Solar power in the "oven"

Remember in school when you used your watch face to reflect a spot of sunlight coming in through the window? A very similar system could become the new frontier in solar energy development.  In Europe we already have the most powerful solar furnace in the world, located in Odeillo, in France. It has a power of 1 MW. The second most powerful is in Tashkent, in Uzbekistan.
In this state-of-the-art laboratory, located in the department of Pyrénées-Orientales, we study the phenomena that occur at very high temperatures and the behaviour of materials subjected to extreme environments by exploiting solar radiation. The location was chosen for its weather (the Sun shines for more than 3,000 hours a year) and the purity of its atmosphere.
The general principle by which the furnace works is the concentration of the Sun’s rays through mirrors. The Odeillo furnace is located in front of a hill on which 63 heliostats direct sunlight towards a parabola, also mirrored, which causes it to converge on the top of the central tower.

But solar furnaces are nothing new in this corner of France; the first, considered the precursor of the technology, was built in 1949 in Mont-Louis. Today it’s a tourist attraction that you can visit to understand the workings of the technology, which is also used in cooking. There are various types of solar furnaces on the market. Portable camping stoves are quite a common example. 

The history of Mont-Louis begins with Félix Trombe (1906–1985). After graduating in industrial chemistry with a thesis on rare metals, he decided to take his knowledge of how metals behave at high temperatures even further. Hence the first solar furnace, with a power of 50 kW. Solar radiation, concentrated using a big, mechanically driven parabolic reflector allows you to follow the Sun’s visible motion and reach extremely high temperatures. Using these you can observe the changes and properties in metals and materials free of the contamination you usually get in traditional furnaces and at lower temperatures.

Based on the first solar furnace’s success, the authorities were convinced to fund the building of a second, with a power of 1 MW, in Odeillo.
Then came the 1980s and the first energy crisis of the century. The French national energy company decided to link the solar furnace up to a thermal power station. A curved mirror concentrating the Sun’s rays on a specific focal point reaches temperatures of about 550 °C. A sort of pipe passes through the focal point, carrying a fluid that can store heat then pass it through an exchanger into a structure that can generate industrial steam or turn a turbine and produce electricity.

Trombe’s work keeps coming back to us in the present day. In his day, the man was a star in the world of solar energy, as even then he realised intuitively the importance it could have for developed and developing countries. The scientist was way ahead of the curve; he was also famous for his Trombe wall, which is still used today to heat buildings with passive solar energy. His passion for solar energy led him to design solar-powered distillers for saltwater, to provide fresh water in dry countries, as well as solar dryers and fridges for perishable food.

Trombe died in 1985, so sadly he did not live to see solar energy come back into vogue after prompting from new energy crises and greater awareness of ongoing climate change. 

Besides building Trombe’s solar furnace in Odeillo, France has not only remembered him but celebrated him with a public information centre, renamed the Héliodyssée in 2006.

The centre’s aim is to spread, especially among student, knowledge of renewable energy and the Sun (learning that would be welcome in Italy too, above all to get on board with the UN’s Sustainable Development Goals).

The road to solar energy is still uphill when it comes to using it on a wide scale. The main obstacle to using concentrating solar parabolas today lies in the energy source itself. The Sun only shines during the day, and at varying and intermittent intensities depending on the weather conditions, latitude and time of year. A system based on concentrating heat from the sun is therefore directly influenced by these variables. That’s not all. The fluid that flows through the pipe, usually molten salt, needs a very high temperature (about 270 °C) if it’s to keep flowing. Below that threshold the salt gets too viscous to flow freely. If it drops below 250 °C it actually solidifies into ceramic-like blocks. That would leave a plant useless. Needless to say, at night and in inclement weather, solar-powered plants can’t work as they’re meant to and need energy to keep their heat-transfer fluids at the right temperature.

The main challenge for researchers when it comes to concentrating parabolas is keeping their consumption down in fallow periods. At Eni’s Research Centre for Renewable Energy and the Environment in Novara, they’ve decided to take up this challenge, starting from four fundamental parameters. The first is of course the mirror, for which they’ve chosen a special film made of PET and silver. It’s far more practical and cheap than the traditional glass models. Secondly, to make the pipe more efficient, they’ve come up with a new one made of multiple layers of steel and ceramic, with optimised thermal properties, namely absorbance of 95% and emissivity of 7% at 550 °C. Then there’s what’s inside the pipe. They’ve gone for a mix of molten salts that solidify at far lower temperatures than their traditional counterparts, between 100 °C and 150 °C. That gives you a much bigger window than the usual temperature of around 250 °C. Finally, in light of the industrial applications and commercial potential of these plants, they’ve gone for the simplest possible solutions, in order to be able to employ local workers to commission, construct and assemble the structures in every country where our Italian group works, even if they’re far from big production sites.
These innovative technologies have gradually brought down the costs of concentrating solar parabolas and aroused great interest in the sector, which bodes well for the next industrial and commercial experiments in it. Also, against a backdrop of decarbonization and a gradual move over to renewable energy sources, solar is one of the most promising and worthy of our attention.

And that’s why Eni has decided to test out the technology at its plant in Gela, the biggest in Sicily.

To spread solar energy, the European Union has taken substantial measures to boost its market share and make solar technologies more accessible for consumers, above all through its clean energy package. Solar energy could step up the pace and become one of the cheapest technologies for producing electricity all over the world. Between 2009 and 2018, production costs fell by 75% as the market went on growing. 

As for Italy, it should be noted that local authorities are key to the energy transition, not only because they own a lot of buildings with great potential for energy efficiency, but because they play a crucial role in stimulating local areas with initiatives for more and better use of renewable resources. Trade associations, and above all Italia Solare and ATER, have asked Prime Minister Giuseppe Conte to invest in renewables, above all photovoltaics, because they say the industry needs renewal, moving beyond the classic solar panels with silicon cells to brand-new OPV printed panels.

We all remember the story of how Archimedes stunned the ancient world when he built giant mirrors to burn the Romans ships as they laid siege to Syracuse. The basic principle is more or less the same: building a parabolic reflector begins with using the same polished bronze shields as the great scientist.

The Sun, whose goodness we all rely on, has symbolised fire and energy since ancient times. But it was always shrouded in unfathomable mystery for men who could not get any closer to it. Until now. This quest for knowledge has NASA and the European Space Agency working on two projects, producing valuable information on the phenomena that govern our star: the Parker Solar Probe, launched in August 2018, and the Solar Orbiter, launched on 10 February 2020. Both will redefine our knowledge of how this immeasurable energy source works.