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A MINIMUM POWER-PROCESSING-STAGE FUEL-CELL ENERGY SYSTEM BASED ON A BOOST-INVERTER WITH A BIDIRECTIONAL BACKUP BATTERY STORAGE



ABSTRACT
When low-voltage unregulated fuel-cell (FC) output is conditioned to generate ac power, two stages are required: a boost stage and an inversion one. In this paper, the boost-inverter topology that achieves both boosting and inversion functions in a single stage is used to develop an FC-based energy system that offers high conversion efficiency, low-cost, and compactness.
The proposed system incorporates additional battery-based energy storage and a dc-dc bidirectional converter to support instantaneous load changes. The output voltage of the boost-inverter is voltage-mode controlled and the dc-dc bidirectional converter is current-mode controlled.
The load low-frequency current ripple is supplied by the battery, which minimizes the effects of such ripple being drawn directly from the FC itself. Analysis, simulation, and experimental results are presented to confirm the operational performance of the proposed system.

A Minimum Power-Processing-Stage Fuel-Cell Energy System Based on a Boost-Inverter

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INTRODUCTION

IN general, energy systems based on photovoltaic (PV) and
fuel cell (FC) generation sources need to be regulated and in
many applications must be supported through additional energy
storage unit to achieve high-quality supply of power [1]–[4].
When such systems are used to power ac loads or be connected
with the electricity grid, an inversion stage is also required.

TOPOLOGY AND CONTROL SCHEME STUDY

Two-Stage FC Energy System

Fig. 1 shows a popular FC energy system that includes two
main power conversion stages between the FC and the load:
an isolated full-bridge dc–dc boost converter and a full-bridge
dc–ac inverter with backup unit for supporting the FC operations.
Due to stability considerations, high-power applications
and low-input voltage have considered the two-power-stage approach.
In the dc-to-dc converter stage, half-bridge, push–pull,
and full-bridge converters are the trend for galvanic isolation and
high boost ratio. From the availability point of view, the fullbridge
converter is introduced as the best solution for the FC
energy system because of its suitability for high-power applications,
low-voltage and minimized current stress of the switches,
reduced turn ratio of the transformer, small input- and outputcurrent
and voltage ripples, and is a favorite topology for zerovoltage
and/or zero-current-switching techniques.

ANALYSIS AND SIMULATION RESULTS

The proposed FC energy system (see Fig. 2), has been designed,
simulated, and experimentally build as a laboratory prototype
and tested to validate its overall performance.
Based on the conditions in Table III, the simulations have
been performed with the PSIM software to validate the analytical
results [27]. The simulation results show the operation of
the boost-inverter and the backup unit.

EXPERIMENTAL RESULTS

The parameters of the prototype for the boost-inverter and
the backup unit are summarized in Table IV. The power stack
consists of three insulated gate bipolar transistor (IGBT) modules
that are used to build the boost-inverter for two modules
and backup unit for one module. The controller unit has
been selected for a number of reasons, such as low cost,
embedded floating point unit, high speed, on chip analog-todigital
converter, and high-performance pulsewidth modulation
(PWM) unit. Fig. 12 shows the experimental setup including
boost-inverter, backup unit, Nexa FC system, and hydrogen
generator.

CONCLUSION

A minimum-stage power-processing FC energy system based
on the boost-inverter topology with a backup-battery-based energy
storage unit is proposed in this paper. The simulated and the
laboratory test results presented in the paper have verified the
operation characteristics of the proposed energy system. In summary,
the proposed FC energy system has a number of attractive
features, such as single main power stage with high efficiency,
simplified topology, low cost, and stand-alone operation.