Toyota Mirai on Display


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 fuel cell stack uses a solid polymer, as in the current Mirai, but has been made smaller and has fewer cells (330 instead of 370). Nonetheless, it sets a new record for specific power density at 5.4 kW/l (excluding end plates). Maximum power has thus risen from 114 kW to 128 kW. Cold weather performance has been improved with start-up now possible at temperatures from as low as -30˚.

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 50% 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 current Mirai and weighs more than a third (34.4%) less.

Lithium-ion battery

The new Mirai is equipped with lithium-ion high-voltage battery in place of the current model’s nickel-metal hydride unit. Although smaller in size, it is more energy-dense, giving higher output and superior environmental performance. Containing 84 cells, it has a 310.8 rated voltage compared to 244.8, and a 4.0 Ah capacity, versus 6.5 Ah. Overall weight has been reduced from 46.9 to 44.6 kg. The output has improved from 25.5 kW x 10 seconds to 31.5 kW x 10 seconds.

The battery’s smaller dimensions have allowed it to be positioned behind the rear seats, avoiding intrusion in the load compartment. An optimised air-cooling path has been designed, with discreet inlets either side of the rear seats.

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