DC-DC Converters Condition Fuel Cell Outputs

Nov 1, 2002 12:00 PM
By Vladimir Brunstein, Advanced Power Associates Corp., New Milford, NJ

The use of fuel cells as an alternative source of clean energy has created a need for a class of specialized power converters. To meet this demand, Advanced Power Associates Corp. has developed a line of buck or boost converters for fuel cells with a power range of 3kW to 15kW. Higher power is obtained by paralleling two or more units.

The table lists the characteristics of a 10kW converter. Converters in this family have fully digital control with a comprehensive diagnostic capability. All setpoints (input/output voltage, current, over/undervoltage limits, etc.) can be programmed through a serial port. Parameters can be changed through software while the unit is running under load, because all transitions are gradual. Some applications require an externally controlled output current. To control the output current through an outer loop, an isolated port is provided for an analog voltage (0Vdc to 5Vdc). Independently regulated dual outputs are optional. A power jumper selects the input range for the boost converter.

High-frequency switching and use of advanced magnetic materials help obtain a compact high-efficiency unit. Some buck converter models have reached efficiencies of 98%.

These converters were successfully used in several of automotive applications for fuel cell-powered electrical buses and cars, including two fuel cell-powered electric cars built by a New Jersey partnership. One of these cars, Genesis, recently established a record in autonomy — running 400 miles on one full tank (photo). Genesis uses H-Power fuel cells with a Millennium Cell Inc. “Hydrogen-On-Demand” installation, which generates H₂ from sodium borohydride in a safe chemical process. Two Advanced Power Associates Corp. converters, model 10K120/400, provided conversion from 120Vdc to 380Vdc to charge a NiMH traction battery.

Fuel cells have emerged as a promising alternative source of clean energy for applications ranging from automotive industry to residential and commercial installations. This has created a need for a class of specialized power converters geared to interface between the fuel cell device and the end-user appliance, often as a battery charger. Specifications for power conversion equipment depends on the fuel cell's physical properties and manufacturing economics.

The economics of fuel cells' manufacturing call for a relatively low output voltage, as the total output voltage is obtained by connecting in series a number of individual cells. Since increasing the cells' surface — which results in higher current capability and, consequently, higher power — is cheaper and less complex than increasing their number, a cost-optimized unit will have a high output current capability and a relatively low output voltage.

The cells' output voltage is dependent on the load; though nonlinear, it's approximately equivalent to a voltage source with series resistance. For a PEM (proton exchange membrane) cell, the voltage may drop from a no-load 1.23Vdc to below 0.5Vdc at full load. Consequently, a converter will have to work with a wide range of input voltages.

A fuel cell stack could be catastrophically damaged if overloaded. Drawing excessive current from any one cell — more than its fuel delivery can produce — can lead to a cell reversal and membrane rupture. Therefore, a converter designed to work in these applications must be controlled based on input voltage and current, as well as output voltage and current — only fully digital control is practical.

Finally, due to the cost of generating power, converter efficiency is one of the most important parameters.

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