Fuel Cells for the Long Haul, Batteries for the Spurts
Just as hybrid vehicles are coming to market, an alternative technology—the fuel cell—has moved a good distance from the laboratory to the road
This is part of IEEE Spectrum's special report: Always On: Living in a Networked World.
Normally home to raucous Formula 1 racing cars, Montreal's Grand Prix racetrack, the Circuit Gilles-Villeneuve, is not the obvious place to look for the latest in environmentally benign vehicles. Still, it was there that Ford Motor Co., Dearborn, Mich., unveiled its first fuel-cell demonstration vehicle less than three months ago: a full-sized Focus sedan powered by a hydrogen fuel-cell system. The choice of venue was not random: Montreal was the host city for EVS-17, the 17th International Electric Vehicle Symposium, held 16-18 October 2000, during which the vehicle made its debut. But it may also have been symbolic--acknowledgment by a major automaker that environmentally sound vehicles were about ripe for the fast track.
A product of Ford Motor's new environmentally focused Think Group, the hydrogen-carrying vehicle, like Toyota's FCHV [see image above], is not and probably never will be commercially available. But it marks a decisive change in the automotive industry's thinking about the future, making it one of the more important technobusiness developments of recent times.
Until only a few years ago, when environmentalists, automakers, and other concerned parties spoke about low-emissions vehicles, they were almost always referring to electric vehicles (EVs)--cars, trucks, and buses powered by batteries of one kind or another. Today, having so far failed in their quest for a battery that could make EVs practical, they are looking more and more to hybrid electric vehicles (HEVs), which automakers had hitherto rejected as merely an interim solution. Batteries are not actually dead, but after more than a century of development, they are still not satisfactory for automotive propulsion. They weigh too much, cost too much, and take too long to charge. But hybrids based on a combination of electric motors and internal combustion engines are attractive for two main reasons: they require no technology breakthroughs and no new infrastructure. They work with existing batteries since they do not rely on them for primary energy storage; rather, they can obtain fuel at any service station.
The battery in a modern HEV serves essentially as a peak leveler, providing bursts of power for acceleration and hill climbing. That allows the internal combustion engine (ICE) to run in the vicinity of the "sweet spot" in its operating map, where its efficiency is high and emissions low. Interestingly, it is the high efficiency, translated into good fuel economy, that appeals to consumers today even though the motivation behind their development initially was the reduction of air pollution.
The first major automaker with a hybrid electric vehicle for sale to the general public was Toyota Motor Corp., of Toyota City, Japan. Its Prius HEV went on sale in 1999 in Japan, where about 40 000 of them have so far been sold. It is now also available in North America. Although the Prius uses its ICE most of the time, it operates as a "pure" (battery-only) EV at very low speeds--when creeping along in stop-and-go traffic, for example--unless its battery is low on charge.
Honda Motor Co., Tokyo, beat Toyota into the North American market with its answer to the Prius, which it called the Insight. The two cars are alike in being parallel hybrids--they can deliver power to the wheels from both the ICE and the electric motor. In a series hybrid--the other kind of HEV--all traction power comes from the motor. The ICE never drives the wheels directly, but instead powers a generator that keeps the battery charged.
The advantages of hybrid EVs over pure EVs are clear. They can be gassed up like any other car, and need no special charging infrastructure since they keep their batteries charged from the engine/generator and via regenerative braking: pressing the brake pedal reverses the action of a vehicle's electric motor, so that it acts as a generator, converting the kinetic energy of the vehicle's motion into reusable electricity instead of wasting it as heat. (For emergency stops, HEVs also have hydraulic brakes, activated when the driver really tromps on the brake pedal.)
But drawbacks remain. Like conventional vehicles, HEVs are not zero-emission vehicles. Worse, as they age (or if their maintenance is neglected) they risk becoming serious polluters. They are more complex than conventional vehicles and may be expected to have many of the same reliability and maintenance problems. And, of course, their use of traditional fossil fuels is a weakness as well as a strength.
Fuel cells outfox technology forecasters
So it's interesting that just as HEVs are entering the market, an alternative technology--the fuel cell--has moved a good distance down the road from laboratory phenomenon to commercial highway product. Like batteries, fuel cells have confounded the technology fortune-tellers, but in the opposite way. Ten years ago the conventional wisdom said that a good EV battery was just around the corner, whereas fuel cells were still a pipe dream. But in what is no doubt some sort of corollary of Murphy's Law, not only has battery progress been disappointing, fuel cell progress has been truly spectacular.
The basic principle of the fuel cell has been known for over 150 years [see "First Among Fuel Cells," below], but a variety of knotty technical and economic problems have kept the devices from the engineering mainstream. They were used in the Gemini and Apollo spacecraft, where they proved reliable but too bulky and expensive for general use. It was in 1995 that the kilowatt-per-liter power-density barrier was first breached, by researchers at Ballard Power Systems Inc., Burnaby, B.C., Canada. They not only beat the state of the art that prevailed just five years earlier but did so by an order of magnitude. Today, the company reports power densities in excess of 1.3 kW/L.
Comparing fuel cells with batteries can be confusing. Batteries store energy. Fuel cells convert the chemical energy in a fuel directly into electricity through an electrochemical reaction. A hydrogen fuel cell will supply electric power so long as it is provided with hydrogen and oxygen. The fuel cell itself does not discharge or become depleted of energy, as a battery does. For a car powered by a fuel cell, the storage function is performed by a fuel tank, as in conventional vehicles.
Storing the hydrogen
This raises a host of interesting issues. First of all, although hydrogen has a very high specific energy--almost 150 megajoules per kilogram--it is so light that a liter of it pressurized to 35 megapascals (about 350 atmospheres) weighs just 31 grams and packs only 4.4 MJ of energy. Gasoline, by contrast, has a lower specific energy of about 50 MJ/kg, but a liter is equivalent to about 30 MJ. Admittedly, fuel cells and electric motors are a lot more efficient than ICEs; nevertheless, to give a fairly efficient car a range of 500 km or so takes about 6 kg of hydrogen. Compressed to 35 MPa, the gas itself would occupy about 200 L. Add tanks, piping, valves, regulators, and so on, and the space required could double.
Several avenues are being explored for dealing with that problem. Ovonic Battery Co., the Troy, Mich., company behind the nickel/metal-hydride battery, has applied its metal-hydride expertise and developed metal alloys that can store about 7 percent of their weight in hydrogen at the fairly low pressure of 200 kPa. With that technology, according to the company, 6 kg of hydrogen can be stored in a system occupying 120 L, or about twice the size of the gas tank on a mid-sized automobile.
But the weight of the storage system remains a problem, and the question remains, where is the hydrogen to come from? No infrastructure exists, and no one seems eager to install one. Valid or not, mental images of the Hindenburg zeppelin disaster contribute to a general feeling that there must be a better way.
Consider an alternative to storing hydrogen, namely, generating it as needed from a more convenient material. Several major automakers have sponsored the development of compact chemical plants, called reformers, for extracting hydrogen from common fuels like methanol and gasoline. Whether gasoline is safer than hydrogen as a traveling companion is open to question, but there is certainly a popular impression to that effect. Moreover, a hydrogen fuel-cell system based on reformed gasoline eliminates the infrastructure problem completely.
On the other hand, reformers tend to be large and bulky. Like ICEs, they are complex and can become serious polluters if not well maintained.
Electricity to the rescue?
But there are still other alternatives: fuels that can be replenished electrically, ideally from hydroelectric or other nonpolluting sources. One such fuel is an aqueous solution of sodium borohydride (NaBH4), which yields hydrogen gas and a sodium borate (NaBO2) solution on exposure to a catalyst. Millennium Cell Inc., Eatontown, N.J., has developed a suitable catalyst, based on rhodium, and has demonstrated the technical viability of the concept.
The fact that the starting point for making sodium borohydride is an irreplaceable mineral--borax--is no drawback, for unlike traditional fossil fuels, it is recyclable. Converted into NaBH4, and used as a fuel, the resulting NaBO2 can be converted back into NaBH4 in an electrochemical process, although, of course, infrastructure for doing so remains to be built. In this scheme, the boron compounds serve more as a mechanism for transporting energy than as a fuel.
The question naturally arises, can any other materials be used in a similar fashion? In a word, yes. Light metals like zinc and aluminum also lend themselves to similar strategies. Metallic Power Inc., of Carlsbad, Calif., is working on a system in which zinc granules 0.5-0.8 mm in diameter are oxidized in a zinc-air fuel cell to generate electricity and zinc oxide. The oxide is later regenerated back into zinc.
In one approach to refueling, a vehicle would be refueled at a recycling/refueling station that would remove zinc oxide from the vehicle and replace it with fresh zinc, pumping both in a stream of flowing electrolyte--an aqueous potassium hydroxide solution. An electrolysis process is used at the station to convert the zinc oxide back to zinc metal pellets.
Meanwhile, back on the East Coast, the folks at EVonyx Inc., Hawthorne, N.Y., have devised a completely solid-state aluminum-air fuel cell based on a proprietary membrane electrolyte. The device uses no liquids, just a rolled up three-layer sandwich--aluminum anode, membrane electrolyte, and air cathode. As a sealed unit, it becomes a rechargeable battery in which aluminum is oxidized on discharge and the resulting oxide is reduced during recharge. Or, as a fuel cell, it has a mechanism for replacing spent rolls of aluminum oxide with fresh rolls of aluminum. Since the device is both a battery and a fuel cell, EVonyx calls it the RPC, for revolutionary power cell.
That's where things stand at the threshold of a new millennium. After years of what can only be described as avoiding the issue, the big automakers finally seem ready to take action with regard to an issue that will affect not only transportation, but public health as well.
To Probe Further
Good Web sites for keeping up with developments in low-emission vehicles are mounted by electric vehicle associations around the world, such as the Electric Vehicle Association of the Americas, at http://www.evaa.org; and the European Electric Road Vehicle Association (Avere) at http://www.avere.org/.
For a discussion of hybrid electric vehicles see "Hybrid electric vehicles take to the streets," David Hermance and Shoichi Sasaki, [Spectrum, November 1998, pp. 48-52].