With the rapid growing aviation industry and the high amount of produced greenhouse gases, the search in more sustainable propulsion methods continues. A possible solution is the use of hydrogen fuel cells, significantly reducing the production of NOx and CO2. Although several studies are being performed for the performance and applications of the fuel cell, for which the Airbus ZEROe program is one of them, the additional air supply system lacks research. Due to the specific operating conditions of the fuel cell and the large difference between ambient conditions, the air supply system should be analysed with care as it can require a lot of energy over an entire flight. This thesis focuses on developing a model capable of analysing the performance of the air supply system, using the Airbus ZEROe fuel cell as a reference. Furthermore, different architectures have been compared in order to analyse the effect of different components. A model has been developed capable of sizing and analysing the performance for different air supply architectures. The compressor, turbine, and heat exchangers have beenmodelled such that they provide a physical representation of what is happening inside the air supply system. Furthermore, as the model requires geometrical parameters as an input, it can be easily adapted to optimise these components for different design conditions. Therefore, the model has been used to optimise four different architectures, consisting of a variation between single or double stage compressors and the inclusion of a recuperator. The optimisation has been performed for several design conditions, analysing not only the effect for different flight conditions but also the altitude, fuel cell pressure, and air mass flow. For the optimisation, it has been found that the architecture resulting in the lowest power required has a single stage compressor and a recuperator, having a power required of 77.5 kW for cruise. However, the addition of the recuperator adds a significant amount of mass, as the total air supply system mass is estimated around 315 kg. Compared to an architecture without the recuperator (168 kg) this is a significant difference. However, the power required for this architecture is significantly higher, with a value of 100 kW. Overall it was found that although adding mass, the recuperator significantly reduces the power required. Next to the recuperator, the addition of a second stage compressor has been analysed as well. Although adding mass, it has been seen that for the higher mass flow and pressure ratio cases, such as top of climb, the double stage compressor performs better than the single stage compressor, although this difference is not as large as for the addition of the recuperator. Apart from the optimisation, the off-design performance has been analysed as well for the four different architectures, taking the cruise-, takeoffand top of climb optimised architectures as a base. It was found that the off-design performance for the different architectures is mostly limited by the choice in design point. The main component that was affected the most by this is the turbine, as a turbine bypass valve is required. The turbine bypass valve regulates the mass flow passing through the turbine, controlling the expansion ratio for a fixed shaft speed. It has been observed that the valve is required for architectures that are optimised for high pressure ratios and low mass flows, as the valve was seen opened for lower pressure ratios and higher mass flows. Although the bypass valve is required, it was observed that the performance of these architectures was always better than an architecture that did not require the turbine bypass valve, such as those optimised for takeoff. Additionally, limiting operating conditions have been identified through off-design analysis. The most critical case for sizing the heat exchangers is the hot day scenario at the top of climb. Due to the high pressure ratio and the resulting high compressor outlet temperature, adequate cooling is required to achieve the necessary fuel cell temperature. It was found that, for other optimised points, architectures with a recuperator face more challenges in this regard compared to those without a recuperator, mainly because of their smaller liquid heat exchangers. Other identified limiting conditions include low mass flow operating points for the compressor, lower-than-design expansion ratios, and higher mass flows for the turbine. These conditions necessitate the addition of bypass valves to ensure proper operation.

The European Commission’s roadmap aims to install 60 GW of offshore wind energy by 2030 and 300 GW by 2050, requiring substantial raw materials. Airborne wind energy (AWE) offers a promising alternative with lower material demand and environmental impact. This thesis assesses the environmental impact of a 100 kW soft-kite AWE system using life cycle assessment (LCA). The target market for these AWE systems includes off-grid remote areas. Thus, a comparative LCA of hybrid power plant (HPP) configurations was conducted using site data from a military base in Marseille, France. Results show the Falcon AWE system has a GWP of 8.6 kg CO2 eq/MWh and CED of 144.1 MJ/MWh, with the ground station being the most impactful component. For the HPP, including diesel generators and batteries reduces the oversizing of renewable components, enhancing sustainability. Future recommendations include developing AWE-specific databases and evaluating more impact indicators.

Given the enduring issues of human-induced climate change, the appeal of strategies aimed at reducing greenhouse gases has never been greater. CO2 is the most concerning greenhouse gas due to the large quantity already emitted into the atmosphere and the ongoing excessive emissions from anthropogenic activities. The transportation sector significantly impacts global warming, as all major transportation modes depend on fossil fuels. The principal fuel used in aviation is kerosene, which is a jet fuel derived from petroleum. Due to the environmental risks associated with petroleum-based fuels, synthetic kerosene is an appealing alternative that could aid the decarbonisation of the aviation sector. This research study aims to conduct a techno-economic analysis of synthetic kerosene, as a potential sustainable fuel for aviation. To address the excessive CO2 concentrations in the atmosphere, the process of extracting CO2 from the air, known as direct air capture, is investigated. A model is developed in Aspen Plus consisting of four main modules, namely direct air capture with NaOH sorbent and calcium looping, syngas production through the reverse water gas shift reaction, Fischer-Tropsch Synthesis and hydrocracking & distillation. There is a substantial gap in the literature concerning the utilisation of direct air capture technologies for synthetic fuel production, and this study aims to narrow this gap.

The decommissioning of offshore wind farms (OWFs) presents a significant challenge due to the environmental impact associated with the process, particularly in terms of greenhouse gas (GHG) emissions. This research aims to develop a model for quantifying and analysing the GHG emissions associated with large-scale OWF decommissioning. It examines key variables, including vessel types, decommissioning activities, transport strategies, and weather conditions, to assess their impact on total emissions.

Using a GHG inventory-based approach, the research applies both deterministic and probabilistic methods to assess emissions across various decommissioning scenarios. The model integrates operational logistics with emission factors, providing a flexible framework that adapts to different OWF projects. The research specifically examines the decommissioning of OWF Lincs Limited.

The results of this study highlight the potential for emissions reduction through optimised vessel strategies, transport methods, and campaign scheduling. While operational activities dominate emissions, the research underscores the importance of addressing external factors, such as weather and campaign timing, to minimise environmental impact. The developed model offers valuable insights to stakeholders aiming to implement effective GHG emission reduction strategies in future OWF decommissioning projects.

Aircraft redesign and flight path optimisation offer promising methods of rethinking how we fly. As the effects of aviation on the planet are becoming better understood, focus has shifted from cost minimisation to climate impact mitigation. In this study, simultaneous aircraft design and trajectory optimisation is used to investigate the trade-off between direct operating costs (DOC) and the global average temperature response (ATR) associated with a typical medium range flight. It is shown for a representative mid-latitude atmosphere and narrowbody aircraft that the ATR can be reduced by 49% with approximately 0.5% increase in operating costs through 2-dimensional flight path optimisation. With simultaneous wing planform optimisation, the DOC-ATR trade-off is even more favourable, leading to a 56% ATR reduction with no increase in operating costs. Results indicate that contrail avoidance is a highly cost-effective method of minimising the climate effects of aviation.

Hydrogen is seen as a potential energy carrier for the next generation of aircraft that will be more climate-friendly than the previous generation of kerosene-powered aircraft. Two types of hydrogen-based propulsion systems are currently foreseen for such aircraft: hydrogen combustion and hydrogen fuel cell. In addition to producing useful electrical power, fuel cell systems produce a considerable amount of heat which must be removed to ensure the continued and efficient operation of the fuel cell stack. This thesis presents a thermal management system sizing methodology for propulsive fuel cell systems onboard CS-23 commuter aircraft. The developed methodology is implemented into an aircraft sizing environment to study the characteristics of air-based thermal management systems and their effects on aircraft design and aircraft performance. The results show that the thermal management system, through its additional mass, parasitic drag and parasitic power, has both a direct and indirect effect on aircraft performance. In addition to the conventional thermal management system, an unconventional system using nanofluids is studied. The use of nanofluids showed no considerable improvement in both system and aircraft level performance when compared to the base fluid. This thesis shows the importance of considering the design of thermal management systems during the conceptual design of fuel cell aircraft.

The need for more efficient aircraft has led to the conception of innovative engine configurations, such as the combined-cycle engine proposed by Delft University of Technology or the Water-enhanced turbofan developed by MTU and Pratt & Whitney. The evaluation of the potential benefits of these novel engine concepts requires detailed performance studies considering weight and drag penalties as figures of merit. At present, no weight estimation tools with sufficient level of accuracy and flexibility are publicly available, apart from WATE++, a tool developed by NASA. However, this software is subject to export control restrictions and cannot be used outside the USA. For this reason, the development of a new component-based preliminary engine design tool, "Weight EStimation of gas Turbine engines" (WEST), was started. The goal of WEST is to predict the weight of novel engine architectures with a reasonable level of accuracy, accounting for design parameters like turbine inlet temperature, overall pressure ratio, mass flow rate, and turbomachinery configuration, with a minimal set of geometry inputs specified by the user. A previous work demonstrated that WEST can effectively predict the weight of turbofan gas generators. As only the main components are modeled, the WEST estimates account for approximately 70-90% of the actual engine weight.

The capabilities of WEST were expanded in this study to allow for the design of small-scale turboshaft engines. To this purpose, a methodology to design radial compressor disks was implemented, and successfully verified against FEM results. Regarding the modeling of the complete turboshaft, it was found that the predictions of the tool account for only 60-70% of the actual engine weight. Such result was, however, expected, as WEST does not take into account the particle separator and the integrated gearbox, which may account for up to $30\%$ of the engine's total weight.

This study analyses the feasibility of an engine architecture using bypass cooled cooling air as a method for reducing specific fuel consumption of current and future high bypass turbofan engines. As cooled cooling air reduces required turbine cooling massflow, a major source of loss in modern aeroengines, it is believed that the concept can improve engine efficiency. No extensive or explicit study on the merits of this concept has been performed, however.
Using the 0-D engine analysis software GSP, engine performance parameters at Take-Off and Cruise, such as, specific fuel consumption and (specific) thrust were evaluated against altering feed cooling air temperature by 0 to -300K and heat exchanger induced pressure losses of 0 to 8% in the bypass. This was done for two test cases; the Leap-1A engine (TOC OPR 50, CRZ BPR 11.1, TO FN 121kN) representing current conventional technology and GTF2050, a geared turbofan engine (TOC OPR 75, CRZ BPR 17.1, TO FN 174kN) with entry into service of 2050. For the Leap-1A engine the specific fuel consumption increased up to 1.9% at cruise and for GTF2050, the increase was up to 4.1%. It is shown that the change in fuel consumption is linear with the decrease in cooling fraction, as for both engines the specific fuel consumption increases by +0.34% per percent cooling air reduction.
With a reduction in cooling fraction, the exhaust pressure of the core increases, resulting in a higher thrust (up to 2.3%). It is shown that the optimal take-off bypass ratio for minimum fuel consumption shifts up, for the GTF2050 engine from 16.4 to 17.5.
Furthermore, scenario tests and an exergy analysis were performed to find that the impact of different mechanisms; the pressure loss in heat exchanger has minor impact on performance; change in turbine efficiency has small impact; the effect of changed turbine massflow and heat rejection into the bypass are the dominant phenomena.
It is concluded that as a method for reducing fuel consumption, cooled cooling air with the bypass as a heat sink is not feasible for conventional architectures even with extreme design parameters such as high OPR and BPR. Heat rejection to the bypass cannot be effectively utilised and no improvement in core efficiency from reduced cooling air can compensate. In the edge case where there is no heat transfer in cruise, there is an observed specific fuel consumption benefit of up to 4%. The achievable benefit will be lower, however, when accounting for installation penalties.
It is recommended to focus turbine cooling studies on the impact and feasibility of active cooling massflow control without heat rejection, investigate alternative heat sinks for cooled cooling air (e.g. cryogenic fuel), and only consider bypass cooled cooling air as last resort when up-flowing cooling
schemes is not permissible or ineffective.

This report covers the investigation of impact of uncertainties on the multidisciplinary design optimization of a medium-range single-aisle turbofan aircraft for minimum global warming impact. The employed workflow for the investigation is a five step process, starting with the implementation of the deterministic climate impact model and carrying out of the design optimization for minimal climate impact. The second step involves the characterisation of uncertainties, where the uncertainties within the climate impact model are identified and quantified. The third step involves the uncertainty analysis, where Monte Carlo simulations are performed to estimate the variability in the average temperature reduction potential of the climate-optimized aircraft with respect to the cost-optimized aircraft. In the fourth step, a robust design optimization is carried out using a non-sorted genetic algorithm to minimize both the average temperature response and variability in average temperature response potential. The sensitivity analysis is carried as the last step using the Morris and variance-based Sobol methods, to identify what the key uncertain parameters are towards the uncertainty in climate impact of the aircraft designs. Scientific uncertainty is identified within the linear climate impact model for the carbon impulse response function parameters, species radiative efficiencies, the NOx and contrail altitude forcing factors, methane lifetime, species efficacies, and are all assigned a probabilistic description. Scenario uncertainty is identified in the future average global CO2 atmospheric concentration projection, for which different realistic future scenarios are characterised. Carrying out the uncertainty analysis has shown that the average temperature response reduction potential of the climate-optimized aircraft is highly uncertain, having a 90% likelihood ranging between 17 and 98 % of the average temperature response of the cost optimized aircraft. This is primarily due to large variability in the estimation of contrail average temperature response. Although the robustness-based optimization did not allow to find any significant improvement in robustness for the climate-optimized aircraft, it did allow to identify an array of robust climate-optimized design solutions. From the sensitivity analysis, it was found that the uncertain parameters showing predominant influence on the output variability are the contrail-related radiaitive efficiency and forcing factors. Additionally, a variability of ±50% in average temperature response apportioned to CO2 emissions was identified due to uncertainty related to future average global CO2 concentration projections.

Zero-carbon-dioxide-emitting hydrogen-powered aircraft have, in recent decades, come back on the stage as promising protagonists in the fight against global warming. Nevertheless, most recent studies agree that hydrogen aircraft would underperform their kerosene counterparts in terms of operative empty mass and specific energy consumption. The main cause for the drop in performance lays in the fuel storage, as not only the liquid hydrogen has to be kept in cryogenic conditions and pressurised, but for the same energy content, it has four times the volume of kerosene. The inevitable consequences are an increase in fuselage size, which adds mass and drag to the aircraft, and the addition of an heavy fuel storage and distribution system. On the other side, hydrogen has 2.8 times higher specific energy, and the consequent reduction in fuel mass could balance the previously mentioned drawbacks. Literature on the topic shows that the optimal fuel storage solution depends on the aircraft mission, but most studies disagree on what solutions are optimal for each aircraft range category.The objective of this research was to identify and compare possible solutions to the integration of the hydrogen fuel containment system on short, medium and long-range airliners. The capabilities of an automated synthesis program for CS-25 aircraft have been expanded with validated structural and thermodynamic physics-based tank design models, to allow for the design and analysis of liquid hydrogen aircraft. Studies were performed on several design options. The effect of using an integral tank structure was found to be negligible for short-range aircraft, but increasingly more beneficial for medium and long-range aircraft. The effect of increasing the fuselage diameter was found to be favourable, especially when seats abreast could be added without the addition of one aisle. The effect of using a combination of an aft and a forward tank was found to be detrimental in terms of operational empty mass, beneficial in terms of specific energy consumption and negligible in terms of maximum take-off mass. The use of spherical tanks was found to be slightly beneficial, but only when compared to a non-spherical tank version using the same tank layout, non-integral tank structure, and same cabin layout. The study on the venting pressure revealed that with increasing aircraft size the optimal venting pressure in terms of main aircraft performance decreases whereas the sensitivity to those same parameters to the choice of venting pressure increases. The use of direct gas venting as a means to contain the pressure rise did not appear to provide significant performance improvements. The optimal designs, in terms of operational empty mass, maximum take-off mass and specific energy consumption, feature increased fuselage diameters, the use of the aft & forward tank layout, non-spherical tanks and no direct venting. The short-range aircraft uses non-integral tanks and high venting pressure, while the medium and the long-range aircraft benefit from an integral tank structure and a lower venting pressure. Nevertheless, the sensitivity to these design choices is not significant, meaning that with a different set of assumptions and/or requirements different design choices may become optimal. The overall best performing LH2 aircraft for the short, medium and long-range categories were found to have respectively 8%, 24% and 22% higher operative empty mass, -2%, 1% and -5% higher maximum take-off mass and 5%, 13% and 5% higher specific energy consumption than their kerosene versions.

In order to meet climate change mitigation targets, the shipping industry must change to an alternative fuel. From a ship design perspective, the switch to an alternative fuel is often a downgrade with respect to the use of conventional fuels. For instance, most alternative fuels have a low energy density, rely on technology that is not fully developed or on systems with a short lifetime. The choice for an alternative fuel is therefore difficult to make, and many aspects from different disciplines must be considered. Though much research is being done on the implications of choosing different alternative fuels, iron powder as a marine fuel is not often considered. The use of iron powder as a fuel formaritime applications is only recently proposed and the technology is in its infancy. It has some suitable properties to be used as a marine fuel though. This research aims to investigate how the choice of iron as a fuel affects the ships design and performance.

To that end, a parametric design model is developed for container ships, with which the design space of these iron fuelled ships can quickly be explored. It was chosen to design the model specifically for container ships, because iron has good volumetric energy density, but poor gravimetric energy density. Because of that, iron suits volume carriers more than deadweight carriers. The model generates a multitude of preliminary ship designs as a function of some operational requirements. Using the results from the model, design trends can be identified withing the design space. The potential performance of the ship designs are assessed using some key performance indicators.

It is found that iron fuelled ships are feasible and can be economically viable for certain operational profiles.Based on contemporary freight rates, iron fuelled ships are best used for short voyages at low speed. This is mostly caused by the high mass of iron, which reduces the viability of operating the ship when a large
energy storage capacity is required. The main dimensions of iron fuelled container ships are driven by the total weight of the ship whilst minimising resistance. That makes iron fuelled container ships deadweight
carriers rather than volume carriers. Finally, the economic performance of iron fuelled ships is compared to that of other alternative fuels. It was found that iron fuelled ships are less profitable than ships running on other alternatives.