By By Pat Smiley, PE, and John Greenlaw, PE
Building designers have had limited options to vertically transport building occupants. Elevators have been either traction type (with cables) or hydraulic type (with pistons) for nearly a century. The traction types have a car and a counterweight on opposite ends of the set of steel cables, the "ropes." A hoist machine with an electric motor turns a "drive sheave" that moves the ropes, counterweight, and car. The hydraulic types have a car moved by one or two hydraulic cylinders beside or under the car and powered with an electric motor and hydraulic pump. Only traction elevators are practical at heights of more than 60 or 70 feet.
Traction and hydraulic technologies have evolved steadily since their invention. Hydraulic elevators became particularly popular after World War II with the spread of low-rise two-story and three-story offices, hotels, and other commercial buildings. Lower first costs made them attractive to developers.
Traction types are relatively energy efficient because the counterweight offsets the weight of the car and nearly half the rated load. Hydraulic types, by contrast, are "brute force" machines where the motor and pump must deliver the energy required to lift the total weight of the car and its load. That difference is substantial, with the hydraulic elevator needing two or three times as much energy to accomplish the same work. In recent years, the gap has widened as more efficient motors and controls have improved the energy efficiency of traction elevators. Hydraulic elevators have not improved at the same rate.
A major change in recent years has been the development of the so-called "machine - room - less" elevators. These are gearless traction elevators with the hoist machines mounted within the hoist ways. One model uses a permanent magnet synchronous direct drive traction motor mounted alongside the car way. They are energy efficient but not more so than conventional machines using the latest technology. They do use somewhat less metal and other materials and therefore have less embodied energy. Some of these new machines are competitive with hydraulic elevators in first cost and have the large energy-efficiency advantage of traction machines. Much of the reduction in first cost relative to other traction elevators is due to a less labor-intensive method of assembly within the hoist way.
Efficiency benefits can accrue from drive selection. Regenerative drive can capture the potential energy stored from lifting a heavy passenger load. With a variable voltage/frequency AC drive system, motor braking produces electricity for productive use elsewhere in the power system instead of dissipating this energy as heat.
Improvements in lighting technology are also now available for the elevator cab. LED lighting systems that create less heat and use less energy compared to conventional systems may be specified for illumination.
Digital technology has improved the sophistication of elevator dispatch systems. These systems now evaluate traffic patterns, monitor floor requests to groups, and deliver passengers to their destinations in the fastest and most efficient way possible. Reducing transport energy is an obvious benefit. Increasing the number of passengers delivered per lift may reduce the number of machines that need to be installed compared to systems using conventional dispatch systems. Elevators can be operated more efficiently and accommodate more traffic, thus reducing the number of lifts required. Construction materials, costs, and energy can all be reduced as a result. These systems may also be retrofitted to existing facilities to improve capacity and efficiency without increasing the number of lifts.
The drive to improve building energy utilization will pressure these systems to continue to evolve. The vertical transportation industry is adapting as well as inventing new technologies to improve the energy use and efficiency of these important systems. Look forward to exciting advancements over the next years and coming decades.
Pat Smiley ([email protected]) manages the vertical transportation department at The Greenbusch Group. He received a degree in aerospace engineering from Iowa State University. He is a member of the American Society for Healthcare Engineering and its Washington State affiliate, and of the NFPA, ASHRAE, and ASPE. John Greenlaw ([email protected]) is a founding partner of The Greenbusch Group and manages the mechanical engineering department. He holds both bachelor's and master's degrees in mechanical engineering from the University of Washington. He has specialized in the design of efficient building systems for the last 20 years.
