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The durability of polymer electrolyte membrane fuel cells (PEMFC) is governed by a nonlinear coupling between system demand, component behavior, and physicochemical degradation mechanisms, occurring on timescales from the sub-second to the thousand-hour. We present a simulation methodology for assessing performance and durability of a PEMFC under automotive driving cycles. The simulation framework consists of (a) a fuel cell car model converting velocity to cell power demand, (b) a 2D multiphysics cell model, (c) a flexible degradation library template that can accommodate physically-based component-wise degradation mechanisms, and (d) a time-upscaling methodology for extrapolating degradation during a representative load cycle to multiple cycles. The computational framework describes three different time scales, (1) sub-second timescale of electrochemistry, (2) minute-timescale of driving cycles, and (3) thousand-hour-timescale of cell ageing. We demonstrate an exemplary PEMFC durability analysis due to membrane degradation under a highly transient loading of the New European Driving Cycle (NEDC).
Bauteilaktivierung
(2015)
Seven cell design concepts for aqueous (alkaline) lithium–oxygen batteries are investigated using a multi-physics continuum model for predicting cell behavior and performance in terms of the specific energy and specific power. Two different silver-based cathode designs (a gas diffusion electrode and a flooded cathode) and three different separator designs (a porous separator, a stirred separator chamber, and a redox-flow separator) are compared. Cathode and separator thicknesses are varied over a wide range (50 μm–20 mm) in order to identify optimum configurations. All designs show a considerable capacity-rate effect due to spatiotemporally inhomogeneous precipitation of solid discharge product LiOH·H2O. In addition, a cell design with flooded cathode and redox-flow separator including oxygen uptake within the external tank is suggested. For this design, the model predicts specific power up to 33 W/kg and specific energy up to 570 Wh/kg (gravimetric values of discharged cell including all cell components and catholyte except housing and piping).
Demand Side Management for Thermally Activated Building Systems based on Multiple Linear Regression
(2015)
Lithium-oxygen cells with organic electrolyte suffer high overpotentials during charge, indicating asymmetric charge/discharge reaction mechanisms. We present a multi-physics dynamic modeling and simulation study of the Li/O2 cell cycling behavior. We present three different multi-step mechanisms of the 2 Li + O2 ⇄ Li2O2 cell reaction, (A) a reversible 5-step mechanism, (B) a partially irreversible 6-step mechanism, and (C) a partially irreversible 8-step mechanism that includes reactions of a redox mediator. Model predictions are compared to experimental galvanostatic cycling data of Swagelok cells without and with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) as redox mediator. All mechanisms are able to predict the discharge behavior in good agreement to the experimental results. The experimentally observed high charge overpotentials as well as their reduction by using a redox mediator can be qualitatively reproduced with the irreversible reaction mechanisms. However, the particular shape of the experimental charge curve with continuously increasing charge overpotential cannot be reproduced with the present mechanisms.
Since 2003, most European countries established heat health warning systems to alert the population to heat load. Heat health warning systems are based on predicted meteorological conditions outdoors. But the majority of the European population spends a substantial amount of time indoors, and indoor thermal conditions can differ substantially from outdoor conditions. The German Meteorological Service (Deutscher Wetterdienst, DWD) extended the existing heat health warning system (HHWS) with a thermal building simulation model to consider heat load indoors. In this study, the thermal building simulation model is used to simulate a standardized building representing a modern nursing home, because elderly and sick people are most sensitive to heat stress. Different types of natural ventilation were simulated. Based on current and future test reference years, changes in the future heat load indoors were analyzed. Results show differences between the various ventilation options and the possibility to minimize the thermal heat stress during summer by using an appropriate ventilation method. Nighttime ventilation for indoor thermal comfort is most important. A fully opened window at nighttime and the 2-h ventilation in the morning and evening are more sufficient to avoid heat stress than a tilted window at nighttime and the 1-h ventilation in the morning and the evening. Especially the ventilation in the morning seems to be effective to keep the heat load indoors low. Comparing the results for the current and the future test reference years, an increase of heat stress on all ventilation types can be recognized.
The current methods used to assess the energy performance of ventilation devices do not consider all the aspects necessary for a comprehensive evaluation of decentralised ventilation concepts and can only be partially adapted to their needs. In order to improve the energy evaluation and to ensure the comparability of different systems, a calorimetric method was developed and implemented in test facilities for the evaluation of two decentralised devices: one equipped with a recuperative counter flow heat exchanger and one with a regenerative heat exchanger. This method, based on direct measurements of the heating load in an insulated test room, includes the effect of the electrical consumption of the fans on the energy performance of the ventilation devices. The calorimetric evaluation method was extended to a seasonal evaluation on the basis of a heating-degree-day method implemented for a warm, a cool and a moderate location in Europe: Athens, Strasbourg and Copenhagen. All the results are above 50% efficiency for both devices, even in Athens where the use of heat recovery ventilation is not usual.
The transformation of the building energy sector to a highly efficient, clean, decentralised and intelligent system requires innovative technologies like microscale trigeneration and thermally activated building structures (TABS) to pave the way ahead. The combination of such technologies however presents a scientific and engineering challenge. Scientific challenge in terms of developing optimal thermo-electric load management strategies based on overall energy system analysis and an engineering challenge in terms of implementing these strategies through process planning and control. Initial literature research has pointed out the need for a multiperspective analysis in a real life laboratory environment. To this effect an investigation is proposed wherein an analytical model of a microscale trigeneration system integrated with TABS will be developed and compared with a real life test-rig corresponding to building management systems. Data from the experimental analysis will be used to develop control algorithms using model predictive control for achieving the thermal comfort of occupants in the most energy efficient and grid reactive manner. The scope of this work encompasses adsorption cooling based microscale trigeneration systems and their deployment in residential and light commercial buildings.
Combined heat and power production (CHP) based on solid oxide fuel cells (SOFC) is a very promising technology to achieve high electrical efficiency to cover power demand by decentralized production. This paper presents a dynamic quasi 2D model of an SOFC system which consists of stack and balance of plant and includes thermal coupling between the single components. The model is implemented in Modelica® and validated with experimental data for the stack UI-characteristic and the thermal behavior. The good agreement between experimental and simulation results demonstrates the validity of the model. Different operating conditions and system configurations are tested, increasing the net electrical efficiency to 57% by implementing an anode offgas recycle rate of 65%. A sensitivity analysis of characteristic values of the system like fuel utilization, oxygen-to-carbon ratio and electrical efficiency for different natural gas compositions is carried out. The result shows that a control strategy adapted to variable natural gas composition and its energy content should be developed in order to optimize the operation of the system.
The energy system of the future will transform from the current centralised fossil based to a decentralised, clean, highly efficient, and intelligent network. This transformation will require innovative technologies and ideas like trigeneration and the crowd energy concept to pave the way ahead. Even though trigeneration systems are extremely energy efficient and can play a vital role in the energy system, turning around their deployment is hindered by various barriers. These barriers are theoretically analysed in a multiperspective approach and the role decentralised trigeneration systems can play in the crowd energy concept is highlighted. It is derived from an initial literature research that a multiperspective (technological, energy-economic, and user) analysis is necessary for realising the potential of trigeneration systems in a decentralised grid. And to experimentally quantify these issues we are setting up a microscale trigeneration lab at our institute and the motivation for this lab is also briefly introduced.
Photovoltaics Energy Prediction Under Complex Conditions for a Predictive Energy Management System
(2015)
A two-dimensional single-phase model is developed for the steady-state and transient analysis of polymer electrolyte membrane fuel cells (PEMFC). Based on diluted and concentrated solution theories, viscous flow is introduced into a phenomenological multi-component modeling framework in the membrane. Characteristic variables related to the water uptake are discussed. A Butler–Volmer formulation of the current-overpotential relationship is developed based on an elementary mechanism of electrochemical oxygen reduction. Validated by using published V–I experiments, the model is then used to analyze the effects of operating conditions on current output and water management, especially net water transport coefficient along the channel. For a power PEMFC, the long-channel configuration is helpful for internal humidification and anode water removal, operating in counterflow mode with proper gas flow rate and humidity. In time domain, a typical transient process with closed anode is also investigated.
The state-of-the-art electrochemical impedance spectroscopy (EIS) calculations have not yet started from fully multi-dimensional modeling. For a polymer electrolyte membrane fuel cell (PEMFC) with long flow channel, the impedance plot shows a multi-arc characteristic and some impedance arcs could merge. By using a step excitation/Fourier transform algorithm, an EIS simulation is implemented for the first time based on the full 2D PEMFC model presented in the first part of this work. All the dominant transient behaviors are able to be captured. A novel methodology called ‘configuration of system dynamics’, which is suitable for any electrochemical system, is then developed to resolve the physical meaning of the impedance spectra. In addition to the high-frequency arc due to charge transfer, the Nyquist plots contain additional medium/low-frequency arcs due to mass transfer in the diffusion layers and along the channel, as well as a low-frequency arc resulting from water transport in the membrane. In some case, the impedance spectra appear partly inductive due to water transport, which demonstrates the complexity of the water management of PEMFCs and the necessity of physics-based calculations.
A 2D-separation of 16 polyaromatic hydrocarbons (PAHs) according to the Environmental Protecting Agency (EPA) standard was introduced. Separation took place on a TLC RP-18 plate (Merck, 1.05559). In the first direction, the plate was developed twice using n-pentane at −20°C as the mobile phase. The mixture acetonitrile-methanol-acetone-water (12:8:3:3, v/v) was used for developing the plate in the second direction. Both developments were carried out over a distance of 43 mm. Further on in this publication, a specific and very sensitive indication method for benzo[a]pyrene and perylene was presented. The method can detect these hazardous compounds even in complicated PAH mixtures. These compounds can be quantified by a simple chemiluminescent reaction with a limit of detection (LOD) of 48 pg per band for perylene and 95 pg per band for benzo[a]pyrene. Although these compounds were separated from all other PAHs in the standard, a separation of both compounds was not possible from one another. The method is suitable for tracing benzo[a]pyrene and/or perylene. The proposed chemiluminescence screening test on PAHs is extremely sensitive but may indicate a false positive result for benzo[a]pyrene.