The impulse to be sustainable — driven by the incessant pressure to lower our emissions — often manifests as lowering consumption and increasing reuse alongside reforms like tweaking consumer behaviour. Electric vehicles are the site of many of these changes, aided by state-led incentives and subsidies. Regenerative braking is an important mechanism in these vehicles that increases their energy use efficiency.
What is braking?
Braking is the mechanism by which an automotive vehicle in motion slows down. A vehicle moving faster has more kinetic energy than a vehicle moving slower, so the process of braking removes (mostly) kinetic energy from the vehicle. The law of energy conservation means this removed energy has to go somewhere.
For example, the disc brake is one type of mechanical brake: it works by pressing brake pads against a disc attached to spinning wheels, and uses friction to convert some of the wheels’ kinetic energy into heat. This is why the discs of disc brakes have holes cut into them, to dissipate heat better.
Another type is the induction brake: a magnet induces circular electric currents in a spinning wheel (made of a conducting material, like metal). These currents produce their own magnetic field, which opposes that of the external magnet. The opposition acts like a drag on the wheel and forces it to slow down. In terms of energy: the metal resists the flow of the circular currents and dissipates heat.
What is regenerative braking?
Regenerative braking is a brake system designed to convert the kinetic energy of the wheels to a form that can be stored and used for other purposes. As such, it creates a process in which at least part of the energy delivered to the vehicle’s wheels can be recovered in a situation when the vehicle doesn’t need it.
Regenerative braking is one type of dynamic braking. In an electric vehicle, of the types becoming common on Indian roads, a battery onboard the vehicle draws electric power from the grid and stores it. When the vehicle moves, the battery powers an electric motor that propels the vehicle, converting electrical to mechanical energy. This motor is called the traction motor.
During regenerative braking, the motor operates as a generator, turning mechanical energy back to electrical energy. In the vehicle, this means an electric current will be produced as the vehicle brakes, which is stored separately in a battery. In some other vehicles, especially trains, the current is fed back into the traction motor.
The other type of dynamic braking is rheostatic braking, where the current is sent to an array of resistors that dissipate the electrical energy as heat. It is often necessary for a vehicle to have both regenerative and rheostatic braking in case the electrical energy recovered can’t be stored or used right away.
How does a motor become a generator?
A motor has two essential parts: a rotor (the thing that rotates) and a stator (the thing that’s stationary). In a rudimentary design, the stator consists of permanent magnets or electromagnets while the rotor consists of current-carrying wires coiled around in loops. The stator surrounds the rotor.
When a charged particle, like an electron, moves inside a magnetic field, the field exerts a force on the particle called the Lorentz force. Whether the force will push or pull the wire in which the electron is moving depends on the direction of the electric current.
This is when the coiling helps. The current at the coil’s two ends moves in opposite directions, so the magnetic fields imposed by the stator will push on one end of the coil and pull on the other. And these opposing forces will continue to act on the two sides of the rotor until the voltage across the wire is constant. Thus, a motor converts electrical energy to rotary motion.
In a generator, mechanical energy from an external source can be fed to the rotor to induce a current in the stator.
Simply speaking, by switching the traction motor between these two configurations, an electric (or hybrid) vehicle can implement regenerative braking.
Does regenerative braking have downsides?
While it is a simple energy recovery mechanism, regenerative braking has some downsides. For example, it alone often doesn’t suffice to bring an electric vehicle to a halt. It has to be used together with a conventional system that dissipates some of the kinetic energy as heat.
Such a system is also required to prevent vehicles from backsliding downhill, which many regenerative brakes won’t prevent.
Another example is that the amount of energy a regenerative brake can recover drops as the vehicle’s velocity drops as well. This said, a regenerative brake can be beneficial for an electric vehicle’s energy-use efficiency in stop-start traffic.
Are there other ways to recover energy?
The design of a regenerative brake depends on the energy form to which the mechanical energy from the wheels is to be converted. An electric vehicle funnels it into a generator and obtains a current, which is stored in a battery or a supercapacitor.
Similarly, the mechanical energy can be used to increase the angular momentum of a rotating flywheel. Flywheels are especially useful because they can receive energy much faster than other such systems. For every unit increase in speed, they also store exponentially more energy. Engineers have been able to build flywheels with carbon-composites that, in a vacuum, can spin at up to 50,000 rpm. The flywheel can be linked to a reciprocating engine to manage or augment its output, like in Formula One racing, or to a gyroscope to help submarines and satellites navigate.
Recovered kinetic energy can also be fed to a pump that compresses air, which can be useful to start internal combustion engines.
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