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Background
Hundreds
of thermochemical cycles have been proposed to split water. The
feasibility of these processes can be assessed through thermodynamic
analysis and experimentation. There is a need to evaluate these cycles
in order to identify the most feasible and economical for further
investigation. The most promising cycles should be demonstrated.
Objectives
The main
objective of the STCH project is to identify a cost-competitive solar-powered
water-splitting process for hydrogen production. Up to three processes
will be demonstrated, including on-sun experimentation.
Achievements
in 2006
The cycle
database and scoring are complete with 353 unique cycles evaluated. Twelve
cycles have been found to be worthy of further experimental study. Five of
those are currently under active study by STCH. These include: 1) zinc oxide
and cadmium carbonate volatile metal oxide cycles; 2) sodium manganese and
cobalt ferrite non-volatile metal oxide cycles; and 3) the hybrid copper
chloride cycle. Metal sulfate cycles were eliminated from consideration when
experiments indicated that no H2 was formed, as side reactions were more
favorable.
The thermal dissociation of ZnO has been investigated in a laboratory
electrically heated transport tube reactor at 1900 K. A refractory alumina
tube was used to prevent competing carbothermal reduction from occurring.
The concentrations of O2, CO, and CO2 are shown in Figure 4.4, indicating
that thermal dissociation of ZnO occurred. For residence times of less than
1 s, an overall 18% conversion of feed ZnO was dissociated to Zn metal.
Product powder collected in a downstream cooled sampling system was analyzed
at >40% conversion. Product powder collected in a final filter was mostly
ZnO, but was formed by recombination of Zn(g) and O2 as indicated by the
powder morphology – via surface area measurements and TEM analysis.
Recombination and materials of construction for a reaction tube on-sun are
the primary obstacles inhibiting scale-up of the ZnO dissociation step. The
Zn hydrolysis reaction step was also evaluated using a transport tube
operating at temperatures below the melting point of Zn. The conversion of
Zn to ZnO was measured to be 28% for residence times of less than 1 second
for process temperatures of 700 K (Figure 4.5). Cross-sectioned TEM
analysis indicated that unreacted core Zn particles were enclosed in a shell
of ZnO. Current research is focused on how to increase the conversion,
overcoming reac-tion rate limitations. Thermogravimetric experiments
indicate that complete conversion can be achieved with longer residence
times.The cadmium carbonate cycle consisting of three steps is being
evaluated: (1) Cd + CO2(g) + H2O → CdCO3 + H2(g); (2) CdCO3 → CdO + CO2(g);
(3) CdO → Cd(g) + ½O2(g). Flow-sheet calcu-lations predict efficiency
potentially as high as 59% (LHV). An experimental program was undertaken to
investigate the H2 production step. No H2 was detected without a catalyst,
i.e. ammonia supplied as NH4HCO3. The reaction mechanism is believed to
progress via a Cd(NH3)n+2 complex. The final solid product was determined to
be CdCO3. Key issues include the rate of reaction and the degree of
conversion (without re-combination) that can be achieved to Cd in the high
temper-ature solar step.
A cobalt-ferrite lattice structure on yttria stabilized zirconia (YSZ) was
constructed and demonstrated for repeatable H2 production. The YSZ
formulation, Co0.67Fe2.33O4 (3:1), was sintered at 1700 K and evaluated in
water splitting cycles. Test data showing repeatable H2 production over 31
cycles are presented in Figure 4.6. The structure is being incorporated
into a Counter Rotating Ring Receiver Reactor Recuperator (CR5) thermo-chemical
engine as depicted in Figure 4.7. The CR5 uses recuperation of sensible
heat to efficiently produce H2 in a two-step thermochemical process.
Research is currently focused on evaluating the reaction kinetics on-sun,
optimizing the reactive material composition, and performing additional
durability testing and characterization.
Progress is also being made in the hybrid copper chloride and sodium
manganese thermochemical cycles. The low-temperature Cu-Cl cycle shows
promising efficiency (~40% LHV), but some critical data must be determined
experimentally and are underway. The first step of the Mn2O3/MnO cycle has
been demonstrated at near 90% conversion in 1 to 2 s residence times. The
hydrogen generation step using NaOH is easily carried out. The real
challenge is recovering NaOH in the process. Some modifications to the
process are being evaluated.
Additional work
is focused on development of a solid particle receiver that can achieve
temperatures in excess of 1300 K. Falling ceramic particles are directly
heated by concentrated solar energy. The complete solar interface includes
two-tank storage and particle lift as well as a heat exchanger to couple the
receiver to the thermochemical process. CFD modeling is being carried out to
aid in the design. First-level component design has been completed and cold-flow
testing has begun.
Finally, an
advanced ultra-high temperature tower is being designed using multiple
heliostat fields and secondary concen-trators and an accompanying cavity
receiver. The baseline sys-tem includes a 200 m tower with 358,100 m2 of
heliostats per field. The CPC secondary has a 23.5 degree acceptance anangle
and a geometric concentration Cgeometric = 6.1. The overall solar
concentration exceeds 7000 suns with 259 MWhth being delivered to drive the
process. The system has been designed for Daggett, California (2679 kWh/m2
available; yearly delivered = 1990 kWh/m2). The system design, size and
performance are being optimized.
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