By Michael Valenti, Senior Editor

The advantages of hydrogen fuel for cars and trucks keep driving efforts to develop ways of handling it. Hydrogen has the highest energy content by weight of any fuel—52,000 Btu per pound. As the simplest and most common element in the universe, though never found naturally in pure form, hydrogen can be produced from a host of available sources, including water, natural gas, coal, biomass, municipal solid waste, or scrap tires.

If burned as a vehicular fuel, hydrogen can be combusted at the high compression ratios and efficiencies demanded by internal combustion engines. When combined with oxygen in automotive fuel cells to generate electricity without combustion, hydrogen fuel raises the car's energy efficiency and produces only heat and water as byproducts. When hydrogen is burned with air in an internal combustion engine, some nitrogen oxides are formed, but fewer than the pollutants generated by fossil fuels, according to the Department of Energy's Energy Efficiency and Renewable Energy Clearinghouse in Merrifield, Va.

Quantum Technologies' TriShield composite cylinders can hold up to 3 kilograms of hydrogen at 5,000 psi, which is sufficient fuel for a 200-kilometer journey in a standard sedan.

Among the obstacles to commercializing hydrogen-powered vehicles—besides production and infrastructure—is the need for storage systems that can contain sufficient hydrogen onboard a car to compete with the range and performance of gasoline-powered autos. Approaches to vehicular hydrogen storage systems can be as different as gas in pressurized tanks, liquid in cryogenic vessels, or a solid in metal hydrides.

All three methods are under study: the first at Quantum Technologies Inc., the second at Lawrence Livermore National Laboratory, and the third at ChevronTexaco Ovonic Hydrogen Systems. Each project intends to capitalize on the strengths and address the limitations of a different hydrogen storage technique.

Fighting Embrittlement

Vessels pressurized to 5,000 psi can hold sufficient hydrogen for a standard hydrogen-powered sedan to be driven 200 kilometers. This is the maximum distance of about 85 percent of the car trips in the United States, according to a report prepared for the Department of Transportation's Office of Highway Information Management.

Conventional industrial hydrogen storage tanks are made of common grade steel and, over time, the gas can migrate into the metal. This makes the metal brittle, fatiguing it to the point that hydrogen can leak from the tank. High-quality steel can prevent embrittlement, but will raise the cost of the tank, and its weight. A premium steel tank holding 3 kilograms of hydrogen would itself weigh 400 kg, definitely cutting into fuel economy.

Quantum Technologies Inc. of Irvine, Calif., developed its composite TriShield hydrogen storage cylinder as an alternative that will be compatible with the cost expectations of the automotive industry, and also meet safety standards. Quantum specializes in developing advanced fuel systems, such as the natural gas and propane systems used on General Motors vehicles, including the J-Car Cavalier Sedan, and the Silverado and Sierra trucks.

The TriShield contains a cross-linked, ultrahigh molecular weight, modified polymer liner that has been optimized to withstand temperatures from -40°F to 190°F. The liner is impermeable. Engineers installed a single boss opening to minimize leak paths and added a redundant hydrogen seal system to enhance the cylinder's reliability.

As a further protection against embrittlement, the liner is surrounded by a carbon fiber inner shell. A hard external shell made of a proprietary fiber/resin system, and impact-resistant polymer domes on each end, protect the cylinder from service damage.

A series of TriShield tanks are mounted on the roof of an experimental fuel cell-powered bus in southern California to demonstrate their ability to store hydrogen fuel safely in field conditions.

The TriShield typically measures 22 by 20 inches, but can be 20 feet long for buses. It holds up to 3 kg of hydrogen pressurized to 5,000 psi, enough for a 200-km trip in a standard sedan. The cylinder can withstand up to 11,250 psi from accidental overfilling before bursting, and has a fatigue life of 45,000 cycles, according to ANSI/CSA NGV2-200 hydrogen tank specifications.

Quantum verified the Tri-Shield's toughness by conducting a series of rigorous tests under auspices of the European Union's European Integrated Hydrogen Project. The tests were completed last November. Manufacturers of storage cylinders must pass the tests to be able to make and sell hydrogen storage cylinders in Europe.

"These tests included placing the cylinder in a crash car, firing armor-piercing bullets at it, dropping the cylinder from six feet onto a concrete surface, placing it in a diesel fire, cycling it thousands of times, and subjecting the cylinder to extreme cold and to corrosive liquids encountered in automotive environments, such as battery acids, saltwater, brake oils, and methanols," explained Neel Sirosh, a mechanical engineer and director of fuel storage systems at Quantum.

The TriShield passed those tests as well as others for
vibration and shock. "These tests involved using accelerometers on test vehicles to obtain the vibration profiles of on-the-road service," Sirosh said. "The vibration profiles were simulated by shake tables in Quantum's testing labs upon which the tanks were mounted."

TriShield tanks have been installed on a Hyundai Santa Fe sedan being tested by the California Fuel Cell Partnership, to demonstrate the tank's ability to feed a fuel cell as well as a combustion engine. Other tanks are mounted on the roof of a fuel cell bus being developed by Thor/ISC International in San Diego.

"Quantum also sees opportunities for the TriShield tanks in augmenting land-based electrical generation," Sirosh said. "Electrolyzers would produce hydrogen at night, when electrical costs are low, and the gas would be stored in groups of tanks. That hydrogen would be tapped during the peak demand in the daytime to generate electricity."

Engineers installed a single boss opening in the TriShield tank in order to minimize leak paths for its pressurized hydrogen contents.

Quantum recently developed a 10,000-psi version
of TriShield, which is now awaiting certification. According to Sirosh, "We adapted the material thickness of the tank to accommodate the greater pressure and increase the tank's energy density by 60 percent. By storing 60 percent more fuel than before in the same-size tank, we make the TriShield competitive with liquefied hydrogen gas storage."

A proprietary regulator resides in the TriShield to keep only the tank at high pressure. As a result, the rest of the fuel system, from lines to engine, can operate under lower pressure, eliminating the precautions needed for high-pressure gas handling.

This contrasts with gaseous fuel systems that typically deliver fuel at high pressure to the engine bay, where it is regulated down to the operating pressure of the fuel-metering device. Thus, the fuel line is filled with high-pressure hydrogen that can explode if the line is breached.

The TriShield's H2R 5000 regulator is a balanced, single-stage, piston-sensed device whose inlet accepts hydrogen at pressures ranging from 300 to 10,000 psi, and feeds it to the vehicle fuel system at 50 to 150 psi.

Preventing Evaporation

Cryogenic pressure vessels offer the low weight and volume to accommodate light-duty vehicles. Five kg of liquid hydrogen would give a high-efficiency hydrogen fuel cell or internal combustion engine vehicle a 600-km driving range, according to Salvador Aceves, an ASME member and associate program leader for transportation at Lawrence Livermore National Laboratory in Livermore, Calif. Aceves is a past chairman of ASME's Advanced Energy Systems division.

However, drawbacks of cryogenic storage are the high electrical consumption required to liquefy the hydrogen, and the evaporative losses that can occur when the low-pressure tanks are filled and when the car is parked. "It would take about 65 kilowatt-hours of electricity to liquefy the 5-kg target for vehicular hydrogen," said Aceves.

The loss of cryogenic hydrogen by evaporation is caused by heat transfer between the tank and environment. According to computer analyses performed by Lawrence Livermore, a standard sedan carrying a conventional, low-pressure (5 bar) cryogenic hydrogen fuel tank would lose 20 percent of its fuel due to evaporation when a car was driven 50 km per day. Evaporation losses increased when the car was driven less.

The Lawrence Livermore researchers also modeled the same vehicle, but one equipped with an insulated cryogenic vessel that can operate at high pressure-250 bar. "If the car was driven 5 km or more per day, there was virtually no evaporative loss," said Aceves.

An insulated cylinder, designed to contain hydrogen chilled to 20 K, has to hold up in a bonfire at Authorized Testing Inc. before the tank is deemed worthy of testing on automobiles.

Based on these findings, Lawrence Livermore is developing insulated, cryogenic pressure vessels whose fuel flexibility enables them to use ambient-temperature, compressed hydrogen for short trips, and cryogenic hydrogen on long trips. "These tanks would require 16 percent less energy when running on compressed fuel than when using cryogenic hydrogen, and would experience less evaporative losses when charged with subzero (20 K) hydrogen by virtue of their insulation," noted Aceves.

Lawrence Livermore determined that commercially available aluminum-lined and composite-wrapped pressure vessels used for natural gas storage offered the desired low weight and affordability for automotive fuel storage. They constructed 1/5-scale tanks for testing.

The researchers coated standard, hydrogen pressure vessels with multiple layers of metallized Mylar, then built an outer jacket to induce a vacuum around the pressure vessel. The vacuum jacket reduces air convection and conduction of heat. Radiant heat is reflected back by the layers of Mylar. Because a tiny portion of heat can pass through a single layer, Lawrence Livermore applies about 80 layers of reflective material on its tanks.

A .30 caliber armor piercing bullet fired from 50 feet punctured Livermore's insulated cryogenic hydrogen tank without fragmenting it, thus meeting DOT and SAE standards.

Aceves and his colleagues built openings in the outer jackets for thermocouples, strain gauges, and a capacitive level sensor to measure pressure, temperature, and level within the tanks. They also equipped the tanks with safety devices to prevent catastrophic failure in case hydrogen leaked into the vacuum. These were relief valves that would open if the pressure limits were exceeded, and rupture discs if the relief valves failed.

The researchers then subjected the tanks to a variety of Department of Transportation and Society of Automotive Engineers tests at Lawrence Livermore's High Pressure Laboratory.

For example, the outer jacket did not leak after being subjected to the stresses of 1,000 vacuum cycles. A cold shock test involved filling the tank with compressed helium to 1.2 times the maximum allowable working pressure for 30 minutes. Then, the vessel was cycled three times in succession from ambient to low, liquid nitrogen temperatures, and finally, leak tested with helium pressurized 25 percent above the maximum allowable working pressure. A mass spectrometer showed no leaks after this phase of testing.

The researchers then tested their insulated pressure vessels by filling them with gaseous, low-temperature hydrogen and cryogenic, liquid hydrogen. The tanks did not leak or become damaged.

Whenever the tanks were filled with volatile hydrogen, they were tested at the national laboratory's High Explosives Facility. "We stayed in a safety bunker while the tanks were tested in the open," said Aceves.

The laboratory subjected the tanks to a variety of certification tests to meet the standards of the U.S. Department of Transportation. These included cycling the tanks, at ambient temperatures, 10,000 times from less than 10 percent of the service pressure to full service pressure.

In a series of environmental cycling tests, the tanks were filled with gaseous hydrogen to determine their performance in extreme climates. First, the tanks were cycled 5,000 times from zero to full service pressure, with an internal tank temperature of 140°F and external ambient air temperatures in 95 percent humidity. Then, the tanks were cycled 5,000 times from zero to service pressure at -60°F tank temperature.

The tanks were subjected to 20 thermal cycles, with tank temperatures ranging from 200°F to -60°F at full service pressure.

All cycling tests were passed successfully, as was the dramatic gunfire test performed by Authorized Testing Inc. in Riverside, Calif. A .30 caliber armor-piercing bullet was fired at the tank from a range of 50 feet. Despite being pierced by the bullet, the tank did not fragment and met the standards. The tanks were placed in bonfires to demonstrate that their safety release device would vent gas to prevent an explosion. The tanks also survived, or suffered acceptable damage, after being dropped from 10 feet onto hard surfaces.

Despite being charred from exposure to a test fire, the Livermore cryogenic tank retains its structural integrity, a key safety consideration.

In the next few months, Lawrence Livermore will fill its insulated tanks with liquid nitrogen and subject them to drop and bonfire testing to earn SAE certification for cryogenic performance. "After we pass the SAE tests, we want to install the insulated cryogenic tanks on vehicles for field testing," Aceves said.

Lawrence Livermore already has partners in getting its insulated hydrogen tanks on the road. They include Structural Composites Industries, a leading manufacturer of pressure vessels based in Pomona, Calif., and Sunline Transit of Thousand Palms, Calif. Sunline is the mass transit agency serving the Palm Springs area and has agreed to install two insulated cryogenic tanks on pickup trucks. "One, a Ford F 250, will carry liquefied natural gas, and the other, a Ford Ranger, will hold liquid hydrogen," Aceves said.

The project is funded by the Department of Energy's hydrogen program and the South Coast Air Quality Management District.

Hydrides on a Diet

Compressed gas tanks store hydrogen as a gas, and their cryogenic counterparts store it as a liquid. A less familiar method of storing hydrogen is as a solid in metal hydrides, alloys of rare earth, transition metal, and magnesium. These granulated materials absorb hydrogen. Because the hydrogen is chemically bonded to the alloys, heat is required to release it.

Among the advantages metal hydride has in fuel storage is its compactness. It holds the target fuel value of 5 kg of hydrogen in one-third the volume of a gaseous hydrogen tank at 5,000 psi, and one-quarter the volume of a hydrogen tank at 3,600 psi.

Metal hydrides are also inherently safe. Hydrogen is held in the hydrides at a low pressure-less than 200 psi-at ambient pressure. In case of a crash, the hydrides will not discharge their hydrogen cargo because it is chemically bonded to the hydrides in a solid state.

These pluses are literally outweighed by metal hydrides' biggest drawback: They are heavy. "A metal hydride storage system that can hold 5 kg of hydrogen, including the alloy, container, and heat exchangers, would weigh approximately 300 kg, which would lower the fuel efficiency of the vehicle," according to Rosa Young, a physicist and vice president of advanced materials development at Energy Conversion Devices in Troy, Mich. "The challenge is to formulate a high-capacity metal hydride alloy whose kinetics and thermodynamic properties, along with more efficient heat exchangers, can be used to make a lightweight hydride storage system," Young explained.

ECD specializes in developing the materials and components for alternative and advanced energy systems for homes, vehicles, and industry. The company formed a 50-50 joint venture with Texaco Energy Systems Inc. at the end of October 2000 to develop and commercialize lightweight metal hydride storage systems for fuel cell vehicles and for internal combustion cars that burn hydrogen. The venture, which was originally called Texaco Ovonic Hydrogen Systems LLC, is now known as ChevronTexaco Ovonic Hydrogen Systems.

Researchers at ChevronTexaco Ovonic Hydrogen Systems are studying the charging speed and desorption rates of the metal hydrides they have developed to store vehicular hydrogen in a solid state.

Young and her colleagues are testing a 2.5-kg hydrogen storage tank using a proprietary metal hydride at the company's laboratory. A full-size, onboard system will consist of one or two tanks. A smaller, European economy car would use one tank to feed a fuel cell that would generate about 35 kW of electricity, while an American fuel cell economy sedan would use two tanks to generate the requisite 70 kW. The fuel supply will provide a driv-ing range of more than 200 miles.

Researchers feed hydrogen directly into the tanks, where it is absorbed by the powdered alloy. As the hydrogen gas is absorbed during charging, the metal hydrides generate heat that is removed by water. When a fuel cell car is driven, the waste heat from the cell is used to release the hydrogen.

Because fuel cells generate a large amount of low-grade heat when they operate, Ovonic designed a metal hydride tank to capture a portion of the waste heat of the fuel cell as well. "Our system is part of the car's radiator, so that the radiator's weight and size can be reduced in size," Young said.

The Ovonic researchers are studying two key parameters of their storage system, namely, charging speed and the desorption rates of hydrogen during simulated driving conditions. "We are modifying the kinetics of the material by metallurgy and by designing a high-efficiency heat exchanger so that it can absorb 5 kg of hydrogen in 10 minutes, and desorb it at ambient temperature," noted Young. "We are also calibrating the use of the waste heat to provide the required release rate of hydrogen for driving."

Young said that her company will have a full-scale prototype on the road for vehicle demonstration within the present year. Because of ChevronTexaco's international presence, the joint venture is also working on hydrogen service stations using metal hydrides for hydrogen bulk storage. It is possible that the prototype onboard system and bulk storage system will be tested in the United States, Europe, and Asia.

© 2002 by The American Society of Mechanical Engineers

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