h a l f b a k e r yNot so much a thought experiment as a single neuron misfire.
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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.
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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. |
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//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 :-) |
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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. |
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Aren't revolving doors resistive enough because of the friction from the sealing around the edges for insulating purposes? |
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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). |
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I read that Thomas Edison had this in his mansion. |
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At last, a way to get some usage out of door-to-door salespersons. |
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