Photovoltaics

The design and installation of photovoltaic farms is one of PowerOn’s main areas of expertise. Our team knows very well the specifics of the Polish energy market, and we know everything there is to know when it comes to issues related to the conversion of solar energy into electricity. We use our knowledge to enable you to build an efficient photovoltaic farm+ allowing you to quickly get a return on your investment. You don’t need to know anything about solar energy – we make sure the whole process runs smoothly and your investment pays off quickly!

We analyse your current situation and needs and then advise you on the most favourable solutions. Our experienced consultants explain the specialised concepts to you in simple terms, advise you on the best farm power, type of connection and choice of plot of land. We carry out an audit of the area for the installation, create an electrical design and an operational cooperation manual, and draw up an application for a licence to the President of the Energy Regulatory Office. We supply all the necessary materials and equipment. We take care of a fast, safe and fully professional installation of the photovoltaic farm – our headquarters are in Poznań, but our installation teams work throughout the country. We prepare the documentation for the Distribution System Operator, test and start the installation, train your team in system operation and take care of regular service of the photovoltaic farm.

A photovoltaic farm (solar power plant) is a specialised installation that uses the sun’s energy to generate electricity. The construction of a photovoltaic farm involves installing the necessary infrastructure (panels, inverters and other structural elements of the installation) and connecting it to the electricity grid. It can range in power from a few tens of kilowatts to several megawatts. The level depends on the number of panels operating within the installation – the more panels there are, the higher the power of the overall system.

A photovoltaic installation uses the photovoltaic effect. The photoelectric phenomenon is made possible by the formation of an electromotive force in a semiconductor (silicon is the most commonly used). The contact of photons (from incident sunlight) with silicon sets its electrons in motion. This creates electricity in the panels. Inverters (solar inverters) connected to the panels convert the direct current produced into alternating current with the parameters used by the public grid. In this way, the energy generated can power the lighting and appliances connected to the farm, be stored in batteries or be fed into the distribution grid and transmitted to other users.

Photovoltaic farms are installations of thousands of panels that are built to produce energy for commercial use. They have a very large capacity and their owners aim to resell the electricity. All the energy generated by the farm is fed into the grid and only from there is it distributed to end users.

Photovoltaic micro-installations are the most popular in the country. Their total output does not exceed 50 kW. The structures are usually installed on the roofs of buildings. Energy is produced here primarily for the owner’s own needs (company, household or agricultural). The electricity generated can be used to power the appliances in use or stored in a battery in the event of cloud cover and an associated reduction in plant performance. Only surplus electricity can be sold back to the grid.

The efficiency of a photovoltaic farm depends very much on the level of sunshine. It is the incident rays that set the electrons of the semiconductor in motion and generate the electricity. It is very important to choose a suitable location for the investment. You should choose a plot of land on flat ground with soil class IV or lower.

An ideal location is a low-quality agricultural land of at least two hectares. The plot must not be shaded by trees or buildings. Ideally, there should be a low- or medium-voltage network in close proximity to the planned development. The type of grid determines the size of the installation that can be connected to it. Investors then find it easier to obtain connection conditions and to conclude a favourable connection agreement.

The installation of photovoltaic devices with a capacity of less than 50 kW does not require a building permit. However, for photovoltaic devices with an installed electrical power of more than 6.5 kW, an obligation to agree in terms of capacity with fire protection requirements on the construction project must be applied.

For photovoltaic farms with a capacity of 50 kW or more, it is necessary to obtain a building permit issued by the poviat starost. It is necessary to prepare a construction project (for micro-installations, a detailed design or technical concept is sufficient). Another point, preceding the investment, is to obtain the connection conditions for the installation. The investor is also obliged to check the local spatial development plan (LSDP) adopted by the municipality where the plot is located. When the municipality does not have an LSDP, it is possible to locate a photovoltaic farm on the plot on the basis of a decision on the location of a public purpose investment. Photovoltaics serve the purpose of environmental protection, therefore the production and distribution of so-called green energy should be considered a public purpose investment by the municipality.

Environmental decision

The construction and installation of elements of a photovoltaic farm may require an environmental decision. When is such a decision necessary? The answer can be found in the Decree of the Council of Ministers of 10 September 2019 on projects that may have a significant impact on the environment. Paragraph 3, item 54 of the ordinance refers to the construction of a solar power plant. The regulations stipulate that photovoltaic systems may potentially have an impact on the environment.

An environmental decision is required for photovoltaic farms that:

1. a) occupy an area of at least 0.5 ha and are located in areas covered by forms of nature protection (including landscape and national parks or reserves) or in the buffer zones of forms of nature protection,
2. b) cover an area exceeding 1 ha (irrespective of the location of the plot).

The Energy Law obliges entrepreneurs who wish to produce electricity as part of their business activities to obtain the appropriate licence. This applies to enterprises for which the production of electricity is to be the main subject of their business activity. The concession for the production of energy from renewable sources is issued by the President of the Energy Regulatory Office. A solar power plant can start its operation after obtaining a licence, registration on the Polish Power Exchange (POLPX) and the commissioning and acceptance of the photovoltaic farm by the energy company.

Legal title to the property

A photovoltaic farm can only be built on a property to which the investor has legal title. The easiest way to do this is for the investor to own the land. In the absence of a suitable plot of land, it is possible to erect a solar power plant on a leased property. It is worth knowing, however, that in approximately 90 per cent of cases, preparation of the land for the construction and installation of the photovoltaic farm will be the responsibility of the owner of the plot (it is the owner who must apply for the relevant permits and payment orders will be sent to their name), which will make the whole procedure much more difficult.

The construction, installation and commissioning of a photovoltaic farm is a profitable investment that can bring high returns. Photovoltaics is an energy revolution that allows electricity to be obtained from the sun’s rays. A PV farm, as a renewable energy source, is environmentally friendly, which is why the so-called green energy and its producers around the world are promoted by international organisations and national governments. At present, Polish investors can take advantage of ministerial, local government and EU subsidies, which allow an earlier return on investment. It is believed that in the future, RES industry subsidies may increase even further.

A photovoltaic installation with a capacity of up to 50 kW can be built by private individuals without the requirement of registering a business and fulfilling additional legal formalities (it is not necessary to obtain a building permit and report it). Farms above 50 kW require a building permit. In both cases, the entire installation must be installed by authorised installers. On the grid connection notification, the installer certifies with its number the correctness of the completed energy source. Once the installation has been built and commissioned, the procedure needed to connect the photovoltaic farm to the grid can begin.

Contact us! We guide you comprehensively through the entire process of connecting your photovoltaic installation to the grid.

Call +48 515 792 391
or send an email to kontakt@e-poweron.pl

Stage 1 Submission of an application for grid connection conditions

An application for grid connection conditions should be submitted in the case of a new RES source and an increase in the connection capacity of an already operating source. The necessary forms of documents for the connection of an electricity source are available on the websites of the Distribution System Operators (PGE Dystrybucja, ENEA, Tauron, Energa), at PGE Dystrybucja S.A. Distribution Customer Service Points and in the Network Development Management Office of the company’s Head Office. Persons wishing to connect a photovoltaic installation or farm to the grid must complete the ‘W-3 Application for Determination of Connection Conditions’ form and an Appendix tailored to the specific RES source (A, B or C). The completed application can be sent to the Branch Headquarters of PGE Dystrybucja S.A. ENEA, Tauron, Energa or deliver it in person to the local entity (the one in whose area the installation is to be connected to the grid).

In the application for definition of the grid connection conditions, you must provide the following information:

• applicant’s data – natural persons provide identification and contact details on the basis of an identity card (or other document confirming identity). Entrepreneurs provide their data on the basis of a current excerpt from the National Court Register or a certificate of entry of the company into the Central Register and Information on Business Activity,
• specification of the facility to be connected to the grid (e.g. photovoltaic power plant) along with its location address,
• expected connection power,
• detailed technical parameters of the generation source.

The application should always be accompanied by:

• a document that confirms the legal title to use the plot, building or premises where the installation or network belonging to the applicant is located and all equipment will be connected,
• technical parameters, operating and operational characteristics of the photovoltaic devices, installation or grid (specification made according to the model in Appendix C),
• data sheets for photovoltaic cells and DC/AC switches.

When you want to connect an inverter of a photovoltaic installation above 1kV to the grid, it is necessary to also attach to the application:

• an electrical internal schematic of the source – this must include a diagram of the source’s electrical substation and the types and lengths of the cables that will supply the source,
• an extract from the local spatial development plan – if there is no such plan in the area where the photovoltaic installation is to be built, a decision on land development conditions must be obtained (if required by law),
• a printout from the National Court Register or the Central Register and Information on Business Activity – applies to entrepreneurs,
• a power of attorney for a representative – if the representative represents the applicant in the process of completing the formalities.

In the case of sources with voltage exceeding 1 kV, the applicant shall pay a connection fee of PLN 30 gross for each kilowatt of connection power (but no more than the expected grid connection fee and no more than PLN 3,000,000). The advance payment shall be made to the bank account of the relevant branch office of the Distribution System Operator. within 14 days from the submission of the application.

The application together with connectors is verified by representatives of the Distribution System Operator. If the applicable appendices are missing, the employees contact the applicant and ask him/her to complete the documentation. The applicant then receives a written confirmation of the submission of a complete application.

Stage 2 Determining the conditions for connection of the source to the grid

The technical staff of the relevant Distribution System Operator verifies the submitted application. For photovoltaic micro-installations, the electricity company has 30 days to analyse the submitted application and issue the connection conditions. For photovoltaic installations with a rated voltage of more than 1 kV, the 30-day period expires after the applicable down payment has been made.

The terms and conditions are then agreed and are incorporated into the connection agreement. A schedule for the implementation of the connection is attached to the agreement. The documents present detailed technical data and information needed to connect the RES source to the grid.

The agreement describes, for example:

• the scope of the equipment to be built, together with the technical parameters,
• obligations of the parties,
• the date of connection of the source to the grid,
• the date of validity of the agreement,
• the persons who will be responsible for coordinating the work,
• the liability of the parties to the agreement,
• the rules for settling any disputes.

In the case of photovoltaic farms with an installed capacity of more than 2 MW, it is necessary to prepare an expert opinion on the impact of the connected RES source on the power system. Only based on this expert opinion can the connection conditions be issued. If it turns out that there are no technical or economic conditions for the connection of a source of a specific capacity to the distribution network of the relevant Distribution System Operator, the applicant shall be notified of the amount of connection capacity that is available in their case. The applicant has 30 days to accept or reject the proposal presented.

The connection conditions issued by the relevant Distribution System Operator are valid for 2 years (calculated from the moment the decision is delivered to the applicant). During this time, the consumer can conclude an agreement based on the conditions issued.

Stage 3. Connection implementation

Based on the analysis of the grid connection conditions, an agreement is prepared between the applicant and the relevant Distribution System Operator Dystrybucja S.A.. The agreement is then signed by both parties.

The following duties rest with the distribution network:

• acceptance of the equipment and the electrical networks that will serve the connection.

The following duties rest with the power generator:

• the construction of the RES source (installation or photovoltaic farm),
• submission of an ‘installation execution declaration’ – for renewable sources with a minimum capacity of 500 kW,
• submission of a ‘declaration of completion of a small installation’ – for a renewable source with an installed capacity of more than 50 kW but less than 500 kW.

The relevant declaration form must be signed by the generator and the network contractor, and the installation contractor of the source to be connected.

When the RES source is constructed in accordance with the agreement, the relevant Distribution System Operator shall issue a ‘confirmation of the possibility to provide the electricity distribution service’. At this point, the applicant can conclude an agreement for the distribution of electricity with the Distribution System Operator’s company and with the seller for the sale of electricity fed into the public distribution network.

Stage 4. Conclusion of the distribution agreement

An important formality that owners of on-grid photovoltaic farms and installations must complete to be able to feed energy into the public grid of the Distribution System Operator is to obtain an energy generation concession. Such a concession is issued by the President of the Energy Regulatory Office. The owner of the RES source must install, at their own expense, a metering system on the generation source.

The next step is to conclude an agreement with the relevant Distribution System Operator for the provision of distribution services for electricity fed into the grid.

Stage 5. Conclusion of a sales agreement with the energy buyer

The energy generated by the photovoltaic installation or farm is sold to the distributor. For this purpose, the owner of the generating source must conclude a sales agreement with the Trading Company buying the generated electricity.

The distribution agreement and the energy sales agreement can be concluded based on the document ‘Confirmation of the possibility to provide electricity distribution service’. This document must be submitted to the Commercial Department of the relevant Distribution System Operator and to the Trading Company. The Trading Company must hold an electricity trading concession. It is also a condition that the selected Trading Company has an agreement with the Distribution System Operator for the provision of services consisting in the distribution of energy.

Stage 6. Commencement of injection of the generated electricity into the Distribution System Operator’s grid

The conclusion of all agreements (required under the provisions of the Energy Law) allows the physical introduction of the electricity generated by the farm or photovoltaic installation into the grid to begin. The readiness of the installation for connection must be notified. The Distribution System Operator (DSO) checks the installation and accepts the installed metering and billing systems made by the energy generator. When all elements are correct, employees will seal the systems. At this point, the photovoltaic owner can start introducing the electricity produced into the Distribution System Operator’s grid.

On-grid installations – when you care about using both energy from the grid and producing your own.

On-grid installations are connected to the electricity grid, so you can sell surplus or draw power from the grid, depending on your needs. To build such an installation you need: photovoltaic modules, inverters, a grid connection and protection. Is it possible to store surplus energy with an on-gird installation? Yes, you just need to install an energy storage facility.

Off-gird installations – when you want complete autonomy.

These installations are not connected to the grid and are therefore autonomous. In this case, batteries to store surplus energy are a must. What do such installations consist of? They consist of photovoltaic modules, an energy storage facility, hybrid inverters and safety devices. Such an installation is suitable for a small facility or where grid connection is impossible or costly.

What do we take into account when we design an installation for you?

We do everything we can to ensure that the solution we propose is the most beneficial for you, so we carry out a detailed analysis which takes into account specific conditions such as:

• location and type of installation,
• amount of sunshine,
• shade risk,
• angle and direction of inclination,
• size of the area for the installation,
• condition of the roof covering or type of land.

Investment tax credit for farmers.

Did you know that, according to current legislation, any farmer who runs a farm and incurs expenses for the reconstruction and upgrading of buildings and equipment for crop production or animal husbandry is entitled to benefit from an investment tax credit and reduce their agricultural tax? This relief is granted upon completion of the investment and consists in the deduction of 25 per cent of the investment expenditure from the agricultural tax payable on land located in the municipality where the investment was made (Article 13 section 2 of the Agricultural Tax Act).

Subsidies

You can also obtain other subsidies for the installation of a photovoltaic installation. You can benefit from: The My Current programme, and preferential loans available under Prosument 2 or the Clean Air programme. Business programmes ? -> add what we can offer in terms of new programmes.

What are photovoltaic panels and how are they constructed?

Photovoltaic panels are devices consisting of a set of modules. Modules are interconnected photovoltaic cells that are used to produce electricity (direct current, which is then converted to alternating current in an inverter) using the conversion of the sun’s rays.

The set of cells is most often sandwiched between two layers of PET film and EVA and encased by a pane of very strong toughened glass. The entire structure is laminated hermetically and then framed. The frame is usually made of aluminium, which makes it rigid but also lightweight. It ensures the high durability of the modules, facilitates their installation and does not impose too great a burden on the building structure (panels in micro-installations are very often installed on roofs).

Types of photovoltaic panels

The different types of photovoltaic panels differ in efficiency (performance), lifetime, properties and price. Panels are divided into two types of cells.

In Poland and Europe, for the construction of micro-installations and photovoltaic farms, the most commonly used are:

• polycrystalline panels (first type),
• monocrystalline panels (first type),
• thin-film panels (second type).

Polycrystalline panels

Polycrystalline panels are made from modules that consist of multiple silicon crystals. The production of such modules is faster and simpler, which contributes to their lower price. Pure semiconductor silicon is dissolved under vacuum under protective glass and poured into special cocrystals in which the oriented silicon solidifies at high temperature. In this way, multicrystalline blocks are formed, from which square posts with a cellular surface are cut. The pillars are then sliced and cleaned with band saws to obtain the form of multicrystalline silicon disks. The discs are 200 mμ thick.

The advantage of polycrystalline panels is the low price (relative to other types) and the wide choice of manufacturers. The disadvantage is lower efficiency – on average between 13 and 19% (the output of polycrystalline panel installations is very much affected by shading). After about 25 to 30 years, they lose about 20% of their power.

Monocrystalline panels

Monocrystalline panels are made from modules that are constructed from single crystals of silicon. The crystal structure is precisely orientated in the cells in one direction. Thanks to this, mono-crystalline panels have the highest solar-to-electric conversion efficiency of any solution currently available on the market (on average from 14 to even over 20 per cent). It is worth noting, however, that module efficiency does not always translate into higher efficiency of the entire system. Due to technological limitations, the modules are polygonal in shape and therefore do not cover 100% of the panel surface.

Their lifetime extends up to 30 years (only after 25–30 years do they lose about 20% of their power). The weaker side of monocrystalline modules is the higher price – but this has been slowly decreasing in recent years, making them even more competitive in the market.

Thin-film panels

Thin-film panels (the second type) are most often made from a single cell and are only a few micrometres thick. This group includes photovoltaic panels with cells made of amorphous silicon, CdTe, CIS and CIGS.

In Europe and Poland, thin-film panels with CIGS cells are the most popular. These are formed from the elements copper, indium, gallium and selenium. They are often installed on the façades of buildings, as they absorb incident, diffuse and reflected radiation very well.

CdTe cells are made from cadmium telluride, which acts as a semiconductor. Here, the entire module is usually made up of only one cell (a thin layer of semiconductor is sprayed onto the glass). Such panels are red and black in colour.

Thin-film panels are cheaper than polycrystalline and monocrystalline panels. However, their efficiency is lower – ranging from about 6 to 14% (depending, for example, on the type of cells used).

How do you choose photovoltaic panels?

Take into account:

• the manufacturing technology – monocrystalline modules achieve a higher production efficiency even under unfavourable conditions,
• module efficiency – the efficiency of thin-film panels ranges up to about 6–14%, polycrystalline from 13 to about 19% and monocrystalline even above 20%,
• resistance to mechanical damage,
• resistance to adverse weather conditions (including elevated temperatures),
• actual power – obtained under actual conditions (the manufacturer provides the maximum power obtained under ideal conditions),
• manufacturer’s product guarantee,
• linear power loss guarantee (power loss over the years).

Polycrystalline panels are extremely stressed in shading zones. The individual cells here are connected in series, so the whole module works as its weakest cell. This problem is less of an issue with monocrystalline panels. Thin-film modules, which also absorb scattered or reflected rays well, are better suited for building façades.

The installation procedure is tailored to individual needs.

Installation systems – on the roof or on the ground?

Photovoltaic panels can be installed on the roof of a building or on a special structure placed directly on the ground. to obtain 1 kW of peak power, cells are needed which cover an area of almost 7 m2. For this reason, few private investors choose to install photovoltaic panels directly on the plot. Photovoltaic farms producing energy for commercial purposes (for resale to the public grid) are the first to resort to this solution.

Micro-installations of up to 10 kW usually go on the roof. This solution is practical, as the installation of photovoltaic panels is then safe, no one is disturbed and the height of the installation provides better access to the sun’s rays. The installation of photovoltaic panels on the roof must be adapted to the shape of the roof and the type of roofing. On a flat roof, the panels can be arranged vertically (one next to the other) or horizontally (one above the other).

Photovoltaic modules can also be installed on the façade of a building or used to build a canopy (e.g. over a garage entrance), but such solutions are only used occasionally.

Installing the panels – on which side of the building?

The highest efficiency and power output of photovoltaic modules is possible when their active surfaces are oriented directly towards the sun. Unfortunately, natural conditions (the constant movement of the Earth around the Sun) only make it possible to achieve the ideal position for some of the time. Experts recommend installing photovoltaic panels in a south-facing position and tilting their level at an angle of about 30–40 degrees. Deviating from this results in a decrease in energy production. Positioning the panels in a south-easterly or south-westerly direction and tilting them about 24–55 degrees from the horizontal will produce a current yield that is 90–95% of the maximum. If the panels are oriented towards the east or west and tilted at an angle of 25–40 degrees, approximately 80–90% of the maximum energy can be obtained.

The direction of fixed-installed panels is best matched to individual energy requirements. Investors with a significantly higher energy intake in the afternoon and evening will get the best performance by orienting the panels towards the south-west. When power consumption is higher in the morning, the panels are better directed towards the south-west. The need to point the panels towards the west with increased afternoon energy consumption will unfortunately reduce the profitability of the investment. In our geographical zone, pointing photovoltaic panels towards the north completely misses the point.

Orienting the panels horizontally is not beneficial due to the associated pollution, which reduces the efficiency of the entire installation. It is assumed that the minimum angle is 20 degrees.

Always consider:

• the position of the building in relation to the cardinal directions (in the southern hemisphere, photovoltaic panels face north and in the northern hemisphere, they face south),
• the angle of inclination of the roof,
• obstacles in the neighbourhood that may cause shading (tall buildings, trees, poles, masts).

How do you avoid shading?

Shadows falling on the panels reduce their efficiency, as less sunlight reaches the photovoltaic cells. The whole photovoltaic system connected in series is only as efficient as its weakest cell. A sticky leaf is enough to shade a small section of one cell so that the others connected to it produce less energy. The slight shading of a section of the modules by masts, poles, trees or even electrical wires can therefore even lead to a temporary downtime of the panels. For this reason, it is necessary to maintain distance from any obstacles when installing the panels and to move movable components (e.g. a TV antenna) to the other side of the roof slope.

Manufacturers of modern photovoltaic modules now use solutions that help avoid the adverse effects of shading on photovoltaic panels. Special bypass diodes (by-pass) are fitted to the panels, which are responsible for automatically switching off the shaded cells. In this way, they do not affect the operation of the other cells, as the current flows through the diode and not the busbar connected to the cell. In crystalline silicon cell panels, typically a row of 10–12 cells is divided into 3 independent sections with separate diodes. Thus, shading of one section will shut down 1/3 of the entire panel.

In thin-film panels, the cells are arranged in parallel in a single row. Here, shading of fragments of multiple cells will only cause a partial drop in the power of the entire module. Such modules usually have a single diode. In practice, this means that if one of the cells is completely shaded, the entire panel is automatically switched off.

Installation conditions for photovoltaic panels

The storage and installation of photovoltaic panels requires strict conditions:

• ambient temperature during operation – from -40 to +85 degrees C
• storage temperature – from -40 to +60 degrees C,
• air humidity – less than 85 rH%,
• installation is not possible during strong wind.

Several methods of mounting photovoltaic panels are used.

1. Installation by means of mounting holes in the frame

The panels are fixed to the frame structure using four pre-drilled mounting holes on the long edges 400 mm from the centre. As standard, metal components, spring washers and flat washers with M8 metric threads are used with a torque of 10 Nm. Higher strengths can be achieved by replacing metal components with galvanised ones. In windier areas and with higher annual snowfall (e.g. foothill areas), it is recommended to use all eight mounting holes to achieve even greater structural strength.

2. Installation by means of clamps

The installation of photovoltaic panels with clamps is also currently used. Mounting the module on the long edge requires clamps to be made along the frame at the mounting holes (maintaining a tolerance of 10% of the total length of the panels to the edge of the frame). When fixing on the shorter edge, the clamps should be located along the frame at the panel edges (with a tolerance of 25% of the panel width to the centre of the frame). When clamping, always follow the panel manufacturer’s individual instructions regarding the pressure on the frame. It is very important that the clamp is only installed on the frame and in the designated locations. Their improper placement could lead to a shading effect and a decrease in the mechanical resistance of the entire panel.

3. Sliding systems

Manufacturers also offer photovoltaic panels with retractable systems on their shorter or longer side (the latter can withstand a maximum snow load of 5400 Pa). The modules slide in from the inside of the rail. PVC frame covers should be used to protect the anodised coating of the frame from damage.

Cooling of photovoltaic panels

The modules are subject to heating by the sunlight falling on them and by the flow of current generated. An increase in the temperature of the photovoltaic cells results in a decrease in their output (1 degree C translates into a decrease in output of approximately 0.5%). In the summer heat, panel temperatures can exceed as much as 50 degrees C. The peak power quoted by the manufacturer is measured when operating at 25 degrees C. For this reason, the installation of photovoltaic panels requires care for adequate ventilation to allow free air flow. It is best to position the panels on a flat roof (or in an open space on the ground). On a sloping roof, the modules are usually mounted parallel to the slope, but a few centimetres above it. Arranging photovoltaic panels directly on the roof (without ventilation from underneath) results in a decrease in energy production of about 10%.

The safe installation of photovoltaic panels requires the use of only certified structures and materials designed for this type of installation. The frames must meet the building requirements for snow and wind resistance. Only the holes made by the manufacturer may be used during installation work – the frame or other module components may not be drilled (this risks voiding the guarantee).

Manufacturers specify a minimum distance of 5 mm between two panels, but increasing this to 20 mm will be optimal. This provides air circulation and ventilation. The panel can be mounted wider or narrower with the edge down – one should pay attention to the configuration of the bypass diodes.

Important:

• installation of photovoltaic panels is only permitted on fire-resistant roofs (fire resistance class A),
• it is recommended to allow access for repair and maintenance work,
• it is always necessary to ensure that the mounted photovoltaic structure is secured against falling under snow pressure or in strong winds.

The installation of a photovoltaic system should only be carried out by professional installers with knowledge and specialised skills. Ideally, the team should be certified as RES installers in the field of photovoltaics.

What are photovoltaic cells?

A photovoltaic cell (photovoltaic cell, solar cell) is a semiconductor element in which the conversion of calorific energy from the sun’s rays into electrical energy (direct current) takes place. The conversion occurs as a result of the photovoltaic phenomenon. In a semiconductor p-n junction, photons (which have a lower energy than the width of the semiconductor’s band gap) cause electrons to move. The result is the creation of an electrical voltage (potential difference).

How are photovoltaic cells constructed?

Cells are formed from a wafer of material that is a semiconductor. Silicon is most commonly used, less commonly cadmium telluride or a mixture of copper, gallium, indium and selenium. The classic cell is made up of two layers of semiconductor. The upper part is a thin, transparent grid – the collecting (negative) electrodes are present here. The bottom part is a metal plate with transfer (positive) electrodes. The top layer is referred to as ‘n’ (negative) and the bottom layer as ‘p’ (positive). The negative surface is separated from the positive surface by a semiconductor ‘p-n’ junction.

Photovoltaic cells with a PERC layer are also available on the market. They have an additional layer on their underside that acts as an electrical insulator. This acts as a reflector that reflects the sun’s rays that have not yet produced an electron. This arrangement increases the efficiency, as the rays have another chance to produce energy.

The cells vary in size (from 4×4 cm to as much as 15×15 cm), with the smallest ones generating 1 W and the largest as much as 7 W. Individual cells are connected in parallel and in series to form photovoltaic panels.

Types of photovoltaic cells

1. 1st generation cells (thick film)

• monocrystalline – are black in colour (may appear dark blue on a sunny day). They are constructed from a silicon monocrystal approximately 30 cm in diameter. The silicon monolith is cut into wafers about 2–3 mm thick. Octagons are cut out of these to minimise material wastage. Monocrystalline cells are distinguished by their high efficiency (efficiency even reaches over 19%) and a lifetime of 25–30 years (only after this time does their output decrease by about 20%),
• polycrystalline – are blue in colour. They are made of square silicon wafers with an irregular structure (a frost-like structure of silicon crystals is visible on them). They are less efficient than monocrystalline cells (they have an efficiency of around 15–18%).

2. 2nd generation cells (thin-film)

These are manufactured from amorphous silicon, cadmium telluride or a mixture of copper, gallium, indium and selenium. A very thin semiconductor layer (approximately 0.001–0.80 mm) is deposited here by sputtering, vapour deposition and epitaxy. They are cheaper than first-generation photovoltaic cells.

Amorphous silicon photovoltaic cells are very popular. They have a brown or maroon tint and are matt. Their efficiency is lower than polycrystalline and monocrystalline cells (it is about 6–8%).

Individual photovoltaic cells vary in shape and in the materials they are made of. However, their general principle of operation is the same. Each photovoltaic cell is made of a semiconductor. The top layer of silicon is negatively doped with phosphorus and the bottom layer is positively doped with boron atoms. The two layers are connected by a neutral zone. A p-n junction (positive – negative, plus – minus) is formed between the two – an electric field is created here.

Light emitted by sunlight falls on the junction. The energy of a photon (the smallest entity of light) releases the electrons and causes them to move. The incident photons have an energy greater than the band gap of the semiconductor. In this way, electron-hole pairs are formed. The magnetic field inside the semiconductor causes the carriers to move in different directions – electrons to the ‘n’ region, holes to the ‘p’ region. An external electrical voltage is created at the junction. The circuit is closed with a receiver of energy. A direct current flows in the circuit (between the top and bottom plates). Photovoltaic installations use an inverter (inverter) to change it into alternating current, which can be used to power electrical equipment.

What photovoltaic cells should I choose?

Monocrystalline photovoltaic cells provide the highest efficiency and longevity of the panels. They work well for building a photovoltaic micro-installation on a small roof. A smaller number of such panels can produce the same amount of electricity as a larger polycrystalline panel area.

Polycrystalline cells are cheaper, which is why they are often chosen by people who care about lower investment costs. Their lower efficiency makes them more suitable for large roofs or on the ground.

Thin-film cells (e.g. made of amorphous silicon) are flexible, which is why they are often recommended for building façade photovoltaic systems.

9. What is the structure and function of inverters, and which one should you choose?

A solar inverter is an essential component in any photovoltaic installation. It is a device whose main function is to automatically convert the energy produced in the panels from DC voltage to AC voltage. In this way, the converted energy is compatible with the low-voltage grid (230/400 V H 50 Hz) and can be used to power electrical appliances, with the surplus sent to the public grid (this function is available depending on the type of inverter).

Inverter functions

The most important function of an inverter is to convert direct current (DC voltage) into alternating current (AC).

However, the inverter in a photovoltaic installation also has other functions:

• allows you to monitor the operation of the plant – the device allows you to access the monitoring of the operation of the entire photovoltaic plant. The amount of electricity produced can be checked on an LCD monitor or remotely (in a special application),
• allows the photovoltaic installation to be synchronised with the public electricity grid – the device counts the energy produced by the photovoltaic modules and that drawn from the external grid. This makes it possible to verify the difference between the current consumed in the building and that sent to the grid (on a daily, weekly, monthly or annual basis),
• has an MPPT system – the inverter with this system tracks the maximum power point of the photovoltaic modules.

How is the inverter built?

A photovoltaic inverter consists of a rectifier, an intermediate stage and a final stage. It also has an automatic control and protection circuit. The rectifier is responsible for rectifying the AC voltage into DC. In the intermediate stage, the DC voltage is stabilised and smoothed. The voltage then goes to the final stage (converter), where the direct current is transformed into an alternating current with strictly defined parameters. A protection system reads out the measurements of the incorrect power, thus protecting the grid from failure.

How does the inverter work?

The inverter monitors the operation of the system and, when parameters change, automatically initiates responses to these changes. Input circuits allow it to be connected to the photovoltaic modules. The inverter constantly monitors the operation of the cells, which determines the maximum operating point of the panels and ensures the safety of the system. The device allows the maximum output power (DC), voltage range (MPPT) and number of inputs to be determined. In the inverter, changes are made that convert DC to AC. It also allows information about the operation of the network to be read out. The protection system checks, among other things, the AC/DC protection, the phase-to-phase output voltage and how the system is cooled.

Type of inverters:

Based on their connection to the grid, inverters are divided into:

• off-grid – devices that do not connect to the grid. All the energy produced is stored in batteries connected to the photovoltaic installation. In Poland, they are most often used in summerhouses, where the demand for energy is small and seasonal,
• on-grid – devices that establish a connection to the power grid. Surplus electricity produced can be fed into the grid and then withdrawn from it (at a reduction of 20% or 30% – depending on the size of the installation). Batteries are then not necessary, as it is the external grid that acts as a kind of energy storage facility. However, it should be remembered that when the supply of electricity from the public grid is interrupted, the building connected to the photovoltaic installation will also be without electricity (due to the need to ensure safe maintenance work),
• hybrid – they can work with the grid or create an island installation working as an off-grid system.

With regard to the size of the installation to be connected, inverters are divided into:

• micro inverters – designed to work with a single photovoltaic panel. This solution is very rarely used (sometimes they are installed in holiday cottages to reduce the electricity bill while on holiday),
• string inverters – designed for photovoltaic installations up to 30 kW. This type of inverter is the most popular for single-family homes,
• central inverters – dedicated to serving installations above 30 kW. This type of inverter is used in photovoltaic farms with capacities reaching up to several hundred kW.

With regard to the connection to the phases, inverters are divided into:

• single-phase – suitable for small installations. The inverter can be connected via the phase (L), neutral (N) and protective (PE) wires. The best effect is obtained when the inverter is connected to the most loaded phase – the current to the other phases is then drawn from the external grid,
• three-phase – dedicated to installations of more than a few kW. The phase wires are divided into 3 phases (L1, L2, L3), neutral (N) and protective (PE). The load is then evenly distributed throughout the building, the network becomes more stable and the current draw to all phases from the public grid is kept to a minimum. An installation with a three-phase inverter distributes the energy produced by the panels to all the appliances operating in the connected building.

When selecting an inverter, pay attention to:

• the efficiency of the device – this parameter determines the ratio of the electric AC output power to the DC output power. High-quality devices from reputable manufacturers achieve an efficiency of approximately 97–98%,
• maximum power point – this parameter determines the position of panels giving the best effect at a given moment. The MPPT system increases the efficiency of the photovoltaic installation by adjusting the power to the light intensity and temperature,
• cooling – provides better operating conditions for the inverter by preventing it from overheating.

The inverter should be selected according to the size of the installation and the associated power produced by the photovoltaic panels. For a single-family house, a low-power string inverter is sufficient. For a photovoltaic farm, a central inverter should be selected. The efficiency of the inverter depends on the instantaneous load – this should vary between 20 and 100%. An oversized inverter will reduce the efficiency of the entire system, so the specialist designing the photovoltaic installation should carefully calculate the energy requirements of the specific building and only on this basis should an inverter be selected.