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ADVANCED E NGINEERING 3(2009)1, ISSN 1846-5900

KINETIC ENERGY RECOVERY SYSTEM

BY MEANS OF FLYWHEEL ENERGY STORAGE Cibulka, J.

Abstract: This paper deals with the design of Kinetic Energy Recovery Systems (KERS) bymeans of Flywheel Energy Storages (FES). KERS by means of FES are currently underdevelopment both for motor sport and road hybrid vehicles. The aim of the work is theoptimalization and implementation to the hybrid and electric road vehicles. Testing equipmentfor the experimental analysis of the simplified FES was designed.

Keywords: Kinetic Energy Recovery System, Flywheel Energy Storage, KineticStorage, Flywheel, Reluctance Motor, Electric Generator / Motor, Regenerative /

Recuperative Braking.

1 INTRODUCTION

1.1 Introduction to Regenerative BrakingA regenerative brake is a mechanism that reduces vehicle speed by converting some of its kinetic energy into another useful form of energy - electric current, compressed air.

This captured energy is then stored for future use or fed back into a power systemfor use by other vehicles. For example, electrical regenerative brakes in electric railwayvehicles feed the generated electricity back into the supply system.

In battery electric and hybrid electric vehicles, the energy is stored in a battery orbank of twin layer capacitors for later use. Other forms of energy storage which may beused include compressed air and flywheels.

Regenerative braking utilizes the fact that an electric motor can also act as agenerator.

The vehicle's electric traction motor is operated as a generator during braking andits output is supplied to an electrical load [Fig. 1.].

It is the transfer of energy to the load which provides the braking effect.

Fig. 1. Regenerative braking kinetic energy stored in a battery

Regenerative braking should not be confused with dynamic braking, whichdissipates the electrical energy as heat and thus is less energy efficient.

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Fig. 2. Mechanism conceptual diagram Fig. 3. System control battery storage

Conceptual diagram of mechanism shows comparison characteristic curve betweenhydraulic and regenerative braking, while driver intentionally brakes [Fig. 2.].Regenerative braking reuse kinetic energy by using its electric motor to regenerateelectricity.

Normally, electric motors are turned by passing an electric current through it.However, if some outside force is used to turn the electric motors, it functions as agenerator and produces electricity. This makes it possible to employ the rotationalforce of the driving axle to turn the electric motors, thus regenerating electric energyfor storage (in the battery) and simultaneously slowing the car with the regenerativeresistance of the electric motors.

The system control coordinates regenerative braking and the braking operation of the conventional hydraulic brakes [Fig. 3.], so that kinetic energy, which is normallydiscarded as friction heat when braking, can be collected for later reuse in normaldriving mode.

Typically, driving in city traffic entails a cycle of acceleration followed bydeceleration. The energy recovery ratio under these driving conditions can therefore bequite high. To take advantage of this situation, the system proactively uses regenerativebraking when running the car in the low speed range.

The regenerative braking effect rapidly reduces at lower speeds; therefore thefriction brake is still required in order to bring the vehicle to a complete halt. Thefriction brake is a necessary back-up in the event of failure of the regenerative brake.

Most road vehicles with regenerative braking only have power on some wheels (asin a 2WD car) and regenerative braking power only applies to such wheels, so in orderto provide controlled braking under difficult conditions (such as in wet roads) frictionbased braking is necessary on the other wheels.

1.2 Introduction to Flywheel Energy StorageKinetic storages, also known as Flywheel Energy Storages (FES), are used in manytechnical fields.

While using this technical approach, inertial mass is accelerating to a very highrotational speed and maintaining the energy in the system as rotational energy. Theenergy is converted back by slowing down the flywheel. Available performance comesfrom moment of inertia effect and operating rotational speed.

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Flywheel mass is either mechanically driven by CVT (Continuously VariableTransmission) gear unit [Fig. 4.] or electrically driven via electric motor / generatorunit [Fig. 5.].

Fig. 4. Mechanically driven composite flywheel Fig. 5. Electrically driven flywheels

Devices that directly use mechanical energy are being developed, but most FESsystems use electricity to accelerate and decelerate the flywheel.

In comparison with other conventional ways of storing electricity (batteries andcapacitors), electric FES systems combined with innovative concept offer essentialadvantages. Especially considering full-cycle lifetime, operating temperature range andsteady voltage and power level, which is independent of load, temperature and state of charge. Thus FES provides minimally much higher power output and energyefficiency.

2 SYSTEM COMPONENTS

[Fig. 6.] refers to KERS components, respectively: Electric Propulsion Motor / Generator, Power Electronics Inverter, and the Quad Flywheel Storage.

Fig. 6. KERS components Fig. 7. Motor / Generator

2.1 Electric Propulsion Motor/GeneratorElectric Propulsion Motor and Generator in one, also known as a MGU - MotorGenerator Unit [Fig. 7.].2.2 System ControlSystem communication is provided via CAN interface (ControllerArea Network).

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Fig. 8. Overview of KERS System control

2.2.1 Power Electronics[Fig. 9.] refers to integrated power electronics liquid cooled Inverter.

Fig. 9. Liquid cooled Inverter

An inverter is an electrical or electro-mechanical device that reversely convertsdirect current DC - from flywheel, to alternating current AC - to MGU. The resultingAC can be at any required voltage and frequency with the use of appropriatetransformers, switching, and control circuits.

2.2.2 Control Electronics[Fig. 10.] refers to flywheel storage subunits equipped with bonding pad for controlelectronics.

Fig. 10. Control electronics ECU Fig.11. Microprocessor of Control unit

Design of bonding pad provides direct connection of control unit, which workssimilar to ECU - Engine Control Unit.

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Hybrid-Bearing is combination of hydrodynamic and ball bearing, works independence on RPM. Ball bearing acts during starting acceleration from low speed.Hydrodynamic bearing starts working contactless at high revolutions.

2.3.2 Safety ConceptSafety concept concerning Control System is following:

Control unit limits rotational speed by a hardware lock in the output stage. Controlsystem monitors all security parameters. During idle operation is no voltage induced.

In case of error messages or breakdown, control system discharge KERS.Controlled and safe discharge of the system is possible by converting rotational energyin thermal energy. In the flywheel storage system, the critical energy is reduced byusing several small storages, coolant ducts and channels in stator [Fig. 14.], [Fig. 15.].

FES is designed as a reluctance motor and its resulting safety benefits arefollowing:

Inner flywheel rotor is designed as homogenous flywheel mass without any addi-tional coil former, windings, magnets or rotor cage. Laminated rotor consists of sheet-metal packet, incl. disc spring and rotor shaft equipped with hybrid bearing [Fig. 17.].

Fig. 17. Flywheel rotor of storage subunit

In case of breakdown, homogeneous flywheel rotor made only from sheet-metalstock has no massive fragmental parts. The stator is also used as a crumple zone andworks as a safety bandage.

Laminated rotor consists of sheet-metal packet has very high bursting strength.

Highest stress of rotor sheets is approximately 70% of Proof stress R p0.2. (Offset YieldStrength).

3 BASIC PRINCIPLES

3.1 Stored EnergyBasic principle of kinetic energy storage is made by rotational energy. While using thistechnical approach, inertial mass is accelerating to a very high rotational speed andmaintaining the energy in the system as rotational energy.

Stored energy is proportional to inertia of rotor and is a quadratic function of

revolution speed:222 R I E S = (1)

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3.3 Boost acceleration - discharge mode

Car is accelerating during boost discharge mode.

The Flywheel rotor is decelerated during boost discharge mode and the energy isconverted back.

Flywheel acts as a generator and sending energy back to electric motor, whichworks as propulsion motor.

Fig.19. KERS boost simulation - Discharge generator boost mode

4 EXPERIMENT - FES MEASUREMENT

Testing equipment for the experimental analysis of the simplified FES was designed inorder to prove the basic principles of discharge generator mode.

Stored energy is proportional to inertia of rotor and is a quadratic function of revolution speed ( 222 R I E S = ).

During boost discharge mode the flywheel rotor acts as a generator and isdecelerated.

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Experimental facility consists of: Power Supply, Power Electronics, ElectricMotor, Flywheel, Amperemeter, Voltmeter and Shunt [Fig. 26.].

Fig. 20. Experimental facility

Fig. 21. Electromotor with elastic coupling

Fig. 22. Theoretical solution Discharging characteristics of unloaded FESStored energy E ak vs. RPM

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Fig. 23. FES on load Propeller

Fig. 24. Experiment Discharging characteristics of FES on load - Generated voltage

5 CONCLUSION

5.1 Comparison with other storage technologiesIn comparison with other battery storage technologies, KERS offers:

Cycle durability [Fig. 25.] - 90% efficiency of flywheel (including power elec-tronics) in both directions during KERS reference duty cycle.

Extensive operating temperature range [Fig. 26.]. Steady voltage and power level [Fig. 27.], which is independent of load, tem-

perature and state of charge. High efficiency at whole working speed range. No chemistry included, thus no environmental pollution and great recycling

capability.

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Fig. 25. Comparative chart in terms of cycle loading

Fig. 26. Comparative chart - Operating temperature range (at appr. 80% Performance)

Fig. 27. Comparative chart in terms of Voltage stability (full cycle)

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5.2 Vision for race and stock carsIn motor sports applications this extra boost energy is used to improve acceleration.

Fig. 28. Race vehicle from Le Mans - Chrysler Patriot equipped with FES

KERS by means of FES are currently under development both for F1 motor sportand road hybrid vehicles.F1 Teams have said they must respond in a responsible way to the world's

environmental challenges. The FIA allowed the use of 60 kW KERS in the regulationsfor the 2009 Formula One season. Energy can either be stored as mechanical energy, asin a flywheel [Fig. 29.], or can be stored as electrical energy, as in a battery orsupercapacitor).

Fig. 29. Williams Hybrid Power F1 KERS Fig. 30. Kinetic storage for hybrid car

Same technology can be applied to road hybrid cars to improve fuel efficiency,especially in city traffic. [Fig. 30.].

Road vehicles with electric or hybrid drive utilizing regenerative braking.Vision for stock car is in convenient hybrid system with high energetic efficiency

and dynamics. Flywheel storage technology provides boost acceleration and braking force.FES supports starting and guarantees light, silent and emissionfree starts of

combustion engine. KERS also supplies all electric appliances, stabilizes on-boardpower supply and offers stable air-condition.

Kinetic recuperation based on braking energy stored in flywheel is without cycle

loading, unlike braking energy repeatedly stored in battery.