Schumann, Marc

The coupling of the three energy grids electric power, gas and heat, via sector coupling devices such as fuel cells or electrolyzers enables the increase of the share of renewable energies in consumption beyond the electricity sector. Correct modeling is crucial for the physically correct description of the behavior of fuel cells and effects on the energy grids, especially regarding transients caused by switching processes or during plant faults. In the paper presented here, the proton exchange membrane fuel cell is introduced as a component for its use in the “Extended Node Method”. It allows a suitable integration into the grid-based energy systems electricity, gas and heat based on node equations. For this purpose, the fuel cell is described as a multi-energy component. Also, for gas and heating grids, electrical analogies are used. The associated systems of equations are constructed in such a way that they can be applied to different topologies. Exemplary operating points are shown for a sample configuration. The results are partly compared with other calculation methods and software and discussed.

Hydrogen fuel cells have become one of the most viable power sources for electric aircraft. Models representing the electrical behavior of the fuel cell stack over the full dynamic operation region are essential for the development of power electronic energy systems powered by fuel cell stacks. This work presents an electrical equivalent circuit model for PEM fuel cell stacks representing the static and dynamic electrical behavior of the fuel cell stack under pulsed loads up to frequencies of 10 kHz. Dynamic phenomena on time scales slower than the considered timescale of power-electronic switching, such as reactant flow, membrane hydration, and temperature effects, are considered stationary. The parameterization method proposed is developed on measured data from a 110 W PEM fuel cell stack and validated with a set of measured data from a 2 kW stack. The time-domain simulated behavior of the parameterized model shows an accurate representation of the measured behavior: the parameterized model reproduces both the static polarization behavior and the behavior under high-frequency pulsed loading with errors of less than 1% with respect to the nominal stack voltage. The model is suitable for dynamic simulation of power electronic systems directly connected to fuel cell stacks and can be parameterized without special electrochemical impedance spectroscopy measurements.

In a future renewable energy system, electrolyzer, as coupling elements between power and gas systems, convert renewable electricity into storable hydrogen, which can be used in further consumption sectors. The general scope of this investigation is a controlled proton exchange membrane electrolyzer at a low voltage grid. For this, the proton exchange membrane electrolyzer as well as the power electronics for grid connection are developed as components for the "Extended Node Method" based on electrical equivalent circuit diagrams. It is shown how these components are especially defined for this method, how their topology affects the node definitions and how the AC/DC and DC/DC transitions are realized. As a study case, a sample model of a 40 kW stack is considered. The setpoints for electrolyzer current as well as reactive power are adjusted and transient calculations are carried out. The results are compared with models in Simulink/Simscape and show general agreement.

Using fuel cells in energy generation makes it possible to provide clean energy in line with the demand. Fuel cells offer a major advantage over other renewable energy sources whose generation is dependent on external influences. However, fuel cells cannot compete economically with conventional energy generation systems such as diesel generators. Such an economical constraint is partly due to the higher energy requirements of hydrogen storage. Metal hydride storage systems offer the possibility of reducing the energy intensity of storage due to low storage pressures. Heat is also required to operate such storage systems, which can be provided from the fuel cell's waste heat. To extract the heat from the fuel cell, a novel cooling circuit structure for large-scale applications is presented and simulated, considering the requirements of the metal hydride storage system regarding temperature (60 °C) and mass flow (60 kg/min). The architecture of the cooling concept consists of a primary and a secondary circuit, whereby the primary circuit is responsible for cooling the fuel cell and the secondary circuit for extracting the heat. Finally, simulation data are presented, which show the system behaviour in the event of changes in the fuel cell's electrical load and the heat consumer's thermal load. This coupling strategy shows that the cooling system is suitable for extracting the waste heat and keeping all essential parameters constant.

Hydrogen fuel cell technology is one of the key focus areas to facilitate the transition from carbon-based fuels to more sustainable solutions in the transportation and mobile power sectors. Transient voltage fluctuations due to load changes and even operation of fuel cells with DC/DC and DC/AC converters are detrimental to the lifetime and this paper proposes a method to deal with these fluctuations. Adding electric field modifier (EFM) electrodes made of gold to the membrane of a fuel cell was proposed elsewhere as a way to influence the short term flux of charge carriers through the membrane. While electrochemical impedance spectroscopy shows a limited capacitance of such electrodes, experiments using square wave excitation of the system in the kHz frequency range show a promising reaction of the cell to this treatment. More in-depth analysis of the used electrode material reveals the need to insulate future EFM electrodes in order to prevent oxidative dissolution. However, this work shows that the principle of using EFM electrodes to manipulate transient oscillations is physically sound.

The dynamic controllability of the fuel cell could be improved by the addition of an electric field modifier (EFM), to selectively boost or attenuate the flux of protons through the membrane and, thereby, influence cell performance. This approach follows the commonly accepted idea of the potential gradient across the membrane being the main driving force behind the proton transport in the membrane. To evaluate the applicability of the idea, a simulation model for a membrane with an integrated EFM is developed to study the effects on the membrane behaviour. First, a modified Poisson-Boltzmann-Model (1D) is developed to characterise the capacitive behaviour of the double layer at the EFM. The approach considers steric restrictions in the membrane pores to estimate the double layer capacitance and the range of the effect at the EFM. Second, the characteristic behaviour of the capacitance is implemented in a secondary current distribution model (2D) as a variable capacitance. In transient simulations, boost of the cell current by up to 82% and attenuation up to a complete reversal of the direction compared to the stationary operation are achieved. Thus, it was possible to show the potential of EFMs to influence the characteristics of fuel cells and electrolysers during transient operation.

This work presents the development and parameterization of a dynamic equivalent circuit model of proton exchange membrane (PEM) fuel cell stacks. The model represents the static and dynamic behavior of the fuel cell stack under pulsed ohmic loading up to switching frequencies of 10 kHz. The model is developed using established mathematical descriptions of fuel cell processes. The model is then parameterized and validated using measurement data obtained from an 11 cell, 110 W PEM fuel cell stack.

Optimized integration of fuel cells into grids or on-board power supplies is necessary to facilitate replacement of conventional energy producers by a reliable and plannable power generation technology. Due to the interdependency between fuel cell current and voltage, integration of fuel cells requires a power conditioning system, which increases integration weight and cost. For this reason, integration of electric field modifier electrodes into the setup of proton exchange membrane fuel cells is a new approach to control the output voltage in order to minimize the subsequent power conditioning system. This approach considers the physics of proton transport through the electrolyte membrane and could offer a lever to control the ohmic resistance. In this paper, a fuel cell model is implemented in MATLAB and extended by electric field modifier electrodes, allowing control of the ohmic resistance through an externally applied voltage. The concept of boosting and attenuating fuel cell voltage is presented along with different setups to enable this behavior. Furthermore, an electrical equivalent circuit for electrically controllable fuel cells is developed and implemented in MATLAB/Simulink. A method to parameterize the developed MATLAB and Simulink models by first experimental results is presented.