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Analysis of Thermodynamics of Two-Step Solar Water Splitting

Roeb, Martin and Gathmann, Nils and Neises, Martina and Sattler, Christian and Pitz-Paal, Robert (2008) Analysis of Thermodynamics of Two-Step Solar Water Splitting. In: ASME 2nd International Conference on Energy Sustainability, ES2008-54240. Pinehurst Technologies, Inc. ASME 2nd International Conference on Energy Sustainability, 2008-08-10 - 2009-08-14, Jacksonville, Fl (USA). ISBN 0-7918-3832-3.

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Abstract

A two-step thermo-chemical cycle for solar production of hydrogen from water has been developed and investigated. It is based on metal oxide redox pair systems, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice. After proof-of-principle, successful experimental demonstration of several cycles of alternating hydrogen and oxygen production, and elaboration of process strategies presented in previous contributions, the present work describes a thermodynamic study aiming at the fine tuning of the redox system, at the improvement of process conditions, and at the evaluation of the potential of the process. For the redox material the oxygen uptake capability is an essential characteristic, because it is directly connected to the amount of hydrogen which can be produced. In order to evaluate the maximum oxygen uptake potential of a coating material and to be able to find new redox materials theoretical considerations based on thermodynamic laws and properties are helpful and faster than actual tests. Through thermodynamic calculations it is possible to predict the theoretical maximum output of H2 from a specific redox-material under certain conditions. Calculations were focussed on the two mixed iron oxides nickel-iron oxide and zinc-iron oxide. In the simulation the amount of oxygen in the redox-material is calculated before and after the splitting step on the basis of laws of thermodynamics and available material properties for the mixed-iron oxides used. For the simulation the commercial Software FactSage and available databases for the necessary material properties were used. The analysis showed that a maximum hydrogen yield is achieved if the regeneration temperature is raised to the limits of the operation range, if the temperature for the water splitting is lowered below 800 °C and if the partial pressure of oxygen during regeneration is decreased to the lower limits of the operational range. The increased hydrogen yield at lower splitting temperature of about 800 °C could not be confirmed in experimental results, where a higher splitting temperature led to a higher hydrogen yield. As a consequence it can be stated that kinetics must play an important role especially in the splitting step.

Document Type:Conference or Workshop Item (Speech)
Title:Analysis of Thermodynamics of Two-Step Solar Water Splitting
Authors:
AuthorsInstitution or Email of Authors
Roeb, MartinUNSPECIFIED
Gathmann, NilsUNSPECIFIED
Neises, MartinaUNSPECIFIED
Sattler, ChristianUNSPECIFIED
Pitz-Paal, RobertUNSPECIFIED
Date:10 August 2008
Journal or Publication Title:ASME 2nd International Conference on Energy Sustainability
Refereed publication:Yes
In ISI Web of Science:No
Page Range:ES2008-54240
Editors:
EditorsEmail
ASME, UNSPECIFIED
Publisher:Pinehurst Technologies, Inc
ISBN:0-7918-3832-3
Status:Published
Keywords:solar hydrogen, thermo chemical cycles, mixed iron oxide, thermodynamics
Event Title:ASME 2nd International Conference on Energy Sustainability
Event Location:Jacksonville, Fl (USA)
Event Type:international Conference
Event Dates:2008-08-10 - 2009-08-14
Organizer:American Society of Mechanical Engineers
HGF - Research field:Energy
HGF - Program:Renewable Energies
HGF - Program Themes:E SF - Solar research (old)
DLR - Research area:Energy
DLR - Program:E SF - Solar research
DLR - Research theme (Project):E - Solare Stoffumwandlung (old)
Location: Köln-Porz
Institutes and Institutions:Institute of Technical Thermodynamics > Solar Research
Deposited By: Dr.rer.nat. Christian Sattler
Deposited On:19 Sep 2008
Last Modified:12 Dec 2013 20:32

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