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Design of Flywheel Rotors

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International Journal of Advanced Engineering Research and Studies
E-ISSN2249–8974
IJAERS/Vol. I/ Issue II/January-March, 2012/299-301
Research Paper
COMPUTER AIDED DESIGN & ANALYSIS ON FLYWHEEL FOR GREATER EFFICIENCY
Sudipta Saha, Abhik Bose, G. Sai Tejesh, S.P. Srikanth
Address for Correspondence
Mechanical Engineering, K.L.University, AP, India.
ABSTRACT
Flywheels serve as kinetic energy storage and retrieval devices with the ability to deliver high output power at high rotational speeds as being one of the emerging energy storage technologies available today in various stages of development, especially in advanced technological areas, i.e., space-crafts. Mainly, the performance of a flywheel can be attributed to three factors, i.e., material strength, geometry (cross-section) and rotational speed. While material strength directly determines kinetic energy level that could be produced safely combined (coupled) with rotor speed, this study solely focuses on exploring the effects of flywheel geometry on its energy storage/deliver capability per unit mass, further defined as Specific Energy. Proposed Computer aided analysis and optimization procedure results show that smart design of flywheel geometry could both have a significant effect on the Specific Energy performance and reduce the operational loads exerted on the shaft/bearings due to reduced mass at high rotational speeds. This paper specifically studies the most common five different geometries (i.e., straight/concave or convex shaped 2D).
INTRODUCTION
A flywheel is a mechanical device with a significant moment of inertia used as a storage device for rotational energy. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when a fluctuating torque is exerted on it by its power source. Flywheels have become the subject of extensive research as power storage devices for uses in vehicles. Flywheel energy storage systems are considered to be an attractive alternative to electrochemical batteries due to higher stored energy density, higher life term, and deterministic state of charge and ecologically clean nature. Flywheel is basically a rechargeable battery. It is used to absorb electric energy from a source, store it as kinetic energy of rotation, and then deliver it to a load at the appropriate time, in the form that meets the load needs. As shown in Fig1, a typical system consists of a flywheel, a motor/generator, and controlled electronics for connection to a larger electric power system.
Theoretical analysis:
Energy is stored in the rotor as kinetic energy, or more specifically, rotational energy Where W is the angular velocity, and I is the moment of inertia of the mass about the center of rotation. The kinetic energy stored in a rotating mass is given as, where
x
is the distance from rotational axis to the differential mass d
mx.;
where I is the mass moment of inertia and W is the angular velocity. Mass moment of inertia is obtained by the mass and geometry of the flywheel and given as, For solid cylindrical disk, I is given as, where
m
is the mass and
r
the radius of the flywheel. Specific energy
E
k,m is obtained by dividing
E
k by the mass to give: If
E
k, is multiplied by the mass density P of the flywheel the energy density is obtained: In this context, the design challenge is to maximize either
E
k,m or
E
k,v, while satisfying the stress constraints. Tangential and radial stresses are given for cylindrical flywheel geometry [10] where the outside radius (
r
o) is assumed to be large compared to the flywheel thickness (
t
)
r
o >10
t
; After careful examination of these formulations, it could be observed that mainly three fully-coupled design factors have significant effect in the overall performance of flywheels. ã
Material strength
; basically stronger materials could undertake large operating stresses, hence could be run at high rotational speeds allow wing to store more energy. Hence could be run at high rotational speeds allow wing to store more energy. ã
Rotational speed
; directly y controls the energy stored, higher speeds desired for more energy storage, b but high speeds assert excessive loads on b both flywheel and bearings during the shaft design. ã
Geometry
; controls the S Specific Energy, in other words, kinetic energy storage capability of the flywheel. Any optimization effort of flywheel C cross-section may contribute substantial improvements in kinetic energy storage capability thus reducing both overall shaft/bearing loads and material failure occurrences.
International Journal of Advanced Engineering Research and Studies
E-ISSN2249–8974
IJAERS/Vol. I/ Issue II/January-March, 2012/299-301
Design Algorithm:
ã
Step 1- a fully parametric model of the flywheel is created to be inputted to ANSYS to form the desired geometry.
ã
Step 2, model obtained in Step 1 is analyzed using ANSYS code, to obtain the stored kinetic energy and mass of the flywheel.
ã
Step 3, the same model is also analyzed using ANSYS, an implicit code, and overall stress distribution of the flywheel obtained and critical stresses and regions identified
ã
Step 4, Optimization
Parameters:
ã
Thickness (t=5.08cm or 2”)
ã
Radius of flywheel (h)= (14.605 or 5.75”)
ã
2D flywheel geometry is constructed with the total of 10 points in the X direction to be less/equal to h.
ã
Material Selection: Although many materials with better strength and low density are available in the market, an example material properties of AISI 1006 Steel (cold drawn), with modulus of elasticity of
E
= 205 GPa, density of 7.872 g/cc, Poisson’s ratio of = 0.29 and yield stress of Y = 290 MPa, is adapted in all cases.
Optimization:
Step 1: Five different flywheel designs are made.
Step 2: A program is made to compute the maximum angular velocity that each design can handle.
Step3: Maximum kinetic energy and specific energy of each case are found out.
Step4: The best design is found comparing the specific energy of each design. Design 1: Design 2: Design 3: Design 4:
International Journal of Advanced Engineering Research and Studies
E-ISSN2249–8974
IJAERS/Vol. I/ Issue II/January-March, 2012/299-301 Design 5: The maximum angular velocities attained by the designed flywheels are found from the output of the program; From we can find the kinetic energy and the specific energy that the flywheel can store.
New values after computation: Results:
ã
Performance doesn’t depend on inner hole radius
ã
Solid disk performs better than the annular disk but highest shaft load is expected since the flywheel mass in this case is the largest.
ã
By adopting simple modifications to the geometry, flywheel specific energy performance could be improved as demonstrated in Case 3 through 5, especially in Case 5 performance of the flywheel performs 50% better than Case 2.
ã
One more thing to note that, Case 5 cross-section also exerts fewer shafts load than Case 1 through 4, since its mass is the smallest. Although this improvement is to be thought small, it still could be crucial for mission critical operations, which require long lasting service life and efficiency. Examining the results shows that using the annular solid disk flywheel yields the lowest Specific Energy performance no matter what the inner hole radius is chosen. Solid disk performs better than the annular disk but highest shaft load is expected since the flywheel mass in this case is the largest. By adopting simple modifications to the geometry, flywheel specific energy performance could be improved as demonstrated in Case 3 through 5, especially in Case 5 performance of the flywheel performs 50% better than Case 2. One more thing to note that, Case 5 cross-section also exerts fewer shafts load than Case 1 through 4, since its mass is the smallest. Although this improvement is to be thought small it still could be crucial for mission critical operations, which require long lasting service life and efficiency.
CONCLUSIONS:
In this design of flywheels, there is still room for research, especially when the performance is the primary objective. The operating conditions impose quite narrow margin of energy storing limitations, even slim amount of improvements may contribute in the overall success. This study clearly depicts the importance of the flywheel geometry design selection and its contribution in the energy storage performance. This contribution is demonstrated on example cross-sections using computer aided analysis and optimization procedure. Overall, the problem objective is formulated in terms of Specific Energy value and its maximization through the selection of the best geometry among the predetermined five cross-sections. Using the available technology at hand, we could very well make fast but crucial improvements in the advanced research areas requiring flywheel utilization, where engineers are frequently confronted with the limitations on magnetic bearing load carrying capacity, size limitations and efficiency.
REFERENCES
1.
Lawrence A. Hawkins “Influence of Control Strategy on Measured Actuator Power Consumption in an Energy Storage Flywheel with Magnetic Bearings”, 6th International Symposium on Magnetic Suspension Technology. 2.
E. Anderson, M. Antkowiak, R. Butt, J. Davis, J. Dean, M. Hillesheim, E. Hotchkiss, R. Hunsberger, A. Kandt, J. Lund, K. Massey, R. Robichaud, B. Stafford, and C. Visser. “A Broad Overview of Energy Efficiency and Renewable Energy Opportunities for Department of Defense Installations”. 3.
Elena AGENJOS, Antonio GABALDON, Francisco G. FRANCO, Roque MOLINA, Mario ORTIZ, Rafael J.GABALDON. “Energy efficiency in railways: Energy storage in diesel electric locomotives”. 20th International Conference on Electricity Distribution. 4.
Marcelo Gustavo Molina, “Dynamic Modelling and Control Design of Advanced Energy Storage for Power System Applications”. Universidad Nacional de San Juan Argentina. 5.
Aleksandr Nagomy, “ High Speed permanent magnet synchronous motor/ generation design for flywheel applications”, National Research council Nasa.
6.
Cesar Pasten, J. Carlos Santamarina. “Energy Geo-Storage
−
Analysis and Geomechanical Implications”. KSCE Journal of Engineering.

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