Solar Energy Perspectives - IEA

Chapter 9: Solar fuels
in line-focus systems such as trough plants using molten salts as both storage medium and
HTFs. This is currently being developed by ENEA in Italy (Figure 9.2).
Figure 9.2 CSP backed by biomass could produce electricity, heat or cold, hydrogen
and fresh water
CH 4
CH + H 4 2
Heat/cold and
electric power
co-generation
CH4
- steam
reforming unit
Storage tank
with integrated
steam generator
550 o C
Hot fluid
Cold fluid
Power block
290 o C
Desalination unit
Sea water
Fresh water
Molten salts
heater fed
by biomass
Source: ENEA-UTRINN-STD.
Key point
Solar fuels can be generated in parallel with electricity and freshwater.
The production of pure hydrogen from water or from both water and biomass would be
considered a superior form of solar hydrogen since it is based on an extremely abundant and
fully renewable resource (hydrogen is recombined in water when used as a fuel) with no CO 2
emissions. It requires, however, much more research.
Solar thermolysis requires temperatures above 2 200°C, and raises difficult challenges. Watersplitting
thermo-chemical cycles allow operation at lower temperature levels (some less than
1 000°C). But they require several chemical reaction steps, and there are inefficiencies
associated with heat transfer and product separation at each step. Thermal cracking of natural
gas will directly produce hydrogen and marketable carbon black. These options too require
long-term research efforts.
More efficient two-step cycles using reversible reduction-oxidation (redox) reactions can also
be used. This can take place, for example, in rotary kilns, as shown on Figure 9.3. Reduced
reactive ceramics or metals are oxidised by water, generating H 2 . The oxidised ceramics are
then exposed to concentrated solar heat and release O 2 .
The two steps of splitting water, instead of taking place at the focus of a concentrating solar
tower, could also be separated in time and place, offering interesting possibilities for their use
in transportation. Dedicated concentrated solar fuel plants would de-oxidise light elements,
which would be easily transported to customer stations or even within vehicles, where their
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