Design aspects of a failure resistant 70 MPa type IV composite overwrapped pressure vessels for hydrogen storage

Abstract

Hydrogen has emerged as a zero-emission solution for light to heavy-duty transport as well as various stationary applications. Type IV composite overwrapped pressure vessels (COPVs) are standardized to store gaseous hydrogen (H2) at a nominal working pressure (pnwp) of up to 70 MPa or more so as to achieve a competitive driving range for commercially available hydrogen fuel cell electric vehicles (HFCEVs) and hydrogen internal combustion engine vehicles (HICEVs). As per various national and international standards, Type IV COPVs are tested up to the burst pressure (pb) which is around 2.25 times of pnwp to provide a sufficient factor of safety for regular operations at pnwp. Although Type IV COPV for H2 storage has been standardized and commercialized, there are several challenges associated with it such as high cost, limited temperature range of usage, safety, etc. which are linked to the materials, design, and manufacturing processes used to fabricate these COPVs. Therefore, a good understanding of the role of materials used for their manufacturing, design along with the manufacturing defects on their failure behavior under critical operational conditions of high-pressure H2 is absolutely necessary. Materials used for manufacturing Type IV COPV include hydrogen embrittlement resistant metals used for the manufacturing boss (required for holding the gas valve), a suitable grade of polymer used for manufacturing liner to control the H2 permeation, and carbon fiber-reinforced plastic (CFRP) that has a suitable matrix material along with the suitable grade of fibers as reinforcement to provide the required strength for holding high-pressure H2. Design aspects of COPVs include length to diameter ratio, shape of dome section and coupling of metallic boss with polymeric liner. Manufacturing defects on the other hand, can include defects in the polymeric liner, gaps at the interface of various components of COPV, sharp thickness variation in CFRP winding, etc. These defects are known to cause several kinds of failures in Type IV COPVs that include buckling of the liner, reduced pb, leakage of gas, as well as an overall reduction in the operational life. To gain a competitive edge on Type IV COPVs, simulation-assisted manufacturing technologies must be developed to reduce test requirements, boost production rates, and reduce rejections or failures under service conditions. To improve such understanding, in this thesis, at first a small Type IV COPV with an internal water volume of 18 liter is designed progressively using netting analysis, classical laminate theory, and finite element analysis, to elucidate the role of various design parameters and defects responsible for its failure at the pb. The failure behavior of Type IV COPV is found to be highly sensitive to the placement of the CFRP lay-up, the outer shape of the metallic boss that interacted with the CFRP winding while the junction points of the liner, boss, and CFRP winding are shown to be the hot spots for failure. Lastly, failure prevention strategies at these hot spots are discussed in terms of the CFRP failure criteria. Next, a novel modeling framework is developed and implemented using UMATHT (user-defined material with heat transfer) subroutines in the commercial FE solver Abaqus. UMATHTisusedtosolve the H2 permeation equation, following the analogy between heat transfer and the diffusion equation. The variation in hydrogen transport properties in liner material based on the morphological properties of polymer with applied pressure has been implemented in the model to investigate the H2 permeation mechanism. The developed modeling framework is first calibrated w.r.t experimental data available in the literature, and thereafter, the model with calibrated constants is extended to simulate the optimal thickness of liner material for a 70 MPa pnwp Type IV COPV. Lastly, to explore the mechanism of H2 permeation and failure based on pre-existing micro-defects in the polymeric liner material, two defect cases are considered for analyzing the role of defects in polymer liner and at liner-composite interface under filling-defilling cycles of Type IV COPVs. A quarter model of the cross-section of cylindrical part of the Type IV COPV with both the defects incorporated is subjected to an operational f illing and defilling cycle typically associated with Type IV COPV. A multi-variable H2 permeation analysis is conducted using the extended governing equations of Fick’s law for hyper-elastic polymeric liner material and defilling-induced blistering model. Filling is carried out up to 70 MPa in 5 minutes, and maintained for several hours, then fast defilling is carried up to a minimum pressure up to 2 MPa as followed in realistic situations. Simulation results provide an understanding of H2 pressure build-up inside defects during the filling and defilling cycle clearly highlighting the effect of location of micro-defects on damage initiation under operational conditions of Type IV COPVs.