Question:

What is the relative energy efficiency of various elevator drive mechanisms? For example, some elevators use "regenerative" motors (when the carriage goes down, the motor acts as a generator and pumps current back into the electrical nework, producing energy savings).

Answer:

The following describes various types of elevators. 

Low-rise buildings may use an elevator driven and supported by a telescoping jack powered by a hydraulic motor. These units operate at a low, fixed-speed, do not feature regenerative braking, and have a limited travel or lift of approximately 25-feet.

For a building with seven or fewer floors, a variation of the hydraulic elevator may be specified that features a constant speed hydraulic pump which imparts pressurized hydraulic fluid onto a hydraulic elevator drive motor. Varying the control valve setting provides the ability to vary the elevator speed. These hydraulic elevators are equipped with counterbalances, but do not have regenerative braking. These units are often found in mid-rise facilities with "residential" usage characteristics, such as apartments, condominiums, or nursing homes.

High-rise buildings are served by geared or gearless traction elevators. These elevators are driven by DC MG sets, silicon controlled rectified (SCR) DC motors, or variable voltage variable frequency (VVVF) drives coupled to AC motors. All of these configurations provide variable and high-speed operation and provide regeneration, but exhibit different operating efficiencies.

Dover Elevators has calculated the average kWh required per day for a single elevator equipped with MG, SCR, and VVVF drives (see table below). Elevators with hydraulic drives would be expected to use about 25% more energy than the SCR drive. The average energy use values are given for elevators of various capacities, serving from 3 to 32 floors, and are based upon an 8-hour operating day and an elevator with a maximum full-load mass of 19,000 pounds.

Note: Average energy consumption values are based on 8 hours of elevator operation per day. If energy consumption for 10-hour operation is desired, multiply the numbers in the table by 1.25

In general, traction elevators, or elevators with regeneration capability, initially cost twice as much as hydraulic units, and are typically found in office buildings with heights of 5 stories or greater.