Robust Design Techniques for Evaluating Fuel Cell Thermal Performance

The National Renewable Energy Laboratory (NREL) and Plug Power Inc. have been working together to develop fuel cell modeling processes to rapidly assess critical design parameters and evaluate the effects of variation on performance. This paper describes a methodology for investigating key design parameters affecting the thermal performance of a high temperature, polybenzimidazole (PBI)-based fuel cell stack. Nonuniform temperature distributions within the fuel cell stack may cause degraded performance, induce thermo-mechanical stresses, and be a source of reduced stack durability. The three-dimensional (3-D) model developed for this project includes coupled thermal/flow finite element analysis (FEA) of a multi-cell stack integrated with an electrochemical model to determine internal heat generation rates. Sensitivity and optimization algorithms were used to examine the design and derive the best choice of the design parameters. Initial results showed how classic design-of-experiment (DOE) techniques integrated with the model were used to define a response surface and perform sensitivity studies on heat generation rates, fluid flow, bipolar plate channel geometry, fluid properties, and plate thermal material properties. Probabilistic design methods were used to assess the robustness of the design in response to variations in load conditions. The thermal model was also used to develop an alternative coolant flow-path design that yields improved thermal performance. Results from this analysis were recently incorporated into the latest Plug Power coolant flow-path design. This paper presents an evaluation of the effect of variation on key design parameters such as coolant and gas flow rates and addresses uncertainty in material thermal properties.

Micromachined Methanol Steam Reforming System Integrated With Catalytic Combustor Using Carbon Nanotubes as Catalyst Supports

In this paper we have successfully demonstrated a new micromachined fuel processing system including vaporizer, catalytic combustor and methanol steam reformer. This fuel processing system utilizes the thermal energy generated from the catalytic hydrogen combustion to heat up the entire system. For the first time, we have used carbon nanotubes as a supporting structure of Pt catalyst for combustion. The catalytic combustor could supply the energy to heat the reformer and maintain its working temperature. We have also developed a new coating method of reforming catalyst (Cu/ZnO/Al2O3) and observed that adequate amount of hydrogen can be generated for PEMFC. We have successfully reported the feasibility of the proposed fuel processing system in each assembled component.

Methanol Reforming Over PT Catalysts for Polymer Electrolyte Membrane Fuel Cells

An experimental investigation on hydrogen generation from methanol using Pt catalysts is presented in this paper. Methanol has the advantages of high energy density, high reforming activity and low CO selectivity at low temperatures. At present Cu-based catalysts are widely used for methanol reforming. But they are pyrophoric and thermally unstable, which causes issues in operating a real system. Pt catalysts dispersed on cerium oxides were tested for methanol reforming to resolve the problems. Steam reforming over Pt/cerium oxides showed the low conversion ratio less than 90% and the high CO concentration of about 15% at 400 °C. Autothermal reforming by adding O2 rapidly promoted the conversion of methanol and reduced the concentration of CO at lower temperature. Increasing the amount of dispersed Pt, the range of 0.5–3.0 wt%, shifted the reforming trends towards lower temperature and decreased the concentration of CO. To achieve more production rate within a given catalyst bed, catalysts coated monolithic honeycomb is prepared. It showed very high conversion at space velocities of up to 60,000/h.

Partial Validation of a Proposed Rating Methodology for Residential Fuel Cell Systems

The National Institute of Standards and Technology (NIST), in conjunction with Virginia Tech, has developed a rating methodology for residential-scale stationary fuel cell systems. The methodology predicts the cumulative electrical production, thermal energy delivery, and fuel consumption on an annual basis. The annual performance is estimated by representing the entire year of climate and load data into representative winter, spring/fall, and summer days for six different U.S. climatic zones. It prescribes a minimal number of steady state and simulated use tests, which provide the necessary performance data for the calculation procedure that predicts the annual performance. The procedure accounts for the changes in performance resulting from changes in ambient temperature, electrical load, and, if the unit provides thermal as well as electrical power, thermal load. The rating methodology addresses four different types of fuel cell systems: grid-independent electrical load following, grid-connected constant power, grid-connected thermal load following, and grid-connected water heating. This paper will describe a partial validation of the rating methodology for a grid-connected thermal load following fuel cell system. The rating methodology was validated using measured data from tests that subjected the fuel cell system to domestic hot water and space heating thermal loads for each of the three representative days. The simplification of a full year’s load and climate data into three representative days was then validated by comparing the rating methodology predictions with the prediction of each hour over the full year in each of the six cities.

Design of Hydrogen Fuel Cell Autorickshaw

In Asia there are less private cars, but there is a high proportion of 2-stroke engines in scooters, motorcycles, auto-rickshaws (Tuk-Tuks), all running on petrol-oil mixtures with levels of hydrocarbon emissions (from partially burnt fuel and oil) well in excess of levels permitted in the USA and Europe. Worldwide Rickshaw/scooter/motorcycle type engine production is estimated at 17 million per year. According to National Transport Research Center (NTRC), the total population of registered (all types) motor vehicles in Pakistan in year 2000 was 4.224 million, out of which more than half of the population is (2.206 million) two wheelers or three wheelers (motorcycle/scooter/auto rickshaw). Almost all auto rickshaws have two stroke power packs and also 60% of motorcycle/scooters are of the same category. Pakistan is a very densely populated developing country, with very loose environment protection rules, which are practically unregulated due to large financial implications. This scenario leads to adverse air quality conditions especially in large cities of the country where the main contributory factors are vehicular traffic, that too, two stroke vehicles Industry, diesel-powered vehicles, and the omnipresent three-wheeled, two-stroke rickshaws all contribute to the extremely dirty air. Taxi/car use is increasing, but rickshaws have the advantage of being able to swarm through the congested car traffic in cities. This explains the over .6 million motorcycles/scooters/rickshaws currently in Pakistan, of which approximately 20% are two stroke Auto-rickshaws of 175 cc. Pakistan’s vehicle fleet has a growth rate of 8.0% (1990–99). The purpose of this study is to examine a particular application of fuel cell technology “The Auto Rickshaws”. They are small three-wheeled vehicles that can carry three people. Due to their small size and low price, rickshaws have traditionally been powered by high power density two-stroke internal combustion engines. Two-stroke engines produce a great deal of pollution and are an object of concern in many Asian countries. Severe pollution from two-stroke engines is a significant driver for cleaner technology. Thus, the target of this study is the Asian urban commuter, since a rickshaw is largely used in many Asian cities and contributes directly to air pollution in major crowded cities of Pakistan also. Countries like China, India, Bangladesh, Taiwan and Pakistan [1] are facing dramatic growth rates in two-stroke vehicle population as bicycle rickshaws are being replaced, so, low-powered but clean rickshaws would be a major step in providing mobility without compromising urban air quality.

Proton Exchange Membrane Fuel Cell System Identification and Control: Part II — H-Infinity Based Robust Control

This paper applies robust control strategies to a PEM fuel-cell system. In Part I of this work [17], a PEM fuel cell was described as a two-input-two-output system with the inputs of hydrogen and air flow rates, and the outputs of cell voltage and current. From the responses, system identification techniques were adopted to model the system transfer function matrix. Then adaptive control methods were applied to control the system with encouraging results. In this paper, the H∞ robust control strategy is proposed due to the highly nonlinear and time-varying characteristics of the system. From the results, it is illustrated to be an efficient control method for the fuel cell systems.

Proton Exchange Membrane Fuel Cell System Identification and Control: Part I — System Dynamics, Modeling and Identification

This paper consists of two parts to address a systematic method of system identification and control of a proton exchange membrane (PEM) fuel cell. This fuel cell is used for communication devices of small power, involving complex electrochemical reactions of nonlinear and time-varying dynamic properties. From a system point of view, the dynamic model of PEM fuel cell is reduced to a configuration of two inputs, hydrogen and air flow rates, and two outputs, cell voltage and current. The corresponding transfer functions describe linearized subsystem dynamics with finite orders and time-varying parameters, which are expressed as discrete-time auto-regression moving-average with auxiliary input models for system identification by the recursive least square algorithm. In experiments, a pseudo random binary sequence of hydrogen or air flow rate is fed to a single fuel cell device to excite its dynamics. By measuring the corresponding output signals, each subsystem transfer function of reduced order is identified, while the unmodeled, higher-order dynamics and disturbances are described by the auxiliary input term. This provides a basis of adaptive control strategy to improve the fuel cell performance in terms of efficiency, transient and steady state specifications. Simulation shows the adaptive controller is robust to the variation of fuel cell system dynamics.

Conversion of Syngas From Biomass in Solid Oxide Fuel Cells

Conversion of biomass in syngas by means of indirect gasification offers the option to improve the economic situation of any fuel cell systems due to lower costs for feedstock and higher power revenues in many European countries. The coupling of an indirect gasification of biomass and residues with highly efficient SOFC systems is therefore a promising technology for reaching economic feasibility of small decentralized combined heat and power production (CHP). The predicted efficiency of common high temperature fuel cell systems with integrated gasification of solid feedstock is usually significantly lower than the efficiency of fuel cells operated with hydrogen or methane. Additional system components like the gasifier, as well as the gas cleaning reduce this efficiency. Hence common fuel cell systems with integrated gasification of biomass will hardly reach electrical efficiencies above 30 percent. An extraordinary efficient combination is achieved in case that the fuel cells waste heat is used in an indirect gasification system. A simple combination of a SOFC and an allothermal gasifier enables then electrical efficiencies above 50%. But this systems requires an innovative cooling concept for the fuel cell stack. Another significant question is the influence of impurities on the fuel cells degradation. The European Research Project ‘BioCellus’ focuses on both questions — the influence of the biogenious syngas on the fuel cells and an innovative cooling concept based on liquid metal heat pipes. First experiments showed that in particular higher hydrocarbons — the so-called tars — do not have an significant influence on the performance of SOFC membranes. The innovative concept of the TopCycle comprises to heat an indirect gasifier with the exhaust heat of the fuel cell by means of liquid metal heat pipes. Internal cooling of the stack and the recirculation of waste heat increases the system efficiency significantly. This concept promises electrical efficiencies of above 50 percent even for small-scale systems without any combined processes.

Fast Design and Manufactured on Complex Flow Channel by Rapid Prototyping for Air-Breathing Polymer Electrolyte Membrane Fuel Cells

The miniature and air-breathing fuel cell has become the globally major design concepts of fuel cell development recently. In this paper, the authors used 3-D drafting software for fast design and utilize rapid prototyping (RP) technology to accelerate the prototype development of new stack designs and optimize the assembly method. A fast design and convenient manufacture tool, i.e., rapid prototyping, has been first successfully applied to the fabrication of the complicated flow channels of both DMFC and PEMFC in this paper. The honeycomb shape methanol reservoir and honeycomb cathode structure design of DMFC and a complex flow distributor design of mono-polar PEMFC stack, which are almost impossibly manufactured by traditional CNC manufacturing, is fabricated by rapid prototyping technology and illustrated for the extraordinary advantages of RP technology. This paper shows that the fast design and manufacture characteristics are more important for the feasibility study of a complicated structure and any new design ideas. Although the performance of air-breathing pseudo-polar DMFC is only 2.16 mW/cm2 in peak power density by using 50% of hydrophobic carbon paper; this poor performance is resort to the MEA of DMFC is not well prepared. The other example of the power density of 188 mW/cm2 (at 0.425 V) in parallel-connection and 123mW/cm2 (at 4.25V) in serial-connection for the air-breathing mono-polar PEMFC stack are achieved. The performance of the stack is close to the state-of-the-art comparing to recently published literatures [6–9].

Technological Aspects of Ethanol Steam Reforming Processors for Molten Carbonate Fuel Cells

Bioethanol, obtained by biomass fermentation, could be an important hydrogen supplier as a renewable source. The availability of active, selective and stable catalyst for bioethanol steam reforming is a key point for the development of processes suitable to this purpose. In this work, the performance of different supported catalysts in the steam reforming of bioethanol at molten carbonate fuel cell (MCFC) operative condition has been focused and a decreasing activity has been related to the formation of carbon. Furthermore catalytic behaviour of a Ni supported catalyst has been tested under reforming condition both distillation industry’s waste and ethanol-water mixture. Results revealed that, superior alcohols (fusel oil) arising from the distillation process influence carbon formation and the presence of oxygen (ATR condition) preserves the catalyst activity which otherwise significantly deactivate mainly due to the carbon formation.