Would You Believe 633 Miles on a 40-Gallon Tank of Liquid Hydrogen?
Visitors and employees at Lawrence Livermore National Laboratory last January might have spotted a white Toyota Prius hybrid vehicle driving continuously around the square-mile site. The car was making history by setting a world record for the longest distance driven on one tank of fuel in a vehicle modified to run on hydrogen.
In setting the distance record, a group of Livermore researchers took another step toward helping create a clean transportation system based on hydrogen. The group made history with a prototype of a concept called cryogenic-compressed tanks. These superinsulated, high-pressure tanks contain extremely cold, liquid hydrogen instead of the room-temperature compressed hydrogen gas typically used in hydrogen test vehicles.
The Prius, which has a combination electric motor and small internal combustion engine, traveled 653 miles on a tank containing almost 40 gallons of liquid hydrogen. The overall fuel economy for the driving conditions used by the Livermore team was about 105 kilometers per kilogram of hydrogen, which is equivalent to about 65 miles per gallon of gasoline. Coincidently, 1 kilogram of hydrogen has about the same energy content as 1 gallon of gasoline.
"One thousand kilometers is a very long range for a hydrogen vehicle because the fuel is difficult to store compactly," says Salvador Aceves, who leads the Energy and Environment Directorate's energy conversion and storage group. Chief technician Tim Ross says, "The range we achieved is better than we expected. We originally anticipated the car would travel about 800 kilometers (500 miles) on 10 kilograms of hydrogen." He describes the group's Prius as a "moving hydrogen-storage technology test bed."
For more than a decade, the Livermore group has been working on alternative, environmentally clean energy technologies for transportation, including hydrogen-fueled vehicles. The group has expertise in mechanical engineering, physics, analytical chemistry, hydrogen storage and usage, combustion engineering and modeling and energy modeling.
The team previously developed a conceptual design for a hydrogen hybrid vehicle that combines a small piston engine with an electrical generator. Other projects have included the development of advanced analysis capabilities for high-efficiency, clean engines and a large engine for stationary energy generation. The team has collaborated with companies such as Ford, BMW, General Atomics, Caterpillar, Navistar International and Cummins.
Dependence on Fossil Fuels
The U.S. transportation sector is almost 100 percent dependent on fossil fuels. Because transportation accounts for more than two-thirds of the petroleum consumed daily in the nation, DOE's hydrogen program focuses primarily on developing hydrogen technology for this sector.
Today, more than 500 hydrogen-powered cars are on the road worldwide. Most use internal combustion engines, which are converted to run on hydrogen with only minor modifications to the fuel-injection system. When burning hydrogen, they generate zero greenhouse gases and only small amounts of nitrogen oxides.
A more energy-efficient use of hydrogen would entail replacing the internal combustion engine with fuel cells and an electric motor. In fuel cells, hydrogen reacts with oxygen, producing electricity to power the vehicle. Water vapor is the only emission. Although research is progressing at a fast pace, fuel cells are still quite expensive.
Hydrogen Offers Big Advantages
Many energy scientists are optimistic that hydrogen-burning vehicles will not only help the nation's energy consumption but also curb the release of greenhouse gases such as carbon dioxide. "Increasing use efficiency is an important first step but may not be enough for steep reductions in petroleum dependence and greenhouse-gas emissions," says Aceves. "We ultimately need to advance to a carbonless energy system using hydrogen fuel.
"Hydrogen-fueled vehicles enable carbonless transportation. We can't collect all the greenhouse gases from the tailpipes of cars and trucks. Instead, we can make hydrogen from natural gas or coal and sequester underground the greenhouse gases that are generated during production." A better choice, he says, is to use nuclear power or renewable energy (wind, solar or biomass) to drive electrolysis, which splits water into hydrogen and oxygen and does not generate any pollutants.
Most prototype hydrogen vehicles use compressed hydrogen stored at room temperature and high pressure (35 to 70 megapascals). Despite hydrogen's stellar fuel efficiency, it is difficult to store compressed hydrogen in the large quantities needed to provide the driving range achieved by gasoline- and diesel-powered vehicles. The energy density of compressed hydrogen is only about one-twelfth that of gasoline. As a result, hydrogen cars use large high-pressure tanks often located in the trunk. According to DOE studies, these cars have a driving range of up to 180 miles, adequate for around town or most commutes but not for a long trip.
Cooling Is Key
Like all gases, compressed hydrogen can be stored more compactly at colder temperatures. Pressurized hydrogen at 35 megapascals becomes twice as dense when cooled from ambient temperature to —€“150°C. Cooling it further to —€“210°C (close to that of liquid nitrogen) triples the energy density. Cooling hydrogen also lowers the potential risk of a sudden tank rupture, increasing the safety factor.
The team has focused on liquid hydrogen (—€“253°C) because it does not require a high-pressure tank, and it takes up one-third the volume of compressed hydrogen at room temperature. Liquid hydrogen can also be delivered in large quantities (up to 4,000 kilograms) cost-effectively by truck. Finally, liquid hydrogen is relatively safe to store in compact, lightweight, low-pressure containers that depend on superinsulation instead of refrigeration to keep the hydrogen extremely cold.
A major drawback to using liquid hydrogen is the significant electricity required to liquefy it (about equal to 30 percent of the energy content of the hydrogen molecule). In addition, liquid hydrogen is extremely sensitive to heat; it expands significantly when warmed only a few degrees. As a result, vehicles that use low-pressure tanks are usually not filled to maximum capacity and must have a system to release some of the hydrogen vapor that accumulates in the tank when the car is not driven for several days.
In a parked car, the tank pressure can build until it surpasses the service pressure (the pressure for which the tank was built). At this point, the hydrogen is released through a safety valve from the vessel. A driver leaving his or her car at the airport for a long time, for example, might find the tank empty upon returning. Drivers whose cars also run on gasoline, such as the new dual-fuel BMWs, would not have this problem because they could use the gasoline fuel.
Alternatively, a high-pressure tank could be used for hydrogen-fuel storage. While researching hydrogen car concepts nearly 10 years ago, the Livermore team designed a high-pressure tank for low-temperature hydrogen that was safe, compact, lightweight and superinsulated. The tank comprises a high-pressure inner vessel made of carbon-fiber-coated aluminum, a vacuum space filled with numerous sheets of highly reflective plastic and an outer jacket of stainless steel.
The Livermore design allows a driver to refuel with liquid hydrogen or compressed hydrogen gas, or with hydrogen at any temperature and pressure in between. If the insulated pressure vessel is fueled with liquid hydrogen, it goes a long way toward solving the problems associated with low-pressure tanks such as having to vent the buildup of hydrogen gas.
A driver could refuel most of the time with room-temperature compressed hydrogen, likely purchased at a lower cost, and also have the flexibility of using liquid hydrogen at any time to greatly extend the driving range. In this way, a driver could use room-temperature compressed hydrogen for short trips and liquid hydrogen for longer trips.
In July 2006, the laboratory acquired a Prius, which had been converted to run on hydrogen by Quantum Fuel Systems Technologies Worldwide, Inc., of Irvine, Calif. "We chose the Prius because it was energy-efficient and required only a small tank of liquid hydrogen," says Aceves. The Prius was also selected because Quantum was already converting about 40 of the cars for use in cities in the Los Angeles area.
The modified Prius arrived with two room-temperature compressed hydrogen tanks rated at a pressure of 34 megapascals. The tanks could store a total of 1.8 kilograms of compressed gaseous hydrogen, enough to travel about 90 miles. Livermore researchers removed the tanks and installed their second-generation tank system, relocating the Prius's electric motor batteries to underneath the car.
The test drive was conducted 25 to 35 miles per hour because of site speed limits. Aceves estimates that under more realistic driving conditions, the Prius would average about 88 kilometers (55 miles) per kilogram instead of the 105 kilometers (65 miles) per kilogram achieved at the low speeds.
One Small Step
Despite the resounding success of the Livermore system, Aceves cautions that a great deal more research is needed before the design is ready for mass-produced hydrogen vehicles. The team is collaborating with industry to design more compact vessels with improved thermal endurance. In addition, the team has had discussions with both domestic and foreign automakers. A third-generation tank design is planned, with even greater performance expected.
Arnie Heller is a science writer at Lawrence Livermore National Laboratory.