A. Lücken

Comparison of different electrical HVDC-architectures for aircraft application

This paper deals with an integration of High Voltage DC (HVDC1) in the onboard power supply system. Conventional aircraft grids have a main voltage of 115 V AC (star voltage) with a variable frequency of 360...800 Hz. Newer aircraft try to integrate 230 V AC generators in the grid to save cable weight. With the “More Electric Aircraft”- concept HVDC grids could be integrated in future airplanes. In all concepts the airport infrastructure has to be considered. The ground power supply is based on mobile Ground Power Units (GPUs) or an inverter supplied by the airport grid (Fig. 1). The ground supply allocates 115 V AC with a fixed frequency of 400 Hz and a maximum power of 90 kVA per unit. Changing to an HVDC-grid has a lot of advantages. Higher voltage means lower cable weight. Furthermore, the electrical converter architecture can be optimized. Especially the converters inside the loads can be built much lighter when using a DC supply. This paper compares different possibilities of HVDC integration. Ancillary conditions like carbon fiber reinforced plastic fuselages and a possible ground power supply with conventional technology are taken into consideration. The attention is focused on an efficient cabin power supply. But to make a global assessment, the entire onboard power supply system is analyzed.

Design and protection of a high voltage DC onboard grid with integrated fuel cell system on more electric aircraft

A major demand of the aviation industry is the reduction of emissions in combination with cost efficiency improvement. One possible approach to face this challenge is the replacement of the conventional auxiliary power unit by a fuel cell system within a “more electric aircraft” design. In order to integrate a fuel cell system into the grid architecture of modern aircraft new protection arrangements have to be developed. For availability and redundancy the fuel cell system has to be split into two subsystems. Furthermore the two subsystems have to be connected in parallel to bundle the power delivery and to balance the power. This complex grid structure requires smart safety arrangements to protect the grid in case of failures. In this contribution two safety concepts for doubly fed ring structures are introduced and analyzed. The directional stepped-curve distance-time protection and the impedance protection are illustrated and simulated. Concluding the results of the two concepts are compared.

Optimized cabin power supply with a +/− 270 V DC grid on a modern aircraft

This paper deals with an onboard power supply of +/− 270 V DC (HVDC, high voltage DC). Conventional aircraft grids have a main voltage level of 115 V AC with a variable frequency of 360…800 Hz. In the future, fuel cells might replace the conventional auxiliary power unit (APU) to optimize the eco efficiency. With that technology HVDC-grids can be integrated in an aircraft. Changing to a HVDC-grid has a lot of advantages. Higher voltage means lower cable weight. Furthermore, the electrical converter architecture can be optimized. Especially the converters inside the loads can be build much lighter when using a +/− 270 V DC supply. This paper gives an overview of the electrical architecture on a modern short- and midrange aircraft and the effect of a +/− 270 V DC power grid thus focusing on the converter architecture in a modern aircraft. Concluding all benefits are summarized.

An empirical approach to calculate short and long term energy storage needs of an electricity system

The share of renewable energies on the net electricity consumption is rising rapidly all over the world and governmental goals of a full renewable electricity supply by 2050 are getting more and more common. Especially intermittent, non-controllable energy sources like wind and sun will play a significant role to reach these targets. The fluctuating nature of these two energy carriers will pose some mature problems to energy supply systems. Solving these problems will be one of the mayor challenges in the next decades. With an installed amount of non-controllable power that exceeds the yearly peak load, situations will occur with a surplus of energy that can be rejected, exported or stored. This paper deals with an empirical approach to estimate the future energy storage needs from an electricity system point of view. The energy storage needs will further be divided into short and long term energy storage.

Fuel Cell System Optimization Using Bypass Converters

The reduction of CO2 emissions, and therefore the increase of ecoefficiency, is one of the most important challenges for the aircraft industry. An approach could be the more electric aircraft concept. Within this concept, the traditional auxiliary power unit could be replaced by a multifunctional fuel cell system. For integration into the ±270 Vdc grid, it is necessary to convert the load-dependent fuel cell output voltage using a direct current-to-direct current converter. The converter usually is designed for the maximum available fuel cell power. This paper deals with an adaption of the fuel cell output voltage to achieve downsizing of the converter. The proposed converter structure with an additional bypass has weight-saving effects and improves overall system efficiency.

Extended power delivery with fuel cell systems

Fuel cell based auxiliary power units (APU) could be a good alternative to conventional, kerosene operated APUs. The eco efficiency could be enlarged by this technology. The dynamic characteristic of fuel cell systems necessitates an adaption of the electrical system. Using short time energy storage power delivery can be extended, demonstrated by a Matlab Simulink Simpower-Systems simulation with two scenarios. The first one is a 3 s storage for load step compensation. The second one is an exemplification of an electrical main engine start with energy storage. This could reduce the fuel cell weight. This paper compares three possible energy storage technologies to consider the requirements and it checks market availability.

Integration scenarios to improve fuel cell dynamics for modern aircraft application

An approach to achieve a high efficiency for energy transfer in modern aircraft could be the More Electric Aircraft concept. According to this, the replacement of the conventional auxiliary power unit by a multi functional fuel cell system reduces the emission of carbondioxide significantly and can increase the efficiency. However, requirements on the electrical system dynamics are higher than currently available fuel cell systems can provide. To improve the electrical dynamics of the fuel cell especially during high peak power demands different methods are considered. This paper compares four possible integration scenarios to improve the system dynamic. Special characteristics as the additional system weight, the stored available energy, the complexity as well as the feasibility under the required conditions are considered. For supplementing the theoretical concepts, a prototype of a super capacitor short-time energy storage was built and integrated into an existing fuel cell test bench. Therefore, a voltage balancing as well as an overvoltage protection is implemented. Measurements show the improved dynamic of the hybrid fuel cell system.

Modellierung und Bewertung einer autarken Notstromversorgung der Bevölkerung unterhalb der KRITIS-Schwelle unter Einsatz von Photovoltaik-Systemen

Im Rahmen dieser Publikation wurde analysiert, wie eine Notstromversorgung nicht-kritischer Infrastrukturen durch Photovoltaik-Systeme verwirklicht werden kann. Dazu wurden vier Szenarien entwickelt und modelliert, die eine Abschatzung des Leistungs- und Energiebedarfs ermoglichen. Dies beinhaltete die Untersuchung relevanter Verbraucher in den einzelnen Szenarien, die Grose des Solargenerators sowie eines Zwischenspeichers. Es wurde zwischen Sommer und Winter sowie der verfugbaren Solarstrahlung in drei Sektoren Deutschlands differenziert. Abschliesend fand eine Analyse des Flachen- und des Kostenbedarfs sowie eine Diskussion der Realisierbarkeit in Anbetracht der raumlichen und finanziellen Moglichkeiten in den einzelnen Szenarien statt. Es wurde ermittelt, dass Unterschiede zwischen dem nordwestlichen Sektor und dem sudlichen Sektor von bis zu 45 % beim Flachenbedarf existieren. Eine Bewertung der benotigten Flachen ergab, dass Notstromversorgungssysteme allein mit Photovoltaik-Systemen oft aus Platzgrunden nicht realisierbar sind. Im Anschluss wurden verschiedene Kostenpunkte einer Photovoltaik-Anlage separat betrachtet. Die Kosten variieren bis zu 41 % zwischen den betrachteten Bereichen Deutschlands.

Concept analysis of an electrical fuel cell integration in modern aircraft

The reduction of pollutant emissions and hence the increasing of the eco efficiency of future aircraft is one of the major challenges for the aviation industry. Executing the More Electric Aircraft concept can be one approach to achieve these high targets. One idea in this context is to substitute the traditional Auxiliary Power Unit by a Multifunctional Fuel Cell System. It is necessary to transform the load-dependent fuel cell stack output voltage to the intended electrical onboard grid voltage level. To increase the efficiency and reduce the system weight an optimized electrical integration is presented. Measurement results on a test bench confirm the theoretical concepts.

Simulation of Electromagnetically and Thermally Controlled Ionic Flow in a Fuel Cell

A major issue related to the use of fuel cells to convert electrical energy in chemical energy in modern power supply concepts are their bad dynamical properties. To overcome these problems, it seems promising to introduce a suitable mechanism to control the ionic flow inside the fuel cell. The purpose of this work is to estimate the potential of certain approaches to controlling the ionic flow inside the fuel cell via magnetic and temperature fields. To this end, mathematical models combining a description of the ionic movement in a hydrogen fuel cell with a model for the effects of an additional magnetic or temperature field, respectively, are proposed. Further the implementation of these models in the context of the finite element method combined with other simulation techniques is discussed, such as, e.g., a molecular dynamic model. Finally, some preliminary results are presented.