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Applications Using ECaSS® |
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++ Is Capacitor Energy Density Smaller? ++ |
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Everybody understands that capacitors have longer lives than batteries, but smaller capacity, or more precisely, energy density (energy per volume) or specific energy (energy per weight). However, this is not correct. The official capacity of a battery is defined at the full discharge value without considering its life. For applications such as a hybrid vehicle, several million large current charge/discharge cycles are required, so a shallow setting of depth of discharge is mandatory to maintain battery life.
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Figure 1: Ragone plots with life factors: Li-ion 7%, NiMH 10%, VRLA 5%, Cap 50%,
and ECaSS® (ECS and ECSnew) 75% [ZOOM] |
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Revised Ragone plots are shown in Figure 1, with specific energy (watt-hours per kilogram) on the vertical axis and specific power (charge/discharge watts per kilogram) on the horizontal axis. The revised portion from conventional methods is that the depth of discharge of each storage device is taken into account. There are good reasons for such shallow use as shown in the comments of the figure, to guarantee the life of batteries for 3 million cycles (500,000 km or 10 years).
Hybrid vehicles usually require a specific power of about 500 W/kg. If the efficiency at this power level is somewhere around 95%, the capacitor will not require forced air-cooling to prevent overheating due to those heavy currents. In Figure 1, power points of 200, 500 and 700 W/kg are marked within a yellow rectangle. It is clearly evident that specific energy of ECaSS® capacitors is not smaller than that of batteries in that range.
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++ How Safe are Capacitors and Batteries? ++ |
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There have been considerable discussions about the safety of Li-ion and NiMH batteries. Comparing those new batteries, capacitors have reputation of being safe, or safer because stored energy is thought to be smaller. However, Figure 1 shows that capacitor energy is not smaller.
The safety, or risk of storage devices can be classified in three ways;
(a)Danger caused by stored electricity
(b)Danger of abnormal chemical reaction due to failure or accident
(c)Danger with generated material caused by leaking or burning
(a) is unavoidable as far as using electricity, and can be protected by using a breaker plus proper handling.
(b) is especially applicable to Li-ion or NiMH. They are almost impossible to extinguish once they start burning hence electronics are required to watch high temperature of the battery, self-heating, and over- or under-charge.
(c) has not been examined enough and the risk must be explained and discussed.
In case of capacitors, (b) is safe enough and depends upon (c). Excluding ECaSS® and other Japanese capacitors, the major capacitor manufactures employing organic electrolyte, use acetonitrile (AN). This is a good solvent for capacitors. By using this, twice the specific energy is obtainable at the same internal resistance compared to propylene carbonate (PC). Unfortunately, AN is flammable with a 5C flash point, and generates cyanide gas on burning. Manufacturers can say, "this is safe as long as it contained in a metal case" or "all the materials are nontoxic except the electrolyte." These descriptions may be true but what will happen in practical, real cases?
Sure, there are opinions that nothing can be productive if you worry too much about collisions or fire, and if you question about the risk of AN, that of Li-ion batteries would be unsafe beyond comparison. However, different from batteries, EDLC has a safer way by avoiding the easy use of AN from electrolyte. It is not only the manufacturers but also the users of those storage devices such as batteries and capacitors, having correct knowledge to safely utilize their full capability.
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Figure 2: Honda FCX will start selling in 2002 in US and Japan
(From Honda international website, by permission of Honda R &D) |
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++ Several Practical Results of Capacitor Storage ++ |
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It is indispensable for specialists in this field to read papers on Electric Vehicle Symposium(EVS-18 in 2002, Berlin) or International Seminar on Double Layer Capacitors in Florida. You can refer to the list of papers and some of our papers are downloadable from the member pages of this website. Commercially produced ECaSS® capacitors are shown in Figure 3. All of them are non-acetonitrile L type cells with about 2ΩF and 6 Wh/kg. Manufacturers are now working hard to get more operating experience and achieve larger scale production to reduce the cost.
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Figure 4: Capacitor 100 kVA UPS by Sizuki Electric. (Permission obtained)
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If you use only the capacitor, as mentioned earlier, you will lose the 4-times multiplication factor from using electronics. Since these parts are described in detail in paper [1], some application results are shown here.
Figure 4 is a photo of a 100 kVA uninterruptible power supply backed up by EDLC, which is manufactured by Sizuki Electric Co. in Nishinomiya, Japan and started being sold on the market since 2001. Three models of 200, 100 and 50 kVA are available. they have been expand 5kVA to 3MVA in 2004.
For passenger vehicles, the Honda FCX shown in Figure 2 has become the first fuel cell vehicle in the world to receive government certification, paving the way for the commercial use of fuel cell vehicles. The capacitor hybrid four seater, hydrogen-fuelled FCX was started sales in the US and Japan in 2002. |
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| Figure 5: Capacitor series hybrid bus at 2000 Tokyo Motor Show |
For commercial vehicles, Nissan Diesel Motor Co. improved their capacitor hybrid bus developed under contract as a NEDO project. Figure 5 is taken a picture taken in 2001, but better mileage rating has been obtained in 2002. In the back of the picture, a capacitor hybrid truck is barely visible.
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Figure 6: Sales started for capacitor parallel hybrid 8-ton truck
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A capacitor hybrid truck has been completed and started sales from June 24, 2002. This parallel hybrid 8-ton truck, which has a diesel engine assisted by capacitors without using any batteries, is probably the world's first product of this type. In Japan, the demonstration run and the capacitor factory were televised over the country, and reported in many magazines newspapers and on the Internet, though almost all coverage was only in Japanese.
ECaSS® has gotten out of the research state at last. Now you can actually buy some of their applied products. Hereafter, the target must be a price that should go down in accordance with the size of production. We wish contribute to the environment, by increasing the adoption of ECaSS® as much as possible.
== Reference ==
[1] M. Okamura and H. Nakamura: "Energy Capacitor System - Part 1 and 2: Capacitors and their Control" The 11th international seminar on double layer capacitors and similar energy storage devices. Dec.3-5, 2001
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(By Michio Okamura)
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