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Research Papers

Efficient and Robust Power Management of Reduced Cost Distributed Power Electronics for Fuel-Cell Power System

[+] Author and Article Information
Sudip K. Mazumder1

Department of Electrical and Computer Engineering, Director of the Laboratory of Energy and Switching-Electronics System, University of Illinois at Chicago, 851 South Morgan Street, 1020 SEO, M/C 154, Chicago, IL 60607mazumder@ece.uic.edu

Sanjaya Pradhan

Philips Lighting, 10275 W. Higgins Road, Rosemont, IL 60018sanjaya.k.pradhan@philips.com

1

Corresponding author.

J. Fuel Cell Sci. Technol 7(1), 011018 (Nov 11, 2009) (11 pages) doi:10.1115/1.3119059 History: Received January 24, 2008; Revised August 09, 2008; Published November 11, 2009; Online November 11, 2009

Batteries in a fuel-cell power system are essential to providing the additional power during the sharp load-transients. This necessitates a power-electronics subsystem (PES), which controls the energy flow between the fuel-cell stack, the battery, and the application load during the transient and in the steady states. In this paper, a distributed PES (comprising a multimodule dc-dc boost converter) is proposed for a fuel-cell and battery based hybrid power system, which provides higher cost effectiveness, efficiency, and footprint savings. This is realized by interfacing both the fuel-cell stack and the battery to the distributed PES using transfer switches, which are so controlled such that during a load transient, power from both the battery power and the fuel-cell stack is fed to the load via the PES while the stack energy input is adjusted for the new load demand. During the steady-state, the control implements a dynamic-power-management strategy such that only an optimal number of power converter modules of the distributed PES are activated yielding improved optimal energy-conversion efficiency and performance. Furthermore, using a composite Lyapunov-method-based methodology, the effect of dynamic change in the number of active power converter modules with varying load conditions on the stability of the PES is also outlined. Finally, the PES concept is experimentally validated by interfacing a multimodule bidirectional dc-dc boost converter with Nexa® proton exchange membrane (PEM) fuel-cell stacks from Ballard Power Systems.

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Figures

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Figure 1

Topology of the distributed PES comprising N dc-dc power-converter modules (with primary MOSFET switches S1 through SN and their complementary counterparts) along with N−1 transfer switches (TS1−TSN−1)

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Figure 2

DPMU for the PES to realize optimal energy-conversion efficiency and transient ride-through. Symbols Gbus and Gi1 through GiN represent the bus-voltage compensator and the current-loop compensators (for the N dc-dc converter modules). Only m(≤N−1) modules are active at any time while the rest of the (N−1−m) modules are turned off. The Nth module is always connected to the battery. Each of the N−1 transfer switches (TS1…TSN−1) also receives input from the DPMU.

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Figure 3

Experimental prototype of the (a) distributed PES and (b) a setup for the overall (two-stack) PEM fuel-cell based power system. (c) The polarization curve (source: Nexa technical specifications) for each PEM stack.

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Figure 4

Post-transient stability of the distributed PES after a load transient from 0.6 kW to 1 kW. Initially one module feeds the load from the stack, and subsequently after the transient, a second module is activated that feeds the additional power from the battery. (a) Minimum eigenvalue of Pki>0 implies that positive Pki is positive definite and that reaching condition 23 is satisfied for all initial power demand for m=2, thereby ensuring convergence of PES dynamics after the second module is activated following the load transient. ((b) and (c)) Experimental validations of reachability and stabilization of the currents.

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Figure 5

Post-transient stability of the distributed PES after a load transient from 0.6 kW to 1.4 kW. Initially one module feeds the load from the stack, and subsequently after the transient, two modules are activated that feeds the additional power from the battery. (a) Minimum eigenvalue of Pki>0 implies that positive Pki is positive definite and that reaching condition 23 is satisfied for all initial power demand for m=3, thereby ensuring convergence of PES dynamics after the second module is activated following the load transient. ((b) and (c)) Experimental validations of reachability and stabilization of the currents.

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Figure 6

Experimental comparison of the PES efficiency with varying stack current level. (top trace) Efficiency when m is varied between 1 through 3 following the optimal criterion outlined in Sec. 2. When m=2 or m=3, the current is shared equally among the modules. (bottom trace) Efficiency when m is always 3 (i.e., no optimal power management implemented) and all the modules share current equally. Clearly, the former demonstrates flatter efficiency profile of the PES leading to better fuel-cell-stack utilization in steady-state.

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