The Green Lantern - Can We Reduce Energy Use by Skipping the Elevator?

(By Mark Alan Stamaty)
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By Nina Shen Rastogi
Thursday, April 9, 2009

How much energy do elevators use, anyway? My co-worker is so proud of herself for taking the stairs every day, as if that's going to save the planet. . . .

Cut her some slack: It's true that if everyone who could take the stairs did take the stairs, we'd see some significant energy savings. Of course, because stair-climbing requires that we expend nine times as much energy as we do standing still, a collective elevator boycott would probably lead us to higher food consumption, which would require more water and fossil fuels and produce more packaging waste.

Before we can go down that rabbit hole, though, we need to have a sense of how much juice goes into operating your average elevator bank. That turns out to be a very, very complicated question, the answer to which depends on a large number of variables: For example, how many people ride the elevator? What kind of drive does it use, hydraulic or traction? Is it geared or gearless? Does the system have the most recent braking technology, which recaptures energy that would otherwise have been lost as heat, funneling it back to the grid? Does it use software that plots the most efficient route possible for each car?

The differences in energy consumption here can be wide. According to figures provided by elevator manufacturer Kone, a typical hydraulic elevator in a three-story office building uses 3,800 kilowatt-hours per year, or about as much as the average American home uses in four months. A traction elevator in a 10-story building might use about 5 1/2 times as much energy. In a 30-story high-rise, it might be 11 1/2 times as much.

Even within a single building, not all elevator rides are created equal. Hydraulic elevators, for example, need a lot more power to go up than to come down. (These systems tend to be used in buildings that are seven stories or shorter.) Trips to the lobby are more efficient, but they're not entirely energy-free: As the car travels down the shaft at a controlled speed, the friction caused by oil passing through the hydraulic valves generates heat, which must then be dissipated by the building's cooling system.

In the traction systems used in taller buildings, counterweighted pulleys raise and lower the cars. These counterweights usually weigh as much as the car itself when loaded to 40 percent capacity. A full car traveling up from the ground floor, then, requires a significant input of energy because it weighs much more than the counterweight. (Imagine a fat kid and a skinny kid on a seesaw. If the fat kid is on the ground, he'll have to push off with his feet in order to travel upward.) A full car traveling downward, on the other hand, is so much heavier than the counterweight that it can move without much help. So in a typical mid-rise or high-rise office building, a full car going up uses more energy than a full car going down, and an empty car going down uses more energy than an empty car going up. The system turns out to be most efficient when the car is 40 percent full -- i.e., when it's perfectly balanced with the counterweight.

How should you interpret all this information? If you work in a tall building where the stairs aren't an option, you might try to be more efficient by taking only empty cars up to your office and full cars back down to the lobby. On the other hand, if everyone followed that rule of thumb, more people would be riding elevators by themselves and the total number of rides would increase, which would likely end up using more energy overall. In that case, the best strategy would be to minimize trips for everyone by elevator-carpooling with your co-workers.

No matter what you do, though, it's nearly impossible for a rider to figure out exactly how much energy his or her elevator habit consumes. One way to get a very rough ballpark figure would be to take the total energy used by a building's elevators in a given workday and divide that by the number of tenants. At the request of the Green Lantern, a ThyssenKrupp consultant ran simulations for typical five-story, 16-story and 42-story office buildings. In each case, the energy use per tenant came out to about 0.3 kilowatt-hour. Is that a significant amount of energy? It's about as much as you'd save in four hours by replacing an incandescent bulb with a compact fluorescent one.

In any case, the elevators in your building will be draining some power no matter what you and your co-workers are doing. Most elevators stay on all day, even if no one is using them. The average standby power rating is between 0.8 and 2 kilowatts, which can really add up: Analysts at ThyssenKrupp who studied a 16-floor office building in Ohio found that roughly one-third of the elevator bank's daily energy consumption occurred during non-business hours. Keeping elevators well-lit is an issue, too: Bulbs might add 1,750 kilowatt-hours a year per cab. No word, though, on how much energy it takes to pump in that soothing Muzak.

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