Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Comprehensive Study and Fundamental Understanding of Lithium Sulfur Batteries for eVTOL(2022) Fisler, Emily; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation is an attempt to bridge the traditional isolation of electrochemistry and aeronautics, particularly rotary-wing aeronautics, where integration of power and platform is vital for the development of a viable aircraft. The dissertation topic addresses the principal barriers of electric-VTOL aircraft: energy and power. It explores a promising alternative to traditional lithium-ion technology; advanced lithium sulfur batteries are projected to supply double the energy of lithium-ion. The principal conclusions are: 1. lithium sulfur can indeed achieve double the energy of lithium-ion but only at low power settings, 2. lithium sulfur exhibits high internal resistance compared to lithium-ion which explains the loss of energy at high power, and 3. discharging lithium sulfur only halfway stabilizes the batteries. A conceptual sizing analysis was developed and verified with NASA. Key battery performance targets were determined for electric VTOL aircraft. Lithium sulfur and lithium-ion coin cells were fabricated with identical overhead for a clear and consistent comparison of specific energy and power. The characteristics measured were discharge cycles, cycle life, impedance under conditions unique to electric vertical takeoff and landing aircraft namely high C-Rates, half cycles, and high transients. Equivalent circuit models were developed and validated to predict the steady-state and transient behavior of these cells. Results show that lithium sulfur provides more than twice the specific energy of lithium-ion up to currents of almost C/2. At 1C, it is comparable. Above 1C it drops drastically and by 4C the energy vanishes almost entirely. This is traced to an order of magnitude higher impedance of these cells. The price to pay for high energy is low cycle life. However, it appears this problem can be eliminated by half cycles. The dynamic behavior of lithium sulfur is richer in comparison to lithium-ion. The response is still capacitative, hence first order, but the complex Warburg and constant phase elements have far greater influence. The behavior is harder to model as it does not fit neatly into linear equivalent circuits. The key conclusion is that lithium sulfur appears to be an attractive alternative to lithium-ion with characteristics that have significant ramifications on future electric VTOL aircraft design and infrastructure.Item GRAMIAN-AWARE CLOSED LOOP FLIGHT CONTROL DESIGN FOR ENERGY HARVESTING THROUGH MODULATING DISTURBANCE SENSITIVITY(2017) Saxena, Utsav; Faruque, Imraan A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many desired micro aerial vehicle missions are significantly larger than the mission endurance of the vehicles. Due to extreme constraints on size, weight and power available, small scale air vehicles are highly sensitive to atmospheric disturbance. This work introduces a control-theoretic framework that models the magnitude of the vehicle's disturbance sensitivity and observability in conjunction with each other under a gramian-based formulation. To implement atmospheric gust response modulatiom, a ``gramian-aware'' flight control law is designed using open loop plant models across various scales and assuming perfect gust measurement. Time-domain system identification was conducted using data collected from repeatable automated flights in a motion capture arena in order to derive the plant model. Closed-loop simulation results as well as experimental data modulating the plant using cruise speed are presented to illustrate that the gramian-based control laws can be utilized to facilitate atmospheric energy scavenging in gusting environments.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.