J.W. Howze

Development of an equivalent circuit model of a fuel cell to evaluate the effects of inverter ripple current

In this paper an impedance model of the proton exchange membrane fuel cell stack (PEMFCS) is proposed. The proposed study employs an equivalent circuit of the PEMFCS derived by frequency response analysis (FRA) technique. An equivalent circuit for the fuel cell stack is developed to evaluate the effects of ripple currents generated by the power-conditioning unit. The calculated results are then verified by means of experiments on three commercially available fuel cells: Avista Labs SR-12 (500 W), Ballard Nexa (1.2 kW) and BCS-Tech (300 W) PEMFC system. The relationship between ripple current and fuel cell performance: such as power loss and fuel consumption is investigated. Experimental results show that the ripple current can contribute up to 10% reduction in the available output power.

Development of a low cost fuel cell inverter system with DSP control

In this paper, the development of a low cost fuel cell inverter system is detailed. The approach consists of a three-terminal push-pull dc-dc converter to boost the fuel cell voltage (48V) to /spl plusmn/200 VDC. A four switch [insulated gate bipolar transistor (IGBT)] inverter is employed to produce 120-V/240-V, 60-Hz ac outputs. High performance, easy manufacturability, lower component count, safety and cost are addressed. Protection and diagnostic features form an important part of the design. Another highlight of the proposed design is the control strategy, which allows the inverter to adapt to the requirements of the load as well as the power source (fuel cell). A unique aspect of the design is the use of the TMS320LF2407 DSP to control the inverter. Two sets of lead-acid batteries are provided on the high voltage dc bus to supply sudden load demands. Efficient and smooth control of the power drawn from the fuel cell and the high voltage battery is achieved by controlling the front end dc-dc converter in current mode. The paper details extensive experimental results of the proposed design on Department of Energy (DoE) National Energy Technology Laboratory (NETL) fuel cell.

Fuel cell powered UPS systems: design considerations

In this paper a 1 kVA fuel cell powered line-interactive UPS system employing modular (fuel cell and power converter) blocks is introduced. Two commercially available PEMFC (25-39 V, 500 W) modules along with suitable DC/DC and DC/AC power electronic converter modules are employed. A supercapacitor module is also employed to compensate for the instantaneous power fluctuations and overcome the slow dynamics of the fuel processor such as reformers. Further energy stored in the supercapacitor is also utilized to handle a momentary overload such as 200% for a short duration. Due to the absence of batteries, the system satisfies the demand for an environmentally friendly clean source of energy. A complete design example illustrating the amount of hydrogen storage required for 1 hr power outage, and sizing of supercapacitors for transient load demand is presented for a 1 kVA UPS. Simulation and experimental results show the validity and feasibility of the 1 kVA fuel cell power plant.

An experimental evaluation of the effects of ripple current generated by the power conditioning stage on a proton exchange membrane fuel cell stack

In this article, an impedance model of the proton exchange membrane fuel cell stack (PEMFCS) is proposed. The proposed study employs an equivalent circuit of the PEMFCS derived by the frequency response analysis technique. An equivalent circuit for the PEMFCS is developed to evaluate the effects of ripple currents generated by the power-conditioning unit. The calculated results are then verified by means of experiments using a commercially available PEMFCS. The relationship between ripple current and fuel cell performance, such as power loss and fuel consumption, is investigated. Experimental results show that the ripple current can contribute up to a 6% reduction in the available output power.

Use of web-based materials to teach electric circuit theory

Beginning fall 2001, the Electrical Engineering Department at Texas A&M University, College Station, significantly altered the instructional philosophy of the ELEN 214 Electric Circuit Theory course by introducing more engineering design into the curriculum and adopting the WebCT-based interactive homework submission system. This paper will discuss the use of the Quiz tool within WebCT for the construction of question banks and their publication to a WebCT server. An example, deriving mathematical expressions, which describes electric circuit behavior and helps customize the homework problems to each individual student, are discussed. In other words, in a class of 200 students, each student is presented with an individual homework assignment with a unique set of problems not repeated to anyone else via WebCT. A help desk staffed by senior undergraduates assists the course students in completing the WebCT-based homework on time. WebCT is an essential ingredient in the delivery of the course. The approach presented in this paper can be adapted to any other course in engineering/science that involves mathematical calculations. So far, the course evaluations suggest that the students are more motivated and excited about electrical and computer engineering as a career.

An approach to improve battery run-time in mobile applications with supercapacitors

In this paper an approach to improve battery run-time in mobile applications with supercapacitors is explored. The performance of a battery-supercapacitor combination is analytically described using simplified equivalent circuit models. It is shown that there is an overall reduction in the internal losses and this translates into increased run-time. Three possible approaches are explored: (a) supercapacitors connected directly across the battery; (b) battery-inductor-supercapacitor connection; and (c) supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented. These results show an increase in runtime of 4-12% is achievable. Also from these results it is shown that the use of a DC-DC converter appears to be a cost effective option, since it allows the use of reduced number of capacitors while maintaining a comparable performance.

Paper: Structural synthesis of multivariable controllers

In this paper the problem of achieving a desired transfer function matrix Hd(s) between external inputs and controlled outputs in a linear multivariable system by connecting proper, stabilizing controllers between measured outputs and control inputs is solved in both transfer function and state space settings. The class Hd of achievable transfer functions is directly and constructively characterized via the theory of transfer function valuations. For each Hd(s)@eHd, the class of synthesizing controllers is determined. Similar valuation conditions are given for the asymptotic tracking and disturbance rejection problem in which Hd(s) is only partially specified. These results extend and complement earlier results. The state space geometric solution of the problem of achieving a desired Hd(s) is obtained by formulating it as an equivalent output feedback disturbance rejection problem. A constructive solvability condition in terms of a pair of measurement and control invariant subspaces is given. This requires a nontrivial generalization of the notion of (C, A, B) pairs. An Hd(s) is shown to be admissible if and only if it induces an appropriate pair of invariant subspaces. The signal flow structure and certain factorizability conditions for a robust synthesis of the output feedback disturbance rejection problem are also given which extend earlier results on robust state feedback disturbance rejection. It is shown that every compensator corresponds to a state feedback control law implemented by an unknown input observer. The results of this paper are expected to be useful in the development of parameter optimization or other computer aided design algorithms where response specifications are to be traded against other design criteria such as sensitivity or stability margins since they explicitly characterize the algebraic variety Hd in which the response transfer function may lie.

Stabilization of linear systems with fixed order controllers

Abstract This paper deals with the problem of stabilizing a linear time invariant plant with a feedback compensator of fixed dynamic order. This is an important unsolved problem in control theory, because existing solutions to the regulator problem can only generate controllers that are of high enough order that arbitrary pole placement becomes possible. In general the dynamic order of controllers can be much lower if only stabilization is desired. In the transfer function domain a new necessary condition for stabilizability by a fixed order controller is derived. This result, which can be used sequentially to derive a lower bound on the order of a stabilizing controller, is based on a classical result (1873) of linear programming known as Gordans theorem of the alternative. The stabilization algorithm successively selects Hurwitz vectors to make the largest stability hypersphere intersect a linear subspace determined by the plant parameters and the order of the controller. Occurrence of such an intersection is a sufficient condition for stabilizability, and the intersection also determines a stabilizing compensator. The algorithm is illustrated by practical examples which demonstrate that a large reduction in controller order is achievable by this method.

Structural Synthesis of Multivariable Controllers

Abstract In this paper the problem of achieving a desired transfer function matrix H d (s) between external inputs and controlled outputs in a linear multivariable system by connecting proper, stabilizing controllers between measured outputs and control inputs is solved in both transfer function and state space settings. The class Hd of achievable transfer functions is directly and constructively characterized via the theory of transfer function valuations. For each H d (s) e H d , the class of synthesizing controllers is determined. Similar valuation conditions are given for the asymptotic tracking and disturbance rejection problem in which H d (s) in only partially specified, The problem of achieving a desired H d (s) is solved by formulating its as an equivalent output feedback disturbance rejection problem. The signal flow structure and certain factorizability conditions for a robust synthesis of the output feedback disturbance rejection problem are also given which extend earlier results on robut state feedback disturbance rejection.

Control system synthesis with response insensitivity

This paper considers the problem of finding a state feedback control law which arbitrarily assigns distinct eigenvalues and achieves output response insensitivity to small plant parameter variations. It is assumed that plant structural parameter dependence is known. Conditions for these objectives are expressed in terms of eigenvectors and equivalent controllability subspaces. A maximum number of possible insensitive terms in the output spectral representation with simultaneous arbitrary eigenvalue assignment is determined, and the problem is shown to be related to the dynamic cover problem. A design procedure employing dynamic compensation is presented. It is shown that dynamic compensation does not influence the number of possible observable, insensitive terms in the output spectral representation, but it can be used to satisfy the required insensitivity conditions. Incorporation of an observer to implement the control law introduces additional sensitivity constraints which can also be satisfied using the design technique.