Center for ElectromechicsThe University of Texas at Austin

Flywheel Safety

Richard Thompson

January 6, 2011

Presentation

• Our approach to flywheel safe and reliable operation

• DARPA Flywheel Safety Program• Flywheel topologies• Flywheel rotor design approach• CEMWIND for advanced prototypes• ANSI/AIAA Flywheel Standard• Rotor testing

– Provides data for model correlation

– Shows understanding of design and fabrication principles

DEFENSE IN DEPTH PHILOSOPHY – Our approach to safe and reliable operation

DARPA Program Team

• Center for Transportation and the Environment, Program administrator

• University of Texas –Center for Electromechanics, Co-program technical manager

• Test Devices, Inc., Co-program technical manager• AFS Trinity Power Corp., Commercial flywheel developer• US Flywheel System, Commercial flywheel developer• Beacon Power Corp., Commercial flywheel developer• Lawrence Livermore National Laboratory• Oak Ridge National Laboratory• Argonne National Laboratory

Program Highlights

• Conducted more than 60 flywheel tests, including

– Flywheel only tests to identify failure modes and structural margins

– Better understanding of safe flywheel designs

– Flywheel burst tests to successfully proof test candidate containment designs

• Demonstrated life of more than 110,000 cycles with a 50% DOD

Flywheel Topologies

Non-Integrated Topology

• Larger than other topologies, butmay have most simple assembly

• Maximum use of conventional M/Gsystems and technology

• Flexible / adaptive design• Power generation outside of vacuum• Requires shaft seal and coupling

Partially-Integrated Topology

• Smaller and more efficient than non-integrated

• Good use of available M/G technology,but integration required

• Good design adaptability• Favors use of PM generator• Heat generation on rotor requires careful engineering

Fully-Integrated Topology

• Most compact system• Special purpose flywheel system• Favors use of PM generator• Heat generation on rotor requiresspecial engineering

• Rotating magnets at large radius• Uses arbor or magnetic bearings tomatch rotor growth

Range of CEM FlywheelSystems Designs

Lab Bearing Amps

Flywheel

Transformer

RectifierAssembly

Lab SafetyDisconnect

Converter

Transit Bus Flywheel150 kW (peak), 100 kW (cont.), 2 kW-h

Advanced Locomotive Propulsion System Flywheel3 MW (peak), 2 MW (cont.), 100 kW-h

Combat Hybrid Power Systems (CHPS) Flywheel5 MW (peak), 350 kW (cont.), 7 kW-h

Space Station Flywheel (FESS)

5.0 kW (peak), 3.66 kW (cont.), 3.66 kW-h

Flywheel Rotor Design Approach

• Coupon-level testing – to determine material allowables

• Component-level testing – first level of test verification

• Prototype build and commissioning

Typical Types of Coupon-Level Tests

Baseline material tests• Tensile• Compression• Shear• Thermal

Residual strength material tests• Hoop tensile

– Thermo-mechanical ultimate– Fatigue (accelerated)– Effects of vacuum/outgassing – Effects of critical flaws

• Creep – preload loss• Stress rupture

• Objective: Obtain material property allowables specific to program requirements

• Lowest level of definition is at the fiber ply level (unidirectional lamina tow)

• Transversely isotropic materials have five independent modulus (~stiffness) components

– 11, 22, 12, 31 and 23

– Measured from induced strain response and calculated stresses

1

2

3

F

F

Tests measure shear strength within the plane of lamination (S21).

Hydroburst Test Method (Circumferential Properties)

To Characterize:• Tensile strength, modulus• Flaw sensitivity• Fatigue properties

– 400,000 cycles– 200 oF

• QA: assess material lot variability

• Pressurized fluid enters through radial feed hole

• Expands Teflon seal• Radial pressure induces hoop strain

in composite ring

Figure 2 Illustration of hydroburst test fixture

Upper steel plate

Lower steel plate

Steel Spacer

Plate with

radial feed hole

ACESE/FESS Fatigue Tests

0.200

0.400

0.600

0.800

1.000

Time ( 1.2 Hz)

Ho

op

Str

ain

, %

Coupon-Level Characterization

Single cycle hydroburst fixture

Fatigue cycle hydroburst fixture

Hydroburst specimen

Fixtures can be configured for elevated temperature testing

Hydroburst method has beenwell reviewed

• Hydroburst method valuable for – CEM has seen good agreement

between hydroburst data and prediction of flywheel rotor test results

– Screening tool for flaw sensitivity and material QA

• ASTM Composite Materials: Testing and Design, 14th Volume, STP 1436, “Hydroburst Test Methodology for Evaluation of Composite Structures”

Typical Stress Allowables(Past Composite Program)

Item Temp Mean UTS

(CEM)

Mean Yld (CEM)

Mean (MIL-17),

All are for uni-tape

Aug Spin Test Stress at 50 Krpm

Previous Baseline

Arbor Design at 50 Krpm

New Baseline Arbor Design at 50 Krpm

New Baseline Arbor Design at 64 Krpm

0F ksi ksi ksi ksi ksi ksi ksi

S11 Tensile 75 400 378 265 230 180 340S11 Compressive 75 200 244 120 5 70-A 110-AS22 Tensile 75 12.6 7 10.5 5.5-H 7.5-H 12-A/2-H 22-A/2-H

S22 Compressive 75 28.3 11.5 43.3 23.5 5 3 6

S33 Tensile 75 12.6 7 1.5 0 0 0

S33 Compressive 75 32.8 15 16 6 3 4

S12 (+/- 68 deg) 75 14.5 7 16.8 21 8 5.5 9S13, applied to s13, s

3175 8.3 8.3 16.3 5.5 1.6 .5 .5

S23, applied to s32, s

2375 8.3 8.3 4.8 1.6 .5 .5

Notes: 1) Fiber Volume is ~ 62 %, zero bleed cure 6) S22/S33 compressive = S22 Tensile2) Lamina operating stresses from CEMWIND

3) S11 tensile from Task C data 7) S12 from +/- 68o flat panel, not recent data4) S11 compressive from vendor data 8) S13 = S23 from uni-directional5) S22 Tensile = S33 Tensile, transverse tensile flat panel

* Yield Transition Pt.

Component-Level Characterization

Arbor spin testArbor static deflection test

Rotordynamics Testing:

Modal frequency test

Creep/Stress Relaxation Tests

• Creep/stress relaxation tests

– Monitor steel rings’ dimensional change vs time and temperature

– Infer change in preload, sr

– Test duration: 2 years

– Projected sr change over 10 years at 200o F is 7%

Final Prototype Build andCommissioning Tests

Final 2 Arbors

In process winding of an arbor pair

BUILD

Rotor shafts with structural and cooling arbor assemblies installed

FINAL PRODUCT

In process winding of a B2 outer banding

Twin rotors shown in incomplete assembly state ( some outer banding assemblies remain)

CEMWIND

Analysis Code for Complex Rotor Structures

Arbor Design Control

• CEMWIND: Filament winding design and fabrication code

• Designer inputs r, z,

• Fabrication checks– Friction– Bridging

• Code attempts to optimize for geodesic wind– No tow slip

CEMWIND Outputs

3

3.5

4

4.5

5

5.5

6

0 2 4 6 8 10 12 14 16 18

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7

CEMWIND

FE Mesh & Material Property Files

Arbor Ply Thickness Build Profile

Fiber motion files for input into CEM’s filament winding machine

CEMWIND Output Showing Arbor Shear Stress Profile

Outputs ply-level stress and strain results. Direct comparison with fiber tow-level material allowables

Arbor Stresses at 50 Krpm: Shear Stress Profile

6

5

4

3

2

6543210-1

-10000

-9000

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Str

ess

(psi

)

543210-1

Axis (in.)

s11

s22

s33

s12

s13

s23

Stress in material principal directions

s12, similar to values for Sep ’03 spin test unit

+s33, prefer full compression

ANSI/AIAA Standard for Flywheels

Government Sponsors:

Kerry L. McLallin, NASA Glenn Research Center

Dr. Jerry Fausz, AFRL - Phillips Lab

Objective

• Develop an industry consensus standard for the certification of flywheel rotors for aerospace applications

• Assure that flywheel rotors developed for government missions can meet safety and life requirements

Center for ElectromechanicsPenn State UniversityLockheed MartinHoneywellFlywheel Energy Systems/CanadaNational Research Council/CanadaToray Composite AmericaBeacon PowerBoeing SeattleBarbour StockwellOak Ridge National LabLincoln CompositesTest Devices, Inc.AFS Trinity

• ANSI/AIAA Standard was accepted in 2004

• S-096-2004, Space Systems – Flywheel Rotor Assemblies

• Being used for non-military flywheel applications

– Performance race vehicles: Formula One

– Others

Examples of Past Arbor Spin Tests

• Pulsed Generator Program

• Successful completion of 1000 fatigue cycles between 7500 rpm to 15,000 rpm.

• Well behaved, stable operation

• Matched analysis predictions

• NASA Arbor Development Program

• Ten spin tests completed• Focus was composite arbor

with high-strength rim• Demonstrated service speed

of 50,000 rpm (1100 m/s)• Well behaved, stable

operation• Matched analysis

predictions• Overspeed test to verify

margins– Demonstrated FoS of 1.5

Excerpt from NASA Glenn’s 2003 NASA

R&T publication:

“The rotor was tested on 9/3/03 and successfully reached 1337 m/s (2990 mph) tip speed.

This represents the highest known attained speed in any useable flywheel configuration.”

Rotor Fatigue Tests

• DARPA Flywheel Program, 2002

• Completed over 112,000 fatigue cycles

• Flywheel speed excursions from 27,000 rpm to 36,000 rpm, with a peak tip speed of 825 meters/second, at about 140o F

• Flywheel test to overspeed - verified no loss in residual stiffness

Goal of test was to better understand flywheels for space applications – Low Earth Orbit

• Significantly increased fatigue test cycles achieved for a full-scale composite flywheel operating in realistic simulated service conditions

• Low earth orbit missions for a 15 year service life require about 90,000 cycles• Test exceeded this cycle requirement• At the time, at 50% DOD, much greater number of cycles than possible with chemical

batteries

Loss of Vacuum Test

• Near instantaneous loss of vacuum from 900 m/s tip speed• No structural damage observed based upon results from follow-up spin tests

R.C. Thompson, J. Kramer, and R.J. Hayes, “Response of an urban bus flywheel battery to a rapid loss-of-vacuum event,” SAMPE (Society for the Advancement of Material and Process Engineering) Journal of Advanced Materials, vol. 37, no. 3, July 2005, pp. 42-50

Comparison of measured and calculated flywheel angular velocity during a loss-of-vacuum event

Comparison of calculated flywheel surface temperature with measured temperature shifted to match room temperature prior to the transient

Methodologies for Composite Flywheel Certification

•G.Y. Baaklini, K.E. Konno, R.E. Martin, and R.C. Thompson, “NDE methodologies for composite flywheels certification,” 2000 Power Systems Conference, San Diego, California, U.S.A., October 31-November 2, 2000, SAE Document Number: 2000-01-3655.

• Collaboration with NASA Glenn

• Rotors were fabricated with flaws

– NDE methods were applied to evaluate their effectiveness for flaw detection

– CT, radiography, ultrasonics

• Also,intentionally seeded delamination, tow break, and foreign materials (bagging materials) into hydroburst rings

– Determine effects of induced flaws on hydroburst material allowables (damage tolerance)