Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
10 results
Search Results
Item Solid Oxide Fuel Cell and Gas Turbine Hybrid Cycles for Aerospace Power and Propulsion(2022) Pratt, Lucas Merritt; Cadou, Christopher P.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid propulsion systems combining gas turbine and solid oxide fuel cells (GT/SOFCs) have the potential to substantially reduce carbon emissions from 737-class aircraft. Many turbine/fuel cell hybrid cycles have been proposed for ground-based energy conversion at the utility scale, and some work has investigated small-scale (<500 kW) fuel cell-based energy conversion systems for aircraft (mostly auxiliary power units). However there is relatively little known about large hybridengine/fuel cell systems capable of providing main propulsive power in large (i.e. 737-class) aircraft. This work takes several important steps toward filling this gap. First, it develops an analytical model of a GT/SOFC system that provides insight into the trends and tradeoffs associated with varying design parameters across a wide design space. Key insights that emerged from this modeling effort are: a)Increasing the fraction of fuel processed by the fuel cell always increases effciency. b) A tradeoff between fuel cell effciency and specific power determines the optimum range of the vehicle. This tradeoff is heavily influenced by the polarization curveof the SOFC. This optimum operating point is different from the maximum power point. c) The GT/SOFC could be used to increase the cycle’s flow specific work, enabling a smaller core to drive the same size fan. This premise is investigated in more detail later in the thesis. d) The fraction of fuel processed by the fuel cell is limited by the ability to cool it. An analytical expression for this limit is derived but in general the maximum power output of the fuel cell is limited to less than half of the total system power output for most hybridization schemes. Second, this work develops an improved thermodynamic model of the hybrid turbine and fuel cell system. The model accounts for off-design performance of the turbomachinery as well as suffcient details of the transport and electrochemistryin the fuel cell to predict the effect of specific design changes (physical dimensions, flow rates, pressure, temperature, etc.) and operating conditions on power output, energy conversion effciency, and system mass. The model is implemented using a NASA-developed tool called Numerical Propulsion System Simulation (NPSS) that is emerging as a standard in modern engine development. While third-party NPSS fuel cell modules are available, they are not suitable for fuel cell design because key performance parameters like utilization, effciency, and specific power are inputs. Our module predicts fuel cell performance from its geometric attributes (channel length, width, height, number) and electrochemical attributes (i.e. temperature, pressure and composition effects on the polarization curve). Such capability is computationally expensive but essential for predicting GT/SOFC performance over varying flight conditions. This work implemented a) ’guardrails’ to prevent solver divergence due to self-reinforcing high or low temperatures, b) an adaptive Newtonsolver damping scheme to improve convergence, c) an electrochemical performance map to find close initial conditions, and d) the option for methane as an additional fuel, amongst other alterations. Taken together, these changes reduced execution time from weeks to hours and greatly improved stability making the thermodynamic model a much more useful tool for design and analysis. Third, the NPSS system model is used to assess the viability of two possible hybridization schemes. The first is a ‘parallel’ hybrid system where an SOFC powers an electric motor that assists the turbine in driving the main fan. The second is a ‘turboelectric’ hybrid system where all of the propulsive power is provided electrically by a fuel cell working in tandem with a mechanical generator attached to the gas turbine. The results show that a parallel hybrid can reduce fuel consumption by 27%, but requires a reformer/fuel cell that achieves > 1kW/kg to achieve range parity with a conventionally-powered B737. This occurs because the thermodynamic effciency of the system increases by 10% and the propulsive effciency increases by 10% due to the higher bypass ratio made possible by the increase in flow specific work associated with hybridization. The turboelectric system reduces fuel consumption by 12% when 25% of power is generated by the SOFC, but requires a reformer/fuel cell that achieves > 1.2kW/kg to achieve range parity with a conventionally-powered B737. This higher specific power requirement occurs because the gas turbine operates at a lower OPR = 15 vs. OPR = 24 to enable recuperation via a heat exchanger. The heat exchanger also improves the thermodynamic performance of both the Brayton cycle and the SOFC (by reducing preheating requirements) even at 30% effectiveness, but adds mass and complexity. Fourth, this work investigates the potential impacts of introducing the fuel cell exhaust—which is hot and contains large amounts of water and combustible reformate—on the Brayton cycle. The system modeling efforts show that the fuelcell exhaust can constitute up to 70% of the total mass flow rate through the system and up to 50% of the total net heat release. Therefore, the effect of the fuel cell exhaust on the operation of the main combustor is expected to be substantial both for integration with traditionally injected fuels, and influencing trades for the SOFC subsystem design choices that affect that exhaust (e.g. fuel utilization). Subsequent chemical kinetic simulations implemented in Cantera show that SOFC exhaust adiabatic flame temperatures can reach as high as 2200K, laminar flame speeds may vary by as much as 500% across a range of fuel utilization targets, ignition delay times with hydrocarbon/air mixtures can reach the millisecond range, and mixed SOFC exhaust can achieve extinction strain rates of over 300,000/s in pressures reasonable for gas turbines. These results suggest that aircraft GT/SOFCs may also require new combustor designs for effective hybridization.Item RADIATION SYNTHESIS OF IONIC LIQUID POLYMER ELECTROLYTE MEMBRANE FOR HIGH TEMPERATURE FUEL CELL APPLICATIONS(2020) Mecadon, Kevin; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this thesis was to design, synthesize and analyze innovative anhydrous fuel cell membranes that can operate at temperatures above 100°C. Operating at this higher temperature region improves performance and reliability of fuel cells: increasing proton mobility, enhancing reaction kinetics, increasing catalysis activity and reducing carbon monoxide poisoning. Traditional polymer electrolyte membrane fuel cells (PEMFCs) do not operate efficiently above 100°C because water is used as a proton conductive medium though the Grotthuss hopping mechanism. Through substituting water with protic ionic liquids and grafting onto fluorocarbon films, a new proton conductive network solid state PEM has been developed. These membranes can perform at high temperature above 100°C. Polymers were selected for grafting based on the following properties: high proton conductivity, low electrical conductivity, high mechanical properties, high chemical resistance, and high temperature and humidity stability. The method used to synthesize these anhydrous polymer electrolyte membranes (PEMs) was radiation grafting using heterocyclic protic ionic liquid monomers and fluorocarbon substrates. PEMs were prepared at the Medical Industrial Radiation Facilities (MIRF) at the National Institute of Standards and Technology (NIST). MIRF is a 10.5 MeV electron beam accelerator, which was used to radiate the fluorocarbon substrate and then indirectly graft heterocyclic protic ionic liquids to create PEMs. After synthesis, the extent and uniformity of PEM composition was analyzed using FTIR microscopy, SEM/EDS, SANS and their proton conductivity as measured by EIS. Through this research, indirect radiation grafting was shown to covalently bond ionic liquids onto fluorocarbon substrates to synthesize PEMs. The resulting ionic liquid PEMs showed proton conductivities greater than 10-3 S/cm above 100°C that behaved independent of humidity. The ionic liquid PEMs also demonstrated a positive correlation of increasing proton conductivity with increasing temperatures above 100°C even after the PEMs are dehydrated. The chemical properties and structure of the grafted ionic liquids greatly affects the proton conductive mechanisms present in the PEMs. These trends found through the course of this research will help the development of future anhydrous PEM with higher proton conductivity, performance, and reliability.Item Gas Turbine / Solid Oxide Fuel Cell Hybrids: Investigation of Aerodynamic Challenges and Progress Towards a Bench-Scale Demonstrator(2019) Pratt, Lucas Merritt; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modern aircraft are becoming more electric making the efficiency of on-board electric power generation more important than ever before. Previous work has shown that integrated gas turbine and solid oxide fuel cell systems (GT-SOFCs) can be more efficient alternatives to shaft-driven mechanical generators. This work advances the GT-SOFC concept in three areas: 1) It develops an improved model of additional aerodynamic losses in nacelle-based installations and shows that external aerodynamic drag is an important factor that must be accounted for in those scenarios. Additionally, this work furthers the development of a lab-scale prototype GT-SOFC demonstrator system by 2) characterizing the performance of a commercial off-the-shelf (COTS) SOFC auxiliary power unit that will become part of the prototype; and 3) combining a scaled-down SOFC subsystem model with an existing thermodynamic model of a small COTS gas turbine to create an initial design for the prototype.Item An Experimental and Analytical Investigation of Hydrogen Fuel Cells for Electric Vertical Take-Off and Landing (eVTOL) Aircraft(2019) Ng, Wanyi; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this thesis is a comprehensive investigation of hydrogen fuel cells for electric vertical take-off and landing (eVTOL) aircraft. The primary drawback of battery powered eVTOL aircraft is their poor range and endurance with practical payloads. This work uses simulation and hardware testing to examine the potential of hydrogen fuel cells to overcome this drawback. The thesis develops steady state and transient models of fuel cells and batteries, and validates the models experimentally. An equivalent circuit network model was able to capture the waveforms and magnitudes of voltage as a function of current. Temperature and humidity corrections were also included. Examination of the results revealed that the transient behavior of batteries and fuel stacks are significant primarily shortly after startup of the fuel stack and at the limiting ranges of high and low power; for a nominal operating power and barring faults, steady state models were adequate. This work then demonstrates fuel cell and battery power sharing in regulated and unregulated parallel configurations. It details the development of a regulated architecture, which controls power sharing, to achieve a reduction in power plant weight. Finally, the thesis outlines weight models of motors, batteries, and fuel cells needed for eVTOL sizing, and carries out sizing analysis for on-demand urban air taxi missions of three different distances -- 50, 75, and 150~mi of cruise and 5~min total hover time. This revealed that for ranges within 75 mi, a light weight (5000-6000~lb gross weight) all-electric tilting proprotor configuration achieves a practical payload (500~lb or more) with current levels of battery specific energy (150~Wh/kg) if high burst C-rate batteries are available (4-10~C for 2.5~min). Either a battery-only or battery-fuel cell (B-FC) hybrid power plant is ideal depending on the range of the mission: For inter-city ranges (beyond approximately 50~mi), the mission is impossible with batteries alone, and fuel cells are a key enabling technology; a VTOL aircraft with a B-FC hybrid powerplant, an aircraft with 6200~lb gross take-off weight, 10~lb/ft$^2$ disk loading, and 10~C batteries, could be sized to carry a payload of 500~lb for a range of 75~mi. For this inter-city range, the research priority centers of fuel cells, as they appear to far surpass future projections of Li-ion battery energy levels based on performance numbers (at a component level), high weight fraction of hydrogen storage due to the short duration of eVTOL missions, and lack of a compressor due to low-altitude missions, with the added benefit of ease of re-fueling. However, for an intra-city mission (within approximately 50~mi), the B-FC combination provides no advantage over a battery-only powerplant; a VTOL aircraft with a battery-only powerplant with the same weight and disk loading as before, and 4~C batteries, can carry a payload of 800~lb for a range of 50~mi. For this mission range, improving battery energy density is the priority.Item EVALUATION AND IMPROVEMENT OF MECHANICAL AND CHEMICAL RESILIENCE OF INTERMEDIATE-TEMPERATURE SOLID OXIDE FUEL CELL ANODES(2017) Hays, Thomas; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cells are in the process of reaching maturity as an energy generation technology, but a number of technical challenges exist, namely mechanical and chemical resilience, that hinder the realization of their full potential and widespread deployment. As more research and development work has been performed on intermediate temperature SOFCs based on gadolinium doped ceria, there persists a number of gaps in the understanding of the behavior of these devices. The mechanical properties of component material and SOFC structures in non-ambient conditions, the nature and degree of damage caused by sulfurized hydrocarbon fuels, and the potential for leveraging produced thermal energy are not satisfactorily characterized for GDC-based SOFCs. Mechanical testing of porous GDC and anode supported SOFC coupons from room temperature to 650°C was performed in air and reducing conditions using a test system designed and built for this application. Spherical porosity was determined to result in the higher strength compared to other pore geometries and a positive linear dependence between temperature and strength was determined for SOFC coupons. Additionally, placing the electrolyte layer in compressive stress resulted in higher strengths. Standard SOFCs were operated while exposed to hydrogen and methane containing ppm level hydrogen sulfide concentration. An infiltration technique was used to deposit a fine layer of GDC on the inner surfaces of some cell anodes, and the results of sulfur expose was compared between infiltrated and unmodified cells. GDC infiltration allowed cells to operate stably for hundreds of hours on sulfurized fuel while unmodified cells were fatally damaged in less than two days. A primary and a resulting secondary degradation mechanism were identified and associated with sulfur and carbon respectively through surface analysis. A novel technique for measuring thermal power output of small-scale SOFCs operating on a variety of fuels was developed and used to evaluate electrical and thermal outputs while operating on simulated anaerobic digester biogas. These findings were used to propose a multi-utility generation system centered on a nominal 10 kW SOFC unit fed by anaerobic digesters and capable of producing clean water and electricity for 50 individuals through direct contact membrane distillation driven by captured waste heat from the SOFC.Item Ceramic Materials Development for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)(2016) Pan, Ke-Ji; Wachsman, Eric D; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy into electric power with high efficiency. Traditional SOFC has its disadvantages, such as redox cycling instability and carbon deposition while using hydrocarbon fuels. It is because traditional SOFC uses Ni-cermet as anode. In order to solve these problems, ceramic anode is a good candidate to replace Ni. However, the conductivity of most ceramic anode materials are much lower than Ni metal, and it introduces high ohmic resistance. How to increase the conductivity is a hot topic in this research field. Based on our proposed mechanism, several types of ceramic materials have been developed. Vanadium doped perovskite, Sr1-x/2VxTi1-xO3 (SVT) and Sr0.2Na0.8Nb1-xVxO3 (SNNV), achieved the conductivity as high as 300 S*cm-1 in hydrogen, without any high temperature reduction. GDC electrolyte supported cell was fabricated with Sr0.2Na0.8Nb0.9V0.1O3 and the performance was measured in hydrogen and methane respectively. Due to vanadium’s intrinsic problems, the anode supported cell is not easy. Fe doped double perovskite Sr2CoMoO6 (SFCM) was also developed. By carefully doping Fe, the conductivity was improved over one magnitude, without any vigorous reducing conditions. SFCM anode supported cell was successfully fabricated with GDC as the electrolyte. By impregnating Ni-GDC nano particles into the anode, the cell can be operated at lower temperatures while having higher performance than the traditional Ni-cermet cells. Meanwhile, this SFCM anode supported SOFC has long term stability in the reformate containing methane. During the anode development, cathode improvement caused by a thin Co-GDC layer was observed. By adding this Co-GDC layer between the electrolyte and the cathode, the interfacial resistance decreases due to fast oxygen ion transport. This mechanism was confirmed via isotope exchange. This Co-GDC layer works with multiple kinds of cathodes and the modified cell’s performance is 3 times as the traditional Ni-GDC cell. With this new method, lowering the SOFC operation temperature is feasible.Item Synthesis of Novel Alkaline Polymer Electrolyte for Alkaline Fuel Cell Applicaitons(2012) Luo, Yanting; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Development of the intrinsically OH- conductive polymeric electrolyte (alkaline polymer electrolyte, APE) is the critical component to enable the wide application of alkaline fuel cell (AFC) technology. Alkaline polymer electrolyte fuel cell (APEFC) based on AFC technology has been revived recently for applications in transportation and portable electronic devices due to its advantages of using non-noble metal catalysts, faster oxygen reduction in alkaline medium, and compact design. The research described in this dissertation aims to synthesize a novel APE, with controlled ionic conductivity and mechanical strength to achieve high fuel cell power density and long durability. Most APEs synthesized up to now use a modification of existing engineering polymer backbones, which are very difficult to balance its mechanical properties with its ionic conductivities. In this research, we copolymerized APE precursor polymers, namely poly (methyl methacrylate-co-butyl acrylate-co vinylbenzyl chloride) (PMBV) from three functional monomers, methyl methacrylate (MMA), butyl acrylate (BA) and vinylbenzyl chloride (VBC), where VBC was the functional group that was attached with trimethylamine (TMA) and was the OH- carrier after ion-exchanging. MMA was used for mechanical support and BA was used to alleviate the brittleness coming from MMA and VBC. We synthesized alkaline polymer electrolytes from bottom-up polymerization of these selected functional monomers using free radical solution and miniemulsion copolymerization techniques. By miniemulsion copolymerization, the properties of the obtained APEs could be precisely controlled by tuning the (1) monomer ratio, (2) glass transition temperature (Tg), (3) molecular weight (MW), and (4) crosslinking the copolymer. The increase in Tg was realized by eliminating BA from monomers, which was a low Tg component. MW was optimized through investigating binary copolymerization kinetics factors (initiator and surfactant). For crosslinking, the newly obtained poly (methyl methacrylate-co-vinylbenzyl chloride) (PMV) was crosslinked as a semi-interpenetrating network (s-IPN) to reduce water uptake and thus enhanced the mechanical strength in a humidified environment for APEFCs. After the optimization, our best quaternized PMBV (QPMBV) series APE membranes could reach a maximum power density of 180 mW/cm2 and the crosslinked QPMV APE could last 420 hours on APEFCs, which was among the best overall performance in APE technologies. In the future, we propose to use fluorinated polymer monomers to redesign the polymer backbone. Another direction in the design of APEs is to reselect the possible functional OH- carrier groups to make APEs more chemically and mechanically stable in a high pH environment. And last but not least, atomic force spectroscopy (AFM) is proposed to observe the APE nanostructure, the ionic conductive path, and the local mechanical strength by applying a small voltage between the tip and stage.Item Modeling the integration of thermoelectrics in anode exhaust combustors for waste heat recovery in fuel cell systems(2011) Maghdouri Moghaddam, Anita; Jackson, Gregory Scott; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recently developed small-scale hydrocarbon-fueled fuel cell systems for portable power under 1 kW have overall system efficiencies typically no higher than 30-35%. This study explores the possibility of using of thermoelectric waste heat recovery in anode exhaust combustors to improve the fuel cell system efficiencies by as much as 4-5% points and further to reduce required battery power during system start-up. Two models were used to explore this. The first model simulated an integrated SOFC system with a simplified catalytic combustor model with TEs integrated between the combustor and air preheating channels for waste heat recovery. This model provided the basis for assessing how much additional power can achieve during SOFC operation as a function of fuel cell operating conditions. Results for the SOFC system indicate that while the TEs may recover as much as 4% of the total fuel energy into the system, their benefit is reduced in part because they reduce the waste heat transferred back to the incoming air stream and thereby lower the SOFC operating temperatures and operating efficiencies. A second model transient model of a TE-integrated catalytic combustor explored the performance of the TEs during transient start-up of the combustor. This model incorporated more detailed catalytic combustion chemistry and enhanced cooling air fin heat transfer to show the dynamic heating of the integrated combustor. This detailed model provided a basis for exploring combustor designs and showed the importance of adequate reactant preheating when burning exhaust from a reformer during start-up for the TEs to produce significant power to reduce the size of system batteries for start-up.Item Performance Prediction of Scalable Fuel Cell Systems for Micro-Vehicle Applications.(2010) St. Clair, Jeffrey Glen; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Miniature (< 500g) bio-inspired robotic vehicles are being developed for a variety of applications ranging from inspection of hazardous and remote areas to environmental monitoring. Their utility could be greatly improved by replacing batteries with fuel cells consuming high energy density fuels. This thesis surveys miniature fuel cell technologies and identifies direct methanol and sodium borohydride technologies as especially promising at small scales. A methodology for estimating overall system-level performance that accounts for the balance of plant (i.e. the extra components like pumps, blowers, etc. necessary to run the fuel cell system) is developed and used to quantify the performance of two direct methanol and one NaBH4 fuel cell systems. Direct methanol systems with water recirculation offer superior specific power (400 mW/g) and specific energy at powers of 20W and system masses of 150g. The NaBH4 fuel cell system is superior at low power (<5W) because of its more energetic fuel.Item Performance Modeling of a H2-Fueled Proton Exchange Membrane Fuel Cell(2007-04-18) Shields, Eric; Jackson, Gregory; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To assist in the development of an integrated proton exchange membrane fuel cell (PEMFC) system, a 2-D fuel cell model has been developed and integrated with supporting zero-D models. The fuel cell model employs a finite-volume discretization of the conservation equations in the gas-phase flow channels, for the gas diffusion layer, and at the electrocatalyst electrolyte interface. The resulting conservation equations are converted into a DAE form for transient integration within MATLAB. The model employs detailed surface thermochemistry within CANTERA for the catalyst and electrolyte surfaces. In this study, the model was used to investigate the isothermal performance of the fuel cell and to assess how steady-state overpotentials depend on operating conditions. These results were validated against existing data supplied by Ballard Power Systems. After validation, the Ballard stack parameters were used in transient integration to evaluate how the fuel cell responds to rapid changes in load and flow conditions.