Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar fuels
Concentrating solar technologies allow producing hydrogen, either from pure water, or from
a carbonaceous feedstock, following various routes (Figure 9.1). Solar towers are the most
obvious candidates to deliver the required high-temperature heat, but large dishes and, in
some cases, line-focus technologies could also be used. The theoretical maximum efficiency
of such an energy conversion process is limited only by the Carnot efficiency of an equivalent
heat engine: with the sun’s surface as a 5 800°K thermal reservoir and the earth as the thermal
sink, 95% of the solar energy could in principle be converted into the chemical energy of
fuels. In practice, however, lower temperatures will be used for material constraints.
Figure 9.1 Routes to hydrogen from concentrating solar energy
H2O-splitting
Decarbonisation
H 2 O
Concentrating
Fossil fuels
solar energy
(NG, oil, coal)
Solar thermolysis
Solar
thermochemical
cycle
Solar electricity
+ electrolysis
Solar reforming Solar cracking Solar gasification
Solar fuels
(hydrogen, syngas)
Optional
CO 2 /C
sequestration
Source: PSI/ETH-Zürich.
Key point
Solar hydrogen can be produced from hydrocarbons or water.
Solar-assisted steam reforming of natural gas, and steam gasification of coal or solid biomass,
can yield syngas, a mixture of carbon monoxide (CO) and hydrogen. This route is certainly
the closest to commercialisation, as steam reforming of natural gas today is the primary route
to producing hydrogen for industrial purposes (mostly manufacturing of ammonia and
fertilisers, and desulphuration of petroleum products). The natural gas is used as both energy
source and feedstock. Replacing natural gas as the energy source saves some gas and reduces
CO 2 emissions. The carbon monoxide can be further processed to give H 2 and CO 2 . The
latter can be easily separated from the former via pressure swing adsorption 1 , while CO 2
mixed in exhaust gases requires more energy and reactants to be captured. However, liquid
transportation fuels, as well as methanol or ammonia, can also be manufactured directly from
syngas using commercially available Fischer-Tropsch processes. Solar-assisted steam
reforming of natural gas can be done at about 850°C, a temperature easily accessible to
point-focus concentrating solar devices. Newly-developed catalytic membrane reformers can
operate at only about 550 to 600°C, making it accessible to the lower concentration factors
1. Pressure swing adsorption is a technology used to separate some gas species from a mixture of gases under pressure according to
the species' molecular characteristics and affinity for an adsorbent material.
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© OECD/IEA, 2011