The year 2004 may well be remembered in the elevator industry as a watershed year. After six years during which Kone fought alone to gain acceptance for high-efficiency machines located within the hoistway (see
Vol. 8, No. 7), now all the major manufacturers are on board. For low-rise applications, these machine-room-less elevators are coming down in cost. “This technology is going to rapidly wipe out hydraulics from the market,” predicts Jim Fortune, chairman of Lerch, Bates and Associates, a leading international elevator consulting firm. In mid-rise buildings, up to 30 stories, they are already replacing the traditional geared, traction elevator: “In five years, I see the geared elevator being almost nonexistent,” says Frank Dugan of Fujitec.
Until recently, low-rise elevators (up to five or six stories) were typically hydraulic, and mid-rise elevators (up to 20 or 30 stories) were geared, traction machines. Hydraulic elevators pump hydraulic fluid, moving a piston that pushes the elevator cab up and lowers it back down. In the U.S. that piston typically sits in a “jackhole” in the ground under the elevator, although various alternative configurations allow for the piston to remain aboveground.
Traction elevators are driven by a motor and suspended from overhead cables. In most mid-rise installations, the motor is connected to a gearbox that in turn drives a sheave—the grooved wheel that guides the cables. In high-rise buildings, a gearless motor turns the sheave directly. The motor and sheaves are typically in a small penthouse machine room that extends above the building’s roofline, but the motor can also be located adjacent to the elevator.
The steel cables that raise and lower the elevator cab wrap around the sheave and are held in place by traction. Traction elevators are inherently much more efficient than hydraulic ones because they use a counterweight to balance the weight of the cab. When the cab and counterweight are in balance, the motor only has to work to accelerate and decelerate the cab, and to overcome friction. More often, however, the cab is either heavier or lighter than the counterweight, so significant energy is needed to move the elevator. When the heavier of the two loads is descending, energy is used for braking—this energy is either dissipated as heat in the machine room or fed back into the electrical system through regenerative braking.
While each manufacturer has its own variations on the machine-room-less technology, the overall trend is toward gearless, permanent-magnet motors using variable-speed, variable-frequency drives. These small motors can be mounted directly in the elevator hoistway, eliminating the need for an overhead penthouse to house the motor (in the case of traction elevators) or a ground-floor machine room (in the case of hydraulic elevators). Their smaller, more sophisticated motors require only one-third as much power, so the size of the electrical service to the elevator is dramatically reduced. They are also easier and quicker to install, and offer a smoother, quieter ride.
The environmental benefits of these machines are also dramatic. The gearless motors with variable-frequency drives are two to three times more efficient than hydraulics, and 30 to 50% more efficient than standard geared elevators. Otis estimates that the typical electricity cost for running their Gen2 gearless elevator might be $1,000 per year, compared with $1,300 for a new geared traction elevator and $1,500 for an older traction elevator. Keeping hydraulic fluid out of the ground is another big benefit, since fluid leaking from jackholes under elevators can contaminate groundwater (unless the fluid is plant-based, in which case toxicity usually isn’t a problem). Hydraulic elevators also require heaters to maintain the fluid at a constant temperature—as National Renewable Energy Lab researchers discovered at Oberlin’s Lewis Center, these heaters use energy around the clock, even if the elevator is rarely used (see
Vol. 11, No. 7). In the case of overhead traction elevators, the roof penetration for the penthouse machine room is a notoriously difficult detail to insulate and air-seal effectively, so avoiding the machine room can reduce heating and cooling loads as well.