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Energy Conversion and Management 49 (2008) 1547–1556

Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles
´ Patrice Charvin a,*, Abanades Stephane a, Lemort Florent b, Flamant Gilles a

Processes, Materials and Solar Energy Laboratory (PROMES-CNRS/UPR8521), 7Rue du four Solaire, 66120 Odeillo-Font Romeu, France b ` ` Commissariat a l’Energie Atomique, BP 17171, 30207 Bagnols sur Ceze Cedex, France Received 19 April 2007; accepted 6 December 2007 Available online 5 February 2008

Abstract This paper presents a process analysis of ZnO/Zn, Fe3O4/FeO and Fe2O3/Fe3O4 thermochemical cycles as potential high efficiency, large scale and environmentallyattractive routes to produce hydrogen by concentrated solar energy. Mass and energy balances allowed estimation of the efficiency of solar thermal energy to hydrogen conversion for current process data, accounting for chemical conversion limitations. Then, the process was optimized by taking into account possible improvements in chemical conversion and heat recoveries. Coupling of the thermochemicalprocess with a solar tower plant providing concentrated solar energy was considered to scale up the system. An economic assessment gave a hydrogen production cost of 7.98$ kgÀ1 and 14.75$ kgÀ1 of H2 for, respectively a 55 MWth and 11 MWth solar tower plant operating 40 years. Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen production; Thermochemical cycle; Metal oxide; Concentrated solarenergy; Process engineering; Economic assessment

1. Introduction Water splitting thermochemical cycles driven by solar energy could potentially lead to a sustainable mass production of hydrogen [1]. Hydrogen is a promising energy carrier that could replace oil in the transportation sector or domestic energy supply. Indeed, fuel cells efficiently convert hydrogen into electricity. Thus, hydrogenenergy may help solve the problem of greenhouse gas emissions causing global warming and stress on the fossil fuel supply and price, provided it is produced by clean processes involving renewable energy. Thermochemical cycles consist of the multi-step thermal dissociation of water into hydrogen and oxygen. Chemical intermediates (oxides, sulphates, halogens. . .) are involved in exothermicreaction(s), generating hydrogen by water

splitting (Eq. (1)), and in endothermic reaction(s), releasing oxygen (Eq. (2)). MOred þ H2 O ! MOox þ H2 T < 1300 K MOox ! MOred þ 1=2 O2 T > 1300 K ð1Þ ð2Þ


Corresponding author. Tel.: +33 4 68 30 77 31; fax: +33 4 68 30 29 40. E-mail address: (P. Charvin).

A database including over 280 cycles was developed and screenedto select the most suitable for coupling with concentrated solar thermal energy [2]. Chiefly, 2 and 3 step metal oxide cycles are the most attractive for high efficiency hydrogen production. High temperatures, which can be provided by concentrated solar energy, are generally required for the endothermic reaction. Laboratory scale experimental devices have been developed to test the chemicalreactions and to determine their conversion and kinetics [3]. In previous studies, a few short thermochemical cycles coupled with solar energy have been thoroughly studied, and most of the research focused on the ZnO/Zn cycle [4,5], the Mn2O3/MnO three step cycle [6,7] and the ferrite cycles [8–10]. These small scale experiments provided accurate data concerning experimental

0196-8904/$ - see frontmatter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.12.011


P. Charvin et al. / Energy Conversion and Management 49 (2008) 1547–1556

Nomenclature Aaperture area of aperture (m2) Amirrors total area of mirrors in heliostat field (m2) C concentration ratio DNI direct normal irradiation (W mÀ2) Fi molar flow rate of stream i (mol sÀ1) HHVH2 high heating value of...
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