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Beacon Power's Flywheel Energy Storage System reduces Material Costs By 50%

Physical testing of prototypes is mainly used as a last check before a new design is approved, providing correlation between simulation and the actual results.

With a 150 lb. rotor spinning at an angular speed in the mid 20 KRPM range, stress, vibration, safety containment and thermal issues all must be addressed when designing a flywheel energy storage system. However, developing prototypes for physical testing is time and cost prohibitive according to David Ansbigian, rotor dynamicist at Beacon Power Corporation, a leading manufacturer of energy storage systems for the telecommunications industry located in Wilmington, Massachusetts. To improve design efficiency, MSC Patran, MSC Nastran, MSC Dytran, were deployed to simulate problems before incurring the expense of prototypes and physical testing. As a side benefit, Beacon Power improved its designs, while reducing material costs 50%.

"Without MSC Software, we would not be able to analyze the dynamic characteristics of the rotor. We've found that MSC Nastran is the only code that can run full 3D finite element models, including gyroscopic stiffening effects when solving complex Eigenvalue analyses, frequency response analyses, and non-linear analyses. The complex Eigenvalue analysis allows us to develop Campbell diagrams for various rotor designs, the frequency response analyses provide rotor support displacements and loads depending on the imbalance, and the non-linear analyses help determine earthquake effects," said Mr. Ansbigian. "We have dramatically reduced our design time, increasing throughput by a factor of at least five times. I can look at two to three design iterations in one day using the MSC Patran /MSC Nastran combination, which was tailored specifically by MSC Software engineers to solve the gyroscopic effects of a rotating structure. We import various designs from our CAD system (Pro-E) directly into MSC Patran, which allows us to create the finite element model. Then the model is executed using MSC Nastran providing a results database, which is then seamlessly imported back into MSC Patran for post-processing. Clearly, you can do more design iterations with the help of MSC Patran and MSC Nastran."

The Background

The flywheel energy storage system, designed and manufactured by Beacon Power Corporation, spins a 250-pound composite rim up to the mid 20's KRPM, while the motor/generator converts electrical energy into stored kinetic energy. Because friction and drag are virtually eliminated with the magnetic bearings and vacuum system, the flywheel spins efficiently for long time periods.

Under normal operation, an electric utility supplies a charging current that drives the flywheel up to operating speed. Because friction is virtually eliminated, only a small amount of power is needed to maintain rated speed. When a power outage occurs, the spinning flywheel instantaneously converts kinetic energy into electrical energy. The conversion continues as the flywheel slows, until it is discharged.

Vibration Analysis

One of the key reasons Beacon Power bought MSC Nastran was for its gyroscopic moment and Eigenvalue analysis capability. These unique features of MSC.Nastran enable analysis of critical speeds and support loads as the spin speed increases. There are certain dynamic requirements that must be met, such that at the maximum operating speed, the forward cylindrical whirl frequency be a certain factor lower than the backward first bending mode. As speed increases, if the backward bending mode approaches a forward whirling mode, the unit will become unstable. Mr. Ansbigian explains, "We had to develop a 3-dimensional finite element model of the rotor, which included the gyroscopic stiffening effect on the rotor as a function of spin speed, and plot a Campbell diagram. This demonstrated the forward cylindrical whirl mode compared to the backward first bending mode as the speed increased. If the backward first bending circular frequency is a certain factor times that of the forward cylindrical whirl, weve got a good design that is stable."

Stability Analysis

Another very important analysis being conducted includes a detailed evaluation of damped rotor stability. In this analysis, the effects of both external viscous and internal hysteretic damping must be incorporated into a finite element model of the rotor. Using MSC Nastran, the finite element simulation of the rotor-stator system can be improved by modeling internal friction effects. This provides an added dimension in studying turbo-machinery stability and can assist engineers in proper selection of bearing characteristics to ensure safe high-speed operation. Mr. Ansbigian explains, "We had to determine how much damping is required to ensure stability at the critical speeds to avoid becoming unstable. MSC Nastran gave us a handle on rotor stability for various external damping values."

Safety Containment Testing

If there is a failure within the rotor at the maximum operating speed, it must be controlled, making safety containment a primary consideration. A few years ago, an underground safety test was conducted. The system was buried with the top of the unit four feet below grade. While spinning at full speed, the rotor was intentionally failed and the subsequent ground motion was measured. MSC.Dytran was used to estimate the energy released from the first safety test and project what would happen if there were only two feet of soil above the unit. Mr. Ansbigian explains, "We built an FEA model of the unit underground and modeled the soil around it and parametrically varied the depth above it. (The finite element model is shown below in Figure 2). We input a pressure time-history into the unit and measured the elevation of the soil motion as a function of time (shown in Figure 3, displacement time history). We were able to match the first safety test with MSC Dytran, which then allowed us to predict what would happen if we had any other depth of earth. The analysis provided us with invaluable insight into what would happen, prior to running a test, allowing us to make design improvements in the installation of our unit. In addition, MSC Dytran showed that we could reduce the depth of burial to £ 2 feet."

50% Material Reduction

With a program to reduce costs in place, a primary issue for consideration is material. The flywheel energy storage system is made out of aluminum and steel with the rotor made of composite material. The stator (a stationary component) was analyzed with MSC.Nastran to determine how thick the stator must be. Mr. Ansbigian explains, "Our software has helped design and analyze the stator for a specific frequency requirements, which definitely affects how manufacturing builds this part. Previous designs were machined out of a billet of material. With cost savings a big driver, we wanted to use a casting with reduced wall thicknesses, so we ran an analysis based on a new structural design configuration. Compared to our first generation, second generation material costs have been reduced by 50%. The next generation is expected to be lower yet."

Physical testing of prototypes is mainly used as a last check before a new design is approved, providing correlation between simulation and the actual results. Mr. Ansbigian explains, "The frequency simulations are within 10% of measured data, which is considered to be very good. And it has reduced our costs and decreased time to market. Right now, we can get a pretty sophisticated result in the same time htat it took to do a first order calbualtion."

The Future

Because of the success that Beacon Power has achieved by utilizing simulation early in the design process, Mr. Ansbigian said, "Most of our engineers are being trained in simulation, using MSC Patran and MSC Nastran, which will result in a stronger technical organization, directly impacting our product and processes, lowering costs and improving our product."

Beacon Power
United States