Jürgen Schumacher

Two-Phase Dynamic Modeling of PEMFCs and Simulation of Cyclo-Voltammograms

A mathematical model is developed that is based on a coupled system of partial differential equations. The model contains a dynamic and two-phase description of the proton exchange membrane fuel cell (PEMFC) and a membrane model that accounts for Schroeders paradox. The mass transport in the gas phase and in the liquid phase is considered as well as the phase transition between liquid water and water vapor. The transport of charges and the electrochemical reactions are part of the model. A potential sweep experiment is simulated using the mathematical model and measured using a test cell with an active area of 1 cm 2 . In this way, the dynamic effect of liquid water formation and transport on the current-voltage characteristic of the fuel cell is investigated. A hysteresis effect is found in the measured time-dependent current-voltage relation. The limiting current density is time-dependent. Qualitative agreement of simulated and measured results is achieved. An analysis of the observed hysteresis of the current-voltage characteristics, based on the modeling results, is given.

Modeling Planar and Self-Breathing Fuel Cells for Use in Electronic Devices

A theoretical study of a planar and self-breathing fuel cell is presented. This work contains the development of a mathematical model for planar self-breathing fuel cells, the validation of the model, and a study of the behavior of this type of fuel cell. The mathematical model presented is two-dimensional and nonisothermal. The validation of the model is performed by comparison of the measured overall performance of a planar self-breathing fuel cell to the predictions of the model. For this type of cell, the maximum power density is in the range between 0.5 and 0.4 V, so the model is applied to study the behavior of the reference cell at a cell voltage of 0.4 V. The results of this study show the gas distribution, the potential distribution, and the temperature distribution are influenced strongly by the geometric design of the cathode end plate. The charge generation rate in the active area of the cathode and anode is affected by the ribs of the cathode end plate. A strong nonuniformity of the current distribution in the cathode is found.

Analysis of one-sun monocrystalline rear-contacted silicon solar cells with efficiencies of 22.1%

21.4% efficient rear-contacted cells (RCC) with interdigitated contact grids processed at the Fraunhofer ISE on 1.25 Ω cm float-zone (FZ) silicon are analyzed in detail. The comprehensive description does not only include a two-dimensional numerical device simulation, but also a detailed analysis of the optical carrier generation using optical ray tracing and determination of the losses due to distributed metal resistance and perimeter currents employing circuit simulation. Bulk and surface recombination losses are separated, combining carrier lifetime and open-circuit voltage measurements with numerical device simulation. The interface surface recombination velocity of the thermally oxidized emitter covering the front surface is deduced to be 1500 cm/s and the bulk diffusion length within the 1.25 Ω cm FZ silicon base is 1200 μm. Despite this excellent bulk diffusion length, the simulations reveal that at a maximum power point 80% of the total recombination is due to Shockley–Read–Hall recombination in t...

Solar cells with mesh-structured emitter

The mesh-structured emitter solar cell (MESC) is introduced as a novel solar cell processing scheme. By the formation of inverted pyramids or microgrooves on a wafer with a homogeneous heavy phosphorus diffusion, a mesh of highly conducting emitter lines is formed. Using this technique, the lateral conductivity of the emitter can be increased, keeping the emitter dark saturation current at a low level. The high phosphorus surface concentration results in a low contact resistance even for screen-printed contacts. Thus, this technique is ideal for solar cells with screen-printed contacts, because the finger spacing of the front contact can be extended, resulting in smaller shadowing losses. Also the processing scheme of high-efficiency solar cells can be simplified, because the formation of the surface texturization and the locally deep diffused emitter can be combined in one step. The first cells with a mesh-structured emitter, evaporated front contacts and local ohmic rear contacts have shown efficien ies up to 21.1%. Lifetime test structures have been used to determine a low dark saturation current of 58 fA cm−2 for the mesh-structured emitter, although the structure is not yet optimized.

Application of an improved band-gap narrowing model to the numerical simulation of recombination properties of phosphorus-doped silicon emitters

Abstract The commonly used band-gap narrowing (BGN) models for crystalline silicon do not describe heavily doped emitters with desirable precision. One of the reasons for this is that the applied BGN models were empirically derived from measurements assuming Boltzmann statistics. We apply a new BGN model derived by Schenk from quantum mechanical principles and demonstrate that carrier degeneracy and the new BGN model both substantially affect the electron–hole product within the emitter region. Simulated saturation current densities of heavily phosphorus-doped emitters, calculated with the new BGN model, are lower than results obtained with the widely used empirical BGN model of del Alamo.

Characterization of silicon solar cells with interdigitated contacts

The characterization of two types of silicon solar cells with interdigitated metal grids processed at Fraunhofer-ISE is presented in this paper: the 19.2% efficient thin-film silicon on insulator (SOI) cell and a 21.4% rear contact cell (RCC). The spectrally resolved reflection and absorption properties of the cells are simulated with the 3D ray tracing program RAYN. Optical generation profiles calculated with RAYN are utilized in the device simulator DESSIS to model the observed electrical properties. The successful modelling allows predictions of the performance of thinner SOI cells and RCC cells processed from lower quality material.

LPE-GaAs and LBSF-Si solar cells for tandem concentrator application

Design and efficiency measurements of a mechanically stacked concentrator tandem solar cell are presented. The GaAs cell was grown by the LPE etch-back-regrowth method and has a diameter of 4 mm. The silicon solar cell is a high efficient LBSF concentrator solar cell of 5 mm diameter. Special care was taken to optimize the antireflection coatings on the back and front side of the GaAs solar cell. Both cells were processed at FhG-ISE in Freiburg. Their short circuit currents were measured at the ISE calibration laboratory under AM1.5d conditions. The efficiency measurements were performed at UPM-IES in Madrid under outdoor conditions. At 27 suns (AM1.5d, 25/spl deg/C) the GaAs cell showed an efficiency of 22.9% and the silicon cell an efficiency of 2.9%. The two single cell efficiencies add to a tandem efficiency of 25.8% for the four terminal operation.

Free open reference implementation of a two-phase PEM fuel cell model

Abstract In almost 30 years of PEM fuel cell modeling, countless numerical models have been developed in science and industrial applications, almost none of which have been fully disclosed to the public. There is a large need for standardization and establishing a common ground not only in experimental characterization of fuel cells, but also in the development of simulation codes, to prevent each research group from having to start anew from scratch. Here, we publish the first open standalone implementation of a full-blown, steady-state, non-isothermal two-phase model for low-temperature PEM fuel cells. It is based on macro-homogeneous modeling approaches and implements the most essential through-plane transport processes in a five-layer MEA. The focus is on code simplicity and compactness with only a few hundred lines of clearly readable code, providing a starting point for more complex model development. The model is implemented as a standalone MATLAB function, based on MATLAB’s standard boundary value problem solver. The default simulation setup reflects wide-spread commercially available MEA materials. Operating conditions recommended for automotive applications by the European Commission are used to establish new fuel cell simulation base data, making our program a valuable candidate for model comparison, validation and benchmarking. Program summary Program Title: MMM1D Program Files doi: http://dx.doi.org/10.17632/2msdd4j84c.1 Licensing provisions: BSD 3-clause Programming language: MATLAB Nature of problem: Steady-state, non-isothermal, two-phase simulation of the coupled through-plane transport of charge, heat and mass within the five-layer membrane electrode assembly of low-temperature proton exchange membrane fuel cells. Solution method: MATLAB’s boundary value problem solver bvp4c , a finite difference solver that implements the 3-stage Lobatto IIIa collocation method with automated mesh selection based on the residual. Additional comments: The complete source code and the license agreement can also be obtained from https://www.isomorph.ch .

A new open-source PEMFC simulation tool for easy assessment of material parameterizations

Institute of Computational Physics (ICP), Zurich University of Applied Sciences (ZHAW), Wildbachstrasse 21, CH-8401 Winterthur, Switzerland vetr@zhaw.ch, schm@zhaw.ch After almost three decades of PEM fuel cell modelling, there is a large need for standardization and establishment of a common basis in the development of PEMFC models, not only for numerical simulation purposes, but also to test and validate MEA material parameterizations from experimental measurements. Until recently, there were only two open-source codes capable of simulating the state of the art in PEMFC modeling at the scale of single cells or MEAs: OpenFCST [1], a rather heavy FEM package consisting of more than 120 000 lines of C++ code (not counting library dependencies), and FAST-FC [2], a finite volume tool built on top of OpenFOAM, consisting of about 12 000 lines of code (not counting the required OpenFOAM). Albeit highly capable, these tools require significant effort and programming know-how to be set up and modified, and they are not well suited for easy substitution of material parameterizations or extensive parameter studies in sufficiently short computation times. We have recently developed the first open standalone MATLAB implementation of a full-blown, steady-state, non-isothermal, macro-homogeneous two-phase MEA model for low-temperature PEM fuel cells [3]. It implements the most dominant through-plane transport processes in a 5-layer membrane electrode assembly (Fig. 1): the transport of charge, energy, gas species and water. With a focus on code simplicity, compactness, portability, transparency, accessibility and free availability, our program is an ideal candidate for the assessment of new material parameterizations that may originate e.g. from experimental data. Thanks to the very short runtime of just a few seconds on an ordinary PC, extensive parameter studies and quick substitution of modeling assumptions or material properties are now possible with our tool without requiring deep programming knowledge or compilation of large software libraries. We demonstrate how the program may be used to quantitatively understand and evaluate PEM fuel cell material properties or measurement data.

Hydrogen for electromobility : a promising energy carrier

Partners References Introduction Electromobility has received important attention in the last few years, but its perception by the public and decision makers is often limited to battery powered vehicles. Alternatives such as hydrogen fuel cells should however be taken into account, as their specific advantages (in particular short refueling times) make electromobility as a whole acceptable by a much broader public. Within the SCCER Mobility, PSI and ZHAW work on a novel fuel cell concept aiming at reducing the major limitation to the deployment of fuel cells: their cost. Hydrogen for Electromobility: A Promising Energy Carrier