The EU energy roadmap for 2050 aims at a 75% share of renewables in the gross energy consumption.

Achieving this target requires a significant share of alternative transportation fuels, including a 40% target share of low carbon fuels in aviation (*1). Therefore the European Commission calls for the development of sustainable fuels from non-biomass non-fossil sources.

In contrast to biofuels, solar energy is undisputedly scalable to any future demand and is already utilized at large scale to produce heat and electricity. Solar energy may also be used to produce hydrogen, but the transportation sector cannot easily replace hydrocarbon fuels, with aviation being the most notable example. Due to long design and service times of aircraft the aviation sector will critically depend on the availability of liquid hydrocarbons for decades to come (*2). Heavy duty trucks, maritime and road transportation are also expected to rely strongly on liquid hydrocarbon fuels (*3). Thus, the large volume availability of ‘drop-in’ capable renewable fuels is of great importance for decarbonizing the transport sector.

This challenge is addressed by the four year solar fuels project SUN-to-LIQUID kicked off in January 2016.


The European H2020 project aims at developing a


as a highly promising fuel path at large scale and competitive costs.



Solar radiation is concentrated by a heliostat field and efficiently absorbed in a solar reactor that thermochemically converts H2O and CO2 to syngas which is subsequently processed to Fischer-Tropsch hydro-carbon fuels. Solar-to-syngas energy conversion efficiencies exceeding 30% can potentially be realized (*4) thanks to favourable thermodynamics at high temperature and utilization of the full solar spectrum (*5).



Expected innovations

The following key innovations are expected from the SUN-to-LIQUID project:

  • Advanced modular solar concentration technology for high-flux/high-temperature applications.
  • Modular solar reactor technology for the thermochemical production of syngas from H2O and CO2 at field scale and with record-high solar energy conversion efficiency.
  • Optimization of high-performance redox materials and reticulated porous ceramic (RPC) structures favourable thermodynamics, rapid kinetics, stable cyclic operation, and efficient heat and mass transfer.
  • Pre-commercial integration of all subsystems of the process chain to solar liquid fuels, namely: the high-flux solar concentrator, the solar thermochemical reactor, and the gas-to-liquid conversion unit.


SUN-to-LIQUID will design, fabricate, and experimentally validate
a large-scale, complete solar fuel production plant

The preceding EU-project SOLAR-JET has recently demonstrated the first-ever solar thermochemical kerosene production from H2O and CO2 in a laboratory environment (*6). A total of 291 stable redox cycles were performed, yielding 700 standard litres of high-quality syngas, which was compressed and further processed via Fischer-Tropsch synthesis to a mixture of naphtha, gasoil, and kerosene (*7).

As a follow-up project, SUN-to-LIQUID will design, fabricate, and experimentally validate a more than 12-fold scale-up of the complete solar fuel production plant and will establish a new milestone in reactor efficiency. The field validation will integrate for the first time the whole production chain from sunlight, H2O and CO2 to liquid hydrocarbon fuels.

SUN-to-LIQUID will realize three sub-systems.

  • A high-flux solar concentrating subsystem
    Consisting of a sun-tracking heliostat field, that delivers radiative power to a solar reactor positioned at the top of a small tower.
  • A 50 kW solar thermochemical reactor subsystem
    For syngas production from H2O and CO2 via the ceria-based thermochemical redox cycle, with optimized heat transfer, fluid mechanics, material structure, and redox chemistry.
  • A gas-to-liquid conversion subsystem
    Comprising compression and storage units for syngas and a dedicated micro FT unit for the synthesis of liquid hydrocarbon fuels.

SUN-to-LIQUID will run a long-term operation campaign
SUN-to-LIQUID will parametrically optimise the solar thermochemical fuel plant on a daily basis over the time scale of months under realistic steady-state and transient conditions relevant to large-scale industrial implementation.


The SUN-to-LIQUID approach uses concentrated solar energy to synthesize liquid hydrocarbon fuels from H2O and CO2. This reversal of combustion is accomplished via a high-temperature thermochemical cycle based on metal oxide redox reactions which convert H2O and CO2 into energy-rich synthesis gas (syngas), a mixture of mainly H2 and CO (*8). This two-step cycle for splitting H2O and CO2 is schematically represented by:

The thermochemical process

Since H2/CO and O2 are formed in different steps, the problematic high-temperature fuel/O2 separation is eliminated. The net product is high-quality synthesis gas (syngas), which is further processed to liquid hydrocarbons via Fischer-Tropsch (FT) synthesis. FT synthetic paraffinic kerosene derived from syngas is already certified for aviation.

SUN-to-LIQUID uses concentrated solar radiation as the source of high-temperature process heat to drive endothermic chemical reactions for solar fuel production (*9). A variety of redox active materials have been explored by different research groups (*10). Among them, non-stoichiometric cerium oxide (ceria) has emerged as an attractive redox active material because of its high oxygen ion conductivity and cyclability, while maintaining its fluorite-type structure and phase.

Reactor configuration

The laboratory-scale solar reactor for a radiative power input of 4 kW has been designed, fabricated, and experimentally demonstrated at ETH Zurich. The reactor configuration, which was used in the FP7-project SOLAR-JET, is schematically shown below.


It consists of a cavity receiver  containing a reticulated porous ceramic (RPC) foam-type structure made of pure CeO2 that was directly exposed to concentrated solar radiation. The production of H2 from H2O, CO from CO2, and high quality syngas suitable for FT synthesis by simultaneously splitting a mixture of H2O and CO2 has been demonstrated (*11).

The main objective of SUN-to-LIQUID is the scale-up and experimental demonstration of the complete process chain to solar liquid fuels from H2O and CO2 at a pre-commercial size, i.e. moving from a 4 kW setup in the laboratory to a 50 kW pre-commercial plant in the field. SUN-to-LIQUID will demonstrate an enhanced solar-to-fuel energy conversion efficiency and validate the field suitability.

SUN-to-LIQUID will demonstrate an enhanced solar-to-fuel energy conversion efficiency and validate the field suitability.

The high-flux solar concentrating subsystem consists of an ultra-modular solar heliostat central receiver that provides intense solar radiation for high temperature applications beyond the capabilities of current commercial CSP installations. This subsystem is constructed at IMDEA Energía at Móstoles Technology Park, Madrid, in 2016. The customized heliostat field makes use of most recent developments on small size heliostats and a tower with reduced height (15 m) to minimize visual impact. The heliostat field consists of 169 small size heliostats (1.9 m x 1.6 m). When all heliostats are aligned, it is possible to fulfil the specified flux above 2500 kW/m2 for at least 50 kW and an aperture of 16 cm, with a peak flux of 3000 kW/m2. A reliable road map for competitive drop-in fuel production from H2O, CO2, and solar energy will be established.