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Regenerative Braking: an Increasingly-Promising Technology for EVs and More

November 30, 2020 by Biljana Ognenova

Regenerative braking systems are able to charge automobiles through the energy developed by their friction, thus promising a future of safer, longer, and greener commuting—particularly for EVs, or electric vehicles. We look at the technology behind this promising solution to vehicle battery charging on the go.

Regenerative—or regen—braking systems are not brand new. Variations of regenerative braking (RB) systems have been historically used in trolley cars, gyrobuses, and rail transport. In fact, even children’s push-and-go toys use reverse friction power to propel forward.

Currently, regen braking is the basis of brakes in electric vehicles, such as the Tesla Roadster. Hybrid and race cars also use RG technology to optimise what would otherwise be energy-inefficient automotive systems. After all, without regenerative braking, much of a vehicle’s useful output energy may be lost in the heat caused by hydraulic braking systems.


Energy Recovery and Extended Battery Life

Of course, it is a widely accepted fact that combustion engine vehicles are a significant contributor to air pollution due to their fossil fuel emissions. The central advantage of regenerative braking systems is that they improve non-electric vehicles’ fossil fuel consumption rates due to their ability to recover the energy lost through the heat created by conventional hydraulics.

As another note, one of the major disadvantages of most electric vehicles remains their low driving range. By restoring lost energy, an EV’s driving range can be extended—by anywhere between 16% and 70%, depending on the user’s driving style, terrain, and other factors.


The mechanics of regenerative brakes are not unlike their hydraulic counterparts. Pictured: a close-up of a car’s exposed wheel arch, which shows both the vehicle’s brake disc and the brake caliper attached to it. Image Credit: Bigstock.


Where the Energy Goes and How to Restore It

Classic cars use hydraulic braking, which uses a lot of kinetic energy; however, once the vehicle stops, the accumulated heat is lost. This is as part of that heat energy goes into mechanical sliding and friction, and another part goes into rolling resistance (and a third part is wasted due to the drag caused by automotive aerodynamics—for instance, consider the aerodynamic drag that impacts less-than-streamlined vehicles).

Fundamentally, when a driver slows down or stops their vehicle, that vehicle produces friction in its brake pads (particularly the car’s brake rotors) and its decelerating tyres.

Without regenerative braking, the heat caused by both of these points of friction cannot be regenerated (in other words, it cannot be re-integrated as useful output energy in the vehicle). This causes wear and tear on the brakes and tyres, meaning that much of the equipment may require frequent replacements.


From a Motor to a Generator and Vice Versa

An electric motor is of course integral to RB systems: when it runs in one direction, it converts electrical into mechanical energy and pulls car wheels into rotational motion.

Moreover, when the motor is upturned or reversed, it transforms into a generator that converts mechanical energy into electrical energy, which is later used to accelerate the vehicle.

The trick is in having the motor running backwards by using the vehicle momentum, i.e. a mechanical force that keeps the vehicle moving once the brakes have been pulled. The energy picked up from the momentum converts into generated electricity that feeds the battery (or a capacitor) for later use.

Using supercapacitors is further recommended, as they are compact, lightweight, and durable, and charge faster than batteries.


Regen Braking Circuits  

Regenerative braking systems use advanced electronics circuits to make multiple conversion efficiency decisions and optimise energy expenditure.

In an RB vehicle, the role of regenerative braking electronics circuits is to determine the moment of switch between a forward and a reverse mode. There are also additional circuits that charge the regenerated electricity back into the battery (or the capacitor).

In hybrid cars with a parallel conventional braking system, the electronics circuits calculate which is the most efficient of the two available braking systems (namely hydraulic or regenerative).

Regenerative braking concept. Pictured: a speedometer with a needle set to ‘charge’. Image Credit: Bigstock.


Two Types of Regen Brake Controllers

In regenerative braking, a brake controller system monitors a driver’s wheel speed and determines the amount of torque necessary to brake, as well as the extra energy that can be converted back for electricity generation.

Some vehicles give the option to drivers to preset the deceleration rate of the braking system. The driver can choose between slowing down to zero speed or moderate deceleration, depending on the weight of the load he or she is towing. These are called time-delayed brake controllers. Each delay occurs when the driver applies the brake, and he or she (by using a control known as a sync switch) can adjust the length of those delays.

Vehicles with pendulum brake controllers, on the other hand, use motion sensing: in keeping with their name, such brake controllers use the physics of their pendulum as a motion sensor, which needs to be calibrated before the journey by the driver.


Flywheel-based Energy Generation

Alongside the regenerative braking technologies already mentioned, there is yet more potential to improve the overall efficiency of electric vehicles and hybrids. Case in point: flywheels.


A cross-section of a flywheel (dual-mass). Image Credit: Bigstock.


A flywheel is a rotating disc that acts as an electromechanical battery. The basis of the energy storage method is rotational inertia.

A flywheel uses its stored kinetic energy to accelerate its rotor to very high speeds and maintain the system energy as rotational energy. (A famous example of a flywheel example is the NASA G2 Module: designed for the laboratory-based testing of components and applications for aerospace energy storage, the G2 is a 60,000 revolutions-per-minute, 525 watt-hour and 1-kilowatt energy storage system.)

A typical modern flywheel-based energy storage device consists of a large rotating cylinder supported on a stator by magnetic bearings. Flywheel systems operate in a vacuum to reduce drag, friction, and energy loss.


Material and Design Considerations for Flywheels

Steel is the most common material used in flywheel production. However, composite materials (such as carbon fibre and magnet-impregnated glass fibre) are becoming progressively popular as the need increases to make lighter, more efficient discs.

Carbon fibre, for instance, is ten times more expensive than steel, five times stronger, and twice as stiff. As an environmentally-benign and chemically resistant material that is tolerant to excessive heat (it also has low thermal expansion), carbon fibre is a promising material for use in heat-efficient braking systems. In combination with graphene, the material can even enable engineers to print electronics directly onto its surface.


The Weaknesses of Regen Braking

Again, the amount of restored energy that is achieved from regen brakes depends on (among other factors) where and how you drive. How useful RB systems are to their users will depend largely on the various actions and experiences of each driver.

On top of this, regen braking systems do have at least one major drawback: they are potentially hazardous on certain fast roads, such as open roads and highways. This is because it is next to impossible for such a regenerative system to restore all lost energy at high speeds or high deceleration rates. When a speeding driver is forced to slam on the brakes, for instance, some of the energy must go into the inevitable, screeching halt—and it then cannot be used to reverse the motor. Such a process involves an intricate balancing act, particularly between inertia and momentum.

Moreover, drivers of regen-braking hybrid cars—given that they use both RB systems and classic hydraulics as a backup—must pay careful attention to the difference in pedal pressures that will be felt based on whether the former or the latter braking system is in use.


Charging electric vehicle concept. A car’s infotainment screen shows a graphic of the car’s charging battery. Image Credit: Bigstock.


The Overall Potential of Regen Braking

Regardless of the imperfections of regen braking, many companies are currently harnessing the potential of the exciting technology. According to Radiant Insights, electric and hybrid vehicle manufacturers show their continued interests in regen braking.

Heavy-load commercial vehicles, public railway transportation, and densely-populated urban areas could all benefit from elastically-paced vehicles with regenerative brakes—not only in terms of air quality but also for road safety through Intelligent Speed Assistance. Plus, of course, the increased driving range that regen braking offers is always welcome, too!

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