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General Information Welcome Overview History Fuel Cell Bus Projects Generation I Generation II Generation III Further Information Partners Links Executives Contact |
Fuel Cell System
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. Fuel Cell Diagram The UTC Fuel Cell Bus uses a phosphoric acid fuel cell, rated for 100 kW (133 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. This process is called reformation, and it requires several steps. First, the methanol (along with water) is vaporized. This mix is then fed into the high temperature steam reformer, where the mixture is converted into a hydrogen-rich gas. However, this new mixture coming out of the reformer contains some carbon monoxide (CO), which can harm the catalyst in the fuel cell stack. So the mixture must also pass through a low temperature shift converter, which oxidizes the CO into CO2. The resultant mixture is called reformate, and is fed into the anode of the fuel cell. 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 high temperature required for the reforming process. Also, water is recovered from the cathode exhaust to maintain the necessary water supply to the reformer. UTC Bus 100 kW PAFC System Diagram Efficiency diagram of the phosphoric acid fuel cell in the UTC bus. Electric Propulsion System The fuel cell on the UTC bus can supply a maximum gross power of about 105 kW (140 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. UTC Bus Hybrid Propulsion System Layout For general information on the UTC Fuel Cell Bus, please visit the General Information page. Or visit the photo gallery of the UTC Bus. Downloadable 2-page brochure for the UTC Bus: UTC Bus Brochure (1.7 MB) Adobe Acrobat Reader is required to view this file. |
