Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 265

active investigation (Haltiwanger et al., 2010; Steinfeld 2002; Melchior, 2009):

ZnO = Zn + ½O 2 (9.2.9)

Zn + H 2 O = ZnO + H 2 (9.2.10)

The redox pairs of metal oxides that have been reported include Fe 3 O 4 /FeO,
TiO 2 /TiO x ,Mn 3 O 4 /MnO, CeO 2 /Ce 2 O 3 ,Co 3 O 4 /CoO, Nb 2 O 5 /NbO 2 ,In 2 O 3 /In,
WO 3 /W, and CdO/Cd, among others (Xiao et al., 2012; Le Gal et al., 2010;
Haltiwanger et al., 2010; Steinfeld, 2002; Charvin et al., 2007). A significant advan-
tage of using these cycles for hydrogen production is that only two chemical reactions
are involved, which reduces the challenges of the system integration that otherwise
occurs for three or more chemical reactions. However, the temperature for the oxy-
gen production reaction is usually in the range of 1,500-2,500 ◦ C. This is a major
challenge for equipment materials, which at the same time are expected to have the
characteristics of high solar absorptance, low thermal emittance, corrosion resistance,
and thermal stability.
Another leading example of fully thermal cycles is the sulfur-iodine (S-I) cycle,
which was first investigated at General Atomics in 1970s (Schultz, 2003; Riccardi
et al., 2011; Khan, 2004). An advantage of the S-I cycle over the metal oxide redox
cycles is its lower temperature requirement of 850 ◦ C. The S-I cycle has been scaled
up from proof-of-principle tests to a larger engineering scale by the Japan Atomic
Energy Agency (JAEA) (Kubo et al., 2004; Lewis et al., 2006; Sakurai et al., 2000;
Terada et al., 2007). The scale of the S-I cycle at JAEA can reach 0.065 kg/day of
hydrogen production at present. Commissariat à l'énergie atomique (CEA, (Anzieu
et al., 2006)) and the Sandia National Laboratory (SNL, (Moore et al., 2007)) are also
active developers of the S-I cycle. There are several types of S-I cycles. Table 9.2.2 shows
a typical three-step purely thermal cycle that is commonly studied (Riccardi et al., 2011;
Khan et al., 2004; Kubo et al., 2004; T-Raissi et al., 2003). The temperatures for each
step of the S-I cycle adopted by different researchers have some differences, depending
on the reactor technology. However, these differences are not significant enough to
have major differences in the S-I cycle.
The presence of iodine-based chemicals in the S-I cycle brings some significant
engineering challenges. For example, great precaution must be taken to process the
mixture of combustible H 2 and I 2 at 450 ◦ C. Also, the separation of HI, H 2 , and I 2 is a
complex multiple-stage process and the distillation of azeotropic HI would significantly
enhance the energy cost of the cycle (Elder et al., 2005; Stewart 2005; Guo et al.,
2011). To avoid these challenges, another sulfur-based thermochemical cycle named
as a “hybrid sulfur cycle'' or “Westinghouse cycle'' has attracted attention (Riccardi
et al., 2011; Hinkley et al., 2011; Monnerie et al., 2011; Corgnale et al., 2011; Roeb
et al., 2010). As shown in Table 9.2.2, in the hybrid sulfur cycle, hydrogen is produced
from the electrolysis of an aqueous solution of SO 2 and the operating temperature is
about 120 ◦ C, which is significantly lower than the HI decomposition temperature of
450 ◦ C for the H 2 production in the S-I cycle.
Both the S-I and the hybrid sulfur cycles require an input temperature of 850 ◦ C,
which still brings many high temperature-related challenges, although the temperature
is significantly lower than redox cycles with metal oxide pairs. In recent years, the