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Plug-in Hybrids and Radical Batteries
Submitted by Dan Sweeney on Sat, 2008-01-26 01:04.
At the beginning of this decade the term plug-in hybrid was a word of art, known only to the precious few who were deeply preoccupied with fuel efficient vehicles. Now it is a household word as is cellulosic ethanol which was similarly obscure back in those times. Amazing what $100 per barrel oil will do to focus public attention on hitherto obscure areas of technology.
Cellulosic ethanol may well be a pipedream as a viable transportation fuel—the economics just aren't there at present—but plug-in hybrids are real and will eventually reach the market—in what numbers remains to be seen. But in order for such designs to become commonplace and accepted, some breakthroughs in battery technology are going to have to occur first. In other words, they don't really provide an answer to the problem of ascending fossil fuel prices in their current state of development.
In a plug-in obviously the electrical system is performing much of the work of traction, and so its ability to absorb and store energy becomes crucially important.
No electrical storage system comes close to equaling a tank full of gasoline in energy density by either volume or mass, and, for this reason, all electric cars made to date have traded off range in order to achieve marginally acceptable weight. An electric car capable of traveling several hundred miles on a charge would weigh tons, and indeed one quickly reaches a point where the weight of the batteries adversely affects the energy efficiency of the vehicle. And obviously that weight will adversely affect fuel efficiency when the hybrid plug-in is running on its internal combustion engine. What is needed to make the technology fully feasible is a much better means of storing electrical energy.
Lately I've been hearing reports of what may be breakthroughs or at least step improvements. But before I get to these, first let's discuss the state of the art in commercial batteries.
Traction Batteries Today
The traditional choice for electrical vehicles has been the same battery chemistry used in conventional automotive batteries for running a gas guzzler's basic electrical system, namely lead acid. While lead acids are likely to remain the battery of choice for conventional automobiles, they leave something to be desired in hybrids and pure electric vehicles.
Lead acid batteries are cheap, durable, and not particularly temperature sensitive, but even the most advanced designs suffer from low energy density. And that isn't likely to change.
Toyota in its very successful Prius hybrid has substituted the considerably more expensive but only incrementally better nickel metal hydride type, but increasing the size of the stack to permit extended all-electric cruising—the defining performance characteristic of the plug-in—results in several hundred pounds of additional avoirdupois minimally and a lot more storage space being given over to the power plant. Hardly the makings of a transportation revolution, as I see it.
Most of the proponents of plug-in hybrids are planning to use lithium ion batteries, if not initially, then in the second generation of cars, and there you're looking at weights and volumes that are only a fraction, albeit a large fraction of the lead acid's. Altair, A123, and Saft have all been researching large format electrical traction batteries based on lithium chemistry, and lithium is already available as a high cost option on ZAP electric cars.
Large format lithium represent some stubborn engineering problems, perhaps foremost the fire hazard they pose, especially in proximity to the high temperature environment of the internal combustion engine, but they're marginally feasible even today and will surely get better. The question is whether the energy density will improve significantly. Academic experiments with nanofabrication have yielded laboratory prototypes with several times the electrical output by mass of commercially available units, but whether they can be manufactured economically remains to be seen.
Meanwhile other researchers are going in other directions.
Over the Horizon
Perhaps most notoriously, a Texas based company calling itself EEStor has been claiming to have developed an ultracapacitor with both energy and power densities that are greatly superior to those of any commercial batteries.
Ultracapacitors, which have been on the market for years, store electricity as an electrostatic charge. Unlike batteries and fuel cells, no chemical ionization process takes place in them. The best designs, such as the costly Skeleton Technologies ultracaps from the Ukraine, have enormous power density, that is, the ability to dump current into a load, but at best only about half the energy density of a lead acid battery of equal weight. Since cruising range equates to energy density and acceleration to power density, such ultracapacitors may be viewed as a sort of electrical booster rocket but not a primary energy source.
I've spoken with nanotechnology experts who believe that ultracapacitors will eventually overtake batteries in energy density, so I'm not prepared to dismiss EEStor out of hand. The fact that they've been able to shake down Kleiner-Perkins Venture Capital for a lot of money also speaks well of them. Kleiner-Perkins, the nation's most successful venture capital firm, is disinclined to back losers and is famous for its prowess in conducting due diligence.
Still, EEStor has failed to produce a product after a number of premature announcements, and they've disclosed nothing of the nature of their purported breakthrough. One can hardly take their claims at face value.
The next contender is a device dubbed an UltraBattery which consists of a combination of a lead acid battery with an ultra-capacitor and an electrical power management system. No one is currently producing such systems, but extensive research has been conducted by CSIRO (Commonwealth Scientific and Industrial Research Organization) of Australia, the Furukawa Battery Corporation of Japan, and the U.S. led Advanced Lead-Acid Battery Consortium, all working in conjunction with one another. While the researchers claim some reduction in weight for the system over conventional lead acid stacks, the main benefits appear to be much longer life and very rapid charging times. One wonders if the same basic approach could be extended to lithium.
The last new technology I'll consider was developed by ReVolt, a Norwegian startup which has succeeded in recruiting an impressive team of scientists and in accumulating a lot of venture capital.
What ReVolt is attempting to do has been tried many times in the past with consistently unsatisfactory results—in other words, they're not offering fundamentally new technology. ReVolt claims to be close to succeeding this time around, however, and in making that old, failed technology practical.
What ReVolt is researching is a class of devices known as rechargeable zinc air batteries or zinc air cathode batteries which may be regarded as semi-fuel cells using atmospheric air as the negative element and stored chemicals as the positive element. Primary zinc air batteries are already widely used in hearing aids and for powering portable military communications equipment and the energy density is typically double that of the best lithiums. But the batteries are fairly expensive, and the fact that they have to be replaced or at least reconditioned at a service center has limited their acceptance.
Lots of companies have tried to make rechargeable zinc air batteries, and one defunct Florida firm whose name escapes me briefly sold such devices on the market for exorbitant sums of money. The batteries worked after a fashion, but could endure only a fairly limited number of recharge cycles, and that combined with their very high cost doomed them in the marketplace.
ReVolt claims their batteries can withstand thousands of cycles and are cheap to manufacture while preserving the extraordinary power density that is the hallmark of this particular battery chemistry. If such claims are accurate, they will have made a real breakthrough though whether that will be sufficient to achieve market success is uncertain.
Zinc air batteries often have rather cumbersome form factors because the internal surface areas exposed to air must be as extensive as possible. Some companies have resorted to forced air ventilation to reduce the contact area, but then the electrical output drops. This basic liability may prevent the design from ever gaining much purchase in the huge market for portable electronic designs but might not pose much of a problem in electric cars. And because no one really owns the market for large format electrical traction batteries today, a startup like ReVolt could have a fighting chance.
Incidentally, even more energy dense batteries may be constructed if the air cathode is combined with a lithium anode rather than one that utilizes zinc. Such batteries have been made experimentally and do indeed excel in energy density, but have a number of serious liabilities such as very poor cold weather performance, extremely poor power density, and rapid degradation when not in use. Whether these shortcomings can be sufficiently ameliorated to permit commercialization remains to be seen.
At this point handicapping the market for advanced electrical storage devices is difficult. Most of the competing technologies have been tried before and in many cases have repeatedly failed to mature—always an ominous sign. It could be that no solution is in sight and that plug-in hybrids will remain problematic. My guess is that lithium will achieve an early dominance and that it will only be improved incrementally in the years to come.