An international research consortium has successfully built a 300-kW pilot plant that uses solar energy to reduce zinc oxide to zinc.
The zinc can be used in zinc-air batteries or be used to produce hydrogen by reacting it with water vapor. In both cases the zinc recombines with oxygen and zinc oxide is produced, which can be reused in the solar reactor to produce zinc once more. (Click on chart at right to enlarge.)
In essence, the process stores solar energy in a transportable metal carrier that then can release the energy as electricity or hydrogen.
The consortium consists of the Paul Scherrer Institute (PSI), the Swiss Federal Institute of Technology Zurich (ETHZ), the Weizmann Institute of Science, and others. Weizmann started working on this in 1997, but it was in 2001 that funding from the EU kicked in to the SOLZINC project.
The first trials of the solar power-plant have used 30% of available solar energy and produced 45 kg of zinc an hour, exceeding projected goals. During further tests this summer the team hopes to achieve a higher efficiency. The consortium projects efficiency levels of 50%–60% for industrial-size plants.
The process uses a two-cavity reactor design. Zinc oxide (ZnO) is combined with coal, coke or carbon biomass and placed into the outer cavity—the reaction chamber. An array of heliostats reflects solar rays to a hyperbolic mirror attached to the solar production tower, which in turn reflect the rays through a secondary concentrator in the reactor.
The solar radiation heats the inner cavity, which then indirectly heats the outer cavity. The sun’s rays are concentrated on this mixture by a system of mirrors. The zinc forms as a gas which is then condensed to a powder.
Straight thermal dissociation of ZnO requires operating temperatures above 1,750ºC. (And PSI is working on a solar reactor for that as well.) However, the use of a carbonaceous material as a reducing agent (e.g., coal, coke, biomass) reduces the required operating temperature to between 1,000ºC–1,400ºC. The SOLZINC process operates at approximately 1,200ºC.
One side-effect of operating at the lower temperature with carbon as a reactant is the release of CO2. The research team determined that:
...compared to the conventional fossil-fuel-based production of Zn, the solar-driven carbothermic process can reduce CO2 emissions by a factor of 5. If biomass is used as a reducing agent, the process has basically zero net CO2 emissions, if the production rate of biomass can be matched to its use as a reducing agent.
PSI SOLZINC project
“Solar Carbothermic Production of Zinc and Power Production Via a ZnO-Zn Cyclic Process,” Proceedings ISES 2003, Göteborg, June 14-19, 2003