Stand-alone Solid Oxide Fuel Cell Research Articles

Exergy assessment and optimization of a cogeneration system based on a solid oxide fuel cell integrated with a Stirling engine

A cogeneration system based on a methane-fed solid oxide fuel cell (SOFC) integrated with a Stirling engine is analyzed from the viewpoints of energy and exergy. The effects on the system performance are investigated of varying four key system parameters: current density, SOFC inlet temperature, compression ratio and regenerator effectiveness. The energy efficiency of the combined system is found to be 76.32% which is about 24.61% more than that of a stand-alone SOFC plant under the same conditions. Considering exergy efficiency as the only objective function, it is found that, as the SOFC inlet temperature increases, the exergy efficiency of the cogeneration system rises to an optimal value of 56.44% and then decreases. The second law analysis also shows that the air heat exchanger has the greatest exergy destruction rate of all system components. The cooling water of the engine also can supply the heating needs for a small home.

Exergy and economic comparison between kW-scale hybrid and stand-alone solid oxide fuel cell systems

Abstract Although hybrid solid oxide fuel cell (SOFC) microturbine systems generate power more efficiently than stand-alone SOFC systems, hybrid systems remain in the demonstration phase. This study compares a hybrid system's exergetic and economic performance with that of a stand-alone system. Both systems meet a university building's kW-scale power demand. The hybrid system operates at 66 % exergetic efficiency, and the stand-alone system operates at 59 % exergetic efficiency. Increasing the fuel cell's operating voltage increases the systems' exergetic efficiencies, and varying the fuel cell's temperature, pressure, and fuel utilization influences the systems' exergetic performances, though to a lesser extent. This study calculates the systems' life cycle costs. We find that the systems' life cycle costs depend significantly on the systems' operation. During baseline operation, the hybrid system costs less than the stand-alone system. After optimizing the systems during cogeneration operation, the hybrid system costs slightly more than the stand-alone system. Overall, our findings support hybrid systems' continued research and development; it is recommended that future work simulate hybrid and stand-alone systems under a range of thermal-to-electric ratios to reflect different building types and operation.

Input–Output Linearization-Based Controller Design for Stand-Alone Solid Oxide Fuel Cell Power Plant

Application of input–output linearization control for improving the dynamic response of a stand-alone solid oxide fuel cell (SOFC) system is presented in this work. The proposed controllers are incorporated in the study system to eliminate the sluggishness in the hydrogen and oxygen partial pressure responses due to variations in the SOFC reference current. Temperature dynamics of SOFC is taken into consideration to replicate the impact of temperature variation on the system dynamics. Eigenvalue study of the linearized zero dynamics is conducted to determine the stability of the unobservable part of the system, and the remaining dynamics is considered for designing the input–output linearization-based controllers. Pole placement technique is adopted to obtain the controller gains which give the desired dynamic response. The performance of the proposed technique is compared with that of the optimized proportional–integral (PI) controller. The proposed controller efficiency is also tested under varying operating conditions and measurement noise. Simulation results show the efficacy and supremacy of the proposed method in improving the dynamic response of the study system.

Optimal design of proportional-integral controllers for stand-alone solid oxide fuel cell power plant using differential evolution algorithm.

This paper proposes the application of differential evolution (DE) algorithm for the optimal tuning of proportional–integral (PI) controller designed to improve the small signal dynamic response of a stand-alone solid oxide fuel cell (SOFC) system. The small signal model of the study system is derived and considered for the controller design as the target here is to track small variations in SOFC load current. Two PI controllers are incorporated in the feedback loops of hydrogen and oxygen partial pressures with an aim to improve the small signal dynamic responses. The controller design problem is formulated as the minimization of an eigenvalue based objective function where the target is to find out the optimal gains of the PI controllers in such a way that the discrepancy of the obtained and desired eigenvalues are minimized. Eigenvalue and time domain simulations are presented for both open-loop and closed loop systems. To test the efficacy of DE over other optimization tools, the results obtained with DE are compared with those obtained by particle swarm optimization (PSO) algorithm and invasive weed optimization (IWO) algorithm. Three different types of load disturbances are considered for the time domain based results to investigate the performances of different optimizers under different sorts of load variations. Moreover, non-parametric statistical analyses, namely, one sample Kolmogorov–Smirnov (KS) test and paired sample t test are used to identify the statistical advantage of one optimizer over the other for the problem under study. The presented results suggest the supremacy of DE over PSO and IWO in finding the optimal solution.

Energy and exergoeconomic evaluation of a new power/cooling cogeneration system based on a solid oxide fuel cell

A new cogeneration system consisting of a hydrogen-fed SOFC (solid oxide fuel cell), a GT (gas turbine) and a GAX (generator-absorber-heat exchange) absorption refrigeration cycle is proposed and analyzed in detail. The electrochemical equations for the fuel cell and thermodynamic and exergoeconomic relations for the system components are solved simultaneously with EES (Engineering Equation Solver) software. Through a parametric study, the influences of such decision parameters as current density, fuel utilization factor, pressure ratio and air utilization factor on the performance of the system are studied. In addition, using a genetic algorithm, the system performance is optimized for maximum exergy efficiency or minimum SUCP (sum of the unit costs of products). The results show that, the exergy efficiency of the proposed system is 6.5% higher than that of the stand-alone SOFC. It is also observed that the fuel cell stack contributes most to the total irreversibility. The exergoeconomic factor, the capital cost rate and the exergy destruction cost rate for the overall system are observed to be 27.3%, 10.63 $/h and 28.3 $/h, respectively. It is observed that for each 6 $/GJ increase in the hydrogen unit cost, the optimum sum of the unit costs of products is increased by around 62.5 $/GJ.

Modeling and Model-based Analysis of a Solid Oxide Fuel Cell Thermal-Electrical Management System with an Air Bypass Valve

For stand-alone solid oxide fuel cell (SOFC) power systems without grid connection, delivering demanded power, maintaining system temperature constraints, and achieving high system efficiency are the key design objectives. A typical SOFC system consists of an SOFC stack and a blower, a heat exchanger, an exhaust-burner as the primary components in its balance of the plant. In this work, a novel SOFC system design with an air bypass valve and a target operating range of 1 to 5kW is considered. A system dynamic model, which captures the spatial electrical and thermal distributions of stack, is developed and optimal operating points to maintain the maximum steady state system efficiency are explored while enforcing four constraints on temperature and temperature gradient. Furthermore, the effects of the control variables, namely the bypass valve opening ratio (BP), voltage (U), fuel utilization (FU), and air excess ratio (AR) in improving system performance and expanding feasible operating range are quantified. In addition, the open-loop optimal operation strategy is obtained. The analysis demonstrates that the design of the stand-alone SOFC system is feasible and the bypass valve plays an important role in improving the overall system performance and operation ability.

SOFC Lifetime Assessment in Gas Turbine Hybrid Power Systems

The adoption of solid oxide fuel cell (SOFC) technology in power generation has been limited, in no small part, by material degradation issues affecting the stack lifetime, and hence, the economic viability. A numeric study was conducted to determine if the life of an SOFC could be extended when integrated with a recuperated gas turbine system. Dynamic modeling tools developed at the National Energy Technology Laboratory (NETL) for real-time applications were applied to evaluate life to failure for both a standalone SOFC and a hybrid SOFC gas turbine. These models were modified using empirical relations to experimental degradation data to incorporate degradation as a function of current density and fuel utilization. For the control strategy of shifting power to the turbine as fuel cell voltage degrades, the SOFC life could be extended dramatically, significantly impacting the economic potential of the technology.

Modeling and Analysis of Transport Processes and Efficiency of Combined SOFC and PEMFC Systems

A hybrid fuel cell system (~10 kWe) for an average family house including heating is proposed. The investigated system comprises a Solid Oxide Fuel Cell (SOFC) on top of a Polymer Electrolyte Fuel Cell (PEFC). Hydrogen produced from the off-gases of the SOFC can be fed directly to the PEFC. Simulations for the proposed system were conducted using different fuels. Here, results for natural gas (NG), dimethyl ether (DME) and ethanol as a fuel are presented and analysed. Behaviour of the proposed system is further investigated by comparing the effects of key factors such as utilisation factor, operating conditions, oxygen-to-carbon (O/C) ratios and fuel preheating effects on these fuels. The combined system improves the overall electrical conversion efficiency compared with standalone PEFC or SOFC systems. For the combined SOFC and PEFC system, the overall power production was increased by 8%–16% and the system efficiency with one of the fuels is found to be 12% higher than that of the standalone SOFC system.

Shuttle-Battery: High Capacity Rechargeable Metal-Air Energy Storage System

Shuttle-Battery is a rechargeable, standalone solid oxide fuel cell (SOFC) that consists of the air electrode, solid electrolyte, anode electrode, and the anode fuel metal in a closed anode compartment with an estimated energy density exceeding 1600 Wh/L. An example of the anode fuel metal is iron. Instead of providing a continuous stream of gaseous fuel, only a small amount of hydrogen is enclosed in the anode compartment that works as an oxygen carrier between the anode electrode and the anode fuel metal. During discharge, hydrogen gas reacts with oxygen anion carried through the solid electrolyte from the air electrode to produce water [Eq. 2]. The water vapor generated at the fuel electrode is readily reduced to hydrogen gas by reacting with the anode fuel metal [Eq. 3]. This reaction causes partial oxidation of the fuel metal. This regenerated hydrogen molecule reacts again with the oxygen anion at the anode electrode, producing water vapors and the electrical power. These cycles are repeated and keep generating electricity until all the metal in anode fuel material is oxidized to the corresponding oxide. For charging, the electrical current is supplied to induce the reversed reactions, re-activating the oxidized fuel metal by reduction with hydrogen [Eq. 3 reverse] generated by electrolysis of water [Eq. 2 reverse]. In Shuttle-Battery, hydrogen is simply carrying oxygen back and forth between the anode electrode and the anode fuel metal. It is noted that this mechanism is different from conventional SOFCs where the gaseous fuel is consumed to create electricity. The discharge/charge reactions of Shuttle-Battery are described in the following schemes (rightward reactions represent the discharge and leftward reactions represent the charge reactions). The overall reaction is simply an oxidation of the anode fuel metal by oxygen from air [Eq. 4]. In that sense, Shuttle-Battery has concept much closer to other metal-air batteries compared to fuel cells.Cathode: ½ O2 + 2e- = O2- [Eq. 1]Anode: H2 + O2- = H2O + 2e- [Eq. 2]Anode compartment: xM + H2O = MxO + H2(M: Metal) [Eq. 3]Total reaction: xM + ½ O2 = MxO [Eq. 4]Since Shuttle-Battery does not require any continuous supply of gaseous fuel or the gas storage, Shuttle-Battery system is much simpler than conventional fuel cell systems, making it less expensive. Elimination of compressed gaseous fuels also makes Shuttle-Battery intrinsically safe. Also, unlike other metal-air batteries such as lithium-air, Shuttle-Battery does not use highly reactive metal fuel that could combust when exposed to air.The capacity of Shuttle-Battery is determined by the amount of the anode fuel metal in the anode compartment. Unlike other rechargeable batteries, the power generation component and the active metal component are physically independent and are connected only by the gaseous oxygen carrier. Thus, it is possible to increase the amount of the anode fuel metal without compromising the performance of the power generation component. The operation voltage is dictated by the redox reaction potential of the oxygen carrier gas. The calculated energy density exceeds 1600 Wh/L. To date, Connexx Systems has proved the concept of Shuttle-SOFC and demonstrated more than 200 cycles with round trip efficiency of 80%, with depth of discharge at 100%. We also developed nano-sized iron powder with the proprietary surface modification technology to extend the cycle life of Shuttle-Battery.

Steady State Analysis of 5kW SOFC System with Novel Configuration

For stand-alone solid oxide fuel cell (SOFC) systems, delivering demanded power, maintaining system operating constraints, and achieving high system efficiency are the primary design objectives. We consider a novel SOFC design that includes an air bypass valve to achieve greater controllability. Through model-based analysis, the steady-state system performance is investigated. And the optimal operating points (OOPs) for different loads are explored to achieve the high system efficiency. The effects of bypass valve are quantified. The analysis demonstrates that the design is feasible and the bypass valve can play a substantial role on improving the overall system performance and operationability.

Thermodynamic analysis of SOFC (solid oxide fuel cell)–Stirling hybrid plants using alternative fuels

A novel hybrid power system (∼10 kW) for an average family home is proposed. The system investigated contains a solid oxide fuel cell (SOFC) on top of a Stirling engine. The off-gases produced in the SOFC cycle are fed to a bottoming Stirling engine, at which additional power is generated. Simulations of the proposed system were conducted using different fuels, which should facilitate the use of a variety of fuels depending on availability. Here, the results for natural gas (NG), ammonia, di-methyl ether (DME), methanol and ethanol are presented and analyzed. The system behavior is further investigated by comparing the effects of key factors, such as the utilization factor and the operating conditions under which these fuels are used. Moreover, the effect of using a methanator on the plant efficiency is also studied. The combined system improves the overall electrical efficiency relative to that of a stand-alone Stirling engine or SOFC plant. For the combined SOFC and Stirling configuration, the overall power production was increased by approximately 10% compared to that of a stand-alone SOFC plant. System efficiencies of approximately 60% are achieved, which is remarkable for such small plant sizes. Additionally, heat is also produced to heat the family home when necessary.

A flexible model integration approach for evaluating tradeoffs between CO2 emissions and cost in solid oxide fuel cell-based building energy systems

A simulation framework for flexible evaluation of various distributed building energy systems based on the integration of component device simulation models is presented. Device technology models were constructed for a solid oxide fuel cell (SOFC), a gas turbine, a double pipe heat exchanger, and a compressor. A scheme is proposed for defining model interfaces in order to improve the flexibility and accessibility of the models. Based on that scheme, interfaces are defined for each device model. The component device models are integrated to construct system models of (1) a hybrid system combining an SOFC and a gas turbine (SOFC/GT system) and (2) a stand-alone SOFC system. The integrated model of the SOFC/GT system is then used to carry out a multi-objective optimization in order to study the tradeoffs between cost and CO2 emissions of the SOFC system operation for a given electricity demand. Through these analyses, the optimal configuration of the SOFC/GT system and the optimal operation conditions of the SOFC system for the given electricity demand were explored. Copyright © 2005 John Wiley & Sons, Ltd.