From sunlight directly to fuel
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. This two-step cycle for splitting H2O and CO2 is schematically shown in Figure 1.
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.
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. This includes moving from a 4 kW lab scale setup, which has been experimentally demonstrated at ETH Zurich in the FP7-project SOLAR-JET (see Figure 2), 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.
Figure 2: Scheme of the solar reactor
P. Furler, J. Scheffe, M. Gorbar, L. Moes, U. Vogt, and A. Steinfeld, Solar thermochemical CO2 splitting utilizing a reticulated porous ceria redox system, Energy & Fuels 26, 7051, 2012.
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 (20 m) to minimize visual impact. The heliostat field consists of 169 small size heliostats (1.9 m x 1.6 m) and provides a average flux density of 2500 kW/m² at the aperture of the 50kW receiver.
The following key innovations are expected from the SUN-to-LIQUID project: