Design and Membrane Selection for Gas to Gas Humidifiers for Fuel Cell Applications
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In its present form, polymer electrolyte membrane fuel cell (PEMFC) technology requires some method of humidification to ensure that high performance and long life of the fuel cell membrane is maintained. External humidification utilizing ‘gas to gas’ membrane based planar humidifiers is one method of humidifying fuel cell reactant gases. This type of humidification offers the benefit of recycling heat and moisture from the fuel cell exhaust, and returning it to the reactants entering the fuel cell. In designing a planar membrane based fuel cell humidifier the two important areas to be considered are: - humidifier channel and plate design; and - humidifier membrane selection. In this work a humidifier design procedure was developed based on prototype humidifier testing. This design procedure involves selection of design parameters based on a dimensionless parameter which describes the ratio of gas residence time, and water diffusion time from the membrane surface. Humidifiers of different flow channel geometries were created with a rapid prototyping technique. These humidifier units were tested at different operating conditions in an attempt to validate the design equations involving a design parameter which is the ratio between the residence times of gas in the humidifier over the diffusion time of water from the surface of the membrane into the channel. This parameter offers a good starting point for humidifier design, the target value of this parameter was found to be between 2.0 and 4.0, with a desired value of 3.0. A fuel cell stack humidifier design procedure and suggestions are presented based this parameter. The design also considers designing a humidifier on limited volume constraints in which the humidifier would have to fit into the fuel cell system. A membrane selection procedure was developed based on design criteria requirements developed during this work for the fuel cell humidifier. This criterion includes high water permeation, low air permeation, good mechanical strength, robust handling, and long lifetime under various operating conditions. . Specific values for membrane selection included a water flux of greater than 14 kg m-2 h-1 in a water permeation test, less than 3 cm3 min-1 cm-2 kPa-1 air permeation when the membrane was dry, and a lifetime of at least 1500 hours of operation without performance degradation. Sixty membranes from various sources were screened for candidacy for use in the humidifier application. Membranes which passed the initial screenings were tested for durability at high and moderate temperature conditions. These membranes were operated until failure, at which time analysis was completed to determine the failure modes of the membrane. Mitigation strategies were proposed when applicable. Recommendations were made for membrane materials for the proposed operating requirements. Suggested membranes materials included those based on UHMWPE and inorganic additives, as well as homogenous membranes based on Nylon 6,6, PEEK, and PFSA.