Search

Due to its ability to perform clean, efficient processes, hydrogen has long been promoted as the energy carrier of the future, but recent activity and advances suggest a tipping point has been reached as hydrogen is now being recognized by much of the world for its unique capabilities to strongly support global efforts in achieving climate neutrality. The coming decades will require a massive increase in the amount of hydrogen used in the energy system as well as a transition from largely thermochemical to predominantly electrochemical processes involving hydrogen. For this to happen, hydrogen will need to globally approach the gigaton-scale on an annual basis. Thus, before hydrogen can fully achieve its potential, challenges that need to be addressed include achieving scale and further research and development (R&D) advances.

The production of energy carriers based on renewable electricity via the Power-to-X (PtX) approach holds the key for a holistic transformation of our global industries from fossil fuels towards renewable energy sources. To compete with cheap fossils, PtX products demand energy-efficient processes and low-cost renewable electricity. Therefore, the import of PtX products from countries with high renewable energy potentials to countries with high energy demand presents a promising pathway. However, the question which set of PtX products qualifies as suitable for long-distance transport has not yet been answered. In this context, this paper assesses the energy and cost efficiency of five PtX energy carriers (methane, methanol, ammonia, liquefied hydrogen and hydrogen bound in LOHC). Furthermore, we evaluate the influence of fluctuating renewables, availability of water and transport distance in a case study for large-scale PtX production in Morocco. Our results show that the evaluated PtX pathway efficiencies vary between 40–52% (base cases) and 44–58% (optimistic cases). None of the pathways assessed is significantly affected in its overall efficiency by a ship transport over an exemplary distance of 4000 km. However, for longer transport distances the cost difference between the assessed pathways increases. The production cost of the PtX energy carriers (124–156 € per MWh) depends on the availability of excess heat, energy density of the product and, if required, liquefaction efforts. In summary, the paper reveals that the long-distance transport and import of PtX products present an interesting option for the ongoing integration of renewable electricity into our energy system and industries. The petrochemical and steel industries in particular, as well as heavy goods transport, shipping and aviation, will be highly dependent on these imported synthetic energy sources.

The utilization of hydrogen and hydrogen-based fuels is a promising option to reduce greenhouse gas emissions of heavy-duty vehicles. Here, the production of sustainable hydrogen plays a key role. Very promising is the production of hydrogen by means of water electrolysis that links fluctuating electricity production from renewable sources with sectors based on chemical energy carriers. Water electrolysis will be one of the key technologies to reduce the energy system transformation costs and to stabilize the electricity grid. Hydrogen can be used directly as a fuel in heavy-duty vehicles or converted with CO2 to fuels like methanol or Fischer-Tropsch products. Methanol can be used both as a fuel and as feedstock for further upgrading to fuels like dimethyl ether (DME), oxymethylene ether (OME), methanol-to-gasoline (MtG) and methanol-to-jet fuel. Recently, also the conversion of hydrogen with nitrogen to ammonia as fuel gains increasing interest in combustion engines. Beyond CO2 reduction exhaust gas emissions have also to be reduced significantly. CatVap® is an innovative fuel processing technology that enables efficient exhaust gas treatment in order to achieve near-zero emission mobility. In addition, hydrogen can be used to operate fuel cell power trains, which is more and more discussed for heay-duty applications. Now it’s the time for a broader market introduction of hydrogen and hydrogen-based fuels in the mobility sector and thus, for cost reduction due the corresponding scale-effects.

Dimethyl ether (DME) is a power-to-liquids product with interesting key characteristics and a broad spectrum of applications as a green fuel, as a compound for the chemical industry and as a chemical building block. At present, DME is produced from fossil resources like natural gas and coal. The shift to renewable production from CO2 and hydrogen will allow emerging pathways for the production of DME to find its way to the market. State-of-the-art and emerging DME production pathways are presented in detail. Particular focus is hereby put on the influence of a renewable production based on CO2 and hydrogen. Last but not least, economic insights into a power-to-DME process are given in the last chapter.

Oxymethylene Dimethyl Ethers (OMEs) are promising diesel fuel alternatives and interesting solvents for various industrial applications. In this report, a well-to-wheel life cycle assessment of short OME oligomers as produced via a Power-to-Liquid (PtL) pathway has been conducted. Variations in electricity and carbon dioxide supply as well as the hardware demand for the PtL plant components (e.g. PEM water electrolysis, carbon capturing, and 36 kta OME plant capacity) have been considered. Conventional diesel fuel is used as the comparative benchmark. In scenarios with a high share of renewable electricity well-to-wheel greenhouse gas emission for OME3–5 fuel is advantageous compared to fossil diesel. For the best case, well-to-wheel greenhouse gas emissions can be reduced by 86%, corresponding to 29 g(CO2eq) km−1 (OME3–5-fuel) compared to 209 g(CO2eq) km−1 (diesel fuel). However, these results are highly sensitive to the applied method with regard to system multifunctionality. A sensitivity analysis indicates that input electricity at ∼50 g(CO2eq) kWhel−1 enables well-to-wheel greenhouse gas emissions of

The synthesis of sustainable methanol based on renewable electricity generation, sustainable hydrogen (H2) and recycled carbon dioxide (CO2) represents an interesting sustainable solution to integrated renewable energy storage and platform chemical production. However, the business case for this electricity based product (denoted hereafter as eMeOH) under current market conditions (e.g. vs. conventional fossil methanol (fMeOH) production cost) and the appropriate implementation scenarios to increase platform attractiveness and adoption have to be highlighted. The aim of the following study was to perform a dynamic simulation and calculation of the cost of eMeOH production (where electricity is generated at a wind park in Germany), with comparison made to grid connected scenarios. Consideration of these scenarios is made with particular respect to the German energy market and potential for the reduction in fees/taxes (i.e. for electrolyser systems). This evaluation and indeed the results can be viewed in light of European Union efforts to support the implementation of such technologies. In this context, CO2 is sourced from EU relevant sources, namely a biogas or ammonia plant, the latter profiting from the resulting credit arising from CO2 certificate trading. Variation in electricity cost and the CO2 certificate price (in the presented sensitivity study) demonstrate a high cost reduction potential. Under the energy market conditions of Germany it is found that eMeOH production costs vary between €608 and 1453 per tonne based on a purely grid driven scenario, whilst a purely wind park supplied scenario results in €1028–1067 per tonne. The reported results indicate that the eMeOH production cost in Germany is still above the present (although variable) market price, with the economical evaluation indicating that electrolyser and H2 storage represent the lion share of investment and operational cost. Substitution of fMeOH results in CO2 avoidance costs between €365 and 430 per tonne of CO2eq avoided for green methanol produced in Germany. The presented assessment indicates that the eMeOH production cost in Germany will reach market parity in ca. 2030–2035 with the price for the avoidance of CO2eq turning from a cost to a benefit at around the same time. Optimistically, the cost is predominantly influenced by rapidly reducing renewable electricity costs as is already the case in South American and Arabic countries offering the potential for methanol production at a cost of

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.

Oxymethylene Dimethyl Ethers (OME) are promising diesel fuel alternatives and interesting solvents for various industrial applications. In this report (Reproduced from [Hank et al. 2019] with permission from the Royal Society of Chemistry), a well-to-wheel Life Cycle Assessment of short OME oligomers as produced via a Power-to-Liquid (PtL) pathway has been conducted. Variations in electricity and carbon dioxide supply as well as the hardware demand for the PtL plant components (e.g. PEM water electrolysis, carbon capturing, 36 kta OME plant capacity) have been considered. Conventional diesel fuel is used as the comparative benchmark.

The manufactures of internal combustion engines face currently a lot of challenges. Among these are the required drastic emission reductions of carbon dioxide (CO2) as well as harmful substances the most prominent. As a consequence of these harsh market conditions solutions that could cope with this framework are urgently needed. In this context the state of the art of synthetic fuels as an essential part of the energy system is discussed and how they could offer solutions.

Methanol synthesis based on renewable electricity generation, sustainable hydrogen (H2) and recycled industrial carbon dioxide (CO2) represents an interesting solution to integrated renewable energy storage and platform chemical production. In this work the technological overview of the methanol synthesis from conventional feedstocks and based on CO2 is provided. The business case for this electricity based product under current market conditions (e.g. vs. conventional fossil methanol production cost) and the appropriate implementation scenarios to increase methanol attractiveness and adoption is highlighted. A complementary ecological evaluation of PtM process is provided and recommendations for this sustainable platform based on these understandings are highlighted.

Electrochemical impedance spectroscopy is used during operation of different polymer electrolyte membrane fuel cell (PEMFC) stack assemblies at various conditions with special interest given to the characteristic time constant tlow-f derived from the low-frequency arc of the spectra which is typically in the range of approximately 15-0.5 Hz. This was done by fitting an equivalent electrical circuit (EEC) consisting of one resistor and two RC-elements to the data. Parameter variation performed on a 90-cell stack assembly suggests that conditions leading to different air flow velocities in the flow channels affect tlow-f while other parameters like humidity influence the impedance spectrum, but not tlow-f. Comparison of the stoichiometry variation between short stack and locally resolved single cell shows similar results with the stack's time constant matching that of the cell's segments which are located off-center toward the outlet. However, a nonlinear dependency between gas flow velocity and tlow-f especially at low stoichiometric values is obvious. Results from stoichiometry variations at different pressure levels suggest that this could be attributed to the different steady-state oxygen partial pressures during the experiments. Comparison of the stoichiometry variation between different stack platforms result in similar dependencies of tlow-f on air flow rate with respect to a reference oxygen partial pressure regardless of size, flow field, geometry, or cell count of the stack. The time constant caused by oxygen diffusion through the gas diffusion layer (GDL), tGDL, was approximated and compared to tlow-f. While it was found that tlow-f ≫ tGDL at low stoichiometric values, tlow-f decreases toward tGDL at very high gas flow rates, suggesting that tGDL offsets tlow-f and becomes dominating if no oxygen concentration variation along the flow channel was present.

The transformation of energy systems towards high shares of renewable energy production while phasing out the use of fossile carbon-based energy carriers requires both a highly resilient energy system and an energy carrier which couples the electricity sector with energy consuming sectors like transportation, industry and chemistry. This is why we now see the advent of large-scale water electrolyzers for the production of sustainable hydrogen. Hydrogen will be an important feedstock for fuel cell cars, busses, trucks and trains, but also an important basic chemical for refineries, for the steel production and for various kinds of sustainable chemicals like fertilizers or polymers. This talk will adress the status quo of electrolyzer technologies and techno-economic analyses of power-to-hydrogen plants with high shares of intermittent power supply. The whole value chain from sustainable power generation to the production of liquid energy carriers will be described. An overview of the research activities of the Fraunhofer institute for Solar Energy Systems (ISE) for the realization of an integrated and sustainable energy system will be given as well. Figure 1

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.

The isothermal operation of single PEM fuel cells below 0 °C is investigated by potentiostatic experiments and analyzed by physical modeling. Current decay is observed during potentiostatic operation, which is caused by the freezing of product water in the porous cathode layers. This paper describes the implementation of the ohmic losses and changes of the active cathode area as a function of the charge transfer density in a physical PEM fuel cell model. The model is comprised of a set of differential equations accounting for the electrochemical reaction and diffusion processes in the cathode of the fuel cell. The model describes the time-resolved current decay during freezing. Thus a better understanding of freezing processes at sub-zero conditions in PEM fuel cells is gained and mechanisms proposed from earlier experimental data and statistical based interpretations are confirmed. Furthermore, the transient water uptake behavior of the membrane is studied in situ at different temperatures.

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.

The catalyst layer is one of the key components of PEMFCs. As the electrochemical reaction occurs at the three-phase interface of the catalyst layer, it is necessary to investigate the hydrophobicity and/or hydrophilicity properties of catalyst layers, which have a large impact on the water transport and even on the performance of the PEMFC. In this paper, time dependent contact angle measurements and porosity analyses have been performed to characterize the catalyst-coated membranes. For the first time, environmental scanning electron microscopy (ESEM) has been used to measure the hydrophilicity and hydrophobicity in the catalyst layer microstructure. The micro-contact angle distribution is given based on the microstructure. It shows a micro-scale hydrophobicity and hydrophilicity study, especially for an inhomogenous material. By means of conventional time-dependent contact angle measurements, porosity measurements and micro-contact angle measurements by ESEM, the hydrophobicity and hydrophilicity of catalyst-coated membranes have been investigated. Useful information has been gained to assist the understanding of catalyst layers from catalyst-coated membranes.

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.

A small, low temperature, direct methanol fuel cell stack for portable applications has been developed. Several flow field designs were investigated with respect to stable operation and high performance. Due to carbon dioxide and water production on the anode and cathode, respectively, methanol and oxygen access to the electrodes is hindered. During single cell operation the effect of both carbon dioxide evolution and water production on the current output was observed. The difference between parallel and serial feeding of both fuel and oxidant to the DMFC stack was also investigated. It was found that it is very important to remove reaction products from the active cell surface in order to ensure stable stack operation at low temperatures. The maximal power realised with the 12-cell direct methanol fuel cell stack was 30 W.

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].

This paper traces briefly the history of this photovoltaic materials and it tries to look at possible future scenarios. A large part of the paper is concerned with silicon although from solid-state physics we know that silicon is not the ideal material for photovoltaic conversion. From the first solar cell developed at Bell Laboratories in 1954 photovoltaics was dominated by silicon. The reasons for this dominating position are investigated. Crystalline silicon today has a market share of 86% which is almost equally distributed between single crystal and cast silicon. Amorphous silicon has another 13%. The main endeavor is to reduce cost. Present trends in the crystalline field are reviewed. The conventional technology still has significant potential for cost reduction but this comes only with increasing volume. A problem to be solved is the supply of solar-grade silicon material. Other future possibilities include thin film crystalline silicon on different substrates. Because of the low absorption coefficient of silicon light trapping is required. True thin film materials need only 1–2 μm of material. Amorphous silicon, copper indium diselenide (CIS) and CdTe are hopeful approaches for very cost-effective solar cells. Some other, more speculative materials and concepts are described at the end of this paper

An interdigitated front grid structure for both the emitter and base was simulated and realized. This contact design is suitable for thin-film solar cells on insulating substrates or insulating intermediate layers. Confirmed efficiencies of up to 18.2% were achieved on a 46 μm thick epitaxial silicon layer which was grown on a SIMOX wafer with an implanted compact SiO 2 intermediate layer. Samples with and without a highly doped back surface field were prepared to study the influence of the back-side recombination velocity. L eff values of 250 and 52 μm, corresponding to S back values of 800 and 10 5 cm/s, respectively, were measured, thus, underlining the importance of a low back-side recombination velocity. The optical confinement properties of the SiO 2 intermediate layers were calculated depending on the angle of the incident rays. An angle from the plane normal which is larger than 23° is necessary in order to achieve the condition of total internal reflection. Future work will focus on recrystallized Si layers on foreign substrates [1]. Since the surface of the silicon layer is fairly rough after the recrystallisation process, another set of masks was designed which is more tolerant to aligning accuracy. This is mainly relevant for the area where the base contacts are located between the locally diffused emitter. The technology for CVD Si-layer deposition, zone melting recrystallization (ZMR), as well as for a simplified solar cell process is under investigation.

The United Nations Framework Convention on Climate Change (COP21) in Paris last year has underlined the urgency of a fast and consequent transformation of the various energy systems from a fossil based energy feedstock towards renewable energies like photovoltaics, wind- and hydropower, in order to drastically decrease the greenhouse gas emissions (GHG). Such a transformation requires a thorough restructuring of the energy systems with all energy carriers and consumer sectors including its intersectoral dependencies, grid structures and energy storage needs. At Fraunhofer ISE we performed time-resolved studies on how to reach the main goal of the German energy transformation to decrease its GHG emissions by at least 80% until 2050 and thus, to widely decarbonize all energy related sectors. Potential transformation pathways based on various scenarios were studied and the required costs were modelled for each scenario. The results of this modelling made very obvious that the ambitious GHG reduction targets can only be fulfilled by a strong rise in electricity generation by fluctuating renewable energies and on the other hand by the installation of large plants for producing synthetic energy carriers from renewable energies. The most versatile energy carrier for such large amounts of energy is hydrogen produced from water electrolysis. Hydrogen can be stored in large amounts in gas tube fields or salt caverns and can be used directly either in fuel cell cars and buses as a fuel or in stationary combined heat and power fuel cell systems. Furthermore the hydrogen can be used in order to hydrogenate CO2 and thus to generate green liquid fuels such as oxymehylenether (OME) or other chemicals in order to replace oil based products. The conversion of electricity into hydrogen is called Power-to-Gas, the further processing of the hydrogen and CO2 towards liquid energy carriers and chemicals is called Power-to-Liquid. With the advent of various Power-to-Gas demonstration projects it becomes clear how important this technology will be in the future energy system. Distributed water electrolyzers in the Megawatt power range will stabilize the grid frequency and voltage or will deliver other ancillary services and thus, will contribute to the secondary balancing market. Alkaline water electrolyzers are applied for more than 100 years in industrial applications and are nowadays complemented by proton exchange membrane (PEM) electrolysers in order to cover the needs for a highly dynamic load in the grid. However, main challenges of the PEM technology are high costs associated with expensive materials and the durability of cells and stacks. With ongoing technological development it is expected that PEM electrolysis systems become a competitive alternative to alkaline systems in the next years. In this talk, the technological base of water electrolyzers will be described and the related performance data, life-time expectation and cost reduction potential will be discussed. Furthermore the impact of distributed water electrolyzers in the Megawatt power range as part of an increasingly renewable energy system in terms of Power-to-Gas modelling scenarios will be given. Figure 1

Understanding losses in polymer electrolyte membrane fuel cells, in the form of ohmic and mass transport, is of great importance to their commercialization. In this study, we use a spatially resolved cell consisting of 49 segments to measure the local current density distribution and high frequency resistance (HFR). A parametric study is used to investigate the effects of cell voltage, inlet relative humidity and flow rate and configuration using a three-channel serpentine flow field. We found that as the cell voltage decreased, the current density increased, while the HFR decreased. However, at a low cell voltage of 200 mV, we found the HFR to be higher than that at 500 mV. This increase is attributed to the increased electro-osmotic drag. This trend is independent of the flow configuration. Further, we found that the effect of the inlet relative humidity on the HFR highly depends on the flow configurations. Finally, a sharp decrease in the current density at some specific bend segments was observed, which correlates with lower OCV values and higher HFR values at this position.

In this work a 50-channel characterisation system for PEMFCs is presented. The system is capable of traditional electrochemical measurements (e. g. staircase voltammetry, chronoamperometry and cyclic voltammetry), and concurrent EIS measurements. Unlike previous implementations, this system relies on dedicated potentiostats for current and voltage control, and independent frequency response analysers (FRAs) at each channel. Segmented fuel cell hardware is used to illustrate the system's flexibility and capabilities. The results here include steady-state data for cell characterisation under galvanostatic and potentiostatic control as well as spatially resolved impedance spectra.

Micro fuel cell and reformer systems will play an important role in upcoming micro energy systems as a supplement or a replacement for batteries for off-grid or portable electronic devices. Micro reformers process hydrocarbon fuels or alcohols into hydrogen rich gases which are subsequently converted into electric power within micro fuel cells. Reformers do play a crucial role when it comes to disseminating fuel cell systems into mass markets, because they allow the end user to use fuels that are more readily available than hydrogen. This is reflected by the great amount of global players in consumer electronics that research micro fuel cells and reformer systems. In this article, an overview of the principles of fuel cells and reformers featuring MEMS based technologies is given. While micro fuel cells are under development with a focus on the realization of high power density and passive operation, research in the area of micro-channel reformers is concentrating on the reforming reactor, burner, heat exchanger and gas treatment design as well as the catalyst development. The current status of the various research activities as well as the major challenges for the realization of miniaturized complete energy systems is described

This paper presents a model-based analysis of a proton exchange membrane fuel cell (PEMFC) with a planar design as the power supply for portable applications. The cell is operated with hydrogen and consists of an open cathode side allowing for passive, self-breathing, operation. This planar fuel cell is fabricated using printed circuit board (PCB) technology. Long-term stability of this type of fuel cell has been demonstrated. A stationary, two-dimensional, isothermal, mathematical model of the planar fuel cell is developed. Fickian diffusion of the gaseous components (O2, H2, H2O) in the gas diffusion layers and the catalyst layers is accounted for. The transport of water is considered in the gaseous phase only. The electrochemical reactions are described by the Tafel equation. The potential and current balance equations are solved separately for protons and electrons. The resulting system of partial differential equations is solved by a finite element method using FEMLAB (COMSOL Inc.) software. Three different cathode opening ratios are realized and the corresponding polarization curves are measured. The measurements are compared to numerical simulation results. The model reproduces the shape of the measured polarization curves and comparable limiting current density values, due to mass transport limitation, are obtained. The simulated distribution of gaseous water shows that an increase of the water concentration under the rib occurs. It is concluded that liquid water may condense under the rib leading to a reduction of the open pore space accessible for gas transport. Thus, a broad rib not only hinders the oxygen supply itself, but may also cause additional mass transport problems due to the condensation of water.

A Monte Carlo Method (MCM) based model has been developed to estimate the radiative energy transfer rate in the optical cavity comprising a TPV power system. The MCM model is augmented by back‐end calculations for the conversion of the photon flux to electrical power based on the TPV cell characteristics. The model has been developed using Mathematica™ and allows for semi‐transparent media, as well as non‐gray spectral and directional characteristics of surfaces in the optical cavity to be included. The overall model was used to corroborate the performance of a real TPV power system that was built at Fraunhofer ISE, and to conduct a preliminary parametric study of the role of emitter type and temperature.

The characterisation of a proton exchange membrane (PEM) fuel cell with a straight channel flow field design is performed. Spatially resolved current distribution measurements at different air flow rates are compared to numerical simulation results. The numerical model is validated by agreement of measured and simulated current distribution. The test cell is segmented. It is operated at steady state conditions and the gas flow rates and cell temperature are controlled. The numerical simulation is realised with a PEM fuel cell model based on the FLUENT T M computational fluid dynamics (CFD) software. It accounts for mass transport in the gaseous phase, heat transfer, electrical potential field and the electrochemical reaction. It provides three-dimensional distributions of, for example, current densities, reactant concentrations and temperature.

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.

The paper reviews the history, the present status and possible future developments of photovoltaic (PV) materials for terrestrial applications. After a brief history and introduction of the photovoltaic effect theoretical requirements for the optimal performance of materials for pn-junction solar cells are discussed. Most important are efficiency, long-term stability and, not to be neglected, lowest possible cost. Today the market is dominated by crystalline silicon in its multi crystalline and monocrystalline form. The physical and technical limitations of this material are discussed. Although crystalline silicon is not the optimal material from a solid state physics point of view it dominates the market and will continue to do this for the next 5-10 years. Because of its importance a considerable part of this review deals with materials aspects of crystalline silicon. For reasons of cost only multicrystalline silicon and monocrystalline Czochralski (Cz) crystals are used in practical cells. Light induced instability in this Cz-material has recently been investigated and ways to eliminate this effect have been devised. For future large scale production of crystalline silicon solar cells development of a special solar grade silicon appears necessary. Ribbon growth is a possibility to avoid the costly sawing process. A very vivid R&D area is thin-film crystalline silicon (about 5-30 mu m active layer thickness) which would avoid the crystal growing and sawing processes. The problems arising for this material are: assuring adequate light absorption, assuring good crystal quality and purity of the films, and finding a substrate that fulfills all requirements. Three approaches have emerged: high- temperature, low-temperature and transfer technique. Genuine thin-film materials are characterized by a direct band structure which gives them very high light absorption. Therefore, these materials have a thickness of only one micron or less. The oldest such material is amorphous silicon which is the second most important material today. It is mainly used in consumer products but is on the verge to also penetrate the power market. Other strong contenders are chalcogenides like copper indium diselenide (CIS) and cadmium telluride. The interest has expanded from CuInSe/sub 2/, to CuGaSe/sub 2/, CuInS/sub 2/ and their multinary alloys Cu(In,Ga)(S,Se)/sub 2/. The two deposition techniques are either separate deposition of the components followed by annealing on one hand or coevaporation. Laboratory efficiencies for small area devices are approaching 19% and large area modules have reached 12%. Pilot production of CIS-modules has started in the US and Germany. Cadmium telluride solar cells also offer great promise. They have only slightly lower efficiency and are also at the start of production. In the future other materials and concepts can be expected to come into play. Some of these are: dye sensitized cells, organic solar cells and various concentrating systems including III/V-tandem cells. Theoretical materials that have not yet been realized are Auger generation material and intermediate metallic band material.

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.

An overview of the high-efficiency solar cell developments with crystalline silicon at Fraunhofer ISE (Germany) is given. Seven types of solar cells are discussed: (1) LBSF cells (local back surface field); (2) RP-PERL cells (random pyramids, passivated emitter rear locally-diffused); (3) RP-PERC (random pyramids, passivated emitter and rear cell); (4) MESC (mesh-structured emitter cell): (5) cells with interdigitated contacts; (6) high-efficiency thin film solar cells based on SOI (silicon on insulator); and (7) mini modules integrated on one 4" SOI-wafer. All these cells show very good efficiencies close to or well above 20%, respectively. The goal of all activities is to develop technologies for efficiencies as high as possible but at the same time as simple as possible. Many of the developments are performed in close cooperation with German PV companies and/or with European research teams in the framework of projects of the European Commission.

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.

not Available.