Stack Optimization and Testing for its Integration in a rSOC-Based Renewable Energy Storage System

Abstract

This work aims at developing an innovative solution for storing renewable energies, so-called “Smart Energy Hub” (SEH) developed by Sylfen, based on reversible Solid Oxide Cell (rSOC) technology. It is able to operate either in electrolysis (SOEC) mode storing excess electricity to produce hydrogen, or when energy needs exceed local production, in SOFC (solid oxide fuel cell) mode producing electricity (and heat) from hydrogen or any other hydrogenated fuel. Particularly this paper focuses on stack development and optimization, partly performed through the REFLEX project supported by European commission funding.To achieve the project’s objectives, in terms of efficiency and increase of system power density, as well as multi-stack operation, optimizing CEA stack is necessary to upgrade a stack originally designed for SOEC research purposes, to a stack reversibly operated, but also integrated as a part of module. This paper presents results of stack optimisations and integration of cells developed within the REFLEX project.First, a reduction in cell thickness of ~20 % led to adapting the stack manufacturing process parameters. Some tests, here presented, demonstrate that the stack performances were not affected by this change.Then, significant effort was devoted to air distribution at the Single Repeat Unit (SRU) scale to minimize pressure drop, undesirable for both global system efficiency and mechanical resistance of stack sealing. At the SRU scale, air path adjustment reduced the pressure drop by a factor 2; at stack scale, the performances obtained in i-V curves demonstrated reactants distributions were unchanged.Optimized cells (so called G2) developed in the frame of REFLEX project were then integrated in a short stack (5 cells vs. 25 full size) to be compared to short stack comprising regular cells produced by Elcogen. Both stacks were tested in terms of durability over more than 1000 hours at 700°C. A first period of about 800 h of operation was conducted alternating from SOFC to SOEC mode by steps of ≈100 h. During this first part, the stack was supplied by 50/50 H2O/H2 at total flow rate of 12.0 NmL/min/cm² on fuel side and air (clean and dry) on oxygen side. Stack performances were checked at each change of operating mode through i-V curves recorded at 800, 750 and 700°C.For the reference cell, durability tests were operated galvanostatic at 0.36 A/cm² in fuel cell mode and -0.51 A/cm² in electrolysis mode. The fuel utilization (FU) was respectively 42% in SOFC, and Steam Conversion (SC) 59% in SOEC. For the optimized cell, tests were carried out at 0.35 A/cm² (FU=41%), and -0.58 A/cm² (SC=67%). A second stage of more than 250h of operation consisted to alternate daily from SOEC to SOFC by cycles of respectively 8h and 16h. Finally, degradation was evaluated and despite the increase in SOEC current density during G2 cell testing, degradation rate was very similar for both cells in SOEC as well as in SOFC mode.A full size (25-cells) stack made of G2 cells was manufactured integrating the fluidic optimizations but also manifold and an electrical insulation necessary to operate stacks serially connected electrically within the REFLEX modules produced by Sylfen. The change of scale was validated by obtaining similar i-V curves recorded for 5-cells and 25-cells stacks.REFLEX Smart Energy Hub will be operated in ENVIPARK Torino using three modules comprising of four stacks each. Stack production has to be as uniform as possible and consequently, manufacturing process has to be optimized. Dispersion in performance, that results of cell production and stack manufacturing process is checked at final stage by i-V curves recording before delivery. Uniformity in production (in terms of tightness, reactant distribution and so initial electrochemical performance) is found to be satisfactory, the dispersion of median cell stack voltage over the stack production is less than the voltage dispersion observed between the whole cells at stack level.For near future needs in rSOC system, an increase of stack power density is required to both reduce the CAPEX and increase system energy efficiency. In that frame cells produced by Elcogen has an active cell area that was extended from 100 to 196 cm² and tested at stack levels of 5 and 25 cells, respectively. Enlarged stack performances are measured via i-V curves and compared to reference 100 cm² cells stacks. As difference in median cell voltage is less than 10 mV up to a current density of -1.25 V/cm² (80% SC), this paper shows that, from a mechanical and fluidic point of view, the enlarged cell were successfully integrated in full-scale stack. Figure 1