KINETIC ENERGY RECOVERY SYSTEM BY MEANS OF FLYWHEEL ENERGY STORAGE

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1 ADVANCED ENGINEERING 3(2009)1, ISSN 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) by means of Flywheel Energy Storages (FES). KERS by means of FES are currently under development both for motor sport and road hybrid vehicles. The aim of the work is the optimalization and implementation to the hybrid and electric road vehicles. Testing equipment for the experimental analysis of the simplified FES was designed. Keywords: Kinetic Energy Recovery System, Flywheel Energy Storage, Kinetic Storage, Flywheel, Reluctance Motor, Electric Generator / Motor, Regenerative / Recuperative Braking. 1 INTRODUCTION 1.1 Introduction to Regenerative Braking A 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 system for use by other vehicles. For example, electrical regenerative brakes in electric railway vehicles feed the generated electricity back into the supply system. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of twin layer capacitors for later use. Other forms of energy storage which may be used include compressed air and flywheels. Regenerative braking utilizes the fact that an electric motor can also act as a generator. The vehicle's electric traction motor is operated as a generator during braking and its 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, which dissipates the electrical energy as heat and thus is less energy efficient. 27

2 Fig. 2. Mechanism conceptual diagram Fig. 3. System control battery storage Conceptual diagram of mechanism shows comparison characteristic curve between hydraulic and regenerative braking, while driver intentionally brakes [Fig. 2.]. Regenerative braking reuse kinetic energy by using its electric motor to regenerate electricity. 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 a generator and produces electricity. This makes it possible to employ the rotational force of the driving axle to turn the electric motors, thus regenerating electric energy for storage (in the battery) and simultaneously slowing the car with the regenerative resistance 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 normally discarded as friction heat when braking, can be collected for later reuse in normal driving mode. Typically, driving in city traffic entails a cycle of acceleration followed by deceleration. The energy recovery ratio under these driving conditions can therefore be quite high. To take advantage of this situation, the system proactively uses regenerative braking when running the car in the low speed range. The regenerative braking effect rapidly reduces at lower speeds; therefore the friction brake is still required in order to bring the vehicle to a complete halt. The friction 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 (as in a 2WD car) and regenerative braking power only applies to such wheels, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels. 1.2 Introduction to Flywheel Energy Storage Kinetic storages, also known as Flywheel Energy Storages (FES), are used in many technical fields. While using this technical approach, inertial mass is accelerating to a very high rotational speed and maintaining the energy in the system as rotational energy. The energy is converted back by slowing down the flywheel. Available performance comes from moment of inertia effect and operating rotational speed. 28

3 Flywheel mass is either mechanically driven by CVT (Continuously Variable Transmission) gear unit [Fig. 4.] or electrically driven via electric motor / generator unit [Fig. 5.]. Fig. 4. Mechanically driven composite flywheel Fig. 5. Electrically driven flywheels Devices that directly use mechanical energy are being developed, but most FES systems use electricity to accelerate and decelerate the flywheel. In comparison with other conventional ways of storing electricity (batteries and capacitors), electric FES systems combined with innovative concept offer essential advantages. Especially considering full-cycle lifetime, operating temperature range and steady voltage and power level, which is independent of load, temperature and state of charge. Thus FES provides minimally much higher power output and energy efficiency. 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/Generator Electric Propulsion Motor and Generator in one, also known as a MGU - Motor Generator Unit [Fig. 7.]. 2.2 System Control System communication is provided via CAN interface (Controller Area Network). 29

4 Fig. 8. Overview of KERS System control 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 converts direct current DC - from flywheel, to alternating current AC - to MGU. The resulting AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits Control Electronics [Fig. 10.] refers to flywheel storage subunits equipped with bonding pad for control electronics. Fig. 10. Control electronics ECU Fig.11. Microprocessor of Control unit Design of bonding pad provides direct connection of control unit, which works similar to ECU - Engine Control Unit. 30

5 2.3 Flywheel Energy Storage Fig. 12. Quad FES consists of 4 subunits [Fig. 13.] refers to one of 4 subunits of Quad FES, which consists of water-cooled housing and heat sink with contact surface for control electronics. Further we could see an electrical configuration for direct connection of control unit. Fig. 13. FES Subunit overview Fig. 14. Cross-section - coolant duct Flywheel Rotor - Reluctance Motor Rotor flywheel mass works as reluctance motor in contrast to common mechanical flywheel. [Fig. 15.] refers to cross-section through storage subunit as reluctance motor. Fig. 15. FES - Reluctance motor Fig.16. Hybrid-Bearing Flywheel energy storage subunit consists of stator, incl. stator windings and channel for coolant backflow. Further we could see flywheel rotor equipped with Hybrid-Bearing [Fig. 16.]. 31

6 Hybrid-Bearing is combination of hydrodynamic and ball bearing, works in dependence on RPM. Ball bearing acts during starting acceleration from low speed. Hydrodynamic bearing starts working contactless at high revolutions Safety Concept Safety concept concerning Control System is following: Control unit limits rotational speed by a hardware lock in the output stage. Control system 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 energy in thermal energy. In the flywheel storage system, the critical energy is reduced by using 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 are following: Inner flywheel rotor is designed as homogenous flywheel mass without any additional coil former, windings, magnets or rotor cage. Laminated rotor consists of sheetmetal packet, incl. disc spring and rotor shaft equipped with hybrid bearing [Fig. 17.]. 32 Fig. 17. Flywheel rotor of storage subunit In case of breakdown, homogeneous flywheel rotor made only from sheet-metal stock has no massive fragmental parts. The stator is also used as a crumple zone and works 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 p 0.2. (Offset Yield Strength). 3 BASIC PRINCIPLES 3.1 Stored Energy Basic principle of kinetic energy storage is made by rotational energy. While using this technical approach, inertial mass is accelerating to a very high rotational speed and maintaining the energy in the system as rotational energy. Stored energy is proportional to inertia of rotor and is a quadratic function of revolution speed: 2 2 E S = 2 I π R (1)

7 3.2 Regenerative braking - charge mode Car is decelerating during recuperative charge mode. Electric motor works as generator and sending energy to flywheel storage. Flywheel rotor is accelerated in recuperative charge mode. Fig.18. KERS braking simulation - Recuperative charge braking mode 33

8 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 is converted back. Flywheel acts as a generator and sending energy back to electric motor, which works as propulsion motor. 34 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 in order to prove the basic principles of discharge generator mode. Stored energy is proportional to inertia of rotor and is a quadratic function of 2 2 revolution speed ( E S = 2 I π R ). During boost discharge mode the flywheel rotor acts as a generator and is decelerated.

9 Experimental facility consists of: Power Supply, Power Electronics, Electric Motor, 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 FES Stored energy E ak vs. RPM 35

10 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 technologies In comparison with other battery storage technologies, KERS offers: Cycle durability [Fig. 25.] - 90% efficiency of flywheel (including power electronics) 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, temperature and state of charge. High efficiency at whole working speed range. No chemistry included, thus no environmental pollution and great recycling capability. 36

11 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) 37

12 5.2 Vision for race and stock cars In 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 sport and 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 regulations for the 2009 Formula One season. Energy can either be stored as mechanical energy, as in a flywheel [Fig. 29.], or can be stored as electrical energy, as in a battery or supercapacitor). 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-board power 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. 38