|The Tiguan HyMotion fuel cell concept uses a VW-developed high-temperature fuel cell stack. Click to enlarge.|
Volkswagen came to the Los Angeles Auto Show to tout the efficiency and performance of its diesel powertrains and the recent market success of the Jetta Clean Diesels in the US, as well as to introduce the new Touareg V6 TDI.
However, the company said it was also casting an eye to the future with its presentation of three fuel cell prototypes: the Tiguan, a compact SUV; the Touran, a compact minivan; and the Passat Lingyu, a sedan customized for use in China (earlier post). In Germany, the Tiguan and Touran are the most successful cars in their class. The same is true of the Passat Lingyu in China. All three zero emissions vehicles were available for test drives at the show.
|Under the hood of the Tiguan HyMotion. Click to enlarge.|
Tiguan HyMotion. The Tiguan HyMotion fuel cell prototype is VW’s first fuel cell-powered SUV. Compared to earlier prototypes, VW has delivered further performance gains in the fuel cell drive, and its efficiency has been optimized.
The fuel cell stack in the Tiguan Hymotion is a VW-developed high-temperature fuel cell (HTFC) stack. (Earlier post.) Low-temperature, conventional PEM fuel cells (LTFC) operate at a membrane temperature of about 80° C. If the temperature rises significantly above this value, fuel cell performance collapses, and the cell experiences irreparable damage. Therefore, prototypes with LT fuel cells have an complicated and expensive cooling system. The radiator surface alone is about three times as large as that for diesel engines, according to VW.
In addition, in a LT system the supply of hydrogen and air gases must be constantly humidified; otherwise energy production breaks down and the fuel cell is permanently damaged. This humidification causes water molecules to be embedded in the membrane, which also introduces undesirable added weight, and the process wastes space and money.
By contrast, the high-temperature membrane developed by Volkswagen, together with a new electrode design, can be continually driven without power loss at temperatures of 120° C without humidification.
In the HTFC, proton conduction occurs via phosphoric acid. This acid has electrolytic properties that are as good as those of water, but it exhibits a higher boiling point. Therefore, the HTFC can operate with a considerably less complicated cooling system and water management, thereby reducing weight and costs. The space requirement of the fuel cell system is also reduced by more than 30%.
HTFCs historically have had an issue with product water formation—the water permeates the membrane and leaches out the phosphoric acid, in turn interrupting the flow of electric current. To prevent this problem, VW modified the electrodes to prevent product water from penetrating the membranes. Using a screen printing machine like the ones used in the semiconductor industry, VW researchers coated carbon fleece elements with a new type of paste.
The 80 kW fuel cell system is integrated in the engine compartment of the Tiguan, along with the 100 kW electric motor. The fuel cell outputs its generated power to the electric motor via a converter and a downstream inverter. Top speed of the Tiguan HyMotion is 87 mph (140 kph); the 1,870 kilogram prototype accelerates from 0 to 100 kph in about 14 seconds.
A 22 kW lithium-ion battery with a charge capacity of 6.8 Ah is recharged via regenerative braking. The battery system is installed in the car’s cargo area under the dual cargo floor that is available on the production Tiguan.
A new 700-bar hydrogen tank is integrated in the space under the rear bench seat and the cargo area. It can hold 3.2 kg of hydrogen—sufficient for a driving range of 125 miles (about 200 km).
Touran fuel cell prototype. The Touran fuel cell prototype also uses the VW-developed stack, but with an 80 kW electric motor. The protoytpe uses a 1.9 kWh NiMH battery instead of a Li-ion battery, because it was conceptualized a generation earlier than the Tiguan, VW said. The battery is recharged by the fuel cell stack or by recovered braking energy.
Both Tiguan and Touran fuel cell vehicles are currently being tested in a durability study conducted as part of the California Fuel Cell Partnership. In parallel, other prototypes are running in Germany. For test purposes, an infrastructure of hydrogen filling stations is being built between Berlin and Hamburg.
|Passat Lingyu with fuel cell system. Click to enlarge.|
Passat Lingyu fuel cell prototype. The prototype was developed in China by Shanghai Volkswagen together with scientists at the Tongji University, where the low-temperature fuel cell stack was developed. This is the fourth generation of a line of fuel cells so far developed at the university.
The 55 kW fuel cell stack is located in the car floor of the Passat Lingyu, which is driven by an electric motor with 88 kW of power and 210 Nm of torque. An 8 Ah, 376 V (approx. 3 kWh) Li-ion battery provides energy storage, and is recharged by regenerative braking and the fuel cell. Hydrogen gas is stored at 350 bar in a carbon fiber reinforced pressure tank.
Shanghai Volkswagen provided 20 Passat Lingyu fuel cell vehicles for use at the Beijing Olympics.