The performance analysis and multi-objective optimization of a typical alkaline fuel cell

Based on the model of a typical alkaline fuel cell (AFC) with circulating potassium hydroxide (KOH) solution as electrolyte and oxygen as oxidant and the experimental data available in the current literature, thermodynamic-electrochemical analyses on the performance of the AFC are carried out, in which multi-irreversibilities such as charger-transfer, concentration and ohmic overpotentials are taken into account. Expressions for the power output and efficiency of the AFC are derived, from which the general performance characteristics of the AFC are discussed in detail. It is found that the power output and efficiency of the AFC first increase and then decrease as the electrolyte concentration is increased, and consequently, there exist the optimal electrolyte concentrations for different temperatures. It is also found that the power output is not a monotonic function of the electric current density while the efficiency is a monotonically decreasing function of the electric current density. According to the performance characteristic curves of the AFC, the optimal operation regions of some main parameters are determined. Moreover, a new multi-objective function is used to further optimize the characteristics of the AFC. Some significant results for the optimal design and operation of practical AFCs are obtained.     

Energy and exergy analyses of a combined molten carbonate fuel cell - Gas turbine system

This study deals with the thermodynamic analysis of molten carbonate fuel cell combined with a gas turbine, based on the first- and second-law of thermodynamics. The mass, energy, entropy and exergy balance equations are written and applied to the system and its components. Some parametric studies are performed to investigate the change of system performance through energy and exergy efficiencies with the change of operating conditions. The irreversibilities occuring in different devices of the integrated system are also investigated through the exergy destruction analysis in these devices. The maximum output work of the MCFC is estimated to be 314.3 kW for an operating temperature of 650 °C. The overall energy and exergy efficiencies achieved for this system are 42.89% and 37.75%, respectively.     

Lifetime optimization of a molten carbonate fuel cell power system coupled with hydrogen production

In this paper, a biogas fuelled energy system for combined production of electricity and hydrogen is considered. The system is based on a molten carbonate fuel cell stack integrated with a micro gas turbine. Hydrogen is produced by a pressure swing absorption system. A multi-objective optimization is performed, considering the electrical efficiency and the unit cost of electricity as the objective functions.The system operation is affected by variations in fuel composition, ambient temperature and performance degradation of the components occurring during its lifetime. These effects are considered while defining the objective functions.     

Performance analysis of new cathode materials for molten carbonate fuel cells

The slow dissolution of the lithiated nickel oxide cathode represents one of the main causes of performance degradation in molten carbonate fuel cells (MCFC). Two main approaches were studied in ENEA laboratories to overcome this problem: protecting the nickel cathode covering it by a thin layer of a material with a low solubility in molten carbonate and stabilizing the nickel cathode doping it with iron and magnesium.Among several materials, due to its low solubility and good conductivity, lithium cobaltite was chosen to cover the nickel cathode and slow down its dissolution. A nickel electrode covered with a thin layer of lithium cobaltite doped with magnesium, was fabricated by complex sol-gel process. To simplify electrode preparation, no thermal treatments were made after covering to produce lithium cobaltite, and during the cell start-up LiMg_0.05Co_0.95O₂ was obtained in situ.To stabilize the nickel cathode, metal oxides Fe₂O₃ and MgO were chosen as dopant additives to be mixed with NiO powder in a tape-casting process (Mg_0.05Fe_0.01Ni_0.94O).On the prepared materials TGA analysis, morphological analysis by scanning electron microscopy (SEM-EDS) and electrical conductivity measurements were carried out.A conventional nickel cathode, the nickel cathode covered by lithium cobaltite precursors and the nickel cathode stabilized by iron and magnesium oxides were each tested in a 100 cm² fuel cell.Polarization curves and internal resistance (iR) measurements were acquired during the cell lifetime (1000 h) and the effect of gas composition variation on the cell performance was studied.From a comparison with the conventional nickel cathode it can be observed that the new materials have similar performance and show a good potential stability during the cell operating time. From the post-test analysis both the nickel cathode covered by lithium cobaltite and the nickel cathode doped with iron and magnesium seem to succeed in reducing nickel dissolution.     

Parameter sizing and stability analysis of a highway fuel cell electric bus power system using a multi-objective optimization approach

Although FC based electric buses are currently popular on urban streets or in short transit routes within large facilities, the version that is designed to operate on a highway, which has much higher dynamic requirements, is yet to be well developed. This research proposes to adopt the NSGA-II based multi-objective optimization scheme to optimize a fuel cell-battery-supercapacitor (SC) based FC power system (FCPS) that is specifically for a FC electric bus operating on the highway fuel economy cycle (HWFET). The optimization objectives are to minimize the FC's fuel consumption, the required battery and SC size and the battery degradation rate. More importantly, the optimization scheme is based on a combined energy management strategy (EMS) software parameter and hardware component sizing approach which is important for guaranteeing dynamically stable responses. This characteristic is achieved by imposing constraints that limit the transient time responses the DC-Bus capacitor voltage electrical parameters upon a generic step change in load power. Results demonstrate that dynamic stability can be guaranteed with proper software parameter and hardware components combinations without any trade-off requirements with the optimizer objectives. Moreover, the system mass and the battery degradation objectives are in trade-off but don't have any dependence to hydrogen consumption.     

Demonstration of coal gas run molten carbonate fuel cell concept

Integrated gasifier‐molten carbonate fuel cell (IG‐MCFC) offers a clean and efficient route for power generation from coal. A molten carbonate fuel cell (MCFC) was assembled and its performance was tested with simulated coal gas. The output and the stability was found to be comparable to that with conventional feed gas. It was also observed that switching from one type of feed gas to another had only a marginal effect on the cell performance. Copyright © 1999 John Wiley & Sons, Ltd.     

Nonlinear predictive control of a molten carbonate fuel cell stack

Operating temperature of a molten carbonate fuel cell stack should be controlled within a special range in order to improve the performance of fuel cell. In this paper, a nonlinear predictive control algorithm based on the Takagi–Sugeno fuzzy model is developed for the temperature of a molten carbonate fuel cell stack. Through predicting the outputs on a Takagi–Sugeno fuzzy model, a discrete optimization of the control action is adopted according to the principle of branch-and-bound method. The simulation results show the potential to introduce the predictive control based on Takagi–Sugeno fuzzy model for the development of fuel cells.     

Overpotential analysis with various anode gas compositions in a molten carbonate fuel cell

The effect of anode gas composition on the overpotential in a 100 cm² class molten carbonate fuel cell is investigated. A total of five different gas compositions are used. They are applied to cross-check the effect of flow rate and composition, e.g., a given composition with different gas flow rates and a total flow rate with different gas compositions. The overpotential at the anode is analyzed via steady-state polarization, inert gas step addition and reactant gas addition methods. The analyses reveal that the anodic overpotential depends on the flow rate of the reactant species rather than the composition. Two identical gas compositions show less overpotential when a larger flow rate is applied. Compositions with large flow rates of CO₂ and H₂O also yield less overpotential due to the gas species. Overpotential analyses show that the three measurements have complementary relationships.     

Performance analysis of a pressurized molten carbonate fuel cell/micro-gas turbine hybrid system

This paper presents the work on the design and part-load operations of a hybrid power system composed of a pressurized molten carbonate fuel cell (MCFC) and a micro-gas turbine (MGT). The gas turbine is an existing one and the MCFC is assumed to be newly designed for the hybrid system. Firstly, the MCFC power and total system power are determined based on the existing micro-gas turbine according to the appropriate MCFC operating temperature. The characteristics of hybrid system on design point are shown. And then different control methods are applied to the hybrid system for the part-load operation. The effect of different control methods is analyzed and compared in order to find the optimal control strategy for the system. The results show that the performance of hybrid system during part-load operation varies significantly with different control methods. The system has the best efficiency when using variable rotational speed control for the part-load operation. At this time both the turbine inlet temperature and cell operating temperature are close to the design value, but the compressor would cross the surge line when the shaft speed is less than 70% of the design shaft speed. For the gas turbine it is difficult to obtain the original power due to the higher pressure loss between compressor and turbine.     

Multi-objective optimization of molten carbonate fuel cell system for reducing CO2 emission from exhaust gases

The aim of this paper is to investigate the implementation of a molten carbonate fuel cell (MCFC) as a CO₂ separator. By applying multi-objective optimization (MOO) using the genetic algorithm, the optimal values of operating load and the corresponding values of objective functions are obtained. Objective functions are minimization of the cost of electricity (COE) and minimization of CO₂ emission rate. CO₂ tax that is accounted as the pollution-related cost, transforming the environmental objective to the cost function. The results show that the MCFC stack which is fed by the syngas and gas turbine exhaust, not only reduces CO₂ emission rate, but also produces electricity and reduces environmental cost of the system.     

Optimization of molten carbonate fuel cell (MCFC) and homogeneous charge compression ignition (HCCI) engine hybrid system for distributed power generation

In a previous study, a new hybrid system of molten carbonate fuel cell (MCFC) and homogeneous charge compression ignition (HCCI) engine was developed, where the HCCI engine replaces the catalytic burner and produces additional power by using the left-over heating values from the fuel cell stack. In the present study, to reduce the additional cost and footprint of the engine system in a hybrid configuration, the possibility of engine downsizing is investigated by using two strategies, i.e. the use of a turbocharger and the use of high geometric compression ratio for the engine design, both of which are to increase the density of the intake charge and thus the volumetric efficiency of the engine. Combining these two strategies, we suggest a new engine design with ∼60% of displacement volume of the original engine. In addition, operating strategies are developed to run the new hybrid system under part load conditions. It is successfully demonstrated that the system can operate down to 65% of the power level of the design point, while the system efficiency remains almost unchanged near 63%.     

The efficient and economic design of PEM fuel cell systems by multi-objective optimization

Since the efficiency of fuel cells is the ratio of the electrical power output and the fuel input, it is a function of power density, system pressure, and stoichiometric ratios of hydrogen and oxygen. Typically, the fuel cell efficiency decreases as its power output increases. In order for the fuel cell system to obtain highly efficient operation with the same power generation, more cells and other auxiliaries such as a high-capacity compressor system, etc. are required. In other words, fuel cell efficiency is closely related to fuel cell economics. Therefore, an optimum efficiency should exist and should result in the definition of a cost-effective fuel cell system. Using a multi-objective optimization technique, the sequential quadratic programming (SQP) method, the efficiency and cost of a fuel cell system have been optimized under various operating conditions. This paper has obtained some analytical results that provide a useful suggestion for the design of a cost-effective fuel cell system with high operation efficiency.     

Effect of sintering procedures in development of LiCoO2-cathodes for the molten carbonate fuel cell

LiCoO₂-powder was synthesized from carbonate precursors by calcination in air. Greentapes were tape-cast using a non-aqueous slurry and 10 μm plastic spheres as pore formers. Sintering was carried out in air at 850–950°C and in argon/air at 500/750°C. The two sintering procedures led to very different sub-micron morphologies, with the primary particles being much smaller in the latter case. The electrochemical performance at 650°C, in terms of overpotential at 160 mA/cm², for the air- and argon/air-sintered electrodes was 57 and 81 mV, respectively. The potential drop due to contact resistance between electrode and current collector was estimated to be 100 and 70 mV, respectively. The electrode materials were characterized by scanning electron microscopy (SEM), Hg-porosimetry, the BET-method (N₂-adsorption), X-ray diffractometry (XRD), flame atomic absorption spectrometry (F-AAS), carbon analysis and a van der Pauw conductivity measurement set-up.     

Theoretical study of a molten carbonate fuel cell stack for pressurized operation

A mathematical stack model of a molten carbonate fuel cell was numerically solved for temperature, gas dynamic pressure, and cell performance. The model assumed a steady state, constant load operation for a co‐flow stack with an external reformer. The numerical computation was done for a two‐dimensional domain with a real size of cell specifications. The effect of two stack operation variables, gas utilization and system pressure, was thoroughly analysed. The computation results were demonstrated in the form of flow fields, temperature contours, axial profiles, and plots of characteristic values. Our analysis began with an underlying fact that a high cathode gas flow is necessary for stack temperature control. The analysis result verified the effect of stack cooling by the cathode gas, and showed various aspects of stack operation and performance under pressurization. The pressurization effect is most significant in a moderate pressure range of 1–5 atm. The gas dynamic pressure, as it inevitably increases at a high gas flow rate, is regulated by pressurization. All the pressurization effects can generally be represented using a dimensionless parameter, named a pressurization factor. The relation between gas dynamic pressure and total system pressure was clarified from the related flow equations. Copyright © 2001 John Wiley & Sons, Ltd.     

Degradation mechanism of molten carbonate fuel cell based on long-term performance: Long-term operation by using bench-scale cell and post-test analysis of the cell

In order to introduce molten carbonate fuel cells (MCFCs) in commercial applications, the target lifetime of a MCFC has been set at 40,000 h. We have carried out long-term operation tests on several bench-scale MCFCs, which include a 66,000-h continuous operation, and clarified the question of voltage degradation in relation to operating time. We have also carried out post-test analyses on the long-term operated cell components including the electrodes, the electrolyte matrix and the current collectors. The results of the long-term operation and the post-test analyses are described in this paper. The degradation mechanisms of voltage and components are discussed.     

Novel multidisciplinary design and multi-objective optimization of centrifugal compressor used for hydrogen fuel cells

One-dimensional (1D) design and optimization of the impeller plays a significant role in performance improvement of the centrifugal compressor. However, most of the concentration has been paid to three-dimensional (3D) optimization of blades, few attention was focused on main control parameters determining aerodynamic performance and their optimal combination. Thus, this study innovatively developed a multidisciplinary design method combined with empirical 1D loss models, statistical analysis, and multi-optimization theory. The preliminary design of 1D parameters was developed based on empirical loss models. Besides, the analysis of variance of signal to noise ratio (SNR) was applied to find the main control parameters according to their contributions. To maximize the total pressure ratio and isentropic efficiency, the multi-objective optimization based on grey relational grade (GRG) was used to find the optimal combination of 1D parameters. The results showed that the impeller outlet width and impeller outlet radius are the most sensitive parameters affecting compressor performance. The optimal combination of 1D parameters is obtained. Compared to the initial design, the optimal impeller can reduce consumed power of 2.99%, enhance the isentropic efficiency of 1.24% at design point, and obtain the maximum increment of isentropic efficiency of 2.16% at 50 g/s operating point at 70,000 rpm.     

Bio-ethanol,a suitable fuel to produce hydrogen for a molten carbonate fuel cell

Catalytic and technological aspects in the use of bio-ethanol as fuel to produce hydrogen in both internal (IR-MCFC) and indirect internal reforming (IIR-MCFC) configurations have been considered. In MCFC conditions, even operating at total ethanol conversion, hydrogen productivity depends on the catalyst efficiency to convert methane formed through a mechanism, which foresees as first step the dehydrogenation of ethanol to acetaldehyde and as a second step the decomposition of acetaldehyde to CO and CH₄. Potassium doped Ni/MgO, Ni/La₂O₃ and Rh/MgO resulted to be the most promising catalysts to be used for the hydrogen production by steam reforming of bio-ethanol. Coke formation represents a serious problem, however, it can be drastically depressed by adding to the reaction stream a low amount of oxygen.     

Mathematical modeling of a molten carbonate fuel cell (MCFC) stack

Mathematical modeling for the parallel flow Molten Carbonate Fuel Cell (MCFC) 150-cell stack has been made. In the 150-cell stack, all cells are connected in a series.     

LCA of a molten carbonate fuel cell system

Fuel cells are recognized by all the scientific community to be ultra low emission energy conversion systems, because the pollutants associated with their operation are very low in concentration, compared to traditional energy systems. On the other hand, fuel cells are mainly fed with hydrogen, a chemical component that is not available as a pure component, but it must be extracted from other compounds. This practice involves energy consumption and emissions related to extraction of fuel, hydrogen conversion, transportation and clean up.In order to evaluate the environmental impact related to the energy production by the use of a fuel cell it is imperative to consider all the processes related to the fuel cell operation, and not only the FC operation itself.Life-cycle assessment (LCA) is a unique approach for evaluating the environmental impact related to the whole life of the system, i.e. considering all the processes associated to the system itself, including construction and decommissioning.In the present study a molten carbonate fuel cell (MCFC) system for electric energy production is considered and the related life-cycle environmental impact is considered. Finally a comparison between traditional energy conversion systems and the MCFC systems is conducted, in order to evaluate which are the advantages and the disadvantages that each supposed scenario can lead to.