Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 266

Table 9.2.2 Major chemical processes of a fully thermal S-I cycle and hybrid sulfur cycle.

Step Process and heat flow Major reaction

Processes in an S-I thermochemical cycle
1 Hydrolysis step I 2 (l + g) + SO 2 (g) + 2H 2 O(g) = 2HI(g) + H 2 SO 4 (l) + Q, at 120 ◦ C

(exothermic)

2 (a) Oxygen production H 2 SO 4 (g) + Q = SO 2 (g) + H 2 O(g) + 0.5O 2 (g), at 800-1000 ◦ C

step (endothermic)

3 Hydrogen production 2HI(g) + Q = I 2 (g) + H 2 (g), at 450 ◦ C

step (endothermic)
Major chemical processes of a hybrid sulfur cycle
A Hydrogen production SO 2 (aq) + 2H 2 O(l) + V E = H 2 SO 4 (aq) + H 2 (g) at 80-120 ◦ C

step (electrolytic)

B (a) Oxygen production H 2 SO 4 (g) + Q = SO 2 (g) + H 2 O(g) + 0.5O 2 (g), at 850 ◦ C

step (endothermic)

Symbols: aq - aqueous, g - gas, l - liquid, Q - heat,V E - electricity
(a) This step can be divided H 2 SO 4 (aq) + Q = SO 3 (g) + H 2 O(g), at 300-450 ◦ C
into two separate steps: SO 3 (g) + Q = SO 2 (g) + 1/2O 2 (g), at 800-1000 ◦ C

copper-chlorine (Cu-Cl) hybrid cycle has gained major attraction due to its lower tem-
perature requirement of 530 ◦ C, which can be accommodated by more technologies of
solar thermal energy (Khan et al., 2004; Xu et al., 2012; Litwin et al., 2010). The
Cu-Cl cycle also has several variations with various numbers of steps from 2 to
5 depending on reaction conditions [Lewis, 2008; Wang et al., 2008, 2009]. The
cycle with 4 steps shown in Table 9.2.3 is a typical hybrid Cu-Cl cycle. The energy
structure and heat requirements are also shown in Table 9.2.3, so as to provide
a basis for the efficiency evaluation of the hybrid cycle. A solarium laboratory
apparatus that can absorb a maximum of 50 kW solar irradiance and tempera-
ture of 800 ◦ C is under development for the study of a solar-based Cu-Cl cycle and
other photochemical processes at the University of Ontario Institute of Technology
(UOIT). The scale-up of the cycle from proof-of-principle to a larger engineering
scale of 3 kg/day is also in progress at UOIT (Wang et al., 2008, 2009; Naterer
et al., 2008) in collaboration with partners that include Atomic Energy of Canada
Limited (AECL).
As suggested in Tables 9.2.2 and 9.2.3, more steps are needed by the fully thermal
S-I and hybrid Cu-Cl cycles than the hybrid metal redox cycles to complete the water
decomposition in a closed loop. Multiple chemical reactors and auxiliary equipment
for chemical reactions and heat transfer are needed. This may increase the capital cost
of the equipment and operating cost. Therefore, the S-I and Cu-Cl thermochemical
cycles are more appropriate for large scale hydrogen production to offset the costs
arising from multiple chemical processes.
Since heat is the major form of energy input to thermochemical cycles, the energy
loss in the conversion of heat to electricity is then avoided. This indicates a great
potential to improve the overall thermal efficiency for hydrogen production. For exam-
ple, it was estimated that the efficiency could reach 40-56% by Zn/ZnO cycles (Xiao
et al., 2012; Haltiwanger et al., 2010; Melchior, 2009; Schunk et al., 2009), 39-45%
by Fe 3 O 4 /FeO cycles (Xiao et al., 2012; Charvin et al., 2008), 35-46% by hybrid sufur