Rising energy requirements and challenges linked to fossil fuel combustion have intensified the focus on more eco-friendly energy alternatives, like solar energy. However, there are still sectors where carbon-based fuels prevail, particularly within the aviation sector. To meet this demand, researchers have been striving to create a method to harness sunlight for generating solar-thermal heating, which could subsequently drive the necessary chemical reactions to produce jet fuel with zero net carbon emissions.
Presently, a group at Caltech, part of a Department of Energy (DOE) Energy Innovation Hub named the Liquid Sunlight Alliance (LiSA), has crafted such a solar-thermal heating apparatus on a limited scale and shown that it can effectively catalyze a crucial reaction for jet fuel synthesis. Entirely run on solar power, the so-called photothermocatalytic reactor integrates a spectrally selective solar absorber to optimize solar-thermal heating production. The reactor’s modular layout leverages contemporary fabrication techniques and the current infrastructure for silicon solar panel production. The team has successfully demonstrated a lab-scale operation of the reactor, and simulations indicate that the technology could potentially scale to sizes that are representative of commercial silicon-based thin-film technologies.
“This apparatus illustrates that the heat produced by abundant solar energy can be directly utilized to facilitate catalytic processes, which have traditionally been accomplished using electricity or fossil fuels,” states Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science, Otis Booth Leadership Chair of the Division of Engineering and Applied Science, as well as the director of LiSA.
The article is available online and will be featured in this month’s printed issue of the journal Device. The lead author of this research is Magel P. Su (PhD ’24), who was responsible for designing and building the solar absorber during her graduate studies in the Atwater Group.
The reactor includes a selective solar absorber designed with multiple layers. The purpose of such an absorber is to capture the maximum amount of the solar spectrum while minimizing heat loss to the environment. “Achieving that with a single material is quite challenging, so we opted for a multilayer architecture,” elaborates Caltech’s Aisulu Aitbekova, a co-author of the new study and a Kavli Nanoscience Institute (KNI) Postdoctoral Scholar Research Associate in Applied Physics and Materials Science. The Caltech team devised a stack of layers composed of materials such as silicon, germanium, and gold meticulously deposited onto a silver substrate. “Each layer has a distinct function, but when combined, they achieve the desired results,” says Aitbekova.
In this setup, a quartz window at the top allows light to penetrate the solar absorber; a vacuum layer aids in minimizing heat losses; and the solar absorber is positioned at the bottom, in direct contact with the chemical reactor. The selective solar absorber reaches a calculated peak temperature of 249 degrees Celsius under one sun illumination and 130 degrees Celsius under standard operating conditions (25 degrees Celsius, 1 atm).
The team harnessed the produced solar-thermal heating to carry out ethylene oligomerization, a chemical process that has conventionally depended on heat generated from burning fossil fuels. The oligomerization reaction commences with ethylene (C2H4), a hydrocarbon consisting of two carbon atoms linked by a double bond, and can be utilized to synthesize longer hydrocarbon chains known as alkenes, which also feature a carbon–carbon double bond. Jet fuels comprise a wide range of hydrocarbon chain lengths, varying from seven to 26 carbon atoms. In the recent research, the Caltech scientists succeeded in producing liquid alkene products with a similar range of carbon atoms using solar energy as the sole driving force.
Unlike concentrated solar technology, the reactor does not need solar tracking. Solar tracking enables a solar collector, reflector, or photovoltaic panel to follow the sun throughout the day to optimize solar radiation absorption. However, solar tracking systems tend to be more costly compared to devices fixed at a specific angle and orientation.
“We are not in competition with concentrated solar technology, which can achieve up to 2,700 suns,” Aitbekova notes. “We are seeking a complementary technology that can be implemented in scenarios where concentrated solar is impractical.”
In this study, the team began with ethylene, which is currently sourced from fossil fuels. However, Aitbekova mentions that the LiSA team has recently published another publication illustrating how to produce ethylene from carbon dioxide (CO2), water, and sunlight. “Now, we present two steps: Initially, we utilize CO2, water, and sunlight to create ethylene, followed by ethylene oligomerization. And solar energy serves as the only energy input for the system.”
Additional contributors to the paper, “A photothermocatalytic reactor and selective solar absorber for sustainable fuel synthesis,” include current Caltech graduate students Matthew Salazar and Fabian J. Williams (MS ’24), former Caltech postdoctoral researcher Xueqian Li, and ex-graduate student Shuoyan Xiong, postdoctoral fellowship trainee Matthew Espinosa, and faculty members from Caltech, Jonas C. Peters and Theodor Agapie (PhD ’07). Peters holds the position of Bren Professor of Chemistry and is the director of the Resnick Sustainability Institute; Agapie serves as the John Stauffer Professor of Chemistry and the executive officer for chemistry. In addition to backing from the Liquid Sunlight Alliance, the initiative benefited from resources and infrastructure provided by the Kavli Nanoscience Institute at Caltech.