Regenerative Braking: Seminar Ii

SEMINAR II

REGENERATIVE BRAKING

INTRODUCTION • For three decades now, the second commandment of every automotive engineer – right behind ‘reduce cost’ – has been ‘reduce fuel consumption.’ The drive to use less fossil fuel has dictated the design of engines, transmissions and control systems for decades. Now, it is pushing the development of completely different technologies for generating power. • In spite of all these truly marvelous improvements, the energy efficiency of the most modern production car is still less than 20 percent. Most of the energy used to move the vehicle at any speed over any distance is literally thrown away as heat. About half of that wasted energy goes through the brakes. Today, almost every manufacturer is developing ways to recover a significant portion of that wasted energy with regenerative braking 2

WHAT IS REGENERATIVE BRAKING? • A regenerative brake is an apparatus, a device or system which allows a vehicle to recapture part of the kinetic energy that would otherwise be 'lost' to heat when braking and make use of that power either by storing it for future use or feeding it back into a power system for other vehicles to use. • Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energy lost during stopping. This energy is saved in a storage battery and used later to power the motor whenever the car is in electric mode. 3

HOW IT WORKS • Regenerative braking does more than simply stop the car. Electric motors and electric generators (such as a car's alternator) are essentially two sides of the same technology. Both use magnetic fields and coiled wires, but in different configurations. Regenerative braking systems take advantage of this duality. Whenever the electric motor of a hybrid car begins to reverse direction, it becomes an electric generator or dynamo. This generated electricity is fed into a chemical storage battery and used later to power the car at city speeds.

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• Regenerative braking takes energy normally wasted during braking and turns it into usable energy. It is not, however, a perpetual motion machine. Energy is still lost through friction with the road surface and other drains on the system. The energy collected during braking does not restore all the energy lost during driving. It does improve energy efficiency and assist the main alternator. • Regenerative braking is a benefit of hybrid electric vehicles, whereby some of the translational energy of the vehicle is captured and returned to an energy storage device when the brake pedal is applied. This is different from conventional braking, in which a car is slowed through friction and the energy of the car is lost through waste heat.

• The energy recaptured by regenerative braking might be stored in one of three devices: an electrochemical battery, a flywheel, or in a regenerative fuel cell. 5

REGENERATIVE BRAKING AND BATTERIES • The electric motor of a car becomes a generator when the brake pedal is applied. The kinetic energy of the car is used to generate electricity that is then used to recharge the batteries. With this system, traditional friction brakes must also be used to ensure that the car slows down as much as necessary. Thus, not all of the kinetic energy of the car can be harnessed for the batteries because some of it is "lost" to waste heat. Some energy is also lost to resistance as the energy travels from the wheel and axle, through the drivetrain and electric motor, and into the battery.

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REGENERATIVE BRAKING AND FLYWHEELS • •

the translational energy of the vehicle is transferred into rotational energy in the flywheel, which stores the energy until it is needed to accelerate the vehicle. • The benefit of using flywheel technology is that more of the forward inertial energy of the car can be captured than in batteries, because the flywheel can be engaged even during relatively short intervals of braking and acceleration. • In the case of batteries, they are not able to accept charge at these rapid intervals, and thus more energy is lost to friction. • Another advantage of flywheel technology is that the additional power supplied by the flywheel during acceleration substantially supplements the power output of the small engine that hybrid vehicles are equipped with. 8

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REGENERATIVE BRAKING AND FUEL CELLS •

• The third system uses what is known as a unitized regenerative fuel cell, • which is designed to both convert hydrogen and oxygen into energy and water, or be reversed to take the energy from the wheels, combine it with water, and produce hydrogen and oxygen. • The system as a single unit is substantially lighter than a separate electrolyzer and generator, which makes this system (known as a URFC) especially beneficial when weight is a factor. • When the URFC is paired up with lightweight hydrogen storage, it's energy density of about 450 watt-hours per kilogram is ten times that of lead-acid batteries and twice as much as any predictions for the energy density of forthcoming chemical batteries. • This means that not only will this technology make lighter hybrids available, it will also give hybrids a driving range that is comparable to that of vehicles today that are equipped with conventional engines. • Further benefits of the URFC is that it will be more cost effective than other vehicles because it will not need to be replaced, and it will provide the additional power needed by an electric engine when accelerating onto a highway. 11

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REGENERATIVE BRAKING AND LAUNCHASSIST • Some of the recovered energy is stored in a package of ultracapacitors instead of the battery pack. • The ultracapacitor is charged with regenerative braking. Voltages are as high in either direction as 300 Vdc and can flow at 100 amps continuously or 200 amps for two minutes. It can be charged and discharged quite rapidly even in urban drive cycles, and it can withstand thousands of charge/discharge cycles with no loss of performance. • In addition to extending the charge state and overall life of the batteries, it also allows regenerative braking even when the batteries are fully charged. At this early point in its development, the ultracapacitor pack is large, heavy and requires its own cooling system. However the launch-assist idea has a lot of potential, and it’s not limited to electric vehicles. 13

HYDRAULIC LAUNCH ASSIST (HLA) • It recovers energy normally lost during deceleration and converts it to hydraulic pressure in an accumulator, where it is available as a source of energy during the vehicle's

next acceleration. • A variable-displacement hydraulic pump/motor is mounted on the transfer case and clutched to the output shaft that powers the front driveshaft. • A valve block mounted on the pump contains solenoid valves to control the flow of hydraulic fluid. A 14-gallon, high-pressure accumulator is mounted behind the rear axle, with an almost identical low-pressure accumulator right behind it to store hydraulic fluid.

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• When the driver presses the brake pedal, a pedal movement sensor signals the control unit, which then operates solenoid valves to send hydraulic fluid from the low-pressure reservoir to the pump. The pumping action slows the vehicle, similar to engine compression braking, and the fluid is pumped into the high-pressure reservoir. • Releasing the brake and pressing hard on the accelerator signals the control unit to send that high-pressure fluid back to the pump, which then acts as a hydraulic motor and adds torque to the driveline. The system can be used to launch the vehicle from a stop or to add torque for accelerating from any speed. • The real advantage of hydraulics is in its power density. Hydraulics is capable of transferring energy very quickly compared to hybrid electric system.

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REGENERATIVE BRAKING ADVANTAGES • Provide greater fuel economy • Greater improvement in emission reduction • Energy savvy (energy conversion will be carefully controlled in the interest of maximum efficiency) • Provide ample drive power and power for sudden acceleration.

• Enhance the efficiency of the engine and the overall economy of the car. 16

REGENERATIVE BRAKING DISADVANTAGES The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the electricity generated with the supply. With DC supplies this requires the voltage to be closely controlled and it is only with the development of power electronics that it has been possible with AC supplies where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).

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