design and analysis of flywheel regenerative braking...

8th International Ege Energy Syposium May 11-13,2016 Afyonkarahisar

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Design and Analysis of Flywheel Regenerative Braking Energy Storage System for Hybrid and Electrical Vehicles

K. Erhan, E. Özdemir, A. Aktaş

Department of Energy Systems Engineering, Kocaeli University, Kocaeli, Turkey

Abstract

Traditional vehicles lose their kinetic energy as heat energy when they want to slow down. However, if vehicles do regenerative braking, they can store some part of this kinetic energy in energy storage system. But most of the recuperated energy is lost as heat in braking mode. In this study, a Flywheel Energy Storage System (FESS) is designed to store vehicle’s kinetic energy, which occurs braking of the vehicle. Mathematical calculations, design of the FESS have been done in a scaled down laboratory prototype. Mechanical production stages of the prototype and preliminary test results are also included in the paper.

© 2016 IEESE. All rights reserved.

1. Introduction

Electric vehicles (EV) use only electric motors for driving vehicle by using stored energy in the battery. A hybrid electric vehicle (HEV) is a type of EV that combines a conventional internal combustion engine (ICE) and electric motor with a battery for driving vehicle. The presence of the electric motor in HEV achieves better fuel economy and performance.

The most significant problem is short range in electrical and hybrid electric vehicles. There are various studies about higher energy density battery technologies and higher efficient internal combustion engines to overcome the range problem. In this study, the kinetic energy storage system designed and experimentally investigated to increase range of an EV and HEV with available battery capacity.

Flywheel is an energy storage system that stores the regenerated energy as kinetic energy on a high speed rotating mass. Vehicle’s electric motor is operated as generator during braking. The regenerated energy is transferred to the FESS’ Motor/Generator (M/G) unit during deceleration of the vehicle. M/G unit is operated as motor mode and energy is stored as kinetic energy by accelerating the FESS. Then the stored energy transferred the vehicle’s DC bus operating M/G unit as generator mode when vehicle needs energy to accelerate the vehicle. Thus there is no loss of kinetic energy converted into heat energy from the vehicle during breaking, is stored to be used again in FESS [2,9].

2. Material and Method

Flywheel used in EV and HEV as an energy storage system. There are two different states related with energy flow. In the first case, Flywheel energy storage system (FESS) directly connected Brushless DC Machine (BDCM). BDCM is operated as motor mode and the energy is stored in the system accelerate the flywheel. In the second case, the stored energy in FESS is transferred to the load by operating BDCM as generator mode. This two state’s diagram can be seen in Fig. 1.


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~

~

+ - M / GM / G

Electrical car

motor unit

(BLDC) RectifierDC motor

driverS1

(mode

change

switch)

S2

(mode

change

switch)

Electrical car

motor driver

Flywheel

Rectifier

Brushless DC

M/G unit

Energy Storage

System

Regenerating of storaged energy mode

~

~

+ -Li-ion

DC-DC converterElectrical car

battery

Regenerative breaking energy flow mode

Fig. 1. General block diagram of the proposed FESS for HEV and EV

2.1. Design of the flywheel energy storage system (FESS)

Designed FESS is a scaled down prototype of the system used in Hybrid and electric car. Energy storage capacity equations for filled cylindrical shape mass can be seen in Eq. (1).

(1)

The descriptions of the variables in the formula are given in Table 1.

Table 1 Description of the major variables in FESS

Bearings should be withstanding the dynamic stresses of the rotor mass when spinning high speed. Therefore special composite bearings have been used in the system. In this system "SKF EnergySeriesE2.6003-2Z/ C3" code number bearings are used [1-9].

Rotors and shafts are made of high-strength steel. Shaft and bearings are mounted to the rotor can be seen in Fig. 2.

Rotor part of the flywheel is centered by flanges and is put in the flywheel cover. All system is painted and assembled on a table. A platform is built to connect the motor generator unit to the system. This platform is processed in a very delicate way not to cause any axial run-out. Then M/G unit is mounted on the platform. Coupling portion which couples M/G unit and flywheel together is designed and made precisely to withstand very high speed.

I The moment of inertia (kg.m2)

w Angular speed (rad/s)

m Mass (kg)

r Radius (m)


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Fig. 2. Shaft and bearings mounted rotor part of the flywheel

When storing energy in the flywheel system, rotor has reached 24,000 rpm. Fig. 3 shows the highest speed is reached in the preliminary tests. The cut off value is 10,000 rpm while consuming the stored energy. Formula (1) shows that FESS stores approximately 2 Wh energy spinning at 24,000 rpm. The remaining energy is calculated 0.52 Wh while spinning at 10,000 rpm. When these results compared, there is no significant energy in the storage system. For this reason power flow is cut off at 10,000 rpm.

Fig. 3.The highest speed of FESS in the pre-tests

With the aid of the motor driver unit, energy is stored in the flywheel at 24,000 rpm, after performing pre-tests. Then, flywheel began to supply the load. When the load connected to the system, speed of the FESS is decreased. Here at the output voltage of the FESS is decreased, too. FESS tests are performed with “Chroma 63103 Programmable DC Load” unit. 100 watts constant power load is supplied by FESS using programmable load unit. Programmable load unit is connected to the output of the FESS. FESS supplies the load 35 seconds. Fig. 4 shows the time-dependent voltage and current waveforms of the FESS.


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Fig. 4. Voltage and current waveforms of the FESS while discharging

3. Results and Discussion

In this study, a FESS production is realized to store energy in EV and HEV. Experimental results show that FESS is reached 24,000 rpm. A programmable load unit is connected to the FESS to stabilize the output power. As a result of the experiments, energy could be stored to FESS successfully. Then stored energy could be transferred to the load. Storing energy in the FESS reaches at 24,000 rpm takes 12-13 second. This data supports the appropriateness of the use FESS in electric and hybrid electric vehicles. FESS could be supply the 100 watt load 35 seconds. This data suggest that the FESS could be store energy during acceleration and deceleration of HEV and EV.

4. Conclusion

Energy recovery is achieved by storing the lost energy during braking. Vehicle’s kinetic energy is stored in FESS by regenerative braking. This stored energy is used for start up the car when needed.

A FESS is developed in this study as a prototype. An experimental setup is designed to test the performances of the FESS. Charge and discharge tests were performed. The results show that FESS could be used in HEV’s and EV’s in the near future.

5. Acknowledgements

This paper was supported by within TUBITAK project number of 113E143.

References

[1] Kozak M., Kozak Ş., “Enerji Depolama Yöntemleri”, SDU International Technologic Science Vol. 4, No 2, pp. 17-29, November 2012.

[2] http://www.flybridsystems.com, 22.05.2015.


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[3] Bachstein J., Daberkow A., “Mechanical energy storage in electric drivetrains for short-time applications”, Power Electronics, Machines and Drives (PEMD 2014), 7th IET International Conference, 2014.

[4] Park C. H., Choi S. K., Son Y. S., Han Y. H., “Development of 5 kWh Flywheel Energy Storage System Using MATLAB/xPC Target”, 2009 World Congress on Computer Science and Information Engineering, 2009.

[7] Kuperman A., Arogeti S., Brand Z., Zhong Q. C., “Robust UDE-Based Control of Active Magnetic Bearing for Flywheel Applications”, Electrical Systems for Aircraft, Railway and Ship Propulsion (ESARS), 2012.

[8] Cross D., Hilton J., “Hıgh Speed Flywheel Based Hybrıd Systems For Low Carbon Vehıcles”, IET HEVC 2008, Hybrid and Eco-Friendly Vehicle Conference, 2008.

[9] http://www.torotrak.com, 22.05.2015.

design and analysis of flywheel regenerative braking...

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