WP6-Brochure-E4 brochure - ELA European Lift Association.

The major cause of energy consumption in traditional hydraulic lifts is, however, the absence
of a counterweight to balance the car load. Until recently, it was only possible to install a
counterweight in roped hydraulic lifts, but this had the disadvantage of increasing the load in
the buildings structure. Instead of pushing the car directly, the piston pulls on the
counterweight in order to move the car up.
Recently, bladder type hydraulic accumulators that act as a counterweight were introduced,
thus allowing for a smaller motor to be used. These high‐pressure gas filled tanks store energy
during the car’s down travel by increasing the pressure of the gas in the accumulator.
Figure 3‐34 shows a simplified representation of a system using hydraulic accumulators.
Figure 3‐34. Representation of lift system using hydraulic counter‐weight (source: Bucher Hydraulics)
In this system, a VVVF drive (3) controls a motor‐pump (2) (1) assembly which pumps oil into a
cylinder (7) to move the lift car. The valves are on/off controlled and the speed of the car is
varied by varying the rotational speed of the pump, thus avoiding throttling losses. The
position of the car is constantly monitored by a fine‐resolution encoder located in the shaft (6).
Variations in viscosity and pressure are compensated by the active control system for car
position. During operation, the hydraulic accumulators act like a spring and release or store
energy depending on the direction of travel, just like a mechanical counterweight would. This
way a smaller motor can be used saving energy and reducing the power supply demand. Since
the potential energy is transferred to the accumulator instead of being dissipated as heat,
cooling is no longer necessary, which results in additional energy saving. Furthermore, since no
significant heating of the hydraulic fluid occurs, these systems are capable of over 120 starts
per hour against a maximum of 45 in conventional systems. Also, high‐pressure is used
(> 15 MPa) and, because of that, a smaller flow rate is needed and smaller components are
used, making possible a compact Machine Roomless Design [16].
Manufacturers claim running energy savings of over 70%, compared to conventional hydraulic
systems, when using the system described above. The energy consumption may be
comparable to modern machine roomless traction lifts.
47