CAR WARS

Fuel cells have the potential to power more energy-efficient vehicles, produce less harmful emissions and increase the United States’ energy independence. But is the American public ready for such change?

Texas Tech University researchers say public acceptance is an overriding factor in any success of the fuel cell in the future. "You can design the best car in the world safety-wise, fuel efficiency-wise and pollution-wise and no one will buy it unless the car is something the public wants," said Michael Parten, Ph.D., Texas Tech professor of electrical engineering.

Scientists debate over which solution is the best. Some scientists indicate a hybrid vehicle that relies on gasoline and electricity is the best option. And others argue that a hydrogen fuel cell is the superior choice. Parten and Timothy Maxwell, Ph.D., Texas Tech associate professor of mechanical engineering, are putting more than 50 years of experience as engineers on the fuel cell.

"The typical internal combustion engine we have now is very good at some things and very bad at other things. And by mating it with an electrical counterpart system, you can mitigate some of the problems, but there’s a limiting factor, the battery. They are big, heavy, expensive and don’t last very long," Parten said. "So how can you get rid of the battery? The ultimate solution is the hydrogen-based fuel cell."

Parten said a small fuel cell could reduce the batteries required for a hybrid vehicle while a large fuel cell could eliminate the need for the internal combustion engine altogether.

The engineers say a hydrogen-based fuel cell could provide the power necessary to give a totally electric vehicle the same range as a modern, gasoline-powered vehicle. In this case, a fuel cell, which has been around for more than 100 years, converts hydrogen into electricity by a simple oxidation reaction. The products of the electrochemical process are electricity, heat and water.

"In a fuel cell powered vehicle, some battery power would be required for start up. The fuel cell would provide the driving power to the wheels. Because the output electrical power of the fuel cell stack is designed to exceed the average power demands of the vehicle, the batteries can be recharged while driving. The vehicle’s range of travel is then tied to the amount of hydrogen or fuel that is on board," Parten said.

Maxwell explained how the fuel cell works. "The system we are using is a proton exchange membrane or PEM fuel cell. The hydrogen is stored in high-pressure tanks and the oxygen is taken from the air. A single fuel cell is comprised of a sandwich-like structure much less than an inch thick and several inches across. On one side of the cell is passage for hydrogen to enter and on the other side is a similar passage for air to enter," he said.

"At that point," Maxwell said, "the hydrogen and air come in contact with porous metal plates, the anode and cathode of the cell. As the hydrogen molecules pass through the porous plate, a catalyst on the surface of the plate causes the electrons to be released, and they pass out through the anode plate to become the electrical current. The remaining hydrogen protons then pass through a membrane material to the cathode porous plate where they meet oxygen atoms coming through the cathode plate. The electrons returning form the electric load to the cathode. When the hydrogen protons, the oxygen atoms and the electrons all meet, water is formed which leaves through the cathode plate and is exhausted with the excess air. Heat is released during the reaction."

Maxwell said the fuel cell stack is made of many cells connected in a series so that the fuel cell stack can provide significant current at a reasonable voltage. "The unit we are installing in the Ford Explorer has six sub-stacks with each containing 105 cells for a total of 630 cells. This unit, which is about the size of a typical V-6 engine and weighs about 300 pounds, will produce about 40 to 50 kilowatts of net electrical power," he said.

However attractive the fuel cell appears, these scientists agree that there are numerous problems to overcome, not the least of which is consumer acceptance.

"Right now, people aren’t concerned. People are going to buy whatever they want to buy, but at some point in time, fuel efficiency and pollution are going to be major issues," Parten said. "It’s just a matter of time."

Maxwell said many technical and economic problems must be solved before fuel cell technology can be fully realized on vehicles. "We face water management issues, hydrogen storage issues, and infrastructure issues to provide hydrogen for vehicles. Also, you have to control the fuel cell to provide the best vehicle performance and efficiency, and you have to make a system that is robust enough to survive in a vehicle environment."

Like most university research, answers to such complex problems are long in coming. Parten and Maxwell have spent five years researching the fuel cell. And still, no certain solution to better fuel economy and less pollution is available.

"The fuel cell has the potential to be a fantastic solution. There are an awful lot of things that have to be worked out," Parten said. "It is an evolutionary process."

Maxwell said vehicle fuel economy depends on a variety of factors including the efficiency of the prime mover, engines, fuel cell, drive train, transmission and electric power that is compatible with electric drive motors.

"Typical internal combustion engines are about 20-25 percent energy efficient whereas a fuel cell can be 40-60 percent efficient in terms of gross electrical power produced per quantity of hydrogen consumed," Maxwell said.

Even more factors must be factored into the equation. "The friction in the wheels and axles, the rolling resistance of the wheels on the road surface, the weight of the vehicle and the aerodynamic resistance of the vehicle must be considered," he said. "Heavier vehicles require more fuel to accelerate and to cruise at a constant speed. However, vehicle weight is primarily a problem during stop-and-go driving conditions."

Maxwell said for a typical mid-size sedan, a Ford Taurus for example, to achieve the much sought after 80 miles per gallon, its entire drive train must be 40 percent efficient in terms of power available per quantity of fuel consumed.

"Currently it is not possible to attain vehicle fuel efficiencies equivalent to the 80 mile per gallon target with an internal combustion engine-based drive train. If we start with a 30 percent efficient engine and add a 90 percent efficient transmission, this provides an internal combustion engine with a 27 percent total drive train – much lower than 40 percent. However, if we instead utilize a 50 percent efficient fuel cell and an 85 percent efficient power conversion and electric motor combination, we could have an overall drive train efficiency of 42.5 percent which is much better than the internal combustion engine-based drive train."

Unlike typical gasoline tanks stationed underneath the vehicle, hydrogen tanks will be mounted in the rear of the vehicle. For example, in a sports utility vehicle where luggage is typically stored, a tank about the size of two large suitcases will be mounted. It will be sealed off for safety, making the floor in the rear of the vehicle higher.

The biggest disadvantage with hydrogen, Maxwell said, is the space required to store it. "It’s the lightest element. Just to get a pound of hydrogen takes up a lot of volume," he said. "Now most people expect their vehicle to travel 300 to 400 miles without stopping to refuel. With hydrogen, they will not like traveling a shorter distance before it’s time to refuel. And larger tanks will take up more room in the vehicle."