Basic Research Needs for Solar Energy Utilization

the reactor system — ZnO surface
temperature reached 2,000K in 2 seconds
— and its resistance to thermal shocks.

Solar Thermal Decarbonization of
Fossil Fuels. The complete substitution of
fossil fuels by solar hydrogen is a long-
term goal. Strategically, it is desirable to
consider mid-term goals aiming at the
development of hybrid solar/fossil
endothermic processes, in which fossil
fuels are used exclusively as the chemical
source for H 2 production and concentrated
solar radiation is used exclusively as the
energy source of process heat. The
products of these hybrid processes are
cleaner fuels than their feedstock because
their energy content has been upgraded by
the solar input in an amount equal to the
enthalpy change of the reaction. The mix
of fossil fuels and solar energy creates a
link between today’s fossil-fuel-based
technology and tomorrow’s solar chemical
technology. It also builds bridges between
present and future energy economies
because of the potential of solar energy to
become a viable economic path once the
cost of energy will account for the
environmental externalities from burning
fossil fuels, such as the cost of greenhouse
gas mitigation and pollution abatement.
The transition from fossil fuels to solar
fuels can occur smoothly, and the lead
time for transferring important solar
technology to industry can be reduced.
Hybrid solar/fossil processes offer a viable
route for fossil fuel decarbonization and
CO 2 avoidance, and further create a
transition path toward solar hydrogen.

Three thermochemical processes are
considered: (1) solar thermal decompo-
sition, (2) steam reforming, and (3) steam
gasification. These processes, depicted in Figure 50, make use of high-temperature solar heat for
driving the endothermic transformations. Since the reactants contain carbon, an optional C/CO 2
sequestration step is added to the scheme for CO 2 -free production of H 2. However, even without

rotating cavity

quartz window

CPC

feede

outlet

quench

ZnO-layer

ZnO

Zn + ½O 2

Concentrated

Solar Power

Figure 49 Schematic of a solar chemical reactor

for the thermal dissociation of ZnO to Zn(g) and O 2

at above 2000K, as part of a two-step water-

splitting thermochemical cycle (Steinfeld and

Palumbo 2001)

Solar Hydrogen

Solar

Decomposition

Solar

Gasification

Solar

Reforming

CO 2 /C

Sequestration

H 2 O Fossil Fuels H 2 O

(NG, oil, coal)

Concentrated Solar Power

Figure 50 Scheme of three hybrid solar

thermochemical routes for the production of

hydrogen by thermal decarbonization of fossil

fuels: solar decomposition, solar reforming, and

solar gasification