Fuel Cell Buses
|Georgetown University Advanced Vehicle Development|
|Learning Centre: Energy|
Fuel cells produce electricity by electrochemically combining hydrogen and oxygen to form water (see diagram below). The hydrogen is broken down into its components (protons and electrons) at the anode. The protons pass through the electrolyte of the fuel cell. The electrons are forced to pass through an external circuit (the light bulb in the diagram), thereby supplying electrical power to external loads. On the Georgetown Fuel Cell Buses, the external loads are the propulsion and auxiliary systems.
Each of the three Test Bed Buses uses a phosphoric acid fuel cell, rated for 50 kW (67 hp) net. The oxygen needed for the cathode of the fuel cell is simply taken from the air. The hydrogen required for the anode must be extracted from liquid methanol, in a process called reformation.
There are two tanks on the Test Bed Buses. One tank contains neat (pure) methanol, and is only used for startup heating and temperature control of the reformer. The main tank contains a defined mixture of methanol and water. This 'premix' is vaporized and then fed into the reformer to generate the hydrogen necessary for the fuel cell to produce power.
Because not all of the hydrogen is consumed in the fuel cell, the anode exhaust is fed back into the reformer burner, which burns the leftover hydrogen and maintains the temperature required for the reforming process. In addition, neat methanol is fed to the reformer burner to help regulate the reformer temperature.
Test Bed Bus 50 kW PAFC System
The efficiency of the phosphoric acid fuel cell in each Test Bed Bus is illustrated by the following chart.
The fuel cell on each TBB can supply a maximum gross power of about 55 kW (73 hp), which is not enough for the operation of a typical urban transit bus. The fuel cell is supplemented by a traction battery pack, which provides surge power for acceleration and hill-climbing. The battery pack also provides a means to recapture energy through regenerative braking. Regenerative braking occurs when the traction motor is operated as a generator; this converts some of the kinetic energy of the bus into electrical energy which is fed into the batteries.
TBB Hybrid Propulsion System Layout
For general information on the Test Bed Buses, please visit the General Information page.
Or visit the photo gallery of the Test Bed Buses.