Photovoltaic Solar Energy

Monocrystalline solar cell during manufacture.

Sustainable homes powered by photovoltaic solar energy in the Vauban solar district (Friburg, Germany).

Mapmundi Solar radiation. The small points on the map show the total area of photovoltaic needed to cover the global demand for energy using solar panels with an efficiency of 8 %.

Photovoltaic solar energy is an energy source that produces electricity from renewable sources, obtained directly from solar radiation through a semiconductor device called a photovoltaic cell, or through a deposition of metals on a substrate called a thin-film solar cell.

This type of energy is mainly used to produce electricity on a large scale through distribution networks, although it can also power countless applications and autonomous devices, as well as supply mountain refuges or homes isolated from the electricity grid. Due to the growing demand for renewable energy, the manufacture of solar cells and photovoltaic installations has advanced considerably in recent years. They began to be mass-produced in the year 2000, when German environmentalists and the Eurosolar organization obtained funding for the creation of ten million solar roofs.

Economic incentive programs, first, and later photovoltaic self-consumption systems and net balance without subsidies, have supported the installation of photovoltaics in a large number of countries. Thanks to this, photovoltaic solar energy has become a the third most important renewable energy source in terms of global installed capacity, after hydroelectric and wind energy. At the end of 2018 the total installed power worldwide reached 500 GW of photovoltaic power, and only in 2018 100 GW were installed.

Photovoltaic energy does not emit any type of pollution during its operation, helping to avoid the emission of greenhouse gases. Its main drawback is that its production depends on solar radiation, so if the cell is not is aligned perpendicular to the Sun, between 10-25% of the incident energy is lost. Due to this, the use of solar trackers to maximize energy production has become popular in grid-connected plants. Production is also affected by adverse weather conditions, such as lack of sun, clouds or dirt that it is deposited on the panels. This implies that to guarantee the electricity supply it is necessary to complement this energy with other manageable energy sources such as plants based on the burning of fossil fuels, hydroelectric energy or nuclear energy.

Thanks to technological advances, sophistication, and economies of scale, the cost of solar photovoltaics has steadily decreased since the first commercial solar cells were manufactured, while increasing efficiency, and achieving that its average cost of electricity generation is already competitive with conventional energy sources in a growing number of geographic regions, reaching grid parity. Currently the cost of electricity produced in solar installations is between $0.05-0.10/kWh in Europe, China, India, South Africa and the United States. In 2015, new records were reached in projects in the United Arab Emirates ($0.0584/kWh), Peru ($0.048/kWh) and Mexico ($0.048/kWh). In May 2016, a solar auction in Dubai reached a price of $0.03/kWh. In 2020, a record high of $0.016/kWh was reached in Saudi Arabia.

History

The French physicist Alexandre-Edmond Becquerel was the discoverer of the photovoltaic effect in 1839, fundamental for the development of photoelectric cells.

Electrical field scheme created in a photovoltaic cell by pn binding between two layers of doped semiconductors.

Basic structure of a silicon-based solar cell, and its operating principle.

The term "photovoltaic" came into use in the United Kingdom in 1849. It comes from the Greek φώς: phos, meaning "light", and from -voltaic, which comes from the field of electricity, in honor of the Italian physicist Alejandro Volta.

The photovoltaic effect was first recognized some ten years earlier, in 1839, by the French physicist Alexandre-Edmond Becquerel, but the first solar cell was not made until 1883. Its creator was Charles Fritts, who coated a semiconductor selenium sample with gold leaf to form the junction. This primitive device had an efficiency of less than 1%, but it demonstrated in a practical way that it was indeed possible to produce electricity with light. Studies carried out in the XIX by Michael Faraday, James Clerk Maxwell, Nikola Tesla and Heinrich Hertz on electromagnetic induction, electric forces and electromagnetic waves, and above all, the work done by Albert Einstein in 1905, for which he was awarded awarded the Nobel Prize in 1921, they provided the theoretical and practical basis for the photoelectric effect, which is the foundation of the conversion of solar energy into electricity.

Principle of operation

When a doped semiconductor is exposed to electromagnetic radiation, an incident photon strikes an electron and tears it away, creating a hole in the atom. Normally, the electron quickly finds another hole to refill, and the energy provided by the photon is therefore dissipated as heat. The principle of a photovoltaic cell is to force the electrons and the holes to advance towards the opposite side of the material instead of simply recombining in it: thus, a potential difference will be produced, and therefore, tension between the two parts of the material, as occurs in a pile.

To do this, a permanent electric field is created, through a pn junction, between two doped layers respectively, p and n. In silicon cells, which are mostly used, there are therefore:

• The upper layer of the cell, which consists of silicon doped type n. In this layer, there is a number of free electrons greater than in a layer of pure silicon, hence the name of doping n, negative. The material remains electrically neutral, since both silicon atoms and doping material are neutral: but the crystalline network has a global greater presence of electrons than in a pure silicon network.
• The lower layer of the cell, which is composed of type p doped silicon. This layer therefore has an average amount of free electrons less than a layer of pure silicon. The electrons are linked to the crystalline network which, consequently, is electrically neutral, but has hollowspositive (p). The electrical conduction is ensured by these load carriers, which move throughout the material.

At the time of creation of the pn junction, free electrons from the n shell instantly enter the p shell and recombine with holes in the p region. Thus, during the whole life of the union, there will be a positive charge in the region n along the union (because there are missing electrons) and a negative charge in the region at p a along the join (because the gaps have disappeared); the set forms the «Space Charge Zone» (ZCE) and there is an electric field between the two, from n to p. This electric field makes the ZCE a diode, which only allows current flow in one direction: electrons can move from region p to region n, but not in the opposite direction, and on the contrary holes they only pass from n to p.

In operation, when a photon tears an electron from the matrix, creating a free electron and a hole, under the effect of this electric field each one goes in the opposite direction: the electrons accumulate in the n region (to become the negative pole), while the holes accumulate in the p-doped region (which becomes the positive pole). This phenomenon is most effective in the ZCE, where there are almost no charge carriers (electrons or holes), since they are cancelled, or in the immediate vicinity of the ZCE: when a photon creates an electron pair -hollow, they split apart and are unlikely to find their opposite, but if creation takes place at a site further from the junction, the electron (turned hole) maintains a great opportunity to recombine sooner to reach zone no. But the ZCE is necessarily very thin, so it is not useful to give a large thickness to the cell. Indeed, the thickness of the n layer is very small, since this layer is only basically needed to create the ZCE that runs the cell. cell. On the other hand, the thickness of the p layer is greater: it depends on a compromise between the need to minimize electron-hole recombinations, and, on the contrary, allow the capture of the greatest possible number of photons, for which a certain minimum thickness is required.

In short, a photovoltaic cell is the equivalent of a power generator to which a diode has been added. To achieve a practical solar cell, it is also necessary to add electrical contacts (which allow the energy generated to be extracted), a layer that protects the cell but allows light to pass through, an anti-reflective layer to guarantee the correct absorption of photons, and other elements that increase its efficiency.

First modern solar cell

The American engineer Russell Ohl patented the modern solar cell in 1946, although other researchers had advanced in its development earlier: the Swedish physicist Sven Ason Berglund had patented in 1914 a method that tried to increase the capacity of solar cells. photosensitive cells, while in 1931, German engineer Bruno Lange had developed a photocell using silver selenide instead of copper oxide.

The modern era of solar technology did not arrive until 1954, when American researchers Gerald Pearson, Calvin S. Fuller, and Daryl Chapin of Bell Laboratories accidentally discovered that semiconductors made of silicon doped with certain impurities were very sensitive to light. These advances contributed to the manufacture of the first commercial solar cell. They employed a diffuse p–n silicon junction, with a solar energy conversion of approximately 6%, an achievement compared to selenium cells that barely reached 0.5%.

Later, the American Les Hoffman, president of the company Hoffman Electronics, through its semiconductor division was one of the pioneers in the manufacture and large-scale production of solar cells. Between 1954 and 1960, Hoffman managed to improve the efficiency of photovoltaic cells to 14%, reducing manufacturing costs to achieve a product that could be marketed.

The International Space Station, which obtains its energy through photovoltaic panels, photographed against the blackness of space and the thin line of the Earth's atmosphere.

Details of the photovoltaic solar panels of the International Space Station.

First applications: space solar power

In the beginning, photovoltaic cells were used in a minority way to power toys and other minor uses, since the cost of producing electricity using these primitive cells was too high: in relative terms, a cell that produced one watt Solar power could cost $250, compared to $2-3 for a watt from a coal-fired power plant.

Photovoltaic cells were rescued from oblivion thanks to the space race and the suggestion of using them in one of the first satellites put into orbit around the Earth. The Soviet Union launched its first space satellite in 1957, and the United States would follow a year later. The first spacecraft to use solar panels was the US Vanguard 1 satellite, launched in March 1958 (now the oldest satellite still in orbit). Solar cells created by Peter Iles in an effort spearheaded by the company Hoffman Electronics were used in its design. The photovoltaic system allowed it to continue transmitting for seven years while the chemical batteries were depleted in just 20 days.

In 1959, the United States launched Explorer 6. This satellite had installed a series of solar modules, supported on external structures similar to wings, made up of 9,600 solar cells from the Hoffman company. subsequently a common feature of many satellites. There was some initial skepticism about how the system would work, but in practice the solar cells proved to be a great success, and they were soon incorporated into the design of new satellites.

A few years later, in 1962, Telstar became the first communications satellite equipped with solar cells, capable of providing a power of 14 W. This milestone generated great interest in the production and launch of geostationary satellites for the development of communications, in which the energy would come from a device for capturing sunlight. It was a crucial development that stimulated research by some governments and prompted the improvement of photovoltaic panels. Gradually, the space industry turned to the use of gallium arsenide (GaAs) solar cells, due to their higher efficiency compared to silicon cells. In 1970 the first highly efficient gallium arsenide heterostructure solar cell was developed in the Soviet Union by Zhores Alfiorov and his research team.

Starting in 1971, the Soviet space stations of the Salyut program were the first manned orbital complexes to obtain their energy from solar cells, attached in structures to the sides of the orbital module, like the North American Skylab station, a few years later.

In the 1970s, after the first oil crisis, the United States Department of Energy and the space agency NASA began studying the concept of solar energy in space, which sought to supply terrestrial energy through space satellites. In 1979 they proposed a fleet of satellites in geostationary orbit, each of which would measure 5 x 10 km and produce between 5 and 10 GW. The construction implied the creation of a large space factory where hundreds of astronauts would continuously work. This gigantism was typical of a time when the creation of large space cities was projected. Technical difficulties aside, the proposal was scrapped in 1981 as insanely expensive. In the mid-1980s, with oil prices low again, the program was cancelled.

However, photovoltaic applications in space satellites continued to develop. The production of Metal Organic Chemical Vapor Deposition (MOCVD) equipment did not develop until the 1980s, limiting the ability of companies to manufacture solar cells. of gallium arsenide. The first company that manufactured solar panels in industrial quantities, from simple GaAs junctions, with an efficiency of 17% in AM0 (zero air mass), was the North American Applied Solar Energy Corporation (ASEC). Double junction cells began their production in industrial quantities by ASEC in 1989, accidentally, as a consequence of a switch from GaAs on GaAs substrates to GaAs on germanium substrates.

Photovoltaic technology, although it is not the only one used, continues to predominate at the beginning of the XXI century in the Earth-orbiting satellites. For example, NASA's Magellan, Mars Global Surveyor, and Mars Observer probes used photovoltaic arrays, as did the Earth-orbiting Hubble Space Telescope. The International Space Station, also in Earth orbit, is equipped with large photovoltaic systems that power the entire space complex, just like the Mir space station in its day. Other space vehicles that use photovoltaic energy to supply themselves are the probe Mars Reconnaissance Orbiter, Spirit and Opportunity, NASA's robots on Mars.

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  Illustration of the Mars Reconnaissance Orbiter probe, equipped with solar panels, in the orbit of Mars.

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  Artistic image of the Juno space probe, equipped with photovoltaic modules, orbiting the planet Jupiter.

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  The Hubble Space Telescope, equipped with solar panels, is placed in orbit from the Discovery ferry cellar in 1990.

Launched in 2004 into orbit toward a comet as far from the Sun as the planet Jupiter (5.25 AU), the Rosetta spacecraft also has solar panels; previously, the furthest use of space solar power had been that of the Stardust probe, to 2 AU. Photovoltaics have also been used successfully on the European unmanned mission to the Moon, SMART-1, powering its Hall effect thruster. The Juno space probe is the first mission to Jupiter to use photovoltaic panels instead of a radioisotope thermoelectric generator, traditionally used in space missions outside the Solar System. The potential of photovoltaics to equip spacecraft orbiting beyond Jupiter is currently being studied.

First land applications

Insulated applications of the electricity grid were one of the first terrestrial uses of solar photovoltaic energy, contributing to a great extent to its development. In the picture, Noup Head lighthouse in the UK.

Since its appearance in the aerospace industry, where it has become the most reliable means of supplying electrical power to space vehicles, photovoltaic solar energy has developed a large number of terrestrial applications. The first commercial installation of this type was carried out in 1966, at the Ogami Island lighthouse (Japan), allowing the use of flare gas to be replaced by a renewable and self-sufficient electrical source. It was the first lighthouse in the world to be powered by solar photovoltaics, and was instrumental in demonstrating the feasibility and potential of this energy source.

Improvement occurred slowly over the next two decades, with the only widespread use being in space applications, where its power-to-weight ratio was greater than that of any competing technology. However, this success was also the reason for its slow growth: the aerospace market was willing to pay any price to get the best possible cells, so there was no reason to invest in lower cost solutions if this reduced efficiency. Instead, the price of cells was largely determined by the semiconductor industry; their migration to integrated circuit technology in the 1960s led to the availability of larger ingots at relatively lower prices. As its price fell, the price of the resulting photovoltaic cells fell by the same amount. However, the cost reduction associated with this increasing popularization of photovoltaics was limited, and in 1970 the cost of solar cells was still estimated at $100 per watt ($/Wp).

Price reduction

Swanson Act, the learning curve of the solar cell from 1976 to 2019

In the late 1960s, American industrial chemist Elliot Berman was investigating a new method for producing the raw material silicon from a tape process. However, he found little interest in his project and was unable to obtain the necessary funding for his development. Later, in a chance meeting, he was introduced to a team from the Exxon oil company who were looking at 30-year strategic projects. The group had come to the conclusion that electrical energy would be much more expensive in the year 2000, and considered that this price increase would make new alternative energy sources more attractive, solar energy being the most interesting among these. In 1969, Berman joined the Exxon laboratory in Linden, New Jersey, called Solar Power Corporation (SPC).

His effort was first directed at analyzing the potential market to identify potential uses for this new product, and he quickly discovered that if the cost per watt were reduced from $100/Wp to around $20/Wp Wp a major lawsuit would arise. Realizing that the "silicon on tape" concept could take years to develop, the team began looking for ways to bring the price down to $20/Wp using existing materials. The realization that existing cells were based on the standard semiconductor manufacturing process was a first step forward, even if it was not an ideal material. The process began with the formation of a silicon ingot, which was cut crosswise into discs called wafers. Subsequently, the wafers were polished and then, for their use as cells, they were coated with an anti-reflective layer. Berman realized that rough cut wafers already had a perfectly valid anti-reflective front surface, and by printing the electrodes directly onto this surface, two important steps in the cell manufacturing process were eliminated.

His team also explored other ways to improve cell assembly on arrays, eliminating expensive materials and manual wiring previously used in space applications. His solution was to use printed circuit boards on the back, acrylic plastic on the front, and silicone glue in between, embedding the cells. Berman realized that existing silicon on the market was already "good enough" for use in solar cells. The small imperfections that could ruin a silicon ingot (or individual wafer) for use in electronics would have little effect in solar applications. Photovoltaic cells could be manufactured from material discarded by the electronics market, which would result in a great improvement in their price.

Putting all these changes into practice, the company began buying rejected silicon from existing manufacturers at very low cost. By using the largest wafers available, reducing the amount of wiring for a given panel area, and packaging them into panels with their new methods, by 1973 SPC was producing panels at $10/Wp and selling them at $20/Wp. Wp, reducing the price of photovoltaic modules to a fifth in just two years.

The maritime navigation market

Maritime Boya operated by the United States National Oceanic and Atmospheric Administration (NOAA).

SPC began to contact the manufacturers of navigation buoys offering them the product, but came across a curious situation. The main company in the sector was Automatic Power, a manufacturer of disposable batteries. Realizing that solar cells could eat into business and profits from the battery industry, Automatic Power bought a solar prototype from Hoffman Electronics only to end up cornering it.. Seeing no interest from Automatic Power, SPC then turned to Tideland Signal, another battery supply company formed by former Automatic Power managers. i>. Tideland introduced a photovoltaic powered buoy to the market and was soon ruining the Automatic Power business.

The timing couldn't be better, the rapid increase in the number of offshore oil rigs and other cargo facilities produced a huge market among oil companies. As Tideland had been successful, Automatic Power then began procuring its own supply of photovoltaic solar panels. They found Bill Yerkes of Solar Power International (SPI) in California, who was looking for a market to sell his product. SPI was soon acquired by one of its largest clients, the oil giant ARCO, forming ARCO Solar. The ARCO Solar factory in Camarillo (California) was the first dedicated to the construction of solar panels, and it was in continuous operation from its purchase by ARCO in 1977 until 2011 when it was closed by the SolarWorld company.

This situation was compounded by the 1973 oil crisis. Oil companies now had huge funds at their disposal due to their huge revenues during the crisis, but they were also well aware that their future success would depend on some other source of energy. In the following years, the big oil companies began the creation of a series of solar energy companies, and were for decades the largest producers of solar panels. ARCO, Exxon, Shell, Amoco (later acquired by BP), and Mobil companies maintained large solar divisions through the 1970s and 1980s. Technology companies also made significant investments, including General Electric, Motorola, IBM, Tyco, and RCA..

Perfecting Technology

Electric vehicle powered by photovoltaic energy, victor South African Solar Challenge.

In the decades since Berman's advances, improvements have brought production costs below $1/Wp, with prices less than $2/Wp for the entire PV system. The price of the rest of the elements of a photovoltaic installation now represents a higher cost than the panels themselves.

As the semiconductor industry developed toward larger and larger ingots, older equipment became available at reduced prices. The cells grew in size when this old equipment became available on the surplus market. The first ARCO Solar panels were equipped with cells from 2 to 4 inches (51 to 100 mm) in diameter. Panels in the 1990s and early 2000s generally incorporated 5-inch (125 mm) cells, and since 2008 almost all new panels have used 6-inch (150 mm) cells. Also the widespread introduction of televisions Flat panel display in the late 1990s and early 2000s led to the widespread availability of large, high-quality sheets of glass, which are used on the front of the panels.

In terms of the cells themselves, there has only been one major change. During the 1990s, polysilicon cells became increasingly popular. These cells offer less efficiency than monosilicon cells, but are grown in large vats that greatly reduce the cost of production. By the mid-1990s By 2000, polysilicon dominated the low-cost panel market.

Applications of photovoltaic solar energy

Photovoltaic solar power meter in Edinburgh, UK.

Basic solar calculator Sharp.

Mountain refugio fueled by photovoltaic energy, in the National Park of Aigüestortes and Lago de San Mauricio (Pyrenees, Spain).

The large-scale industrial production of photovoltaic panels took off in the 1980s, and its many uses include:

Telecommunications and signaling

Solar photovoltaic energy is ideal for telecommunications applications, including local telephone exchanges, radio and television masts, microwave repeater stations, and other types of electronic communication links. This is because, in most telecommunications applications, storage batteries are used and the electrical installation is normally carried out in direct current (DC). In hilly and mountainous terrain, radio and television signals may be interfered with or reflected due to undulating terrain. In these locations, low power transmitters (LPT) are installed to receive and retransmit the signal among the local population.

Photovoltaic cells are also used to power emergency communications systems, for example in SOS (Emergency Telephone) posts on highways, railway signaling, beacons for aeronautical protection, weather stations or environmental data monitoring systems and of water quality.

Isolated devices

The reduction in the energy consumption of integrated circuits, made possible in the late 1970s the use of solar cells as a source of electricity in calculators, such as the Royal Solar 1, Sharp EL-8026 or Teal Photon.

Also, other fixed devices that use photovoltaic energy have seen their use increase in recent decades, in places where the cost of connecting to the electrical network or the use of disposable batteries is prohibitively expensive. These applications include, for example, solar lamps, solar streetlights, water pumps, parking meters, emergency telephones, trash compactors, temporary or permanent traffic signals, charging stations, or remote surveillance systems.

Rural electrification

In isolated environments, where little electrical power is required and access to the network is difficult, photovoltaic panels have been used as an economically viable alternative for decades. To understand the importance of this possibility, it should be taken into account that approximately a quarter of the world's population still does not have access to electricity.

In developing countries, many villages are located in remote areas, several kilometers from the nearest power grid. Due to this, photovoltaic energy is increasingly being incorporated to provide electricity supply to homes or medical facilities in rural areas. For example, in remote parts of India a rural lighting program has provided lighting using solar-powered LED lamps to replace kerosene lamps. The price of the solar lamps was about the same as the cost of supplying kerosene for a few months. Cuba and other Latin American countries are working to provide photovoltaic energy in areas far from conventional electricity supply. These are areas in where the social and economic benefits for the local population offer an excellent reason to install photovoltaic panels, although usually these types of initiatives have been relegated to specific humanitarian efforts.

Pumping systems

Photovoltaic pumping systems can be used to provide water in irrigation systems, drinking water in isolated communities or livestock abbreviations.

Photovoltaics are also used to power pumping facilities for irrigation systems, drinking water in rural areas and troughs for cattle, or for water desalination systems.

Photovoltaic pumping systems (as well as those powered by wind energy) are very useful where it is not possible to access the general electricity network or it is prohibitively expensive. Their cost is generally cheaper due to their lower operating and maintenance costs, and have a lower environmental impact than pumping systems powered by internal combustion engines, which also have less reliability.

The pumps used can be both alternating current (AC) and direct current (DC). Normally direct current motors are used for small and medium applications of up to 3 kW of power, while for larger applications alternating current motors are used coupled to an inverter that transforms the direct current from the photovoltaic panels for use. This allows sizing systems from 0.15 kW to more than 55 kW of power, which can be used to supply complex irrigation or water storage systems.

Solar-diesel hybrid systems

Due to the decrease in the cost of photovoltaic solar energy, the use of hybrid solar-diesel systems is also spreading, which combine this energy with diesel generators to produce electricity in a continuous and stable manner. These types of facilities are equipped normally with auxiliary equipment, such as batteries and special control systems to achieve the stability of the electrical supply of the system at all times.

Due to its economic feasibility (transporting diesel to the point of consumption is usually expensive) in many cases old generators are replaced by photovoltaics, while new hybrid installations are designed in such a way that they allow the use of solar resources as long as is available, minimizing the use of generators, thus decreasing the environmental impact of power generation in remote communities and at facilities that are not connected to the power grid. An example of this is constituted by mining companies, whose exploitations are normally located in the open field, far from large population centers. In these cases, the combined use of photovoltaics makes it possible to greatly reduce dependence on diesel fuel, allowing savings of up to 70% in the cost of energy.

This type of system can also be used in combination with other sources of renewable energy generation, such as wind power.

Transportation and maritime navigation

Equipment Nuna 3, competing solar vehicle in the World Solar Challenge.

Although photovoltaics is not yet widely used to provide traction in transportation, it is increasingly being used to provide auxiliary power in ships and automobiles. Some vehicles are equipped with air conditioning powered by photovoltaic panels to limit the interior temperature on hot days, while other hybrid prototypes use them to recharge their batteries without having to connect to the electricity grid. The possibility has been well demonstrated. practice of designing and manufacturing vehicles powered by solar energy, as well as ships and airplanes, road transport being considered the most viable for photovoltaics.

The Solar Impulse is a project dedicated to the development of an airplane powered solely by photovoltaic solar energy. The prototype can fly during the day powered by the solar cells that cover its wings, while charging the batteries that allow it to stay airborne at night.

Solar energy is also commonly used in lighthouses, buoys and beacons for maritime navigation, recreational vehicles, charging systems for electric batteries on ships, and cathodic protection systems. Electric vehicle charging is becoming increasingly important.

Building-integrated photovoltaics

Solar Marquesin located in the car park of the Universidad Autónoma de Madrid (Madrid, Spain).

Many photovoltaic installations are often located in buildings: usually they are placed on an existing roof, or they are integrated into elements of the building's own structure, such as skylights, skylights or facades.

Alternatively, a photovoltaic system can also be located physically separate from the building, but connected to the building's electrical installation to supply energy. As of 2010, more than 80% of the 9,000 MW of PV Germany had in operation at the time had been installed on rooftops.

Building-integrated photovoltaics (BIPV) are increasingly being incorporated as a primary or secondary electrical power source in new residential and industrial buildings, and even in other architectural elements, such as bridges. Tiles with integrated photovoltaic cells are also quite common in this type of integration.

According to a study published in 2011, the use of thermal imaging has shown that solar panels, as long as there is an open gap through which air can circulate between the panels and the roof, provide a passive cooling effect on the buildings during the day and also help keep the heat accumulated during the night.

Grid connection photovoltaics

One of the main applications of photovoltaic solar energy, which has been most developed in recent years, consists of power plants connected to the grid for electricity supply, as well as photovoltaic self-consumption systems, of generally lower power, but equally connected to the power grid.

Floating photovoltaics

Although solar panels are usually installed on land, it is possible to install them floating on the water of reservoirs or calm lakes. Although it is more expensive, it has many advantages: it reduces the losses due to evaporation of the dammed water, it improves its quality (because it grows less algae), the installation is simpler, the cooling of the panels themselves is facilitated (thus increasing the energy they produce) and it is an alternative way for hydroelectric dams to generate electricity, without wasting the water they store or occupying additional land.

Components of a photovoltaic solar plant

A photovoltaic solar plant has different elements that allow it to function, such as photovoltaic panels for capturing solar radiation, and inverters for transforming direct current into alternating current. There are others, the most important they are mentioned here below:

Photovoltaic solar panels

Photovoltaic cell.

Generally, a photovoltaic module or panel consists of an association of cells, encapsulated in two layers of EVA (ethylene-vinyl-acetate), between a front sheet of glass and a back layer of a thermoplastic polymer (frequently the tedlar) or another sheet of glass when you want to obtain modules with some degree of transparency. Very often this set is framed in an anodized aluminum structure in order to increase the mechanical resistance of the set and facilitate the anchoring of the module to the structures support.

The cells most commonly used in photovoltaic panels are made of silicon, and can be divided into three subcategories:

• Monocrystalline silicon cells are made up of a single silicon crystal, usually manufactured by the Czochralski process. This type of cells has a uniform dark blue color.
• Polycrystalline silicon cells (also called multicrystalline) are made up of a set of silicon crystals, which explains that their performance is somewhat lower than that of monocrystalline cells. They are characterized by a more intense blue color.
• The amorphous silicon cells. They are less efficient than crystalline silicon cells but also less expensive. This type of cells is, for example, the one used in solar applications such as clocks or calculators.

The current versus voltage curve of a module gives us useful information about its electrical performance. Manufacturing processes often cause differences in the electrical parameters of different PV modules, even cells of the same type. Therefore, only the experimental measurement of the I–V curve allows us to accurately establish the electrical parameters of a photovoltaic device. This measurement provides very relevant information for the design, installation and maintenance of photovoltaic systems. Generally, the electrical parameters of PV modules are measured by indoor testing. However, the outdoor test has important advantages such as no costly artificial light source is required, no limitation on sample size, and more homogeneous illumination of samples.

The performance of a photovoltaic (PV) module depends on the environmental conditions, mainly the global incident irradiance G in the plane of the module. However, the p–n junction temperature T also influences the main electrical parameters: the short-circuit current ISC, the open-circuit voltage VOC and the maximum power Pmax. In general, it is known that VOC shows a significant inverse correlation with T, while for ISC this correlation is direct, but weaker, so that this increase does not compensate for the decrease in VOC. As a consequence, Pmax decreases when T increases. This correlation between the output power of a solar cell and the working temperature of its junction depends on the semiconductor material, and is due to the influence of T on the concentration, lifetime, and mobility of the intrinsic carriers, that is, electrons and holes. inside the photovoltaic cell.

Sensitivity to temperature is usually described by temperature coefficients, each of which expresses the derivative of the parameter to which it refers with respect to the junction temperature. The values of these parameters can be found in any PV module data sheet; are the following:

- β: Coefficient of variation of VOC with respect to T, given by ∂VOC/∂T.

- α: ISC variation coefficient with respect to T, given by ∂ISC/∂T.

- δ: Coefficient of variation of Pmax with respect to T, given by ∂Pmax/∂T.

Techniques for estimating these coefficients from experimental data can be found in the literature.

Investors

A solar investor installed on a network connection plant in Speyer, Germany.

The direct electrical current provided by the photovoltaic modules can be transformed into alternating current by means of an electronic device called an inverter and injected into the electrical network (for sale of energy) or into the internal network (for self-consumption).

The process, simplified, would be the following:

• Energy is generated at low voltages (380-800 V) and in continuous current.
• It is transformed with an inverter in alternating current.
• Power plants below 100 kW inject energy directly into the low voltage distribution network (400 V in triphasic or 230 V in monophasic).
• And for powers above 100 kW a transformer is used to raise energy to medium voltage (up to 36 kV) and is injected into the transport networks for its subsequent supply.

In the initial stages of the development of photovoltaic inverters, the requirements of the operators of the electrical networks to which they were connected requested only the contribution of active energy and the disconnection of the inverter from the network if it exceeded certain limits voltage and frequency. With the progressive development of this equipment and the increasing importance of smart power grids, inverters are already capable of providing reactive power and even providing stability to the power grid.

Solar trackers

Solar plant located at the Nellis Air Force Base (Nevada, United States). These panels follow the tour of the Sun on an axis.

The use of trackers with one or two axes allows a considerable increase in solar production, around 30% for the former and an additional 6% for the latter, in places with high direct radiation.

Solar trackers are quite common in photovoltaic applications. There are several types:

• In two axes: the surface is always perpendicular to the Sun.
• In a polar axis: the surface rotates on a south-facing axis and inclines an angle equal to latitude. The spin is adjusted for the normal to the surface to coincide at all times with the Earth meridian containing the Sun.
• In an azimutal axis: the surface rotates over a vertical axis, the angle of the surface is constant and equal to latitude. The spin is adjusted for the normal to the surface to coincide at all times with the local meridian that contains the Sun.
• In a horizontal axis: the surface rotates in a horizontal axis and oriented towards north-south. The spin is adjusted for the normal to the surface to coincide at all times with the Earth meridian containing the Sun.

Wiring

Connectors of a solar panel, used to transport the continuous current generated by the same to the investor, where it is usually transformed into alternating current for subsequent use.

It is the element that transports electrical energy from its generation, for its subsequent distribution and transport. Its sizing is determined by the most restrictive criterion between the maximum allowable voltage drop and the maximum allowable intensity. Increasing the conductor sections obtained as a result of theoretical calculations provides added advantages such as:

• More downloaded lines, which extends the useful life of the cables.
• Possibility to increase the power of the plant without changing the driver.
• Better response to possible shorts.
• Improved installation performance.

Photovoltaic concentration plants

Solar follower with photovoltaic concentration panels, capable of producing 53 kW. Next you will find the Tesla Roadster electric vehicle, allowing you to appreciate its scale.

Another type of technology in photovoltaic plants are those that use a concentration technology called CPV for its acronym in English (Concentrated Photovoltaics) to maximize the solar energy received by the installation, as well as than in a solar thermal power plant. Concentrated photovoltaic installations are located in locations with high direct solar irradiation, such as the countries on both shores of the Mediterranean, Australia, the United States, China, South Africa, Mexico, etc. Until 2006, these technologies were part of the research field, but in recent years larger facilities have been launched, such as the ISFOC (Institute of Concentrating Photovoltaic Solar Systems) in Puertollano (Castilla-La Mancha) with 3 MW supplying electricity to the electrical grid.

The basic idea of photovoltaic concentration is the replacement of semiconductor material by reflective or refractive material (cheaper). The degree of concentration can reach a factor of 1000, in such a way that, given the small solar cell surface used, the most efficient technology can be used (triple junction, for example). On the other hand, the optical system introduces a loss factor that recovers less radiation than flat photovoltaics. This, together with the high precision of the monitoring systems, constitutes the main barrier to be solved by concentration technology.

The development of large plants (above 1 MW) has recently been announced. Concentration photovoltaic plants use a double axis tracker to allow maximum use of the solar resource throughout the day.

The development of photovoltaic solar energy in the world

Between 2001 and 2016 there has been an exponential growth in photovoltaic production, doubling approximately every two years. The total installed photovoltaic power in the world (connected to the grid) amounted to 16 gigawatts (GW) in 2008, 40 GW in 2010, 100 GW in 2012, 180 GW in 2014, 300 GW in 2016 and 500 GW in 2018.

Historically, the United States led the installation of photovoltaics from its inception until 1996, when its installed capacity reached 77 MW, more than any other country to date. In subsequent years, they were overtaken by Japan, which held the lead until Germany in turn overtook it in 2005, holding the lead ever since. At the beginning of 2016, Germany was close to 40 GW installed. However, around that time, China, one of the countries where photovoltaics is experiencing the fastest growth, surpassed Germany, becoming the largest producer of photovoltaic energy since then. in the world. It is expected to multiply its current installed capacity to 200 GW in 2020.

World production

Total installed capacity already accounts for a significant fraction of the electricity mix in the European Union, covering an average of 3.5% of electricity demand and reaching 7% in periods of greatest production. In some countries, such as Germany, Italy, the United Kingdom or Spain, reaches maximums of over 10%, the same as in Japan or in some sunny states in the United States, such as California. The annual production of electrical energy generated by this source of energy worldwide was equivalent in 2015 to about 184 TWh, enough to supply the energy needs of millions of homes and covering approximately 1% of the world demand for electricity.

Chinese

Market share of major photovoltaic cell producing countries between 1995 and 2013.

Photovoltaics has become one of the largest industries in the People's Republic of China. The Asian country is the world leader in photovoltaic capacity, with installed power at the beginning of 2019 of more than 170 GW. It also has some 400 photovoltaic companies, including Trina Solar, Jinko Solar and JA Solar, world giants in the manufacture of solar panels. In 2014, it produced approximately half of the photovoltaic products manufactured in the world (China and Taiwan together account for more than 60% share). The production of photovoltaic panels and cells in China has increased significantly over the last decade: in 2001 it held a share of less than 1% of the world market, while at the same time, Japan and the United States accounted for more than 70% of the total. World production. However, the trend has been reversed and China now far outperforms other producers.

Chinese solar panel production capacity nearly quadrupled between 2009 and 2011, even exceeding global demand. As a result, the European Union accused the Chinese industry of dumping, that is, selling its panels at prices below cost, imposing tariffs on the import of this material.

The installation of photovoltaic energy has developed spectacularly in the Asian country in recent years, even exceeding initial forecasts. Due to such rapid growth, the Chinese authorities have been forced to reassess their PV power target on several occasions.

The total installed capacity in China grew to 77 GW at the end of 2016, after connecting 36 GW in the last year, according to the country's official statistics. In 2017, China had exceeded the target set by the government for 2020, a photovoltaic capacity of 100 GW. For this reason, at the end of 2018 it was announced that China could raise its solar target for 2020 to more than 200 GW.

This growth reflects the steep decline in the cost of photovoltaics, which is now becoming a cheaper option than other energy sources, both at retail and commercial prices. Chinese government sources have stated that photovoltaics will present more competitive prices than coal and gas (also providing greater energy independence) by the end of this decade.

United States

Barack Obama during a visit to a photovoltaic facility on the roof of the Denver Museum of Nature and Science in February 2009.

Since 2010, the United States has been one of the countries with the greatest activity in the photovoltaic market, it has large companies in the sector, such as First Solar or SolarCity, as well as numerous grid connection plants. At the beginning of 2017, the United States had more than 40 GW of installed photovoltaic capacity, enough to provide electricity to more than 8 million homes, after doubling its solar capacity in less than two years.

Although the United States does not maintain a uniform national energy policy across the country for photovoltaics, many states have individually set renewable energy targets, including solar power in varying proportions. In this sense, California Governor Jerry Brown has signed legislation requiring that 33% of the state's electricity be generated by renewable energy by the end of 2020. These measures have been supported by the federal government with the adoption of the Investment Tax Credit (ITC), a tax exemption established in 2006 to promote the development of photovoltaic projects, and which has recently been extended until 2023.

A private report states that photovoltaic solar energy has expanded rapidly during the last 8 years, growing at an average of 40% each year. Thanks to this trend, the cost per kWh produced by photovoltaics has been greatly reduced, while the cost of electricity generated by fossil fuels has not stopped increasing. As a result, the report concludes that PV will achieve grid parity with conventional power sources in many regions of the United States by 2015. But to achieve a 10% share of the power market, the report continues, PV companies they will need to further streamline the installations so that solar energy becomes a “plug-and-play” technology. In other words, it is easy to acquire the components of each system and their interconnection is simple, as is their connection to the network.

Currently, most of the facilities are connected to the grid and use net balance systems that allow the consumption of electricity at night from energy generated during the day. New Jersey leads the States with the least restrictive net balance law, while California leads in the total number of solar-powered homes. Many of them were installed during the million solar roof initiative.

The current trend and rate of growth indicate that in the coming years a large number of photovoltaic plants will be built in the south and southwest of the country, where available land is abundant, in the sunny deserts of California, Nevada and Arizona. Companies are increasingly acquiring large areas in these areas, with the intention of building larger-scale plants.

Japan

Photovoltaics in Japan has expanded rapidly since the 1990s. The country is one of the leading manufacturers of photovoltaic modules and ranks among the top ranked in terms of installed power, with more than 23 GW at the end of 2014, most of it connected to the grid. Irradiation in Japan is optimal, standing between 4.3 and 4.8 kWh m² day, making it an ideal country for the development of this type of energy.

The sale of PV modules for commercial projects has grown rapidly after the Japanese government introduced a PV incentive fee in July 2012 following the Fukushima nuclear accident and the shutdown of most power plants nuclear power that the country has.

Most of the modules come from local manufacturers, among which Kyocera, Sharp Corporation, Mitsubishi or Sanyo stand out, while a small part are imported, according to data from the Japan Photovoltaic Energy Association ( Japan Photovoltaic Energy Association, JPA). Traditionally, the photovoltaic market has been heavily shifted to the residential segment, accounting for up to 97% of installed capacity across the country as of 2012. Although this trend is reversing, still more than 75% of the cells and modules sold in Japan at the beginning of 2012 were for residential projects, while about 9% were used in commercial PV installations.

In 2014, the total installed photovoltaic power in the country was around 23 GW, which contributed approximately 2.5% of the country's electricity demand. During the summer of 2015, it was reported that the photovoltaic production in Japan had covered at certain times 10% of the total national demand. Two years later, in 2016, it stands at around 42 GW, and the forecast indicates that the Japanese photovoltaic market will grow even more in the next few years. coming years.

Germany

At the beginning of 2016, Germany had an installed capacity of close to 40 GW. In 2011 alone, Germany installed close to 7.5 GW, and photovoltaics produced 18 TW h of electricity, 3% of the total consumed in the country.

Solar panels on the roof of several buildings, in the solar district of Vauban, in Freiburg, Germany.

The photovoltaic market in Germany has grown considerably since the beginning of the XXI century thanks to the creation of a regulated tariff for the production of renewable energy, which was introduced by the «German Renewable Energy Act», a law published in the year 2000. Since then, the cost of photovoltaic installations has fallen by more than 50% in five years, since 2006. Germany has set itself the goal of producing 35% of electricity from renewable energy by 2020 and reaching 100% by 2050.

In 2012, the tariffs introduced cost Germany around €14 billion per year for both wind and solar installations. This cost is distributed among all taxpayers through a surcharge of €3.6 cents per kWh (approximately 15% of the total cost of electricity for the domestic consumer).

The considerable installed power in Germany has set several records in recent years. During two consecutive days in May 2012, for example, the photovoltaic solar plants installed in the country produced 22,000 MWh at noon, which is equivalent to the generation power of twenty nuclear power plants working at full capacity. Germany smashed this record on July 21, 2013, with an instantaneous power of 24 GW at noon. Due to the highly distributed nature of German PV, approximately 1.3-1.4 million small PV systems contributed to this new record. Approximately 90% of the solar panels installed in Germany are located on roofs.

In June 2014, German PV again broke multi-day records, producing up to 50.6% of all electricity demand in a single day, breaking the previous instantaneous power record of 24.24 GW.

In early summer 2011, the German government announced that the current FIT scheme would end when installed power reached 52 GW. When this happens, Germany will apply a new injection tariff scheme whose details are not yet known.

However, aware that energy storage through batteries is essential for the massive deployment of renewables such as wind or photovoltaic energy, given their intermittency, on May 1, 2013, Germany launched a new aid program to encourage photovoltaic systems with storage batteries. In this way, photovoltaic installations of less than 30 kW that install batteries and accumulate electricity are financed, with 660 euros for each kW of battery storage. The program is endowed with 25 million euros per year distributed in 2013 and 2014, and in this way it is possible to have energy when the resource is not available —there is no wind or at night—, in addition to facilitating the stability of the system electric.

Indian

Total photovoltaic capacity installed in India until 2016.

India is densely populated and also has high solar irradiation, which makes the country one of the best candidates for the development of photovoltaics. In 2009, India announced a program to accelerate the use of solar installations in government buildings, as well as hospitals and hotels.

The drop in the price of photovoltaic panels has coincided with an increase in the price of electricity in India. Government support and the abundance of the solar resource have helped drive the adoption of this technology.

The 345 MW Charanka solar park (one of the largest in the world) was commissioned in April 2012 and expanded in 2015, along with a total of 605 MW in the Gujarat region. The construction of other Large solar parks have been announced in the state of Rajasthan. Also the 40 MW Dhirubhai Ambani solar park was inaugurated in 2012.

In January 2015, the Indian government significantly increased its solar development plans, setting an investment target of $100 billion and 100 GW of solar capacity by 2022.

At the beginning of 2017, the total installed capacity in India was above 10 GW. India expects to quickly reach 20 GW installed, meeting its goal of creating 1 million jobs and reaching 100 GW in 2022.

Italy

Italy is among the first countries to produce electricity from photovoltaic energy, thanks to the incentive program called Conto Energia. Growth has been exponential in recent years: installed power tripled in 2010 and quadrupled in 2011, producing in 2012 5.6% of the total energy consumed in the country.

This program had a total budget of €6,700 million. Once this limit has been reached, the Government has stopped encouraging new installations, as network parity has been reached. A report published in 2013 by Deutsche Bank concluded that grid parity had indeed been reached in Italy and other countries around the world. The sector has come to provide jobs for around 100,000 people, especially in the field of design and installation of said solar plants.

Since mid-2012, new legislation has been in force that requires the registration of all plants greater than 12 kW; those with lower power (rooftop photovoltaics in residences) are exempt from registration. At the end of 2016, the total installed power was above 19 GW, assuming such an important energy production that several gas plants were operating at half their capacity. its potential during the day.

United Kingdom

Solar power in the UK, although relatively unknown until recently, has taken off very rapidly in recent years, due to the sharp drop in the price of photovoltaic panels and the introduction of feed-in tariffs from April 2010. In 2014, there were already some 650,000 registered solar installations in the British Isles, with a total capacity of close to 5 GW. The largest solar plant in the country is located on the Southwick Estate, near Fareham, and has a capacity of 48 MW. It was inaugurated in March 2015.

In 2012, David Cameron's UK government committed to powering four million homes with solar power in less than eight years, which is equivalent to installing around 22 GW of photovoltaic capacity by 2020. In early 2016, the UK had installed more than 10 GW of solar PV.

Between April and September 2016, solar produced more electricity in the UK (6,964 GWh) than coal produced (6,342 GWh), both accounting for around 5% of demand.

France

The French market is the fourth most important within the European Union, after the markets of Germany, Italy and the United Kingdom. At the end of 2014 it had more than 5 GW installed, and is currently maintaining sustained growth, estimating that in 2015 it will connect an additional 1 GW to the current capacity to the electricity grid. Recently, the French country increased the quota of its energy auctions photovoltaic from 400 to 800 MW, as a result of government recognition of the increasing competitiveness of solar energy.

France is home to one of the largest photovoltaic plants in Europe, a 300 MW project called Cestas. It came on stream at the end of 2015, providing the photovoltaic sector with an example for the rest to follow of the European industry.

Photovoltaic facade in the MNACTEC building (Tarrasa, Spain).

Spain

Spain is one of the countries in Europe with the highest annual irradiation. This makes solar energy in this country more profitable than in others. Regions such as the north of Spain, which are generally considered unsuitable for photovoltaic energy, receive more annual irradiation than the average in Germany, a country that has maintained leadership in the promotion of photovoltaic solar energy for years.

Since the early 2000s, in accordance with the support measures for renewable energies that were being carried out in the rest of Europe, the regulation that establishes the technical and administrative conditions had been approved, and that marked the beginning of a slow takeoff of photovoltaics in Spain. In 2004, the Spanish government removed the economic barriers to connecting renewable energy to the electricity grid. Royal Decree 436/2004 equalized the conditions for its large-scale production, and guaranteed its sale through generation premiums.

Thanks to this regulation, and the subsequent RD 661/2007, in 2008 Spain was one of the countries with the most installed photovoltaic power in the world, with 2,708 MW installed in a single year. However, later modifications in the sector's legislation slowed down the construction of new photovoltaic plants, in such a way that in 2009 only 19 MW were installed, in 2010, 420 MW, and in 2011 354 MW were installed, corresponding to 2%. of the total of the European Union.

In terms of energy production, in 2010 photovoltaic energy covered approximately 2% of electricity generation in Spain, while in 2011 and 2012 it represented 2.9%, and in 2013 3.1% of the electricity generation according to data from the operator, Red Eléctrica. In 2018, the share of photovoltaic solar energy in Spain reached 3.2% of all energy produced nationwide.

At the beginning of 2012, the Spanish Government approved a Royal Decree-Law that halted the installation of new photovoltaic power plants and other renewable energies. At the end of 2015, the photovoltaic power installed in Spain amounted to 4,667 MW. In 2017, Spain fell for the first time from the list of the ten countries with the highest installed photovoltaic capacity, being surpassed by Australia and South Korea. However, in July 2017, the Government organized an auction that awarded more than 3,500 MW of new photovoltaic power plants, which will allow Spain to achieve the renewable energy generation objectives established by the European Union for 2020. As a novelty, neither the construction of the awarded plants nor their operation will entail any cost for the system, except in the event that the market price falls below a floor established in the auction. The great drop in costs of photovoltaic energy has allowed large companies to bid at market price.

In 2019, photovoltaics has increased the installed power in Spain by more than 3,000 MW with a total installed power of 7,800 MW. Spain has the largest connected photovoltaic plant in Europe, located in the town of Mula (Murcia), with 494 MW.

Latin America

In Latin America, photovoltaics has begun to take off in recent years. The construction of a good number of solar plants has been proposed in various countries, throughout the region.

Brazil

Pirapora solar complex, the largest in Brazil and Latin America with 321 MW.

Photovoltaic solar energy is expanding in Brazil, while in 2020 the country had 7.8 GW of installed solar energy, the fourteenth country in the world in terms of this energy, as of October 2022 the installed capacity reached a total of 21 GW, with an average capacity factor of 23%. In 2023, Brazil will be among the 10 countries in the world with the most solar energy installed..

Mexico

Mexico is the second Latin American country with the highest installed capacity (7.0 GW in 2021), and it still has enormous potential in terms of solar energy. 70% of its territory has an irradiation greater than 4.5 kWh/m²/day, which makes it a very sunny country, and implies that using current photovoltaic technology, a 25 km² solar plant anywhere in the state of Chihuahua or the Sonoran desert (which would occupy the 0.01% of the surface of Mexico) could provide all the electricity demanded by the country.

The Aura Solar project, located in La Paz (Baja California Sur), inaugurated at the beginning of 2014, which intended to generate 82 GWh per year, enough to supply the consumption of 164,000 inhabitants (65 % of the population of La Paz), but it was devastated by Hurricane Odile in September of the same year and the plant stopped operating for several months. In 2016, the reconstruction of the plant was carried out, which ended at the end of the same year and from 2017 to date it is in operation again.

Another 47 MW photovoltaic plant is in the planning phase in Puerto Libertad (Sonora). The plant, originally designed to house 39 MW, was expanded to allow the generation of 107 GWh/year.

Mexico already has more than 3,000 MW installed. It is expected to experience further growth in the coming years, in order to reach the goal of covering 35% of its energy demand from renewable energy by 2024, according to a law passed by the Mexican government in 2012.

Chili

Until a few years ago, Chile led solar production in Latin America (today it is in third place - 4.4 GW in 2021). The first photovoltaic solar plant in Chile was El Águila, a 2.2 MWp plant located in Arica, completed in 2012. This country inaugurated a 100 MW photovoltaic plant in June 2014, which became the largest to date in Latin America. The high price of electricity and the high levels of radiation that exist in northern Chile have promoted the opening of an important subsidy-free market. At the end of 2018, the Andean country had 2,427 photovoltaic MW in operation. Chile has a potential of more than 1,800 GW of possible solar energy in the Atacama desert, according to a study carried out by the German GIZ in Chile (German Society for International Cooperation, 2014). The Atacama desert is the place with the highest irradiation in the world with global irradiation levels (GHI), above 2700 kWh/m²/year.

Other markets

Other Latin American countries have begun to install large-scale photovoltaic plants, including Argentina (1.0 GW in 2021), Honduras (0.51 GW in 2021), Puerto Rico (0.49 GW in 2021), Republic Dominican Republic (0.49 GW in 2021), El Salvador (0.47 GW in 2021), Panama (0.46 GW in 2021), Peru (0.33 GW in 2021), Uruguay (0.25 GW in 2021), Colombia (0.18 GW in 2021) and Bolivia (0.17 GW in 2021).

In the Altiplano of Bolivia is the Caracollo Solar Plant, which is the highest photovoltaic solar plant in the world, at 3725 meters above sea level.

Temporal evolution

The following table shows the detail of global installed power, broken down by each country, from 2002 to 2019:

Potencia total instalada (MWp) por país​​​ ​ ​ ​ ​ ​​

País	Total 2002	Total 2003	Total 2004	Total 2005	Total 2006	Total 2007	Total 2008	Total 2009	Total 2010	Total 2011	Total 2012	Total 2013	Total 2014	Total 2015	Total 2016	Total 2017	Total 2018	Total 2019	Total 2020	Total 2021	Total 2022
Total mundial	2220	2798	3911	5340	6915	9443	15 772	23 210	39 778	73 745	104 015	139 523	176 089	222 213	296 155	389 411	488 739	587 134	713 970
China	-	-	-	-	-	-	-	-	893	3 108	6 719	17 759	28 399	43 549	77 809	130 822	175 287	204 996	254 355
Estados Unidos	212,2	275,2	376	479	624	830,5	1168,5	1255,7	2519	5 644	8 613	13 045	17 651	23 442	34 716	43 115	53 184	60 682	75 572
Japón	636,8	859,6	1132	1421,9	1708,5	1918,9	2144	2627	3617	4 890	6 430	12 107	19 334	28 615	38 438	44 226	55 500	61 526	67 000
Alemania	278	431	1034	1926	2759	3835,5	5340	9959	17 320	25 916	34 077	36 710	37 900	39 224	40 679	42 293	45 158	49 047	53 783
India	-	-	-	-	-	-	-	-	189	566	982	1 499	3 673	5 593	9 879	18 152	27 353	35 089	39 211
Italia	22	26	30,7	37,5	50	120,2	458,3	1157	3502	13 136	16 790	18 190	18 600	18 907	19 289	19 688	20 114	20 871	21 600
Australia	39,1	45,6	52,3	60,6	70,3	82,5	104,5	183,6	504	2 473	3 799	4 568	5 287	5 946	6 689	7 354	8 627	13 252	17 627
Vietnam	-	-	-	-	-	-	-	-	-	5	5	5	5	5	5	8	105	4 898	16 504
Corea del Sur	5,4	6	8,5	13,5	35,8	81,2	357,5	441,9	662	730	1 024	1 555	2 481	3 615	4 502	5 835	7 130	10 505	14 575
España	7	12	23	48	145	693	3354	3438	3892	5 432	6 569	6 994	7 001	7 008	7 017	7 027	7 068	11 277	14 089
Reino Unido	4,1	5,9	8,2	10,9	14,3	18,1	22,5	29,6	72	1 000	1 753	2 937	5 528	9 601	11 914	12 760	13 073	13 346	13 563
Francia	17,2	21,1	26	33	43,9	75,2	179,7	335,2	1025	3 004	4 359	5 277	6 034	7 138	7 702	8 610	9 691	10 804	11 733
Países Bajos	26,3	45,7	49,2	50,7	52,2	52,8	57,2	67,5	97	149	287	650	1 007	1 526	2 135	2 911	4 608	7 177	10 213
Brasil	-	-	-	-	-	-	-	-	2	2	3	8	20	41	148	1 296	2 470	4 615	7 881
Ucrania	-	-	-	-	-	-	-	-	3	188	372	748	819	841	955	1 200	2 003	5 936	7 331
Turquía	0,9	1,3	1,8	2,3	2,8	3,3	4	5	6	7	12	19	41	250	834	3 422	5 064	5 996	6 668
Sudáfrica	-	-	-	-	-	-	-	-	40	6	11	262	1 163	1 352	2 174	3 447	4 801	4 905	5 990
Taiwán	-	-	-	-	-	-	-	-	32	130	231	410	636	884	1 245	1 768	2 738	4 150	5 817
Bélgica	-	-	-	-	-	-	-	574	803	1 979	2 647	2 902	3 015	3 132	3 329	3 621	4 000	4 637	5 646
México	16,2	17,1	18,2	18,7	19,7	20,8	21,8	25	31	39	60	82	116	173	389	674	2 555	4 440	5 644
Polonia	-	-	-	-	-	-	-	-	-	1	1	2	27	108	187	287	562	1 539	3 936
Canadá	10	11,8	13,9	16,7	20,5	25,8	32,7	94,6	200	497	766	1 210	1 843	2 517	2 661	2 913	3 100	3 310	3 325
Grecia	-	-	-	-	-	-	-	55	206	612	1 536	2 579	2 596	2 604	2 604	2 606	2 652	2 834	3 247
Chile	-	-	-	-	-	-	-	-	-	-	2	15	221	576	1 125	1 809	2 236	2 654	3 205
Suiza	19,5	21	23,1	27,1	29,7	36,2	47,9	73,6	111	223	437	756	1 061	1 394	1 664	1 906	2 203	2 589	3 118
Tailandia	-	-	-	-	-	-	-	-	28	79	382	829	1 304	1 425	2 451	2 702	2 967	2 988	2 988
Emiratos Árabes Unidos	-	-	-	-	-	-	-	-	-	13	13	126	133	134	141	355	598	1 918	2 539
Austria	10,3	16,8	21,1	24	25,6	27,7	32,4	52,6	103	174	337	626	785	937	1 096	1 269	1 455	1 702	2 220
República Checa	-	-	-	-	-	-	-	463,3	1953	1 913	2 022	2 064	2 067	2 075	2 068	2 070	2 075	2 086	2 073
Hungría	-	-	-	-	-	-	-	-	-	4	12	35	89	172	235	344	728	1 400	1 953
Egipto	-	-	-	-	-	-	-	-	-	35	35	35	35	45	59	180	765	1 662	1 694
Malasia	-	-	-	-	5,5	7	9	11	15	1	25	97	166	229	279	370	536	882	1 493
Israel	-	-	0,9	1	1,3	1,8	3	24,5	66	196	243	426	676	772	872	975	1 076	1 438	1 439
Rusia	-	-	-	-	-	-	-	-	-	-	-	1	5	61	76	225	535	1 064	1 428
Suecia	3,3	3,6	3,9	4,2	4,8	6,2	7,9	9	11	12	24	43	60	104	153	244	428	714	1 417
Rumanía	-	-	-	-	-	-	-	-	-	1	41	761	1 293	1 326	1 372	1 374	1 386	1 398	1 387
Jordania	-	-	-	-	-	-	-	-	-	-	-	-	-	6	296	406	809	1 101	1 359
Dinamarca	1,6	1,9	2,3	2,7	2,9	3,1	3,3	4,6	7,1	17	402	571	607	782	851	906	998	1 080	1 300
Bulgaria	-	-	-	-	-	-	-	5,7	18	154	1 013	1 020	1 026	1 029	1 028	1 036	1 033	1 048	1 073
Filipinas	-	-	-	-	-	-	-	-	-	2	2	3	28	173	784	908	914	973	1 048
Portugal	1,7	2,1	2,7	3	3,4	17,9	68	102,2	131	172	238	296	415	447	513	579	667	901	1 025
Argentina	-	-	-	-	-	-	-	-	1,2	1	6	8	8	9	9	9	191	442	764
Pakistán	-	-	-	-	-	-	-	-	-	19	46	101	165	266	589	655	679	713	737
Marruecos	-	-	-	-	-	-	-	-	-	34	35	35	40	200	202	204	734	734	734
Eslovaquia	-	-	-	-	-	-	-	0,2	145	496	513	533	533	533	533	528	472	590	593
Honduras	-	-	-	-	-	-	-	-	-	4	5	5	5	393	414	454	514	514	514
Argelia	-	-	-	-	-	-	-	-	-	25	25	25	26	74	244	425	448	448	448
El Salvador	-	-	-	-	-	-	-	-	-	-	-	13	15	26	28	126	206	391	429
Irán	-	-	-	-	-	-	-	-	-	-	-	1	9	9	43	184	286	367	414
Arabia Saudita	-	-	-	-	-	-	-	-	-	3	14	22	24	24	24	34	84	409	409
Finlandia	-	-	-	-	-	-	-	5	7	7	8	9	11	17	39	82	140	222	391
República Dominicana	-	-	-	-	-	-	-	-	-	-	2	8	15	25	73	106	205	305	370
Perú	-	-	-	-	-	-	-	-	-	18	103	109	134	139	146	298	325	331	331
Singapur	-	-	-	-	-	-	-	-	-	5	8	12	25	46	97	116	160	272	329
Bangladés	-	-	-	-	-	-	-	-	-	43	66	94	119	145	161	185	201	255	301
Eslovenia	-	-	-	-	-	-	-	9	36	57	142	187	223	238	233	247	247	264	267
Uruguay	-	-	-	-	-	-	-	-	-	-	1	2	4	65	89	243	248	254	256
Chipre	-	-	-	-	-	-	-	3,3	6,2	10	17	35	65						200
Panamá	-	-	-	-	-	-	-												198
Luxemburgo	-	-	-	-	-	-	-	27	27	30	76	100	110	120					195
Malta	-	-	-	-	-	-	-	2	2	11,5	18	25	54						184
Cuba	-	-	-	-	-	-	-	s.d.	s.d.	s.d.	s.d.	s.d.	s.d.	s.d.	s.d.	65			163
Noruega	6,4	6,6	6,9	7,3	7,7	8	8,3	9	9	10	10	s.d.	13						152
Lituania	-	-	-	-	-	-	-	-	-	-	6	68	68						148
Bolivia	-	-	-	-	-	-	-	-	-	-									120
Colombia	-	-	-	-	-	-	-	-	-	-									107
Guatemala	-	-	-	-	-	-	-	-	-	-	-	-							101
Croacia	-	-	-	0,5	1,2	3,2	5,6	12,1	16,4	16,4	21,7	24,7	33						85
Ecuador	-	-	-	-	-	-	-	s.d.	s.d.	s.d.	s.d.	s.d.	26						28

Long-term forecasting

It is estimated that installed photovoltaic power has grown by about 75 GW in 2016, and China has taken the lead over Germany, already being the largest producer of photovoltaic energy. By 2019, it is estimated that total power worldwide will reach 396 GW (moderate scenario) or even 540 GW (optimistic scenario).

The consulting firm Frost & Sullivan estimates that photovoltaic power will increase to 446 GW by 2020, with China, India and the United States being the fastest growing countries, while Europe will see its capacity double from current levels. Grand View Research, a consulting firm and market analyst based in San Francisco, published his estimates for the sector in March 2015. The photovoltaic potential of countries such as Brazil, Chile and Saudi Arabia has not yet been developed as expected, and is expected to be developed during the next few years. next years. On top of that, increased manufacturing capacity in China is expected to continue to help ease declining prices further. The consultancy estimates that the global photovoltaic capacity will reach 490 GW in 2020.

The organization PV Market Alliance (PVMA), a consortium made up of various research entities, estimates that global capacity will be between 444-630 GW in 2020. In the most pessimistic scenario, it forecasts that the annual installation rate will slow down. between 40 and 50 gigawatts by the end of the decade, while in the most optimistic scenario it is estimated that between 60 and 90 GW per year will be installed over the next five years. The intermediate scenario estimates that they will be between 50 and 70 GW, to reach 536 GW in 2020. The PVMA figures are in line with those previously published by Solar Power Europe. In June 2015, Greentech Media (GTM) published its Global PV Demand Outlook report for 2020, which estimates that annual installations will increase from 40 to 135 GW, reaching a total global capacity of almost 700 GW. in 2020. GTM's estimate is the most optimistic of all those published to date, estimating that 518 GW will be installed between 2015 and 2020, which is more than double that of other estimates.

For its part, EPIA also estimates that photovoltaic energy will cover between 10 and 15% of Europe's demand in 2030. A joint report by this organization and Greenpeace published in 2010 shows that by 2030, a total of 1,845 GW of photovoltaics could generate approximately 2,646 TWh/year of electricity worldwide. Combined with energy efficiency measures, this figure would represent covering the consumption of almost 10% of the world population. By the year 2050, it is estimated that more than 20% of the world's electricity could be covered by photovoltaics.

Grid connection photovoltaic plants

In Europe and in the rest of the world, a large number of large-scale photovoltaic plants have been built. Currently, the largest photovoltaic plants in the world are, according to their production capacity:

Lauingen Energy Park, 25.7 MW in Suabia (Baviera, Germany).

Solar park in Waldpolenz, Germany.

Project	Country	Location	Power	Year
Bhadla Solar Park	India	27°32′22.81′N 71°54′54.91′′E / 27.5396694, 71.9152528 (Bhadla Solar Park)	2245 MW	2020
Qinghai Solar Plant	China		2200 MW	2020
Pavagada Solar Park	India	14°05′49′N 77°16′13′′E / 14.09694, 77.27028 (Pavagada Solar Park)	2050 MW	2019
Tengger Desert Solar Park	China	37°33′00′N 105°03′14′′E / 37.55000, 105.05389 (Tengger Desert Solar Park)	1547 MW	2016
Ben Ban Solar Park	Egypt	24°27′21.6′N 32°44′20.4′′E / 24.456000, 32.739000 (Benban Solar Park)	1500 MW	2019
Noor Abu Dhabi	United Arab Emirates	24°24′11′N 55°16′07′E / 24.40306, 55.26861 (Noor Abu Dhabi)	1177 MW	2019
Solar Kurnool	India	15°40′53′N 78°17′01′′E / 15.681522, 78.283749 (Kurnool Solar Park)	1000 MW	2017
Datong Solar Power Top Runner Base	China	40°04′25′N 113°08′12′′E / 40.07361, 113.13667 (Datong Solar Power Top Runner Project)	1000 MW	2016
Yanchi Solar PV Station	China	38°09′48′N 106°45′40′′E / 38.1633714, 106.7611986	1000 MW	2016
Longyangxia Hydro-solar PV Station	China	36°10′54′′N 100°34′41′′E / 36.18167, 100.57806 (Longyangxia Dam Solar Park)	850 MW	2013-2017
Villanueva Solar Park	Mexico	29°22′21′N 101°2′38′′ / 29.37250, -101.04389 (Villanueva Solar)	828 MW	2018
Rewa Ultra Mega Solar	India	24°32′N 81°17′E / 24.53, 81.29 (Rewa Ultra Mega Solar)	750 MW	2018
Charanka Solar Park	India	23°54′N 71°12′E / 23.900, 71.200 (Charanka Solar Park)	690 MW	2012-2019
Kamuthi Solar Power Project	India	9°21′16′N 78°23′4′′E / 9.35444, 78.38444 (Kamuthi Solar Power Project)	648 MW	2016
Mohammed bin Rashid Al Maktoum	United Arab Emirates	24°45′17′N 55°21′54′′E / 24.75472, 55.36500 (Mohammed bin Rashid Al Maktoum Solar Park)	613 MW	2019
Solar Star	United States	34°49′50′N 118°23′53′′O / 34.83056, -118.39806 (Solar Star)	579 MW	2015
Copper Mountain Solar Facility	United States	35°47′N 114°59′O / 35.783, -114.983 (Copper Mountain Solar Facility)	552 MW	2016
Desert Sunlight Solar Farm	United States	33°49′33′N 115°24′08′′O / 33.82583, -115.40222 (Desert Sunlight Solar Farm)	550 MW	2015
Topaz Solar Farm	United States	35°23′N 120°4′O / 35.383, -120.067 (Topaz Solar Farm)	550 MW	2014
Núñez de Balboa Solar Plant	Spain	38°27′10.58′N 6°13′30.5′′O / 38.4529389, -6.225139 (Planta Solar Núñez de Balboa)	500 MW	2020
Mula Solar Plant	Spain	38°02′31′N 129′26′′O / 38.04194, -1.49056 (Mula Solar Plant)	494 MW	2019

The largest solar plants in the world are located in China and India. Kurnool Solar, in the Indian state of Andhra Pradesh, houses 1 GW of capacity, equivalent in power to a nuclear power plant. The Yanchi Solar plant in the province of Qinghai (China) also has this capacity. Also among the top ranks is Longyangxia Hydro-Solar PV Station, located next to the Longyangxia Dam in China. It consists of a 1,280 MW macro hydroelectric complex, to which a 320 MW photovoltaic plant was later added, completed in 2013. At the end of 2015, a second phase of 530 MW was inaugurated, bringing the total power of the solar plant to the 850 MW.

Other large-scale projects are located in the United States. Solar Star, has a capacity of 579 MW and is located in California. The Topaz Solar Farm and Desert Sunlight Solar Farm plants in Riverside County, also in California, also has a capacity of 550 MW. The Blythe Solar Power project consists of a 500 MW photovoltaic plant, also located in Riverside County, which is scheduled to be built soon. In Europe, the largest plant is located in Murcia (Spain). It has a capacity of 494 MW and came into operation in 2019.

There are many other large-scale plants under construction. The McCoy Solar Energy Project, in the United States, will have a capacity of 750 MW once completed. In recent years, the construction of several plants with capacities greater than 1,000 MW have been proposed. in different parts of the world. The Quaid-e-Azam Solar Park plant, located in Pakistan and whose first phase is already operational with 100 MW, plans to expand its capacity to 1,500 MW. The United Arab Emirates also plans to build a plant of 1,000 MW. The Ordos Solar Project, located in China, will reach 2,000 MW. The Westlands Solar Park project has a planned capacity of 2,700 MW, to be completed in several phases. The Ladakh project in India plans to host 5 GW of photovoltaic capacity.

As regards rooftop photovoltaic installations, the largest installation is at the Renault Samsung Motors facilities in Busan (South Korea), and has 20 MW distributed over the different roofs, car parks and infrastructure of the complex. Opened in 2013, it provides power to the factory and thousands of nearby homes.

Storage of photovoltaic energy using batteries

Energy storage presents itself as a major challenge to allow a continuous supply of energy, since solar energy cannot be generated at night. Rechargeable batteries have traditionally been used to store excess electricity in isolated systems. With the advent of grid-connected systems, excess electricity can be transported through the electrical grid to consumption points. When renewable energy production accounts for a small fraction of demand, other energy sources can adjust their production appropriately to support the variability of renewable sources, but with the growth of renewable sources, control is necessary. more suitable for balancing the grid.

With the decline in prices, photovoltaic plants begin to have batteries to control the output power or store excess energy so that it can be used during the hours that renewable plants cannot generate directly. This type of batteries makes it possible to stabilize the electrical network by softening demand peaks for minutes or hours. It is anticipated that in the future these batteries will play an important role in the electrical grid, as they can be charged during periods when generation exceeds demand and feed that energy into the grid when demand is greater than generation.

For example, in Puerto Rico a system with a capacity of 20 megawatts for 15 minutes (5 megawatt hours) is used to stabilize the frequency of the grid on the island. Another system of 27 megawatts for 15 minutes (6.75 megawatt hours) with nickel-cadmium batteries was installed in Fairbanks, Alaska in 2003 to stabilize the voltage of the transmission lines.

Most of these battery banks are located next to the photovoltaic plants themselves. The largest systems in the United States include the 31.5 MW battery at the Grand Ridge Power plant in Illinois, and the 31.5 MW battery at Beech Ridge, Virginia. Notable projects include the 400 MWh system (100 MW for four hours) of the Southern California Edison project and a 52 MWh project in Kauai (Hawaii), which allows the complete displacement of the production of a 13MW plant for use after sunset sol. Other projects are located in Fairbanks (40 MW for 7 minutes using nickel-cadmium batteries) and in Notrees (Texas) (36 MW for 40 minutes using lead-acid batteries).

In 2015, a total of 221 MW with battery storage were installed in the United States, and the total capacity of this type of systems is estimated to grow to 1.7 GW in 2020. Most installed by the companies themselves wholesalers in the US market.

Self-consumption and net balance

Photovoltaic rooftop installation in a residence in Boston (Massachusetts, United States).

Example of integration of photovoltaic solar energy on the roof of a house.

Photovoltaic self-consumption consists of small-scale individual production of electricity for own consumption, through photovoltaic panels. This can be complemented with the net balance. This production scheme, which allows electricity consumption to be offset by what is generated by a photovoltaic installation at times of lower consumption, has already been successfully implemented in many countries. It was proposed in Spain by the Association of the Photovoltaic Industry (ASIF) to promote renewable electricity without the need for additional financial support, and was in the project phase by IDAE. Later it was included in the 2011-2020 Renewable Energy Plan, but it has not yet been regulated.

However, in recent years, due to the growing boom in small renewable energy installations, self-consumption with a net balance has begun to be regulated in various countries around the world, being a reality in countries such as Germany, Italy, Denmark, Japan, Australia, the United States, Canada and Mexico, among others.

Among the advantages of self-consumption over grid consumption are the following:

• With the breakdown of self-consumption systems and the rise in electrical rates, it is increasingly more profitable for oneself to produce its own electricity.
• The dependence of electrical companies is reduced.
• Photovoltaic self-consumption systems use solar energy, a free, inexhaustible, clean and environmentally friendly source.
• It generates a distributed system of electricity generation that reduces the need to invest in new networks and reduces energy losses by the transport of electricity through the network.
• The energy dependence of the country with the outside is reduced.
• Problems are avoided to supply all demand in peak time, known by power cuts and voltage rises.
• The impact of electrical installations in your environment is minimized.
• Companies reduce their energy costs, improve their image and reinforce their commitment to the environment.

In the case of photovoltaic self-consumption, the return time of the investment is calculated based on how much electricity is no longer consumed from the network, due to the use of photovoltaic panels.

For example, in Germany, with electricity prices at €0.25/kWh and insolation of 900 kWh/kWp, a 1 kWp installation saves about €225 per year, which with installation costs of €1,700/kWp means that the system will be amortized in less than 7 years. This figure is even lower in countries like Spain, with higher irradiation than in the north of the European continent.

Efficiency and costs

The effect of temperature on photovoltaic modules is usually quantified by means of coefficients that relate the variations of the open-circuit voltage, of the short-circuit current and of the maximum power to changes in temperature. In this article, comprehensive experimental guidelines for estimating temperature coefficients

Chronology of conversion efficiency achieved in photovoltaic solar cells (source: National Renewable Energy Laboratory of the United States).

Price evolution of photovoltaic crystal silicon cells (in $/Wp) between 1977 and 2015 (source: Bloomberg New Energy Finance).

Solar cell efficiencies range from 6% for those based on amorphous silicon up to 46% for multi-junction cells. Conversion efficiencies of solar cells used in commercial photovoltaic (silicon) modules monocrystalline or polycrystalline) are around 16-22%.

The cost of crystalline silicon solar cells has fallen from $76.67/Wp in 1977 to about $0.36/Wp in 2014. This trend follows the so-called Swanson's law, a prediction similar to that well-known Moore's Law, which states that the prices of solar modules fall by 20% every time the capacity of the photovoltaic industry doubles.

In 2014, the price of solar modules had fallen by 80% since the summer of 2008, putting solar for the first time in an advantageous position with respect to the price of electricity paid by the consumer in a good number of sunny regions. In this sense, the average cost of electricity generation from solar PV is already competitive with that of conventional energy sources in a growing list of countries, particularly when the hour of generation is considered. of this energy, since electricity is usually more expensive during the day. There has been a stiff competition in the production chain, and further falls in the cost of photovoltaic energy are expected in the coming years, which means a increasing threat to the dominance of fossil fuel-based generation sources. As time passes, renewable generation technologies are generally cheaper, while f ossils become more expensive:

The more the cost of photovoltaic solar energy decreases, the more favorable it competes with conventional energy sources, and the more attractive it is for electricity users worldwide. Small-scale photovoltaic can be used in California at prices of $100/MWh ($0.10/kWh) below most other types of generation, even those that work with low-cost natural gas. Reduced costs in photovoltaic modules also represent a stimulus in the demand of private consumers, for which the cost of photovoltaic is already compared favorable to that of the final prices of conventional electricity.

By 2011, the cost of photovoltaics had fallen well below that of nuclear power, and is expected to continue to fall:

For large-scale installations, prices have already been reached below $1/watt. For example, in April 2012, a price of photovoltaic modules was published at 0.60 Euros/watt ($0.78/watt) in a 5-year framework agreement. In some regions, photovoltaic energy has reached the network parity, which is defined when the costs of photovoltaic production are at the same level, or below, of the electricity prices that the final consumer pays (although in most of the occasions still above the costs of generation in coal or gas plants, without having the distribution and other costs induced). Photovoltaic energy is generated during a period of the day very close to demand peak (previously) in electrical systems that make great use of air conditioning. More generally, it is clear that, with a carbon price of $50/tonned, which raises the price of coal plants to 5 cent./kWh, photovoltaic energy will be competitive in most countries. The downward price of the photovoltaic modules has been quickly reflected in a growing number of installations, accumulating in all 2011 about 23 GW installed that year. Although some consolidation is expected in 2012, due to cuts in economic support in the major markets in Germany and Italy, strong growth will most likely continue for the rest of the decade. In fact, a study already mentioned that total investment in renewable energy in 2011 had exceeded investments in coal-based electricity generation.

The trend is for prices to decline further over time once PV components have entered a clear and direct industrial phase. By the end of 2012, the average price of PV modules had fallen to 0. 50 $/Wp, and forecasts suggest that its price will continue to drop to 0.36 $/Wp in 2017.

In 2015, the German Fraunhofer Institute for Solar Energy (ISE) carried out a study that concluded that most of the scenarios predicted for the development of solar energy underestimate the importance of photovoltaics. The study carried out by the institute Fraunhofer estimated that the levelized cost (LCOE) of photovoltaic solar energy for grid-connected plants will be between €0.02 and €0.04/kWh in the long term, levels lower than those of conventional energy sources.

Extract from the conclusions of the Fraunhofer ISE study: Current and Future Cost of Photovoltaics. Long-term Scenarios for Market Development, System Prices and LCOE of Utility-Scale PV Systems (current and future cost of photovoltaic energy. Long-term scenarios for market development, price systems and LCOE of photovoltaic network connection systems)— February 2015:

1. Photovoltaic solar energy is now a low-cost renewable generation technology. The cost of large-scale photovoltaic plants connected to the network fell in Germany from values above 0.40 €/kWh in 2005 to 0.09 €/kWh in 2014. Even lower costs have been published in other sunny regions of the rest of the world, as much of the components of photovoltaic plants are marketed in global markets.
2. Solar energy will soon become the cheapest source of energy in many regions of the world. Even assuming conservative projections and considering that there will be no significant technological advances, there is no expectancy in the reduction of costs currently occurring. Depending on the annual irradiation of the chosen location, the cost of the photovoltaic will be between 0,04-0,06 €/kWh for 2025, reaching 0.02-0,04 €/kWh before 2050 (conservative estimate).
3. The financial and regulatory environment will be the key to future cost reductions of this technology. The cost of components in global markets will fall regardless of the local conditions of each country. But inappropriate regulation can mean a cost increase of up to 50% due to the higher cost of funding. This can even make a negative difference in the fact that there is a greater solar resource in some areas.
4. Most of the scenarios planned for the development of solar energy underlie the importance of photovoltaic. Based on outdated cost estimates, most of the projections for the future of domestic, regional and global energy systems provide only a small production of solar energy. The results of our analysis indicate that a fundamental review of this aspect is necessary to achieve cost optimization.

Thin film photovoltaic energy

Fine layer photovoltaic laminates being installed on a roof.

Market share of different photovoltaic technologies: The predominance of silicon cells is observed. In 2010, the share of thin-layed technology fell by 30% due to the growth of the most efficient single silicon and polycrystalline technologies that dominate the market (they are represented in light blue and intense).

Another low-cost alternative to crystalline silicon cells is thin-film photovoltaics, which is based on third-generation solar cells. They consist of a solar cell that is manufactured by depositing one or more thin layers (thin film) of photovoltaic material on a substrate.

Thin-film solar cells are usually classified according to the photovoltaic material used:

• Lovely silicon (a-Si) and other thin movie silicios (TF-Si)
• Cadmium Teluro (CdTe)
• Copper Indian galio and seleniuro (CIS or CIGS)
• Solar cells sensitized by coloring (DSC) and other organic solar cells.

The International Low-Cost Solar Energy Conference in Seville, held in February 2009, was the first showcase in Spain for these. This technology caused great expectations in its beginnings. However, the sharp drop in the price of polycrystalline silicon cells and modules since late 2011 has forced some thin-film manufacturers to exit the market, while others have seen their profits greatly reduced.

U.S. President Barack Obama made a speech during the inauguration of a photovoltaic solar plant in May 2009.

Environmental benefit

The amount of solar energy reaching the Earth's surface is enormous, at about 122 petawatts (PW), which is nearly 10,000 times more than the 13 TW consumed by humanity in 2005. This abundance suggests that there is no It will be a long time before solar energy becomes humanity's main source of energy. Additionally, photovoltaic electricity generation has the highest energy density (a global average of 170 W/m²) of all renewable energies.

Unlike fossil fuel-based power generation technologies, photovoltaic solar energy does not produce any harmful emissions during its operation, although the production of photovoltaic panels also has a certain environmental impact. Final waste generated during the component production phase, as well as factory emissions, can be managed through existing pollution controls. In recent years, recycling technologies have also been developed to manage the different photovoltaic elements at the end of their useful life, and programs are being carried out to increase recycling among photovoltaic producers.

The energy return rate of this technology, for its part, is increasing. With current technology, photovoltaic panels recover the energy necessary for their manufacture in a period between 6 months and 1 and a half years; Taking into account that their average useful life is more than 30 years, they produce clean electricity for more than 95% of their life cycle.

Greenhouse gas emissions

Greenhouse gas emissions throughout the life cycle for photovoltaics are close to 46 g/kWh, and may even be reduced to 15 g/kWh in the near future. In comparison, a combined cycle gas plant emits between 400-599 g/kWh, a diesel plant 893 g/kWh, a coal plant 915-994 g/kWh or with carbon capture technology about 200 g/kWh (excluding emissions during coal mining and transportation), and a high-temperature geothermal power plant, between 91-122 g/kWh. The emissions intensity for the life cycle of hydropower, wind and nuclear energy is less than that of photovoltaic energy, according to data published by the IPCC in 2011.

Like all energy sources whose emissions depend primarily on the construction and transportation phases, the transition to a low-carbon economy could further reduce carbon dioxide emissions during the manufacturing of solar devices.

A photovoltaic system of 1 kW of power saves the combustion of approximately 77 kg (170 pounds) of coal, avoids the emission into the atmosphere of about 136 kg (300 pounds) of carbon dioxide, and saves monthly use of about 400 liters (105 gallons) of water.

Degradation of photovoltaic modules

The power output of a photovoltaic (PV) device decreases over time. This decrease is due to its exposure to solar radiation as well as other external conditions. The degradation index, which is defined as the annual percentage loss of output power, is a key factor in determining the long-term production of a photovoltaic plant. To estimate this degradation, the percentage decrease associated with each of the electrical parameters. It should be noted that the individual degradation of a PV module can significantly influence the performance of a complete string. In addition, not all the modules of the same installation decrease their benefits at exactly the same rate. Given a set of modules exposed to long duration outdoor conditions, the individual degradation of the main electrical parameters and the increase in their dispersion must be considered. As each module tends to degrade differently, the behavior of the modules will be more and more different over time, negatively affecting the overall performance of the plant.

There are several studies dealing with the analysis of power degradation of modules based on different photovoltaic technologies available in the literature. According to a recent study, the degradation of crystalline silicon modules is very regular, oscillating between 0.8% and 1.0% per year.

On the other hand, if we analyze the performance of thin-film photovoltaic modules, an initial period of strong degradation is observed (which can last several months and even up to 2 years), followed by a later stage in which the degradation is stabilizes, being then comparable to that of crystalline silicon. Strong seasonal variations are also observed in such thin-film technologies because the influence of the solar spectrum is much greater. For example, for modules made of amorphous silicon, micromorphic silicon or cadmium telluride, we are talking about annual degradation rates for the first years of between 3% and 4%. However, other technologies, such as CIGS, have much lower degradation rates, even in those early years.

Recycling of photovoltaic modules

A photovoltaic installation can operate for 30 years or more with little maintenance or intervention after its commissioning, so after the initial investment cost necessary to build a photovoltaic installation, its operating costs are very low in comparison with other existing energy sources. At the end of its useful life, most of the photovoltaic panels can be treated. Thanks to technological innovations that have been developed in recent years, up to 95% of certain semiconductor materials and glass can be recovered, as well as large amounts of ferrous and non-ferrous metals used in modules. Some private companies and Non-profit organisations, such as PV CYCLE in the European Union, are working on end-of-life panel collection and recycling operations.

Two of the most common recycling solutions are:

• Silicone panels: Aluminium frames and connection boxes are manually dismantled at the beginning of the process. The panel is crushed and the different fractions are separated: glass, plastics and metals. It is possible to recover more than 80% of the incoming weight and, for example, the extracted mixed glass is easily accepted by the glass foam and insulation industries. This process can be carried out by flat glass waste pickers, as the morphology and composition of a photovoltaic panel is similar to the flat glass used in the construction and car industry.
• Panels of other materials: Today there are specific technologies for recycling photovoltaic panels that do not contain silicon, some techniques use chemical bathrooms to separate the different semiconductor materials. For the cadmium teluro panels, the recycling process starts by crushing the module and then separating the different parts. This recycling process is designed to recover up to 90% of the glass and 95% of semiconductor materials. In recent years, some private companies have launched recycling facilities on a commercial scale.

Since 2010, an annual conference has been held in Europe that brings together producers, recyclers and researchers to discuss the future of photovoltaic module recycling. In 2012 it took place in Madrid.