Mechanical reliability and durability of SOFC stacks. Part I : Modelling of the effect of operating conditions and design alternatives on the reliability

Abstract

Creep Contact analysis a b s t r a c tElectrochemical and mechanical aspects in solid oxide fuel cell stack must be understood to meet the reliability targets for market implementation. This study presents a stack modelling framework that combines thermo-electrochemical models, including degradation and a contact finite-element thermo-mechanical model. It considers rate-independent plasticity and creep of the component materials and proposes periodic boundary conditions to model the stacking of repeating units. This Part I focuses on the effects of the operating conditions and design alternatives.In the present conditions, the stresses in both the anode and the cathode contribute to the probability of failure (P f ), which can be lowered by adjusting the operating conditions.The requirements for mechanical reliability are here opposite to those that alleviate electrochemical degradation. Gas-diffusion layers (GDL) and interconnect design alternatives and stacking have a lower impact on the P f , but affect the contact pressure on the GDLs, which can cause electrical contacting challenges.Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. IntroductionSufficient mechanical reliability is a major hurdle to overcome for the commercial viability of solid oxide fuel cell (SOFC) devices [1]. Despite the evidence of this shortcoming, structural issues in SOFC stacks are practically solved by progressive design adjustments while efforts to understand the underlying processes have been limited. The need for a more solid, comprehensive and versatile background is now acknowledged, as the SOFC technology approaches market introduction. Strong coupling between the phenomena characterises the SOFC field. Stack and system design and operation must be integrated with structural and lifetime analysis to meet performance, reliability, durability, versatility and availability requirements, in the view of the lowest price of electricity delivered to the end-user. Numerous trade-offs govern the development of a functional SOFC system. They act at all scales and affect to similar extents the choice of an electrode microstructure and material, and that of a system layout. In a simplified view, the risks of structural failure scale with power density and number of thermal cycles. Effective mitigation requires the precise knowledge of the mechanisms. Available online at www.sciencedirect.com journal h om epa ge: www.elsev ier.com/locate/he i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 4 9 e9 2 6 8

Progressive activation of degradation processes in solid oxide fuel cells stacks: Part I: Lifetime extension by optimisation of the operating conditions

“…Our structural analyses [27,28] have furthermore shown that all these manipulations can be opposite to those that promote mechanical reliability and durability, resulting in trade-offs between electrochemical and mechanical considerations. Fig.…”

“…The lack of knowledge on mechanical failures in SOFC stacks, and the variety of their nature, prevent their simplification to indicators, hence simple implementation in the optimisation problem. For future structural analysis [27,28], discrete values of the decision variables that mainly govern the temperature profile in an SRU embedded in a stack are enforced, within the typical range for intermediatetemperature, anode-supported SOFCs:…”

“…In the present conditions, a value of 2000 h is short enough to avoid numerical failures in the high system specific power regime, yet sufficient to capture longer-term characteristics, since the degradation of the ionic conductivity of 8YSZ and of the anode due to nickel particle growth are close to their plateau at this time [20]. Fixed gas channel geometry, rather than constant system pressure is enforced, as the focus is on the operating conditions rather than on SRU design, and for consistency with structural analysis [27,28].…”

“…Operating conditions are optimised for co-flow and counter-flow configurations, along the trade-off between system power and efficiency, first for the best initial, then for the best longterm electrical efficiency during operation at a fixed system power, to verify whether both cases generate similar requirements or not. The methane conversion fraction in the reformer and the allowable maximum solids temperature are handled as discrete decision variables, for further structural analysis [27,28], as these are believed to significantly affect the thermomechanical behaviour of the SRU. The lifetime predictions during operation at constant system power are compared with those at fixed current density, voltage or stack power, which are more common in stack testing.…”

Mechanical reliability and durability of SOFC stacks. Part II: Modelling of mechanical failures during ageing and cycling

“…Part I [46] focuses on the influence of the SOFC stack operation parameters on the mechanical reliability, whereas this Part II investigates the combined or separate effects of electrochemical and mechanical degradation on the structural reliability of a planar stack with anode-supported cell. The analysis encompasses rate-independent plasticity and creep deformation of the components and shrinkage of the anode material, together with electrochemical degradation to gain knowledge on the detrimental phenomena acting during combined steady-state operation, performance characterisation and thermal cycling, under practical operating conditions.…”