Energy Efficiency
The Rexroth solution for the Eiffel Tower West Elevator drive system provides 25% savings on annual power consumption compared to the historical solution
Historic Solution: principle of operation
The transfer pump ensures constant re-pumping of the hydraulic fluid from the low-pressure (LP) accumulators to the high-pressure (HP) accumulators.
When ascending, the ascent valve opens and allows the hydraulic fluid to flow from the HP accumulators in the lift cylinders.
When descending, the descent valve opens and allows the hydraulic fluid to flow from the lift cylinders in the LP accumulator (energy recovery).
For the installation to work, each of the valves must be sized to allow the flow required for the maximum speed of the cabin in the most critical individual case of the smallest pressure drop.
Thus, the ascent valve must allow this flow when the cabins are fully loaded.
The descent valve must allow it when the cabins are empty.
Therefore, in the following two cases of operation, the valve for regulating the motion must create a pressure drop by throttling the flow that it allows to pass through (= loss of energy, converted into heat):
- Ascent with empty cabins
- Descent with fully loaded cabins
Deployed solution: principle of operation
The motor-pump units are fitted with variable displacement pumps, capable of generating bi-directional flow, driven by an electric motor at a constant rotational speed.
They are located in a semi-closed hydraulic circuit, some being connected to the accumulator cylinders, others to the hydraulic tank. All of them are connected on their other port to the lift cylinders.
When ascending, the pumps are all controlled by inclination in the direction of a flow from the accumulators or from the hydraulic tank to the lift cylinder.
When descending, they are simply driven in the opposite direction, the fluid flows from the lift cylinder to the accumulators or the hydraulic tank.
Thus, it is the inclination of the pump that regulates the speed of movement of the cabins, and the power extracted from the power network by the electric drive motors varies depending on the load to be moved, i.e., as a function of the cabin load. The load carried by the accumulator cylinders, constant, is determined in turn to limit power consumption peaks in the case of extreme operation – empty/fully loaded cabins.
An exemplary illustration of the Rexroth 4EE philosophy
The energy approach to the design of the overall system implements an energy efficiency principle: It is no longer necessary to systematically convert energy into heat through a regulator to control cabin speed, which was how the original solution worked (“Energy System Design”).
Moreover, this principle supports the optimization of energy recovery during cabin descent phases (“Energy Recovery”).
Continuing this approach shows that the functional equipment in the cabin movement phases can be completely stopped during floor stop phases (which represent about 60 to 70% of the cycle time), and only started for the movement phases, thereby reducing auxiliary energy losses (“Energy on Demand”).
Finally, the choice of optimized performance components, and the special care applied to the minimization of losses in line and through the hydraulic blocks, helps to increase the energy efficiency of the solution (“Efficient Components”).
Consequences
The overall contribution of the various above-mentioned energy features of the implemented solution has been compared with the historic solution, assessing daily total energy consumption for a typical cycle of operation.
Result: the consumption of the deployed solution proved to be 25% lower than the historic solution.
Nevertheless, Bosch Rexroth’s solution remains consistent, in spirit, with the historic solution:
- It is based on the same system architecture, which implements the two lift cylinders, with the same dimensions, and the three accumulator cylinders, installed exactly as the historic accumulator cylinders,
- It enables recovery of the movement of these three accumulator cylinders, one of which was no longer functional under the last principle, currently in operation in 2008,
- It restores the accumulator cylinders to their original role of energy recovery during phases of descent and storage for the ascent phases,
- It also uses a principle that prioritizes the energy conservation approach, completely revamped using current technologies.