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PHOTOVOLTAIC SOLAR PANELS INFORMATION

If you are interested in solar energy, you have come to the right place and continue reading and you will find more PHOTOVOLTAIC SOLAR PANELS INFORMATION.

Photovoltaic solar panels are undoubtedly one of the best modern inventions, besides being, probably, the invention that contributes the most to ecology.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

Solar panels are modules that use the energy that comes from solar radiation, and there are several types, such as those for domestic use that produce hot water or photovoltaic solar panels that produce electricity.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

 

The  photovoltaic panels  are one of the  renewable energies  most used worldwide. This data is not surprising since this clean energy is very efficient and the  price of photovoltaic panels decreases year after year.

Photovoltaic solar panels are made up of cells that convert light into electricity. These cells take advantage of the photovoltaic effect, through which the light energy produces positive and negative charges in two nearby semiconduct of different types, so that an electric field with the ability to generate current is produced.

Photovoltaic solar panels can also be used in solar vehicles. The standardized parameter to classify its power is called peak power, and corresponds to the maximum power that the module can deliver under standardized conditions, which are:
– radiation of 1000 W / m²
– cell temperature of 25 ° C (not temperature ambient).

How are single panels divided?

The photovoltaic panels are divided into:

Crystal clear

Crystal Monkey: they are composed of sections of a single silicon crystal (Si) (recognizable by its circular or octagonal shape, where the 4 short sides, if you can see it in the image, it is seen that they are curved, because it is a circular cell trimmed).

Crystal poly:

when they are formed by small crystallized particles.
Amorphous: when the silicon has not crystallized.

Its effectiveness is greater when the crystals are bigger, but also their weight, thickness and cost. The performance of the former can reach 20% while the latter may not reach 10%, however its cost and weight is much lower.

The cost of photovoltaic panels has been reduced steadily since the first commercial solar cells were manufactured and their average cost of electric generation is already competitive with conventional energy sources in a growing number of geographical regions, reaching network parity .

 

PHOTOVOLTAIC SOLAR PANELS INFORMATION

 

The term photovoltaic comes from the Greek φώς: phos, which means «light» and voltaic, which comes from the field of electricity, in honor of the Italian physicist Alejandro Volta, (who also provides the term voltio to the unit of measurement of the difference of potential in the International System of Measures). The term photovoltaic began to be used in England since 1849.

The photovoltaic effect was recognized for the first time in 1839 by the French physicist Becquerel, but the first solar cell was not built until 1883.

Its author was Charles Fritts, who coated a semiconductor selenium sample with a gold leaf to form the splice. This primitive device had an efficiency of only 1%.

In 1905 Albert Einstein gave the theoretical explanation of the photoelectric effect. Russell Ohl patented the modern solar cell in 1946, although Sven Ason Berglund had previously patented a method that sought to increase the capacity of photosensitive cells

Operating principle

Theoretical principles of operation. Simplified explanation.
Some of the photons, which come from solar radiation, hit the first surface of the panel, penetrating it and being absorbed by semiconductor materials, such as silicon or gallium arsenide.
The electrons, atomic subparticles that are part of the outside of the atoms, and which are housed in orbitals of quantized energy, are struck by the photons (interact) freeing themselves from the atoms to which they were originally confined.This allows them, subsequently, to circulate through the material and produce electricity. The complementary positive charges that are created in the atoms that lose the electrons, (similar to bubbles of positive charge) are called holes and flow in the opposite direction of the electrons, in the solar panel.

It has to be commented that, just as the flow of electrons corresponds to real loads, that is to say, loads that are associated with real mass displacement, the gaps, in reality, are charges that can be considered virtual since they do not imply displacement of mass real.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

Representation of the potential difference, or voltage of current with respect to time in direct current

A set of solar panels transform the solar energy (energy in the form of radiation and depends on the frequency of the photons) in a certain amount of direct current, also called DC (acronym of the English Direct Current and which corresponds to a type of current electrical, which is described as a movement of charges in one direction and one direction, through a circuit, electrons move from the lowest to the highest potentials).

Optional:
The direct current is carried to an electronic converter circuit (inverter) that transforms the direct current alternating current, (AC) (type of current available in the power supply of any home) of 120 or 240 volts.

The power of AC enters the electrical panel of the house.

The electricity generated is distributed, almost always, to the distribution line of the lighting devices of the house, since they do not consume excessive energy, and are adequate to work properly with the current generated by the panel.
The electricity that is not used can be routed and used in other facilities.
Generation of current in a conventional plate
PHOTOVOLTAIC SOLAR PANELS INFORMATION


Electric scheme.

The photovoltaic modules work, as has been seen in the previous section, by the photoelectric effect. Each photovoltaic cell is composed of at least two thin sheets of silicon.

One doped with elements with less valence electrons than silicon, called P and another with elements with more electrons than silicon atoms, called N.

Those photons coming from the light source, which have adequate energy, impact on the surface of the P layer, and when interacting with the material they release electrons from the silicon atoms which, in movement, pass through the semiconductor layer, but can not return.

The N layer acquires a potential difference with respect to the P. If electrical conductors are connected to both layers and these, in turn, are attached to a device or electrical element that consumes energy, usually and generically called load , a continuous electric current will start.

This type of panels produce electricity in direct current and although their effectiveness depends both on their orientation towards the sun and their inclination with respect to the horizontal, they usually mount panel installations with orientation and fixed inclination, for savings in maintenance.

Both the inclination and the orientation, to the south, are fixed depending on the latitude and trying to optimize it to the maximum using the recommendations of the corresponding ISO standard.

The most usual solar cell is made of silicon and configured as a large pn junction area. A simplification of this type of plates can be considered as a n-type silicon layer directly in contact with a layer of p-type silicon.

In practice, the pn junctions of the solar cells are not made in the above manner, rather, they are made by diffusing a type of dopant on one of the faces of a p-type wafer, or vice versa.

If the piece of p-type silicon is located in intimate contact with a piece of n-type silicon, diffusion of electrons from the region with high concentrations of electrons (the n-type face of the junction) to the region of low concentrations of electrons (face type p of the union).

When the electrons diffuse through the pn junction, they recombine with the holes in the p-type face. However, the diffusion of the carriers does not continue indefinitely. This separation of charges, which the diffusion itself creates, generates an electric field caused by the imbalance of the charges stopping, immediately, the subsequent flow of more charges through the union.

The electric field established through the creation of the pn junction creates a diode that allows the flow of current in only one direction through said junction. The electrons can pass from the n-type side to the interior of the p-side, and the holes can pass from the p-type side to the n-type side. This region where the electrons have diffused into the junction is called the depletion region because it contains nothing more than some mobile charge carriers. It is also known as the cargo space region.

Efficiency factors of a solar cell

Maximum power point


A plate or solar cell can operate in a wide range of voltages and current intensities.

This can be achieved by varying the resistance of the load, in the electric circuit, on the one hand, and on the other, by varying the impedance of the cell from the zero value (short-circuit value) to very high values ​​(open circuit) and can be determined the maximum theoretical power point, that is, the point that maximizes V and time versus I, or what is the same, the load for which the cell can deliver the maximum electrical power for a certain level of radiation.

The maximum power point of a photovoltaic device varies with incident lighting.

For quite large systems, an increase in the price can be justified by the inclusion of devices that measure the instantaneous power by continuous measurement of the voltage and the current intensity (and hence the transferred power).

Use this information to adjust, dynamically, and in real time, the load so that the maximum possible power is always transferred, despite the variations in light that occur during the day.

Efficiency in energy conversion

The efficiency of a solar cell ( , «eta»), is the percentage of power converted into electrical energy from the total sunlight absorbed by a panel, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, Pm, divided by the light reaching the irradiance cell (E, in W / m²), under standard conditions (STC) and the surface area of ​​the solar cell (Ac in m²).

The STC specifies a temperature of 25 ° C and an irradiance of 1000 W / m² with a spectral air mass of 1.5 (AM 1.5). This corresponds to the irradiation and spectrum of incident sunlight on a clear day on a solar surface inclined with respect to the sun at an angle of 41.81 ° above the horizontal.

This condition represents, approximately, the position of the midday sun at the spring and autumn equinoxes in the continental states of the USA with a surface oriented directly to the sun. In this way, under these conditions a typical solar cell of 100 cm2, and an efficiency of approximately 12%, is expected to be able to produce a power of 1.2 watts.

Filling factor

Another term to define the efficiency of a solar cell is the fill factor (FF), which is defined as the ratio between the maximum power point divided by the open-circuit voltage (Voc) and the short-circuit current Isc :

TONC
Nominal Operating Temperature of the Cell, defined as the temperature reached by the solar cells when the module is subjected to an irradiance of 800 W / m2 with AM 1.5 G spectral distribution, the ambient temperature is 20 ° C and the wind speed of 1 m / s.


Power and costs

On a sunny day, the Sun radiates about 1 kW / m2 to the surface of the Earth. Considering that the current photovoltaic panels have a typical efficiency between 12% -25%, this would imply an approximate production of between 120-250 W / m² depending on the efficiency of the photovoltaic panel.

On the other hand, great advances are taking place in photovoltaic technology and experimental panels already exist with yields exceeding 40%.

At mid- and northern latitudes, taking into account the diurnal cycle and atmospheric conditions, they reach the earth’s surface 100 W / m² on average in winter and 250 W / m² in summer. With a conversion efficiency of approximately 12%, one can expect to obtain 12 and 30 watts per square meter of photovoltaic cell in winter and summer, respectively.

With the current costs of electricity, 0.08 $ / kWh (USD), one square meter will generate up to 0.06 $ / day, and one km² will generate up to 30 MW, or $ 50,000 / (km².day). To compare, the depopulated Sahara extends for 9 million km², with less clouds and a better solar angle, being able to generate up to 50 MW / km², or 450 TW (terawatt) in total.

The current energy consumption of the terrestrial population is close to 12-13 TW at any given time (including petroleum derivatives, coal, nuclear and hydroelectric power).


Manufacture of conventional panels

Generally they are made of silicon, the element that is the main component of the silica, the material of the sand.

Currently, world production of photovoltaic cells is concentrated in Japan (48%), Europe (27%) and the USA (11%). The consumption of silicon in 2004 for photovoltaic applications amounted to 13,000 tons.


Uses of solar photovoltaic cells

They owe their appearance to the aerospace industry, and have become the most reliable means of supplying electrical power to a satellite or a probe in the inner orbits of the Solar System. This is thanks to the greater solar irradiation without the impediment of the atmosphere and its low weight.

On land, they are the most popular solar source in small installations or in buildings, compared to the method of heliostat mirror fields used in large solar power plants.

Together with an auxiliary battery, it is commonly used in certain low consumption applications such as buoys or devices in remote territories, or simply when the connection to a power plant is impractical.

Its use on a large scale is restricted by its high cost, both purchase and installation. So far, photovoltaic panels occupy a small portion of global energy production.

Experimentally they have been used to power cars, for example in the World solar challenge through Australia. Many yachts and land vehicles use them to charge their batteries away from the power grid.

Large-scale incentive programs, offering financial rewards such as the possibility of selling excess electricity to the public grid, have greatly accelerated the progress of solar photovoltaic cell installations in Spain, Germany, Japan, the United States and other countries .

The experience in production and installation, the technological advances that increase the efficiency of the solar cells, the economies of scale in a market that grows 40% annually, together with the increases in the prices of fossil fuels, make them start. to contemplate photovoltaics for basic electricity production, in power plants connected to the grid.

PHOTOVOLTAIC SOLAR PANELS INFORMATION
Solar panels formed with photovoltaic modules, Expo 2005 Aichi Japan, Japan.

List of applications where photovoltaic panels are used:

Centrals connected to the network.
Self-consumption systems photovoltaic.
Microwave and radio repeater stations, including internet access through Wimax.
Electrification of villages in remote areas (rural electrification).
Medical facilities in rural areas.
Electric current for country houses.
Emergency communications systems.
Monitoring systems for environmental data and water quality.
Beacons, buoys and maritime navigation beacons.
Pumping for irrigation systems, drinking water in rural areas and drinking troughs for livestock.
Beacon for aeronautical protection.
Cathodic protection systems.
Desalination systems.
Recreational vehicles powered by solar electricity captured in motion.
Railway signaling.
Systems to load the accumulators of boats.
Energy source for spacecraft.
SOS posts (emergency roadside telephones).
Parking meters
Recharge of bicycles, scooters and other electric vehicles.

No doubt there is still much to advance in the technology of solar panels, especially in terms of efficiency, but this energy is considered one of the cleanest energy sources available today, and increasingly becomes more competitive compared to other types of energy. energies, which augurs great development in the solar panel industry in the future.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

The  cost of photovoltaic solar panels  has decreased by 75% in 2016, considering  solar energy  much cheaper than other sources of energy such as oil and gas according to a report by Bloomberg Technology (December, 2016) that analyzes the energy industry.

Although there is still a lot to work on, since the profitability and amortization of  photovoltaic panels  is still long term.

The  photovoltaic technology  is perfect to start in the  electric self-consumption  and to forget the electric bill and its high costs.

There are  increasingly cheaper  photovoltaic kits that can help us save a lot of energy every month.

The  photovoltaic self-consumption in homes  is highly recommended in countries with many hours of sun and with a remarkable solar radiation.

The evolution of  photovoltaic solar energy  is so positive and with such a prosperous future that it deserves a detailed article about  everything you need to know about Solar Photovoltaic Panels .

 

1. Definition of photovoltaic solar panels

1.1. How are photovoltaic solar panels different from thermal ones?
1.2. Installation of photovoltaic solar panels

2. Buy photovoltaic solar panels

2.1. How much does a solar photovoltaic panel cost
2.2. Prices and types of photovoltaic solar panels
2.3. Photovoltaic self-consumption

3. Maintenance of photovoltaic panels

PHOTOVOLTAIC SOLAR PANELS INFORMATION

 

1. Definition of photovoltaic solar panels

The  solar energy  is  clean energy  from the sun, more specifically its solar radiation having particles called photons with different amounts of energy. And  solar photovoltaic panels  use these photons to convert it into electrical energy.

Therefore,  what is a photovoltaic solar panel?  It is a technology that, based on  solar energy,  generates clean, unlimited, and increasingly economical electricity.

And,  how does the photovoltaic panel work?  the transformation of  solar energy  into electrical energy is done through the  photovoltaic cells  (also called  solar cells ) of the  photovoltaic panels , which are made of semiconductor materials that have the power to absorb the energy coming from the sun (more specifically the particles called photons) and that through their electrons generate electrical energy.

The most recognized and highest quality material used in  solar cells  is silicon, the more pure, the higher the photon absorbs. The absorbed photons are combined with the electrons that the semiconductor material contains, thus producing electrical energy.

1.1. How are photovoltaic solar panels different from thermal ones?

P solar thermal aneles  are not the same as  photovoltaic solar panels , although they have a very similar aesthetic. The  differences of the photovoltaic panels and thermal panels  are many, but mainly it is that the  thermal plates   are used to generate sanitary hot water and heating, while the  photovoltaic panels  generate electrical energy.

1.2. Installation of photovoltaic solar panels

Before  installing a photovoltaic solar panel , we have to analyze the terrain and the needs that are to be met when installing this  renewable technology .

It is very important to make a preliminary study by a  professional or company specialized in photovoltaic energy , so that it indicates the right place and inclination for the  photovoltaic system to  be as efficient as possible.

The places where the photovoltaic solar panels are placed   should be where the most hours of sunlight and irradiation exist. The choice of a  photovoltaic panel  or another is also decisive.

Once the terrain has been analyzed and site-specific data are available, the  photovoltaic panel is installed .

In a very simple way, we could say that the  installation of a photovoltaic system  is done in this way:

  1. The  photovoltaic panel  must be placed in a structure (the structure will depend on the orientation and site where we decide to put the  photovoltaic panel )
  2. The  photovoltaic panels  are connected in series, then must be connected to the charge controller which is an electronic device that has the function of protecting the photovoltaic batteries (also called solar battery) preventing an overcharge or discharge, increasing its useful life. The charge regulator prevents more energy from accumulating when the batteries are full, and when the energy stored in the batteries is used, the regulator again allows energy to accumulate in the  photovoltaic batteries .
  3. In turn, the charge controller is connected to the  photovoltaic batteries  (which are also connected in series) from which energy is obtained for consumption. For safety, the charge controller should be as close as possible to the battery.
  4. An inverter is installed in the charge regulator when one of the electronic devices that we are going to use with this clean energy is AC. This is due to the fact that the  photovoltaic systems  are of direct current, so it has to be transformed to alternating current.

2. Buy photovoltaic solar panels

To know  what type of photovoltaic solar panel to buy , we must know all the  types of photovoltaic panel that exist, their prices, their efficiency and their quality.

The most important thing that we should know in the  purchase of a photovoltaic panel , is the need we want to cover, our budget and the land that we have to place the  photovoltaic panels , because of all this will depend on the  purchase  of the photovoltaic solar panel , because it is not to buy the best one but the one that best satisfies the demand.

Below we show the 7 most important data that we have to know about a  photovoltaic solar panel  that is known thanks to its  photovoltaic technical sheet .

  1. Number of cells and tension.  If we are going to buy a  solar battery  or we are going to carry out a photovoltaic selfconsumption project,   this data is very important. This is due to the fact that, depending on the capacity of the  photovoltaic batteries  , a solar panel  or another is chosen  , for example, for two monoblick batteries and for  self-consumption projects  , 24V photovoltaic panels  must be used  .
  2. Output power.  It is the data that determines how much electrical power the photovoltaic panel generates  . But this parameter has been calculated with solar conditions (hours of sun and solar radiation) that hardly occur in reality.
  3. Output power at nominal operating temperature.  This parameter determines the power output but in conditions that can occur in reality. However, this data is hardly going to correspond to the reality of the weather condition found in the land where a photovoltaic system will be  installed .
  4. Tolerance.  It is the data that determines how much the output power of a photovoltaic solar panel can vary  . For example, if a  100W  photovoltaic panel has a tolerance of +/- 5%, it is possible that the 100W panel could become 95W or 105W
  5. Efficiency.  It is the data that determines the power that a  photovoltaic solar panel  per square meter receives an irradiation of 100W / m2.
  6. Power temperature coefficient.  This parameter determines the power lost by the  photovoltaic module  for each degree received, provided it is above 25ºC.
  7. Nominal operating temperature of the cell (NOCT).  The less the photovoltaic panel overheats  , the better it will work. This data determines the temperature of the  cells of the photovoltaic module  and generally the parameters used for this data are an ambient temperature of 20 ° and an irradiation of 800W.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

 

Once these 7 important parameters of the  photovoltaic panels are known,  mainly reflected in their  photovoltaic technical sheet , we will explain the  prices and types of photovoltaic panels  that exist and which we should opt depending on the situation.

 

2.1. How much does a photovoltaic solar panel cost? 

How much does a photovoltaic panel cost?  The  price of a photovoltaic solar panel  is difficult to give lightly, since it depends on many variables such as its size, its  efficiency , the  kind of  photovoltaic panel  it is, and even the year of its purchase, as they pass Over the years,  photovoltaic solar panels are cheaper .

As we explained at the beginning of the article, the  cost of photovoltaic panels is  becoming cheaper, because their development is progressing and new ways of  manufacturing  cheaper photovoltaic panels are increasingly being discovered .

Below is a graph of the evolution of  photovoltaic  and wind prices  in the Boomberg Technology report (December, 2016) and its evolution.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

 

2.2. Prices and types of photovoltaic solar panels

There are 3  types of photovoltaic panels  that differ mainly for their quality, manufacture and price.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

 

Types of photovoltaic panel. Source: Solar panels

Silicon monocrystalline photovoltaic solar panel

It is the  best solar photovoltaic panel  because it has the purest silicon, although it is also the most expensive because it has a different manufacturing process, where cylindrical blocks of very pure silicon are obtained. They are easy to distinguish because their  photovoltaic cells  are cut on their 4 sides to reduce the costs of their manufacture.

It is the  photovoltaic panel  that works best on days when it is not very sunny, so your purchase is extremely important in some areas where there is not much  solar energy  available.

The  monocrystalline panels  are recommended when the project has a tight budget and high energy demand.

Silicon polycrystalline photovoltaic solar panel

This  photovoltaic panel  also has silicon, but it is not so pure, so its efficiency is lower. The  prices of this solar panel  are lower than the monocrystalline silicon. As a general rule, the  best photovoltaic panel  for the home that is recommended are  polycrystalline panels .

Its manufacturing process is less expensive than that of monocrystalline panels, the silicon is cast in a square mold, cooled there and split into sheets. This manufacturing gives a perfectly rectangular appearance to this  type of  photovoltaic panels  that makes them differentiate themselves from the rest.

Thin-film photovoltaic panel

It is the  cheap photovoltaic solar panel , the  low cost photovoltaic . The low cost is due to the fact that it has lower quality materials and easier manufacturing.

They are the  most economical solar panels  but at the same time the least efficient, however, the  purchase of this cheap photovoltaic panel  is recommended when a lot of land is available.

 

2.3. Photovoltaic self-consumption

The  photovoltaic own consumption  is also highly recommended, especially in areas with lots of sunshine and a lot of sunlight.

There are two types of photovoltaic self –  consumption .

Photovoltaic self-consumption connected to the network

It is a  photovoltaic self-consumption installation  where the  photovoltaic system is interconnected to the network, it  does not suppose the total supply of a dwelling (generally), but of a part.

By remaining connected to the network, the electricity network can be used when stored photovoltaic energy is not available  . However, when we have   fully filled photovoltaic batteries and more  photovoltaic energy is produced , it will be invested in the network so that other users can use it. This transfer of energy in some countries is remunerated and in others it is not.

Photovoltaic self-consumption isolated

The  isolated consumption  is the complete self – supply of housing, therefore dissociates itself from the traditional power grid because there is no need to stay connected.

In this case we completely forget the electricity bill, as a general rule a house of these characteristics not only has some  photovoltaic panels , but other elements of  energy efficiency , such as having a  thermal insulation system  or even another source of  energy. renewable energy .

The  isolated self-consumption kits  are very varied depending on our energy needs, but a  self-consumption photovoltaic kit  that produces 1400 W / day in summer and in winter 7000 W / day would cost between € 9,500 and € 11,000 (these watts per day depend a lot on radiation solar and the hours of sun). For example, a  kit for self-consumption photovoltaic  for € 9.341 with 24 solar batteries, inverter, 9 polycrystalline solar panels and electrical components that are used for installation.

PHOTOVOLTAIC SOLAR PANELS INFORMATION

 

 

But, how do we know that  solar photovoltaic kit  suits us for  photovoltaic self-consumption ? It is necessary to do a previous study, to know the  solar energy  that can be obtained in the land and the electrical demand that must be satisfied. Only in this way can we obtain the photovoltaic solar energy  that best suits our circumstances.

To give us an idea of ​​the energy consumed by a house and the  photovoltaic kit  we need, the IDAE (Institute for Diversity and Energy Saving) has conducted a study in 2016 that reveals that a Spanish house uses approximately 9,922 kwh / h.

This figure in single-family homes in isolated houses doubles to 15,513 kWh of annual average, while the floors or blocks of housing are in approximately 7,544 kWh.

 

4.Maintenance of photovoltaic panels

The choice and  purchase of the photovoltaic panel  is not the only important thing, so is its maintenance. The more we take care of these small details, the more possibilities we have of maximizing the useful life of  the photovoltaic solar panel before the  solar panel is recycled .

The cleaning of the  photovoltaic cells  can be done simply by exposing the  photovoltaic panel  to rain and wind, thus eliminating dirt and impurities. However, acid rain and sea wind do not clean the  solar  panels , but rather damage them.

Maintenance and cleaning can be done by the end user simply with clear water and completely free of residues, under no circumstances use solvent or detergent. Cold water is not useful as it can irreversibly damage the  photovoltaic solar panel  or pressurized water.

The frequency at which cleaning and maintenance is carried out depends very much on the place where the solar installation is located . For example, in areas near the sea, it should be carried out more frequently by the impurities of the sand and salt. While if the  photovoltaic panels  are close to vegetation, we must be more careful, because through the wind can accumulate leaves in the  photovoltaic panels  that limit their efficiency.

There is also a  company specializing in maintenance of photovoltaic solar panels  that have this homologated service that can perform a very professional cleaning and maintenance.

Many times you opt for a company because you can lose the legal guarantee of  the photovoltaic equipment . Below is a video of how the solar panels are cleaned by the   Kärcher company