The Odeillo Solar Furnace: France's Mountain Mirror That Reaches Thousands of Degrees

High in the eastern Pyrenees, in the commune of Font-Romeu-Odeillo-Via in the French department of Pyrénées-Orientales, a building wears a giant concave mirror on one face like a satellite dish the size of an apartment block. Facing it, on a terraced hillside, stand dozens of flat mirrors that turn slowly through the day to follow the sun. Together they make up the Odeillo solar furnace, often described as the largest solar furnace in the world. On a clear day it can deliver about one megawatt of concentrated sunlight onto a target smaller than a dinner table and push sample temperatures past 3,000 °C.

The furnace is not a power station. It does not sell electricity, and it was never meant to. It is a scientific instrument, run by the French national research organisation CNRS through its PROMES laboratory, and its job is to recreate extreme heat in a clean, controllable way so that researchers can study how materials behave when they are pushed close to their limits. Over more than half a century it has been used to test heat shields, investigate high-temperature chemistry, develop solar fuels and help train generations of solar engineers.

This article explains where the furnace came from, how it works, what it has been used for, how it compares with other solar technologies, and what its limits are. It also clears up some common confusion, including the frequent mix-up between Odeillo, the older Mont-Louis furnace nearby and the Thémis solar tower a few kilometres away.

Quick facts

The figures below come from CNRS and PROMES descriptions and from encyclopedia summaries that draw on them. Where sources disagree, the range is given rather than a single number.

Why put a furnace on a mountain?

A solar furnace needs one thing above all: strong, direct sunlight that is not scattered by clouds, haze or humidity. Diffuse light, the soft glow that still reaches the ground on an overcast day, cannot be focused by mirrors. Only the direct beam coming straight from the sun's disc can be concentrated.

The Cerdagne plateau, where Odeillo sits, is unusually good for this. It is a high, wide basin on the French side of the Pyrenees, close to the Spanish border. French-language sources describe more than 2,400 hours of direct sunshine a year at the site, while the English-language Wikipedia entry gives more than 2,500 hours. Either way, it is among the sunniest places in mainland France. The altitude helps too. At around 1,500 metres there is less atmosphere above the mirrors, the air is drier and generally cleaner, and the direct beam is correspondingly stronger. Direct sunlight at Odeillo can reach roughly 1,100 watts per square metre, according to the French Wikipedia description of the site.

The region had another advantage by the 1960s: experience. French scientists had already been working with concentrated sunlight a few kilometres away at Mont-Louis since 1949. Odeillo was not chosen at random. It was the next step for a research team that already knew the climate, the logistics and the science.

Origins: from a wartime searchlight mirror to Mont-Louis

Félix Trombe and the Meudon experiments

The story begins just after the Second World War with Félix Trombe, a French chemist working for the CNRS on rare earths and high-temperature materials. Trombe was interested in a practical problem. To make very pure refractory materials, the kind that survive extreme heat, you need to melt or treat ores at very high temperatures. Conventional furnaces can do that, but they introduce contamination: combustion gases, electrodes, crucible walls and other materials all touch or react with the sample.

Concentrated sunlight offered a way around that. Light carries energy but no chemical baggage. If you can focus enough of it onto a sample, you heat the sample and nothing else.

In 1946, Trombe and colleagues at Meudon, near Paris, carried out the first experiments using a parabolic mirror taken from an anti-aircraft searchlight. PROMES's own history of the laboratory names Marc Foëx and Charlotte Henry La Blanchetais as part of the early team and describes this first "solar heating station" as a 2 kW device on a polar mount. The PROMES history also frames the work as a revival of 18th-century experiments by the chemist Antoine Lavoisier, who had used large lenses to concentrate sunlight.

Mont-Louis, 1949

The Meudon results were promising enough that the team looked for a sunnier site. In 1949 they built a solar furnace inside the fortified citadel of Mont-Louis, a small fortified town in the Pyrenees. The Mont-Louis furnace had a thermal power of about 50 kW. It is widely described as the first facility of its kind in the world, and PROMES says Trombe saw it as a model for a future industrial-scale solar furnace.

Mont-Louis used a two-mirror design: a flat, steerable mirror (a heliostat) caught sunlight and redirected it onto a fixed parabolic mirror, which focused the light onto a target. That arrangement solved a basic problem. A parabolic mirror only focuses properly when light arrives parallel to its axis. Rather than try to swing a huge parabola around to follow the sun, you keep the parabola still and move a flat mirror instead. The same idea, scaled up enormously, became Odeillo.

The Mont-Louis furnace still exists. In 1975 it was moved from the centre of the citadel to a spot near the town walls, and today it is used for education and demonstrations, according to IEEE Spectrum. Visitors can watch it ignite wood or melt metal in seconds.

Building the big one

Through the 1950s the CNRS team at Mont-Louis refined its techniques and its ambitions. Construction of the much larger installation at Odeillo began in 1962 and finished in 1968. French sources name Henri Vicariot as the architect of the building. In 1968 the CNRS laboratories moved from Mont-Louis into the new solar furnace building, beginning what PROMES calls a period of adjustment and qualification. The furnace was commissioned in 1969. Some CNRS photo captions and IEEE Spectrum give 1970 as the year it went into service, which probably reflects the difference between first commissioning and routine scientific operation.

How the Odeillo furnace works

The basic optical chain

The furnace has three main parts, and sunlight passes through them in order.

1. The heliostat field. Sixty-three flat mirrors, called heliostats, stand in rows on the terraced slope facing the main building. Their combined reflective area is about 2,835 square metres, according to PROMES. Each heliostat tracks the sun across the sky so that the light it reflects always travels in the same fixed direction: horizontally towards the big curved mirror. French sources give distances between the heliostats and the parabola from about 104 metres for the nearest rows to 314 metres for the farthest.

1. The parabolic concentrator. The heliostats light up a huge concave mirror built onto the north face of the laboratory building. PROMES describes it as a truncated, faceted parabolic mirror of about 1,830 square metres with a focal length of 18 metres. "Faceted" means it is not one single curved sheet of glass but an assembly of thousands of small mirror pieces, each set at a precise angle so that together they approximate a paraboloid. The English Wikipedia entry gives a total of 9,600 reflecting mirrors for the installation.

1. The focal tower. Eighteen metres in front of the parabola, at its focus, stands a tower that holds the target. This is where samples, reactors and test equipment are placed. The concentrated beam converges there into a hot spot. PROMES gives a focal spot of about 80 cm diameter for the full 1 MW; other summaries mention 40 cm or roughly 1 m, depending on how the edge of the spot is defined.

The result is a two-stage concentration. The heliostats do not concentrate light at all; they are flat. They simply redirect it. All of the concentration happens at the parabola, which takes a broad, parallel beam and squeezes it into a small area.

Why the parabola faces north

The layout can seem backwards at first: the giant mirror faces away from the sun. That is deliberate. In the northern hemisphere the sun is in the southern half of the sky. Heliostats placed north of the building face south towards the sun, catch its light and send it back north-to-south horizontally into the concave mirror, which faces north towards them. The parabola never has to see the sun directly. This arrangement also keeps the parabola's axis fixed and horizontal, which makes it possible to build a very large, rigid mirror attached to a building.

Tracking the sun

The sun moves about 15 degrees across the sky every hour. For the beam reflected by each heliostat to stay pointed at the parabola, the heliostat has to turn by half the change in the sun's angle on two axes. At Odeillo the heliostat field is controlled from the tower by what PROMES describes as an open-loop coordinate servo system. In plain terms, a computer calculates where the sun will be at each moment from astronomical formulas and drives the mirrors to the matching angles, rather than relying primarily on sensors that "look" for the brightest spot.

The system is very energy-efficient. French sources note that only a few kilowatts of electricity are needed to drive the heliostats, compared with the roughly 1,000 kW of thermal power they deliver.

Controlling the power

A furnace that can only be "on" at full power would be of limited use in a laboratory. Researchers need to dial in a specific flux and to ramp it up and down. At Odeillo, power can be adjusted in several ways, as described in French-language summaries of the facility:

• by choosing how many heliostats are used, from one to all 63;
• by choosing which heliostats are used, since those farther away contribute differently to the spot;
• by opening or closing large shutters, like a venetian blind, placed in the beam path in front of the focus;
• and, unavoidably, by the weather and the time of day, which determine how strong the direct sunlight is.

Shutters are particularly important for experiments that study sudden heating or cooling. Because they can open and close quickly, the flux on a sample can change in a fraction of a second, which is much harder to achieve in an electric or fuel-fired furnace.

What "10,000 suns" means

Unconcentrated sunlight at the Earth's surface on a clear day delivers on the order of 1,000 watts per square metre, a value often called "one sun". PROMES gives the peak flux density at Odeillo's focus as up to 10 MW per square metre, or 10,000 times that reference value. That is the source of the popular description of the furnace as concentrating "10,000 suns".

It is worth being careful with the phrase. It refers to the intensity of light at the very centre of the focal spot, not to the total amount of energy. The total thermal power is about 1 MW, about the same as a small industrial boiler. What makes the furnace remarkable is not the total but the density: a megawatt squeezed onto an area well under a square metre.

There is also a physical ceiling on how far sunlight can be concentrated. The sun is not a point; seen from Earth it is a disc about half a degree across. Light from different parts of the disc arrives at slightly different angles, and no mirror can focus all of it to a single point. The laws of optics and thermodynamics limit concentration of sunlight in air to roughly 46,000 times. Real systems, with imperfect mirrors, tracking errors and atmospheric effects, stay well below that. Odeillo's peak figure is impressive but sits comfortably within the physics.

How hot does it get?

Different sources give different maximum temperatures, and the reasons are instructive.

• The English Wikipedia entry and several summaries say the furnace can reach 3,500 °C.
• French sources say up to 3,300 °C, and specify that this figure was reached on a graphite sample.
• The same English source notes that temperatures above 2,500 °C can be reached within a few seconds.
• PROMES itself describes its solar facilities as able to produce temperatures above 3,000 °C.

The furnace does not have a single "temperature" the way an oven does. The temperature reached depends on what is being heated. A sample's final temperature is the point where the energy it absorbs from the beam equals the energy it loses by radiation, conduction and convection. A dark, heat-resistant material such as graphite absorbs a lot of light and can survive very high temperatures, so it can get very hot. A shiny metal reflects much of the light and may melt or vaporise first. So a number such as 3,300 °C is best read as "the hottest temperature measured on a suitable target", not as a setting on a dial.

The PROMES laboratory

Who runs it

The furnace belongs to the CNRS and is operated by PROMES, a research unit (UPR 8521) attached to the CNRS Institute of Engineering and Systems Sciences and linked by agreement to the University of Perpignan Via Domitia. The laboratory works across three sites:

• Odeillo-Font-Romeu, home of the 1 MW solar furnace;
• Targasonne, site of the Thémis solar tower, on land belonging to the Pyrénées-Orientales departmental council;
• Perpignan, at the Tecnosud technology park.

PROMES organises its research into three broad themes: materials for energy and space; next-generation solar power plants; and solar chemistry and energy storage.

A short institutional history

PROMES's own history describes several phases after the move to Odeillo.

• 1971 to 1976: the 1,000 kW furnace was used mainly for high-temperature materials testing.
• After the 1973 oil crisis: interest in solar energy surged worldwide, and Odeillo's work shifted towards testing solar boilers for proposed solar tower power plants. PROMES says its heliostat field was, at that time, the only one in the world.
• 1976 to 1984: work expanded into producing energy carriers such as hydrogen or metals with solar heat, and into chemical storage of solar energy.
• 1984 to 2003: most of the solar furnaces returned to high-temperature materials research, and work for space applications grew. In 1986 the laboratories were unified as the Institute of Science and Engineering of Materials and Processes (IMP), which included University of Perpignan teams.
• 2004 onward: the laboratory was renamed PROMES, adding "solar energy" to its title, and took part in a series of European research infrastructure projects that opened the furnaces to visiting scientists.

Not just one furnace

The big furnace is the famous one, but Odeillo is home to a whole family of solar concentrators. PROMES lists eleven medium-sized solar furnaces in addition to the 1 MW installation. Nine sit in the main building and two in an annex. Their powers range from 1.5 kW to 6 kW. The 6 kW unit has a concentration ratio of about 4,500 and a focal spot about 8 cm across. The 2 kW and 1.5 kW units reach concentration ratios of about 15,000 with focal spots around 1 cm across. A PROMES brochure counts twelve solar furnaces at Odeillo in total, served by 68 heliostats, with powers from 1.5 to 1,000 kW.

The smaller furnaces matter because many experiments do not need a megawatt. Testing a centimetre-scale sample, measuring how a coating's optical properties change at high temperature, or running a small chemical reactor can be done more cheaply and more often on a small furnace. The big furnace is reserved for work that needs a large hot area or a lot of total power, such as testing full-size components.

What the furnace is used for

Materials under extreme heat

The core use of the furnace from the start has been materials science. Because only the sample is heated, and only by light, the furnace avoids the contamination that comes from flames, electrodes or furnace walls. Researchers can also surround the sample with a controlled atmosphere, or a vacuum, to see how it behaves in specific conditions.

Typical questions include:

• At what temperature does a ceramic or composite begin to degrade?
• How does a coating's ability to absorb or reflect light change as it gets hotter?
• What happens to a material's structure when it is heated very quickly and then cooled?
• How do materials react chemically with a surrounding gas at high temperature?

Thermal shock

The ability to switch the beam on and off rapidly makes Odeillo well suited to thermal shock testing. Thermal shock occurs when one part of an object heats or cools much faster than another, producing internal stresses that can crack it. Spacecraft components, engine parts and furnace linings all face this problem. In a conventional furnace, heating is gradual. In a solar furnace, a shutter can expose a sample to intense flux almost instantly.

Space and re-entry

Some of the best-known work at Odeillo has been related to aerospace. French sources describe experiments in controlled atmospheres that simulate the vacuum of space or conditions of atmospheric re-entry on Earth and Mars. IEEE Spectrum reports that CNRS uses the furnace for testing thermal shielding for spacecraft. A CNRS photo caption describes a reactor placed at the furnace's focus to study the thermo-radiative properties of materials for NASA's Parker Solar Probe, which flies closer to the sun than any previous spacecraft. A European project record describes the installation's ability to run large-scale simulations of materials and components under extreme conditions, including near-sun space environments and re-entry.

The English Wikipedia article also states that the high-temperature materials division has used the furnace to study radome survival during re-entry of multiple-warhead missiles (MIRVs) and materials under intense thermal radiation of the kind produced by nuclear explosions. Defence-related testing of this sort has been one of the less-advertised uses of solar furnaces in several countries, because few other tools can produce such intense radiant heat on demand.

Nuclear materials

IEEE Spectrum mentions experiments on cladding for the inside of nuclear reactors. French sources also list waste treatment, including radioactive waste, among the research themes associated with the furnace. In both cases the attraction is the same: very high temperatures in a clean environment.

Solar fuels and chemistry

After the oil shocks of the 1970s, and again in recent decades with interest in decarbonisation, the furnace has been used for solar thermochemistry. The idea is to use concentrated sunlight not to make electricity but to drive chemical reactions that need high temperatures. French sources list water splitting to make hydrogen and the production of synthetic fuels from biomass among the research topics. IEEE Spectrum describes work on "carbon-free methods of producing synthetic fuels".

The appeal is that chemical fuels can be stored and transported in a way that sunlight cannot. If sunlight can drive the reactions that turn water, carbon dioxide or biomass into fuel, solar energy becomes something you can put in a tank. The practical challenge, which Odeillo helps researchers study, is finding materials and reactor designs that survive repeated cycles of extreme heat and still convert energy efficiently.

European research infrastructure

Odeillo is not just a French national asset. Through a series of European Union-funded programmes, including SOLFACE, SFERA and their successors, researchers from other countries have been able to apply for time on the 1 MW furnace and the medium-sized furnaces. A CORDIS project page describes SOLFACE as giving visiting researchers access to the MegaWatt Solar Furnace and the eleven medium-sized solar furnaces. This model, where a few rare facilities serve a whole continent's research community, is common for expensive instruments such as particle accelerators and research reactors.

Advantages of a solar furnace

Compared with other ways of reaching very high temperatures, the Odeillo approach has several clear strengths.

The cleanliness of heating is the main scientific reason solar furnaces exist. The speed of heating and cooling is the second. The low running cost is a welcome bonus but was never the primary motivation.

Limits and drawbacks

The furnace is a remarkable instrument, but it has real constraints.

It depends on the weather

The most obvious limitation is that the furnace only works when the sun is shining directly on the mirrors. Clouds, haze, rain or snow stop experiments. Winter days are short, and the sun is low. Researchers have to plan around the weather, and an experiment can be delayed for days. This is very different from an electric furnace that runs whenever it is switched on.

It is not uniform

The flux at the focus is not perfectly even. It is highest at the centre and falls off towards the edges. For many experiments that is fine, and it can even be useful. For others, especially those that need a large area heated evenly, it creates complications. Researchers often place sensors, calibrated targets or secondary optics at the focus to measure and shape the beam.

Measuring temperature is hard

At thousands of degrees, ordinary thermometers do not survive. Temperatures are often measured optically, using pyrometers that infer temperature from the light a hot object emits. But the sample is also bathed in intense reflected sunlight, which can confuse optical measurements. Separating the light the sample emits from the light it reflects is a technical challenge in its own right and one of the reasons solar furnace research involves so much measurement science.

It does not scale to industry easily

Trombe originally imagined solar furnaces as the basis for industrial processing. That vision has not been realised on a large scale. The quantity of material that can be heated at the focus is small, and building larger concentrators gets expensive quickly. The furnace remains a research tool. Industrial-scale use of concentrated sunlight today tends to take other forms, such as solar tower power plants that heat a fluid and generate steam, rather than furnaces that heat a solid sample to thousands of degrees.

Maintenance

Thousands of mirror facets and dozens of heliostats are exposed to mountain weather: wind, snow, freeze-thaw cycles, dust and ultraviolet light. Keeping them clean, aligned and in good condition is an ongoing job. Small misalignments degrade the focus. The facility is now more than half a century old, and keeping it working requires constant care.

Odeillo and its neighbours

The Cerdagne is unusual in having several solar installations within a few kilometres of each other. People often mix them up.

Mont-Louis

Mont-Louis is the parent of Odeillo, both technically and historically. It is much smaller, about a twentieth of the power, but uses the same two-mirror principle. Its current use is educational: it hosts guided tours and demonstrations, and has been involved in technology-transfer work aimed at bringing solar ovens to villages in countries such as Morocco, according to Wikipedia's article on the site.

Thémis

Thémis is a different kind of machine. It was built as an experimental solar power station by the French electricity utility EDF. Construction began in 1979, and it produced power from 1983 to 1986. Instead of a parabolic mirror, it used a field of 201 heliostats, each about 53.7 square metres, to concentrate sunlight on a receiver at the top of a tower roughly 100 metres high. Molten salt carried heat from the receiver to a steam generator, which drove a turbine. The plant closed in 1986, partly because its cooling system was hard to manage and partly because political and financial support ran out. It lay largely idle for years, was used for a period for gamma-ray astronomy, and since a 2004 rehabilitation programme has been used again for solar research coordinated by CNRS and PROMES, including projects on gas turbines driven by solar-heated air and on particle receivers that also store heat.

The key difference is purpose. Thémis is about converting sunlight into electricity at power-station scale. Odeillo is about producing extreme heat for experiments. Their research programmes overlap, which is why the same laboratory now works with both, but they are different machines.

How Odeillo compares with other solar technologies

It helps to place the furnace among the main ways people use sunlight.

These categories are broad, and specific systems vary. The point is that Odeillo sits at the extreme end of concentration. It sacrifices scale and continuous operation for intensity and cleanliness.

There are a handful of other large solar furnaces in the world. CNRS photo captions describe Odeillo as the most powerful, alongside a large solar furnace at Parkent in Uzbekistan, built in the Soviet era and opened in 1981 according to the Wikipedia article on solar furnaces. Research centres in Spain, Germany, Switzerland, the United States and elsewhere operate smaller high-flux solar furnaces and solar simulators. Many of the design principles used in those facilities trace back to the Mont-Louis and Odeillo work.

Visiting and public engagement

For decades the furnace was also a tourist attraction. From 1990 a public information centre operated on the site, independent of the CNRS laboratory. It was originally called the "Exposition du Grand Four Solaire d'Odeillo" and was renamed Héliodyssée in 2006. It explained solar energy and other renewables to visitors of all ages.

The centre closed on 31 October 2017 after a change of management. The laboratory itself is a working research facility and is not generally open for tours. According to current encyclopedia summaries, some outdoor areas and information panels remain accessible, so visitors can still see the giant mirror and the heliostat field from outside. Anyone planning a trip should check current access arrangements locally, because they have changed over the years.

The furnace has also become something of a regional icon. CNRS has featured it in photo collections and in a 2024 educational video in its "VaSavoir" series presented by Maxime Labat, with researchers Emmanuel Guillot and Jean-Louis Sans of PROMES. The building was listed as a French historic monument in 2009, a recognition of its importance as a piece of 20th-century scientific architecture.

Common misconceptions

"It generates electricity for the region."

No. The Odeillo furnace is a research instrument. The concentrated sunlight is used to heat samples, not to drive a generator. The nearby Thémis plant was built for electricity generation, but that is a separate installation, and it ran as a power station only from 1983 to 1986.

"The big curved mirror tracks the sun."

The parabolic mirror does not move. It is built onto the building. The 63 flat heliostats on the hillside do the tracking, and they send a parallel beam to the fixed parabola. This is the key design choice that made such a large concentrator practical.

"It is the hottest place on Earth."

The furnace can heat suitable samples to more than 3,000 °C, which is extraordinarily hot. But laboratory techniques such as electric arcs, plasmas, lasers and particle collisions can produce higher temperatures, sometimes far higher, though usually over smaller volumes or shorter times. The furnace's distinction is producing very high temperatures cleanly, with sunlight, over a relatively large area.

"10,000 suns means 10,000 times the energy of the sun."

The phrase describes intensity, not total energy. It means that at the centre of the focus, each square metre receives roughly 10,000 times as much power as a square metre of ordinary sunlit ground. The total power is about 1 MW, which is a tiny fraction of the sunlight falling on the whole Cerdagne plateau.

"It could be used as a weapon."

Concentrated sunlight has fascinated people since ancient stories of Archimedes supposedly burning Roman ships at Syracuse with mirrors, a legend that historians doubt. A solar furnace is the opposite of a weapon in practice: it only focuses at one fixed point, 18 metres in front of its mirror, and the focus cannot be aimed elsewhere. It does have a history of defence-related research, as noted above, but that involves testing materials, not projecting heat.

"Mont-Louis and Odeillo are the same thing."

They are related but separate. Mont-Louis (1949, about 50 kW) came first and is now used for education. Odeillo (1960s, about 1 MW) is the larger research furnace. Thémis, a third installation, is a solar tower power plant.

Why the Odeillo furnace still matters

A long-lived research instrument

Few experimental facilities stay useful for more than half a century. Odeillo has done so because the basic need it meets, very high temperatures in very clean conditions, has never gone away. New materials for spacecraft, reactors, engines and energy systems all need testing. The furnace's capabilities have been extended by modern instruments, sensors and control systems, while the core optical design has remained essentially unchanged.

Training and know-how

Odeillo and its sister facilities have trained researchers and engineers in solar concentration, high-temperature measurement and materials testing. That expertise feeds into the wider field of concentrating solar power and solar chemistry. When European projects work on new solar receivers, storage systems or solar reactors, Odeillo's furnaces are often where components are tested before they are scaled up.

A bridge between research and energy transition

Interest in concentrated solar heat has grown again as countries look for ways to decarbonise heavy industry. Many industrial processes, such as making cement, metals, glass and chemicals, need very high temperatures that are difficult to supply with electricity alone. Solar thermochemistry and high-temperature solar heat are among the options researchers are studying. Odeillo is not going to supply industrial heat itself, but it is one of the places where the materials and reactor concepts that might do so are tried out.

Frequently asked questions

Where exactly is the Odeillo solar furnace?

It is in Font-Romeu-Odeillo-Via, in the Pyrénées-Orientales department of the Occitanie region in southern France, in the high Cerdagne plateau close to the Spanish border. Its coordinates are roughly 42°29′N, 2°01′E, and it sits at about 1,535 metres altitude.

When was it built?

Construction ran from 1962 to 1968. The furnace was commissioned in 1969, and some sources date its entry into service to 1970.

Who built and runs it?

It was developed by the French CNRS research organisation, building on the work of chemist Félix Trombe and his team at Meudon and Mont-Louis. It is operated today by the CNRS PROMES laboratory in partnership with the University of Perpignan Via Domitia.

How powerful is it?

About 1 MW of thermal power at full strength, with a peak flux density of around 10 MW per square metre at the centre of the focus.

How hot can it get?

Sources cite maximum sample temperatures from about 3,300 °C, measured on graphite, to 3,500 °C. Temperatures above 2,500 °C can be reached within seconds.

How many mirrors does it have?

There are 63 flat heliostats on the hillside, with a total area of about 2,835 m², and a large parabolic concentrator of about 1,830 m² made up of many small mirror facets. The English Wikipedia entry gives 9,600 reflecting mirrors in total.

Does it work on cloudy days?

No. Solar furnaces need direct sunlight, which cannot penetrate thick cloud. Experiments are scheduled around clear weather.

Can you visit it?

The research laboratory is not generally open to the public, and the Héliodyssée visitor centre closed in October 2017. Some outdoor areas reportedly remain accessible. The older Mont-Louis furnace nearby is used for public demonstrations.

Is it the largest solar furnace in the world?

It is widely described that way, and CNRS describes it as the most powerful along with the furnace at Parkent in Uzbekistan.

Is it a historic monument?

Yes. It was listed as a monument historique in 2009 and carries the "Patrimoine du XXe siècle" label for 20th-century heritage.

Why not just use an electric furnace?

Electric and fuel-fired furnaces are useful, and they work regardless of weather. But they can contaminate samples and often cannot heat or cool as fast. The solar furnace heats only the sample, only with light, and can switch on or off almost instantly. For certain experiments those properties are hard to match.

Has anything been sent into space after testing at Odeillo?

Odeillo has been used to study materials for spacecraft heat shields. A CNRS image record describes a reactor at the focus used to study the thermo-radiative properties of materials for NASA's Parker Solar Probe. Many test campaigns are not publicised in detail, so a full list of flight hardware tested there is not publicly available.

Summary

The Odeillo solar furnace is a giant optical instrument that turns mountain sunlight into a megawatt of tightly focused heat. It grew out of Félix Trombe's experiments with a wartime searchlight mirror at Meudon in 1946 and his first solar furnace at Mont-Louis in 1949, and it was built between 1962 and 1968 on a sunny, high plateau in the eastern Pyrenees. Sixty-three tracking heliostats feed a fixed parabolic mirror of about 1,830 square metres, which focuses up to about 10 MW per square metre onto a target in a tower 18 metres away. On suitable samples, temperatures above 3,000 °C are possible.

Its value lies not in producing energy but in producing clean, controllable, rapidly changing extreme heat. For more than fifty years it has been used to test materials for spacecraft and reactors, to study thermal shock, to develop solar fuels and to train solar scientists. It depends entirely on clear skies, cannot easily scale to industry and requires constant maintenance. Even so, it remains one of the world's most important tools for high-temperature research, and a striking piece of 20th-century scientific architecture.

Sources

• Odeillo solar furnace (Wikipedia)
• Four solaire d'Odeillo (Wikipédia, French)
• PROMES-CNRS: Solar concentrators
• PROMES-CNRS: History of the laboratory
• PROMES-CNRS laboratory home page
• IEEE Spectrum: The Godzilla of solar ovens (2016)
• CNRS Le journal: Four solaire d'Odeillo, dompter le feu sacré (2024)
• CNRS Images: Odeillo solar furnace photo records
• CORDIS: SOLFACE project
• Mont-Louis Solar Furnace (Wikipedia)
• Solar furnace (Wikipedia)
• Themis solar power plant (Wikipedia))