Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 267

Table 9.2.3 Major chemical processes of a hybrid copper-chlorine cycle and energy distribution.

Step Process and heat flow Major reaction

Processes in Cu-Cl thermochemical cycle
I Electrolytic hydrogen 2CuCl ( s ) + 2HCl (aq) + V E = H 2 (g) + 2CuCl 2 (aq) in aqueous

production solution, at 30 ∼ 100 ◦ C

II Drying of cupric chloride CuCl 2 (aq) + n f H 2 O (l) + Q = CuCl 2 · n h H 2 O( s ) + (n f − n h )H 2 O,

(endothermic) where n f > 7.5, n h = 0 ∼ 4, depending on temperature.

Below 80 ◦ C, crystallization; at 100 ∼ 200 ◦ C, spray drying.

III Hydrolysis of cupric 2CuCl 2 · n h H 2 O (s) + H 2 O (g) + Q = CuOCuCl 2 (s) + 2HCl (g) +

chloride (endothermic) n h H 2 O (g), at 400 ◦ C

IV Oxygen production CuOCuCl 2 (s) + Q = 2CuCl ( molten ) + 0.5O 2 (g), at 530 ◦ C

(endothermic)
Symbols: aq - aqueous, g - gas, l - liquid, n f - number of free water, n h - number of hydrated water,
Q - heat, s - solid,V E - electricity
Energy distribution: In the total energy input, thermal energy and electricity occupy 70-90%
and 10-30%, respectively.
Heat requirements for various hydrogen production scales
H 2 production rate, tonnes/day 0.001 (1 kg/day) 1 50 100 200

Heat requirement, MW th 0.00263 (2.63 kW th ) 2.63 132 263 525

cycles (Hinkley et al., 2011; Monnerie et al., 2011; Corgnale et al., 2011; Summers
et al., 2009), and 40-60% by S-I and Cu-Cl cycles. These efficiencies have the potential
to compete with current steam methane reforming (Lewis, 2008; Wang et al., 2008,
2009). Another advantage of thermochemical cycles is that water decomposition may
utilize separate facilities that are independent of the capturing and processing of solar
thermal energy. Therefore, the design and maintenance of the hydrogen production and
solar thermal energy facilities can be separately performed. The solar thermal energy
plant can be designed in a compact fashion that mainly aims at efficiently capturing
and concentrating the solar irradiance, wherein a solar tracking system can be readily
utilized.
If the captured solar thermal energy needs to be transported by a heat transfer
fluid over a distance from a solar tower to the thermochemcial hydrogen production
cycle, the heat losses must be controlled to below 30% so as to compete with water
electrolysis and steam methane reforming. The pipeline diameter (including the thermal
insulation) for the heat transport between solar thermal and thermochemical hydrogen
production plants must be large, either utilizing molten salt or pressurized helium as
the heat transfer fluid when the heat transport lies in the range of 100-700 MW th
which corresponds to 40-200 tonnes of hydrogen production per day. A long distance
( > 10 km), heat transport is not suggested.

9.2.4 Water electrolysis
The energy input for Equation (9.2.2) could also be in the form of electricity. This
means the solar energy must be converted to electricity and the corresponding facilities
should then be designed for the distribution of electrical current. Regarding the usage
of electricity as the major energy input, the water molecule is split by an imposed