Jay T. Pukrushpan

Control of fuel cell breathing

In this article we analyzed and designed air flow controllers that protect the fuel cell (FC) stack from oxygen starvation during step changes of current demand. The steady-state regulation of the oxygen excess ratio in the FCS cathode achieved by assigning an integrator to the compressor flow. Linear observability techniques were employed to demonstrate improvements in transient oxygen regulation when the FCS voltage is included as a measurement for the feedback controller. The FCS voltage signal contains high frequency information about the FC oxygen utilization, and thus, is a natural and valuable output for feedback. We used linear optimal control design to identify the frequencies at which there is a severe tradeoff between the transient system net power performance and the stack starvation control. The limitation arises when the FCS system architecture dictates that all auxiliary equipment is powered directly from the FC with no secondary power sources. This plant configuration is preferred due to its simplicity, compactness, and low cost. The FCS impedance given the closed-loop air flow and perfect humidification and temperature regulation captures the FC current-voltage dynamic relationship. It is expected that the closed-loop FCS impedance will provide the basis for the systematic design of FC stack electronic components.

Control-oriented modeling and analysis for automotive fuel cell systems

Fuel Cells are electrochemical devices that convert the chemical energy of a gaseous fuel directly into electricity. They are widely regarded as a potential future stationary and mobile power source. The response of a fuel cell system depends on the air and hydrogen feed, flow and pressure regulation, and heat and water management. In this paper, we develop a dynamic model suitable for the control study of fuel cell systems. The transient phenomena captured in the model include the flow and inertia dynamics of the compressor, the manifold filling dynamics (both anode and cathode), reactant partial pressures, and membrane humidity. It is important to note, however, that the fuel cell stack temperature is treated as a parameter rather than a state variable of this model because of its long time constant. Limitations and several possible applications of this model are presented.

Modeling and control for PEM fuel cell stack system

A nonlinear fuel cell system dynamic model that is suitable for control study is presented. The transient phenomena captured in the model include the flow characteristics and inertia dynamics of the compressor, the manifold filling dynamics, and consequently, the reactant partial pressures. Characterization of the fuel cell polarization curves based on time varying current, partial oxygen and hydrogen pressures, temperature, membrane hydration allows analysis and simulation of the transient fuel cell power generation. An observer based feedback and feedforward controller that manages the tradeoff between reduction of parasitic losses and fast fuel cell net power response during rapid current (load) demands is designed.

Simulation and Analysis of Transient Fuel Cell System Performance Based on a Dynamic Reactant Flow Model

Fuel cell stack systems are under intensive development by several manufacturers since they complement heat engines and reduce the ubiquitous dependence on fossil fuels and thus have significant environmental and national security implications. To compete with ICE engines, however, fuel cell system must operate and function at least as well as conventional engines. Transient behavior is on of the key requirements for the success of fuel cell vehicles. The fuel cell system power response depends on the air and hydrogen feed, flow and pressure regulation, and heat and water management. During transient, the fuel cell stack control system is required to maintain optimal temperature, membrane hydration, and partial pressure of the reactants across the membrane in order to avoid degradation of the stack voltage, and thus, efficiency reduction. In this paper, we developed a fuel cell system dynamic model suitable for control study. The transient phenomena captured in the model include the flow characteristics and inertia dynamics of the compressor, the manifold filling dynamics (both anode and cathode), and consequently, the time-evolving reactant partial pressures, and membrane humidity. The effects of varying oxygen concentration and membrane humidity on the fuel cell voltage were included. Simulation results are presented to demonstrate the model capability.Copyright

Control of natural gas catalytic partial oxidation for hydrogen generation in fuel cell applications

A fuel processor that reforms natural gas to hydrogen-rich mixture to feed the anode field of fuel cell stack is considered. The first reactor that generates the majority of the hydrogen in the fuel processor is based on catalytic partial oxidation of the methane in the natural gas. We present a model-based control analysis and design for a fuel processing system (FPS) that manages natural gas flow and humidified atmospheric air flow in order to regulate 1) the amount of hydrogen in the fuel cell anode and 2) the temperature of the catalytic partial oxidation reactor during transient power demands from the fuel cell. Linear feedback analysis and design is used to identify the limitation of a decentralized controller and the benefit of a multivariable controller. Further analysis unveils the critical controller cross coupling term that contributes to the superior performance of the multivariable controller.

Simple motion planning strategies for spherobot: a spherical mobile robot

Mobile robots have been traditionally designed with wheels and few have explored designs with spherical exo-skeletons. A spherical mobile robot that offers to have a number of advantages, is proposed in the paper. The success of our design is contingent upon development of control strategies for reconfiguration of the sphere. We address the open-loop control problem and present two strategies for reconfiguration. The first strategy uses spherical triangles to bring the sphere to a desired position with a desired orientation. The second strategy uses a specific kinematic model and generates a trajectory comprising straight lines and circular arc segments. As compared to existing motion planners, our strategies require less computation and provide scope for easy implementation.

Dynamics of low-pressure and high-pressure fuel cell air supply systems

Choosing an operating pressure is a critical decision that defines many characteristics of the fuel cell system. High stack power density is the main selling point of high-pressure systems. On the contrary, low-pressure systems have a benefit of low parasitic loss on air flow devices and less sealing and membrane fracture problems. Here, we look at the dynamic characteristics of both high-pressure and low-pressure systems. The model of a low-pressure air supply system with low-speed blowers is developed. We then perform a dynamical analysis comparing the low pressure system with the high-pressure system equipped with an air compressor. The differences in the transient behavior of the two systems are investigated and their sensitivity to the flow device inertia and the supply manifold volume are explored.

Fuel Cell APU for Silent Watch and Mild Electrification of a Medium Tactical Truck

This paper investigates the opportunities for improving truck fuel economy through the use of a Fuel Cell Auxiliary Power Unit (FC APU) for silent watch, as well as for powering electrified engine accessories during driving. The particular vehicle selected as the platform for this study is a prototype of the Family of Medium Tactical Vehicles (FMTV) capable of carrying a 5 ton payload. Peak stand-by power requirements for on-board power are determined from the projected future digitized battlefield vehicle requirements. Strategic selection of electrified engine accessories enables engine shutdowns when the vehicle is stopped, thus providing additional fuel savings. Proton Exchange Membrane (PEM) fuel cell is integrated with a partial oxidation reformer in order to allow the use of the same fuel (JP8) as for the propulsion diesel engine. The APU system is modeled and linked with the complete vehicle system simulation, and accessory duty cycles are derived for both silent watch and driving. The results indicate six-fold improvements of the silent watch fuel economy with the FC APU compared to main-engine idling, and relatively modest improvements from the mild electrification and FC APU use during the driving cycle. Combined fuel economy benefits calculated over the hypothetical daily military mission with a combined 10 hour highway/local/off-road driving and 10 hour silent watch are 20.1%.

Control-Oriented Model of an Integrated Fuel Cell Stack and Fuel Processor System

Abstract A control-oriented dynamic model of a catalytic partial oxidation-based fuel processor is developed using physics-based principles. The fuel processor converts a hydrocarbon fuel to a hydrogen rich mixture that is directly feed to the PEM-FC. Cost and performance requirements of the total powertrain typically lead to highly integrated designs and stringent control objectives. Physics based component models are extremely useful in understanding the system level interactions, implications on system performance and in model-based controller design. The model can be used in a multivariable analysis to determine characteristics of the system that might limit performance of a controller or a control design. The model can also be used to assist in measurement selection and to develop a plant observer to predict or estimate critical plant variables and conditions.

Control of Natural Gas Fuel Processor

During changes in the stack current, the fuel processor needs to (i) quickly regulate the amount of hydrogen in the fuel cell stack (anode) to avoid starvation or wasted hydrogen [79] and (ii) maintain a desired temperature of the CPDX catalyst bed for high conversion efficiency [91]. Accurate control and coordination of the fuel processor reactant flows can prevent both large deviation of hydrogen concentration in the anode and large excursion of the CPDX catalyst bed temperature.