atomic_fungus (atomic_fungus) wrote,

#575: Off by a factor of 10. Mea culpa.

Here I asserted that an electric car with a super-capacitor would be impossible to charge in 5 minutes. And I was wrong.


What's worse, it's a simple arithmetical error. Argh.

After re-crunching the numbers I realized that it would only require an electric cable that can handle 100A of current, not 1,000A. A 100 amp cable is doable in a non-industrial setting; if you've ever handled a set of jumper cables you've handled a cable which could handle 100A for five minutes. (They might get rather warm, though.)

So my primary criticism of an electric car with a super-capacitor in place of a battery is pretty much kaput. Rather than 1.1 megawatt-hours in five minutes, the charger would have to handle only 110 kilowatt-hours in five minutes: 22 kw-h/minute rather than 220 kw-h/minute.

This is why "back of the envelope" calculations for engineering projects are so important, though. You need to know if an idea is possible before you invest time and money in pursuing it.

I'm glad those guys are better at arithmetic than I am. grumble

* * *

The only thing which the super-capacitor does is make electric cars more convenient. A five-minute recharge is within the realm of a quick stop at a gas station, and a 500-mile range is within the realm of the typical car's range. This is a big step forward for electric cars.

But even so, there is still the rather "small" problem of supply. In many parts of the country, both generating capacity and the power grid itself are already operating somewhere rather close to maximum capacity.

22 kilowatt-hours is about what a large air conditioning unit consumes. Imagine the strain on the power grid of Everytown when 50,000 cars are plugged in for a morning pre-commute fill-up. Now imagine the strain when 50,000 cars are plugged in in the summertime, when 50,000 air conditioners are already running. Those popping sounds you hear are line fuses blowing out. Those have to be replaced by linemen, one-by-one.

(Actually, when they blow, they make a sound like a shotgun. These are big fuses.)

Conventional electric cars are not really any better. They merely spread the load out over a longer period. It's true that you can plug your e-car in at night, when electricity use is likely to be lower; but it's also true that the electricity you're pumping into the car's batteries must come from somewhere and you're not likely to get much out of a solar panel at night, so you're going to have to plug the thing in.

And 50,000 cars doing the slow-charge thing will end up consuming a lot of power. Maybe not as much as in the prior example, because the range is ten times shorter and the energy density is lower, but it will still run to quite a few megawatts.

Power costs money. Electric cars won't save you any money unless you're an early adopter; once everyone has an electric car, power rates will rise because the demand must be met and it takes money to do build a grid capable of transmitting that much power. Besides, when electricity is used instead of gasoline, states will tax electricity at a higher rate to make up for the shortfall caused by the drop in gasoline usage. (Gasoline itself will become a lot cheaper, too.)

Where will the power come from? It's clear that more generating capacity will need to be brought on-line. Perhaps the drop in gasoline use will be offset by a rise in oil-fired power plants. Maybe a few new coal plants can go on-line, as well. It'll have to come from somewhere and it's likely to take literal decades to get any new nuclear plants on-line.

Let us dispense with the fantasy that all this electricity will come from solar panels and windmills, okay? In the real world, windmills generate usable quantities of power about 1/10th of the time, and their output is unpredictable at best. Solar power is also useless for commercial power generation since you only get about 100w per square meter of solar panel, and that's when the sun is shining directly on the solar panel without any clouds in the way. I can just about guarantee that you'd need to cover your roof with solar panels just to supply the power you use during the day; and where does the power you use at night come from?

To say nothing, by the way, of the environmental problems that manufacturing that quantity of solar panels would cause. Making solar panels requires toxic chemicals, and the manufacturing process generates toxic waste. What do you do with it? You can only go so "green" with that process without making the panels too expensive for people to afford; and solar panels are expensive enough as it is, believe me.

No. If we want electric cars, we're going to need more generating plants. If we're doing this to "save the Earth from man-made global warming" those plants will need to be nuclear plants, because there are no other methods of generating electricity which are reliable, controllable, and plentiful enough to meet the demand for electricity that electric cars would generate, which do not also require the burning of gobs of fossil fuels.

I have always thought that eliminating our need to burn fossil fuels was a laudable goal. Oil is too useful for other things; we shouldn't be burning it, but we don't have much of a choice at present.

Oh well.

  • #8645: Four feet??

    Read a story about an explosion at a chocolate factory in Pennsylvania--not Hershey's--and the story mentioned that the explosion was violent enough…

  • #8644: The hard science of power generation

    It says windmills are no good. a wind speed of 20mph, the power produced by a wind turbine is 600 watts per square metre at full efficiency.…

  • #8643: Well, we've lost Wonderduck

    He died earlier this month, apparently. His blog hasn't updated since November, so there are very few details, but he'd had a number of health…

  • Post a new comment


    default userpic

    Your reply will be screened

    Your IP address will be recorded 

    When you submit the form an invisible reCAPTCHA check will be performed.
    You must follow the Privacy Policy and Google Terms of use.