Source: Die Weltwoche
Aldo Steinfeld next to the Solar Refinery 2024
ETH professor Aldo Steinfeld is one of the pioneers of synthetic fuels. He explains how to capture the solar energy in such a way that it can be poured into the fuel tank.
When will synthetic fuels, which can be handled like gasoline, diesel or kerosene, and that are not of fossil origin but are climate neutral, finally arrive? The automotive and aviation industries are eagerly speculating about such super-remedies. Are they coming?
We ask an expert who is working on it: Aldo Steinfeld is a Full Professor of Renewable Energy Carriers at the Department of Mechanical and Process Engineering of ETH Zurich. He is a prominent researcher in the field of solar technology and solar fuels, with extensive scientific publication activity, 27 patents, and the supervision of sixty doctoral theses.
His research group Professorship of Renewable Energy Carriers (PREC) at ETH Zurich has developed technological innovations in the field of solar energy conversion and produced two successful spin-offs: Climeworks and Synhelion. Climeworks is focused on the technology for direct CO2 capture from air, Synhelion markets the technology for the production of solar fuels, as shown here on the previous pages.
Weltwoche: Professor Steinfeld, in your scientific career you have focused primarily on the generation of solar fuels. How did you come to it? Was there a decisive experience that brought you on this path?
Aldo Steinfeld: Thank you for this question. As I approach retirement at the end of the spring semester of this year, it is a good time to reflect on past experiences. It was an amazing journey. Since the early days of my career, the signature portion of my research has been the production of solar fuels. The motivation was clear: with only 0.1 percent of the Earth’s land area, one could collect more than enough solar energy to meet the energy demand of the world’s population. Furthermore, the solar energy reserve is unlimited and its use is ecologically benign. Good enough reasons to expect the vast utilization of solar energy, if it were not for some very serious drawbacks, namely: the solar radiation that reaches the Earth is very dilute, intermittent and unequally distributed. I asked myself: How can we get hold of a sunbeam such that it can be stored, transported and used to propel our cars, ships and planes?
Weltwoche: According to the motto: “Put the sun into the tank”?
Steinfeld: Yes, this question motivated me to search for recipes that convert sunlight into storable chemical energy, in the form of fuels. Airplanes flying with fuels made according to such a recipe would actually fly with solar energy, even if it is rainy night.
Weltwoche: For many, hydrogen is considered the fuel of the future. Do you agree?
Steinfeld: Initially, my focus was on the solar production of hydrogen from water for the purpose of decarbonising transportation and other industrial sectors. However, in view of the challenges of storing, distributing and using H2 as a fuel for transportation, I have re-focused my research on the solar production of so-called drop-in fuels from water and CO2.
Weltwoche: So basically what you can pour into canisters?
Steinfeld: Absolute, into canister or directly in the tank. Drop-in fuels are synthetic liquid hydrocarbons, such as kerosene, gasoline or diesel, which can utilize the existing massive transportation infrastructure for the storage, distribution and end-use of fuels, and therefore do not require any new technologies beyond the production chain. Drop-in fuels can therefore readily replace fossil fuels, and in particular solar kerosene can make aviation more sustainable.
Weltwoche: How did you get to ETH in Switzerland?
Steinfeld: After my doctoral studies at the University of Minnesota in the USA and a postdoctoral research stay at the Weizmann Institute of Science in Israel, I joined the Paul Scherrer Institute, PSI, in 1991, where I later served as the Head of the Solar Technology Laboratory. It was a unique opportunity to help build a research group in the emerging and promising field of solar chemistry. In 1999, I was appointed Professor at ETH Zurich and gradually transferred my research from PSI to ETH. My research program at ETH for the last 25 years has been aimed at the further development of the chemical engineering sciences applied to solar energy technologies.
Weltwoche: Did “Swiss” play a role in the selection of your research topics, or is the university a completely global institution?
Steinfeld: Both. My research was guided by the main challenges in energy and the environment at the global scale and in Switzerland.
Weltwoche: Many people imagine that in the future the transport sector can use synthetic liquid fuels, such as gasoline, diesel or kerosene today, with the advantage that these new substances do not harm the climate. A dream combination. What is your comment about these hopes?
Steinfeld: My comment on this is that this dream combination, this dream, is becoming a reality. In 2019, we demonstrated for the first time worldwide the production of sustainable drop-in fuels from two ingredients: sunlight and air. This pioneer demonstration was carried out under real field conditions in our solar mini-refinery mounted on the roof of the ETH Machine Laboratory in Zurich. In 2024, Synhelion commissioned the first industrial solar refinery. And in 2027, Synhelion will operate a ten times larger solar refinery in Spain and ramp up commercial production capacity.
Weltwoche: A central step in the production of drop-in fuels is to remove hydrogen from the water, snatch the H-atoms from H2O, then further steps are taken with the hydrogen. Can you briefly describe the process steps and what the resulting products are?
Steinfeld: The solar refinery for the production of drop-in fuels integrates in series three thermochemical conversion units, which are shown in Figure 1 schematically and with some technical language: Station 1 is the direct air capture (DAC) unit, which co-extracts CO2 and H2O from ambient air. An alternative option to DAC is the use of biogenic CO2 from biomass. Station 2 is the solar redox unit (SR), which converts CO2 and H2O into a desired mixture of CO and H2 (synthesis gas) using concentrated solar energy. Station 3 is the gas-to-liquid (GTL) unit, which converts synthesis gas into liquid hydrocarbon products such as kerosene, gasoline, diesel or methanol. These are the drop-in fuels that are ready for use in the existing global transportation infrastructure.
Weltwoche: And these are climate neutral?
Steinfeld: Yes, these are carbon-neutral fuels because solar energy is used for their production and because they release only as much CO2 during their combustion as was previously extracted from the air for their production. The life cycle assessment of the production chain for solar kerosene indicates an 80 percent avoidance of greenhouse gas emissions with respect to fossil kerosene, and approaching zero emissions when construction materials such as steel and glass for the heliostat field are manufactured using renewable energy.
Weltwoche: Does your system work, in simple terms, by directing solar energy with mirrors and concentrating it on a certain point, where it becomes very hot, and then this heat drives the important chemical reactions?
Steinfeld: Correct. In the solar refinery of Synhelion in Jülich, a field of sun-tracking parabolic mirrors, called heliostats, focusses the incident sunlight into a solar receiver mounted on a tower [see adjacent photo]. In this solar receiver, the concentrated solar radiation is efficiently absorbed and converted into heat at over 1200 degrees Celsius, which in turn is delivered to the chemical reactor for the thermochemical production of fuels. This pathway to solar fuels uses the entire solar spectrum and thus offers potentially high production rates and efficiency.
“Ready for use in the existing global transportation infrastructure.”
“Robustness of the technology”: Synhelion’s solar refinery “Dawn” in Jülich, Germany.
Weltwoche: How close are the systems that your scientists model from practical maturity? Is the start-up Synhelion a typical example?
Steinfeld: Yes, the systems are approaching technical maturity. Indeed, the technology is being scaled-up for commercial application by Synhelion – a pioneer in the field of solar fuels. In 2024, Synhelion commissioned the first industrial-scale solar refinery, called “Dawn”, to demonstrate the robustness of the technology. In 2027, Synhelion will operate a ten times larger solar refinery called “Rise” in Spain with an annual thruput of a 1000 tons of solar fuel. Commercial production capacity will be ramped up with the aim of producing one million tons per year by 2033 and covering around half of the European demand for Sustainable Aviation Fuel, called SAF, by 2040.
Weltwoche: Which drop-in products would be well suited for aviation, shipping, commercial vehicles, cars, stationary stores?
Steinfeld: Drop-in fuels such as synthetic gasoline and diesel are fully compatible with the internal combustion engines of passenger cars, trucks and maritime shipping, while synthetic kerosene is fully compatible with the jet engines of aircrafts. It is important to note that these drop-in fuels, when produced using solar energy and CO2 from the air or from a biogenic source, are sustainable fuels.
Weltwoche: Which of these products are the simplest to use in practice?
Steinfeld: As already mentioned, all these drop-in fuels can use the existing transportation infrastructure for storage, distribution and end-use and therefore do not require new technologies beyond the production chain. This means that drop-in fuels can replace fossil-derived gasoline, diesel, or kerosene or be blended with them. Due to the challenge of decarbonizing aviation, solar kerosene is the most attractive as a SAF.
Weltwoche: The production of drop-in fuels with solar energy is more expensive than fossil fuels. Which factors are decisive for this cost disadvantage? Can this disadvantage be compensated by the benefits of solar fuels? In which constellations would this be possible?
Steinfeld: Techno-economic analyses of the entire process chain described in Figure 1 estimated a long-term cost of solar kerosene in the range of one to two euros per litre. These cost values are predominantly sensitive to the energy efficiencies of the solar redox unit, the CO2 costs by DAC or biomass processing and the manufacturing costs of the heliostat field shown in Figure 1.
Weltwoche: But for the time being the cost difference is much higher?
Steinfeld: For the time being yes, but it will come down, first through scaling effects and process optimizations, and then through mass production of key components and learning-by-doing. I believe that the most appropriate instrument for bringing solar fuels to market would be a quota system that mandates airlines to introduce a minimum share of SAF in their total kerosene volume. This quota would initially be small and rise each year, leading to new solar refineries and that in turn to falling costs, as we observed with photovoltaics. For example, the EU has already adopted a plan to impose a quota of 2 percent SAF this year, rising to 20 percent by 2035 and to 70 percent by 2050.
Weltwoche: Are you optimistic about the future?
Steinfeld: Certainly. The technologies developed in our laboratories at ETH can be scaled-up to competitive costs and at a global scale, and can therefore contribute to the decarbonization of the aviation sector.
Original article in German:
“Der Traum wird Wirklichkeit”
https://weltwoche.ch/story/der-traum-wird-wirklichkeit/
