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'Cars can power supermarkets'

  1. Jun 15, 2009 #1
    I came across this article today.
    http://http://www.dailymail.co.uk/news/article-1193070/Shoppers-cars-soon-able-power-supermarkets.html" [Broken]

    Does the energy they get from the cars come from the car losing gravitational potential energy, in which case the cars will have to expend more energy to get out of the dip (unless it is going downhill) but the energy from the engine does not go directly into producing power for this mechanism, or the cars' kinetic energy from the engines putting the energy in, in which case it cannot be 'green' energy. I think it's the first one. but I showed it to someone, and they think that the enrgy comes from the kinetic energy of the car, and it would slow down, thus requiring more energy to be put into the car.

    Please explain your answers thouroughly.
     
    Last edited by a moderator: May 4, 2017
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  3. Jun 15, 2009 #2

    russ_watters

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    The link doesn't work for me....
    [edit] Found it: http://www.dailymail.co.uk/news/article-1193070/Shoppers-cars-soon-able-power-supermarkets.html

    Basically, the car rolls over plates, pushing them down, and that works as a reciprocating motor, spinning a flywheel and generator. Yes, any energy that is generated that way comes from the kinetic energy of the car. The only way it wouldn't be a waste and not green would be to locate the pads in a place where you know for certain that cars are braking anyway, such as on an off ramp of a highway.

    Note also, that in addition to a car not generating power anywhere near as efficient as a power plant, you are also harnessing it in a particularly inefficient way. When the car bounces up and down, the engine isn't just adding energy to lift it back up, the shocks and tires are absorbing kinetic energy from the sides of the slope, losing even more energy.

    Also, they say it generates "30 kW of green energy an hour", which is a mismatch of units, but if they mean it generates 30 kW of power (30 kWh of energy an hour), that would be an enormous amount of power, and I'm highly dubious of that. 30 kW is a good fraction of the power output of a car engine, and more than a typical car uses maintaining a moderate constant speed on a highway.

    I'd be interested in getting more real details of this, though - altogether, it isn't a very useful article.
     
    Last edited: Jun 15, 2009
  4. Jun 15, 2009 #3

    Pengwuino

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    http://www.dailymail.co.uk/news/article-1193070/Shoppers-cars-soon-able-power-supermarkets.html

    Sigh....
     
  5. Jun 15, 2009 #4

    russ_watters

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    Lol @ Penguino, I missed that one! That's hilarious!

    Here's a blogger that picked up the story and has some interesting anecdotes about other half-baked energy saving ideas similar to that one: http://www.climate-skeptic.com/2009/06/is-it-green-or-is-it-just-theft.html

    They include:
    -Running a freezer to freeze water to cool a cooler so you don't have to run a refrigerator.
    -Taking ice from an ice machine at work to cool your refrigerator (ie, steal the energy)
     
  6. Jun 15, 2009 #5

    Pengwuino

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    Wow some people need a science class or 2! I really really hope this is just god awful news reporting.

    I also don't understand what they mean by the plates harvesting enough rainwater to help flush toilets?
     
  7. Jun 15, 2009 #6

    russ_watters

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    Lol, that's a completely separate conservation effort having nothing to do with the plates. "It" in that sentence is the store, not the plates.
     
  8. Jun 15, 2009 #7

    Pengwuino

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    Ohhh oops :rofl: :rofl:
     
  9. Jun 17, 2009 #8
  10. Jun 17, 2009 #9

    Pengwuino

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    No. There's no free lunch in thermodynamics. Unless the freezer is superior in efficiency, there is nothing to be gained really.
     
  11. Jun 17, 2009 #10

    russ_watters

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    ...and the laws of thermodynamics dictate that the freezer is not of superior efficiency. The efficiency of a heat engine is a function of the temperature difference between the hot and cold reservoirs. So since a freezer creates colder air than an air conditioner, it will run less efficiently.

    An additional factor that reduces the efficiency is the type of cooling done. Air run through a regular air conditioner can be cooled a certain amount before moisture starts condensing out of it. Once it reaches that point, further temperature reductions require something like three times as much energy. In other words, it is like you are running a dehumidifier when you don't need to be.
     
  12. Jun 18, 2009 #11
    I can see what you're saying, but I thought that the additional amount of energy lost as heat via friction in the faster-moving 'single' system would be less than the amount of energy lost through friction in two systems, if you were doing the same work with them both running at a lower speed than with one. Of course, it would depend on how different the efficiencies between the two different types of systems were, which I was unsure of, so I presumed they would be of a fairly similar efficiency.

    I'm not saying you're wrong, I'm just explaining why I thought that.
     
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