The investigated two-step MxOy/ MxOy−1 solar thermochemical cycles consist of two redox reactions. Net result is watersplitting with concentrated solar energy as the source of high temperature process heat:
1)Solarreduction:
MxOyMxOy1+1/2O2
(about1700°Catatmosphericpressure,
endothermal)
2)Hydrolysis:
MxOy1+H2OMxOy+H2
(about400°C,exothermal)
The MxOy−1 species produced in reaction (1) is gaseous in the case of the ZnO/Zn cycle. The oxide (ZnO) is injected in a solar thermochemical reactor and undergoes a thermal reduction reaction (oxygen release). Dilution/quenching with a neutral gas at the reactor exit yields nanoparticles of metal by condensation. The particles have a high specific surface area that leads to a high reactivity in the 2nd step. The reduced species (Zn) can then be fed to another reactor to react with water steam. The reaction produces pure H2 and forms the original metal oxide. A high-temperature lab-scale solar reactor prototype was designed, constructed and operated, allowing continuous metal oxide processing under controlled atmosphere. It is based on a cavity-type rotating receiver absorbing solar radiation. The reactant powder is injected continuously inside the cavity and the produced particles (Zn) are recovered in a downstream filter. The solar reduction of ZnO has been achieved, the reaction yields were quantified, and a first concept of solar reactor was qualified.
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