Dual-Voltage Electrical System with a Fuel Cell Power Unit 2000-01-3067

Fuel cells show great promise in generating electrical power for a variety of uses. In the automotive realm, one focus has been on the use of fuel cells for primary vehicle propulsion. Another emerging application is the fuel cell as the primary provider of electrical power to the vehicle, augmenting or replacing the traditional alternator, while producing higher power levels. The advantage of the fuel cell in this role is that the fuel cell operation is de-coupled from that of the engine. High power levels can be achieved independent of engine speed and power can be produced without the engine running.

This paper examines the application of a fuel cell auxiliary power unit (APU) to a dual-voltage 42V/14V automotive electrical system meeting the evolving 42V PowerNet specifications. An architecture for this electrical system is presented, followed by a sizing analysis to properly match the fuel cell stack to the voltage of the PowerNet and to a 42V battery pack. Simulation data is presented showing the fuel cell power unit performance under heavy load conditions and battery charging conditions for five possible stack cell quantities. Data from a load transient simulation is also presented, showing the advantage of a battery pack with low source/sink impedance in absorbing load transients seen by a fuel cell, which will likely have slow transient response. For the parameters assumed in this paper, a 52-cell stack is determined to be the most desirable for compatibility with the 42V PowerNet and the lithium-polymer battery pack.