Fuel cells start to look real
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Energy Conversion Devices
Energy Conversion Devices Inc. (ECD) of Troy, MI, is developing a hydrogen-storage technology that could be a key to future zero-emissions fuel-cell-powered vehicles. The technique relies on metal hydrides - special alloys that incorporate hydrogen atoms in their crystalline structure when heat is removed. Hydrogen is released when heat is applied to the alloy. "A tank displacing about 120 L (31 gal) and containing around 120 kg (265 lb) of magnesium-based metal-hydride powder could store about 6 kg (13 lb) of hydrogen, giving an advanced fuel-cell vehicle about a 480-km (300-mi) range," said Bob Stempel, ECD President.
In most hydrogen-powered prototypes, pure hydrogen is stored as either a supercooled liquid that must be kept chilled at very low temperatures or as a compressed gas at pressures up to 34 MPa (5000 psi). In the former case, a cryogenic tank holds 31 g (1.1 oz) of hydrogen for every liter of storage volume. In the latter, a compressed gas system stores about 71 g (2.5 oz) of hydrogen for each liter of volume (depending on the pressure). By contrast, ECD's metal-hydride system can capture 103 g (3.6 oz) of hydrogen per liter volume.
In general, metal-hydride storage systems comprise three main parts: hydrogen gas, engineered metallic materials, and the interface region between them, according to ECD's Vice President of Advanced Materials Development, Rosa Young. "The metal alloys we have formulated are in loose, dry powder form," he said. "Hydrogen gas entering the storage vessel adsorbs onto the interface regions of the powder. Hydrogen molecules dissociate into individual hydrogen atoms and metal hydride is formed when these atoms arrange in a specific pattern with the metal atoms. Heat is also a factor. Removing heat drives the adsorption process. Adding heat reverses the chemical reaction and causes the hydrogen atoms to reform as hydrogen molecules."
In ECD's fuel-storage design, the hydrogen is pumped into a tank containing racks of canisters filled with a powdered magnesium-based alloy compressed into cakes. The system reportedly operates at relatively low pressures - around 2.4 MPa (350 psi). The gas is desorbed from the hydride at a temperature of 286°C (547°F), with heat generated on startup by a "catalytic burner" in each canister. Up to 20% of the hydrogen is consumed in starting the release process. Later, the heat would be supplied from the reaction in the fuel cell itself. The storage system also includes a refueling heat exchanger and high-efficiency insulation. Company engineers have put the technology through more than 2000 fill-and-release cycles, enough to power a vehicle for several hundred thousand miles.
"Until recently, the (weight%) limit was 2 or 3 g (0.07 or 0.10 oz) of hydrogen per 100 g (3.5 oz) of hydride," said Stanford R. Ovshinsky, Co-founder of ECD. "By using a high percentage of magnesium with several other metals in our patented hydride powders, ECD is capable of storing 7 g (0.25 oz) of hydrogen per 100 g (3.5 oz) of hydride (7 weight%)." Most current metal hydrides can store 2 or 3% of their own weight in hydrogen.
Stempel says that ECD researchers are working on ways to boost the amount of hydrogen the magnesium-based metal-hydride alloys can store. "We're looking to increase the active surface area of the powders by increasing the porosity, and by adding small amounts of carbon and other useful additives," he said.
"We still have a lot of work to do on this technology before it's ready for the market," said Stempel. One drawback to metal-hydride storage is that it uses some energy in its cycling operation. The key to that issue is how to best capture the heat so the total system efficiency remains high. Another issue concerns speeding up the rate at which the hydride soaks up and releases the hydrogen to cut the wait at the pump. "We've resolved the thermodynamic issues of storing and releasing hydrogen from a metal hydride so a typical fill-up would require only 3 or 4 minutes," claimed Ovshinsky.
Shrinking both the size and weight of the tanks as well as the high cost are other concerns. At $4/lb, the magnesium-hydride powder alone would add up to $1000 to the vehicle's cost. Stempel acknowledges that the hydrogen storage technology ECD envisions is expensive, but on a systems basis, he thinks it can be cost-competitive.
Yet another technical challenge is ensuring the safety of the magnesium-based powder. Onboard tanks would be designed to resist puncture and fires, but the magnesium dust would be dangerously explosive during processing.
Last year, Texaco Inc. invested nearly $68 million in ECD, announcing it was primarily interested in ECD's proprietary fuel-cell technology and its hydrogen-storage system. More recently, Texaco Energy Systems Inc. (TESI) and ECD formed Texaco Ovonic Hydrogen Systems LLC, a 50-50 joint venture to further develop and advance the commercialization of ECD's metal-hydride technology. Under the terms of the joint venture agreement, ECD will provide proprietary technology, while TESI will provide additional technological support and funding during the product development and pre-production phase of the company's operations.
"ECD's proprietary metal-hydride hydrogen-storage technology has the potential to overcome one of the key challenges of making fuel cells and other hydrogen-dependent energy sources practical, efficient, and safe," said William M. Wicker, Texaco Senior Vice President. "We are confident that the formation of this joint venture will move us forward to achieving this important goal."
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