the green lantern

Take a step in the right direction

By Nina Shen Rastogi
Tuesday, November 30, 2010

How much energy do escalators use? They keep those things running all day long! Should I take the elevator instead?

When it comes to energy use, all escalators are not created equal. The bigger an escalator is - the higher it rises and the wider its steps - the more juice it needs to go trudging along its endless, circular path. The amount of traffic it gets also makes a difference.

As with elevators, escalators can vary widely when it comes to energy consumption. According to a representative of Power Efficiency Corp., which designs energy-saving devices for escalators, your average shopping mall unit - which has a 7.5 horsepower motor, rises 15 feet, and is kept running 14 hours a day, six days a week - might use about 7,500 kilowatt-hours of electricity in a year. An escalator that runs all day and all night in a convention center or hotel - say, a 20-foot-high unit with a 20-horsepower motor - would use roughly 31,000 kWh annually. A continuously running escalator of the kind you'd find in airports or subway stations - 35 feet high with a 40 horsepower motor - would use about 60,000 kWh annually. (For comparison's sake, the average American home consumes 11,040 kWh in a year.)

So how do moving stairs stack up against elevators? It's a bit of an apples-to-oranges comparison, as the two machines have different jobs.

Escalators are great when you're dealing with throngs of people, since they can carry many passengers at once. That's why they're so popular in transit stations, where you often have a wave of people all exiting at the same time. But during periods of light traffic, elevators come out ahead, because they can speedily move small groups of passengers. Plus, they sit idle - and thus save energy - when not being used. (You can also rig an escalator to stop moving when no one's on it, but that's not a recommended tactic, in part because people who see a stopped escalator tend to assume it's broken.)

Still, escalators seem to be the bigger electricity users, particularly as buildings get taller. Elevator technology consultant Jim Bos modeled two theoretical office buildings, one with three floors and one with six, each with a peak traffic flow of 35 people per minute. He assumed that the equipment in these buildings was reasonably modern and that the escalators run only 12 hours a day during the workweek. (During the weekend, the escalators in this comparison are turned off while the elevators remain on standby.)

With the caveat that there are a whole lot of variables in an assessment of this nature, Bos estimates that the shorter building would require either five elevators, which would collectively use 68,000 kilowatt-hours a year, or two pairs of escalators, which would use 75,000 kWh a year. (That includes lighting in the elevator cabs.) To handle the traffic in the six-floor building, you would need only one additional elevator, but you would have to add three more pairs of escalators. In this scenario, the elevator bank might use 130,000 kWh annually, while the escalators use 187,000 kWh.

Escalators do offer some big opportunities for saving energy if you can figure out how to cut power consumption during periods when they chug along without any passengers. Option 1 is to slow it down. These intermittent or variable-speed escalators are popular in Europe and Asia, but they haven't gained much traction in the United States, thanks to a national safety code that forbids escalators from changing speed. (The forthcoming update to the code will lift that injunction for new machines.) The actual savings on a given escalator will depend on how often the unit is idle, but a recent European study estimated that installing variable-speed drives on all the region's escalators could reduce electricity use by about 28 percent.

The second option is to install a controller on the escalator motor that improves its efficiency.

Escalator motors are designed to move a massive amount of weight: A full escalator can have 150 to 300 pounds of passenger on each step. Alternating current motors, the kind you find in escalators, are most efficient when they're moving a full load. But escalators are almost never totally occupied; most of the time, the motor is drawing more power than it needs. Efficiency controllers try to match a motor's power supply with its power needs at any given moment. Manufacturers of these devices claim they can reduce energy use by 15 to 35 percent. (These controllers can also be installed on elevators.)

In the meantime, there's a simple way to reduce the footprint of your personal escalator habit: Use the stairs on your way up, and ride the escalator on the way down. On an up escalator, each additional passenger makes the motor work a little harder to pull the steps up. But on a down escalator each additional passenger eases the motor's task, thanks to the action of gravity. So riding the up escalator costs a little energy, while riding the down escalator saves a little. On the up journey, you'll save the most energy by skipping the escalator entirely, but you can still save a bit by walking up the moving staircase: You'll finish your journey more quickly, and thus the motor won't spend as long straining to haul you up.

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