Solar hydrogen from iron oxide based thermochemical cycles

In 1977 T. Nakamura proposed the production of hydrogen from water by means of an iron oxide thermochemical cyle. To lower the temperature of the solar thermal reduction, we extended the original approach to mixed iron oxides [1, 2]. To advance the understanding on the solar thermal reduction of these metal oxides, solar chemical reactors were developed [3-5]. They are clearly suited for any high-temperature reactivity studies on condensed phase materials. In addition, an experiment to investigate the water-splitting reaction was developed and the kinetics of the reaction of pure and mixed iron oxides was studied in depth [2, 6].

Literature

[1]        J. J. Ehrensberger. Inaugural-Disseration, Universität Zürich, Zürich, 1995.

[2]        K. Ehrensberger, A. Frei, P. Kuhn, H. R. Oswald, and P. Hug. Comparative Experimental Investigations of the Water-Splitting Reaction with Iron Oxide Fe_1-yO and Iron Manganese Oxides (Fe_1-xMnx)_1-yO. Solid State     Ionics, 1995, 78, 151-60.

[3]        J. Ganz, E. Steiner, and M. Sturzenegger. Powder Cloud Reactors – An Attractive Concept to Run Solar High-Temperature Reactions. Journal de Physique IV France, 1999, 9, Pr3 361-6.

[4]        T. Frey, E. Steiner, D. Wuillemin, and M. Sturzenegger. TREMPER — A Versatile Tool for High-Temperature Chemical Reactivity Studies under Concentrated Solar Radiation. Journal of Solar Energy Engineering, 2001, 123, 147-52.

[5]        T. Frey, C. Guesdon, and M. Sturzenegger, Determination of the Residence Time Distribution for a Solar Reactor and its Use for Deriving Reaction Kinetics, PSI Scientific Report 2000/Volume V, Paul Scherrer Institut,         Villigen PSI/Switzerland March 2001.

[6]        K. Ehrensberger, P. Kuhn, V. Shklover, and H. R. Oswald. Temporary Phase Segregation Processes During the Oxidation of (Fe_0.7Mn_0.3)_0.99O in N₂-H₂O Atmosphere. Solid State Ionics, 1996, 90, 75.

Solar hydrogen from a manganese oxide thermochemical cycle

The development of iron oxide based thermochemical cycles is challenged by the need of temperatures above 2000 ºC to accomplish the solar thermal reduction of iron oxide. Manganese oxides are reduced at temperatures as low as 1600 ºC. Consequently, a manganese oxide based thermochemical cycle for producing hydrogen from steam requires an additional chemical reaction to fulfill the entropy requirements of the water-splitting reaction [1]. Based on these considerations we proposed a thermochemical cycle where conversion of solar light into hydrogen is achieved by a set of three chemical reactions. Investigations on the individual reactions [2, 3] and an efficiency analysis [4] suggest that implementation of a viable hydrogen production requires an improved separation of the manganese oxide from the ternary intermediate.

Literature

[1]        P. Nüesch. PhD Thesis, Universität Zürich, Zürich, 1998.

[2]        M. Sturzenegger, J. Ganz, P. Nüesch, and T. Schelling. Solar Hydrogen from Manganese Oxide Based Thermochemical Cycle. Journal de Physique IV France, 1999, 9, Pr3 331-6.

[3]        T. Frey, E. Steiner, D. Wuillemin, and M. Sturzenegger. Investigations on the Solar Thermal Reduction of Manganese Oxide, in Proceedings of the 10th SolarPACES Int. Symp. on Solar Thermal Conc. Technologies: Solar Thermal 2000, Sidney/Australia, H. Kreetz, K. Lovegrove, and W. Meike, Eds.; 2000 pp 309-14.

[4]        M. Sturzenegger and P. Nüesch. Efficiency Analysis for a Manganese Oxide Based Thermochemical Cycle. Energy, 1999, 24, 959-70.

Photochemistry at High Temperatures

The direct photochemical conversion of concentrated radiation (implying high temperatures) was attempted with the phototcatalyst TiO₂ to drive the photoreduction of CO₂/H₂O. The photoproducts CO, H₂, CH₄, and some higher hydrocarbons were detected in small quantities at temperatures up to 700 K [1]. To perform these measurements a miniaturized photoreactor (V=100ml), withstanding temperatures up to 700 K, was designed [2] and the theoretical framework was developed how to measure quantum efficiencies with this apparatus [3]. ZnO was assessed as high temperature photocatalyst [4] and it was demonstrated how limitations due to thermodynamics and kinetics can be determined from photo-luminescence measurements.

It was shown that photochemical energy storage at high temperatures generally suffers from the following drawbacks: The chemical potential m, i.e. the maximum energy that can be stored, decreases with temperature as does the lifetime of the excited state. The latter ultimately requires adsorbed reactands thus setting an absolute upper limit for the working temperature. High temperatures, however, are beneficial to the rate of thermally activated steps that follow the photochemical step such as the desorption of the products. An optimal working temperature of 440 K was determined for the  TiO₂ /CO₂/H₂O system.

Literature

[1]     F. Saladin, I. Alxneit. Temperature Dependence of the Photosynthetic Reduction of CO2 with H2O at the Solid/Gas Interface of TiO2. J. Chem. Soc. Farad. Trans. 1997, 93, 4149.

[2]     F. Saladin, A. Meier, I. Kamber. Miniaturized Reactor for Photocatalysis and for Simultaneous in situ Spectroscopy. Rev. Sci. Instrum. 1996, 67, 2406.

[3]     M. Corboz, I. Alxneit, G. Stoll, H.R. Tschudi. On the Determination of Quantum Efficiencies in Heterogeneous Photocatalysis. J. Phys. Chem. B. 2000, 104, 10569.

[4]     M. Schubnell, I. Kamber, P. Beaud. Photochemistry at High Temperatures - Potential of ZnO as a High Temperature Photocatalyst. Appl. Phys. A. 1997, 64, 109.

 

Pyrometric Temperature Measurements in a Solar Furnace

Accurate temperature information at high temperatures is crucial for process control in solar chemistry. The thermally emitted radiation is a convenient source for this information and pyrometry is an established technique to measure surface temperatures. But ordinary pyrometry yields inaccurate results if the surface emittance is not precisely known or if external light sources strongly interfere. Both instances typically occur in solar furnaces.

At PSI, two different approaches were developed to overcome these difficulties. The first method [1] uses an additional light source and a calibrated reflectance standard to determine in situ the spectral reflectance of the sample and, consequently, also its spectral emittance, quantities that often depend on temperature. The thermally emitted and the reflected solar radiation can be separated in this way and the surface temperature can be calculated even in the presence of strong interfering light sources. A characteristic non-solar application is described in [2]. The second approach consists in a solar-blind pyrometer, a one-colour pyrometer being sensitive solely in a spectral range where the solar radiation is absorbed by the atmosphere [3].  A solar-blind pyrometer according to our specifications is commercially available.

Literature

[1]        H. R. Tschudi and M. Schubnell. Measuring temperatures in the presence of external radiation by flash assisted multi-wavelength pyrometry. Rev. Scientific Instr., Vol. 70, 2719 – 2727, 1999.

[2]        B. Bitnar, W. Durisch, J.-C. Mayor, H. Sigg, H. R. Tschudi, Characterisation of rare earth selective emitters for thermophotovoltaic applications. Solar Energy Materials & Solar Cells, Vol. 73, 221 – 234, 2002.

[3]        H. R. Tschudi and G. Morian, Pyrometric Temperature Measurements in Solar Furnaces. J. of Solar Energy Engineering, Vol. 123, 164 – 170, 2001.

Theoretical Aspects of the Design of Solar Chemical Reactors

The design of solar chemical reactors touches problems in different branches of physics such as non-imaging optics, fluid-dynamics combined with radiative transfer, heat conduction, chemical reactions etc. In Ref. [1], the general solution of a mathematical problem is given that emerges if two-dimensional non-imaging concentraters are designed using the edge-ray principle. In Ref. [2], the limits of stable operation of a simple solar chemical reactor are discussed.

Literature

[1]        H. R. Tschudi, Mapping of two families of rays by a reflecting contour. J. Opt. Soc. Am. A, Vol. 13, 1117 – 1120, 1996.

[2]        H. Ries, H. R. Tschudi, W. Spirkl, On the Stability of Solar Chemical Particle Receivers.  J. of Solar Energy Engineering, Vol. 120, 96 – 100, 1998.