Half a croissant, on a plate, with a sign in front of it saying '50c'
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Revolving Door Power Generator

Generate electricity by a revolving door
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A revolving door in a busy place usually spins for hours every day. Much of the energy used to turn the door is wasted. A generator at the door could convert the mechanical energy from the spinning door into electrical energy. It may be a little harder to turn the door, but people will burn more calories. Little kids running in circles in a revolving door will actually be generating lots of energy. Most of the subways in New York City have revolving doors that are used continuously all day. Think of all the energy that could be harnessed from the doors.
kewldude471, Jul 24 2005

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       This would just make it harder to turn the door, so that weak and lazy people would think it was stuck, and would use the 'standard' door instead. Besides, how much energy could the door really produce, even over a day? I suppose you could have a lightbulb over the door that stays alight when people go through.
dbmag9, Jul 24 2005
  

       //how much energy could the door really produce//
Easy. Suppose that the door needs a hefty push (say, 100N, equivalent to lifting 10kg) over a distance of 1 metre. Then, the energy expended in pushing the door is 100Nm, or 100 Joules.

Now make a guess as to the efficiency with which the energy can be recovered - say 50% (which is very very optimistic). Therefore, 50J of energy is available. This will light a 50W lightbulb for 1 second, or about a third of the time it takes a person to go through.
Now to find the total daily energy yield. This depends on how often the door is used, but let's be generous and assume the door is in continuous rotation at all times. Of course, it might have several people in it sometimes (and they could, collectively, push with more force, if the mechanism were smart enough to increase its resistance), but on the other hand in reality there will be quiet times, so a continuous one-person push seems reasonable. In this case, the average power output of the door over the whole day is about 17 Watts, or about 1.5MJ per day.

I have no idea of the energy involved in building and installing this mechanism. I would imagine that it would be at least a 100MJ (energy to make and press the steel components, to refine the copper for the wires.....). If the guy who delivers and fits it it has to drive for 1hr in total, in a 200hp vehicle, that adds another 540MJ to the energy bill. So, energy-wise, the unit will pay for itself in just over 1 year (assuming no energy-expensive maintenance call- outs), after which you'll be able to run your 17 watt lightbulb whenever the heck you please :-)
Basepair, Jul 24 2005
  

       would be very useful, but should only be used on revolving doors that are heavily used. anywhere else it won't generate enough power for its installation to be practical.
FireElf, Jun 11 2006
  

       Aren't revolving doors resistive enough because of the friction from the sealing around the edges for insulating purposes?
BJS, Jun 11 2006
  

       Problem with any animal-powered generation is that it is a terrible use of a complex energy-consuming "machine" to produce a small amount of energy. How many lightbulbs will be powered by the first lawyer whose client is injured from one of these increased-friction doors? If you really want to power up a few lightbulbs with people exerting the energy, at least go to cooperative participants -- try a fitness club (think treadmills and weight machines).
thekohser, Jun 11 2006
  

       I read that Thomas Edison had this in his mansion.
pashute, Jun 30 2013
  

       At last, a way to get some usage out of door-to-door salespersons.
not_morrison_rm, Jul 01 2013
  
      
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