United Technology Corporation's (UTC) PEM (proton exchange membrane) design offers many technical advantages over competing fuel cell technologies. The fuel cell operates at ambient pressure, which not only eliminates the need for a compressor, but also eliminates the need to develop power to run a compressor, thereby substantially increasing the efficiency of the fuel cell. For the ElDorado National bus, the team achieved its goal of over 50% net efficiency of the fuel cell system, giving it a decisive advantage over any conventional power plant. Even the best of diesel engines can only achieve 40%, and then only in constant speed and power operation.

The UTC fuel cell stack also contains an integral humidification system that eliminates the need for complicated external systems. The system is capable of full power operation at temperatures up to 120°F without loss of water balance. This is extremely important for buses operated in regions of the country that experience very high temperatures.

ISE has developed and evolved a proprietary CAN based communication system for the fuel cell, drive system, energy storage system, operator controls, accessory drives, and standard bus controls. This required the development of numerous gateways, translators, IO modules, and power distribution boxes. The fuel cell controller controls the fuel cell subsystem, including cooling, fuel, hydrogen monitors, ground fault monitoring, and other related functions. It is also the main interface to the drive system, sending information to the vehicle detailing power available and any fault conditions.

Below is a schematic showing the flow of energy within the major components of ThunderVolt® fuel cell hybrid system.