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The climate impact of aviation can be reduced using powertrains based on hydrogen fuel cells and batteries. Combining both technologies in a direct-hybrid without a DC/DC converter is a promising approach for light-weight systems. Depending on the power demand, both the fuel cell and battery are used to provide power or only the fuel cell is connected to the powertrain. The system voltage in a direct-hybrid is determined by the fuel cell and battery, but the performance of fuel cells is affected by low-ambient pressure at high altitudes and the battery voltage is affected by state of charge and discharge rate. Taking this into account, the presented work demonstrates how a direct-hybrid system must be designed based on a scaled mission profile of a 40-seater aircraft. The fuel cell and battery are configured and sized according to the power demand in different flight phases while considering voltage limits given by the powertrain. The energy requirement from the fuel cell and the battery is calculated for a flight based on a realistic mission profile and different battery and fuel cell configurations are evaluated. By optimizing the battery and fuel cell size, the energy required from the battery was reduced by 57% and the total weight of the fuel cell and battery was reduced by 11%. [ABSTRACT FROM AUTHOR]

Hybrid electric aircraft with a powertrain based on fuel cells and batteries can reduce climate-active emissions in aviation. In a direct hybrid powertrain, the fuel cell and the battery are connected in parallel, without a DC/DC converter balancing their voltage levels. Switches make it possible to select different operational modes (fuel cell only, hybrid or battery charging) depending on the power demand during different flight phases. To exploit the high specific energy of hydrogen, the system should change from Hybrid Mode during take-off to Fuel Cell Mode in cruise. During descent, the battery can be charged if Charging Mode is selected. To avoid voltage and current peaks and consequent damage to components when switching between modes, certain conditions must be fulfilled. Those switching conditions were defined, and switching procedures for changing from one mode to the other during flight were developed and tested in a lab system. In a direct hybrid, the system voltage depends on the required power. When switching from Hybrid Mode to Fuel Cell Mode, a short reduction in power of 65% is necessary for the examined system to meet the switching requirements. It is also shown how this power loss can be reduced to 25% by distributed propulsion with a second powertrain or even eliminated by a change in the hybrid ratio. [ABSTRACT FROM AUTHOR]

The development of fully electric and hybrid electric aircraft using fuel cells and batteries has lately received much attention since the technology can contribute to achieving an emission-free world. For hybridisation of fuel cells and batteries, the challenge of integrating different voltage sources to supply power to different loads at different voltage levels has to be met. Presently, this is achieved by using a combination of complex passive circuitry and DC–DC converters. In this paper, a new galvanically isolated DC–DC converter topology is proposed; it integrates a battery and fuel cells as input sources and supplies power to a high voltage (HV) and a low voltage (LV) load simultaneously. The converter enables power flow from the sources to the load, as well as between the sources and power flow back towards the battery during recuperation for recharging the battery during flight. The functionality of the converter design is verified using a real-time (RT) hardware-in-the-loop (HiL) test. The proposed concept is easily scalable and can be adopted in any fuel cell and battery drive train of all-electric flights of different sizes. [ABSTRACT FROM AUTHOR]

Fuel-cell-based, hydrogen-driven aircraft can significantly reduce the climate impact of aviation but the low ambient pressures at high altitude negatively affect the performance of fuel cells. The influence of low pressures on the behavior of a direct hybrid-system that connects a fuel cell and a battery without a DC/DC converter, is examined and compared to a fuel-cell-only system. Low ambient pressure leads to a reduced fuel cell voltage and power output. The voltage change between maximum voltage and the voltage level at the operational power increases with a decreasing air pressure for both the hybrid-system and the fuel-cell-only system. However, in a direct hybrid, the battery stabilizes the system voltage level and the voltage decrease is significantly smaller for the hybrid-system than for the fuel-cell-only system. At an ambient pressure of 300 mbar, the voltage variation of a 4 kW system is 96% lower for a hybrid-system compared to a fuel-cell-only system. The lower pressure also reduces the power output of the fuel cell. In the direct hybrid the battery contributes a demand related share of power and the contribution from the fuel cell decreases with air pressure as the contribution of the battery rises. [Display omitted] • Influence of pressure dependent fuel cell behaviour on direct hybrid system is shown. • Influence of low pressure on voltage variation within a direct hybrid is analysed. • Low ambient pressure increases voltage variation especially in fuel cell only systems. • Direct hybridisation stabilizes the voltage. • Low ambient pressure reduces power contributed by fuel cell in a direct hybrid. [ABSTRACT FROM AUTHOR]

• New Power Management and Control Module for hybrid drivetrains in airplanes. • Safety increase due to redundancy and other features in fuel-cell-battery-hybrid. • Functionality demonstrated in flight. • 5 operational modes possible including recharging of battery during flight. • Major contribution to enabling emission free flight. Hybrid electric aircrafts using fuel cells and batteries are an option to reduce emissions in air travel. State of the art concepts either connect fuel cell and battery using a direct current/direct current (DC/DC) converter or as a direct hybrid. Converters are flexible to control but direct hybrids are lighter and easy to build. A new power control module is presented which is easier to control than a system with DC/DC converters and more flexible and efficient than a direct hybrid. The new control module was developed to assure high aircraft safety criteria due to its redundancy and additional safety measures. It includes the possibility to charge the battery passively and actively from the fuel cell or by recuperation during flight reducing the required battery size. The functionality of the system was simulated and demonstrated in flight. This new concept can be adapted to power different kinds of hybrid aircrafts and offers a guideline to long-term all-electric flights since it is scalable for different aircraft sizes including single-aisle aircrafts. It is additionally usable for all kind of hybrid drive trains using fuel cell and battery. [ABSTRACT FROM AUTHOR]