Solar fuel breakthrough: sustainable fuels from water, carbon dioxide and sunlight

Madrid Institute for Advanced Studies in Energy (IMDEA Energy)
  • European-Swiss project demonstrates the feasibility of large-scale production of renewable hydrocarbon fuels from water, concentrated sunlight and carbon dioxide.
  • The developed technology by SUN-TO-LIQUID has the potential to contribute significantly towards the reduction of net CO2 emmissions allowing to decrease reliance on fossil fuels
  • The public and private sector have worked hand-to-hand to bring this project to success


Liquid hydrocarbon fuels are ideal energy carriers for the transportation sector due to their exceptionally high energy density and convenient handling. Using a fuel supply of sustainable origin, if compared to conventional fossil-derived jet fuel, net CO2 emissions to the atmosphere could be reduced by over 90%. Additionally, it is advantageous that no changes need to be made to the existing global infrastructure for their use.

Regarding the obtention of liquid hydrocarbon fuels via sustainable production, at present virtually all renewable hydrocarbon fuels originate from biomass. Alas, the feasibility of meeting all the global fuel demand and the related environmental impact with these approaches is a matter which is subject of discussion, in part in connection with the difficulties associated to large-scale biomass-based fuel production.

The transport industry is heavily dependent on hydrocarbon fuels. Above, a freight ship at the Georgia-South Carolina border.

Still, the transition from fossil to renewable fuels is one of the most important challenges to be tackled for the transition towards a green economy. EU and Switzerland fund the SUN-to-LIQUID project, which takes on this challenge. The initiative to produce renewable transportation fuels from water and CO2 by means of concentrated sunlight successfully demonstrated the first synthesis of solar kerosene.


Renewable fuels made from concentrated sunlight, water and carbon dioxide

The approach followed by SUN-to-LIQUID has the potential to be able to cover future fuel consumption needs, as it establishes a non-biomass, non-fossil-fuel path to obtain by synthesis the renewable liquid hydrocarbon fuels required, using abundant feedstocks such as of H2O, CO2 and solar energy.

The driving force to enable the synthesis of the renewable fuels is concentrated solar radiation, which enables to conduct a thermochemical redox cycle which inherently needs to operate at high temperatures, and which utilizes the full solar spectrum. This provides a thermodynamically favourable path to solar fuel production with high energy conversion efficiency and high economic competitiveness.

SUN-TO-LIQUID (S2L) solar field. Sunlight is concentrated at the tower reactor in orden to produce hydrocarbon fuels.

The first-ever production of solar jet fuel had yet been experimentally demonstrated at laboratory scale using a solar reactor containing a ceria-based reticulated porous structure undergoing the redox cyclic process. This took place during the project SOLAR-JET, which developed the technology and achieved the first-ever production of solar jet fuel in a laboratory environment. The SUN-to-LIQUID project scaled up this technology for on-sun testing at a solar tower, a facility where sunlight is concentrated via an extensive network of mirrors that directs sunlight towards the reactor.


SUN-to-LIQUID takes solar fuel technology from the laboratory to the field

“The SUN-to-LIQUID core solar technology and the integrated chemical plant were experimentally validated under real field conditions relevant to industrial implementation,” said Prof. Aldo Steinfeld of ETH Zurich, who leads the solar thermochemical reactor development. “This technological demonstration can have important implications for the transportation sectors, especially for the long-haul aviation and shipping sectors which are strongly dependent on drop-in hydrocarbon fuels,” announced project coordinator Dr Andreas Sizmann of Bauhaus Luftfahrt, “we are now a step closer to living on a renewable energy income instead of burning our fossil energy heritage. This is a necessary step to protect our environment.”

For the purpose of SUN-To-LIQUID, a unique solar concentrating plant was built at the IMDEA Energy Institute in Móstoles, Spain. “A sun-tracking field of heliostats concentrates sunlight by a factor of 2,500 – three times higher than current solar tower plants used for electricity generation,” explains Dr Manuel Romero of IMDEA Energy.

Heliostats are devices with mirrors that reflect sunlight towards a specific target, in the case of SUN-TO-LIQUID, the reactor.

The intense solar flux concentrated at the tower, verified by the flux measurement system developed by project partner DLR, allows to reach reaction temperatures of more than 1,500°C within the solar reactor positioned at the top of the tower. The solar reactor, developed by project partner ETH Zurich, produces synthesis gas, a mixture of hydrogen and carbon monoxide, from water and CO2 via a thermochemical redox cycle. An on-site gas-to-liquid plant that was developed by the project partner HyGear processes this gas to kerosene.

Other expected innovations from the project include an advanced high-flux ultra-modular solar heliostat field, a 50 kW solar reactor, and optimized redox materials to produce synthesis gas that is subsequently processed to liquid hydrocarbon fuels. The complete integrated fuel production chain will be experimentally validated at a pre-commercial scale and with record high energy conversion efficiency. The ambition of SUN-to-LIQUID is to advance solar fuels well beyond the state of the art and to guide the further scale-up towards a reliable basis for competitive industrial exploitation. Large-scale solar fuel production is expected to have a major impact on a sustainable future transportation sector.

SUN-to-LIQUID - Renewable Transportation Fuels from Solar Energy

SUN-to-LIQUID received funding by the European Union’s Horizon 2020 research and innovation programme and the Swiss State Secretariat for Education, Research and Innovation (SERI). SUN-to-LIQUID joined leading European research organizations and companies in the field of solar thermochemical fuel research, namely ETH Zurich, SOMMa member IMDEA Energy, DLR, Abengoa Energía and HyGear Technology & Services B.V. The coordinator Bauhaus Luftfahrt e.V. was also responsible for technology and system analyses. ARTTIC supported the Research Consortium with project management and communication.



Image credits:

Freight ship picture was downloaded from Flickr and licensed via a Creative Commons Attribution 2.0 Generic (CC BY 2.0) license.

Solar tower picture SOLUCAR PR10 was downloaded from Flickr and licensed via a a Creative Commons Attribution 2.0 Generic (CC BY 2.0) license.

Heliostat picture was downloaded from Wikipedia and licensed via a Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.

Ongoing collaboration