Modeling and optimization of a typical fuel cell–heat engine hybrid system and its parametric design criteria

A theoretical modeling approach is presented, which describes the behavior of a typical fuel cell–heat engine hybrid system in steady-state operating condition based on an existing solid oxide fuel cell model, to provide useful fundamental design characteristics as well as potential critical problems. The different sources of irreversible losses, such as the electrochemical reaction, electric resistances, finite-rate heat transfer between the fuel cell and the heat engine, and heat-leak from the fuel cell to the environment are specified and investigated. Energy and entropy analyses are used to indicate the multi-irreversible losses and to assess the work potentials of the hybrid system. Expressions for the power output and efficiency of the hybrid system are derived and the performance characteristics of the system are presented and discussed in detail. The effects of the design parameters and operating conditions on the system performance are studied numerically. It is found that there exist certain optimum criteria for some important parameters. The results obtained here may provide a theoretical basis for both the optimal design and operation of real fuel cell–heat engine hybrid systems. This new approach can be easily extended to other fuel cell hybrid systems to develop irreversible models suitable for the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Optimization and efficiency analysis of methanol SOFC-PEMFC hybrid system

In order to improve the power generation efficiency of fuel cell systems employing liquid fuels, a hybrid system consisting of solid oxide fuel cell (SOFC) and proton exchange membrane fuel cell (PEMFC) is proposed. Utilize the high temperature heat generated by SOFC to reform as much methanol as possible to improve the overall energy efficiency of the system. When SOFC has a stable output of 100 kW, the amount of hydrogen after reforming is changed by changing the methanol flow rate. Three hybrid systems are proposed to compare and select the best system process suitable for different situations. The results show that the combined combustion system has the highest power generation, which can reach 350 kW and the total electrical efficiency is 57%. When the power of the tail gas preheating system is 160 kW, the electrical efficiency can reach 75%. The PEM water preheating system has the most balanced performance, with the electric power of 300 kW and the efficiency of 66%.     

A new analytical approach to evaluate and optimize the performance of an irreversible solid oxide fuel cell-gas turbine hybrid system

Based on the current models of the solid oxide fuel cell and gas turbine, a generic model of a solid oxide fuel cell-gas turbine hybrid system is established, in which the multiple irreversibilities existing in real hybrid systems are taken into account. Expressions for the efficiencies and power outputs of both the subsystems and the hybrid system are analytically derived. The general performance characteristics of the hybrid system are revealed and the optimum criteria of the main performance parameters are determined. The effects of some key irreversibilities existing in the fuel cell and gas turbine on the performance of the hybrid systems are discussed in detail. The results obtained may be directly used to discuss some special cases of the hybrid system.     

Grid connected fuel cell and PV hybrid power generating system design with Matlab Simulink

Renewable energy sources have been taken the place of the traditional energy sources and especially rapidly developments of photovoltaic (PV) technology and fuel cell (FC) technology have been put forward these renewable energy sources (RES) in all other RES. PV systems have been started to be used widely in domestic applications connected to electrical grid and grid connected PV power generating systems have become widespread all around the world. On the other hand, fuel cell power generating systems have been used to support the PV generating so hybrid generation systems consist of PV and fuel cell technology are investigated for power generating. In this study, a grid connected fuel cell and PV hybrid power generating system was developed with Matlab Simulink. 160 Wp solar module was developed based on solar module temperature and solar irradiation by using real data sheet of a commercial PV module and then by using these modules 800 Wp PV generator was obtained. Output current and voltage of PV system was used for input of DC/DC boost converter and its output was used for the input of the inverter. PV system was connected to the grid and designed 5 kW solid oxide fuel cell (SOFC) system was used for supporting the DC bus of the hybrid power generating system. All results obtained from the simulated hybrid power system were explained in the paper. Proposed model was designed as modular so designing and simulating grid connected SOFC and PV systems can be developed easily thanks to flexible design.     

Canadian remote community power generation: How reformer and fuel cell systems compare with diesel generators

More than three quarters of Canadian remote communities rely solely on diesel generators for electricity generation. The diesel dependency of remote communities has inflated local per capita greenhouse gas emissions and resulted in rising and inconsistent electricity prices that have made community viability reliant on government subsidies. As the diesel generators approach the end of their lifespan replacement, technologies must be considered that will help transition Canadian remote communities from diesel to renewables. Replacing diesel generators with steam reformer and solid oxide fuel cell systems would allow for more efficient diesel generation and would benefit the future implementation of renewable power. A model was generated in Honeywell's UniSim Design Suite to simulate the performance of a diesel fed steam reformer and solid oxide fuel cell system. System operating parameters in the model were optimized to minimize the expected payback period. The system model outputs were compared with expected diesel generator performance for a test case remote community. The test community demonstrated that replacing diesel generators with the proposed steam reformer and solid oxide fuel cell system would result in annual net efficiency improvements of 32%. The efficiency improvement could potentially translate to reductions in carbon dioxide equivalents of over 258 kt and 20‐year savings of over $450 million if all diesel‐reliant Canadian remote communities switched to steam reformer and solid oxide fuel cell systems. In addition to immediate environmental and economic savings, the improved low load performance of the reformer and fuel cell system would allow for the future integration of renewable energy to create highly efficient diesel‐renewable hybrid power plants.     

Design performance analysis of pressurized solid oxide fuel cell/gas turbine hybrid systems considering temperature constraints

This study presents critical aspects and their influence on the performance of hybrid power systems combining a pressurized solid oxide fuel cell (SOFC) and a gas turbine (GT). Two types of hybrid system configurations with internal and external reforming have been analyzed. In order to examine the effect of matching between the fuel cell temperature and the turbine inlet temperature on the hybrid system performance, we considered air bypass after the compressor as well as additional fuel supply to the turbine side. This study focuses on the limitation of the temperature difference at the fuel cell stack and its influence on the performances of the two hybrid systems. Performances of the hybrid systems are also compared with those of simple SOFC systems, and the extent of performance enhancement is evaluated. The system with internal reforming gives better efficiency and power capacity for all design conditions than the system with external reforming under the same constraints. Its efficiency gain over the SOFC only system is considerable, while that of the system with external reforming is far less. As the temperature difference at the cell becomes smaller, the system performance generally degrades. The system with internal reforming is less influenced by the constraint of the cell temperature difference.     

Evaluation of the power generation efficiency and the amount of CO2 discharge from a microgrid using a fuel cell cascade system

The purpose of this study was to combine fuel cells with different operating temperatures into fuel cell cascade systems in order to analyze their power generation efficiency and environmental impact (CO₂ emissions). Nine fuel cell cascade systems were investigated by numerical analysis. We also proposed the use of these systems in microgrids. The power generation efficiency of a compound system containing a solid‐oxide fuel cell, a micro‐gas turbine, a reformer, and a proton‐exchange membrane fuel cell showed great improvement compared with simplex operation of each component of the system. Moreover, a fuel cell cascade system can use alcohol fuels with low CO₂ emission factors. The fuel cell cascade systems tested showed that CO₂ emission reductions are possible. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance analysis and parametric optimum criteria of a class of irreversible fuel cell/heat engine hybrid systems

Based on the current models of solid oxide fuel cells and two-heat-source heat engines consisting of two isothermal and two polytropic processes, a general model of a class of fuel cell/heat engine hybrid systems is established, in which multi-irreversibilities existing in real hybrid systems are taken into account. Expressions for the efficiency and power output of the hybrid systems are analytically derived from the model. The curves of the efficiency and power output of the hybrid systems varying with the current density and the efficiency versus power output curves are represented through numerical calculation. The general performance characteristics of the hybrid systems are revealed and the optimum criteria of the main performance parameters are determined. The effects of some key irreversibilities existing in the fuel cell, regenerator and two-heat-source heat engine on the performance of the hybrid systems are discussed in detail. The results obtained here are very general and may be directly used to derive the various interesting conclusions of the hybrid systems which are operated under different special cases.     

A unified model of high-temperature fuel-cell heat-engine hybrid systems and analyses of its optimum performances

On the basis of the models of various developed high-temperature fuel-cell heat-engine hybrid systems, a unified model of hybrid systems is proposed. General expressions for the power output and efficiency of hybrid systems, high-temperature fuel cells such as solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs), and heat engines including the Brayton, Otto, Diesel, Atkinson, Braysson, and Carnot engines are, respectively, derived by using the theories of electrochemistry and non-equilibrium thermodynamics. The effects of main irreversible losses existing in real fuel cells and heat engines on the performance of hybrid systems are investigated. The general performance characteristics and optimal operating regions of some of the key parameters of hybrid systems are discussed in detail. A variety of special typical cases are discussed. The important results in the literature can be readily reproduced, and the interesting findings of our study are presented.     

Hybrid power generation system of solar energy and fuel cells

This paper introduces the methods of integration of solar energy and low‐temperature solid oxide fuel cells. On the one hand, we design the system that integrates the solar photovoltaic cells and fuel cells. On the other hand, solar energy is concentrated to heat up the fuel cell and supply the working temperature at hundreds Celsius degrees by Fresnel lens. Then the fuel conversion efficiency is increased because of gain from the solar energy. Moreover, integration of solar thermal energy power system with the fuel is a good method for resolving the instability of solar energy. CHP (combined heat and power) is another aspect to enhance the design hybrid system overall efficiency. Finally, we present a novel device but built on different scientific principle. It can convert solar energy and chemical energy of fuel to electric energy simultaneously within the same device to integrated solar cell and fuel cell from the device level. Copyright © 2016 John Wiley & Sons, Ltd.     

Fuzzy neural control of a hybrid fuel cell/battery distributed power generation system

An innovative control strategy is proposed of hybrid distributed generation (HDG) systems, including solid oxide fuel cell (SOFC) as the main energy source and battery energy storage as the auxiliary power source. The overall configuration of the HDG system is given, and dynamic models for the SOFC power plant, battery bank and its power electronic interfacing are briefly described, and controller design methodologies for the power conditioning units and fuel cell to control the power flow from the hybrid power plant to the utility grid are presented. To distribute the power between power sources, the fuzzy switching controller has been developed. Then, a Lyapunov based-neuro fuzzy algorithm is presented for designing the controllers of fuel cell power plant, DC/DC and DC/AC converters; to regulate the input fuel flow and meet a desirable output power demand. Simulation results are given to show the overall system performance including load-following and power management of the system.     

Externally reformed solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid systems fueled by methanol and di-methyl-ether (DME)

Solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid power plants integrate a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200–500 kW). The SOFC–MGT systems currently developed are fueled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and di-methyl-ether (DME) (200–350 °C) is significantly lower than that of natural gas (700–900 °C), the reformer can be sited outside the stack. External reforming in SOFC–MGT plants fueled by methanol and DME enhances efficiency due to improved exhaust heat recovery and higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio (SCR)) must be carefully chosen to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fueled hybrid plant is approximately 240 °C, giving efficiencies of about 67–68% with a SOFC temperature of 900 °C (the efficiency is about 72–73% at 1000 °C). Similarly, for DME the optimum reforming temperature is approximately 280 °C with efficiencies of 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around one for methanol and 1.5–2 for DME.     

U.S. DOE fossil energy fuel cells program

The U.S. Department of Energy's (DOE) Office of Fossil Energy's (FE) National Energy Technology Laboratory (NETL), in partnership with private industry, educational institutions and national laboratories, is leading the development and demonstration of high efficiency, high temperature solid oxide fuel cells (SOFCs) and fuel cell turbine (FCT) hybrid power generation systems for stationary markets including auxiliary power units (APUs), distributed generation (DG) and large, coal-based central power plants. The DOE FE fuel cells program has three aspects: the Solid State Energy Conversion Alliance (SECA), Fuel Cell Coal Based Systems for central power, and the High Temperature Electrochemistry Center (HiTEC). The SECA goal is to decrease SOFC system cost to US$ 400 per kilowatt (kW) by 2010 for stationary markets. DOE FE is ultimately concerned with coal-based central power plants such as FutureGen. The goal is to aggregate SECA-type fuel cells into larger systems and to produce a very high efficiency megawatt-class FCT hybrid for testing at FutureGen. The low-cost, US$ 400 kW⁻¹ SECA FCT hybrid is a key component to achieving 60% efficiency by 2020. Advanced aspects of solid oxide technology are part of HiTEC R&D. Technical progress and advances are discussed for all three program aspects.     

Part load operation of SOFC/GT hybrid systems: Stationary analysis

In a global energetic context characterized by the increasing demand of oil and gas, the depletion of fossil resources and the global warming, more efficient energy systems and, consequently, innovative energy conversion processes are urgently required. A possible solution can be found in the fuel cells technology coupled with classical thermodynamic cycle technologies in order to make hybrid systems able to achieve high energy/power efficiency with low environmental impact. Moreover, due to the synergistic effect of using a high temperature fuel cell such as solid oxide fuel cell (SOFC) and a recuperative gas turbine (GT), the integrated system efficiency can be significantly improved. In this paper a steady zero dimensional model of a SOFC/GT hybrid system is presented. The core of the work consists of a performance analysis focused on the influence of the GT part load functioning on the overall system efficiency maintaining the SOFC power set to the nominal one. Also the proper design and management of the heat recovery section is object of the present study, with target a global electric efficiency almost constant in part load functioning respect to nominal operation. The results of this study have been used as basis to the development of a dynamic model, presented in the following part of the study focused on the plant dynamic analysis.     

An analysis of SOFC/GT CHP system based on exergetic performance criteria

In this study, we first consider developing a thermodynamic model of solid oxide fuel cell/gas turbine combined heat and power (SOFC/GT CHP) system under steady-state operation using zero-dimensional approach. Additionally, energetic performance results of the developed model are compared with the literature concerning SOFC/GT hybrid systems for its reliability. Moreover, exergy analysis is carried out based on the developed model to obtain a more efficient system by the determination of irreversibilities. For exergetic performance evaluation, exergy efficiency, exergy output and exergy loss rate of the system are considered as classical criteria. Alternatively, exergetic performance coefficient (EPC) as a new criterion is investigated with regard to main design parameters such as fuel utilization, current density, recuperator effectiveness, compressor pressure ratio and pinch point temperature, aiming at achieving higher exergy output with lower exergy loss in the system. The simulation results of the SOFC/GT CHP system investigated, working at maximum EPC conditions, show that a design based on EPC criterion has considerable advantage in terms of entropy-generation rate.     

Model of a novel pressurized solid oxide fuel cell gas turbine hybrid engine

Solid oxide fuel cell gas turbine (SOFC-GT) hybrid systems for producing electricity have received much attention due to high-predicted efficiencies, low pollution and availability of natural gas. Due to the higher value of peak power, a system able to meet fluctuating power demands while retaining high efficiencies is strongly preferable to base load operation. SOFC systems and hybrid variants designed to date have had narrow operating ranges due largely to the necessity of heat management within the fuel cell. Such systems have a single degree of freedom controlled and limited by the fuel cell. This study will introduce a new SOFC-GT hybrid configuration designed to operate over a 5:1 turndown ratio, while maintaining the SOFC stack exit temperature at a constant 1000 °C. The proposed system introduces two new degrees of freedom through the use of a variable-geometry nozzle turbine to directly influence system airflow, and an auxiliary combustor to control the thermal and power needs of the turbomachinery.     

Analysis of the design of a pressurized SOFC hybrid system using a fixed gas turbine design

Design characteristics and performance of a pressurized solid oxide fuel cell (SOFC) hybrid system using a fixed gas turbine (GT) design are analyzed. The gas turbine is assumed to exist prior to the hybrid system design and all the other components such as the SOFC module and auxiliary parts are assumed to be newly designed for the hybrid system. The off-design operation of the GT is modeled by the performance characteristics of the compressor and the turbine. In the SOFC module, internal reforming with anode gas recirculation is adopted. Variations of both the hybrid system performance and operating condition of the gas turbine with the design temperature of the SOFC were investigated. Special focus is directed on the shift of the gas turbine operating points from the original points. It is found that pressure loss at the fuel cell module and other components, located between the compressor and the turbine, shifts the operating point. This results in a decrease of the turbine inlet temperature at each compressor operating condition relative to the original temperature for the GT only system. Thus, it is difficult to obtain the original GT power. Two cell voltage cases and various degrees of temperature difference at the cell are considered and their influences on the system design characteristics and performance are comparatively analyzed.     

Hybrid acid–base polymer membranes prepared for application in fuel cells

For application in fuel cells, a series of hybrid acid–base polymer membranes were prepared by blending sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) with (3-aminopropyl)triethoxysilane (A1100) through a sol–gel process. As indicated by scanning electron microscopy, energy-dispersive X-ray analysis, and thermogravimetric analysis, the acid–base interaction improves not only the membrane homogeneity and thermal stability but also the mechanical strength and flexibility. Apart from the low cost, the developed membranes exhibit high proton conductivity and low methanol permeability as compared to Nafion^® 117. Further, the optimal membrane shows better performance than Nafion^® 117 in a single cell test. All these properties make the hybrid membranes suitable for application in fuel cells.     

Synergistic integration of a gas turbine and solid oxide fuel cell for improved transient capability

A theoretical solid oxide fuel cell–gas turbine hybrid system has been designed using a Capstone 60 kW micro-gas turbine. Through simulation it is demonstrated that the hybrid system can be controlled to achieve transient capability greater than the Capstone 60 kW recuperated gas turbine alone. The Capstone 60 kW gas turbine transient capability is limited because in order to maintain combustor, turbine and heat exchangers temperatures within operating requirements, the Capstone combustor fuel-to-air ratio must be maintained. Potentially fast fuel flow rate changes, must be limited to the slower, inertia limited, turbo machinery air response. This limits a 60 kW recuperated gas turbine to transient response rates of approximately 1 kW s⁻¹. However, in the SOFC/GT hybrid system, the combustor temperature can be controlled, by manipulating the fuel cell current, to regulate the amount of fuel sent to the combustor. By using such control pairing, the fuel flow rate does not have to be constrained by the air flow in SOFC/GT hybrid systems. This makes it possible to use the rotational inertia of the gas turbine, to buffer the fuel cell power response, during fuel cell fuel flow transients that otherwise limit fuel cell system transient capability. Such synergistic integration improves the transient response capability of the integrated SOFC gas turbine hybrid system. Through simulation it has been demonstrated that SOFC/GT hybrid system can be developed to have excellent transient capability.     

Load-Following Strategies for Evolution of Solid Oxide Fuel Cells Into Model Citizens of the Grid

Proper converter design can allow solid oxide fuel cells operated as distributed generators to mutually benefit both the load and the electric utility during steady-state conditions, but dynamic load variations still present challenges. Unlike standard synchronous generators, fuel cells lack rotating inertia and their output power ramp rate is limited by design. Two strategies are herein investigated to mitigate the impact of a large load perturbation on the electric utility grid: 1) external use of ultracapacitor electrical storage connected through a dc–dc converter and 2) internal reduction of steady-state fuel utilization in the fuel cell to enable faster response to output power perturbations. Both strategies successfully eliminate the impact of a load perturbation on the utility grid. The external ultracapacitor strategy requires more capital investment while the internal fuel utilization strategy requires higher fuel use. This success implies that there is substantial flexibility for designing load-following fuel cell systems that are model citizens.