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Solar Hydrogen Production from a ZnO/Zn
Thermo-chemical Cycle |
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Participants:
Contact:
Funding:
Duration:
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Schematic of the solar chemical reactor configuration: 1 = rotating cavity
lined with sintered ZnO tiles,
2 = 80%Al2O3-20%SiO2 insulation,
3 = 95%Al2O3-5%Y2O3 CMC,
4 = alumina fibres, 5 = Al reactor shell, 6 = aperture, 7 = quartz window, 8
= dynamic feeder,
9 = front cone, 10 = rotary joint,
11 = chain wheel.
Solar power
input, cavity temperature, and O2 mass flow rate in the product gases, with
one feed-cycle of 398 g of ZnO. Evidence for ablation mode: ZnO dissociation
reaches its maximum rate indicated by the peak O2 rate before the cavity
wall temperature attains its stationary value of 1850 K.
Schematic of the
“Solar TG” reactor used for testing quench units for Zn(g)-O2 separation.
Publications:
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Abanades S.,
Flamant G. (2006) Thermochemical hy-drogen production from a two-step
solar-driven water-splitting cycle based on cerium oxides, Solar Energy
80(12), 1611-1623.
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Karlsson M.,
Alxneit I., Ruetten F., Wuillemin D., Tschudi H.R. (2007) A Compact
Setup to Study Homogeneous Nucleation and Condensation, Rev. Sci.
Instrum. 78, 31102.
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Background:
Hydrogen
production from water using solar energy in a two-step thermochemical cycle
is being investigated. The first, endothermic step is the thermal
dissociation of ZnO(s) into Zn(g) and O2 at temperatures above 2000 K using
concentrated solar energy as the source of process heat. The second,
non-solar, exothermic step is hydrolysis of Zn at 700 K to form H2 and ZnO(s);
the latter separates naturally and is recycled to the first step. H2 and O2
are derived in different steps, thereby eliminating the need for high-temperature
gas separation.
Objectives:
- Solar
chemical reactor technology for the production of Zn by thermal
dissociation of ZnO
- solar
chemical reactor modeling using CFD and Monte Carlo ray-tracing
simulations;
- basic
research on the reoxidation and quenching of Zn(g)
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production of H2 by hydrolysis of Zn.
Achievements
in 2006
The
engineering design of the solar chemical reactor for thermal
dissociation of ZnO at above 2000 K has been improved, eliminating the
materials problems in the previous reactor. It features a rotating
cavity-receiver lined with ZnO particles that are held by centrifugal
force. With this arrangement, ZnO is directly exposed to concentrated
solar radiation and serves simultaneously the functions of radiant
absorber, chemical reactant, and thermal insulator. The multi-layer
cavity is made of sintered ZnO tiles placed on top of a porous 80%Al2O3-20%SiO2
insulation and reinforced by a 95%Al2O3-5%Y2O3 ceramic matrix composite,
providing mechanical, chemical, and thermal stability and a diffusion
barrier for product gases. 3D CFD was employed to determine the optimal
flow configuration for an aerodynamic protection of the quartz window
against condensable Zn(g). Experimentation was carried out in PSI’s high
flux solar simulator with a 10 kWth reactor prototype subjected to
radiative heat fluxes over the aperture exceeding 3000 suns (mean) and
5880 suns (peak). The reactor was operated in a transient ablation mode
with semi-batch feed cycles of ZnO particles, characterized by a rate of
heat transfer – predominantly by radiation – to the layer of ZnO
particles undergoing endothermic dissociation that proceeded faster than
the rate of heat transfer – predominantly by conduction – through the
cavity walls.
Part of the
continued reactor development work is aimed at optimizing the quench unit
for avoiding Zn re-oxidation. Sepa-ration by rapidly quenching the gaseous
products is a promising strategy requiring control of complex physical and
chemical processes such as homogeneous nucleation of zinc vapor and
oxidation of zinc droplets. Two complementary laboratory ex-periments have
been set up:
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A test rig
for studying the homogeneous nucleation and condensation of zinc vapor
using adiabatic expansion in a Laval nozzle will be put into operation
in spring 2007. It is based on a much simpler device previously used to
verify the measurement principle by reevaluating the homogeneous
nucleation of n-butanol
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A “Solar TG”
(Solar Thermal Gravimeter; see Figure 4.10) is equipped with a balance
allowing for continuously monitoring the mass loss of thermally
dissociated ZnO using concentrated radiation. The product gas contain-ing
Zn(g) and O2 exits the reactor through a quench device that is tested
under different conditions for maximum Zn yield. Work is in progress.
A comprehensive
model describing the complex phenomena occurring during quenching of a Zn(g)-O2
mixture has been implemented. The model analyzes the chemical reactions in a
mixture of zinc vapor, oxygen and inert gas subjected to a temporal
temperature gradient. It predicts key properties of the product, such as
chemical composition, i.e. average degree of oxidation, or particle size
distribution. The model allows integrating the results of both experimental
studies into a common framework and is used to identify and assess possible
quenching strategies for the gaseous product exiting the solar ZnO
dissociation reactor.
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