Toyota’s new fuel cell stack and fuel cell power converter (FCPC) have been developed specifically for use with the GA-L platform. The designers have been able to bring all the elements together in the stack frame (including the water pumps, intercooler, air conditioning and air compressors and the hydrogen recirculation pump) with each part made smaller and lighter, while at the same time improving performance. The stack case itself has been made smaller by using Friction Stir Welding, reducing the gap between the fuel cell and casing.
The packaging improvements have made it possible to relocate the FC stack in the Mirai’s front compartment (engine bay), helping realise the benefits of an optimum 50:50 front:rear weight balance. On the previous generation model, the unit was beneath the front seat
The fuel cell stack uses a solid polymer, as in the previous Mirai, but has been made smaller and has fewer cells (330 instead of 370). Its weight bas been brought down from 56 to 52 kg. As well as achieving savings in size and weight, it also sets a new record for specific power density at 5.4 kW/l (4.4 kW/l excluding end plates). Maximum power from the FC stack has consequently been increased from 155 DIN hp/114 kW to 174 DIN hp/128 kW. Cold weather performance has been improved with start-up now possible at temperatures from as low as -30˚C.
By concentrating the system connections within the case, fewer components are needed, again saving space and weight.
Focusing on innovation and improvement in every component has delivered a 42% weight reduction yet a 12% increase in power. New measures include relocation of the manifold, reducing the size and weight of the cell optimising the shape of the gas channel separator and using innovative materials in the electrodes.
The unit also incorporates the Fuel Cell DC-DC Converter (FDC) and modular high-voltage parts, while achieving a 21% reduction in size compared to the current system. Weight has been cut by 2.9 kg to 25.5 kg. Advanced technology has contributed to the space-saving, with Toyota’s first-time use of a next generation silicon carbide semiconductor material in the intelligent power model (IPM) transistors. This enables an increase in output and lower power consumption while using fewer transistors, which in turn allows the FCPC to be made smaller.
The same size and weight-saving approach has been applied to other parts of the FC stack. The air intake is designed for low pressure loss and contains sound-absorbing material so that noise from the air inlets is unnoticeable in the cabin. The exhaust uses a resin pipe and is designed to allow for a large amount of air and water to be discharged; a larger-capacity silencer contributes to the quieter cabin. The complete air system is almost 30% smaller than in the previous Mirai and weighs more than a third (34.4%) less.
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