Honda FCX Clarity - Core Technology

V Flow FC Stack

Smaller. Higher Output. The fuel cell vehicle has evolved.

The new V Flow FC Stack is smaller and more powerful - a giant leap forward

Honda fuel cell vehicles continue to lead the industry. Recognizing that the fuel cell stack is the key to the fuel cell vehicle's evolution, Honda has continued to improve performance and packaging. In 2003, Honda unveiled the Honda FC Stack, capable of sub-zero startup and with outstanding production feasibility. Its design, featuring stamped metal separators and an aromatic electrolytic membrane revolutionized the fuel cell. Next, Honda took up the challenge of structural innovation - creating the V Flow FC Stack with an original cell structure that delivers decreased weight, improved performance and even more compact design. The V Flow FC Platform is the next generation in package design. It makes compelling styling and packaging feasible in a fuel cell vehicle for the first time. It's the breakthrough in stack technology that gives the FCX Clarity its elegant design.

The new V Flow FC Stack: high output in a lightweight, compact design

The V Flow FC Stack features an entirely new cell structure that achieves a higher output of 100 kW, smaller size and lower weight, with a 50% improvement in output density by volume, and a 67% increase in output density by mass, compared to the 2005 FCX.

Layers of cells producing lots of energy

The Honda V Flow FC Stack uses a proton exchange membrane fuel cell (PEMFC) electrical generation system that directly converts chemical energy produced in hydrogen-oxygen reactions into electrical energy. The extremely thin proton exchange membrane (electrolytic membrane) is sandwiched between pairs of electrode layers and diffusion layers (the hydrogen and oxygen electrodes) to form a membrane electrode assembly (MEA). The MEA is enclosed between two separators to form a cell - a single electrical generation unit. Several hundred cells are stacked together to form a fuel cell stack. As with batteries, these individual cells are connected in series to produce a high voltage.

How electricity is generated

  • Hydrogen gas is passed over the hydrogen electrode. Each hydrogen atom is converted into a hydrogen ion in a catalytic reaction with the platinum in the electrode, releasing an electron.
  • Having given up its electron, the hydrogen ion passes through the electrolytic membrane, where it joins with oxygen from the oxygen electrode and an electron arriving via an external circuit.
  • The released electrons create a flow of direct current in the external circuit. The reaction at the oxygen electrode produces water as a byproduct.
  • Because the electrolytic membrane must be kept continually damp, it is necessary to humidify the supply of hydrogen and oxygen. The water byproduct is recycled for this purpose. Unneeded water and air are released as exhaust.

Electrical power on demand

The main components of the fuel cell vehicle's power plant are the fuel cell stack, which generates electricity from hydrogen, the hydrogen tank, the lithium ion battery, the electric drive motor, and the Power Drive Unit (PDU), which governs the flow of electricity. Because the vehicle is propelled by an electric motor, it delivers smooth, powerful acceleration and quiet operation, without the noise and vibration associated with an internal combustion engine. During startup and acceleration - when a large amount of power is required - the electricity from the fuel cell stack to the drive motor is supplemented with electricity from the lithium ion battery to provide powerful performance. During deceleration, the drive motor works as a generator, converting kinetic energy into electricity, which is stored in the lithium ion battery along with any excess electricity produced by the fuel cells. When the vehicle is stationary, an idle stop system shuts down electrical generation in the fuel cell stack. Electricity from the lithium-ion battery ensures continued operation of the air conditioner and other devices. The system optimally controls electrical power, resulting in highly efficient operation.

Another key advance: a vertical gas flow cell structure is combined with wave flow-channel separators for an even more compact, lightweight design.

Until now, hydrogen and air flowed horizontally through the cells of Honda fuel cell stacks. The new V Flow FC Stack introduces a cell structure in which hydrogen and air flow vertically, and gravity is used to facilitate more efficient drainage of the water byproduct from the electrical generating layer. The result is greater stability in power generation. The new structure also allows for a thinner flow channel and reduction in the stack's size and weight. And Honda's innovative and original wave flow-channel separators provide a more even and efficient supply of hydrogen, air and coolant to the electrical generating layer. The results are higher generating performance, optimal cooling characteristics and major reductions in size and weight. More compact, the new stack has far fewer parts and can be fitted into a single box. It's also much easier to manufacture and can be fitted into a single box.

V Flow cell structure for greater stability in electrical generation and a thinner design

In addition to allowing hydrogen and air to flow vertically, the V Flow design also means that water drainage is assisted by gravity. Water doesn't collect on the electrical generation layer, ensuring constant power generation. This also allows flow channel depth to be reduced by 17% - a major contributing factor in creating thinner cells and a more compact stack.


Wave flow-channel separators enable a smaller stack design

The fuel cell consists of a membrane electrode assembly (MEA) - an electrolytic membrane sandwiched between the pairs of electrode layers and diffusion layers forming the hydrogen and oxygen electrodes - which are in turn enclosed between separators containing flow channels for hydrogen, air and coolant. The V Flow FC Stack incorporates wave-shaped vertical flow channels for the hydrogen and air, with horizontal coolant flow channels weaving between them. The wave flow channels provide greater flow length per channel than straight channels, while the resulting turbulent flow within the channel promotes improved hydrogen and air distribution. As a result, the hydrogen and air are spread over the entire electrode layer, making more efficient use of the compact electrical generation layer and achieving approximately 10% higher generating performance than with straight flow channels. The horizontal coolant flow also ensures more even cooling over the entire electrical generation layer, allowing for a reduction in the number of cooling layers to half that of previous stacks. The previous stack had one cooling layer for each cell. The new stack needs only one cooling layers per two cells. This results in a 20% reduction in stack length and a 30% weight reduction - a major breakthrough in compact, lightweight stack design.

Improved heat mass allows startup at -30°C

Improved water drainage due to the V Flow cell structure facilitates better output immediately after startup. The reduced coolant volume and single-box design made possible by the wave flow-channel separators results in heat mass 40% lower than previous stacks. As a result, the amount of time required to achieve 50% output after startup at -20°C (-4°F) is only one-quarter that of the previous stack. Startup is now possible at temperatures as low as -30°C (-22°F).

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