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Solar thermal energy . Parabolic trough power plant . Solar power tower . Photovoltaic . Photovoltaic cell

Solar Energy

The Sun is a tremendous source of energy. Each year, the amount of solar energy received by the Earth corresponds to about 7,000 times that consumed by humankind. It arrives in luminous form (light), conveyed by photons generated by the nuclear fusion of hydrogen within the Sun. Sunlight has the potential to supply all our energy needs — provided that it is converted efficiently and economically. Hence the development of two main avenues for the exploitation of solar energy, namely photovoltaic and thermal technology. While the latter relies directly on the heat created by the Sun’s radiation, the former — possibly the most familiar to the general public — converts solar radiation into electricity.

1 - Solar thermal energy
At the 1878 Universal Exposition in Paris, a professor of mathematics named Augustin Mouchot presented a 20 m2 solar concentrator, which earned him a gold medal. The former schoolteacher-cum-researcher was convinced that the world’s coal reserves would eventually be exhausted and that the Sun was the energy source of the future. Four years later, his associate, an engineer named Abel Pifre, used a similar solar concentrator to power a steam engine for printing newspapers. Today, some 70 concentrating solar power plants around the world rely on the same principle to heat and vaporize water, using the steam to power a turbine and generate electricity. With Spain leading the way, this concentrating solar power technology, also known as thermodynamic solar power, has expanded rapidly throughout the world over the past ten years. France was a pioneer in this field. In 1949, the chemist Félix Trombe began the construction of a solar furnace in the southwestern town of Mont-Louis, and supervised that of a larger furnace in Odeillo-Font-Romeu. Commissioned in the early 1970s, this furnace is still being operated and developed and remains a worldwide reference. With 63 movable mirrors and a 54-meter wide by 40-meter high parabolic mirror, it can concentrate the Sun’s heat by a factor of 10,000 to reach temperatures as high as 3,500°C (6,332°F) in just a few seconds, thus creating the extreme conditions required to study new materials and produce synthetic fuels. In 1979, not far from Odeillo, CNRS and the French electricity company EDF set out to build the Thémis solar power tower, which began producing electricity in 1983. However, France’s energy policy revolved around nuclear power and Thémis was closed three years later. Another reason for the shutdown was the country’s relatively low solar potential: concentrating solar plants are only viable in parts of the world that receive a great deal of direct sunlight — the so-called “Solar Belt.” Even in those sunny zones, the skies can cloud over above the power plants. To ensure a steady supply of electricity throughout the day, these installations can use secondary fuels — oil, natural gas or even wood — whenever necessary to keep their turbines running. In addition, some of them can supply electricity after sunset, with tanks of molten salts storing the collected heat for a few hours. Within the next decade, other heat storage solutions using refractory ceramics or phase-change materials should also become available. In the longer term, concentrating solar power plants could produce dihydrogen-based fuels to power cars, instead of generating electricity. Solar energy storage would no longer be an issue: it could be stockpiled in huge tanks in the form of solar fuel. Storage is also one of the key challenges taken up by another technology called "low-temperature solar thermal energy". Solar collectors, usually installed on rooftops, are used to heat water to supply taps and radiators. In 1891, the first solar water heaters were successfully introduced in the United States: by 1941 more than half the houses in Miami were using solar hot water. But the breakout of World War II put an end to the industry, which never really picked up again due to lower electricity costs. In Europe, the use of low-temperature solar thermal energy became more widespread in the wake of the two oil crises of the 1970s. Interest in the technology was renewed 20 years later as the world became aware of the environmental impact of fossil fuels. Since 2008, the sector has again been undermined by the economic crisis compounded by sharp cutbacks in governmental financial incentives. Installing an individual solar water heater requires a substantial investment by the homeowner. The system’s lifespan exceeds 20 years, which in France affords a return on investment in six to twelve years. This is more than in countries like Germany or Poland, where gas and electricity are more expensive. Denmark has overcome this economic obstacle by building some 20 low-temperature solar thermal plants to supply urban heating networks. The future of this technology also depends on the research and development of seasonal storage solutions that would make it possible to collect heat in the summer and use it in the winter. The payoff would be considerable: in 2009, the energy used to heat residential and tertiary buildings amounted to a quarter of France’s total energy consumption. Industrial systems powered by low-temperature solar thermal energy, including for refrigeration, will also be developed over the coming decades. In fact, solar thermal collectors connected to air conditioners have already been used to cool hotels, airports and offices for the past 20 years. While these systems remain quite expensive, the market has great potential, especially in the warm countries.
Experts : Gilles Flamant - PROMES – Perpignan; Philippe Papillon – CEA INES - Grenoble

ANIMATION - Functioning of a parabolic trough power plant
A concentrating solar power plant of the “parabolic trough” type is equipped with parallel rows of long rectangular concave mirrors, usually running north to south. The mirrors pivot on an east-west axis to follow the path of the Sun. Their curvature makes it possible to concentrate the Sun’s radiation by a factor of up to 100, focusing it on a central absorber tube that runs the length of each row. This tube collects and transports the Sun’s energy in a heat transfer fluid that gradually reaches a temperature of about 400°C (752°F). The process is then the same as for other types of power plants: the heat is transmitted by the fluid to the water, changing it into steam, which in turn powers a turbine that drives a generator to produce electricity. Some of these plants also have huge tanks filled with molten salts that can store the heat for a number of hours, making it possible to continue generating electricity at night or in cloudy weather.
Expert : Gilles Flamant

ANIMATION - The functioning of a solar power tower
A “solar tower” concentrating solar power plant consists of a tower and a field of slightly concave mirrors. Called heliostats, these mirrors rotate individually around two axes to follow the path of the Sun and reflect its rays onto a zone at the top of the tower. The concentration is therefore not linear, as in the case of parabolic trough mirrors, but isolated (point focus). It is also much greater: the top of the tower receives up to about 1,000 times more radiation than it would without heliostats. A heat transfer fluid is pumped to the top of the tower, where it circulates to collect the heat. After reaching a temperature of about 500°C (932°F) and over, it is brought back down and routed through a heat exchanger. The process is then the same as for other types of power plants: the heat is transmitted by the fluid to the water, changing it into steam, which in turn powers a turbine that drives a generator to produce electricity. Some systems, such as those using molten salts, can store the heat for a number of hours, making it possible to continue generating electricity at night or in cloudy weather.
Expert : Gilles Flamant

SLIDE SHOW: Some concentrating solar plants
Today, in 2014, more than 90% of the world’s concentrating solar plants that are used to generate electricity rely on parabolic trough concentrators, the first such technology to have become commercially available. However, the proportion of concentrating solar power towers is expected to rise in the coming decades, because they can reach higher temperatures (500°C/932°F, compared with 400°/752°F for cylindro-parabolic mirrors) and offer much greater heat storage potential. Power plants using a variant of parabolic trough concentrators called Fresnel mirrors are also gaining ground. The temperature reached is lower (300°C-350°C/572°F-662°F), but the technology is more affordable for many “Solar Belt” countries.

1 - The SEGS complex of nine parabolic trough power plants (US)
The Solar Electric Generating Systems (SEGS) complex consists of nine parabolic trough power plants commissioned between 1984 and 1990 in the Mojave Desert of California (US). As of June 2013, this complex, the oldest of its type in the world, was still the most powerful, with an installed capacity of 354 MW. Placed end to end, its more than 900,000 mirror assemblies (modules) and absorber tubes would cover a distance of 370 km (229 miles). About 3,000 mirrors (0.0,3%) are broken by the wind every year. Along with absorber tubes, they are usually replaced with more recent technologies to prevent the plants from becoming obsolete. An automatic washing system removes sand from the reflecting surfaces at regular intervals, thus ensuring the highest possible energy yield.

2 - The Andasol-1 parabolic trough power plant (Spain)
In operation since 2008 with an output of 50 MW, Andasol-1 was Europe’s first parabolic trough power plant, located at an altitude of 1,100 m in southern Spain. Its 624 rows of mirrors cover a total surface of 51 ha. (126 acres), which is the equivalent of 70 professional football pitches. Andasol-1 is also the world’s first power plant to use molten salts for heat storage. Two tanks 36 m in diameter by 14 m high, containing 28,000 tons of liquid nitrate salts, can compensate for the absence of sunlight for a maximum of 7.5 hours. Another two 50 MW plants are now operating at the same site: Andasol-2 since 2009 and Andasol-3 since 2011.

3 - The Shams-1 parabolic trough power plant (United Arab Emirates)
When it went online in March 2013, Shams-1 became the first concentrating solar power plant in operation in the Middle East as well as the most powerful in the world, with a capacity of 100 MW. Located in the Emirate of Abu Dhabi and jointly run by Total and the Spanish industrial group Abengoa, it uses more than 258,000 parabolic trough modules in 768 rows of 150 meters in length. A dry cooling system condenses the steam as it comes out of the turbines to reduce water consumption — an important consideration in this desert region. Shams-1 also has two gas burners that are used to superheat the steam, increasing the yield of the thermodynamic cycle and thus the quantity of electricity generated. Seven smaller burners are used to preheat the heat transfer fluid (synthetic oil) when starting up the plant in the morning and during cloudy periods.

4 - The Ain Beni Mathar hybrid solar and fossil fuel power plant (Morocco)
The Ain Beni Mathar site in Morocco is one of only five hybrid solar power plants in the world. Its 56 rows of parabolic troughs supply 20 MW of power that is used during the day to superheat the water vapor produced by the steam cycle of a combined cycle gas power plant with an output of 450 MW. Unlike the other plants mentioned here, this one uses solar energy as its secondary rather than primary resource. In the next few years, Morocco intends to take advantage of its exceptional solar potential to reduce its high dependence on imported hydrocarbons while limiting its greenhouse gas emissions. In May 2003, construction began on a 160 MW parabolic trough power plant near Ouarzazate, due to go online in 2015. An extension scheduled by 2020 will raise its total capacity to 500 MW.

5 - The Fresnel mirror solar power plant in Llo (France)
Construction by the French company CNIM of a 9 MW solar power plant using Fresnel mirror technology is currently underway in Llo (southwestern France). Slightly concave rectangular mirrors, capable of pivoting on a horizontal axis to follow the Sun’s path, will be positioned in 25 parallel rows of 340 m in length. These mirrors are easier and less costly to manufacture than parabolic troughs, although the temperature reached by the heat transfer fluid is lower, reaching a maximum of only 300°C (572°F). Water will be used to avoid the safety and environmental problems associated with synthetic oils (which are flammable) and molten salts (which are corrosive). France also has plans for another demonstration plant of this type (12 MW planned capacity), to be built by the Solar Euromed company in Alba Nova, Corsica. Two Fresnel mirror plants, built by other companies, have already been supplying electrical power in Australia and Spain since 2012. Another plant using this technology began operations in Australia in 2013.

6 - The PS10 tower at Solnova solar power station (Spain)
Erected in southern Spain near Seville, Planta Solar 10 (PS10) was the first commercial tower concentrating solar power plant to be commissioned in Europe, in 2007. With a total surface of 120m2, its 624 movable mirrors, called heliostats, concentrate the Sun’s rays onto the top of a 155 m tower, generating 11 MW of power. PS20, another tower plant at the same site, has been delivering 20 MW of power since 2009, with a 160 m tower facing 1,255 heliostats. Less than three kilometers away, three parabolic trough power plants with a total power of 150 MW went online at the Solnova solar complex in 2010.

7 - The Gemasolar power tower (Spain)
Commissioned in 2011 near Seville with an output of 20 MW, Gemasolar is Spain’s third and most recent tower power plant. Its 2,650 pivoting mirrors, each with a reflecting surface of 120m2, concentrate the Sun’s radiation onto the top of a 140 m tower where the heat transfer fluid, consisting of molten salts, can reach a temperature of 500°C (932°F). Gemasolar's molten salt tanks can produce energy for up to 15 hours in the absence of sunlight, making it the world’s first solar power plant to supply electricity around the clock with no other energy sources (gas, fuel oil, coal) — at least in the summer (the rest of the year, the plant operates about 20 hours a day).

8 - Ivanpah (US)
Since October 2010, three tower power plants with an expected output of 377 MW have been under construction in the Ivanpah dry lake bed in California’s Mojave Desert. The site spans more than 1,400 hectares (3,459 acres). Its 170,000 movable mirrors, each with a surface of 15m2, is operational since February 2014.

9 - The Thémis prototype high-output solar power plant (France)
Built from 1979 in southwestern France, Thémis was the forerunner of Spain’s Gemasolar project: the world’s first tower power plant to use molten salt as the heat transfer fluid. Linked to the national power grid in 1983, it was closed three years later when the price of crude oil dropped and France began focusing its energy policy on nuclear power. Since 2006, with the backing of the Languedoc-Roussillon and Pyrénées-Orientales regional governments, the CNRS Pégase project has brought Thémis back to life. As of 2013, more than 100 movable mirrors, each with a surface of 52m2, are once again operational, and a new solar receiver is under construction at the top of a 105 m tower. The result of extensive research, notably as regards the choice of materials, the receiver should be able to withstand fluid temperatures nearing 800°C (1472°F). In this case, the heat transfer fluid consists of compressed air. The Pégase project is aimed at testing a prototype high-output hybrid plant combining solar and gas energy for a total power of 1.8 MW. Compressed air heated to 800°C in the solar receiver will be routed to the combustion chamber of a gas turbine to increase its temperature to 1000-1200°C (1832-2192°F). This technology paves the way for combined cycle systems that can optimize the yield of the thermodynamic cycle to nearly 55%, compared with current yields of 30 to 32%.
Expert : Gilles Flamant - PROMES – Font Romeu-Odeillo

Storing thermodynamic solar energy in the form of heat
The Sun heats the Earth’s surface during the day, but thermodynamic solar power plants also need to supply electricity at night, and keep their turbines running in cloudy weather. Two solutions are envisaged within the next 10 years, based on high-temperature heat storage. The first option relies on materials that do not change state when their temperature increases, storing heat in what scientists call “sensible” form. As of today, several dozen thermodynamic solar power plants store heat in massive tanks filled with nitrate salts, which can remain liquid at 250 to 560°C (482 to 1040°F). This system can compensate for the absence of sunlight for a maximum of 15 hours. However, this technology will not be viable in the long term: the world production of nitrate salts is not sufficient to keep pace with the expected development of solar power in the years to come. In addition, no other liquid offers the advantage of being safe, non-toxic and available in large quantities, while avoiding dependence on supplier countries. Current research is therefore focusing on solids, especially ceramics produced from industrial wastes like fly ash from incinerators and coal-fired power plants, metallurgical by-products and asbestos-bearing waste. Available in large quantities, these have no use and can be transformed into inert ceramics capable of storing sensible heat at temperatures of up to 1100°C (2012°F). Industrial tests are underway to assess their performance in the heat transfer fluid tanks used in solar power plants, for example in the form of honeycomb structures or corrugated sheets a few centimeters thick. The second storage solution relies on materials that change state at a given temperature. Potassium nitrates, for example, go from solid to liquid at approximately 300°C (572°F), absorbing heat from their ambient environment. This energy is not lost, but stored in the liquid as so-called “latent” heat. When the salts become solid again, for example when the ambient temperature drops, they release the energy in the form of heat that can be absorbed by the heat transfer fluid. Various materials (metal alloys, carbonates, nitrates, paraffins , sulfates, etc.) are being tested both in the laboratory and on an industrial scale. This method requires lesser quantities of materials than sensible heat storage.
Expert : Xavier PY - PROMES – Perpignan

The production of solar fuels
The concentration of sunlight by the tower power plants that are currently in operation generates temperatures of up to 500°C (932°F). Yet under experimental conditions, researchers have achieved temperatures of 750-800°C (1382-1472°F) and hope to be able to reach 900-1000°C (1652-1832°F) in the near future. Temperatures of this magnitude open up the possibility of new applications, such as the use of very high temperature chemical reactions to produce dihydrogen (H2)-based fuels. Capable of powering cars without emitting greenhouse gases or other pollutants, these “solar fuels” would be produced in reactors installed at the top of solar power towers, which would no longer be thermodynamic — i.e. used to generate electricity — but thermochemical. Various techniques are being investigated using different basic precursors, the first of which is methane (CH4), a hydrocarbon found in abundance in natural gas and biogas resulting from the fermentation of animal or plant matter. Temperatures of between 1000 and 1500°C (1832 and 2732°F) are needed to extract dihydrogen from methane, and to produce solid carbon for the manufacturing of tires, batteries, etc. The second potential precursor is water (H2O). The use of metal oxides to produce intermediary reactions now makes it possible to reduce the temperature needed to break down the water molecule from 3000 to 1500°C (5432 to 2732°F). The third possibility involves two precursors: water and carbon dioxide (CO2), which — when heated to a temperature approaching 1500°C (2732°F) — are transformed into a “syngas,” a combination of carbon monoxide (CO) and dihydrogen (H2). This syngas can easily be transformed into methanol (CH3OH) or other synthetic fuels (including dimethyl ether, petrol, diesel and kerosene aviation fuel) that can power the engines of present-day vehicles. In addition, this solution offers the advantage of recycling CO2 emissions from cement plants, refineries and coal-fired power plants. Research is also underway for using biomass, for example from wood, to produce syngas in solar reactors, which would require temperatures of about 800-1000°C (1472-1832°F).
Expert : Stéphane ABANADES PROMES-CNRS – Odeillo Font-Romeu

Low-temperature solar thermal energy
Low-temperature solar collectors use both direct sunlight and diffuse solar radiation, most often to supply residential buildings with hot water and heat (or cooling). In warm countries, simple solar panel "mats" are usually sufficient: the water slowly heats as it circulates through a network of parallel flow tubes. Flat-plate solar collectors are more commonly used in temperate zones. Solar radiation trapped by the outer glass pane creates a greenhouse effect that heats a network of metal tubes containing a coolant (mixture of water and antifreeze). When it reaches the maximum output temperature of 80 to 90°C (176-194°F) this fluid is routed to a tank, where it circulates through a metal coil to transfer its calories to the water. The coolant can then be pumped directly to floor heating panels that are adapted to the output temperature, and whose thermal inertia makes it possible to store the heat. Solar collectors that use vacuum tubes can reach temperatures as high as 110°C (230°F). More effective but also more expensive, they are mostly found in commercial and industrial buildings with greater heating needs, like offices, hospitals, airports and hotels. These receivers can also be used to power air conditioning systems that convert the water’s heat into cool air. In this case, unlike heating devices, the periods of greater sunlight coincide with times of demand.
Expert: Lingai LUO, LTN

Heat storage in the home
In France, low-temperature solar collectors now supply between 40 and 80% (depending on the region) of the energy requirements for hot water and 25 to 60% of the combined needs for hot water and heating. In order to achieve total solar coverage, solutions for storing heat over several months are being investigated and tested on prototypes at different scales in the laboratory. The objective is to store the excess thermal energy produced in the summer and release it in the winter, in particular for heating living areas. The researchers are focusing on thermochemical processes based on reversible reactions that alternatively absorb heat (endothermic reactions) or release it (exothermic reactions). The reactions considered involve dehydration in the summer and hydration in the winter, using solid or liquid reagents. Storage time is unlimited, as long as the dehydrated compounds and recondensed water are kept separate in different tanks. When they are recombined, weeks or months later, the energy released can be used to heat the air in the home directly and supply hot water for floor heating panels, bathing, etc. However, the choice of reagents is very limited: they must be cost-effective and non-toxic, with a low environmental impact. Their energy density is crucial as the size of the tanks should not exceed 5 -10m3. Researchers are also working on high-performance reactors and heating systems that can make the most efficient use of this type of stored heat.
Experts : Kevyn Johannes, CETHIL; Lingai Luo, LTN; Gwennyn Tanguy, INES

2 - Photovoltaic

In 1839, the French scientist Edmond Becquerel discovered the photovoltaic effect, a phenomenon used today to produce electricity directly from sunlight via photovoltaic solar cells. In 1905, Albert Einstein proposed a theoretical explanation based on the concept of “quanta of energy” carried by light particles, later called photons. The first efficient photovoltaic cells, based on silicon, were developed at Bell Laboratories in the US in 1954. Four years later, they were used to power one of the two transmitters of Vanguard I, the second American satellite to be successfully placed in orbit. Since then, photovoltaic modules using advanced technology have become the norm. Their high cost is offset by undeniable advantages, including autonomy, efficiency and longevity. In the wake of the first oil crisis in 1973, the use of photovoltaic cells for the large-scale production of electricity was envisaged as an alternative to the continued use, and inevitable exhaustion, of the world’s fossil resources. Much research effort was put into enhancing their efficiency while lowering their cost. Before long, this momentum slowed as an increasing number of countries shifted their focus to nuclear power, and the price of crude oil dropped in the 1980s, making fossil fuel energy affordable again. The use of photovoltaics was then limited to isolated sites such as lighthouses and light buoys, oil drilling platforms, relay antennas and weather stations, plus a few niche markets like calculators. When environmental and climate concerns came to the fore front in the 1990s and 2000s, Japan and Germany adopted “green” policies that changed perspectives. By the early 2000s, the introduction of feed-in tariffs for photovoltaic electricity boosted the sector. Other countries followed suit, including France in 2006, and the industry took off. Between 2000 and 2010, worldwide production of solar panels increased 100-fold. Meanwhile, the price of crystalline silicon panels, which represent 90% of total sales, plummeted due to a scale effect. Now photovoltaic energy represents more than 5% of electricity consumption in several countries like Germany or Italy, and it reaches 1% in France. Other technologies have also been developed, such as the thin layer technique, including for example the CIGS layers made of copper, indium, gallium and selenium, whose efficiency in industrial modules ranges from 7 to 14%, compared with 14 to 20% for crystalline silicon modules. This lesser performance is offset by lower production costs due to a less complex manufacturing process consisting in applying thin layers of material to a glass substrate instead of cutting silicon ingots whose production requires very high temperatures. Other new and potentially cheaper types of solar cells are expected to reach the market by the end of the decade, including organic and dye-sensitized cells, whose record output in the laboratory, as of 2013, reached 11% and 12% respectively. They can be manufactured by coating the substrate at low temperature and ambient pressure, which could open the way for flexible plastic solar cells once the remaining technological hurdles are overcome to make industrial production viable. Which technology will win the race for best efficiency-to-cost ratio in the next decade? There could be as many winners as there are niches on the market: - Silicon and thin layer panels to be installed on rooftops; - Multi-junction cells, with their record efficiency rate of 44%, for use in space and, in combination with concentration optics, on solar farms; - Organic and dye-sensitized cells, to be integrated in the façades of buildings or at bus stops, for example, or for recharging smartphones and tablets. In addition to the ever-increasing role of photovoltaics in our everyday lives, another major change should take place by the end of the decade: the advent of grid parity. In other words, photovoltaic solar electricity will cost no more than that supplied by the grid to private customers, as is already the case in Germany and southern Italy. Researchers are already working on other types of solar cells that could prove even more efficient, using photonic crystals, nanowire arrays, quantum dots — a plethora of innovative concepts that mobilize many laboratories. At the same time, the research community keeps an eye on costs, while closely collaborating with industry to improve existing technologies.
Expert : Daniel Lincot - IRDEP

ANIMATION - How a photovoltaic cell works
Photovoltaic cells rely on the photovoltaic effect to produce electricity from sunlight. They are made using materials called semiconductors, which have special properties: - they absorb light easily; and - they have the capacity to generate both an electric field and a voltage. This electric field is obtained by superimposing two layers of a semiconductor, usually made of silicon, that has been “doped”—i.e. whose electrical conductivity has been improved by the addition of “dopant elements”, also called “impurities”. One of the silicon layers is doped with phosphorus atoms. Phosphorus is an element that has more peripheral electrons than silicon. One of these electrons is not involved in molecular bonds and is therefore "free" to be released. Doping introduces atoms that can release free electrons into the layer, which is called the “N-layer” because its mobile charge is negative. The other semiconductor layer is doped with boron, an electron-deficient element. Since it has fewer peripheral electrons than silicon and lacks an electron to form bonds, it “accepts” an electron from the rest of the material. This electron leaves behind an “electron hole”, which is mobile and has a positive charge. In this case, doping introduces atoms that can create mobile holes into the silicon layer, called the “P-layer” because its mobile charge is positive. The electrons and holes move by diffusion within the two layers. Near their interface, this diffusion induces a recombination between the holes and electrons, which annihilate each other. This phenomenon produces an electrically non-neutral zone of phosphorus atoms with a positive charge and an electrically non-neutral zone of boron atoms, with a negative charge. This creates an electric field within the semiconductor, and this region is called the P-N junction. The electric field is accompanied by the appearance of an internal electrical potential between the N-zone and the P-zone. When sunlight strikes a photovoltaic cell equipped with two terminals, the photons transmit their energy to some electrons engaged in the chemical bonds between the atoms. They let these bonds reach a higher energy level, while leaving an empty space, called hole, in the chemical bonds. These holes are equivalent to particles with a positive charge. The electrons and holes that are formed will tend to recombine giving heat, but when the phenomenon occurs in or near the P-N junction the electric field will, conversely, tend to separate the positive and negative charges: the electrons are pushed back toward the N-layer and the holes toward the P-layer. As the electrons try to combine with the holes, they are forced to go through an external circuit in order to reach the holes, creating a continuous electrical current called a “photocurrent.” The electrons are delivered by the voltage generated in the P-N junction, which is known as photovoltage. The product of the two corresponds to the electric power delivered by the cell, which thus directly converts part of the Sun’s luminous energy into electric energy. The ratio of light energy to electric energy is called the cell conversion efficiency.
Expert : Daniel Lincot - IRDEP

SLIDESHOW - The many applications of photovoltaic solar energy
The applications of photovoltaic solar energy are wide-ranging, including both off-grid (standalone) and on-grid installations. The earliest applications were off-grid: at first, photovoltaic energy was used to power man-made satellites, portable electronic devices like calculators, watches and computers, and isolated installations like maritime beacons, road signs, pay-and-display parking meters and mountain shelters. On-grid installations came later and are divided into three categories: they can be part of a building (individual home; apartment, office, or farm building), integrated into a structure (car park, noise barriers) or part of a solar power plant producing energy for direct use on the grid.

This slideshow presents some of these applications.
1 - Photovoltaic solar panels integrated into the balconies of an eco-friendly building complex in Helsinki (Finland).
In the Scandinavian countries, the sun’s low angle of incidence makes it cheaper to install solar panels on the buildings’ southern façades rather than on their roofs. Building-integrated solar panels are also an option to meet aesthetic requirements.
© Wikimedia Commons/Photo Pöllö
2 - La Halle Pajol in northern Paris (France).
Opened in 2013, the complex is the country’s first urban solar power station. Its roof is equipped with 3,500 m2 . (38,000 sq.ft.) of photovoltaic solar panels, with an annual output of 390 MWh of electricity, plus 200 m2. (2,150 sq.ft.) of thermal solar panels used to heat water for a youth hostel located in the premises. La Halle Pajol is part of the ZAC Pajol project for the rehabilitation of disused railway infrastructure in the district of La Chapelle.
© Marc Verhille / Mairie de Paris
3 - A car park with photovoltaic panels on the roof, University of Arizona (USA).
 © Wikimedia Commons/Photo Kevin Dooley
4 - A noise barrier bordering a motorway in Freising (Germany).
Commissioned in 2003, the sound wall is the largest installation of its type in the world and has an electrical production capacity of 500 kWp.
© Wikimedia Commons/Photo
5 - A barn in Rieupeyroux, in the Lozère region (southern France).
The photovoltaic roof of this livestock building located at an altitude of 800 m is part of a joint effort by 77 farmers raising calves for the “Veau d’Aveyron et du Ségala” quality label, who have devoted 30,000 m2 (323,000 sq.ft.) of roof space to electrical production, with an installed capacity of 3.2 MW.
© CNRS Images/ Marcel Dalaise
6 - An Amonix 7700 solar power generator in California (USA).
The Amonix 7700 is a concentrating solar generator made up of multi-junction photovoltaic cells and hundreds of lenses that concentrate sunlight by a factor of about 500. Generators of this type use either optical lenses or spherical mirrors. Intended for use primarily in dry, sunny regions, they are more efficient than conventional photovoltaic systems. In August 2013, the Amonix company announced a record photovoltaic module efficiency of nearly 36%.
© Wikimedia Commons/Photo Mbudzi
7 - A solar farm located on the Colle des Mées plateau in Provence (southern France).
Commissioned in 2011, this photovoltaic power plant is one of the largest in France, with more than 110,000 solar modules spanning 66 hectares (163 acres). With a capacity of 31 MWp, it generates enough electricity to power 12,000 households. The land occupied by the solar installation is sown with nectar plant seeds in the spring in order to help preserve the bee population and promote organic agriculture. When the site becomes obsolete, it will be converted into farmland and its panels will be recycled.
© Wikimedia Commons/Photo C. Pinatel de Salvator
8 - A photovoltaic power plant in Toul-Rosières, in the Meurthe et Moselle region (northeastern France).
The Toul-Rosières site is the largest photovoltaic solar power plant in France. It is located near the city of Nancy on a former French Air Force base whose land has been cleared of pollution and partially reforested. An area of 120 hectares (nearly 300 acres) is covered with thin-film solar panels with a total capacity of 135 MWp — enough to power a city of 60,000. This vast installation was commissioned in stages starting in May 2012.
© Photo Emile Pol/SIPA
9 - The Sevilla low-concentration photovoltaic plant in Sanlúcar La Mayor (Spain).
The site spans 30 hectares (74 acres), with 5,913 m2 (63,647 sq.ft.) of photovoltaic panels. It has an installed electrical capacity peaking at 1.2 MW and generates 2.1 GWh per year.
© Wikimedia Commons/Photo afloresm
10 - The Mildura concentrated solar power plant in the state of Victoria (Australia).
The largest concentrated photovoltaic (CPV) site in Australia, it was commissioned in July 2013 and has an output of 1.5 MW. It is a demonstration plant built to assess the economic feasibility of CPV parabolic trough technology.
© Photo Solar Systems-Silex
11 - The Altersonne, a solar-powered tour boat in Hamburg (Germany).
© Wikimedia Commons/Photo KMJs
12 - The Sunseeker solar-powered airplane flying over the California desert in 2005 (USA).
© Wikimedia Commons/Photo Ccoonnrraadd
13 - A Mercedes-Benz automobile built in 1985 for a solar-powered vehicle race between Lake Constance and Lake Geneva. Mercedes-Benz Museum, Stuttgart, Baden-Württemberg (Germany).
© Wikimedia Commons/Photo Morio

Photovoltaic silicon
In 2012, crystalline silicon technology represented more than 85% of all solar modules sold worldwide. Silicon is the second most abundant element in the Earth’s crust, after oxygen. They combine in the form of silicon oxide. It is extracted primarily from the silica in sand and silicates in rock. Refined to a purity that can reach 99.999999%, for electronic applications, the silicon is crystallized in the form of cylinders (monocrystalline silicon) or blocks (multicrystalline silicon) weighing several hundred kilos. It is then cut into ingots and ultimately into wafers 180 to 200 micrometers thick. This complex manufacturing process requires very high temperatures (around 1500°C/2732°F), which means that the cells’ energy payback time is about two years in southern France and three years in the north. Numerous university and industrial laboratories are investigating ways to reduce the amount of energy and materials required in the manufacturing process. Proposed solutions include increasing the size of silicon ingots (> 600kg) by using monocrystalline silicon seeds and controlling the melting and cooling temperatures, drawing ribbons 60 to 100 micrometers thick from a liquid silicon melt, thus avoiding the loss resulting from cutting wafers from ingots, and converting condensed silicon powder into wafers. Other projects focus on the structure of the photovoltaic cells: placing the electrical contacts only on the back side to eliminate the shadows cast by metal components on the exposed front face; using bifacial cells with metal grids on both sides to collect more energy; coating the front side with a layer of amorphous hydrogenated silicon, etc. The aim is to reach the theoretical maximum efficiency of crystalline silicon cells, which is around 30%. Researchers are also investigating systems using silicon nanoparticles and photonic networks, which could enable them to convert certain photons more efficiently or confine as many of these “light particles” as possible within the cell.
Expert : Abdelilah Slaoui - ICube - Strasbourg

Thin layers and silicon nanowires

In 2012, hydrogenated amorphous silicon cells accounted for 3.8% of the photovoltaic cells sold in the world. This type of silicon, with its amorphous (non-crystalline) structure, offers a tremendous advantage: it takes only a very thin layer to absorb visible light: 0.25 μm compared with 200 μm for crystalline silicon. Even more importantly, this thin layer can be deposited on a wide range of substrates, including glass, plastic and metal, using low-temperature (around 200°C ) plasma processes. Although less costly to produce, these cells do not convert light into electricity efficiently due to the amorphous nature of the semiconductor material. With industrial module efficiency of 7%, compared with 14% for multicrystalline silicon, this type of cell is losing market share. But recent research has led to the emergence of new technologies and concepts. Today, thin layers of hydrogenated amorphous silicon are used to coat thicker crystalline silicon cells (HIT technology ), achieving a record yield of 24.7% . In the laboratory, researchers have been able to grow crystalline silicon using low-temperature plasma processes . Prospective projects are also underway for testing the efficiency of silicon nanowires. In this case, the cell no longer consists of layers of superimposed materials, but of a “labyrinth” of tiny radial structures, themselves made up of a succession of layers. This type of structure makes it easier to trap light and "recover" electrons. In 2013, researchers were able to achieve 8.2% efficiency with cells composed of 108 nanowires/cm2, whose thickness did not exceed 0.1 μm. They are now hoping to increase this percentage to 15% by 2015, using tandem radial junction structures. Another challenge is to grow, on large surfaces, straight nanowires whose optimal density, size, shape and diameter remain to be determined.
Experts: Jean-Paul Kleider, LGEP; Pere Roca i Cabarrocas, LPICM

CIGS thin layer solar cells
In 2012, CIGS cells accounted for 2.9% of the photovoltaic cells sold in the world. The acronym CIGS (or CIGSe) refers to the semiconductor material, which consists of copper, indium and gallium combined with sulfur or selenium. This material’s light absorption capacity is 100 times higher than that of silicon — so powerful that a 2 μm layer is enough to absorb all the photons in the visible range. With current technology, CIGS is vacuum deposited on a glass substrate at temperatures of about 550°C (1022°F). In an effort to reduce production costs, researchers are working on chemical or electrochemical coating methods that can operate at low temperature and atmospheric pressure. They are also investigating the possibility of replacing the glass substrate — which makes up about 30% of the price of a cell — with metal or plastic, and the indium with more common elements like zinc or tin. Also in demand for use in microelectronics, indium is a rare element and is likely to remain costly in the coming decades. To make supplies last longer, the thickness of the CIGS layer could be reduced to as little as 0.1 microns while retaining good light absorption through nanophotonic techniques. Another challenge for this technology is to replace the buffer layer of cadmium sulfide with a more eco-friendly film that does not contain toxic cadmium. This ultra-thin buffer layer (50 nm thick) connects the CIGS active layer to the cell's outer layer. The latter contains zinc oxide, which serves as a pre-conductor contact and is “doped” to enable the separation of electrons and holes in the CIGS layer. Unlike a silicon cell, in a CIGS cell the junction is not made up of a single material doped differently, but rather of two materials doped differently, or what is called a “heterojunction.” As of June 2013, the maximum efficiency achieved with a CIGS cell was 20.4% .
Experte : Negar Naghavi - IRDEP

Multi-junction cells
A silicon cell can convert, at most, only 30% of the light energy it receives into electrical energy. This is because a ray of sunlight contains photons of many different energy levels, and only those with an energy greater than 1.1 eV can excite the electrons in silicon. The weaker photons pass through the silicon with no effect. This energy is inversely proportional to the light’s wavelength: a red photon of 700 nm has less energy than a blue light photon of wavelength 450 nm. Crystalline silicon is capable of converting a wide range of wavelengths (from 400 to 1100 nm) into electricity, spanning the visible light and near-infrared spectrum. In order to improve the efficiency of photovoltaic cells by using infrared photons between 800 and 1800 nm without sacrificing the conversion of higher-energy photons, one solution is to stack cells made of semiconductors that react to photons of different wavelengths, thus covering more of the solar spectrum. These are called multi-junction or tandem cells. The most efficient ones comprise three junctions: the top cell is made of a gallium-indium-phosphorus material that converts ultraviolet photons to green wavelengths; the middle cell — composed of gallium arsenide — converts yellow and red photons, and the bottom cell is made of germanium to convert infrared photons. As of June 2013, these cells had achieved 37.7% record conversion efficiency under standard illumination and 44% at a concentration of 942 suns (their efficiency increases logarithmically in proportion to the luminous intensity). By 2020, new triple-junction cells are expected to reach 50% efficiency under concentrated sunlight.
Expert : Jean François Guillemoles- IRDEP

Concentrating photovoltaics (CPV)
The more light a solar cell receives, the more current it delivers. This phenomenon is directly proportional: if the photon flux is increased by a factor of 500, so is the current. In the 1970s, researchers began to experiment with solar receivers to concentrate light on very small solar cells, with surfaces between 1.mm2 and 1 cm2. However, the increase in generated power was not sufficient to offset the cost of optical concentration systems (parabolic troughs or Fresnel lenses) and associated solar tracking devices. In particular, the efficiency of the silicon cells, in use 40 years ago — including the best performing — was too low, and concentration could not exceed 200 suns. However, interest in this type of system was revived in the 1990s with the advent of new gallium arsenide multi-junction cells. With an overall efficiency of nearly 30%, the cost of concentrated photovoltaic (CPV) conversion today is still 50% higher than that of silicon installations, but with great potential for improvement. As of June 2013, around a hundred CPV power plants were connected to the grid worldwide, for a total installed capacity of less than 100 MW. By 2020, this should reach 1 gigawatt with the commissioning of new installations in sunny and generally cloud-free regions such as northern Africa, the southwestern US, Australia, etc. Indeed, the so-called “Solar Belt” offers the best locations for a technology that uses only direct sunlight. Operating at concentrations of 500 to 700 suns, the vast majority of present-day CPV plants use a “passive” cooling system: heat is dissipated through high thermal conductivity receivers onto which photovoltaic cells are inserted, while a finned radiator at the back of the modules facilitate heat dissipation via the ambient air. A few prototypes are being tested for operation at concentrations of 1000 to 1500 suns. These installations use “active” cooling systems to keep the cells from melting, for example dense networks of microchannels engraved on the receivers through which a fluid , introduced in the form of micro-jets, circulates at high speed. Beyond commercial considerations, research is also underway to improve understanding of the processes, achieving concentrations of up to 10,000 suns at the Odeillo solar furnace in southwestern France. Other projects target “micro-CPV” systems using modules made of micro-concentrators that focus sunlight on microcells whose surface area does not exceed 0.1 mm2.
Expert : Alain Dollet - PROMES

Organic thin layers
In an organic solar cell, the photons are absorbed by small molecules (with no more than about 50 atoms) consisting mainly of carbon and hydrogen atoms, or by very long molecules with a repeated pattern of “mer units,” called organic semiconducting polymers. First introduced in the early 1990s, this type of cell had reached a record efficiency of 12% as of June 2013 . In order to improve this performance, researchers are experimenting with molecules that absorb a larger portion of the solar spectrum (including infrared and red photons) as well as compatible “electron-accepting” materials. Indeed, all organic solar cells contain two intertwined materials: an electron “donor” and an electron “acceptor,” the acceptor being indispensable for the separation of the electron-hole pair. The internal structure of the cell is a key factor: it must multiply the interface zones between the two materials at the nanometric scale. The most commonly used electron acceptor is a fullerene derivative that can take on different geometric shapes, becoming round like a football (C60) or oval like a rugby ball (C70). The possibility of replacing it with polymers or inorganic nanoparticles is now being tested in the laboratory. However, industrial production will depend on the development of low-cost electrodes and transparent encapsulation materials that can be used in manufacturing processes at temperatures below 150°C (302°F). The modules must be encapsulated to provide lasting protection against water and oxygen, which can break down the organic substances. The lifespan of these cells is currently estimated at about 10 years.
Experts : Solenn Berson – CEA /INES; Cyril Brochon – LCPO/Bordeaux University

Dye-sensitized solar cells
Created in a Swiss laboratory in 1991 and also called “Grätzel cells” after their inventor, dye-sensitized solar cells mimic the photosynthesis process of plants. Yet rather than using chlorophyll pigments to absorb photons, they contain organometallic, organic or inorganic dyes, like perovskites, that have the capacity to bond to titanium dioxide nanoparticles. The assembly is immersed in a liquid or solid electrolyte that regenerates the initial electronic state of the dye. In contrast to silicon cells, the light-absorbing function (performed by the dye) and the charge-transporting function (performed by the titanium dioxide for the electrons and the electrolyte for the holes) are separate. This type of cell had achieved a record efficiency of 14.1% as of June 2013. In theory, this figure could reach 33% with the use of new dyes capable of converting near-infrared photons, improved “electron-acceptor” materials and new electrolytes. Tandem cells, on the other hand, have achieved maximum yields of 43%. Research is also underway on the mechanisms that cause the dyes to break down, a phenomenon that currently limits the cells’ estimated lifespan to about 15 years. They could be recycled by replenishing the dye and electrolyte without replacing the glass and titanium oxide conductive shells, which are quite durable and account for 70% of the total cost of the cell. Recycling capabilities are an important consideration for the future integration of clear and colored solar cells into buildings’ façades or glazing.
Expert : Frédéric Sauvage – LRCS

Nanophotonics and PV

Today’s silicon photovoltaic cells are coated with an anti-reflective material and a multitude of small “inverted pyramid” structures that facilitate light penetration into the absorption layer. Nanophotonic techniques could be combined with these conventional optical methods to make solar cells thinner and more efficient. One example is photonic crystals — tiny holes 10 to 100 nm deep, spaced at regular intervals. Engraved on the outer surface of the cell, they direct a range of wavelengths transversally into the semiconductor material, thus increasing the absorption of the desired photons. Other concepts under investigation in the lab include depositing small nanostructured metal disks on the surface of the cell. Less limited in terms of the light spectrum, this technique uses the “plasmon effect” (a plasmon is a photon-metal electron pair) to trap light in tiny cavities, making it possible to reduce the thickness of the absorption layer by a factor of 10. It is even possible to modify the spectrum of the incident light. Deposited on the surface of solar cells, rare earths (erbium, ytterbium, etc.) can “break down” very high-energy ultraviolet or violet photons into multiple red photons that can then generate at least twice as much electron movement. Researchers are also testing “photon addition” by inserting specific materials into the cells’ junctions. The idea is to convert two low-energy photons that are normally not absorbed by the semiconductor into a single higher-energy photon. Some of the techniques and materials used are still costly, but research is underway to find substitutes.
Experts : Stéphane Collin - LPN; Alain Fave - INL

PV systems
The viability of photovoltaic electricity does not depend solely on the efficiency of the solar cells and the location of the installation. It also depends on the performance of the other components of the system, such as electrical cables, circuit breakers, power inverters, etc. The latter transform the direct current supplied by solar cells into alternating current at a voltage and frequency that can be used in the grid (in Europe, 230 V and 50 Hz). New control laws for adapting the supplied current in real time and new configurations of the power inverters’ internal electronic components have significantly improved efficiency, but there is still considerable room for progress. For example, improving the inverters’ performance could increase the production of photovoltaic energy from 5% to 15%. Another challenge is to triple the lifespan of this equipment (which is currently about 10 years, compared with 25 years for the modules) in order to cut the cost and environmental impact of photovoltaic systems as a whole. This entails rethinking the internal structure of the inverters to reduce the effects of stress and designing components from new, longer-lasting materials that are more resistant to temperature and power variations. It would also require the emergence of an industry supplying electronic components specifically for photovoltaics, so that PV systems would no longer need to rely on those developed for household applications, such as washing machines and refrigerators.
Experte : Corinne Alonso - LAAS

The PV boom
Grid parity should be achieved in France by 2020. At that point, it will be cheaper for a household to consume the photovoltaic electricity produced on its own roof than to buy power from the grid. Likewise, buildings like schools and government or corporate offices will be able to buy surplus electricity from nearby households. The interaction among users, operators, distributors, producers, etc., will also be much more complex than it is today, with a single operator, EDF, currently playing a predominant role. The development of photovoltaics also affects the basic economics of electrical power: the intermittent nature of its production causes volatile fluctuations in the price of electricity on the wholesale market, even resulting in negative electricity prices during extremely sunny periods — a situation that had occurred once as of June 2013. This could lead to an “energy revolution” combining production, consumption and storage depending on each region's energy resources and needs, which would inevitably require expanded storage solutions. For many years, hydroelectric power plants have used their surplus electricity to pump water into their reservoirs, ready to produce more electricity during peak consumption periods. Other forms of storage are being tested on the scale of individual buildings through the use of “smart grid” techniques: for example, the batteries of the electric vehicles parked in a building store electricity during the day and release it at night, or household electrical appliances are momentarily turned off by the energy supplier (EDF) to reduce consumption surges. It remains to be determined whether motorists and consumers would accept such constraints and under which conditions. In the future, new technologies like fuel cells (e.g. using hydrogen) and pneumatic tanks (to store energy in the form of a compressed gas) could become widespread. Another possibility would be to store the summer electricity surplus as heat in water tanks under buildings, and use it for heating in the winter.
Expert : Gilles Debizet - PACTE

CNRS    sagascience