Schematic of solar thermochemical fuel production path. Direct solar radiation is concentrated by a field of heliostats and drives the high-temperature thermochemical conversion of H2O and CO2 to H2 and CO (syngas). The syngas is stored and finally converted into jet fuel via the FT process. Click to enlarge.

The solar thermochemical fuel path is based on the high-temperature conversion of water and CO₂ into syngas (hydrogen and carbon monoxide) and oxygen mediated by ceria redox reactions. To reach the reduction temperatures of 1500 °C and above that are usually required for redox reactions of metal oxides, solar energy is concentrated into the aperture of a thermochemical reactor. A solar tower or dish concentration system can deliver the required level of radiative flux.

Solar syngas is then converted into liquid hydrocarbon fuels by the Fischer−Tropsch process. The produced synthetic paraffinic kerosene is certified for use in commercial aviation in mixtures with a share of up to 50% with conventional jet fuel according to ASTM D7566.

All of the process are fully developed and established in a commercial environment—with the exception of the solar thermochemical production of syngas. Several research groups are currently tackling this.

The plant in their baseline case produces 1,000 bpd of jet fuel with 865 bpd of naphtha as a co-product. The publicly supported solar stand-alone facility, i.e., without external sources of heat or electricity, is located in a region with 2500 kWh/(m² a) of direct normal irradiation, where the concentration facility is a tower system. Thermochemical conversion efficiency is 20%. CO₂ is supplied by an air capture unit located at the plant site and H₂O by a seawater desalination unit 500 km away. Transportation of the fuel is assumed to be carried out over 500 km via pipeline.

Key drivers for the economic and ecological performance of the plant are the thermochemical energy conversion efficiency; the level of solar irradiation; operation and maintenance; and the initial investment in the fuel production plant.

For the baseline case of a solar tower concentrator with CO₂ capture from air, jet fuel production costs of €2.23/L (US$11.83/gallon) and life cycle greenhouse gas (LC GHG) emissions of 0.49 kg_CO2‐equiv/L are estimated.

Capturing CO₂ from a natural gas combined cycle power plant instead of the air reduces the production costs by 15% but leads to LC GHG emissions higher than that of conventional jet fuel.

The use of CO₂ and electricity from a NGCC power plant reduces the costs of jet fuel production as the unit cost for CO₂ provision is lower compared to air capture; however, it considerably increases the life cycle GHG emissions due to the fossil origin of CO₂ used for the fuel synthesis. Emissions of the production process and of fuel combustion can thus not be counterbalanced by negative emissions of CO₂ as in the baseline case using CO₂ capture from the atmosphere. The production of solar thermochemical fuels presents only a viable option over conventional fuels if the CO₂ is captured from renewable sources such as the atmosphere and not from flue gases of a fossil power plant.

—Falter et al.

While the favorable assumptions analysis noted above yields the low price and almost zero LC GHG emissions, even lower production costs may be achieved if the commercial value of oxygen as a byproduct is considered.

Resources

• Christoph Falter, Valentin Batteiger, and Andreas Sizmann (2015) “Climate Impact and Economic Feasibility of Solar Thermochemical Jet Fuel Production” Environmental Science & Technology doi: 10.1021/acs.est.5b03515