Let’s say an elevator in a skyscraper is at the top floor, and a bunch of people get on & they all want to go to the ground floor. That’s a lot of potential energy to get rid of, where exactly does it go? Is it just converted to heat energy in the motor? Does any elevator company use this energy in a useful way, such as feeding it back into the building?
Keep in mind that many cable-suspended elevators have large counterweights so there’s actually a lot less PE change going on. You also get to use a smaller motor since it doesn’t have to hoist the entire weight of the car.
Having said that, potential energy will be transformed into heat (mostly) and sound.
I’m aware of the use of counterweights; this has nothing to do with the amount of potential energy that must be dispersed.
I think, as Valgard said, it all goes into heat from friction and the motor. (Even the sound ends up as heat.)
And why not? You’re taking the potential energy of the elevator car and transferring it to the counterweights (or vice-versa, if the car is going up, instead.)
Or rather, some of the potential energy. Any left over is converted to heat in various parts of the system.
Why do you say that? The potential energy of one is transferred into potential energy of the other.
Not exactly; the people leave the building & go on their way. The counterweights remain in the building. If another bunch of people on the top floor suddenly decide they have to leave as well, the counterweights make the same trip - but their mass is the same, and it takes less energy to bring an empty elevator back to the top floor than is released by bringing that next carload of people down. If there were a large number of weights on the bottom floor, consistent with the amount of weight of all the people living on the top floor, and you changed the counterweights repectively, then you could say that the energy was transferred…but I doubt that elevators are built that way. As far as I know, the counterweights do not change, and therefore you are not transferring the energy to them, you are merely offsetting part of the weight, and this has nothing to do with conservation of energy.
I think I’m doing a poor job of explaining myself, so please let me elaborate a bit.
First, I’m making a couple of assumptions, which are:
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The counterweight weighs as much as the elevator plus the average number of passengers. This way, during most usage, the motor is doing the least amount of work.
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The counterweight does not change weight.
Of course you’re putting some of the energy into the counterweights. Let’s say a full load of people weighs 500 pounds more than the counterweight. Since the weight can’t go up any more than the elevator was in the first place, that’s 500 pounds times the height of the elevator worth of energy that can’t “fit” into the counterweight.
Of course, any difference in weight between the elevator and weights means more work for the motor. If an empty elevator is on its way down, the motor has to work to pull those weights up. Would you consider being able to transfer the energy from the elevator to the counterweight in this case? Of course not.
Flying_Monk;
I didn’t mean to imply that the counterweight magically balances the system in all circumstances, just that it is there to handle the bulk of the energy change, so that the motor and braking system don’t have to work as hard. Like you said, the counterweight is probably set equal to the mass of (elevator plus average cargo) as that’s the most efficient way to go.
Consider the case where the counterweight is exactly equal in mass to the elevator + cargo. As the elevator moves up or down, the change in PE of the elevator will be exactly equal to the change in PE of the counterweight. The only energy to account for will be the friction in the system (what it takes to turn the wheels, motor running the system, etc). Friction will come out as heat and noise.
Now when the cargo is different from “average” you’ll have some excess PE to think about (suppose it’s one extra person). If the elevator is going down then it loses more PE than the counterweight gains (an amount equal to the mass of the extra persongh). That extra PE will be turned into more KE which must be reduced by the braking system and this will again wind up as heat and some noise.
Valgard, I agree completely. My OP was basically in the vein of discovering whether or not the occasional energy surplus was put to any use, or if it was just wasted. Seems to me it wouldn’t be difficult at all to feed any excess energy back into the building’s wiring, through the motor. I was also thinking of wacky energy sources…and I thought of the possibility of a downwards escalator, reaping energy while enabling people to be lazy.
If the motor is controlled by a Variable Frequency Drive then some of the power used by the system can be routed either back into the system or back to the power company depending on the complexity of the Drive.
Some engineer will try to explain this better than me.
When I was a CE major I did talk to a structural engineering professor about another tall building problem - toilets. When you flush, that’s a bunch of mass dropping 100 floors (even if it’s not a straight shot all the way down. It’s still a lot of PE to be dealt with) and some people were wondering if it could be put to use (heck, you spent a lot of energy pumping the water up that high, might as well recover some part of it).
Waterwheels were discussed in all seriousness. I think that running a plumber’s snake to clear a drain would be a little challenging…
I guess that you could recover some elevator energy but I don’t think it is done currently - the elevators are made by one company, the building wiring is done by someone else, dunno if you’d need some kind of big storage mechanism (batteries?)…it adds a layer of complexity and I don’t know whether the payoff would make it worthwhile.
Ask Otis what they’re working on.