The unique solar fuel research facility at IMDEA Energía in Móstoles, Madrid, Spain (Figure 1) achieved a new record of > 5% in solar-to-syngas energy conversion efficiency and a pioneering long-term routine operation of > 100 cycles. It demonstrated
1) the tracking performance of the high-flux solar concentrating subsystem
2) the stability and performance of advanced redox materials in the up-scaled solar thermochemical reactor subsystem, and
3) the full integration of the gas-to-liquid (GtL) conversion subsystem for on-site solar fuel production.
A field of 169 sun-tracking heliostats delivered 50 kW of concentrated solar radiative power to the solar reactor positioned on top of the solar tower, driving the thermochemical co-splitting of H2O and CO2 via a redox thermochemical cycle at temperatures of 1500°C, and producing high-quality syngas which was further processed to kerosene. The system performance under realistic field conditions represents a pioneering achievement and demonstrates the technical feasibility of the solar technology for producing drop-in fuels at an industrial scale.
The solar reactor, designed and fabricated by ETH Zürich consisted of a cavity-receiver containing a porous redox structure directly exposed to the incoming concentrated solar radiation. Balance of plant and control instrumentation were implemented for the execution and evaluation of the solar redox cycles. The solar flux characterization a flux distribution measurement system was contributed by DLR, and also a calorimeter of identical aperture was integrated at the platform as well. The Fischer-Tropsch GtL subsystem from HyGear collected and converted the solar syngas to hydrocarbon fuel.
Advanced material development at ETH Zürich achieved superior porous structures made of ceria – the redox material with fast redox kinetics. The thermodynamic properties of undoped and doped ceria were determined and their redox performance were evaluated, and reticulated and hierarchically ordered porous structures were experimentally assessed for their ability to volumetrically absorb high-flux solar irradiation. Advanced heat management and heat recovery concepts were analysed with CFD modelling. Materials for a heat accumulator were experimentally tested by Abengoa.
For future development directions, Abengoa and DLR led a MW-scale plant study that investigated the receiver-reactor performance for a wide set of operational and design parameters. As a result of the analysis, several key factors for high-performance systems were identified: implementation of advanced redox structures, solid heat recovery, coherent optimization of the solar concentration system in combination with the actual receiver-reactor array design and operational strategy.
The techno-economic and environmental performance analyses of future commercial plants showed an emission reduction potential of 80% compared to conventional jet fuel, at estimated production costs of 2.0 €/L in the baseline case and of 1.2 €/L under favorable conditions.
The objectives of the project were fully reached. Solar fuels from H2O and CO2 were successfully produced in a solar tower and the highest solar-to-fuel energy efficiency for a thermochemical fuel plant of this kind was shown in June 2019. The joint press release of the consortium resulted in widespread visibility through media coverage with over 100 news articles and TV coverage in leading news channels. Social media activities complemented the project events. The project results were presented at conferences and published in high-impact refereed journals to set the scientific background for future R&D work. The SUN-to-LIQUID technology now stands for European leadership in this field at the time of completion of the project.
