Do escalators going down put electricity back into the grid?

Question

Where I work there's a station with lots of escalators to move people up and down as they use the station.

In the morning this means loads of people going up at the same time, which must use loads of electricity.

It got me thinking... in the afternoon when there are more people going down, do the escalators convert the downward mass into electricity and feed it back into the power grid?

Answer 1

In theory, yes.

A escalator with no one on it uses about 25 percent of the motor's rating just to turn the "belt" linkage of steel steps.

Adding people to an "up" escalator causes it to do more work.  The motors are designed to handle a continuous stream of people on every step.  150+ lbs per step.

The worst case electrical load on the "down" escalator is when it is empty.  People getting on the down escalator cause it to go into a synchronous braking mode to maintain the constant speed.  The result is the escalator requires less than 25 percent of the motor's rating to operate.  

Let's assume you get the motor load down to 10 percent of nameplate.  Even if you got it to generate power back to the system, that would be used to operate other equipment in the facility.  You'd most likely never get your electric meter to spin backwards.

Factories that built and test escalators actually use large racks of resistors to dump the excess energy during testing.  One of the critical tests is related to a power failure of the down escalator with people on it.  Without the utility's 60 Hz (or 50 Hz) to maintain a constant speed, the escalator would speed up.  They have brakes to hold the steps in place when the power is off.

Perhaps we should develop a fitness routine where people take the stairs up, and then the escalator back down?  It would be far more productive that the expensive stairs-machines at the fitness center.

Answer 2

In order to generate electricity, you need a generator and a means of feeding the electricity back to the grid. Clearly escalators, up or down, do not have these, so NO!  

Answer 3

No, and this is because there is a lot of friction involved with the escalator.  While you usually have the weight of the people on the escalator bushing down, you also have to lift up the metal stairs that are going up.  An escalator that only goes down will still use electricity.

Answer 4

Interesting...but I don't think so

Then again I can't give a proper reason why not

Answer 5

In theory you should be able to do it, as it's similar to how pumped-storage hydroelectric stations work. You just have a flow of people rather than water.

Perhaps more similar is the situation with cable car systems. When there's a lot of load coming down a mountain, it's not unusual for ballast to be added to the cars at the bottom station, (and removed at the top station) to help counterweight the mass coming down. This ballast absorbs power in its climb. When it's allowed back down this power is released.

The mass of belt/steps/ropes/cables/cars etc doesn't need to be considered as that going "up" balances that coming "down" (unless there's stretch of course)

The raw power released when a object falls is as follows

"Gravitational"_Power[W]= mass_flow_rate[Kg/s] * gravitational_constant[N/kg]

(use a negative value for flow if mass going UP)

For an escalator/cable car you plug it into the follow power equation to determine if you need to add, or take power out, of the system to maintain a constant speed.

Gravitational_power+Motor_power_IN - Power_to_overcome friction=0

If the gravitational_power is greater than friction then energy HAS to be extracted from the system or else the escalator would speed up. If the gravitational_power is less than friction then energy HAS to be added.

A brake simply transfers energy out of the moving system (which why brakes they heat up!).

So the problem would be one of extracting useful power...

Motors can be arranged to produce power when they are driven by a load (rather than driving the load) Problem is these dual purpose machines aren't as efficient as dedicated motors/generators. Costs are likely to higher, and there's more to go/wrong fix. Probably isn't economically viable.

Answer 6

No.  The escalators are actually using power even for the trip down.  You notion would be valid if the escalator is just free falling.  However, it is not.  The downward motion is controlled and slowed by using energy.  

Answer 7

no

.