In an age where recycling is socially acceptable and increasingly mandatory, the auto industry remains one of the largest environmental offenders. Some authorities blame auto emissions for 60% of all air pollution in U.S. urban areas. Small wonder, then, that automakers are embracing electric, hybrid-electric systems, and fuel cells as eco-friendly sources of power for future vehicles.
Who’s jumping on the green bandwagon? Ford has developed the Ranger EV, an electric pickup which boasts zero emissions, and joined Daimler-Benz and fuel-cell producer Ballard Power Systems Inc., Vancouver, B.C., in the quest for economical fuel cells. According to Daimler-Benz, the focus is on pushing fuel-cell technology to operational maturity by the middle of the coming decade. In 1997, Honda introduced the EV Plus electric vehicle to California, also a zero-emissions vehicle, but which carries 1,000 lb of batteries. Toyota is marketing Prius, said to be the world’s first hybrid vehicle, which runs on an internal-combustion engine and a battery-powered electric motor. And, not to be outdone by the Daimler-Benz partnership, General Motors and Toyota Motors also expect to have fuel-cell vehicles in production by 2004-05.
However, none of these technologies are as yet ready for large-scale production. Details must be worked out such as improving power and energy efficiency of full-size battery cells, as well as safety issues and high costs. Developers of power plants, such as the compression-ignition direct-injection engine, struggle with reducing nitrogen oxide and particulate emissions, costs, and overall vehicle weight. Costs, methods of mass manufacturing, and fuel selection are challenges remaining for fuel-cell development.
The ultimate clean machine
Electric vehicles store electricity in large, rechargeable batteries. A controller sends juice to an electric drive motor when the driver hits the accelerator. “Refueling” comes via a charge plug inserted into a 120 or 240-V charge receptacle. Regenerative brakes use the motor to help brake the vehicle, generating electricity and charging the battery.
EVs today use relatively inexpensive lead-acid batteries, but with the downside of a 50-mile range. Battery technologies such as nickel-metal hydride or lithium ion could potentially double the range of EVs, and equal the performance of gasoline-powered vehicles. Both are advanced batteries, producing more energy per unit of weight than a lead-acid battery. Lithium-ion batteries have about 40% more energy per unit of weight than nickel-metal hydride.
Lithium-ion batteries transport lithium ions back and forth between a metal or graphite solution of lithium and magnesium oxide. The downside of lithium-ion batteries is that they retain a memory of the number of cycles completed, which shortens battery life. Nickel-metal hydride batteries generate hydrogen gas. But the batteries retain no cycling knowledge, resulting in long life. For example, on a vehicle battery, nickel-metal hydride batteries complete about 1,200 full cycles compared to 400 for lithium ion.
According to John Wallace, director of Alternative Fuel Vehicles for Ford Motor Co., the best battery for a production electric vehicle is nickel-metal hydride. “The vast majority of manufacturers will use the nickel-metal hydride battery because it’s durable,” he says. “It withstands a fair amount of abuse and has a long life.” The challenge with nickel-metal hydride batteries is the high cost of materials. Lithium-ion batteries have low material costs, but present manufacturing difficulties because they are expensive and difficult to build. Safety issues include temperature limits for lithium-ion batteries, and preventing the generation of explosive gases, such as hydrogen, in nickel-metal hydride batteries.
In 1991, the United States Advanced Battery Consortium was formed. It consists of the Big Three automakers, the U.S. Department of Energy, the Electric Power Research Institute, and several battery manufacturers. The USABC focuses on developing battery technologies to make electric vehicles practical on a large scale. The partnership’s long-term goals include a battery with 400 W/kg of power, 200 W/kg of energy, and a 10-yr life, at a cost of $100 or less per kW hr. What this translates to is 0 to 60 mph in 9 sec, a driving range of 200 miles, and a $4,000 battery cost.
Hybrid-electric vehicles contain electric motors and fuel-based engines. While not as clean as EVs, an HEV is comparable in performance to internal-combustion-engine vehicles, but with much lower emissions. Two power sources sit onboard. Sources such as an internal-combustion engine, gas turbine, or fuel cell, convert fuel into energy. An electric motor powered by an energy storage device, such as a battery, lowers the demand placed on the first power source.
If the two power sources are arranged in parallel, a vehicle can use either one or both simultaneously, depending on the driving situation. For example, the electric motor might power the vehicle in the city or over flat terrain. When accelerating and climbing hills, the two sources work together. The electric motor can also operate as a generator to slow or stop the vehicle, capturing energy lost during braking and regenerating it into electricity for later use. HEVs use high-power batteries such as nickel-metal hydride or lithium ion to store the energy powering the electric motor.
As part of the HEV system, researchers are considering direct-injection engines, where fuel is injected directly into each cylinder. The DI engine works with the electric motor, turning itself off automatically when not needed, increasing mileage, and reducing emissions.
Another HEV option is the compression-ignition direct-injection engine. Combustion is triggered by highly compressing the air-fuel mixture so it self-ignites instead of using spark plugs. When coupled with synthetic fuels made from natural gas such as dimethyl ether or Fischer Tropsch, the CIDIs produce almost no particulates. The goal of CIDI technology is to achieve a 0.04 gm/mile particulate emissions level or better. This is being accomplished by limiting engine controls that reduce exhaust gas recirculation to minimize energy-out particulate emissions, and by lowering nitrogen oxides by using catalytic aftertreatment. The reduction of nitrogen oxides remains one of the biggest challenges for the CIDI engines.
The principle behind fuel cells is simple. A chemical reaction occurs between hydrogen and oxygen, triggered by a catalyst, to form electricity, water, and heat. A fuel cell consists of two electrodes, the anode and cathode, separated by an ion-conducting membrane electrolyte. The electrodes have a thin layer of platinum or nickel catalyst on one side. Hydrogen is fed to the anode while oxygen is fed to the cathode. The hydrogen fuel disassociates into free electrons and protons at the anode’s catalyst. The free electrons become electric current through an external circuit, while the protons migrate through the electrolyte to the cathode. Oxygen, electrons flowing through the external circuit, and protons combine at the cathode to form pure water and heat.
A key part of the fuel cell is the polymer membrane electrolyte which is sandwiched between channeled-graphite flow-field plates. On one side, the field-plate channels let hydrogen flow through to the anode while the platinum catalyst separates it into electrons and protons. On the opposite side, the channels conduct the air to the cathode where oxygen attracts the hydrogen protons through the membrane. Air removes the water created in the electrochemical process and is the system’s only emission besides heat.
There are five types of fuel cells, each using different materials as electrolytes. The electrolyte in proton-exchange-membrane (PEM) fuel cells is a solid polymer membrane. These cells feature 1 kW per liter of volumetric power density, operate at 85°C, respond immediately to changes in electrical demand, and will not leak or corrode. Low operating temperatures compared to other types of cells let manufacturers use less-expensive materials, a bonus for mass production. PEM fuel cells are considered the most appropriate for automotive applications.
Phosphoric-acid fuel cells use this corrosive liquid as an electrolyte and generate power at 200°C. They have operating efficiencies of 40 to 55%. Because of complex designs, high operating temperatures, and high cost, these cells work well in mid to large stationary power plants.
Molten-carbonate fuel cells get their name from a low-melting electrolyte combination of lithium carbonate and potassium carbonate. Though they boast efficiencies of 55 to 80%, operating temperatures of 650°C, expensive materials, potential leakage, vaporization, and corrosion of the electrolyte will probably keep commercial development at least 10 years away. The cells are expected to be suitable for base-load electric generation.
Solid-oxide fuel cells operate at 1,000°C, with high energy efficiencies of 55 to 80%. The electrolyte is a thin layer of yttria-stabilized zirconia that tolerates relatively impure gases. Commercialization of these cells is said to be 20 years away.
Alkaline fuel cells first debuted in the Apollo space program. An improved version is in the works to power electrical systems in the Space Shuttle. Potassium hydroxide serves as the electrolyte but has an extremely low tolerance to carbon dioxide. This limits applications to space programs.
Fuel cells versus ICEs
By 2003, California will require 10% of registered vehicles to have zero emissions. Fuel cells could easily accomplish this task. The pollutants coming from internal-combustion engines include sulphur dioxides, nitrogen oxides, carbon monoxide, particulates, and reactive organic gases. Vehicles that convert liquid methanol to hydrogen onboard are not strictly zero-emission vehicles because their fuel cells release tiny amounts of nitrogen oxides and carbon monoxide during reformation. However, these emissions are minor compared to the pollutants emitted by ICEs.
Fuel cells excel in a variety of ways compared to internal combustion engines. ICEs burn fuel to create heat which is converted into mechanical energy and motion. They have efficiencies in the 15 to 20% range. The conversion process loses efficiency by heat and friction loss. Conversely, fuel cells can be refueled similar to ICEs and have a long life because they contain no moving parts. Fuel cells using hydrogen gas boast efficiencies of 50 to 60%. Those that use liquid methanol as fuel exhibit about 40% efficiency because of losses in the process required to obtain hydrogen from methanol. Factors such as the need to come up to temperature and cool down reduce the efficiency of the fuel-cell system. But for vehicles having the potential of 80 mpg, some difficulties are expected.
Fuel cells also look like a better bet than batteries. Because batteries only store and do not produce energy, they must be recharged from an outside source, reducing efficiency. And despite decades of research, they are also plagued by short travel ranges and lower power densities than fuel cells.
Hurdles to overcome
Though fuel-cell technology continues to grow, obstacles remain. According to Ken Dircks, marketing manager for the Ballard Automotive division of Ballard Power Systems Inc., the biggest challenge facing fuel-cell development is fuel selection. “We are looking at a fuel that will work for an early introduction, but also allow expansion into mass-market acceptance,” says Dircks. Fuel cells consume hydrogen, but it’s tough to store gaseous hydrogen onboard a passenger vehicle. High-pressure storage of hydrogen requires large tanks and presents potential leaking and refueling problems. Hydrogen fuel is also not readily available for mass manufacturing and distribution.
Methanol is in the running as a fuel source. It’s a liquid hydrocarbon pumped onboard a vehicle, just like conventional gasoline, and does not require the massive storage tanks needed for hydrogen gas. Another advantage is its low reforming temperature. Methanol yields hydrogen through a process called reforming, where methanol and water are converted into hydrogen gas and carbon dioxide. Although it is a greenhouse gas, carbon dioxide is generated only in small amounts. This makes methanol look like the winner if it can be produced in sufficient volumes.
The process of getting hydrogen out of conventional hydrocarbon fuel is a subject of intense research. “To convert regular gasoline into a hydrogen fuel onboard a vehicle is not particularly easy,” says Dircks. “So we’re looking at other hydrocarbons as well, such as naptha.” Naptha is a homogenous, clear-liquid gas, similar to camping-stove gas. Gasoline has additives, making it a heterogeneous fuel. Thus, naptha is easier than gasoline to reform onboard a vehicle. The same distribution system and most of the refineries existing today could be used to produce naptha fuel.
Numerous companies are working on fuel processing, among them International Fuel Cells Corp., South Windsor, Conn. According to Bill Hahn, vice president for automotive fuel cells at IFC, the idea of reforming gasoline is a sound one. “The fuel must come from somewhere, and petroleum is already an established source. However, if you’re going to have a synthetic fuel, the concern lies with the process and what it does. This results in two challenges: how onboard fuel reformation takes place, and how to reduce the cost and size of the reformer.” Adding a reformer to a fuel-cell power plant makes the overall system more complicated. Simultaneously striving to reduce cost, size, and weight of reformers and fuel cells doubles the challenge.
Cost is another issue biting at the heels of fuel-cell development. Costs must be cut if fuel cells are to compete with ICEs. Currently, fuel cells cost approximately $500/kW and automakers need that figure to be at least $50/kW or lower. Experts say vehicles require at least an 80-kW fuel cell for good performance. Thus, the price-per-kilowatt must drop for vehicles to be affordable.
“The general feeling in the automotive industry is that it’s very difficult to sell a vehicle at a premium and have it be environmentally friendly,” says Dircks. “Therefore, environmentally friendly vehicles need to be at the same cost or preferably lower than current vehicles because the market will not bear higher costs.” On a positive note, costs of fuel-cell materials have dropped, in part because manufacturers have been able to reduce the platinum concentration in catalysts. Material improvements are also taking place in membrane technology, flow plates, and advanced catalysts.
Putting aside cost and fuel issues, left looming is the giant hurdle of internal-combustion-engine history. There are decades of manufacturing, parts, and maintenance networks to battle. And, there is the psychological stigma associated with zero-emissions vehicles. The absence of moving parts in batteries and fuel cells means there’s no noise, eliminating the familiar rev of ICEs. And, to some people, that just isn’t right.
Plastics get inside fuel cells
Increasing performance and keeping costs down are important factors in fuel-cell development. This involves the use of new materials, such as plastics, to increase performance while minimizing costly graphite used in flow-field plates.
Ballard Power Systems Inc. turned to Mack Plastics Corp., Bristol, R.I., to design and manufacture the component, a plastic manifold. The manifold is positioned on the ends of the flow-field plates in the fuel cell. The plates transport fuel and air to a membrane electrode assembly. Three intricately placed holes let air, hydrogen, and water flow through the manifold. Equally spaced apart, the holes have a contoured ridge that prevents hydrogen from escaping.
The part requires a material flexible enough to maintain height tolerances and form a contoured seal. Inconsistent wall thickness and tight flatness requirements add to the challenge. Engineers from Ballard use Ultem polyetherimide (PEI) resin from GE Plastics, Pittsfield, Mass., for its high-temperature resistance and dimensional stability.
Molding the parts was even a challenge for tool designers. Filling the large, thick-walled parts was difficult because of features such as 15 small, thin-walled tubes in one side of the manifold. Concerns included minimizing sinks and voids in thick sections, filling the tubes, and limiting flash, while maintaining the dimensional and flatness tolerances. GE Plastics and Polymer Solutions collaborated to address molding concerns. They collectively identified critical-molding parameters, running extensive mold-flow analyses on tool models to determine optimum gating and other design parameters.