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Beyond the capacitor there is Ultracapac And you always thought that the Farad was a ridiculously large unit . . . Start thinking in KILOFarads! by Ross Tester S ome time in the not-very-distant future you will pick up your cordless drill and start drilling away – with more power than you ever thought possible. And it will keep on drilling for much longer than you thought possible. The drill will look and feel the same as current cordless drills but it will have one major difference: it won’t contain a battery. Instead, it will get its power from a capacitor. Needless to say, it won’t be a “normal” capacitor. In fact, it’s so abnormal it has a new name: an ultracapacitor (or sometimes a supercapacitor). While the terms have been somewhat interchangeable, they’re starting to be used more selectively, with ultracapacitors denoting the larger values. Already (at least in the US) there are rechargeable tools on the market which use ultracapacitors instead of batter- ies, such as the Coleman Flash Cell Screwdriver and the Superior Tool Co Ultracut Cordless Tube Cutter. And without realising it, you may well be using one already: many computers these days use an ultracapacitor, or at least a supercapacitor, in place of the batteries once used for CMOS backup. They’re also found in many other devices doing the same task – video recorders and even digital alarm clocks, for example. We’re already seeing ultracapacitors starting to be used in a wide variety of industrial applications, such as lifts (elevators) and electric forklifts. In both of these, power is used to lift a load and until now, power has been required to limit the downward travel, or at best hydraulics used, with the energy wasted. Now ultracapacitors are finding their way into new designs. When the lift descends, its motor-generator pumps A hybrid test car on a test track in England, powered by the CSIROdeveloped UltraBattery – a combination of an ultracapacitor and leadacid battery. Photo courtesy Advanced Lead-Acid Battery Consortium. 12  Silicon Chip siliconchip.com.au citor! power back into an ultracapacitor. When a forklift brings a pallet down from the warehouse shelf, its motor converts to a generator and recharges the ultracapacitor. Another interesting ultracapacitor application, already in use, is in wind turbines. Some now have ultracapacitors to supply the power needed to turn the blades into the wind or adjust blade angles when they themselves are not supplying power, or to smooth out the variations caused by changing windspeed. Start thinking big! The backup supercapacitors used in computers, DVRs etc, are midgets compared with those used in vehicles and industrial tasks. You can already find supercapacitors at your local lolly shop, with ratings of perhaps 0.5F to 50F and voltages up to (usually) 5.5V. Ultracapacitors are still rather harder to get (and very much more expensive). They tend to start at about 100F and go up into the kFarads – but more importantly, voltage ratings are up into the 100V+ range and if you believe recent publicity from one US manufacturer, well up into the thousands of volts. That becomes very important, as we shall see. By the way, you did read that correctly: Farads. Not nF or even mF. Not so long ago, a 10,000mF capacitor was regarded as very big. And remember when you started in electronics and wondered why the Farad was the unit of capacitance, when everyone knew it was a huge value and you always had to divide by a million or more to get to useable values? Not any more! Electric vehicles In the future, both hybrid electric vehicles (HEVs) and plug-in electric vehicles (PEVs, ie, electric power only) may be powered by ultracapacitors, perhaps instead of batteries but just as likely, as current research suggests, in tandem with them. The photo on the facing page shows a Honda Insight HEV fitted with an UltraBattery, developed by Australia’s CSIRO, built by the Furukawa Battery Company of Japan and tested in the United Kingdom through the Americansiliconchip.com.au Ultracapacitor + Lead Acid Battery = UltraBattery Australia’s CSIRO has combined a supercapacitor and a lead acid battery in a single unit, creating a hybrid car battery that lasts longer, costs less and is more powerful than current technologies used in hybrid electric vehicles (HEVs). Tests have shown the UltraBattery has a life cycle that is at least four times longer and produces 50% more power than conventional battery systems. It’s also about 70% cheaper than the batteries currently used in HEVs. By marrying a conventional fuelpowered engine with a battery to drive an electric motor, HEVs achieve the dual environmental benefit of reducing both greenhouse gas emissions and fossil fuel consumption. The UltraBattery also has the ability to provide and absorb charge rapidly during vehicle acceleration and braking, making it particularly suitable for HEVs, which rely on the electric motor to meet peak power needs during acceleration and can recapture energy normally wasted through braking to recharge the battery. Over the past 12 months, a team of drivers has put the UltraBattery to the test at the Millbrook Proving Ground in the United Kingdom, one of Europe’s leading locations for the development and demonstration of land vehicles. CSIRO’s ongoing research will further improve the technology’s capabilities, making it lighter, more efficient and capable of setting new performance standards for HEVs. The UltraBattery test program for HEV applications is the result of an international collaboration. The battery system was developed by CSIRO in Australia, built by the Furukawa Battery Company of Japan and tested in the United Kingdom through the Americanbased Advanced Lead-Acid Battery Consortium. UltraBattery technology also has applications for renewable energy storage from wind and solar. CSIRO is part of a technology start-up that will develop and commercialise battery-based storage solutions for these energy sources. (CSIRO) The Coleman “Flash Cell” cordless screwdriver, now available in the US, uses a 5.4V ultracapacitor instead of a battery. It has a 90-second recharge. MA arch pril 2008  13 based Advanced Lead-Acid Battery Consortium. The most significant aspect of the photo, taken at the Millbrook Proving Ground in the UK, is that this Honda has exceeded 160,000km on the test track. Like just about all ultracapacitor manufacturers, the CSIRO and the consortium are keeping the details of the UltraBattery pretty close to their collective chests but as we saw in the article in SILICON CHIP February 2008, it is not too-difficult a task to add significant battery capacity (or in this case UltraBattery capacity) to the Honda to give it a much greater range on battery power alone. Incidentally, Honda has also developed an ultracapacitor in conjunction with a fuel cell in their quest for the perfect HEV/PEV. Yet another use of ultracapacitors is in electric buses and trains, where ultracapacitors not only supply accelerationfrom-rest (ie, peak) power but can also handle and store the regenerative braking energy which batteries find much more difficult, thus saving up to 30% of total energy. The same system is very likely to find its way into HEVs and PEVs as they start to become more popular. A huge amount of research is currently under way around the world into these vehicles as the search for an alternative to fossil fuels hots up. Despite the fact that ultracapacitors appear to be a recent development, they have been around for decades – at least in the laboratory and in some specialised (expensive!) applications. They got a big “kick along” late last century when NASA realised that they would be very useful as peak-power enhancers in spacecraft. Fuel cells used in spacecraft are somewhat similar to batteries: great at supplying base-load power but needing help to supply peakload power. Ergo, ultracapacitors. Why the hype? OK, so what is the big advantage of ultracapacitors over rechargeable batteries? There are several: (1) They offer much better peak power performance than a battery. A battery’s output is basically limited by the rate of the chemical reaction inside it without overheating. An ultracapacitor has no chemical reaction so peak currents can be much higher. (2) There is less heat to dissipate. (3) They can be discharged much more deeply than a A 2500F (or 2.5kF) Maxwell Power Cache Ultracapacitor. Note the low operating voltage (2.7V) – this means that many of these must be used in series to obtain any type of reasonable voltage rating. The photo at right show the same capacitors installed in a Honda EV conversion. (Photos courtesy www.metricmind.com). battery (in fact, usually to zero, long past the point where most batteries will have been irreparably damaged if not destroyed). To be fair, that’s also long past the point where the ultracapacitor can supply any useful power. (4) They can be charged very, very quickly – with many ultracapacitors already in use in road vehicles, the time to charge is not too dissimilar to the time to fill fuel tanks (a few minutes or so). Battery recharge time is usually measured in hours. (5) They are lighter (sometimes very significantly so) than batteries of similar ratings and occupy no more space (usually less). (6) They can be cycled many, many more times than a battery. With careful cell management, most rechargeable battery systems are limited to perhaps 10-20,000 charge/ discharge cycles. Ultracapacitors are usually rated at between 100,000 and 500,000 cycles (and we’ve seen some claims of a million). (7) Overall life expectancy is a lot longer than a battery – the guarantee is usually 10 years but this would be regarded as a minimum. Theoretically, the life of an ultracapacitor is indefinite. (8) Ultracapacitors do not deteriorate anywhere near as much in performance as they age. With batteries, the chemical reaction decreases as they age. And the negatives? Having digested all that, there must be some disadvantages. Yes, there are a few: Honda’s ultracapacitor module was designed for the Honda FCX Clarity hydrogen fuel-cell-powered car (shown right), which goes on limited lease in the US around the middle of this year. 14  Silicon Chip siliconchip.com.au TELGESIS ETRX2 ETRX2 Actual size - 37.75 x 20.45mm WORLD’S FIRST ZigBee PRO COMPLIANT MODULE (1) Until now, ultracapacitors have not had the energy density of batteries. However, that may be changing – and significantly – if recent manufacturers’ announcements come to fruition. (2) Voltage ratings of the current crop of ultracapacitors are very low. With PEVs operating anywhere up to a few hundred volts, you need to put a lot of ultracapacitors in series. And when you connect capacitors in series, the capacitance decreases. (3) The moment you start using power from an ultracapacitor, the voltage starts to drop and keeps dropping. It obeys all the usual laws of capacitors! This may or may not be a problem, depending on the device being powered. When used in conjunction with a battery as a peak-load supply, as soon as the peak load is delivered the battery will recharge the ultracapacitor. (4) They’re expensive! So are high-power rechargeable batteries, of course – and the price of both will come down as volumes increase. (5) Finally, there is a lot of hype. Some amazing claims have been made by ultracapacitor manufacturers (to impress investors?) and in too many cases, they have turned out to be vapourware. What’s inside an ultracapacitor? No one has re-invented the laws of physics when it comes to ultracapacitors. They are still capacitors and they obey all the rules we’ve learned long ago. First, let us refresh your memory about the construction of capacitors. Here, two conductive “plates” are separated by an insulating material which we refer to as a dielectric. The capacitance is directly proportional to the size of the plates and the dielectric constant of the insulating material. At the same time, the capacitance is inversely proportional to the distance between the plates; the smaller the spacing, the larger the capacitance. In the case of electrolytic capacitors, the “plates” take a different form. The capacitor is a wound element of aluminium foil which has been etched to greatly increase its surface area. At the same time, its surface has a very thin oxide layer produced during manufacture. Finally, it is wound with a porous paper layer which is impregnated with a conductive electrolyte paste and this makes the siliconchip.com.au PRESS RELEASE ZigBee module makes the Pro league The Telegesis ZigBee module product range has successfully achieved certification based on the ZigBee Pro feature set of the ZigBee standard. The widely-acclaimed Telegesis AT Command layer has been tested by US test house National Technical Systems, where Telegesis was granted ZigBee certification. This makes the Telegesis ETRX2 the world's first ZigBee Pro compliant module. The ETRX2 is a low-cost low-power ZigBee transceiver that can be embedded in a wide range of devices to produce self-organising self-healing wireless meshes. All Telegesis ZigBee Pro products are built around the Ember EM250 chip with core mesh networking technology provided by Ember Corporation's EmberZnet 3.1 software. The new meshing RF module will allow companies to tap into the lucrative ZigBee market for wireless control and monitoring products with no in-house RF engineering experience. ZigBee is now a technology that is truly ready for use in real-world applications and Telegesis envisage rapid expansion throughout 2008 as ZigBee is applied to a wide range of wireless solutions. TELEGESIS ETRX2: EXCLUSIVELY FROM 01010101 Telelink Communications e-mail Jack Chomley – jack<at>telelink.com.au or call (07) 4934 0413 or 0428 199 551 www.telelink.com.au April 2008  15 the total mass. Taken together, that means the ultracapacitor achieves one quarter of the theoretical capacitance based on electrode area and ion size. The future of ultracapacitors This illustration, courtesy of electronicdesign.com, shows the inside of an ultracapacitor. It’s easy to see why they are regarded as two capacitors in series. electric connection to the can of the capacitor. So in an electrolytic capacitor, the positive “plate” is the wound aluminium foil element and the aluminium oxide “skin” is the dielectric. Finally the electrolyte paste is the negative “plate”. This combination of a very large surface area (ie, the etched surface) together with a very thin dielectric (aluminium oxide) layer gives rise to the very large values of capacitance that we have come to expect with electrolytic capacitors. But ultracapacitors far surpass electrolytics! Ultracapacitors also contain two “plates” of sorts. But the “plates” are formed on the surfaces of nano-porous materials, typically activated charcoal or carbon nanotubes surrounded by an electrolyte. These nano-porous materials have much larger surface areas than the etched aluminium foils of electrolytic capacitors. Nor do ultracapacitors have a conventional dielectric, as such. They are based on a structure that contains an electrical “double layer”. In a double layer, the effective thickness of the “dielectric” is exceedingly thin – in the order of nanometres – and that, combined with the very large surface area, is responsible for their extraordinarily high capacitances. When a DC voltage is applied across the porous carbon, compensating accumulations of cations or anions develop in the solution around the charged electrodes. If no electron transfer can occur across the interface, a “double layer” of separated charges (electrons or electron deficiency at the metal side and cations or anions at the solution side of the interface boundary) exists across the interface. The amount of capacitance depends on the area of those porous carbon electrodes and the size of the ions in solution. The capacitance per square centimetre of electrode double layers is roughly 10,000 times larger than those of ordinary dielectric capacitors. That’s because the separation of charges in double layers is about 0.3-0.5nm, a lot less than the 10-100nm in electrolytics and the 1000nm in polystyrene or mica types. However, you never get something for nothing. Effectively, you have two capacitors in series. So straight away capacitance is halved. The double-layer configuration reduces the potential capacitance of a practical device yet again because the ultracapacitor consists of a pair of electrodes, each with half 16  Silicon Chip We alluded to some pretty amazing claims earlier. EEstor, one of the up-and-coming performers of the US stock market recently, has been researching nanotube technology and have also announced what amounts to breakthrough technology in their ultracapacitors. EEstor use barium titanate coated with aluminium oxide and glass to achieve a level of capacitance claimed to be much higher than anything else currently available in the market. While yet to be independently verified, the claimed energy density is a whopping 1.0MJ/kg – actually higher than a battery. Existing commercial supercapacitors typically have an energy density of the order of 0.01MJ/ kg and a lithium-ion battery has an energy density of 0.54-0.72MJ/kg. If true, there is a rather significant downside: a PEV using such ultracapacitors could not, using existing technology and domestic wiring, plug in! To transfer that amount of energy in the times claimed would melt the local substation. OK, slight exaggeration perhaps – but the point is real. It has been suggested that a second EEstor ultracapacitor could be used to slowly charge, using cheap off-peak power – and that plug into the PEV to transfer the energy in say 5-10 minutes. Someone must believe EEstor because they have had some significant money invested in them, including the Canadian ZENN motor company (which plans to release an EEstor-powered electric vehicle this year) taking an EEstor licence worth an estimated $US3-5 million and venture capitalist house KPBC putting in another $US3 million. Incidentally, EEstor are not the only ones researching ultracapacitors. Australia’s own CSIRO is also one of the main players in the game (see press release earlier) and there are many organisations around the world trying to come up with their version of the holy grail. The leading manufacturers of ultracapacitors today are Maxwell Technologies in the United States, NESS Capacitor Company in South Korea, Okamura Laboratory in Japan, and EPCOS in Europe. Energy Finally, we mentioned earlier that the voltage rating of a capacitor (including, of course, an ultracapacitor) is very important. The reason for this lies in the formula for energy stored in a capacitor: E = 0.5CV2 It’s the V2 term that makes all the difference. Doubling the voltage doesn’t simply double the energy – it quadruples it. So running an electric vehicle, a drill, a forklift – anything – from a higher voltage is very advantageous. The problem is, as we have seen, ultracapacitors have a very low voltage rating. Ultracapacitor cells must be stacked in series to lift that rating and as every electronics student knows, you get lower capacitance that way. Some researchers are claiming much higher cell voltage ratings: the world is waiting to see if they can deliver! SC siliconchip.com.au