Interview with Ignacio Mártil de la Plaza, Doctor in Physics and Professor of Electronics at the Complutense University of Madrid.
What are the main challenges ahead of us in the energy transition?
There are many challenges, but I would essentially highlight two: electrifying the economy and mobility.
Electricity is part of our daily lives. It is present in everything. For example, we are doing this interview by videoconference because there is electricity, our cell phones work because they have a battery that supplies electrical energy, lights work thanks to electricity, etc.
However, electricity represents only 15% to 25% of total energy consumption, depending on the country. If we want to decarbonize the economy, 100% of the energy we consume must be electrical energy. Why? Because we can obtain it from sources that do not emit CO2.
We still have a long way to go to achieve 100% electric power. This requires, on the one hand, electrifying a large part of industry and, on the other, electrifying transport, especially aviation and maritime transport. The challenge ahead is enormous.
What role can renewable energies play?
Renewable energies play an essential role since, together with nuclear fission energy, they are the only ones that do not emit CO2.
Renewable energies play an essential role because, together with nuclear fission energy, they are the only ones that do not emit CO2. All renewable energies are needed, especially wind and photovoltaic, as they go hand in hand. In winter there is not so much sun but the wind blows, and in summer there is little wind but a lot of sun. One of the great evils always blamed on renewables is that they are intermittent and unpredictable, but if you look at the whole set of renewables as a single energy-producing element, those problems are drastically minimized. So we need both.
There is also talk of fission and nuclear fusion as other energy sources that could allow us to solve the problem of decarbonization. For me, however, they are not real alternatives. Nuclear fusion still has a long way to go before it becomes viable and profitable, both in terms of research and economics. And we don’t have time to waste.
As for nuclear fission, it has been in the energy mix since the 1960s and has been stalled for a long time for several reasons, including safety and investment, both in terms of time and money. In the most optimistic case, it will take between 10 and 15 years to start up a nuclear fission reactor. This energy will never represent a higher percentage than it does today, that is, about four percent. It is a very small amount.
What are the main challenges of renewable energies?
At this point, I am going to focus on wind and solar energy, both photovoltaic and thermoelectric.
We talked earlier about intermittency. Another big challenge related to this is energy storage. We need to have efficient and cost-effective procedures that allow us to store energy at times when we are producing it but do not need it. That, for example, is the main challenge for photovoltaics. As long as photovoltaic energy is not being used to decarbonize the economy, that is, to make an industry work, for a car factory, a large steel mill, or a cement plant for example, that energy that is being produced is either dumped into the grid or we lose it. We need to be able to store it efficiently.
There is a lot of talk these days about green hydrogen and other processes, although it remains to be seen in what timeframe it can be feasible. In the future it will be obtained with 100% renewable processes, it will be able to be stored and transported, but as of today this is not yet the case.
There is another challenge that has more to do with political than scientific issues. In this case I am talking specifically about Spain. On the one hand, we need the energy regulatory framework to be stable, i.e., it must not depend on the ups and downs of whoever wins or loses the elections. And on the other hand, we need to bet not only on self-consumption but also, and above all, on large plants, both photovoltaic and wind. Land is needed for this, and it must be done well.
What lines of research are you currently working on in photovoltaic solar energy?
There are several lines of work, although the most important is to improve the efficiency of the dominant technology, which is silicon. Solar photovoltaic energy is completely efficient. It is almost at the very limit of what the theory predicts can be obtained and, according to the International Energy Agency, it is currently the cheapest way of producing electricity.
The problem is that silicon is reaching its theoretical limit, which is around 29%-30% efficiency. This means that out of 100 units of energy received from the sun, only 29 or 30 are transformed into electricity.
Today there are laboratory solar cells that have an efficiency of over 27%, and there are photovoltaic modules on the market that are at levels of 24%. We are already very close to the efficiency limit, but we need more.
Why do we need more? Solar photovoltaic energy, like all renewable energies, is a low-energy density energy, i.e., to obtain a certain amount of energy you need many production units. Therefore, efficiency needs to be increased.
This can be achieved by combining silicon with another solar cell to complement it. This is what perovskite does. Perovskite takes its name from a material, a rock, found in nature called calcium titanate (calcium, oxygen, and titanium) and is named after the Russian mineralogist Lev Perovski. However, the perovskite used specifically in solar cells is synthesized in the laboratory.
It is a material that absorbs part of the solar spectrum much more efficiently than silicon. If we combine, in a structure we call a tandem, a silicon solar cell on the bottom and a perovskite solar cell on top, the perovskite absorbs the most energetic part of the solar spectrum and, at the same time, lets through the less energetic part that will be absorbed by the silicon. In this way, we combine the best of both worlds.
In the laboratory, solar cells with this tandem structure have achieved an efficiency of 31%-32%, compared to the 27% efficiency of silicon alone. And at the semi-commercial level there are tandem modules with an efficiency of 28%, compared to 24% for silicon.
In no more than two or three years there will be commercial silicon and perovskite tandem modules with efficiencies of 30%-32%. That is the efficiency of a nuclear power plant or a gas-fired power plant.
Can these efficiency levels be increased in the future?
If we compare the advances in silicon and perovskite efficiency, we can certainly increase this efficiency.
Silicon solar cells emerged in the late 1950s and had an efficiency of 15%. Today they have an efficiency of 26%. In other words, their efficiency has doubled in 70 years.
If we analyze perovskite, the first solar cell appeared in 2009 and had an efficiency of 4%. Today, perovskite cells alone, without silicon, reach an efficiency of 26%. With this data, imagine what can be achieved in the next five years.
Apart from efficiency, another great advantage of laboratory-synthesized perovskite is that the chemical elements of which it is composed (carbon, hydrogen, nitrogen, lead, etc.) are very abundant in nature. The drawback is that precisely one of these elements, lead, is highly toxic. Hence, one of the lines of work in the field of perovskites is to find an alternative to lead that allows the same levels of efficiency to be maintained. It has been tried to be replaced by tin, but so far, the results have not been very promising.
Can solar panels be recycled?
Today, recycling is one of the biggest businesses emerging from the renewable energy world. Currently, 90% of the components of silicon solar cells are recycled. And in a few years this figure will reach 100%.
Silicon photovoltaic solar energy uses very abundant and non-toxic chemical elements. On the one hand, silicon, which is the second most abundant chemical element in the earth’s crust after oxygen. On the other hand, aluminum, which is the third most abundant element in the earth’s crust. It also uses silver, which, although not as abundant as the previous elements, is also abundant. Finally, it also incorporates very small amounts of other elements such as phosphorus or boron, which are also abundant. In short, all its elements are abundant and not very toxic, which makes recycling much easier.
Will renewable energies completely replace fossil fuels in the future?
This is a big debate. Many people doubt that it is possible to achieve a 100% renewable mix throughout the planet. Today there are countries like Iceland that have achieved this, but of course, it is a country with few inhabitants and spectacular geothermal energy. Achieving this at a global level is a huge challenge. But we have no choice but to move in this direction, even if neither we nor our children will ever see it.
Another of the great challenges of any energy transition is that investments in energy are very long-term investments, i.e., the inertia of energy systems is enormous. Transforming an energy mix is a very long and slow process because the investments are very large, and their amortization is also long.
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