Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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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 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 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.