Abstract

Fuel cells are electrochemical devices that combine hydrogen and oxygen from air to produce electric current, with water and heat as the main co-products. The management of liquid water from either the internal chemical reactions or externally humidified reactants is an important design consideration for proton exchange membrane (PEM) fuel cells because of the effects on both cell performance and durability. To achieve proper water management, significant effort has been devoted to developing new fuel cell materials, hardware designs, and appropriate stack operating conditions. However, water management in the region of the channel-to-manifold interfaces has received limited attention. This region covers the ends of the bipolar plate from where liquid water exits the active area to the entrance of the stack exhaust manifolds where excess reactant flows from individual cells are combined and leave the stack. For practical applications, especially in the anode flow field, there is a small driving force to expel liquid water in this region. Under severe operating conditions such as freezing temperatures, the buildup of water may cause channel-scale blockage. This work investigated the water management of PEM fuel cells in the flow field by both ex-situ experiments and in-situ neutron imaging technique to provide a comprehensive two-phase transport model and propose a water mitigation strategy by flow field surface modification method. The results demonstrate the effects of small variations in cell temperature on water accumulation, which translate into significant changes in cell voltage under some conditions. This water can also influence the pressure drop across both anode and cathode flow fields, and it was found that a small amount of water flow can significantly affect the differential pressure, but further increases in water flux appeared to have an incrementally smaller influence. Additionally, the ex-situ experiments also investigated the water distribution of the inlet non-active area, active area, and outlet non-active area, which confirmed the significance of water management in the channel-to-manifold region. A new empirical correlation was developed to characterize the variation of two-phase friction multiplier (i.e., ratio of two-phase to single-phase ΔP) with gas and liquid flow rates. In cases where water accumulates in the non-active cell region downstream of the active area, it was determined that hydrophilic bipolar plate coating was effective in reducing or eliminating full-channel water blockages, thus minimizing the start-up time and energy under freezing conditions. The novel research contributions from this part of the dissertation research include:

• Assessed PEM fuel cell water management behavior in the low non-freezing temperature range (200C to 400C), which significantly affects the reliability and durability of PEM fuel systems, but has received very little attention in the literature.

• Analyzed water management in the non-active region of the bipolar plate, which not only affects the channel-to-channel water distribution within the fuel cell flow field, but also the cell-to-cell water distribution in a fuel cell stack. This research concluded that water management should focus on the anode side, especially in the outlet channel-to-manifold region.

• Quantified the water content in a PEM fuel cell flow field using measurements of channel two-phase flow quality and differential pressure. The two-phase transport model developed in this research is capable of quantifying the water volume in PEM fuel cell flow field, and the results showed good agreement with neutron imaging data.

• Evaluated the water mitigation effectiveness of PEM fuel cell for various surface energy modification locations, and concluded for the first time that only one hydrophilic coated channel in the anode channel-to-manifold transition could substantially facilitate the fuel cell cold start-up process.

In addition, there is a significant global activity in assessing and optimizing distributed energy systems in so-called “microgrid” architectures, which in principle enable operation completely independent of the primary electrical grid. A shortcoming of such an approach is that many renewable energy systems are intermittent by nature, and thus supply and demand are often out of phase. This necessitates the implementation of energy storage, but few options exist for cost-effective, large-scale storage. One attractive alternative is to use hydrogen as an energy storage medium, because it offers the possibility for storage at relatively high volumetric density, and hydrogen is readily utilized in various energy applications of immediate interest in large product distribution centers. The dissertation work explored the economic impact of PEM fuel cell material handling equipment (MHE) with comparison to the conventional lead acid battery MHE and the emerging lithium-ion battery MHE. Using data obtained directly from large product distribution centers, it was determined that fuel cells are the low-cost option in installations with large MHE vehicle fleets, multi-shift facilities, and relatively high grid electricity costs. The novel contributions of this analysis stem from it being the first to consider lithium-ion batteries with lead acid batteries and fuel cells as competing MHE propulsion technologies. Moreover, it is the only known study to date to account for the time value of money in the economic analysis, and to consider the target facility fleet configuration using data acquired directly from large product distribution centers in various U.S. locations.

Publication Date

5-2018

Document Type

Dissertation

Student Type

Graduate

Degree Name

Sustainability (Ph.D.)

Department, Program, or Center

Sustainability (GIS)

Advisor

Thomas A. Trabold

Advisor/Committee Member

Nabil Nasr

Campus

RIT – Main Campus

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