Research Papers

J. Electrochem. En. Conv. Stor.. 2018;15(3):031001-031001-15. doi:10.1115/1.4038601.

The state-of-the-art conventional technology for postcombustion capture of CO2 from fossil-fueled power plants is based on chemical solvents, which requires substantial energy consumption for regeneration. A promising alternative, available in the near future, is the application of molten carbonate fuel cells (MCFC) for CO2 separation from postcombustion flue gases. Previous studies related to this technology showed both high efficiency and high carbon capture rates, especially when the fuel cell is thermally integrated in the flue gas path of a natural gas-fired combined cycle or an integrated gasification combined cycle plant. This work compares the application of MCFC-based CO2 separation process to pulverized coal fired steam cycles (PCC) and natural gas combined cycles (NGCC) as a “retrofit” to the original power plant. Mass and energy balances are calculated through detailed models for both power plants, with fuel cell behavior simulated using a 0D model calibrated against manufacturers' specifications and based on experimental measurements, specifically carried out to support this study. The resulting analysis includes a comparison of the energy efficiency and CO2 separation efficiency as well as an economic comparison of the cost of CO2 avoided (CCA) under several economic scenarios. The proposed configurations reveal promising performance, exhibiting very competitive efficiency and economic metrics in comparison with conventional CO2 capture technologies. Application as a MCFC retrofit yields a very limited (<3%) decrease in efficiency for both power plants (PCC and NGCC), a strong reduction (>80%) in CO2 emission and a competitive cost for CO2 avoided (25–40 €/ton).

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031002-031002-10. doi:10.1115/1.4039141.

Proton exchange membrane fuel cells (PEMFCs) require mechanical compression to ensure structural integrity, prevent leakage, and to minimize the electrical contact resistance. The mechanical properties and dimensions of the fuel cell vary during assembly due to manufacturing tolerances and during operation due to both temperature and humidity. Variation in stack compression affects the interfacial contact pressures between components and hence fuel cell performance. This paper presents a one-dimensional equivalent stiffness model of a PEMFC stack capable of predicting independent membrane and gasket contact pressures for an applied external load. The model accounts for nonlinear component compression behavior, thickness variation due to manufacturing tolerances, thermal expansion, membrane expansion due to water uptake, and stack dimensional change due to clamping mechanism stiffness. The equivalent stiffness model is compared to a three-dimensional (3D) finite element model, showing good agreement for multicell stacks. Results demonstrate that the correct specification of gasket thickness and stiffness is essential in ensuring a predictable membrane contact pressure, adequate sealing, and avoiding excessive stresses in the bi-polar plate (BPP). Increase in membrane contact pressure due to membrane water uptake is shown to be significantly greater than the increase due to component thermal expansion in the PEMFC operating range. The predicted increase in membrane contact pressure due to thermal and hydration effects is 18% for a stack containing fully hydrated Nafion® 117 membranes at 80 °C, 90% relative humidity (RH) using an eight bolt clamping design and a nominal 1.2 MPa assembly pressure.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031003-031003-11. doi:10.1115/1.4039298.

Water management is a critical issue for a direct methanol fuel cell (DMFC). This study focuses primarily on the use of a super-hydrophilic or super-hydrophobic cathode porous flow field to improve the water management of a passive air-breathing DMFC. The flow field layer was made of an in-house copper-fiber sintered felt (CFSF) which owns good stability and conductivity. Results indicate that the super-hydrophilic flow field performs better at a lower methanol concentration since it facilitates water removal when the water balance coefficient (WBC) is high. In the case of high-concentration operation, the use of a super-hydrophobic pattern is more able to reduce methanol crossover (MCO) and increase fuel efficiency since it helps maintain a lower WBC due to its ability in enhancing water back flow from the cathode to the anode. The effects of methanol concentration and the porosity of the CFSF are also discussed in this work. The cell based on the super-hydrophobic pattern with a porosity of 60% attains the best performance with a maximum power density of 18.4 mW cm−2 and a maximum limiting current density of 140 mA cm−2 at 4 M.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031004-031004-12. doi:10.1115/1.4038634.

Operating points of a 300 kW solid oxide fuel cell gas turbine (SOFC-GT) power plant simulator are estimated with the use of a multiple model adaptive estimation (MMAE) algorithm. This algorithm aims to improve the flexibility of controlling the system to changing operating conditions. Through a set of empirical transfer functions (TFs) derived at two distinct operating points of a wide operating envelope, the method demonstrates the efficacy of estimating online the probability that the system behaves according to a predetermined dynamic model. By identifying which model the plant is operating under, appropriate control strategies can be switched and implemented. These strategies come into effect upon changes in critical parameters of the SOFC-GT system—most notably, the load bank (LB) disturbance and fuel cell (FC) cathode airflow parameters. The SOFC-GT simulator allows the testing of various FC models under a cyber-physical configuration that incorporates a 120 kW auxiliary power unit and balance-of-plant (Bop) components. These components exist in hardware, whereas the FC model in software. The adaptation technique is beneficial to plants having a wide range of operation, as is the case for SOFC-GT systems. The practical implementation of the adaptive methodology is presented through simulation in the matlab/simulink environment.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031005-031005-10. doi:10.1115/1.4038627.

Water management in proton-exchange membrane fuel cells (PEFCs) has a large impact on the performance of the device, as liquid water affects the transport properties of the gas diffusion layer (GDL). In this study, we develop an ensemble-based model of the liquid water distribution inside the GDL. Based on a water injection experiment, the wet structure of the porous medium is inspected via X-ray tomographic microscopy and, after an image segmentation process, a voxel-based meshing of the fiber, air, and water domains is obtained. Starting from the obtained dry fiber structure, a Metropolis-Hastings Monte Carlo algorithm is used to obtain the equilibrium distribution of liquid water that minimizes the surface free energy of the ensemble. The different water distributions from the Monte Carlo (MC) simulation and water injection experiment are identified as solution for different physical mechanisms both of which are present in a running fuel cell. The wet structure is then used to calculate saturation-dependent effective transport properties using the software geodict. Thereby, a strong influence of the saturation gradient on the macrohomogeneous transport properties is found.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031006-031006-7. doi:10.1115/1.4039045.

In this study, the European biogas market and its potential are analyzed. The four countries with the biggest biogas production and potential, namely Germany, Italy, France, and the UK, are studied in detail. Particular attention is paid to their agricultural characteristics (livestock population, average number of cattle per agricultural holding, number of farms), as well as their policy and market conditions (feed-in tariffs, average biogas investment costs). A financial model is built and used to compare the four countries, based on the net present value as the performance indicator. Solid oxide fuel cells (SOFCs) are considered as a more efficient alternative for valorizing agricultural-derived biogas, which, at the moment, is vastly done with cheaper conventional engines. After an analysis of the economic merits, target farm sizes are recommended for each country. It is shown that the low valorization of manure-derived biogas in Europe offers a big opportunity for the commercialization of SOFCs and for a push toward mass production. Considering the high installation costs, it is still primordial that policy-makers incentivize the installation of biogas plants throughout the EU.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031007-031007-15. doi:10.1115/1.4039356.

This paper evaluates a state-space methodology of a multi-input multi-output (MIMO) control strategy using a 2 × 2 tightly coupled scenario applied to a physical gas turbine fuel cell hybrid power system. A centralized MIMO controller was preferred compared to a decentralized control approach because previous simulation studies showed that the coupling effect identified during the simultaneous control of the turbine speed and cathode airflow was better minimized. The MIMO controller was developed using a state-space dynamic model of the system that was derived using first-order transfer functions empirically obtained through experimental tests. The controller performance was evaluated in terms of disturbance rejection through perturbations in the gas turbine operation, and setpoint tracking maneuver through turbine speed and cathode airflow steps. The experimental results illustrate that a multicoordination control strategy was able to mitigate the coupling of each actuator to each output during the simultaneous control of the system, and improved the overall system performance during transient conditions. On the other hand, the controller showed different performance during validation in simulation environment compared to validation in the physical facility, which will require a better dynamic modeling of the system for the implementation of future multivariable control strategies.

Commentary by Dr. Valentin Fuster

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