25th June 2009 - 05:35 PM
As of now, the EEESTER seems to be the most promising super-capacitor technology for auto application... over 40 Farads, 3500 volts in it's present form, with up to 10,000 volts of charge allegedly possible via the technology. I’ve followed the eeester story in passing for a couple of years… eeester was pretty quiet, and last I had noticed NASA had interest in the technology for possible use in space probes (obviously, an intersteller space environment would render operating temperature ranges moot.) Then, it was recently announced that a car, the (Chinese-built?) Zenn, will be powered electrically using only the EE Ester, and I thought “Wow, it’s for real.”
But a few questions come to mind.
Assuming the eeester works as claimed, what’s irked me all along was the power conversion strategy they intended to use (I only found that out recently in reading about the Zenn.) It’s a buck-boost converter.
The buck-boost converter would seem to be the most simple, direct, and straightforward approach to the problem of adjusting output from a capacitor charged with great pressure, yet this strategy would have a poor duty cycle due to parasitic resistance… thus, a degree of inefficiency would seem unavoidably inherent to such a design. Now, to get even nominal performance out of even a compact car will require between 50 KVA and 100 KVA of power (converting from horsepower, assuming efficiency to be fairly high.) If a 12 volt DC motor were used, very large currents would be necessary to produce the electromotive forces required by a compact car.
The alternative to such would seem be a higher-voltage AC motor (perhaps 120 or 240 VAC, 60 Hz) with the conditioning/inverting power supply consisting of either (1) a power transistor array, or (2) vacuum tubes. Either option could be microprocessor-controlled to invert the DC capacitor output to AC, while improving system efficiency while continually monitoring loads, yet the second option impresses me as possibly being more practical, even though vacuum tubes have become antiquated for most applications.
First, I’ll summarize the possible obstacles of using a power transistor array, as I see them. The most robust semiconductors commercially available can handle about 1 KVA. Thus, it would take (at minimum) 50-100 of the largest power transistors readily available- operating in tandem- to meet the aforementioned drive requirements of a small car. Constantly varying high loads, in addition to harmonics which would be introduced into such a system during city driving (in addition to power regeneration strategies proposed for braking to recapture the car’s momentum as current during braking) produce the kind of conditions that make semiconductors vulnerable to damage and failure. Longevity or reliability could initially present design challenges. Such a power conversion system would necessarily be complex, it would require cooling (probably liquid) and it would likely be relatively expensive to produce.
Suppose vacuum tubes were used. The disadvantages would include perhaps 70% efficiency, and a need for water-cooling (likewise for a semiconductor array… similar to a radiator and water pump as on gasoline engines.) 2 EESTERS could form a +/- 3500 VDC supply, the control grids could be semiconductor controlled via a microprocessor to provide 120 or 240 VAC with supplemental conditioning components, for instance, which could drive an “off the shelf” motor of high-reliability using far less current. Perhaps with an array of 2,4,or 8 pentodes, the power being directed to the pair most compatible with the charge-state of the capacitor. The microprocessor could be interfaced to monitor loads and adjust grid potentials, and which tubes were powered to maximize efficiency.
The advantages to vacuum tubes in this application, as I see them, are:
1. Even high-power tubes are compact and light.
2. Due to their design simplicity, they can be manufactured very inexpensively.
3. Tubes are virtually impervious to damage from shorting, overloading, and harmonics.
4. Tubes can handle 10 megawatts or more, with those rated over 10KVA water-cooled.
5. The technology is simple, and has already been developed to an advanced stage, with the operation theory very well-understood.
6. Even very high-power vacuum tubes are compact, light, have few internal parts, are reliable, and can last up to 10 years.
7. Manufacturing methods, tools, and instruments for commercial production already exist and have been highly-refined.
8. Their physical designs can easily be made highly-durable… for example, a high-strength polymer enclosure with inner ceramic or metal cylinders, in a rectangular enclosure including channels for coolant flow would be one possible configuration.
Anyone have any thoughts on the matter?