Daniel Silberhorn

Hydrogen-fueled fuel cell aircraft require powerful thermal management systems to operate. The sizing and integration of these systems remain a key challenge for this type of propulsion system. This study investigates the impact of a fuel cell cooling system on a parallel-hybrid regional aircraft by means of integrating a cooling system model within a conceptual aircraft design workflow. The workflow includes key design parameters of the fuel cell and the thermal management system for co-optimization towards minimum aircraft CO2 emissions. The multi-objective optimization of the thermal management system includes a ranking of drag, mass and power consumption impacts. The thermal management system drag is found to have the largest impact at aircraft level. It is most affected by the heat exchanger fin geometry and frontal area. A wide-ranging drag-specific heat rejection of 1.8 to 14 kW/N is observed for the clean-sheet aircraft design during cruise flight. Larger frontal areas and the implied substantial reduction in thermal management drag outperform the drag increase in larger fairings that contain the heat exchanger. It is also shown how the optimization of the fuel cell load and operating temperatures can minimize the system penalty. This highlights the need for an integrated design approach to reduce the system impact.

Liquid Hydrogen (LH2) as an energy carrier for passenger aircraft has the potential to combine low climate impact and high lifecycle energy efficiency. Due to its significantly different physical properties compared to kerosene, the integration of LH2 fuel storage and distribution systems interacts with the general configuration of the aircraft. In order to assess promising configuration combinations quantitatively, an aircraft design and assessment framework is further developed. These additions are aimed at capturing the interdependencies originating from the fuel system integration choices at the aircraft level and quantifying the effect of trim drag. The framework is applied to a selection of LH2 mid-to-long-range aircraft designs. A comparison of the mass breakdown, aerodynamics breakdown and performance indicators such as specific energy consumption is carried out for the framework-generated aircraft models. A trim drag induced block fuel penalty is quantified for the aircraft selection as well as a mitigation strategy based on operational constraints.

This paper presents the development and assessment of a 250-seat battery-electric aircraft with range extenders, designated D250-PHEP, developed within the DLR project EXACT. The concept investigates how hybrid-electric propulsion can combine the high efficiency of battery-electric operation on short routes with the range flexibility granted by gas-turbine-based range extenders. The propulsion system features four electrically driven propellers powered either by onboard batteries or by two gas turbines operating through a partially turbo-electric drive. In its base configuration, the aircraft carries a large battery enabling highly efficient hybrid operation up to 700–800 nautical miles. For improved performance at longer ranges, the design allows most battery modules to be removed, creating a mild-hybrid configuration with substantially lower mass and extended range capability. The modelling framework developed within EXACT enables a direct comparison with a turbofan and a turboprop baseline aircraft under consistent boundary conditions. The results indicate that large-scale battery-based energy storage becomes feasible once high-energy battery technology suitable for aviation reaches a pack-level specific energy of roughly 400 Wh/kg.

Preliminary design trades for the liquid hydrogen storage system of a short-range aircraft are presented. Two promising insulation methods, namely rigid foam and multilayer insulation, are identified as main design drivers. In addition, the maximal pressure and the shape of the hydrogen storage tank influence the aircraft performance and the insulation efficiency. In this study, the hydrogen storage tanks are integrated in wing pods. The main effects driven by the design parameters are addressed using conceptual and preliminary methods: models are carried out for the storage mass, additional drag, propeller efficiency loss and the dynamical thermodynamic behavior of the liquid hydrogen storage. These effects are coupled making an integrated design method necessary. For the sizing of the liquid hydrogen storage, a multidisciplinary workflow is set up including the aircraft sensitivities on the design mission block fuel. The trade-off study reveals the opposing trend between insulation efficiency and aircraft performance. For the insulation architecture based on rigid foam, the penalties implied by the storage tank on aircraft level and the penalties due to vented hydrogen can be balanced and result in minimal block fuel for the design mission. The application of multilayer insulation avoids venting during the design mission, but has an increased penalty on the aircraft performance compared to rigid foam insulation. Besides the criterion of minimal block fuel, the dormancy time is compared, indicating the thermal efficiency. Applying multilayer insulation, the dormancy time can be increased significantly calling for a discussion of operational requirements for hydrogen-powered aircraft.

One of aviation’s major challenges for the upcoming decades is the reduction in its climate impact. As synthetic kerosene and green hydrogen are two promising candidates, their potentials in decreasing the climate impact is investigated for the mid-range segment. Evolutionary advancements for 2040 are applied, first with an conventional and second with an advanced low-NO_x and low-soot combustion chamber. Experts and methods from all relevant disciplines are involved, starting from combustion, turbofan engine, overall aircraft design, fleet level, and climate impact assessment, allowing a sophisticated and holistic evaluation. The main takeaway is that both energy carriers have the potential to strongly reduce the fleet level climate impact by more than 75% compared with the reference. Applying a flight-level constraint of 290 and a cruise Mach number of 0.75, causing 5% higher average Direct Operating Costs (DOC), the reduction is even more than 85%. The main levers to achieve this are the advanced combustion chamber, an efficient contrail avoidance strategy, in this case a pure flight-level constraint, and the use of CO₂ neutral energy carrier, in a descending priority order. Although vehicle efficiency gains only lead to rather low impact reduction, they are very important to compensate the increased costs of synthetic fuels or green hydrogen.

Green liquid hydrogen (LH2) could play an essential role as a zero-carbon aircraft fuel to reach long-term sustainable aviation. Excluding challenges such as electrolysis, transportation and use of renewable energy in setting up hydrogen (H₂) fuel infrastructure, this paper investigates the interface between refueling systems and aircraft, and the impacts on fuel distribution at the airport. Furthermore, it provides an overview of key technology design decisions for LH2 refueling procedures and their effects on the turnaround times as well as on aircraft design. Based on a comparison to Jet A-1 refueling, new LH2 refueling procedures are described and evaluated. Process steps under consideration are connecting/disconnecting, purging, chill-down, and refueling. The actual refueling flow of LH2 is limited to a simplified Reynolds term of v · d = 2.35 m²/s. A mass flow rate of 20 kg/s is reached with an inner hose diameter of 152.4 mm. The previous and subsequent processes (without refueling) require 9 min with purging and 6 min without purging. For the assessment of impacts on LH2 aircraft operation, process changes on the level of ground support equipment are compared to current procedures with Jet A-1. The technical challenges at the airport for refueling trucks as well as pipeline systems and dispensers are presented. In addition to the technological solutions, explosion protection as applicable safety regulations are analyzed, and the overall refueling process is validated. The thermodynamic properties of LH2 as a real, compressible fluid are considered to derive implications for airport-side infrastructure. The advantages and disadvantages of a subcooled liquid are evaluated, and cost impacts are elaborated. Behind the airport storage tank, LH2 must be cooled to at least 19K to prevent two-phase phenomena and a mass flow reduction during distribution. Implications on LH2 aircraft design are investigated by understanding the thermodynamic properties, including calculation methods for the aircraft tank volume, and problems such as cavitation and two-phase flows. In conclusion, the work presented shows that LH2 refueling procedure is feasible, compliant with the applicable explosion protection standards and hence does not impact the turnaround procedure. A turnaround time comparison shows that refueling with LH2 in most cases takes less time than with Jet A-1. The turnaround at the airport can be performed by a fuel truck or a pipeline dispenser system without generating direct losses, i.e., venting to the atmosphere.