Influence of current densities in SOFC and PEFC stacks on a SOFC–PEFC combined system

A solid oxide fuel cell (SOFC)–polymer electrolyte fuel cell (PEFC) combined system was investigated by numerical simulation. Here, the effect of the current densities in the SOFC and the PEFC stacks on the system's performance is evaluated under a constant fuel utilization condition. It is shown that the SOFC–PEFC system has an optimal combination of current densities, for which the electrical efficiency is highest. The optimal combination exists because the cell voltage in one stack increases and that of the other stack decreases when the current densities are changed. It is clarified that there is an optimal size of the PEFC stack in the parallel-fuel-feeding-type SOFC–PEFC system from the viewpoint of efficiency, although a larger PEFC stack always leads to higher electrical efficiency in the series-fuel-feeding-type SOFC–PEFC system. The 40 kW-class PEFC stack is suitable for the 110 kW-class SOFC stack in the parallel-fuel-feeding type SOFC–PEFC system.     

5 kWe + 5 kWt reformer-PEMFC energy generator from bioethanol first data on the fuel processor from a demonstrative project

A power unit constituted by a reformer section, a H₂ purification section and a fuel cell stack is being tested c/o the Dept. of Physical Chemistry and Electrochemistry of Università degli Studi di Milano, on the basis of a collaboration with HELBIO S.A. Hydrogen and Energy Production Systems, Patras (Greece), supplier of the unit, and some sponsors (Linea Energia S.p.A., Parco Tecnologico Padano and Provincia di Lodi, Italy). The system size allows to co-generate 5 kW_e (220 V, 50 Hz a.c.) + 5 kW_t (hot water at 65 °C) as peak output. Bioethanol, obtainable by different non-food-competitive biomass, is transformed into syngas by a pre-reforming and reforming reactors couple and the reformate is purified from CO to a concentration below 20 ppmv, suitable to feed a proton exchange membrane fuel cell (PEMFC) stack that will be integrated in the fuel processor in a second step of the experimentation. This result is achieved by feeding the reformate to two water gas shift reactors, connected in series and operating at high and low temperature, respectively. CO concentration in the outcoming gas is ca. 0.4 vol% and the final CO removal to meet the specifications is accomplished by two methanation reactors in series. The second methanation step acts merely as a guard, since ca. 15 ppmv of CO are obtained already after the first reactor.     

Biodiesel fuel processor for APU applications

Tail pipe emission reduction, increased use of renewable fuels and efficient supply of auxiliary power for road vehicles using fuel cells have been the main drivers of the European project BIOFEAT (biodiesel fuel processor for a fuel cell auxiliary power unit for a vehicle). Within the project a biodiesel fuelled heat integrated fuel processor for 10 kW_e capacity has been designed and constructed. Demonstration tests showed a high quality reformate with less than 10 ppm of CO and a gross efficiency of 87%.     

Exergetic analysis of solid oxide fuel cell and biomass gasification integration with heat pipes

This paper presents an exergetic analysis of a combined heat and power (CHP) system, integrating a near-atmospheric solid oxide fuel cell (SOFC) with an allothermal biomass fluidised bed steam gasification process. The gasification heat requirement is supplied to the fluidised bed from the SOFC stack through high-temperature sodium heat pipes. The CHP system was modelled in AspenPlus™ software including sub-models for the gasification, SOFC, gas cleaning and heat pipes. For an average current density of 3000 A m⁻² the proposed system would consume 90 kg h⁻¹ biomass producing 170 kW_e net power with a system exergetic efficiency of 36%, out of which 34% are electrical.     

MCFC-based marine APU: Comparison between conventional ATR and cracking coupled with SR integrated inside the stack pressurized vessel

In the present work the implementation of MCFCs as auxiliary power units on-board large vessels, such as cruising, passengers or commercial, ships was investigated. The MCFC stack was designed to supply 500 kW_e and was fed with diesel oil undergoing a reforming process. The system modelling of the plant was performed in steady-state and aimed at assessing the power efficiency for different reforming strategies, process configurations and constituting items thermal integrations. The code Matlab/Simulink was used to this end. Two major fuel processing strategies were examined: “auto-thermal reforming” and “inside vessel steam reforming”. The latter consisted of a pre-reforming unit in which the liquid fuel underwent a catalytic cracking in mild conditions; subsequently, the resulting gas mixture made of light hydrocarbons was mixed with steam and fed into a steam reformer inside the MCFC stack vessel, where conversion to syngas occurred. Due to the high temperature (650 °C) exothermic level of MCFC, the stack was compatible with a syngas steam reforming production thermally self sustained. This allowed to increase the global electrical efficiency from 32.7% (for the ATR-based system) up to 44.6%. The process was then designed aiming at increasing the overall efficiency by thermally integrating the outlet flue gases with the pre-heating section. This lead to efficiencies equal to 39.1% and 50.6% for the “auto-thermal reforming” and “inside vessel steam reforming”, respectively. Finally, the process was upgraded from an auxiliary power unit (APU) to a combined heat and power unit (CHP), since the residual heat in the flue gases was recovered for heating purposes (sanitary water production) and the demineralised water recirculation was implemented to reduce the water make-up and the process environmental footprint.     

Evaluation of a low temperature fuel cell system for residential CHP

CHP (combined heat and power) is a technology that allows to provide electrical and thermal energy. CHP is normally used in systems that produce wasted heat at high temperature to recover energy and increase overall system efficiency. The aim of this work is to investigate the possibility to recover heat produced by a 5 kW PEFC system for residential applications (hot water and building heating). As known, PEFCs work at low temperature (60-90 °C) and the experiments have been carried out in order to improve the overall system efficiency by reusing heat that is normally wasted.The work was developed during an Italian National project PNR-FISR “Polymeric and Ceramic Fuel Cell” coordinated by CNR-ITAE. A 5 kW PEFC system, developed with NUVERA Fuel Cells in the framework of the project, was tested in cogeneration configuration recovering wasted heat with a heat exchanger directly connected to cathode out.Tests on PEFC system were carried out in the range 2.5-5 kW, maintaining the working stack temperature at 71 °C. Heat, produced at different power levels, was removed from the system by using a regulated water flow in the heat exchanger. A peculiar feature of the system is the so-called “direct water injection” at the cathode, that allows simultaneous cooling and humidification of the stack. This characteristic permitted the recovery of most of the waste heat produced by the fuel cell.The performance of the PEFC unit was analyzed in terms of electrical, thermal and total efficiency. Tests showed that it is possible to obtain water at about 68 °C under different power levels. Moreover, experimental data showed that heat recovered was maximum when heat exchanger worked at nominal power and, under these conditions, the overall system efficiency increased up to 85%.     

Dynamic behaviour of Li batteries in hydrogen fuel cell power trains

A Li ion polymer battery pack for road vehicles (48 V, 20 Ah) was tested by charging/discharging tests at different current values, in order to evaluate its performance in comparison with a conventional Pb acid battery pack. The comparative analysis was also performed integrating the two storage systems in a hydrogen fuel cell power train for moped applications. The propulsion system comprised a fuel cell generator based on a 2.5 kW polymeric electrolyte membrane (PEM) stack, fuelled with compressed hydrogen, an electric drive of 1.8 kW as nominal power, of the same typology of that installed on commercial electric scooters (brushless electric machine and controlled bidirectional inverter). The power train was characterized making use of a test bench able to simulate the vehicle behaviour and road characteristics on driving cycles with different acceleration/deceleration rates and lengths. The power flows between fuel cell system, electric energy storage system and electric drive during the different cycles were analyzed, evidencing the effect of high battery currents on the vehicle driving range. The use of Li batteries in the fuel cell power train, adopting a range extender configuration, determined a hydrogen consumption lower than the correspondent Pb battery/fuel cell hybrid vehicle, with a major flexibility in the power management.     

Numerical evaluation of a parallel fuel feeding SOFC–PEFC system using seal-less planar SOFC stack

We propose a system that combines a seal-less planar solid oxide fuel cell (SOFC) stack and polymer electrolyte fuel cell (PEFC) stack. In the proposed system, fuel for the SOFC (SOFC fuel) and fuel for the PEFC (PEFC fuel) are fed to each stack in parallel. The steam reformer for the PEFC fuel surrounds the seal-less planar SOFC stack. Combustion exhaust heat from the SOFC stack is used for reforming the PEFC fuel. We show that the electrical efficiency in the SOFC–PEFC system is 5% higher than that in a simple SOFC system using only a seal-less planar SOFC stack when the SOFC operation temperature is higher than 973 K.     

Modeling and analysis of a 5 kWe HT-PEMFC system for residential heat and power generation

We present a high-temperature proton exchange membrane fuel cell (HT-PEMFC) system model that accounts for fuel reforming, HT-PEMFC stack, and heat-recovery modules along with heat exchangers and balance of plant (BOP) components. In the model developed for analysis, the reaction kinetics for the fuel reforming processes are considered to accurately capture exhaust gas compositions and reactor temperatures under various operating conditions. The HT-PEMFC stack model is simplified from the three-dimensional HT-PEMFC CFD models developed in our previous studies. In addition, the parasitic power consumption and waste heat release from the various BOP components are calculated based on their heat-capacity curves. An experimental fuel reforming reactor for a 5.0 kW_e HT-PEMFC system was tested to experimentally validate the fuel reforming sub model. The model predictions were found to be in good agreement with the experimental data in terms of exhaust gas compositions and bed temperatures. Additionally, the simulation revealed the impacts of the burner air-fuel ratio (AFR) and the steam reforming reactor steam-carbon ratio on the system performance and efficiency. In particular, the combined heat and power efficiency of the system increased up to 78% when the burner AFR was properly adjusted. This study clearly illustrates that an HT-PEMFC system requires a high degree of thermal integration and optimization of the system configuration and operating conditions.     

Exergy analysis and optimization of a biomass gasification, solid oxide fuel cell and micro gas turbine hybrid system

A hybrid plant producing combined heat and power (CHP) from biomass by use of a two-stage gasification concept, solid oxide fuel cells (SOFC) and a micro gas turbine was considered for optimization. The hybrid plant represents a sustainable and efficient alternative to conventional decentralized CHP plants. A clean product gas was produced by the demonstrated two-stage gasifier, thus only simple gas conditioning was necessary prior to the SOFC stack. The plant was investigated by thermodynamic modeling combining zero-dimensional component models into complete system-level models. Energy and exergy analyses were applied. Focus in this optimization study was heat management, and the optimization efforts resulted in a substantial gain of approximately 6% in the electrical efficiency of the plant. The optimized hybrid plant produced approximately 290 kW_e at an electrical efficiency of 58.2% based on lower heating value (LHV).     

Hybrid polymer electrolyte membrane fuel cell–lithium‐ion battery powertrain testing platform – hybrid fuel cell electric vehicle emulator

A testing and validation platform for hybrid fuel cell (FC)–lithium‐ion battery (LIB) powertrain systems is investigated. The hybrid FC electric vehicle emulator enables testing of hybrid system components and complete hybrid power modules up to 25 kW for application in electric light‐duty vehicles, light electric vehicles and so forth. A hybrid system comprising a 10‐kW_el low‐temperature polymer electrolyte membrane FC stack and an 11.5‐kWh LIB pack is installed. The system supplies power to a 20‐kW permanent magnet synchronous motor and a 25‐kW alternating current asynchronous, electrically programmable dynamometer is used to simulate the vehicle load during testing at dynamic drive cycle. The steady‐state performance tests of the direct current (DC) motor, DC/DC converter, low‐temperature polymer electrolyte membrane FC stack and LIB are performed as well as dynamic tests of the complete hybrid system. The Economic Commission for Europe driving cycle is selected as a reference cycle to validate the investigated hybrid FC–LIB powertrain. An efficiency of 83% and 95% is measured for electric motor and DC/DC converter, respectively. An average stack efficiency of 50% is achieved. An average hydrogen consumption of 3.9 g * km^?1 is reached during the Economic Commission for Europe driving cycle test. Copyright © 2017 John Wiley & Sons, Ltd.     

PEM stack test and analysis in a power system at operational load via ac impedance

This paper presents a method for collecting ac impedance data from a commercial PEFC power system at operational loads. The PEM fuel cell stack in the power system, including 47 MEAs, was operated using room air and pure hydrogen (>99.99%). For a stack test in the power system, the power source for the embedded controller board was simultaneously switched from the fuel cell stack to a similar external power source after the system reached a steady temperature. The PEM fuel cells in the stack were tested by collecting the ac impedance data at different current levels. By using ac impedance, a single fuel cell, a group of fuel cells, and a complete stack were then tested without the embedded control devices for ohmic, activation, and mass transport losses. The ohmic resistance for each cell component in the stack was obtained as 117 mΩ cm² at operational loads from 2.5 A to 35 A. The membrane thickness was further estimated as ca. 51–89 μm. Resistances from ohmic conduction, anode/cathode activation, and mass transport were measured and discussed using the Nyquist plots from the ac impedance spectra.     

Modeling and parametric study of a 1 kWe HT-PEMFC-based residential micro-CHP system

A detailed thermodynamic, kinetic and geometric model of a micro-CHP (Combined-Heat-and-Power) residential system based on High Temperature-Proton Exchange Membrane Fuel Cell (HT-PEMFC) technology is developed, implemented and validated. HT-PEMFC technology is investigated as a possible candidate for fuel cell-based residential micro-CHP systems, since it can operate at higher temperature than Nafion-based fuel cells, and therefore can reach higher cogeneration efficiencies. The proposed system can provide electric power, hot water, and space heating for a typical Danish single-family household. A complete fuel processing subsystem, with all necessary balance-of-plant components, is modeled and coupled to the fuel cell stack subsystem. The micro-CHP system’s synthesis/design and operational pattern is analyzed by means of a parametric study. The parametric study is conducted to determine the most viable system/component design based on maximizing total system efficiency, without violating the requirements of the system. Four decision variables (steam-to-carbon ratio, fuel cell operating temperature, combustor temperature and hydrogen stoichiometry) were parameterized within feasible limits to provide insight on their effect on the overall performance of the proposed system under study and also to provide input on more efficient design in the future. The system is designed to provide maximum loads of 1 kW_e and 2 kW_th. A sensitivity analysis is applied to investigate the influence of the most important parameters on the simulated performance of the system.     

Modelling of an HTPEM-based micro-combined heat and power fuel cell system with methanol

A fuel cell-based combined heat and power system using a high temperature proton exchange membrane fuel cell has been modelled. The fuel cell is fed with the outlet hydrogen stream from a methanol steam reforming reactor. In order to provide the necessary heat to this reactor, it was considered the use of a catalytic combustor fed with methanol. The modelling aims to fit the hydrogen production to the demand of the fuel cell to provide 1 kW_e, maintaining a CO concentration always lower than 30,000 ppm. A system with 65 cells (45.16 cm² cell area) stack operating at 150 °C and hydrogen utilization factor = 0.9 (with O₂/methanol ratio = 2 at combustor; H₂O/methanol ratio = 2 and temperature = 300 °C at reformer) needed a total methanol flow of 23.8 mol h⁻¹ (0.96 L h⁻¹) to reach 1 kW_e, with a system power efficiency (LHV basis) ca. 24% and a CHP efficiency over 87%. The ability to recycle the non-converted hydrogen from the fuel cell anode to the combustor and to use the heat produced at the fuel cell for obtaining hot water increased the global energy efficiency.     

Experimental results from a 5 kW PEM fuel cell stack operated on simulated reformate from highly diluted hydrocarbon fuels: Efficiency,dilution, fuel utilisation,CO poisoning and design criteria

The present article analyses the effects of dilute biogas on efficiency, fuel utilisation, dynamics, control strategy, and design criteria for a polymer electrolyte fuel cell (PEFC) system. The tested fuel compositions are exemplified by gas compositions that could be attained within various Swedish biofuel demonstration projects. Experimental data which can serve as a basis for design of PEFC biogas systems operating in load-following, or steady-state mode, are reported for a 5 kW PEFC stack.     

Thermodynamic assessment of solid oxide fuel cell system integrated with bioethanol purification unit

A solid oxide fuel cell system integrated with a distillation column (SOFC–DIS) has been proposed in this article. The integrated SOFC system consists of a distillation column, an EtOH/H₂O heater, an air heater, an anode preheater, a reformer, an SOFC stack and an afterburner. Bioethanol with 5 mol% ethanol was purified in a distillation column to obtain a desired concentration necessary for SOFC operation. The SOFC stack was operated under isothermal conditions. The heat generated from the stack and the afterburner was supplied to the reformer and three heaters. The net remaining heat from the SOFC system (Q_SOFC,Net) was then provided to the reboiler of the distillation column. The effects of fuel utilization and operating voltage on the net energy (Q_Net), which equals Q_SOFC,Net minus the distillation energy (Q_D), were examined. It was found that the system could become more energy sufficient when operating at lower fuel utilization or lower voltage but at the expense of less electricity produced. Moreover, it was found that there were some operating conditions, which yielded Q_Net of zero. At this point, the integrated system provides the maximum electrical power without requiring an additional heat source. The effects of ethanol concentration and ethanol recovery on the electrical performance at zero Q_Net for different fuel utilizations were investigated. With the appropriate operating conditions (e.g. C_EtOH = 41%, U_f = 80% and EtOH recovery = 80%), the overall electrical efficiency and power density are 33.3% (LHV) and 0.32 W cm⁻², respectively.     

Simulation analysis of a system combining solid oxide and polymer electrolyte fuel cells

We evaluated the performance of system combining a solid oxide fuel cell (SOFC) stack and a polymer electrolyte fuel cell (PEFC) stack by a numerical simulation. We assume that tubular-type SOFCs are used in the SOFC stack. The electrical efficiency of the SOFC–PEFC system increases with increasing oxygen utilization rate in the SOFC stack. This is because the amount of exhaust heat of the SOFC stack used to raise the temperature of air supplied to it decreases as its oxygen utilization rate increases and because that used effectively as the reaction heat of the steam reforming reaction of methane in the stack reformer increases. The electrical efficiency of the SOFC–PEFC system at 190 kW ac is 59% (LHV), which is equal to that of the SOFC-gas turbine combined system at 1014 kW ac.     

Conceptual design of a novel ammonia-fuelled portable solid oxide fuel cell system

A novel portable electric power generation system, fuelled by ammonia, is introduced and its performance is evaluated. In this system, a solid oxide fuel cell (SOFC) stack that consists of anode-supported planar cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode is used to generate electric power. The small size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. The results predicted through computer simulation of this system confirm that the first-law efficiency of 41.1% with the system operating voltage of 25.6 V is attainable for a 100 W portable system, operated at the cell voltage of 0.73 V and fuel utilization ratio of 80%. In these operating conditions, an ammonia cylinder with a capacity of 0.8 l is sufficient to sustain full-load operation of the portable system for 9 h and 34 min. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required in the SOFC stack, the first- and second-law efficiencies, the system operating voltage, the excess air, the heat transfer from the SOFC stack, and the duration of operation of the portable system with a cylinder of ammonia fuel, are also studied through a detailed sensitivity analysis. Overall, the ammonia-fuelled SOFC system introduced in this paper exhibits an appropriate performance for portable power generation applications.     

Application of adaptive neuro-fuzzy inference system techniques and artificial neural networks to predict solid oxide fuel cell performance in residential microgeneration installation

This study applies adaptive neuro-fuzzy inference system (ANFIS) techniques and artificial neural network (ANN) to predict solid oxide fuel cell (SOFC) performance while supplying both heat and power to a residence. A microgeneration 5 kW_el SOFC system was installed at the Canadian Centre for Housing Technology (CCHT), integrated with existing mechanical systems and connected in parallel to the grid. SOFC performance data were collected during the winter heating season and used for training of both ANN and ANFIS models. The ANN model was built on back propagation algorithm as for ANFIS model a combination of least squares method and back propagation gradient decent method were developed and applied. Both models were trained with experimental data and used to predict selective SOFC performance parameters such as fuel cell stack current, stack voltage, etc.     

Design,testing and mathematical modelling of a small-scale CHP and cooling system (small CHP-ejector trigeneration)

Trigeneration is the production of heat, cooling and power from one system. It can improve the financial and environmental benefits of combined heat and power (CHP) by using the heat output from the CHP unit to drive a cooling cycle, as demonstrated in existing large-scale installations. However, small-scale systems of a few kW_e output present technological challenges. This paper presents the design and analysis of possible trigeneration systems based on a gas engine mini-CHP unit (5.5 kW_e) and an ejector cooling cycle. Analysis shows that an overall efficiency around 50% could be achieved with systems designed for applications with simultaneous requirements for heat and cool. While using part of the CHP electrical output into the cooling cycle boosts the cooling capacity, it does not improve the overall efficiency and increases the CO₂ emissions of the system. Emissions savings compared to traditional systems could be achieved with improvements of the heat transfer from CHP to cooling cycle.