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Fueling aircraft with hydrogen is one compelling method for achieving net-zero emissions in the aviation sector. However, many questions must first be answered to make the path to commercial viability clear---questions involving technical design, economics, policy, infrastructure, and more. There are four technical questions that are central to this dissertation. Firstly, which flights might be well-suited to hydrogen aircraft? Secondly, how should liquid hydrogen be handled on aircraft? Thirdly, how should aircraft be designed around hydrogen's storage requirements? Finally, how can the heat produced by hydrogen fuel cells be efficiently rejected?
To address the first question, a rapid aircraft performance estimation tool is created that predicts the fuel consumption and weight of different net-zero emission aircraft types, given top-level aircraft design requirements. The tool is used to study the amount of low-carbon electricity that would be needed to generate the fuel to power different types of net-zero emission aircraft on nearly all commercial missions currently flown. A sensitivity study investigates the influence of certain technology parameters on the results to identify high-impact technology improvements.
For the second question, a liquid hydrogen tank thermodynamic model is developed to better understand and predict how a liquid hydrogen tank will behave in different scenarios. The model is the first aircraft liquid hydrogen tank model developed specifically for use with gradient-based optimization. This enables it to be incorporated into large system architecture optimization problems so that engineers can better understand how the liquid hydrogen management requirements might impact the overall hydrogen propulsion system design.
The third question involves a massive design space of all possible aircraft configurations. The work in this dissertation focuses on two commonly-studied configurations for hydrogen aircraft: the tube and wing and blended wing body concepts. It compares how the fuel energy requirements of each change as they adapt from kerosene to hydrogen. A sensitivity study demonstrates how the results are impacted by different model assumptions.
The final question deals with the use of hydrogen in the propulsion system. Low-temperature fuel cells, an option introduced by using hydrogen fuel, could potentially achieve lower climate impacts and greater efficiencies than combustion-based propulsion systems. However, fuel cells produce large quantities of low-temperature waste heat, which is challenging to efficiently reject. If not properly designed, the waste heat management could introduce losses that eat into the fuel cell's advantages. The work in this dissertation demonstrates a design methodology to reduce these losses. It uses gradient-based optimization to design low-drag and lightweight ducted heat exchangers, the systems responsible for transferring waste heat to the surrounding environment.