Basic Research Needs for Solar Energy Utilization

H 2 SO 4 at 1,130K, and the University of Tokyo Cycle #3 (UT-3) four-step cycle based on the
hydrolysis of CaBr 2 and FeBr 2 at 1,020 and 870K.

In recent years, significant progress has been made in the development of optical systems for
large-scale solar concentration; such systems are capable of achieving mean solar concentration
ratios exceeding 2,000 suns (1 sun = 1 kW/m^2 ). Present efforts are aimed at reaching
concentrations of 5,000 suns (Steinfeld and Palumbo 2001). Such high radiation fluxes allow the
conversion of solar energy to thermal reservoirs at 2,000K and above, which are needed for
efficient water-splitting thermochemical cycles using metal oxide redox reactions (Steinfeld
2005). This two-step thermochemical cycle (Figure 48) consists of a first-step solar endothermic
dissociation of a metal oxide and a second-step nonsolar exothermic hydrolysis of the metal. The
net reaction is H 2 O = H 2 + 0.5O 2 , but since H 2 and O 2 are formed in different steps, the need for
high-temperature gas separation is thereby eliminated.

H 2

H 2 O

½ O 2

recycl e

HYDROLYSER

xM + yH 2 O = MxOy+ yH 2

SOLAR REACTOR

MxOy= xM + y/2 O 2

MxOy

M

MxOy

Concentrated

Solar Energy

H 2

H 2 O

½ O 2

recycl e

HYDROLYSER

xM + yH 2 O = MxOy+ yH 2

SOLAR REACTOR

MxOy= xM + y/2 O 2

MxOy

M

MxOy

Concentrated

Solar Energy

Figure 48 Solar hydrogen production by water-splitting

thermochemical cycle via metal oxide redox reactions

This cycle was examined for the redox pairs Fe 3 O 4 /FeO, Mn 3 O 4 /MnO, Co 3 O 4 /CoO, and mixed
oxides (Steinfeld 2005 and citations therein). One of the most favorable candidate metal oxide
redox pairs is ZnO/Zn. Several chemical aspects of the thermal dissociation of ZnO have been
investigated (Palumbo et al. 1998). The theoretical upper limit in the energy efficiency, with
complete heat recovery during quenching and hydrolysis, is 58% (Steinfeld 2002). In particular,
the quench efficiency is sensitive to the dilution ratio of Zn(g). Alternatively, electrothermal
methods for in situ separation of Zn(g) and O 2 at high temperatures have been demonstrated
(Fletcher 1999); these enable recovery of the sensible and latent heat of the products. Figure 49
shows a schematic of a solar chemical reactor concept that features a windowed rotating cavity-
receiver lined with ZnO particles that are held by centrifugal force. With this arrangement, ZnO
is directly exposed to high-flux solar irradiation and simultaneously serves the functions of
radiant absorber, thermal insulator, and chemical reactant. Solar tests carried out with a 10-kW
prototype subjected to a peak solar concentration of 4,000 suns proved the low thermal inertia of