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 14 th November 2017
Presentation Outline 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 2
Electrical Flywheel storage: How it works or Grid connection Store energy by spinning a rotor of moment of inertia I f to speed w f Energy stored = ½ I f w f 2 Usable Energy = ½ I f (w 2 max-w 2 min) 3
Electrical flywheel storage: How it works The key technology is the flywheel rotor : Must operate at high peripheral speeds E = ½ MV 2 (linked to E = ½ Iw 2 ) so to get low M, need high mean V Must have low frictional losses Vacuum essential with level depending on V V In theory, lasts forever, no capacity fall off w Above all must be safe - rotor design and containment must be considered together kwh/kg and kwh/litre must be for both 4
Design choices for components Three main choices for flywheel rotors : Solid monolithic (one piece) steel Carbon fibre composite Laminated steel 5
Design choices for components 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) 6
Design choices for components Carbon fibre composite rotors Upside 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 7
Design choices for components 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 8
Electrical flywheel storage: How it works 540 mm 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) V V Ø800 mm 135 mm Ø400 mm N max = 10,000 rpm N max = 20,000 rpm Rotor mass of both around 500kg Can add a motor-generator of 100 s kw to either 9
Flywheel storage: How it works- rotors Laminated steel rotor verses composite Attribute Carbon Fibre, V max = 790 m/s a = 2/3 b Steel Laminate V max = 427 m/s a = 0 (no hole) b a Mass 1 4.53 Volume 1 0.503 But, this is just for the rotor, casing for steel laminated design is thinner and smaller than a safe containment for a composite rotor 10
Electrical flywheel storage: How it works 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 11
Design choices for components Three main choices for flywheel bearings: Mechanical (rolling element) Passive magnetic Active magnetic 12
Design choices for components 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 13
Design choices for components Passive magnetic Bearings Radial bearing Upside Simple Low loss (theoretically zero) Radial configuration has low capacity Axial bearing Downside Not possible to use these alone must be hybridised Forces are strong care must be taken in assembly 14
Design choices for components Active magnetic bearings Upside Low loss Can vary stiffness useful for rotor dynamics Downside External power is required to energise bearing Expensive Limited suppliers 15
Design choices for components Three main choices for motor-generator (M/G) Permanent magnet Switched reluctance Asynchronous induction 16
Design choices for components 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 17
Design choices for components 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 18
Flywheel storage: How it works mot/gen 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 19
Flywheel storage: How it works mot/gen 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 20
Comparison with other storage technologies Ragone plot Ref (Xing Luo, Jihong Wang, Mark Dooner, Jonathan Clarke, Overview of current development in electrical energy storage, 2014. 21
Comparison with other storage technologies 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 22
Comparison with other storage technologies Flywheels excel in applications where the following is needed: Attribute Score/10 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 Cost 4 Power 10 Energy 8 Response 10 Efficiency 9 Discharge 7 Life 10 Env. 10 Downscaling 10 Can be good to hybridise flywheels with Li-ion or other mechanical systems which are usually slow response 23
Comparison with other storage technologies 24
The Gyrotricity flywheel Developed initially with an InnovateUK project Vehicle application heavy hybrid passenger vehicle 25kW, 250kJ specification (C100) 25
The Gyrotricity flywheel 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 26
The Gyrotricity flywheel 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 27
The Gyrotricity flywheel Ground power/grid application bank (C20 rating) 28
The Gyrotricity flywheel 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 29
The Gyrotricity flywheel 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 30
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 31
City, University of London Northampton Square London EC1V 0HB United Kingdom T: +44 (0)20 7040 3475 E: k.pullen@city.ac.uk http://www.city.ac.uk/people/academics/keith-robert-pullen Thank you for listening Questions Mustafa E. Amiryar Mustafa.Amiryar.2@city.ac.uk Keith R Pullen k.pullen@city.ac.uk 32