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Arctic Leaf

Can supercapacitors replace batteries?

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The electric car depends on batteries, and before EVs become a large chunk of our automotive fleet, there are probably going to be some changes.

Right now, Elon Musk is betting he can produce millions of small lithium-ion batteries not much bigger than the ones you put in your flashlight and string them together to power a $35,000 Tesla Model E over a range of 200 miles at speeds of up to 70-80 mph. The Model E also will also need an infrastructure of roadside “filling stations” and home chargers, although the best superchargers still take more than 20 minutes to achieve 80 percent capacity.

But there is another way to store electricity, long familiar to the designers of electrical circuits. It’s the capacitor, a device that stores a small current by static electricity rather than a chemical reaction. Capacitors sit in all of your electrical devices, from radio circuits to the most sophisticated laptops, and are essential to providing the steady electric current needed to run such electronics. But what if the concept of capacitors could be scaled up to the point where they could help power something as big as an electric vehicle? Granted, it’s a long, long way from the 1.5-volt capacitor in your iPad and powering a 4,500-pound Tesla along the Interstate, but researchers are out there probing and are already thinking in terms of a breakthrough.

Right now there’s a huge separation between the things that batteries can do and the things that capacitors can do. In a way they are complementary — the strengths of one are the weaknesses of the other. But researchers are working toward a convergence — or perhaps just a way of using them in tandem.

A battery employs chemistry by splitting ions in the electrolyte so that the negative ones gather on the cathode and the positive on the anode, building up a voltage potential. When they are connected externally an electric current flows. Batteries have a lot of energy density. They can store electricity up into the megawatt range and release a flow of electricity over long periods of time. The process can also be reversed, but, because the reaction is (once again) chemical, it can take a long time.

Capacitors store electrons as static electricity. A thundercloud is a great big capacitor with zillions of electrons clinging to the almost infinite surface area of individual raindrops. And as everyone knows, this huge stored capacity can be released in a “bolt of lightening.” Capacitors can be recharged almost instantly but also they release their energy almost instantly, rather than the even flow of a battery. One of their major uses is in flash photography. But their capacity for storing power is also limited. On a pound-for-pound basis, the best capacitors can only store one-fifth to one-tenth the equivalent of a chemical battery. On the other hand, batteries can start to wear out after five years, while supercapacitors last at least three times as long.

Back in the 1950s, engineers at General Electric, and later at Standard Oil, invented what have come to be called “supercapacitors.” Basically, a supercapacitor changes the surface material and adds another layer of insulating dielectric in order to increase storage capacity. Surface area is the key and engineers discovered that powdery, activated charcoal vastly increased the capacity of the storage plates. Dielectrics were also reduced to ultra-thin layers of carbon, paper or plastic, since the closer the plates can be brought together, the more intense the charge. Since then they have begun experimenting with graphene and other advanced materials that may be able to increase surface area by orders of magnitude. All of this means that much more electricity can be stored in a much smaller space.

But the problem of low energy density remains. Even supercapacitors can only operate at about 2.5 volts, which means they must be strung together in series in vast numbers in order to reach voltage levels required to power something like an electric car. This creates problems in maintaining voltage balance. Still, some supercapacitors are already being employed in gas-electric hybrids and electric buses in order to store the power siphoned off from braking.

Researchers in the field now see some possibility for convergence. Most exhilarating is the idea that the frame of the car itself could be transformed into a supercapacitor. Last month, researchers from Vanderbilt University published an online paper entitled, “A Multifunctional Load-Bearing Solid-State Supercapacitor,” in which they suggested that load-bearing materials such as the chassis of a car or even the walls of your house could be transformed into supercapacitors to store massive amounts of electricity on-site. Combined with advances in evening the flow of electrons from supercapacitors, this opens up whole new avenues of approach to the electric car.

All of these developments are a long way off, of course. Still, supercapacitors support the possibility of pulling out of your driveway in the morning and returning at night in your EV without needing to gas up with foreign oil at your nearest filling station.

Arctic Leaf

From lab to market, it’s a long haul

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The Energy Information Administration has done us an enormous favor by producing a simple chart to make sense of where the development of energy storage technology is going. Energy storage, as the EIA defines it, includes heat storage, and a quick look at the chart reveals that those forms that involve sheer physical mechanisms – pumped storage, compressed air and heat reservoirs – are much further along than chemical means of storage, particularly batteries.

The EIA divides the development of technologies into three phases – “research and development,” “demonstration and deployment” and “commercialization.” It also ranks them according to a factor that might be called “chances for success,” which is calculated by a multiple of capital requirements times “technological risk.”

As it turns out, only two technologies that could contribute to transportation are in the deployment stage while three more are in early development. The two frontrunners are sodium-sulfur and lithium-based batteries while the three in early stages are flow batteries, supercapacitors and hydrogen. The EIA refers to hydrogen as one of the ways of storing other forms of energy generation, particularly wind and solar. But hydrogen is also being deployed in hydrogen in hydrogen-fuel-cell vehicles that have already been commercialized.

Other than building huge pumped-storage reservoirs or storing compressed air in underground caverns, the chemistry of batteries is the most attractive means of storing electricity, which is the most useful form of energy. Batteries have always had three basic components, the anode, which stores the positive charge, the cathode, which stores the negative charge, and the electrolyte, which carries the charge between them. Alexander Volta designed the first “Voltaic pile” in 1800 by submerging zinc and silver in brine. Since then, battery improvements have involved finding better materials for all three components.

Lead-acid batteries have become the elements of choice in conventional batteries because the elements are cheap and plentiful. But lead is one of the heaviest common elements and becomes impractical when it comes to loading them aboard a vehicle.

The great advantage of lithium-ion batteries has been their light weight. The lithium substitutes for metal in both anode and cathode, mixing with carbon and iron phosphate to create the two charges. Li-ion, of course, is the basis of nearly all consumer electronics and has proved light and powerful enough to power golf carts. The question being posed by Elon Musk is whether they can be ramped up to power a Tesla Model S that can do zero-to-60 with a range of 300 miles.

Tesla is not planning any technological breakthrough, but will use brute force to try to scale up. Enlarging li-ion batteries tends to shorten their life so the Tesla will pack together thousands of small ones no bigger than a AA that will be linked by a management system that coordinates their charge and discharge. Musk is betting that economies of scale at his “Gigafactory” will lower costs so that the Model X can sell for $35,000. According to current plants, the Gigafactory will be producing more lithium-ion batteries than are now produced in the entire world.

In the sodium-sulfur battery, molten sodium serves as the anode while liquid sodium serves as the cathode. An aluminum membrane serves as the electrolyte. This creates a very high energy density and high discharge rate of about 90 percent. The problem is that the battery must be kept at a very high temperature, around 300 degrees Celsius, in order to liquefy its contents. A sodium-sulfur battery was tried in the Ford “Ecostar” demonstration vehicle as far back as 1991, but it proved too difficult to maintain the temperature.

Flow batteries represent a new approach where both the anode and cathode are liquids instead of solids. Recharging takes place by replacing the electrolyte. In this way, flow batteries are often compared to fuel cells, where a steady flow of hydrogen or methane is used to generate a current. The great advantage of flow batteries is that they can be recharged quickly by replacing the electrolyte, rather than taking up to 10 hours to recharge, as with, say, the Chevy Volt. So far flow batteries have relatively low energy density, however, and their use may be limited to stationary sources. A German-made vanadium-flow battery called CellCube was just installed by Con Edison as a grid-enhancement feature in New York City this month.

Supercapacitors use various materials to expand on the storage capacity devices in ordinary electric circuits. They have much shorter charge-and-discharge cycles but only achieve one-tenth of the energy density of conventional batteries. As a result, they cannot yet power vehicles on a stand-alone basis. However, supercapacitors are being used to capture braking energy in electric trams in Europe, in forklifts and hybrid automobiles. The Mazda6 has a supercapacitor that uses braking energy to reduce fuel consumption by 10 percent.

The concept of “storage” can be also be expanded to include hydrogen, since free hydrogen is not a naturally occurring element but can store energy from other sources such as wind and solar. That has always been the dream of renewable energy enthusiasts. The Japanese and Europeans are actually betting that hydrogen will prove to be a better alternative than the electric car. Despite the success of the Prius hybrid, Toyota, Honda and Hyundai (which is Korean) are putting more emphasis on their fuel cell models.

Finally, methanol can be regarded as an “energy storage” mechanism, since it too is not a naturally occurring resource but is a way to transmit the potential of our vast reserves of natural gas. Methanol proved itself as a gasoline substitute in an extensive experiment in California in the 1990s and currently powers a million cars in China. But it has not yet achieved the recognition of EVs and hydrogen – or even compressed natural gas – and still faces regulatory hurdles.

All these technologies offer the potential of severely reducing our dependence on foreign oil. All are making technical advances and all have promise. Let the competition begin.