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Following initial performance analysis in 2-D, Ricardo used Opera 3-D to investigate end-effects to minimize interaction with the flywheel housing.

Construction equipment benefits from advanced electromagnetic design tool

Ricardo may look back on 2014 as THE year for TorqStor technology. Though it had been in development for about seven years, this was the year it won a Tech Award from SAE International as one of the top five technologies on display at the SAE 2014 World Congress. TorqStor also took on heavy-duty overtones earlier this year when a 200-kJ prototype was on display at ConExpo. 

TorqStor technology consists of a carbon-fiber composite flywheel contained within a permanently sealed vacuum chamber, which can be scaled in size to provide energy-storage solutions for a variety of off-highway equipment. The flywheel is linked by a magnetic coupling and gearing system, and rotates at 45,000 rpm.

Ricardo estimates that a "well-integrated solution in an excavator would deliver fuel savings in the region of 10% at typical operating speeds." Integrated with a machine’s hydraulic power system, TorqStor stores the gravitational energy that is currently unused during the downward movement of the machine’s arms or booms, and then uses it to help power subsequent upward movement. It is this improvement that contributes to operational efficiency and fuel savings, and may potentially allow for the use of smaller capacity engines for the same load-moving capabilities.

When Ricardo first started development of magnetically coupled kinetic energy storage devices, it looked at a number of electromagnetic simulators on the market to help shorten design-to-manufacture times. The company says the Opera package from Cobham Technical Services emerged as the favorite because of its scripting flexibility for automating design investigations and ease of interfacing to other software tools. It is now considered Ricardo’s electromagnetic FEA tool of choice.

The first two generations of Ricardo’s flywheel energy storage were designed for integration with the powertrains of hybrid and all-electric vehicles such as cars and buses, to provide traction power. Ongoing research indicating that even higher efficiency gains could be achieved in applications involving frequent energy storage and demand cycles is what led to the TorqStor development.

Ricardo set a three-month target for all magnetic analyses for the TorqStor project. During this period the development team simulated more than 100 individual design concepts using Opera, each involving successive changes to a wide range of parameters such as material properties and magnet temperatures to examine their effect on torque capacity and magnetic losses. The entire operating cycle of the device needed to be accurately simulated to fully evaluate its performance, so relative rather than absolute parameter changes were employed to help minimize analysis time.

To further accelerate analysis, Ricardo used the 3-D version of Opera to initially configure a model and derive data such as the end loss factor of the permanent magnets in the TorqStor coupling rotors, then switched to the less computationally intensive 2-D version for the bulk of simulation tasks, such as deriving the torque per unit length of the flywheel for different configurations.

Despite the complexity of the models, each non-linear 2-D simulation run typically takes less than 30 minutes, with overnight batch processing further helping to shorten the process. Solutions that met specific criteria in the design space were then modeled and simulated using Opera 3-D to fine-tune performance and provide detailed confirmation of results.

The energy storage flywheel in the TorqStor system contains a large number of permanent magnets mounted radially around its end face. These then couple via stationary pole pieces in the housing to an external rotor that is fitted with a smaller number of magnets, providing non-mechanically linked power transfer and gearing functions.

“The lack of mechanical symmetry means that we cannot segment the design and use partial simulation to accelerate the process. Also, the magnetic gears are geometrically linked, which makes it difficult to do a sweep of parameter changes and means that we cannot use auto-optimization tools to eliminate the need for manual intervention," said Joshua Dalby, a Development Engineer at Ricardo.

Opera’s "near-seamless mesh control capabilities" require minimal input and allow any angular alignment of rotating and stationary parts to be considered, added Dalby.

"The necessary air gap re-meshing can be controlled from a command script file, which we have found especially helpful—it has enabled us to evaluate the effect of the two air gaps between the inner and outer rotors and the pole pieces of TorqStor very accurately,” he said. 

A video of the system can be viewed here.

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