Have I got this right? If I have an elevator car, and I want to move it up inside a building over and again by lifting from above, then I’ll be using a lot of energy each time the car ascends.
But if I go to the one-off energy expense of attaching a counterbalancing weight to my car via a pulley system, and if this counterweight exactly matches the weight of my elevator car, then I can send my box up the building (without payload) as many times as I want and the only energy required would be that needed to offset the friction on the pulley and cable system.
Have I got this wrong? It seems that I shouldn’t be able to remove the energy required to move something from significant, to negligible, that easily.
While your ascents use less energy, you also now need to put energy into going down, which is free in a non-counterbalanced system (although you lose much of the recovered energy controlling the descent). And you can’t balance the weight exactly (because people randomly get on and off, and they all weight different amounts), but using a counterweight system does drastically reduce the energy requirements. There are often more efficient ways to do things, but you have made the system heavier (twice the weight) so it has more inertia, and there are more things to maintain (you really don’t want the counterweight slipping from it’s track into the path of the elevator car). The elevator void is also bigger, the cables are longer, and you have a more complex safety system.
I remember seeing a show about a canal lift, which rotates a lock full of water and canalboat, and uses a few kilowatts (about the same as boiling a kettle of water, according to the report) per lift. They were able to adjust the counterbalance by adding/removing water, though.
As an object (in this case the elevator car) gains altitude, it increases its gravitational potential energy, and that energy has to come from somewhere. Without a counterweight, the energy comes from a really, really big electric motor that has to pull hard to raise the entire weight of the elevator car.
The reverse is also true: if something loses altitude, it decreases potential energy, and that energy has to go somewhere. So by adding a counterweight to the elevator system, the car and the counterweight basically shuffle gravitational potential energy back and forth between each other. With the counterweight pulling up on the car just enough to balance its weight, now you only need a modest electric motor cable of accelerating the car/counterweight system with reasonable alacrity and lifting the weight of the passengers with acceptable speed.
Think of two kids on a seesaw: same idea, they’re just passing gravitational potential energy back and forth.
To put some emphasis on something **si_blakely **mentioned, but only in passing …
If you simply had a drum at the top and a cable down to the elevator car with no counterweight, then the motor would have to expend a bunch of actual electrical energy to move the car to the top, converting some of that into frictional losses and the rest into gravitational potential energy.
A naive 1880s elevator would use a friction brake to control the descent, converting 100% of that gravitational potential energy into heat.
A smart 21st century elevator would use regenerative braking & electrical storage to capture most of the gravitational potential as reusable eletrical energy. You’d still have frictional losses, but since the system would weight 1/2 as much,they ought to be proportionally less as well.
As of today, the low-tech solution of a counterweight is cheaper / better. But at least in theory, we could produce a counterweight-less system which was lighter & as energy efficient over a round trip bottom-top-bottom.
Ball bearings and greasy cables have very low frictional losses. And pig iron counterweights by themselves are low mainenance and low cost.
Meanwhile energy conversion, either electrical-to/from-mechanical or electrical-to/from-storage device, is lower efficiency, ranging from 70-90%.
Which is why elevators today have counterweights. But 30 years from now ???
Other thought …
overall, there is nothing magic about
For any problem, there are smart & naive ways to solve it. A wheelbarrow vs. a wheelbarrow-like bin minus the wheel is an example of a smart vs. naive solution. And yes, there can be massive gains in efficiency using smart but still low tech solutions.
ETA: Joe’s more concise message wasn’t there when I opened this thread. Sorry for the partly redundant redundancy.
You’re probably thinking of the Falkirk Wheel in Scotland. Interestingly, the adjustment required to maintain the balance is automatic. When the boat moves into its side of the lock, an equivalent weight of water automatically moves out (since the boat displaces its weight in water), and similarly when the boat moves out of the lock.
Older elevators did not worry about energy use, low cost. The counter weight ballances an elevator when it is 1/3 loaded. Drum elevators without counter weights need larger motors. They used Motor generator sets and a DC motor. The generators has complex fields, same for the motors. The control cabinets could have over 200 relays. And timing was everything.
The new elevators use electronics and computers to control variable speed AC motors. And they are designed to use energy effeciently. (side line they are not as dependable).
Just a side bar. I love the movies that show an elevator falling down to the bottom of the shaft when the breaks fail. They fall up unless loaded.
Or the bad guy who cuts the lifting cables and the car free falls. They have safety breaks that will stop them.