

Tile/tile roof

There are various systems that allow for minimum interference with the roof tiles and maximum stability. The most common system is the use of roof hooks, which are attached directly to the rafters. This method avoids direct loading of the roof tiles and ensures that the load of the solar modules is evenly distributed over the load-bearing elements of the roof. The roof hooks are mounted under the roof tiles, which enables seamless integration without visible interference. Sometimes parts of the roof tile have to be sawn out/cut in order to get the roof hooks in the right place. Aluminum profiles are then screwed onto the roof hooks, onto which the modules are then attached, usually with module clamps.

Sheet metal/bitumen roof

In contrast to tiled roofs, it is not possible to maintain the upper roof layer without drilling holes in sheet metal and bitumen roofs. For this reason, subsequent sealing is used, for example with self-sealing screws, patches/tapes, liquid bitumen or liquid plastic. Hanger bolts are often used here to ensure under-ventilation, onto which aluminum profiles are then mounted. In the case of folded and some trapezoidal sheets, installation can also be carried out on the vertical parts. Leading suppliers of assembly systems have already developed solutions here. Appropriate systems are also available for barrel roofs.

Flat roof/garage roof/carport

As flat roofs by definition have no or a very slight gradient, elevations are normally used here. If the roof is stable enough, these can simply be weighted down with paving slabs or other weights. Where this is not possible, there is normally no way around a screw connection. Here, too, care must be taken to ensure that the density and integrity of the roof is not compromised, which is something a specialist can best assess.

In most cases of roof installation, a roof penetration is necessary in the end, because the cables for the connection to the house electrics usually have to be routed through the roof. These penetrations must also be professionally sealed to prevent leaks and water damage. Special lead-through elements, which differ depending on the type of roof, ensure a watertight connection.

Facade/balcony

The balcony or façade is usually the most obvious place to install solar modules to generate your own energy. Due to the mostly vertical installation, slightly less yield can be expected overall than on the roof, but even such systems can pay for themselves financially after a few years. This is also due to the fact that in winter they deliver slightly higher yields than roof systems, as the sun is lower and therefore has a better angle of incidence on vertical modules. Sometimes systems with an elevation angle are also available, but due to the possible stronger wind load, you should pay close attention to the information on the wind zone, mounting height and terrain category or ask for it.

For façade mounting, systems with aluminum rails are usually used, which are screwed to the façade and on which the modules are then fixed with module clamps or linear systems. If a building is already planned with solar elements during construction, this is referred to as “BIPV” (Building integrated PV). In this case, special building law requirements for the solar modules must be observed.

There are a number of different systems that can be used for mounting on balconies, from brackets that enclose the handrail to mounting rails that can be clamped or screwed. If balconies need to be newly built or renovated, parapet elements with integrated solar modules can also be used. These are usually designed in a semi-transparent, frameless glass-glass version and are particularly aesthetically pleasing.


Note for tenants/property owners
:
Attachment to a balcony or façade impairs the appearance of the façade. In addition, the installation of solar modules often requires structural changes such as drilling holes in the parapet/building envelope or similar. In this case, according to the current legal situation, the consent of the landlord or the community of owners must be obtained in advance. However, this does not apply, for example, to a solar module located on balcony areas or roof terraces that cannot be seen.
In addition, the legal situation is currently changing. A “privileging” of certain forms of photovoltaics in tenancy and residential property law is to be expected in the near future, which will include a fundamental approval of such projects. Nevertheless, landlords or owners’ associations can still specify how the project is to be implemented. It is still unclear how far these requirements can go.

Terrace/garden/property

If you have enough space for a PV system on a terrace or in the garden, there is now a wide range of options available. Solar roofs for patios, for example, are popular and are sometimes offered as a complete product. As with the balcony parapet elements, semi-transparent modules are often used here to ensure a beautiful lighting atmosphere. For PV systems in the garden or other parts of the property without buildings, simple elevations are usually used and, depending on the conditions, concrete brackets, weighted elevation triangles, fixed feet or module trays are used. Particular attention must be paid here to the wind loads to be expected at the place of use.

A more recent development is solar fencing, which is available either as a complete system or as a retrofit kit for existing fences. The use of bifacial modules can be worthwhile here, especially for a fence that runs from north(west/east)den to south(west/east)en. These also generate energy on the back, which can be used for the morning/evening hours.

Inverter

The inverter is the heart of a photovoltaic system. It is thanks to it that the direct current from the solar modules can be fed into the grid at all, because it converts it into the sinusoidal alternating current that is found in the grid. Solar systems are primarily divided into two camps in terms of the inverters used: those with string inverters and those with module inverters. These in turn differ according to the number of “MPP controllers”.

MPP controller

MPP stands for “Maximum Power Point”. As the output (power) is the product of current and voltage, and the current and voltage of the solar modules vary depending on the type, solar radiation and outside temperature, the MPP controller in the inverter ensures that there is always the optimum ratio between current and voltage in order to generate the maximum output. Inverters with one or two MPP controllers are available for solar systems. The models with one MPP controller are divided into string and micro inverters (also known as module inverters), while inverters with two (or more) MPP controllers are always micro inverters.
As the name suggests, string inverters work with strings, i.e. strings of several solar modules that are connected in series (always the plus plug of one module to the minus plug of the other module). This series connection ensures that the voltage of the individual modules is added together. While this would mean instant death for micro inverters, string inverters can withstand this high voltage without any problems. However, they have a decisive disadvantage: all modules must face in the same direction and must not be shaded differently. The following applies to a modular string: The string is only as strong as its weakest part. If a module is shaded, the output of the entire string still drops to that of the shaded module, even if the sun is shining on the other modules.
Microinverters do not have these problems. They do not work with strings but only offer space for one module per connection. Each connection in turn has its own MPP controller, so that the MPP is still controlled separately for each module in an inverter with two connections. It therefore makes less of a difference if a module is in the shade. The other still continues to supply energy.
In addition, micro inverters are also much smaller and more weatherproof thanks to simpler temperature management at lower voltages. The micro inverters can then be coupled together for solar systems. The current is looped through so that at the end a string of micro inverters only needs one connection, which then bundles the power of all the individual inverters.

This makes it clear that module inverters may be the better choice for systems with foreseeable shading. However, string inverters are usually cheaper in comparison, as only one is normally needed, which is why they are much more common.

Conformity

Inverters must have a number of properties in order to be operated in the German power grid. For example, they must not form unwanted stand-alone grids with other inverters, must provide reactive power and must have grid and system protection (NA protection). These properties help to protect the electricity grid and ensure its stability. In view of the rapidly increasing number of energy feed-ins via inverters, this also makes sense. Proof that the inverters have these properties is provided in the form of certificates of conformity issued by an independent testing institute. These certify the compliance of the generating unit and NA protection with the current industry standards, in particular the VDE AR-N-4105:2018-11 standard.

Communication standards

Modern inverters not only convert energy but also record the corresponding performance data. There are different ways in which these are passed on. Whereas in the past this usually required cables, today communication usually works via Wi-Fi, smart home standards or in some cases even via 5G frequencies. These are used, for example, to transmit the current output via the manufacturer’s servers to apps on your own smartphone so that you can always see live how much the power plant is currently generating. However, the data can also be used to control other control systems such as smart home managers or power hubs. For example, it is possible to start the washing machine automatically when there is sufficient solar energy to operate it as cheaply as possible.
Quality products also observe the rules of data security and have strict encryption strategies as well as data centers within European borders. Particularly in view of the future requirements for a smart and adaptable electricity grid, care should be taken today to ensure that communication interfaces such as W-LAN or 5G are available, which can also be used for integration into energy communities, virtual power plants and flexibility offerings.

What return can a PV system generate?

For photovoltaic systems without energy storage, you can typically expect returns, i.e. an annual financial return in relation to the investment costs incurred, of between 5 and 8 percent. In some cases, even up to 10 percent is possible. This makes investing in a photovoltaic system a financially attractive investment option. However, the return on a photovoltaic system varies greatly depending on a number of factors.
Here are some calculation examples: (20-year operating period from 2024)

Size of the PV system

5 kWp

10 kWp

Size of the PV system

Yield without storage, 35% self-consumption

5 kWp

6,5 %

10 kWp

7,5 %

Size of the PV system

Yield with storage, 70% own consumption

5 kWp

7 %

10 kWp

10 %

Source: Stiftung Warentest yield calculator

Conditions: Surplus feed-in, electricity price: 32 cents, operation: 2%, electricity price increase: 2%, yield p.a.: 950kWh/kWp, no replacement of the storage system

The conditions that must be observed for calculating the potential return are briefly described below.

Acquisition, installation, connection

When purchasing a PV system, the question arises as to how much of the installation you can do yourself. This has a direct impact on the acquisition and installation costs.
If the system is installed on an open area or on a flat roof with elevations, you can do the installation yourself and save a lot of money. However, climbing onto the roof and bending roof tiles is not everyone’s cup of tea. If you would like to use the services of a specialist for the installation, then you are often referred to the respective specialist company when purchasing the components. They sometimes still make a return on the sale of these and therefore often do not offer the installation of a system purchased by the operator himself. Safety and warranty reasons also play a role here.
A specialist should always be consulted for the electrical connection. In most cases, it makes no difference whether the system was purchased and installed by yourself or by a specialist company.

If you buy a PV system yourself, it is currently available for well under € 1,000.00 per kWp. Prices for storage systems are falling continuously and are often already well below € 1,000.00 per kWh. Therefore, it is primarily the costs for labor and equipment such as lifting platforms that drive up the overall price. It is therefore worth doing it yourself and obtaining comparative offers.

Operation, maintenance, cleaning

The annual operating costs mainly comprise maintenance and repair costs, which can amount to around € 3,000 in the first 20 years of operation. Regular cleaning of the system is possible, but often not economical, as the modules are usually cleaned by rain and snow. The decision to insure the system depends on its size; in general, larger systems are more likely to justify insurance.

Modern LiFePo4 batteries have a service life of around 15-20 years. If these have to be replaced, this may reduce the return considerably. However, it is still unclear how storage costs will change over the coming decades. A further drastic reduction is very likely but of course never certain.

Yield, location, orientation

The yield of a photovoltaic system, which depends on the size of the system and the location of the house, is also decisive for the return on investment. In Germany, one kilowatt peak of photovoltaic output generates an average of 1,000 kWh of electricity per year, although systems in southern Germany can generate up to 1,300 kWh per kWp and systems in northern Germany can generate as little as 900 kWh per installed kilowatt peak in rainy years.

A purely south-facing orientation produces a higher yield, but an east-west orientation can increase the proportion of self-consumption, as there is no midday peak and generation is distributed more evenly throughout the day.

The optimum elevation for a south-facing orientation is between 20° and 40°, depending on the latitude (steeper in the north, flatter in the south). Elevation systems often tend to be at 20°, as this also keeps the area exposed to the wind small and thus the necessary ballasting lower.

Share of own consumption

Self-consumption of solar power increases the return on investment, as one kilowatt hour of solar power can be generated for just over 10 cents, whereas electricity from the supplier costs an average of around 32 cents per kilowatt hour. It is therefore worthwhile to monitor your own consumption and generation.
An electricity storage system can drastically increase the self-consumption rate by utilizing daily surpluses after sunset. However, it should match your own consumption behavior and the size of your system. The rule of thumb is: 1 kWh capacity per kWp module output of the system. Precise calculations are possible with several free online tools. We recommend the HTW Berlin independence calculator.

Electricity costs & electricity price increases

To calculate the financial gain from using your own solar power, multiply the current price per kilowatt hour (kWh) by the amount of electricity you use yourself. Electricity prices in Germany have been rising continuously for years. On average, the increase is between 1% and 3% per year. Of course, this also increases your annual savings through self-consumption. An average annual profit increase of 2 % for self-consumed electricity is a common assumption.

Remuneration for electricity fed into the grid

The remuneration that you receive for the part of your solar power that you feed into the grid naturally also influences the profitability of your photovoltaic system. For small systems on house roofs, the feed-in tariff is currently around 8 cents per kWh. In recent years, the remuneration for new systems has mostly been continuously reduced. This is expected to be the case again in the future. Therefore, the motto is: It’s not worth waiting!

Service life and power losses

The remuneration that you receive for the part of your solar power that you feed into the grid naturally also influences the profitability of your photovoltaic system. For small systems on house roofs, the feed-in tariff is currently around 8 cents per kWh. In recent years, the remuneration for new systems has mostly been continuously reduced. This is expected to be the case again in the future. Therefore, the motto is: It’s not worth waiting!

Financing with a loan

If you plan to finance your investment with a loan, the interest on the loan must be included in the profitability calculation. Special solar loans are available for this purpose, which are offered at interest rates of between 5 % and 6 %.

Conclusion

The actual profitability of PV systems depends on a whole range of factors. Fortunately, in many cases you can influence this yourself. Free tools help with initial orientation. But the good thing is: in the end, almost every PV system pays for itself after a few years.
Our friendly staff will be happy to answer specific questions about your project.

Registration of the solar system

PV systems must be registered, even if they are not intended to generate remuneration. The registration serves to better estimate the real share of renewable energies in the German electricity grid and also helps to correctly calculate the electricity demand in the distribution grid. If more electricity is purchased than the grid needs, this has to be compensated for at great expense by means of flexibilities such as controllable consumers or the curtailment of generation plants. In the end, this is reflected in the electricity costs of all consumers.

Registration takes place in two places.

Distribution system operator

The distribution grid operator manages the lines, electricity boxes, substations, distribution stations and other infrastructure of the low-voltage grid that supplies households. It is not itself the electricity supplier but is independent of them. In particular, the grid operator knows whether the connection of a solar system is technically feasible in the respective grid area or whether it may overload it. To have this clarified, you can submit a “grid connection request”, which can then be answered positively or negatively. The final registration forms for the solar installation are standardized nationwide and are set out in a VDE application guideline (VDE AR N 4105). Many network operators already offer online registration portals for entering the data required there. In addition to data such as name, address, contact details, meter number and performance data, there are some technical details that are better left to specialists. The existence of certificates for the inverter must also be confirmed, which confirm the conformity of the generation unit and grid and system protection (NA protection) with the Low Voltage Directive DIN VDE AR-N-4105. This is also really relevant, because only certified inverters switch off safely when disconnected from the grid. You should therefore check for the presence of certificates at the time of purchase.
However, the distribution system operator is not the only body that you currently have to register with. A further registration must be made with the Market Master Data Register.

Market master data register

The market master data register is the central online directory for devices and systems for the generation and storage of renewable energy. It is operated by the Federal Network Agency, the supreme monitoring authority for German grids. PV systems must be registered there within 4 weeks of commissioning. First, the user of the PV system is entered as a so-called “market player” and then the PV system is entered as a “unit”. Some of the same data must be provided here as with the grid operator, including information on the metering concept, controllability, etc., which is almost impossible for laypersons to answer. Here too, it is therefore advisable to leave this to a specialist.

Non-registration

Registration is a legal requirement, even if you do not wish to receive EEG remuneration. Both the grid operator and the market master data register are therefore subject to penalties for non-registration. The grid operator may charge € 10.00 per kWp of module output per month for the period in which it can be proven that the module was not used as registered. For a power plant with 5kWp, that would be €600.00 per year. There is even talk of fines of up to 50,000 euros for the market master data register. The exact height must be proportionate, so it is naturally lower for smaller systems. It is best to avoid both penalties by registering in good time.

If you have any questions about registration, please contact our friendly team.

Battery storage for the PV system

The washing machine doesn’t only run when the sun is shining. Even if it is worth consuming the electricity when the PV system is generating it for free, this is not always possible. In particular, it is rarely possible to consume all the electricity generated immediately. With sufficient solar radiation, the appliances in the household are usually not sufficient to utilize the energy generated. So there are lots of surpluses. Instead of giving this away to the grid for just 8 cents/kWh, it makes sense to store it temporarily in a storage facility and use it again after sunset or at times of high consumption. Depending on the price of electricity, this will save you many times over. A lot has therefore happened in the solar storage sector in recent years and different systems have become established, each of which has advantages and disadvantages. These differ primarily according to the way in which the electricity is stored and how it is made available again after storage.

Types of storage

Lithium iron phosphate (LiFePo4) storage systems have now become the standard for the chemical composition of battery storage systems. They combine high cycle stability and therefore service life with safety in operation. This means that they outperform lead-based storage systems and also the related lithium-ion (Li-ion) batteries. Other storage technologies such as compressed air storage, gravity storage, salt water batteries and many others are not fully developed or are usually not suitable for the typical application of a PV system for various reasons.

Battery storage systems are charged and discharged with direct current. In order to make their energy usable in the household, which is operated with alternating current, they require an inverter, just like the PV system itself. There are storage systems that are charged directly from the PV system with direct current (DC coupling) and those that are connected to the domestic grid independently of the PV system (AC coupling). With the latter, the mains current has to be converted back into direct current to charge the battery, which means additional losses. On the other hand, an AC-coupled storage system can also be charged with grid power when the sun is not shining. This can be an advantage if you use flexible electricity tariffs, for example. Cheap electricity can then be stored for times of high electricity prices. An AC-coupled storage system can also be useful with regard to future economic models such as flexibility trading or P2P trading. DC-coupled systems, on the other hand, also work when there is no mains supply, i.e. in the event of a power failure. They can then be used via an off-grid or hybrid inverter for a self-sufficient power supply. However, this then runs via a separate connection and not via the usual household electricity.
If you want to make the entire household capable of providing emergency power, you need larger storage units and a professionally installed electrical system with additional technology. A completely self-sufficient power supply requires a large amount of PV area, which cannot be realized on conventional properties. This has to do with the greatly reduced solar output in winter, which can only be compensated for with a large number of solar modules. In addition, a storage system with high output power and large capacity is required for peak loads.

Types of use

Usually, the PV storage system feeds exactly the amount of energy into the household that is needed thanks to a measurement of the household consumption at the house connection and a control signal based on this. This ensures high self-consumption and therefore low grid consumption.

If an energy management system is also used, this can be increased even further. The system uses control algorithms to ensure that the PV system, consumer and storage system are optimally coordinated so that even the last kilowatt hour of solar power can be used as efficiently as possible. There are already systems that incorporate the battery charge level, predicted consumption and even weather forecasts into the charging and discharging processes.

The simultaneous use of an electric car and a PV system with storage can exploit further synergies.

Economic efficiency

The greater the difference between the feed-in tariff and the electricity price, the more profitable a battery storage system is. As electricity prices have been rising continuously for many years, a storage system pays for itself more and more over time.

Many factors play a role in the calculation of profitability. The size ratio between the PV system and the storage system and their relationship to your own consumption behavior are particularly relevant. The higher the electricity consumption during the day and the smaller the PV system, the more of the PV electricity can be consumed directly, for example, which is detrimental to the profitability of a storage system. For larger PV systems or consumption that mainly takes place in the evening, however, a storage system is almost a must. Particularly in the long term, a PV storage system for PV systems from a size of 5 kWp almost always pays for itself within its lifetime. However, it should not be too large or too small, but should always be in proportion to the actual size of the system.

Conclusion

In most cases, battery storage systems are a sensible extension for PV systems and may pay off even more in the future, not only due to the foreseeable rise in electricity prices but also due to other economic models.

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