flywheel energy storage a robust solution for high power ... · flywheel energy storage –a robust...

Flywheel Energy Storage – A robust

solution for high power, high cycle

applications

Mustafa E. Amiryar and Keith R. Pullen

Mechanical and Aeronautics Department, Energy Systems, City

University of London, UK

14th November 2017

2

Electrical flywheel storage: How it works

Design choices for components

Comparison with other storage technologies

Grid-level applications: Why this is a viable choice

The Gyrotricity flywheel with applications in

Transport and grid storage

Presentation Outline

Electrical Flywheel storage: How it works

3

or Grid connection

Store energy by spinning a rotor of moment of inertia If to speed wf

Energy stored = ½ If wf2

Usable Energy = ½ If (w2max-w

2min)

The key technology is the flywheel rotor : Must operate at high peripheral speeds

E = ½ MV2 (linked to E = ½ Iw2 ) so to get low M, need high mean V

Must have low frictional losses

Vacuum essential with level depending on V

In theory, lasts forever, no capacity fall off

Above all must be safe - rotor design and containment must be considered together

kWh/kg and kWh/litre must be for both


4

V

w


5

Three main choices for flywheel rotors :

Solid monolithic (one piece) steel

Carbon fibre composite

Laminated steel


6

Solid monolithic steel rotors

Upside

Rotor is one single piece

Material is low cost, properties well understood

Outgassing minimal

Downside

A failure by fatigue crack will release 2-3 massive chunks

Solid construction creates a triaxial stress promoting fracture

Difficult to check inside the rotor for defects

Very high strength steels in less available in large diameter bar

stock

Only safe with bunker containment or if speeds are kept low

leading (leads to high weight and volume)


7

Carbon fibre composite rotorsUpside

Rotor has high specific energy (kW/kg) due to higher V

In a failure the rotor disintegrates in to small particles not chunks

Downside

Material and manufacturing is more expensive

Explosive failure mode may occur and requires strong containment to mitigate – must be in bunker or thick containment

Difficult to check rotor for defects due to its nature

Rotor will outgas due to plastic matrix material

Speeds have to be over twice as high as steel – requires;

- Higher switching frequencies for power electronics

- Finer motor laminations on motor - generator

- Harder vacuum needed

- Higher speeds for bearings


8

Laminated steel rotors

Upside

If crack occurs, small pieces released so

containment can be thinner and lighter

Does not need to go inside a bunker

Material is low cost, properties well understood

High strength steel available at low cost in sheets

Downside

Rotor construction is more complex – needs

innovative solution to hold discs together

Requires reasonable economies of scale to obtain

the lower cost


9

Other considerations: Rotor sizes and speeds

Need to design for a peripheral speed;

For a steel flywheel with 420 m/s of 5 kWh (min speed = 50% max)

Nmax = 10,000 rpm

Ø800 mm

540

mm

Ø400 mm

135 mm

Nmax = 20,000 rpm

VV

• Rotor mass of both around 500kg• Can add a motor-generator of 100’s kW to either

Flywheel storage: How it works- rotors

10

Laminated steel rotor verses composite

Attribute Carbon Fibre, Vmax = 790 m/s

a = 2/3 b

Steel LaminateVmax = 427 m/sa = 0 (no hole)

Mass 1 4.53

Volume 1 0.503

ab

But, this is just for the rotor, casing for steel laminated design is

thinner and smaller than a safe containment for a composite rotor


11

Other considerations: kWh per rotor?

Have more smaller machines?

Safer since one failure releases less energy

Easier to transport and install

Cost of mass producing more smaller items less

Market larger – min size dictated by smallest unit

Have fewer larger machines?

Easier to control

Shaft speeds are lower


12

Three main choices for flywheel bearings:

Mechanical (rolling element)

Passive magnetic

Active magnetic


13

Mechanical (rolling element)

Upside

Simple

Low cost and available from many suppliers

Grease or oil for vacuum operation available

High overload capacity

Downside

Requires maintenance – change of oil/grease or

replacement of bearing


14

Passive magnetic Bearings

Upside

Simple

Low loss (theoretically zero)

Radial configuration has low capacity

Downside

Not possible to use these alone – must be hybridised

Forces are strong – care must be taken in assembly

Axial bearing

Radial bearing


15

Downside

External power is required to energise bearing

Expensive

Limited suppliers

Active magnetic bearings

Upside

Low loss

Can vary stiffness – useful for

rotor dynamics

Three main choices for motor-generator (M/G)

Permanent magnet

Switched reluctance

Asynchronous induction


16


17

Permanent magnet M/G

Upside

Highest efficiency

Easiest to act operate as a

generator

Downside

Large free running loss

Additional cost of magnets

Magnets can demagnetise if

overheated


18

Switched reluctance M/G

Upside

In between IM and PM in

efficiency

Robust rotor, no magnets

Low free running loss

Downside

Some rotor loss

Power electronics more difficult

Flywheel storage: How it works – mot/gen

19

Asynchronous induction M/G

Upside

Lowest cost - most common type of motor

Robust

Easiest to power as a motor

Very low free running loss

Downside

Lowest efficiency

Larger rotor losses

More difficult to operate as a generator

Flywheel storage: How it works – mot/gen

20

Other considerations:

Separate motor-generator

Full flexibility in design

Easier to find suppliers

Simpler to vary power rating

Integrated motor-generator

Can be more compact

Use flywheel rotor to hold magnets

But danger of failing rotor in an

overheat

Bespoke so more expensive


21

Ragone plot

Ref (Xing Luo, Jihong Wang, Mark Dooner, Jonathan Clarke, “Overview of current development in electrical energy storage,” 2014.

22

List of key attributes and relative comparison


Li Ion S Cap FW

Low cost per kWh

Low cost per kW

Power density (in and out) per kWh (kg/litre)

Energy density per kW (kg/litre)

Full power response time

Efficiency in/out

Self discharge

Calendar and cycle life

Environmentally incl. recyclability

Downscaling ability to few kWh/kW

Maintenance (incl. replacement) over 25 years

Thermal resilience and effect on life

23


Flywheels excel in applications where the following is needed:

High number of daily cycles (> 5)

High power, ie 5 < C > 200 (C = kW/kWh)

High cycle and calendar life

(20k < cycles < , > 25 years)

High certainty in state of health needed

Thermally challenging applications

Fast response

Can be good to hybridise flywheels with Li-ion or other

mechanical systems which are usually slow response

Attribute Score/10

Cost 4

Power 10

Energy 8

Response 10

Efficiency 9

Discharge 7

Life 10

Env. 10

Downscaling 10

24


The Gyrotricity flywheel

25

Developed initially with an InnovateUK project

• Vehicle application – heavy hybrid passenger vehicle

• 25kW, 250kJ specification (C100)


26

Flywheel safety case analysis and testing• Fail safe design proven by experiment

• One laminate inserted with major crack and burst at full speed

• No distortion/damage to casing, only light surface damage

• No damage to other laminates

• Burst captured on Photron high speed camera (50,000 fps)

• Results simulated using dynamic Finite Element Analysis


27

Rail Application

• DBS funded by RSSB with support of City, Tata, TPS, Deutsche Bahn, Porterbrook and Sellick Rail to develop laminated steel flywheel for DMU rail

• Simulation study shows up to 40% fuel saving and many other benefits when deployed in DMUs

• Hardware to be tested late autumn 2017, on vehicle testing spring 2018


28

Ground power/grid application bank (C20 rating)


29

Comparison with other flywheel developers

• Stephentown, New York is the site of Beacon Power’s first 20 MW plant

• Operating commercially for 6 years

• Similar plants in 4 places in the US


30

Comparison with other flywheel developers

• Derived from the Urenco Uranium centrifuge technology

• Centrifuges have operated for decades

• Containerised above ground solution

• Needs substantial steel containment

31

Summary points The key attributes and components of flywheel electrical energy storage has

been explained

Flywheels offer a viable alternative for grid-level applications and can be configured to any power and storage level in arrays

Rotors and motor-generators can be matched to give any combination of energy and power

All share the fundamental benefit of high cycle life-can survive 100’s k cycles

Some designs are better for low running loss

Costs for vary for different solutions - potential for very low cost possible depending on production levels given low material costs

The Gyrotricity flywheels offers significant benefits over alternatives by avoiding bunkering and is a compact technology suitable for grid balancing and transport applications

City, University of London

Northampton Square

London

EC1V 0HB

United Kingdom

T: +44 (0)20 7040 3475

E: [email protected]

http://www.city.ac.uk/people/academics/keith-robert-pullen

32

Thank you for listening

Mustafa E. Amiryar

[email protected]

Questions

Keith R Pullen

[email protected]

flywheel energy storage a robust solution for high power ... · flywheel energy storage –a robust...

Documents