1. Cody Hyman HC399
Regenerative Braking

2. What is Regenerative Braking?
Conventional braking systems convert kinetic energy into heat, usually via friction.
This wastes a great deal of energy!
Regenerative braking systems reclaim and storing the kinetic energy in a reusable manner
Many modern electric drive vehicles including electric locomotives and HEVs have regenerative braking systems

3. Regen Braking Principles in Electric Transportation
EVs use their drive motors to convert kinetic energy into electromagnetic energy
Motors and generators operate under the same principle and can be used interchangeably.
The First Law of Thermodynamics dictates that the EV will slow down as this occurs.
The generated electrical energy can be dissipated through a resistive network (Dynamic Braking) or stored (Regen Braking)

4. How Regen Braking Works Part 1: The Mechanical Aspect
Vehicle has forward momentum
The wheels are coupled to the rotors of electric traction motors in an EV
Instead of a current being applied to the motor to turn the rotors, the rotors are turned by the wheels of the EV
The rotors experience opposing torque as current is induced in the motor coils
This opposing torque slows the vehicle
The generated electrical energy is stored

5. Diagram of a Regen Braking System

6. Part 2: Brake Control Circuitry
Complex electronic circuits that handle braking functions
Coordinates motor and friction brakes under a wide range of situations
The controller will take the following parameters into account and determines how the vehicle will brake
Speed of vehicle
Driver input (pedals)
Storage medium state-of-charge
Routes and regulates generated power
Depending on the desired output, the brake controller must be able to supply vehicle batteries or capacitors with the proper polarity, current, and voltage for safe charging (depends on battery state of charge)

7. Options for storing the Energy
Recharging vehicle battery packs
State of charge influences how much of the generated current can be safely stored
Ubiquitous method for hybrid cars
Charging an array of capacitors/super-capacitors
Pro: Very high charge/discharge rate (high power density)
Con: Much lower energy density than batteries
Allows for regen braking if batteries are at a high state of charge
Implemented on some buses
Non-electrical methods
Fluid compression
Flywheels

8. Mechanical Methods Hydraulic Regenerative Braking
Slows the vehicle by compressing gas and storing it in an accumulator
Pressure is used to assist the engine upon forward acceleration
Potentially more efficient than electric regenerative brake systems
A VW Beetle converted to a hydraulic hybrid that claims up to 80% regeneration efficiency
Source:

9. Regenerative Braking in Rail Vehicles
Dynamic Braking has been frequently used in rail vehicles to reduce brake wear
Diesel-Electric locomotives require onboard energy storage for regeneration
Impractical
Some electric rail/overhead line locomotives with regenerative braking can send power back through supply line
Power can be received by other trains connected to the line or sent back to the grid

10. Differences Between AC and DC Rail Lines
AC Locomotives
Can feed power back into the grid without the need for large power inverters
The New Delhi Metro, after implementing regenerative capabilities, cut down its power consumption by 30%, a total savings of 112MWh between
Other AC traction lines in the UK and NZ have achieved improvements of 15-17%
DC Locomotives
Cannot easily feed power back to external networks but it can power other trains on the same line
Efficiency depends on the number of nearby locomotives that can receive power from regeneration

11. DC Railway Regeneration Estimates
% Energy recapture figures for DC Catenary Locomotives
Theoretical potential
Potential if additional technologies are used
Potential without additional technology
Main lines
15%
11% 2%
Regional lines
35%
25%
10%
Local lines
45%
32%
16%
Freight lines
20%
14% 3%
Source: Institute for Futures Studies and Technology Assessment

12. Regen Braking in Road Vehicles
Road vehicles require onboard energy storage as well as relatively quick braking compared to rail vehicles
Regen braking improves the efficiency in stop and go city traffic more than high speed travel
Efficiency gains vary greatly from vehicle to vehicle and under the driving conditions
Prius regen efficiency ≈ 30%

13. Road Vehicle Efficiency Data
Source:

14. Cars with Regenerative Braking
Toyota Prius
Honda Insight
Ford Escape Hybrid
Tesla Roadster
Chevy Volt
It actually does have regen braking*
Toyota Prius
Tesla Roadster

15. Regenerative Braking is not All That New
The electric vehicle in this 1906 had regenerative braking capabilities
Image source:

16. Benefits of Regen Braking
Increase of overall energy efficiency of a vehicle
Increases vehicle range
Cuts down on pollution related to electricity generation
Increases the lifespan of friction braking systems
Less use of traditional mechanical brakes leads to less wear over time

17. The Downsides Added complexity of brake control system
Only works for wheels connected to motors
Most vehicle operation is done in 2WD
Friction brakes are still necessary
Safety
Motor braking power decreases as the kinetic energy of the vehicle decreases

18. Conclusion
Regenerative braking is an effective method of improving vehicle efficiency and longevity
Is already in use in many EVs
The technology to do it exists and is often well worth it
Mostly dependent on the wider adoption of EVs or further development of hydraulic regeneration systems

19. Sources