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Sektorenkopplung | Der Umbau der Energiesysteme ist in vollem Gange – hierzulande und weltweit. Immer mehr fossile Kraftwerke werden abgeschaltet, Windenergie- und Photovoltaikanlagen sind auf dem Vormarsch. Der wachsende Anteil des erneuerbaren Stroms muss jedoch in Einklang mit dem Bedarf gebracht werden. Künftig wird daher eine Stärkung der vierten Säule des Energiesystems nötig: Neben Erzeugung, Transport und Verbrauch treten Speichertechnologien. Große Vorteile bietet hier der chemische Energieträger Wasserstoff – er ist flexibel herstell- und einsetzbar und kann große Anteile erneuerbarer Energien auch in die Sektoren Mobilität, Wärme und Industrie bringen. Ein aktueller Blick auf Technologie, Potenziale und die rasante internationale Entwicklung.

A design of experiments (DOEs) coupled with a mathematical model was used to quantify the factors affecting methanol crossover in a direct methanol fuel cell (DMFC). The design of experiments examined the effects of temperature, cathode stoichiometry, anode methanol flow rate, clamping force, anode catalyst loading, cathode catalyst loading (CCL), and membrane thickness as a function of current and it also considered the interaction between any two of these factors. The analysis showed that significant factors affecting methanol crossover were temperature, anode catalyst layer thickness, and methanol concentration. The analysis also showed how these variables influence the total methanol crossover in different ways due to the effects on diffusion of methanol through the membrane, electroosmotic drag, and reaction rate of methanol at the anode and cathode. For example, as expected analysis showed that diffusion was significantly affected by the anode and cathode interfacial concentration, by the thickness of the anode catalyst layer and membrane, and by the diffusion coefficient in the membrane. Less obvious was the decrease in methanol crossover at low cathode flow rates were due to the formation of a methanol film at the membrane/cathode catalyst layer interface. The relative proportions of diffusion and electroosmotic drag in the membrane changed significantly with the cell current of the cell.

A hydrogen fuelled, 30 W proton exchange membrane fuel-cell (PEMFC) system is presented that is able to operate at an ambient temperature between −20 and 40 °C. The system, which comprises the fuel-cell stack, pumps, humidifier, valves and blowers is fully characterized in a climatic chamber under various ambient temperatures. Successful cold start-up and stable operation at −20 °C are reported as well as the system behaviour during long-term at 40 °C. A simple thermal model of the stack is developed and validated, and accounts for heat losses by radiation and convection. Condensation of steam is addressed as well as reaction gas depletion. The stack is regarded as a uniform heat source. The electrochemical reaction is not resolved. General design rules for the cold start-up of a portable fuel-cell stack are deduced by the thermal model and are taken into consideration for the design. The model is used for a comparison between active-assisted cold start-up procedures with a passive cold start-up from temperatures below 0 °C. It is found that a passive cold start-up may not be the most efficient strategy. Additionally, the influence of different stack concepts on the start-up behaviour is analysed by the thermal model. Three power classes of PEMFC stacks are compared: a Ballard Mk902 module for automotive applications with 85 kW, the forerunner stack Ballard Mk5 (5 kW) for medium power applications, and the developed OutdoorFC stack (30 W), for portable applications.

Air-breathing PEMFCs consist of an open cathodic side to allow an entirely passive supply of oxygen by diffusion. Furthermore, a large fraction of the produced water is removed by evaporation from the open cathode. Gas diffusion layers (GDLs) and the opening size of the cathode have a crucial influence on the performance of an air-breathing PEMFC. In order to assure an unobstructed supply of oxygen the water has to be removed efficiently and condensation in the GDL has to be avoided. On the other hand good humidification of the membrane has to be achieved to obtain high protonic conductivity. In this paper the influence of varying cathodic opening sizes (33%, 50% and 80% opening ratios) and of GDLs with different wetting properties are analysed. GDLs with hydrophobic and hydrophilic properties are prepared by coating of untreated GDLs (Toray ® carbon paper TGP-H-120, thickness of 350 μm). The air-breathing PEMFC test samples are realised using printed circuit board (PCB) technology. The cell samples were characterised over the entire potential range (0–0.95 V) by extensive measurements of the current density, the temperature and the cell impedance at 1 kHz. Additionally, measurements of the water balance were carried out at distinct operation points. The best cell performance was achieved with the largest opening ratio (80%) and an untreated GDL. At the maximum power point, this cell sample achieved a power density of 100 mW cm −2 at a moderate cell temperature of 43 °C. Furthermore, it could be shown that GDLs with hydrophilic or intense hydrophobic properties do not improve the performance of an air-breathing PEMFC. Based on the extensive characterisations, two design rules for air-breathing PEMFCs could be formulated. Firstly, it is crucial to maximise the cathode opening as far as an appropriate compression pressure of the cell assembly and therewith low contact resistance can be assured. Secondly, it is advantageous to use an untreated, slightly hydrophobic GDL.

A measurement set-up is presented containing a multichannel frequency response analyser system that allows the measurements of impedance spectra of single cells in a fuel cell stack simultaneously. As a proof of reliability, the sum of the impedances of the single cells is compared to the measured impedance of the whole stack. A good correlation was found. The absolute deviation is less than 2.5%. A measurement of the stack during low load operation is presented. Differences in the individual cell voltages are investigated. A flooding event in a single cell is observed by means of the impedance measurement minutes before the polarisation shows a voltage drop.

To date, the available equipment for characterising the microstructure of membrane electrode assembly (MEA) is still not well developed. For example, applying the normal scanning electron microscope (SEM) only provides information on the dry structure of MEAs. This paper presents a microstructure analysis method of MEAs in proton exchange membrane fuel cells (PEMFC). The microstructure analysis in this paper utilises the environmental scanning electron microscope (ESEM), which shows its advantage on the sample microstructure analysis in wet mode. When water is present, the characteristics of the MEA, especially the hydrophobic and/or hydrophilic properties, are distinguishable on the ESEM images. With proper temperature and pressure control, the water distribution within both the membrane and the catalyst layer can be viewed by ESEM. Based on ESEM measurement and mercury porosity measurement, the distributions of hydrophobic and hydrophilic pores in MEA have been analyzed. By means of ESEM and energy dispersive X-ray (EDX), a degraded MEA is characterized. The microstructure change of the degraded MEA has been discussed. The results provide helpful information for the understanding of MEAs in PEMFC.

A measurement set-up is presented containing a multichannel frequency response analyser system together with a multichannel potentiostat that allows measurements of the local electrical impedance spectra of segmented fuel cells simultaneously. For evaluation the current and impedance distribution is combined with optical spatially resolved measurement and numerical simulation. Furthermore the multichannel frequency response analyser system is used for the measurement of impedance spectra of single cells in a fuel cell stack simultaneously.

The flow-field design can have great influence on the operating performance of both proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). Inhomogeneous transport of reactants to and products from the active area of these low-temperature fuel cells result in loss of power. Newly designed fractal structures are tested as flow-fields in PEMFCs and DMFCs for portable applications. To achieve a uniform fluid distribution and simultaneously minimize energy demand for mass transport (pressure loss), a computer algorithm is developed to provide a given area with a multiple ramified fluid network. By virtue of the self-similarity, the structures of such a network are called fractals. These are investigated and compared with common serpentine and parallel flow-fields. For both PEMFCs and DMFCs fractal flow-fields show similar performance to parallel designs. The most stable and highest power output is reached with the serpentine flow-field.

A fuel cell has been designed that combines three methods of spatially resolved measurements. The experimental setup allows to perform a simultaneous evaluation of current, temperature and water distribution in a polymer electrolyte fuel cell (PEFC) under operation. The test cell has a segmented anode flow field for current distribution measurement. The back plate of the cathode flow field is made of an optical window, which is transparent for infrared (IR) as well as for visible wavelengths. This allows infrared thermography and optical surveillance of water droplets and flow field flooding.

The effect of the diffusion layer on the performance and mass transport in a direct methanol fuel cell at ambient conditions is reported. Carbon cloths with variable Teflon contents and pore sizes, carbon paper and a metal wire cloth were investigated. Membrane-electrode-assemblies (MEAs) for direct methanol fuel cells (DMFCs) prepared after an in-house receipt are used, giving reproducible results after a pre-treatment involving polarisation with hydrogen and air. Long-term effects and methanol crossover were also briefly investigated. Adding Teflon to the diffusion layer leads to better gas transport, as gas and liquid transport takes place in different paths. Thus, the fuel cell power output is more stable. The same effect was seen with increasing pore size. Carbon paper is found not suitable as a diffusion layer for low temperature DMFC. The metal wire cloth yielded best performance giving 15.8 mW cm−2.

Fuel cells are under discussion for a large variety of applications as efficient and clean power generators today. In general, fuel cells are easily scalable due to their modular design and furthermore, the storage capacity and the output power can be combined independently. This enables the integration into portable devices or even in micro systems, which is favourable in those cases when batteries do not meet the respective demands. In particular, fuel cells offer advantages like variable geometry, high storage capacity, no self discharge and the potential for low production costs which make them interesting candidates for an environmentally friendly energy source [1].

The use of planar PEMFCs in printed circuit board (PCB) technology with a thickness of less than 3.5 mm are presented. This planar design consists of an open cathode side which allows a completely passive, self-breathing operation of the fuel cell. Power densities of 100 mW/cm 2 at 500 mV with hydrogen as a fuel were achieved. A steady operation with this type of fuel cell was demonstrated over a week without any cell flooding. Since the electrical conducting elements of PCBs usually are made of copper, corrosion in the wet environment of a PEMFC is expected. Thus, a significant degradation in performance of fuel cells made of plain copper PCBs was seen in long-term operation. In order to avoid corrosion, the copper layer has to be coated. Fuel cells in PCB design with diverse coatings were tested in long-term operations up to 1000 h under load. Moreover, corrosion currents have been determined by the use of potentiodynamic scans. Promising coatings are electroplated Cr and Ni and a combination of Ni/Au.

Two-phase flow and transport of reactants and products in the cathode of a transparent proton exchange membrane fuel cell (PEMFC) are studied experimentally. Images of water formed inside the cathode gas channels are presented to explain the phenomenon of water flooding. Effects of air stoichiometry, temperature, air humidity and different characteristics of diffusion layers are discussed. The influence of hydrophobic and hydrophilic diffusion layers compared to standard carbon papers on water transport is investigated. Mathematical verification of a threshold current density corresponding to first appearance of liquid water at the membrane/cathode interface confirms the experimental results.

A new type of planar fuel cell based on printed circuit board (PCB) technology has been developed. This planar design consists of an open cathode side which allows a completely passive, self-breathing operation of the fuel cell. Power densities of 100 mW/cm2 at 500 mV with hydrogen were achieved. Long-term operation for more than 1500 h has been demonstrated. The operation behavior of planar hydrogen in free-breathing PEMFC concerning stability, water management and temperature distribution are examined. The effects of diffusion layer thickness and cathode opening ratios on the performance of this fuel cell type were characterized. Moreover, the amount of water removed from the anode and the temperature distribution of the cathode side were determined.

Realistic modelling of a TPV system is a very demanding task. For a rough estimation of system limits many assumptions simplify the complexity of a thermophotovoltaic converter. It is obvious that real systems cannot be described in this way. An alternative approach that can deal with all these complexities such as arbitrary geometries, participating media, temperature distributions, etc is the Monte Carlo method (MCM). This statistical method simulates radiative energy transfer by tracking the history of a number of photons beginning with the emission by a radiating surface and ending with absorption on a surface or in a medium. All interactions are considered in this way. The disadvantage of large computation time compared to other methods is no longer a weakness with the speed of today's computers. If additional heat transfer modes conduction and/or convection are of similar magnitudes the complexity significantly increases. This paper points out different ways for realistic TPV system simulation focusing on statistical methods. This paper includes an example calculation for a combined radiation–conduction problem and a literature survey.