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Research Papers

Control Oriented Modeling of Solid Oxide Fuel Cell Auxiliary Power Unit for Transportation Applications

[+] Author and Article Information
Marco Sorrentino1

Department of Mechanical Engineering, University of Salerno-Italy, Via Ponte don Melillo 1, 84084 Fisciano, Salerno, Italymsorrentino@unisa.it

Cesare Pianese

Department of Mechanical Engineering, University of Salerno-Italy, Via Ponte don Melillo 1, 84084 Fisciano, Salerno, Italypianese@unisa.it

Electric efficiency was calculated by referring to the fuel higher heating value because it is assumed throughout the paper that water enters the fuel cell system in liquid phase.

1

Corresponding author.

J. Fuel Cell Sci. Technol 6(4), 041011 (Aug 14, 2009) (12 pages) doi:10.1115/1.3081475 History: Received June 15, 2007; Revised September 23, 2008; Published August 14, 2009

This paper reports on the development of a control-oriented model for simulating a hybrid auxiliary power unit (APU) equipped with a solid oxide fuel cell (SOFC) stack. Such a work is motivated by the strong interest devoted to SOFC technology due to its highly appealing potentialities in terms of fuel savings, fuel flexibility, cogeneration, low-pollution and low-noise operation. In this context, the availability of a model with acceptable computational burden and satisfactory accuracy can significantly enhance both system and control strategy design phases for APUs destined to a wide application area (e.g., mild-hybrid cars, trains, ships, and airplanes). The core part of the model is the SOFC stack, surrounded by a number of ancillary devices: air compressor/blower, regulating pressure valves, heat exchangers, prereformer, and postburner. Since the thermal dynamics is clearly the slowest one, a lumped-capacity model is proposed to describe the response of SOFC and heat exchangers to load (i.e., operating current) variation. The stack model takes into account the dependence of stack voltage on operating temperature, thus adequately describing the typical voltage undershoot following a decrease in load demand. On the other hand, due to their faster dynamics the mass transfer and electrochemistry processes are assumed instantaneous. The hybridizing device, whose main purpose is to assist the SOFC system (i.e., stack and ancillaries) during transient conditions, consists of a lead-acid battery pack. Battery power dependence on current is modeled, taking into account the influence of actual state of charge on open circuit voltage and internal resistance. The developed APU model was tested by simulating typical auxiliary power demand profiles for a heavy-duty truck in parked-idling phases. Suited control strategies also were developed to avoid operating the SOFC stack under severe thermal transients and, at the same time, to guarantee a charge sustaining operation of the battery pack. In order to assess the benefits achievable by introducing the SOFC-APU on board of a commercial truck, the simulated fuel consumption was compared with the fuel consumed by idling the thermal engine. From the simulation carried out, it emerges how the SOFC-APU allows achieving a potential reduction in fuel consumption of up to 70%.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Hierarchical approach for modeling and simulation of planar SOFCs (tasks 1 and 2, carried out in Refs. 11-13), SOFC-APU modeling and simulation (task 3), control strategies definition (tasks 4 and 5), and implementation (task 6) of an SOFC-APU. The gray smooth-cornered box highlights the tasks accomplished in this paper.

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Figure 2

Block diagram of a typical reformate-fed SOFC-APU

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Figure 3

Comparison between reference data and model outputs to assess the accuracy of the voltage black-box model (the reference voltage values were obtained on a single SOFC by varying the operating conditions in the ranges: Ts,in∊[650;700]°C, λ∊[5;9], Uf∊[5;9], J∊[0;1] A/cm2, and ps=1 bar)

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Figure 4

Single cell simulated voltage (a) and outlet temperature (b) trajectories after a positive load step, from 40 A to 70 A

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Figure 5

Experimental compressor efficiency map (27)

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Figure 6

Equivalent circuit of the battery pack ((a) discharge; (b) charge)

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Figure 7

Variation in battery internal resistance in charging and discharging as function of state of charge (16)

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Figure 8

Gross and net ac power as function of current for a single SOFC

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Figure 9

Plant schematic of the SOFC system with description of energy and mass flows for the most efficient point (i.e., I=25 A)

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Figure 10

Comparison between gross and net SOFC efficiencies

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Figure 12

Supervisory control map

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Figure 13

Simulated cold-start dynamics

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Figure 14

Cell voltage (a) and excess air (b) trajectories, simulated under controlled operation after a positive load step from 40 A to 70 A

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Figure 15

Random power profile, which is representative of typical auxiliary power demands on a commercial truck in parked-idling phase

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Figure 16

Simulated state of charge trajectories in Cases 1 and 2

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Figure 17

Case 1 power sharing

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Figure 18

Case 2 power sharing

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Figure 19

Temperature trajectories simulated in Case 1

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Figure 20

Temperature trajectories simulated in Case 2. Note that the curves are to be referred to the same legend as in Fig. 1.

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Figure 21

Comparison between the two efficiency distributions simulated in Cases 1 and 2

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Figure 22

Comparison between the voltage trajectories simulated in Cases 1 and 2

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