Water and thermal management have been identified as technical hurdles to the successful implementation of low temperature, polymer electrolyte membrane (PEM), fuel cell power systems. In low power applications, miniature PEM fuel cells show significant promise as a competitor to lithium-ion batteries. Significant design work is underway to improve the specific power and energy densities of these fuel cells. However, little attention has been given to characterizing transient response in these miniature applications to enable gains in system design, optimization and control. This work develops, calibrates and experimentally validates two different dynamic control-oriented models for open-loop temperature state observation in miniature PEM fuel cells. Of critical importance, these estimators target operation under dry conditions with no reactant pre-treatment. Operational conditions are then identified for which each model architecture is more suitable, specifically targeting minimal model complexity. A sensitivity analysis was completed that indicates necessary sensor measurements with sensor frugality in mind. The dynamic response under changes in load and fuel stoichiometry are well captured over a range of operating conditions.
We present the design of a multi-cell, low temperature PEM fuel cell for controlled meteorological balloons. Critical system design parameters that distinguish this application are the lack of reactant humidification and cooling due to the low power production, high required power mass-density and relatively short flight durations. The cell is supplied with a pressure regulated and dead ended anode, and flow controlled cathode at variable air stoichiometry. The cell is not heated and allowed to operate with unregulated temperature. Our prototype cell was capable of achieving power densities of 43 mW/cm2/cell or 5.4 mW/g. The cell polarization performance of large format PEM fuel cell stacks is an order of magnitude greater than for miniature PEM fuel cells. These performance discrepancies are a result of cell design, system architecture, and reactant and thermal management, indicating that there are significant gains to be made in these domains. We then present design modifications intended to enable the miniature PEM fuel cell to achieve power densities of 13 mW/g, indicating that additional performance gains must be made with improvements in operating conditions targeting achievable power densities of standard PEM fuel cells.
Engineers for a Sustainable World (ESW) is a US-based non-profit network that designs and implements technical sustainability projects through a network of collegiate and professional chapters. Although founded around international development, today the organization focuses primarily on domestic projects situated within local campuses and communities. Similarly, ESW operates as a network rather than a hierarchical organization, an organizational model with its own lessons for service learning. This paper outlines the basic structure and function of ESW, and uses three case study projects combined with ESW’s recent history to emphasize the importance of service learning organizations on sustainability education. It examines lessons and the value of ESW and similar organizations at the individual level and for the university community, and examines challenges in replicating this model. We show that the diversity of members and autonomy of chapters in ESW’s network leads to several benefits, including lower costs per student and entry points for any discipline or career status. Nonetheless, this requires extra effort to build effective engagement and collaboration across chapters and needs continuous monitoring to balance support with control.
Numerous applications exist requiring power for small loads (<5W) with minimal mass operating in extreme ambient conditions. Making progress toward reducing stack mass, we investigate the influence of flow field channel depth and endplate compression on cell performance. The best performance was found at endplate compressions of 139 psi, cathode channel depths of 0.032 in and anode channel depths of 0.032 in. The maximum power mass-density achieved with these 4.84 cm2 cells was 16.8 mW/g in a single cell stack. If deployed in a multicell stack, this same performance would translate to a power mass-density of 45.3 mW/g, nearing the performance of off-the-shelf lithium ion batteries (approximately 70 mW/g).
Polymer electrolyte membrane (PEM) fuel cells have been explored as a clean battery replacement in portable and miniature applications where total system mass and specific energy density (Wh/kg) are critical design constraints. By coupling a boost (step-up) DC/DC converter with a miniature PEM fuel cell stack, the total power system mass can be reduced while providing voltage regulation capabilities not available with a fuel cell alone. This configuration is applied to the design of a controlled meteorological (CMET) balloon power system as a case-study. In this work, we designed and tested three different micro-power DC/DC boost converters that were deployed in series with a PEM fuel cell stack. Testing of the converters revealed a transition region in which the converter output voltage is hysteretic, not well regulated, and dependent on the input voltage. As a result, it is important to identify the minimal stable and reliable input voltage to a given DC/DC converter in order to minimize the fuel cell power system mass. An optimization strategy is presented here that enables the minimization of PEM fuel cell stack mass by identifying the appropriate DC/DC converter input voltage subject to the dimension constraints of the fuel cell components. Prototype DC/DC converters were then experimentally tested in direct connection to a miniature two-cell PEM fuel cell stack.
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