Study of an Energy Storage and Recovery Concept Based on the W/WO 3 Redox Reaction: Part I. Kinetic Study and Modeling o
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THE proposed grid-scale energy storage and recovery system is based on thermally integrating a solid-state tungsten/tungsten-oxide (W/WO3) energy storage and recovery cycle with a reversible solid oxide electrochemical cell (RSOEC) stack. The system stores energy as tungsten metal when the RSOEC stack is operated as a steam electrolyzer using renewable and off-peak sources of electrical energy. The RSOEC electrolyzer produces hydrogen, which reduces the WO3 to W (Figure 1) through the following reaction: WO3(s) + 3H2(g) fi W(s) + 3H2O(g); DG = 5.7 kJ @ 1023 K (750 C) and –5.6 kJ @ 1173 K (900 C).[1] During periods of peak demand or to maintain base load conditions for renewable energy sources, the RSOEC stack is operated as a fuel cell. The hydrogen needed for the fuel cell is obtained by reducing steam ROMAIN HABOURY, formerly Graduate Student, E´cole Normale Supe´rieure de Chimie de Paris, 7, rue Thibaud, 75014 Paris, France, is now Visiting Student, Department of Mechanical Engineering, Boston University, Boston, MA 02215. UDAY B. PAL and SOUMENDRA N. BASU, Professors, PETER A. ZINK, Research Assistant Professor and Lecturer, and SRIKANTH GOPALAN, Associate Professor, are with the Division of Materials Science and Engineering, Department of Mechanical Engineering, Boston University. Contact e-mail: upal@ bu.edu Manuscript submitted September 12, 2011. Article published online April 7, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
with the energy stored in the tungsten metal (Figure 1) through the following reaction: W(s) + 3H2O(g) fi WO3(s) + 3H2(g); DG = –5.7 kJ @ 1023 K (750 C) and 5.6 kJ @ 1173 K (900 C).[1] In both the energy storage and recovery modes, steam is recycled within the system. It is clear from the preceding equations that the change in standard free energy for the storage and recovery reactions is either slightly positive or slightly negative between 1023 K and 1173 K (750 C and 900 C), which is the operating temperature range of RSOEC. This indicates that the equilibrium constants for the storage and the recovery reactions are near unity, which means that both of these processes will operate at near 50 pct conversion efficiency. The current work focuses on the reduction of tungsten oxide (WO3) with hydrogen. This publication forms the first part of the study aiming at designing and building a research scale W/WO3 energy storage and recovery system. Studies of tungsten oxidation by water vapor and the feasibility and stability of extended and repeated cycling between the oxidation and the reduction reactions will be presented in forthcoming publications. In the existing literature, large amounts of data are available on the hydrogen reduction of the various oxides of tungsten.[2–10] The application of thermogravimetric methods for the determination of the activation energies of solid–gas reactions and the advantages of such methods have been discussed in several VOLUME 43B, AUGUST 2012—1001
Fig. 1—Schematic of the storage and recovery concept.
publications.[2,3] However, a lack of i
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