Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar fuels
oxidation with water produces hydrogen (and heat). Oxides are then returned to the solar
plants. Aluminium, magnesium and non-metallic elements such as boron are good candidates
as energy carriers in such schemes, although zinc seems to offer the best properties for
efficient reduction in solar receivers. These solar fuels will thus be solids, easier to store and
transport than gases, while not intended to being used as solids when final, useful energy is
to be delivered.
Figure 9.3 Two-step water splitting based on redox reactions generating H 2
from sun and water
Endothermic
reaction
Reduced reactive
ceramics
Separation
MO = MO + 1/20 (g)
ox red 2
O2-releasing
reaction cell
H O 2
H 2
O 2
Concentrated
solar heat
Ar carrier gas
H2-generation
reaction cell
Oxidised reactive
ceramics
MO
red
+ H2O(g) = MOox + H2(g)
Note: M = metal.
Source: Tamaura, Kaneko et al., Solutions Research Laboratory, Tokyo Institute of Technology.
Key point
Small-scale production of solar hydrogen from water has been demonstrated.
This way of carrying and storing hydrogen and its huge energy content per unit of mass is
likely to be more effective than compressing or liquefying hydrogen, both processes
entailing important energy losses. If oxidation does not take place in cars and planes, it
could take place in refuelling stations or larger plants and serve road and maritime
transportation, as well as some industrial uses. In fact, the products are metals which are
usable as fuels to generate either high-temperature heat via combustion or electricity via
fuel cell and batteries (for example, zinc-air batteries), or to generate hydrogen and use it
in various forms.
As scientists from the Paul Scherrer Institute Aldo Steinfeld and Robert Palumbo note,
“hydrogen can be further processed to make other fuels or it can be used directly for
producing electricity or other forms of power. Once the hydrogen is expended, it will convert
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© OECD/IEA, 2011