Cody Hyman
HC399
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

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)
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

Diagram of a Regen Braking
System
http://static.howstuffworks.com/gif/regenerativebrake-diagram.jpg
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)
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/supercapacitors
 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
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
Source: http://www.hybridcars.com/relatedtechnologies/hydraulic-hybrids.html
A VW Beetle converted to
a hydraulic hybrid that claims
up to 80% regeneration
efficiency
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
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 2004-2007
 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
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
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%

Road Vehicle Efficiency Data
Source: http://www.privatenrg.com/
Cars with Regenerative Braking
Toyota Prius
 Honda Insight
 Ford Escape Hybrid
 Tesla Roadster
 Chevy Volt

Toyota Prius
 It actually does have
regen braking*
Tesla Roadster
http://www.nytimes.com/2009/11/22/automobiles/au
toreviews/22-chevy-volt.html?_r=1&ref=technology
Regenerative Braking is not All
That New
 The electric vehicle in this 1906 had
regenerative braking capabilities
Image source: http://www.shorpy.com/node/5734
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
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
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

Sources
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9.
http://auto.howstuffworks.com/auto-parts/brakes/braketypes/regenerative-braking.htm
http://www.hybridcars.com/components/regenerativebraking.html
http://www.hybridcars.com/related-technologies/hydraulichybrids.html
http://www.railway-technical.com/brake1.shtml
http://www.shorpy.com/node/5734
http://www.railwaygazette.com/news/singleview/view//regenerative-braking-boosts-green-credentials.html
http://www.railwayenergy.org/static/Regenerative_braking_in_DC_systems_103.
php
http://privatenrg.com/
http://cleantech.com/news/4201/delhi-metro-earns-carboncredits-re