Solar Technology Laboratory
Solar thermal ZnO-decomposition |
| Contact: Aldo Steinfeld, Anton Meier, |
| Start: 1998 |
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Funding:
Swiss Federal Office of Energy |
DescriptionThe direct solar thermal decomposition of ZnO to its elements is an attractive process for the storage of solar energy. In this process, concentrated sunlight provides high-temperature process heat for the endothermic reaction ZnO (s) --> Zn (g) + 1/2 O 2 (for T > 2000 K) At temperatures near 2000 K, the reaction proceeds. Solar radiation is thereby directly converted into the chemical energy of Zn and O2. To avoid their recombination at high temperature, the gas phase products are separated with a rapid cooling process known as a quench. The solar energy stored in the condensed Zn phase may be used as the fuel in a fuel cell or battery. When hydrogen is the desired fuel, it has also been suggested that the Zn be used to split water in an exothermic reaction. In either scenario, the ZnO is recycled to the solar furnace. Figure 1 shows a schematic representation of the two-step water-splitting cycle using the Zn/ZnO redox system for the solar production of hydrogen. |
 Figure 1: Schematic representation of the two-step water-splitting cycle using the Zn/ZnO redox system for the solar production of hydrogen [2]. |
| The Solar Technology Laboratory is investigating the industrial potential of this process. Fundamental research is on going where we are investigating the reaction kinetics of the ZnO decomposition reaction, the reverse re-oxidation reaction of the products, and radiation heat transfer processes coupled to the reaction kinetics of the
decomposition reaction. In parallel, we are developing engineering methods for effecting the quench of the products as well as developing reactor concepts for effecting the decomposition reaction. Figure 2 is an example of a reactor concept for the decomposition of ZnO. It is a reactor closed to air.[1] The main component is a rotating conical cavity-receiver (1) made of inconel steel that contains the aperture (2) for access of concentrated solar radiation through a quartz glass window (3). The cavity approaches a black body absorber. The solar flux may be further augmented by incorporating a CPC (4) in front of the aperture. Both the copper window mount and the aluminum CPC are water cooled and integrated into a concentric non-rotating conical shell (5). The reactants are ZnO particles that are fed continuously along the axis into the rotating cavity by means of a powder screw feeder located at the rear of the reactor (6). The centripetal acceleration forces the ZnO powder to the wall where it forms a thick layer of ZnO (7) that reduces the thermal load on the inner cavity walls. The gaseous products Zn and O2 are swept out of the chamber by a continuous flow of inert gas that enters the cavity-receiver tangentially at the front (8) and exits via an outlet port (9) to a quench device (10). The purge gas also keeps the window cool and clear of particles or condensable gases. |
 Figure 2: Example of a reactor concept for the decomposition of ZnO [2]. |
| With this arrangement, concentrated sunlight impinges directly on the top surface of the ZnO layer. This efficient heating condition leads to a system with a low thermal inertia and excellent thermal shock resistance. The ZnO serves simultaneously as radiation absorber, thermal insulator, and chemical reactant. The reactor is currently being tested. To date one can say that it meets many of the design objectives. It responds
quickly to solar input as shown in Fig. 3 [1]. |
 Figure 3: Solar input and temperature during an experiment. |
It can operate continuously at temperatures greater than 2000 K.[1] However, Zn yields are currently low; they are on the order of 25 molar percent.[1] As stated above, fundamental kinetic work and engineering design work is in progress for improving the yield. Also the reactor is currently being modified to lessen heat losses. Both tasks are directed at improving its overall efficiency for converting sunlight into chemical energy as depicted in Figure 1. The latest research results with regard to reactor development
can be found in references 2 and 3.References
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