ADVANCED METAL HYDRIDE FUEL TANK FEATURED
IN MAZDA HR-X HYDROGEN-POWERED CONCEPT VEHICLE
With its advanced metal hydride fuel tank and extraordinarily low levels of emissions, the Mazda HR-X concept vehicle shows just how close a hydrogen- powered future can be.
As the world's automakers search for clean-burning substitutes for fossil fuels, it is not surprising that Mazda and a few other companies are looking into hydrogen. Hydrogen is one of the most abundant elements on Earth and, when bumed, its primary emission is water vapor.
Using hydrogen as a fuel, however, is not a new idea: research into hydrogen combustion has taken place since the 1920s. What is new is how Mazda has overcome the long-standing technical obstacles -- storage of the gas and controlled combustion -- to practical hydrogen use in its revolutionary concept vehicle, the HR-X.
METAL HYDRIDE BREAKTHROUGH
For hydrogen to be a practical alternative fuel, Mazda first had to develop a safe and effective way to store the gas on board the vehicle. Metal hydride tanks had been used before, but were limited by a storage capacity that tended to diminish over time. Through perseverance, the engineers at Mazda developed a metal hydride that uses magnesium and can be refueled up to 500 times during its useful life. The metal hydride tank used in the HR-X weighs 617 pounds (280 kg). In this type of tank, the metal hydride absorbs hydrogen into a metal lattice, storing the gas in the form of a metallic compound. When heated, the metal hydride releases the hydrogen. In the HR-X heated coolant from the vehicle's radiator is circulated through tubes in the metal hydride tank, releasing the hydrogen gas.
Refueling the HR-X is just as easy, requiring only cooling water to be circulated through the system so that the metal hydride will absorb hydrogen from an outside supply.
One advantage of the metal hydride used in the Mazda HR-X is that it is granular, meaning that the tank can be made in a wide variety of shapes -- a flexibility that designers of vehicles fueled by electricity or compressed natural gas can only dream about.
One popular concern about hydrogen has always been its extreme volatility. In the HR-X this potential safety concern has been addressed by the metal hydride tank. With the metal hydride, the hydrogen is stored in the form of a metallic compound, not as a gas. In addition, it is stored at low pressures, so that even in the event of a tank rupture, only a small amount of hydrogen would be released, and this amount would leak out slowly. The ability of the tank to be placed anywhere in the vehicle also allows it to be placed in the safest possible position in the vehicle.
One additional advantage of the HR-X's metal hydride tank is that once its useful life is over, the entire tank can be melted down and recycled. As good as the metal hydride tank is, Mazda is investigating further refinements that will permit greater hydrogen storage capacity, as well as other storage options.
VERY LOW EMISSIONS
Hydrogen is seen as an ideal fuel for the future not only because of its abundance but also because it is extremely clean-burning. Its primary emission is water vapor.
When hydrogen is burned in an internal combustion engine, like the rotary engine in the HR-X there are measurable amounts of oxides of nitrogen (NOx) and minute amounts of hydrocarbons (HC) and carbon monoxide (CO). These emissions are due to the lubricants used inside the engine, as well as the nitrogen naturally found in air.
To control these trace emissions, Mazda is researching the use of a three-way catalyst (similar to those used on current vehicles) or lean-burn technology.
Mazda is interested in developing the hydrogen-fueled engine as a Zero-Emission Vehicle (ZEV).
While Mazda's efforts to date have centered on the company's unique rotary engine, research is also under way into using hydrogen with conventional reciprocating engines. This research, if successful, would allow Mazda to meet its goal of developing hydrogen technology that could be shared with all automobile manufacturers in finding a permanent replacement for fossil fuels.