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

3.8 Efficient Hydraulic Lifts
In conventional hydraulic lifts, the electricity consumption, when the lift is moving, is much
higher than in traction lifts. A number of factors contribute to their lower energy efficiency,
namely:
• Losses in the pump.
• Losses in the valve unit due to pressure drop.
• The potential energy is dissipated as heat when travelling down.
• Absence of counterweight to balance the potential energy needed to lift the car.
• Since the oil pressure and flow to the cylinder are controlled by returning oil to the
tank through a bypass valve, the pump works at constant flow, wasting energy
especially during acceleration and deceleration.
• Because oil viscosity changes with temperature, oil cooling or heating is sometimes
required to maintain ride quality and performance. Heating is sometimes necessary
when the machine room is subject to low ambient temperatures (e.g. basements,
below stairs) leading to an increase in oil viscosity. On the contrary, heavy duty
situations lead to the dissipation of large amounts of heat into the oil and cooling may
be necessary.
Hydraulic lifts only consume energy to lift the car. During downwards travel the car descends
due to gravity and controlled oil flow. One simple way to reduce the energy consumption of
hydraulic lifts is to adjust the travel speed without compromising the round‐trip time, to make
use of this characteristic. This is done by reducing the up travel speed and raising the down
travel speed. This way a smaller motor can be used and energy consumption can be reduced
by around 20% [15].
Another simple way to reduce the energy consumption is to reduce the weight of the car by
using lighter materials.
Mechanical hydraulic valves, where the flow of oil is controlled by internal hydraulic feedback,
have problems in compensating for variations in oil viscosity and pressure. Electronic sensing
of the flow of oil using proportional solenoids fully compensates for these variations while
providing better efficiency.
Nowadays, products that use VVVF drives to power the pump are becoming available. By
varying the speed of the pump, only the amount of oil necessary to move the lift is supplied as
opposed to conventional systems where the amount of oil pumped is constant and partially
fed‐back into the tank. Another advantage is that the starting current is reduced by decreasing
the demand on the power supply. Savings of over 30% can be realised in VVVF controlled
hydraulic lifts when the lift is running. VVVF drives can add to the stand‐by power consumption
and actually increase the overall energy consumption of the installation, especially in lifts with
very low usage, like some residential lifts.
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