Solar energy will be at the heart of the ecological transition

Interview with Daniel Lincot

CNRS researcher Daniel Lincot has been involved in photovoltaic solar energy research since 1978, where he has contributed to significant advances. His specialty is the interface between chemistry, materials and photovoltaics. He is the author of over three hundred publications and twenty-two patents.
He has been invited to hold the Technological Innovation Liliane Bettencourt Chair for 2021-2022.

Photovoltaic solar energy has been on a roll for the past fifteen years. Why was it so neglected for so long?

Daniel Lincot: I think that the consumption model established by fossil fuels is skewing our fellow citizens' perception of solar energy. We have to realize that what has been done with coal and oil is quite extraordinary. Indeed, since the rise of industrial societies, i.e. since the early ^19th century, the use of these "instant" energy sources has led us to forget the real constraints of nature, with which we have lived for millennia. One of these is intermittency, and this is specific to renewable energies: the wind doesn't blow everywhere, not all the time and, at night, the sun sets. Some people point to this as a major problem, but it's not really. If you offer someone the choice of growing their own tomatoes or buying them ready-prepared from the supermarket, chances are the supermarket route will be preferred. It's the same for energy: we can choose the more restrictive route, which will adapt to the intermittence of renewable energies, or the "earthy" supermarket route, which will consume the reserves, which is fundamentally less sustainable. So why not imagine an energy economy in which we adapt directly to the source, namely the Sun? Our societies have strayed from this philosophy, and have sometimes come to deny the advantages of solar energy, even though it is probably the most universal!

Why is it universal?

It's universal, and very democratic indeed, because, despite its constraints, it's everywhere, in abundance, all the time, and available to everyone like the air we breathe. It has been calculated that the solar energy that reaches us, after reflection by the atmosphere, is on the order of seven thousand times the human consumption of energy on earth. So we have a veritable fountain of energy at our disposal, and a renewable one at that. On a sunny day in Paris, we get around a thousand watts of light per square metre. This means that, in a square of blue sky measuring one kilometer by one kilometer, we have the equivalent power of a nuclear reactor (1 GW), or around five hundred and fifty thousand reactors for the whole of France, on a fine day! For me, it's a no-brainer to use it. To do so, you need the means to harness it and the tools to use it. It's a question of adaptation: you have to constantly adapt to the constraints imposed. The use of fossil fuels - themselves derived from solar energy, it should be remembered - coupled with alternators to produce electrical energy is the result of the same logic of adaptation. The new element, the "breakthrough" as we call it, is that today there is a technology capable of transforming solar energy directly into electricity, without going through any intermediate stages, of "plugging directly into the Sun": photovoltaic solar cells. When Edmond Becquerel discovered the photovoltaic effect in 1839, and the first cells were subsequently developed, few people believed in it; they didn't understand how a system could produce electricity without movement, without any physical activity visible to the naked eye. Yet technology has continued to evolve ever since, and today we're faced with something far more powerful, far more exploitable, than the pioneers could have imagined nearly a century ago.

Today, the best silicon cells achieve record efficiencies of 26.7%. Can we imagine this yield continuing to rise in the years to come?

To imagine this, we need to understand the mathematical relationships behind these efficiencies. The transformation of thermal energy into mechanical or electrical energy depends on the temperature of both the source and the receiver. For this, we have Carnot's famous relationship, established for steam engines in 1824, which gives us a theoretical efficiency fixed solely by these two values. It consists in subtracting from 1 the ratio between the temperature of the receiver and that of the source, i.e. :

In this situation, we take into account the temperature in kelvin, i.e. the temperature in degrees Celsius, to which we add 273. For a solar cell operating in a 20°C environment, i.e. at 293 K, and with the Sun as the emitting source, whose surface temperature is known to be 6,000 K, we obtain a theoretical efficiency of 95%, i.e. a considerable capacity for conversion into useful energy - such as electricity. More advanced theories show that the theoretical limit is closer to 85%, which is still high. If you put a solar panel with this capacity perpendicular to the Sun, the thousand watts per square metre it receives will produce eight hundred and fifty watts of electricity. The grail for researchers is to get as close as possible to this value, and that takes time. With the discovery of modern silicon cells in the 1950s, we went from 1% to 6% efficiency, and here we are today with a record 26.7%. Better still, using much more complex, multi-junction technologies, we've even reached a record 47.1% recently. That's the beauty of this field: it illustrates the combined march of research and technology. We sometimes stagnate for years, and then a discovery or technical advance suddenly boosts yields. It is widely accepted within the research community that we should pass the symbolic 50% threshold within a few years. This will also have an impact on the technologies that can be used by everyone.

What about other types of solar cells, such as perovskite?

In the emerging photovoltaic field, we're seeing a profusion of innovations, such as organic photovoltaics and the spectacular take-off of a new sector based on perovskite cells. This technology was born around 2008 and, in less than fifteen years, its yields have soared to an unprecedented 25.5%! The whole point of perovskite is that it's a gifted material. Researchers are deploying a wealth of creativity in synthetic and materials chemistry. This technology could be combined with current silicon technology to produce so-called "tandem" double-junction cells with a theoretical efficiency of 43%, compared with 29% for silicon alone. What's striking is that research is taking place at the same time as industrialization.

Isn't there a risk that the democratization of solar energy will be accompanied by a depletion of the materials used to manufacture the cells?

I can't envisage the large-scale development of solar energy without it being fully integrated into a logic of respect for the environment and biodiversity, as well as a social logic. Only under these conditions will photovoltaics be able to really take off, and be seen as an innovation in its own right. Some people point to the problem of exploiting "rare earths" to make cells, but today, a solar cell is 99% silicon, the second most abundant element in the earth's crust (around 28%). It is found mainly in the form of silica, in sand, and some projects aim to manufacture silicon cells from desert sand. This raises the question of cost, as it's more expensive to purify sand than to extract pure silicon from silica mines, but it's the right way forward. It's true that certain types of cell require elements that are a little rarer; I work with indium in particular. But it's a question of learning to work with a clear understanding of the facts: by setting up systems to save these materials. Incidentally, when these rare (or non-rare) elements are used in photovoltaics, they are not "consumed" in the same way as petrol is consumed by a car engine. The material remains, reusable ad infinitum.

Can these components of photovoltaic panels be efficiently recycled?

Recycling is an essential issue for all our technologies. A solar panel consists of a glass plate - a material that can be recycled very easily - and silicon wafers that can be crushed (or recovered directly). What you end up with is a kind of ore that's much purer than the original, as extracted from the mines. It's called "urban mining", when one piece of technology at the end of its life is used to make another. Some companies are innovating: they are starting to design their panels without glue, so that they can be dismantled and their various components re-used. Today, the minimum lifespan of a solar panel is around twenty-five years, and could go well beyond that (forty or fifty years), it's a proven and extremely resilient technology, thanks in particular to the advances catalyzed by space research.

What about the carbon footprint of photovoltaics, from panel manufacture to energy storage?

To find out, we carry out life-cycle analyses. Let's take a panel with an efficiency of 20%, which works for twenty-five years, and for which each square meter receives one megawatt-hour (MWh) per square meter per year. Next, let's take into account the carbon and material footprints. We then divide the quantity in kilograms of_CO2 required to produce the panel by the quantity in megawatt-hours of energy produced over its lifetime to obtain "grams of_CO2 emitted per kilowatt-hour (kWh) of energy produced". Today, this value is around 15-45 g of_CO2 per kilowatt-hour for photovoltaics, 10 for wind power and 6 to 12 for nuclear power. Fossil fuels, on the other hand, range from 0.5 kg (gas) to 1 kg (coal). No pun intended, but there's no contest! Photovoltaics is an exceptional low-carbon technology. Incidentally, the carbon footprint depends on the source of electricity. If the panels are manufactured using electricity that already comes from decarbonized sources, such as photovoltaic, wind or nuclear power, the carbon footprint will be lower. It's a virtuous circle.

What role can we expect solar energy to play in the energy mix over the coming decades?

Worldwide, cumulative installed photovoltaic capacity has been growing exponentially for years, reaching 750 GW, with 143 GW installed by 2020 alone. In Germany, photovoltaic solar power already accounts for over 10% of electricity generated, and 2.5% in France. Renewable energies are gradually gaining in importance, and in some countries, such as Denmark, they are even becoming the majority. The urbanized part of France corresponds to around 50,000 km² - 9% of the territory is built-up. To obtain the equivalent of today's electricity production - around 500 TWh - we would need around 5,000 km² of surface area equipped with photovoltaic panels! When we say that, many people imagine that we'd need to cover an entire département. In reality, this surface area already exists in a completely decentralized way on roofs and in artificial areas! When an architect designs a new building, he should include solar panels from the outset. Farmers could and should become energy producers, because they have the resource of space! As the concept of agri-photovoltaics develops rapidly, we could extend it to systems of panels that deploy reversibly at the end of harvest to capture light. I call this concept "photovoltaic harvesting". There is also considerable potential for floating photovoltaics, which are fast becoming a major innovation. Its development at sea also offers great prospects. We could also imagine low-altitude aerial solar power (solar balloons) and even more futuristic developments with space photovoltaics. This is not a pipe dream, and the key issue today is simply the economic competitiveness of current systems. In France, photovoltaic tenders are currently priced at around six cents per kilowatt-hour, compared with five cents in Germany and even two cents in Saudi Arabia. Very few would have imagined this situation just ten years ago. With equally spectacular progress in battery storage and green hydrogen, synergies with electrification and energy efficiency, and tomorrow's access to chemical transformation technologies inspired by photosynthesis, we can imagine a global energy system based on solar energy. It's already at the heart of the energy transition and, beyond that, will be at the heart of the ecological transition!

Interview by William Rowe-Pirra, science journalist