No abstract

From an aerodynamic point of view, the electric turbocharger for the air supply of an automotive fuel cell faces difficult requirements: it must not only control the pressure level of the fuel cell, but it also has to operate with very high efficiency over a wide range. This paper explores features for the compressor and the turbine of an existing electric turbocharger, which are intended to meet the specific requirements of a fuel cell in an experimentally validated numerical study. Adjustable diffuser or nozzle vanes in the compressor and turbine achieve wider operating ranges but compromise efficiency, especially because of the necessary gaps between vanes and end walls. For the turbine, there are additional efficiency losses since the pivoting of the nozzle vanes leads to incidence and thus to flow separation at the leading edge of the nozzle vanes and the rotor blades. An increase in the mass flow and a slight efficiency improvement of the turbine with the low solidity nozzle vanes counteracts these losses. For the compressor, a reduction in the diffuser height and its influence over the operating range and power consumption yields an increase in surge margin as well as in maximum efficiency.

No abstract

In this study, an electric turbocharger for fuel cell applications is investigated with regards to the extension of both compressor and turbine operating range by means of geometric changes to the turbomachinery components, namely variable nozzle and diffuser vane angles. Therefore, the interaction of the electric turbocharger subsystem with the fuel cell stack is investigated over the full operating range to judge the overall efficiency, system dynamics and stability. Initially, selected options for extending the performance maps of both compressor and turbine are presented and discussed. The numerical methods used for predicting the performance maps are then described. Subsequently, the entire machine is simulated under both steady-state and transient operating conditions using the in-house tool ASTOR (AircraftEngine for Transient Operation Research). Based on the wider operating ranges of compressor and turbine, promising setups are identified and investigated in further detail to select the best choice for the operation of the electric turbocharger. The impact of isolated component modifications is shown initially and substantial improvements of the operating range are shown. The modification of the compressor diffuser leading edge angles in a fixed-geometry diffuser increases the range of covered operating points from 4 to 7 while at the same time improving the compressor surge margin during steady state operation above the required safety margin of 20%. Additionally, the system efficiency can be increased by 0.2%. The application of a positive angle variable nozzle turbine significantly shifts the operating line towards higher mass flows, thus increasing the surge margin especially in the high speed range where the transient effects during deceleration of the machine are the greatest. By applying combined modified compressor with pivoting vanes and and variable nozzle turbine geometry, the operating range can be extended even further. The surge margin can be kept above 20% during the initial critical part of a transient deceleration manoeuvre. Nevertheless, a decrease of 0.5% in overall system efficiency has to be accepted due to the measures.

Um die Brennstoffzelle im Kraftfahrzeugsektor als wirtschaftliche Energiequelle weiter zu etablieren, ist es notwendig, die Luftversorgung des Brennstoffzellensystems an die auftretenden Betriebsbedingungen anzupassen. Das Vorhaben ARIEL befasst sich deshalb in interdisziplinärer Zusammenarbeit von mehreren Instituten und Industrieunternehmen mit der Luftversorgungseinheit eines Brennstoffzellensystems. Ziel ist es, den verwendeten, elektrisch angetriebenen Luftverdichter bezüglich Bauraum, Gewicht und Wirkungsgrad zu optimieren.