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RMS current load is one of the most important design criteria for the dc-link capacitors of voltage source converters. In the literature, determination of this rms current load is well documented for state-of-the-art modulation schemes, such as space vector modulation. But the methods presented cannot be applied for direct current control algorithms. Therefore, this article introduces a new approach especially for direct current controllers, which reflects and compares to the state-of-the-art calculation method for space vector modulation. As direct current control is a feedback control, no closed-form solution can be given. Instead, the dc-link capacitor rms current load is determined by an iterative calculation scheme as a function of the modulation level and the phase angle. Although the scheme presented is applied to a three-phase two-level voltage source converter, it can be adapted to any voltage source converter topology. An example is presented applying the calculation method to one specific direct current control algorithm. The calculated results are verified via a digital simulation model as well as test bench measurements showing good correlation. For correct comparison, different effects are considered and discussed between simulation, calculation, and measurement results. These contain transients, the commutation process between insulated gate bipolar transistor (IGBT) and diode, the current-dependent voltage drop of the semiconductors, and the stationary current error of the hysteresis control. The dc-link capacitor current load for space vector modulation is calculated analytically as a reference by using the state-of-the-art method. The comparison of results obtained in this article and the reference are very similar, which is discussed in the conclusion.

The successful market introduction of the solid oxide fuel/electrolysis cell technology for power-to-gas applications requires the reduction of the degradation rates and the better understanding of the degradation mechanisms of the stacks. Therefore, the paper reports and compares the long-term behavior of a solid oxide cell stack in electrolysis and reversible fuel cell/electrolysis operation. The 30-cell stack with electrolyte supported cells was supplied by Sunfire GmbH (Dresden/Germany) in the German funded RSOC Project. The stack was operated for 3370 h in electrolysis and afterwards for 2500 h in reversible fuel cell/electrolysis mode, each at 70 % gas conversion. In the beginning of the test, the stack showed high gas tightness, good performances and high efficiencies in both SOEC and SOFC operations. During 3370 h of SOEC operation a low degradation of +0.5 %/1000 h was measured. During 2500 h of reversible fuel cell/electrolysis cycling, the gas tightness of the stack slightly decreased, which led to a temperature increase, and higher degradation rates were observed. The increase of the ohmic resistance contributed mostly to the degradation. Optimized operating conditions for reversible cycling and increasing the purity of the supplied water are foreseen in order to minimize stack degradation in reversible operation.

The successful market introduction and public acceptance of the solid oxide fuel/electrolysis cell technology requires high performance, long-term stability and low costs of the corresponding cells and stacks. However, especially the degradation of SOEC stacks during long-term operation for several thousand hours in electrolysis and reversible SOFC/SOEC mode are not yet well understood. The paper presents the long-term behavior and the degradation results of a 30-cell stack with electrolyte supported cells operated in reversible SOFC/SOEC mode. The stack was supplied by Sunfire GmbH (Dresden/Germany) as part of the German funded project Reversible Solid Oxide Cell. The reversible cycles with duration of 24 h consist of 9 h SOFC operation at 70 % fuel utilization, 9 h SOEC operation at 70 % steam conversion and two switching phases, each with duration of 3 h. The stack is currently being operated for 6300 h under these reversible SOFC/SOEC cycling conditions. After the initial performance test, current-voltage curves and electrochemical impedance spectra were recorded every 2000 h. The electrochemical properties such as open circuit voltages, area specific resistances, power densities, fuel utilization, steam conversion rate and impedance values of the stack and of selected repeat units are presented. The homogeneity among the repeat units is discussed in context with the measured temperature gradients in the stack. The degradation and the change of the resistances, e.g. the ohmic resistances, the polarization resistances of the electrodes and the gas concentration resistances, during the reversible SOFC/SOEC long-term operation are presented and compared to each other. Furthermore, the degradation mechanisms of the different cell and stack components are discussed.

To further increase the performance of polymer electrolyte membrane fuel cells (PEMFCs) at high current densities, a reduction of mass transport losses by an enhanced evacuation of water from the cell and an improved supply of gases to the reactive sites are necessary. The application of three-dimensional flow field structures in PEMFCs is a promising measure to promote the evacuation of excess liquid water from the cell while boosting the transport of reactants from the gas channel in the direction of the cell. Expanded metal meshes offer a three-dimensional geometry, which predestines them to be used as flow field structures in PEMFCs. In the past, their application as flow fields has primarily been investigated for direct methanol fuel cells (DMFCs) [1, 2, 3] and electrolyzers [4, 5]. To gain a profound understanding of the impact of such three-dimensional flow field structures on the electrochemical properties of PEMFCs, the physicochemical loss processes of the cell have to be analyzed in detail. In this contribution, the results of an in-depth analysis of the influence of three-dimensional flow field structures consisting of expanded metal meshes on the physicochemical loss processes in PEMFCs and their effect on the cell performance is presented. To assess the impact of expanded metal meshes in comparison to a conventional channel rib flow field design, the physicochemical loss processes were examined for both flow field designs. The operating conditions (j, RH, T, pO2) of the incremental PEM fuel cells (1 cm2 active cell area) [6] were varied systematically and the cells were analyzed by means of polarization curves and electrochemical impedance spectroscopy (EIS). The physicochemical loss processes of the cells were identified by a subsequent impedance data analysis by the distribution of relaxation times (DRT) [7, 8]. This investigation aims to determine the physicochemical loss process in PEMFCs with expanded metal meshes as flow field structures and compare them to the results of a conventional flow field design. Based on the physical interpretation of the results, consequences and conclusions for the further development of the expanded metal flow field geometries are discussed. References: [1] Yuan et al., International Journal of Hydrogen Energy 43, 19711-19720 (2018). [2] Wang et al., Applied Energy 208, pp. 184-194 (2017). [3] Mallick et al., Electrochimica Acta 243, pp. 299-309 (2017). [4] Stiber et al., Adv. Energy Materials 11, 2100630 (2021). [5] Lafmejani et al., J. Power Sources 397, pp. 334-342 (2018). [6] M. Heinzmann et al., J. Power Sources 402, pp. 24-33 (2018). [7] H. Schichlein et al., J. Appl. Electrochem. 32, pp. 875-882 (2002). [8] E. Ivers-Tiffée et al., J. Ceram. Soc. Japan 125, pp. 193-201 (2017).

Performance prediction for large-sized polymer electrolyte membrane fuel cell (PEMFC) stacks necessitates consideration of spatially deviating operating conditions on the nonlinear electrochemical behaviour. This interaction between operating conditions and electrochemistry is best described by complex CFD-models. But high computing power excludes stack and system modelling in real time applications. We address this challenge by a multi-physical stack model, which couples (i) the non-linear electrochemistry within the cell, (ii) the fluid pressure drop along the gas channels and (iii) the thermal behaviour within the stack. The spatial resolution focusses on the most relevant directions and thus limits the computational effort. Simulation runtime is further reduced by modelling the electrochemical behaviour by a physico-chemically meaningful equivalent circuit model (ECM) [1], which relies on a data set of electrochemical impedance spectroscopy (EIS) measurements performed on incremental cells [2]. Individual impedance contributions are identified by the distribution of relaxation times (DRT). ECM model parameters are subsequently quantified by a CNLS-fitting procedure [3,4] and transferred to a nonlinear, zero-dimensional DC-performance-model. The magnitude of the modelled pressure depends on the gas flow within the channel and considers the change of gas composition, whereas the local gradients in current density cause gradients in released heat within the cell itself. This effect along the gas flow, the convection between fluids and solid parts of the stack (bipolar plates and cell) and the internal heat conduction between the solid control volumes are considered in the modelled thermal behaviour. In this contribution a multi-physical stack model considering gradients in temperature, pressure and gas composition is presented. The interdisciplinary interactions and dependencies within the different physical domains, especially the influence of pressure and temperature on the non-linear electrochemical model, are shown. A concise validation based on measured data and conclusions for the possibilities of further applications as a system model will be discussed. [1] D. Klotz et. al., ECS Transactions 25, pp. 1331-1340 (2009) [2] M. Heinzmann et al., J. Power Sources 402, pp. 24-33 (2018). [3] H. Schichlein et al., J. Appl. Electrochem. 32, pp. 875-882 (2002). [4] S. Dierickx et al. Electrochimica Acta 355, 136764 (2020)

The TerraSAR-X mission, based on two satellites, has produced SAR data products of high quality for a number of scientific and commercial applications for more than ten years. To guarantee the stability and the reliability of these highly accurate SAR data products, both systems were first accurately calibrated during their respective commissioning phases and have been permanently monitored since then. Based on a short description of the methods applied, this paper focuses on the radiometric performance including the gain and phase properties of the transmit/receiver modules, the antenna pattern checked by evaluating scenes acquired over uniformly distributed targets and the radiometric stability derived from permanently deployed point targets. The outcome demonstrates the remarkable performance of both systems since their respective launch.

The presented paper evaluates measurement results and reports about further development from the vehicle to grid system presented in [1] and [12]. It uses a number of mass-production electric vehicles in sink and source mode to balance the load of an industrial plant by cutting off its peak loads. The actual load is detected and the balancing power is calculated by a predictive algorithm which keeps the total load below the setpoint. Analysis show that the goal of peak load shaving is reached using the presented control strategy. Although mean availability of balancing performance is lower than assumed. Improvements were made, reducing DC link ripple current by passively low pass filtering. After evaluating proportion of stations' active to passive times, a standby mode was implemented improving efficiency of overall system.

Transponders (also known as polarimetric active radar calibrators or PARCs) are commonly used for radiometric calibration of synthetic aperture radars (SARs). Currently three methods for the determination of a transponder's frequency-dependent radar cross section (RCS) are used in practice. These require either to measure disassembled transponder components, or a separate radiometric measurement standard (like a flat, metallic plate or a corner reflector), leading to additional uncertainty contributions for the calibration result. In this paper, a novel method is introduced which neither requires disassembly nor an additional radiometric reference. Instead, the measurement results can be directly traced back to a realization of the meter, lowering total measurement uncertainties. The method is similar in approach to the well known three-antenna method, but is based on the radar equation instead of Friis transmission formula. The suitability of the method is demonstrated by a measurement campaign for DLR's three new Kalibri C-band transponders, completed by an uncertainty analysis. The method is not universally applicable for all transponder calibrations because (a) three devices are necessary (instead of only one for the known methods), and (b) the transponders must provide certain additional features. Nevertheless, these features have become standard in modern SAR calibration transponder designs. The novel, potentially more accurate three-transponder method is thus a viable alternative for transponder RCS calibration, ultimately contributing to synthetic aperture radars with a reduced radiometric measurement uncertainty.

A synthetic aperture radar (SAR) system requires external absolute calibration so that radiometric measurements can be exploited in numerous scientific and commercial applications. Besides estimating a calibration factor, metrological standards also demand the derivation of a respective calibration uncertainty. This uncertainty is currently not systematically determined. Here for the first time it is proposed to use hierarchical modeling and Bayesian statistics as a consistent method for handling and analyzing the hierarchical data typically acquired during external calibration campaigns. Through the use of Markov chain Monte Carlo simulations, a joint posterior probability can be conveniently derived from measurement data despite the necessary grouping of data samples. The applicability of the method is demonstrated through a case study: The radar reflectivity of DLR’s new C-band Kalibri transponder is derived through a series of RADARSAT-2 acquisitions and a comparison with reference point targets (corner reflectors). The systematic derivation of calibration uncertainties is seen as an important step toward traceable radiometric calibration of synthetic aperture radars.

One of the most important requirements for grid-connected inverters is an accurate control of the line currents. Applying the “switched diamond hysteresis control (SDHC)” to multilevel inverters combines the advantages of both techniques and is an enabling step towards a better inverter performance than using state of the art controllers. The hypotheses are further investigated in a real hardware test setup. Experimental results proof the excellent dynamic of the SDHC control algorithm.

The technological advancement of the synthetic aperture radar (SAR) principle leads to an innovative challenge for the calibration as well. In order to provide an active reference target for an accurate absolute radiometric calibration the knowledge of the target's backscattering characteristics is essential. For the recently developed DLR C-band transponder named “Kalibri” several strategies for an accurate determination of the radar cross section (RCS) have been analyzed. Based on a comparison with respect to accuracy and feasibility, several recommendations for the best transponder calibration strategy were established. The resulting RCS of the transponders retrieved from the most suitable measurement method is presented as well as a cross-validation to prove the plausibility of these results.

A SAR system has to be calibrated for quantitative measurement. During this process all essential parameters of a SAR image are linked to their geophysical quantity. This includes the location of the image pixel, its backscattering characteristics (in amplitude and in phase) and polarimetric information. The Microwave and Radar Institute of the DLR has gained a lot of experience in these calibration procedures during the last decades and has developed special methods and dedicated reference targets for spaceborne SAR system calibration. In example of calibration results obtained for different spaceborne SAR systems the DLR SAR Calibration Center is presented.