N. Woudstra

Analytical fuel cell modeling; non-isothermal fuel cells

Abstract The isothermal fuel cell model, given in an earlier publication, will be generalized to describe the behaviour of non-isothermal fuel cells of co-flow type. To this end the temperalure distribution inside a fuel cell in steady state is investigated analytically. A simplified relation between the local temperature and the fuel utilization is derived and its practical significance elucidated. Furthermore, it is shown that the solution of the non-isothermal model is accurately approximated by analytical expressions obtained from a so-called quasi-isothermal approach. This new approach yields a similar expression for the cell voltage as derived from the isothermal model. The quasi-isothermal approach is also used to make a clear comparison between the isothermal and the non-isothermal fuel cell model.

Flexible Coproduction of Hydrogen and Power Using Internal Reforming Solid Oxide Fuel Cells System

Within the framework of the Greening of Gas project, in which the feasibility of mixing hydrogen into the natural gas network in the Netherlands is studied, we are exploring alternative hydrogen production methods. Fuel cells are usually seen as the devices that convert hydrogen into power and heat. It is less well known that these electrochemical energy converters can produce hydrogen, or form an essential component in the systems for coproduction of hydrogen and power In this paper, the coproduction of hydrogen-rich syngas (that can be converted into hydrogen) and power from natural gas in an internal reforming fuel cell is worked out by flow sheet calculations on an internal reforming solid oxide fuel cell system. The goal of this paper is to study the technical feasibility of such a system and explore its possibilities and limitations for a flexible coproduction. It is shown that the system can operate in a wide range of fuel utilization values at least down to 60% representing highest hydrogen production mode up to 95% corresponding to standard FC operation mode.

The Thermodynamic Evaluation and Optimization of Fuel Cell Systems

Energy conversion today is subject to high thermodynamic losses. About 50% to 90% of the exergy of primary fuels is lost during conversion into power or heat. The fast increasing world energy demand makes a further increase of conversion efficiencies inevitable. The substantial thermodynamic losses (exergy losses of 20% to 30%) of thermal fuel conversion will limit future improvements of power plant efficiencies. Electrochemical conversion of fuel enables fuel conversion with minimum losses. Various fuel cell systems have been investigated at the Delft University of Technology during the past 20 years. It appeared that exergy analyses can be very helpful in understanding the extent and causes of thermodynamic losses in fuel cell systems. More than 50% of the losses in high temperature fuel cell (molten carbonate fuel cell and solid oxide fuel cell) systems can be caused by heat transfer. Therefore system optimization must focus on reducing the need for heat transfer as well as improving the conditions for the unavoidable heat transfer. Various options for reducing the need for heat transfer are discussed in this paper. High temperature fuel cells, eventually integrated into gas turbine processes, can replace the combustion process in future power plants. High temperature fuel cells will be necessary to obtain conversion efficiencies up to 80% in the case of large scale electricity production in the future. The introduction of fuel cells is considered to be a first step in the integration of electrochemical conversion in future energy conversion systems.

Modeling of a Direct Carbon Fuel Cell System

Direct carbon fuel cells DCFCs, also known in literature as direct carbon conversion cells, DCCCs offer specific thermodynamic advantages compared with other fuel cell types. DCFCs electrochemically convert solid carbon fuel into CO2 and/or CO. Because the entropy change in the overall cell reaction is around zero for CO2 as reaction product or even positive for CO as product, the theoretical reversible electric efficiency commonly defined as G /H of the overall cell reaction of a DCFC can be around 100%. The development of DCFCs has been limited by low anode reaction rates, accumulation of impurities in the electrolyte and logistics of refueling the cell. These problems are being addressed by recent developments in the production of clean and highly reactive carbon materials e.g., from natural gas, lowcost techniques for separation of ash from coal, the possibility of pneumatic distribution of solid particulate fuel to the cells or the use of a slurry of carbon particles in a molten carbonate, and the availability of technology developed for the MCFC developments electrodes and electrolytes1. The DCFC configuration and theoretical principles resemble those of the MCFC and solid oxide fuel cell SOFC. The reactions that take place in the cell are comparable and cathode, electrolyte, and current collector materials are similar. This paper will not focus on cell design, cell configuration, and cell material details. We will take the design presented in Ref. 1 and assume that it can be scaled up and operated at reasonable current density 100‐150 mA/cm 2 . Research and development R&D has been focused on development of the cell itself on a lab scale and bench scale. Steinberg and co-workers 2,3 have proposed a number of system configurations with a DCFC, but up till now no flow sheet calculations on a DCFC system have been performed. In this study, we will evaluate the DCFC system performance by flow sheet calculations using the program CYCLE-TEMPO

The application of a fuel cell-electrolyzer arrangement as a power balancing set-up in autonomous renewable energy systems

Small-scale autonomous renewable energy systems have gained attention during the last years due to growing concerns in relation to an increasing world energy demand and to constraints in CO2 emissions. Polymer electrolyte membrane fuel cells (PEMFC), wind turbines and solar panels are promising zero-emission devices to be incorporated into these systems. In order to integrate them, appropriate control designs are necessary, among other aspects. This paper presents a configuration that allows a proper operation of the fuel cell while the system is able to handle the power fluctuations produced by the wind turbine and the load. An electrolyzer is used to take advantage of the power surplus. The system is designed to supply 10 households, but can be easily extended. The objective of this study is to evaluate the technical feasibility of implementing such a power balancing set-up in DENLab, a renewable energy laboratory at Delft University of Technology, the Netherlands.

Flowsheet calculation of a combined heat and power fuel cell plant with a conceptual molten carbonate fuel cell with separate CO2 supply

Abstract A new type of MCFC with a separate CO 2 supply (improved or i-MCFC) is previously presented, which has the potential for reducing NiO cathode dissolution and system enhancement by CO 2 removal from fuel gas. This article presents the first flowsheet calculations of an i-MCFC system that utilizes the potential of reducing NiO dissolution. A submodel that simulates energy and massflows of the i-MCFC is built using standard flowsheeting components. The performance of the i-MCFC is assumed to be equal to the MCFC and differences in Nernst potentials and irreversible losses are neglected. To compare the differences in concept, a MCFC combined heat and power (CHP) system flowsheet is modified and the MCFC model substituted by the i-MCFC submodel. The overall efficiencies of both fuel cell systems are calculated using a flowsheeting program. The calculated results are compared and the differences analyzed. The overall system performance of this i-MCFC CHP system is slightly lower than the MCFC CHP reference system (about 0.1% point in average). The difference in performance is ascribed to the change in gas composition and heat capacity of the cathode gas. The change in heat capacity increases the total massflow through the i-MCFC resulting to an increase in overall auxiliary power consumption. The low CO 2 content of the cathode gas should reduce the NiO cathode dissolution to a negligible level.

Internal Reforming SOFC System for Flexible Coproduction of Hydrogen and Power

In the framework of the project Greening of Gas, in which the feasibility of mixing hydrogen into the natural gas network in the NL is studied, we are exploring alternative hydrogen production methods. Fuel cells are usually only seen as devices that convert hydrogen into power and heat. It is less well known that these electrochemical energy converters can produce hydrogen, or form an essential component in systems for co-production of hydrogen and power. Co-production of hydrogen and power from NG in an Internal reforming fuel cell (IR FC) is worked out by flow sheet calculations on an Internal reforming Solid Oxide fuel cell (IR-SOFC) system. It is shown that the system can operate in a wide range of fuel utilization values at least from 60% representing highest hydrogen production mode to 95% corresponding to ‘normal’ fuel cell operation mode. For the atmospheric pressure system studied here hydrogen and CO content increase up to 22.6 and 13.5 % respectively at a fuel utilization of 60%. Total system efficiency (power + H2 /CO) is increasing significantly at lower fuel utilization and can reach 94 %. Our study confirms that the calculations of Vollmar et al1 ) on an IR-SOFC stack also hold for a complete FC system. Notably that paradoxically a system with the same fuel cell stack when switched to hydrogen production mode can yield more power in addition to the H2 and CO produced. This is because the hydrogen production mode allows for operation at high current and power densities. The same system can double its power output (e.g. from 1.26 MW to 2.5 MW) while simultaneously increasing the H2 /CO output to 3.1MW). Economics of these systems is greatly improved. These systems can also be considered for hydrogen production for the purpose of mixing it with natural gas in the natural gas grid in order to reduce CO2 emissions at the end users, because of the ability to adopt the system rapidly to fluctuations in natural gas/hydrogen demand.Copyright

Designing Solid Oxide Fuel Cell Gas Turbine Hybrid Systems Using Exergy Analysis

In conventional gas turbine systems combustion results in high exergy losses (∼30%) of fuel exergy input. Replacing the combustor with a high temperature fuel cell, like the Solid Oxide Fuel Cell (SOFC), will significantly reduce these exergy losses. As the SOFC electrochemically converts the natural gas, exergy losses are far lower (∼10%) compared to combustion. Natural gas entering a SOFC system has to be reformed first to hydrogen and carbon monoxide by steam reforming. Here it is chosen to use the heat generated by the fuel cell to drive the endothermic reforming reactions: internal reforming. The SOFC-GT system has the advantage that both fuel cell and gas turbine technology contribute to power production. In earlier work [1] several fuel cell system configurations with PEMFC, MCFC or SOFC, were analyzed studying the exergy flows. Here is focused on the SOFC-GT configuration, to get a detailed understanding of the exergy flows and losses through all individual components. Several configurations, combining the SOFC with the GT are possible. The selected operating conditions should prevent carbon deposition. Systems studies are performed to get more insight in the exergy losses in these combined systems. Exergy analysis facilitates the search for the high efficient SOFC-GT hybrid systems. Using exergy analysis, several useful configurations are found. Exergy losses are minimized by varying pressure ratio and turbine inlet temperature. Sensitivity studies, of equivalent cell resistance and fuel cell temperature, show that total system exergy efficiencies of more than 80% are conceivable, without using a bottoming cycle.Copyright

Modeling of a Methane Fuelled Direct Carbon Fuel Cell System

Energy conversion today is subject to high thermodynamic losses. About 50 to 90 % of the exergy of primary fuels is lost during conversion into power or heat. The fast increasing world energy demand makes a further increase of conversion efficiencies inevitable. The substantial thermodynamic losses (exergy losses of 20 to 30 %) of thermal fuel conversion will limit future improvements of power plant efficiencies. Electrochemical conversion of fuel enables fuel conversion with minimum losses. Various fuel cell systems have been investigated at the Delft University of Technology during the past twenty years. It appeared that exergy analyses can be very helpful in understanding the extent and causes of thermodynamic losses in fuel cell systems. More than 50 % of the losses in high temperature fuel cell (MCFC and SOFC) systems can be caused by heat transfer. Therefore system optimisation must focus on reducing the need for heat transfer as well as improving the conditions for the unavoidable heat transfer. Various options for reducing the need for heat transfer are discussed in this paper. High temperature fuel cells, eventually integrated into gas turbine processes, can replace the combustion process in future power plants. High temperature fuel cells will be necessary to obtain conversion efficiencies up to 80 % in case of large scale electricity production in the future. The introduction of fuel cells is considered to be a first step in the integration of electrochemical conversion in future energy conversion systems.Copyright

Solid oxide fuel cell integrated with biomass gasification for power generation for rural areas in China

Fuel cells coupled with biomass gasifiers can offer the potential of highly efficient and renewable power generation in an environmentally friendly and CO2-neutral manner. Three key aspects, i.e. the fuel resources and fuel cell types as well as gas cleaning for this combined system were discussed to present the feasibility of applying biomass gasification (BG) based solid oxide fuel cell (SOFC) system for distributed power generation in developing countries. The performance of BG-SOFC based combined heat and power (CHP) system and future work was also presented.