Potential Impact of Regenerative Braking

Introduction

Fig. 1: A Toyota Prius, which uses regenerative braking. (Source: Wikimedia Commons)

Transportation across land, of both people and goods, primarily takes place using automobiles, like trucks, cars, and busses, and locomotives, like trains. These vehicles mainly use energy to accelerate to a steady speed, and to maintain that speed against friction and air resistance. When the vehicles need to stop, they use brakes to convert all of this kinetic energy into heat energy through friction. Since this heat is lost to the environment, all of the energy that accelerates vehicles and maintains their speed is normally lost as well.

However, there are many technologies, like water wheels and wind turbines, that are able to convert kinetic energy from movement into electric energy that can be stored and used later. The same conversion of kinetic energy into useful energy should be possible with these vehicles. Indeed it is already in use in some electric cars and trains, for example the Toyota Prius in Fig. 1, and the London Underground train in Fig. 2.

In these electric vehicles, a power source generates a current through a magnetic field, which causes a force that drives the motor pushing the vehicle forward. If done correctly, this process can also happen backward: the forward motion of the vehicle turns the motor inside of a magnetic field, which causes resistance, slowing the motor down, and also generates a current which is sent back to charge the power source. This power source can be an individual battery, or an entire power grid. Regenerative braking can be used on any transportation method that utilizes mechanical brakes, in particular automobiles and locomotives.

Automobiles

Road transportation is the oldest form of transportation, and also the most used. Today, the primary mode of road transportation is by automobile. Many families own multiple cars, and most drive personal vehicles both to and from their workplace. Additionally, large freight trucks are a major part of the transportation of goods, as they can access virtually anywhere connected to the road infrastructure.

City driving involves much more braking than highway driving. Regenerative braking in cities shown to correspond to result in an 8-25% decrease in energy usage. [1] On average, the decrease in energy usage is predicted to be 20%. [2] Since road transport accounts for 2.8 × 10¹⁹ Joules yr^-1 of energy consumption worldwide, this corresponds to a total potential energy savings of 5.6 × 10¹⁸ Joules yr^-1. [3]

Locomotives

Fig. 2: A Stock S8 train from the London Underground that utilizes regenerative braking. (Source: Wikimedia Commons)

In contrast to automobiles, trains are mostly used to transport freight, although about a third of the train activity is passenger transport. [3] As expected by experience, passenger trains stop much more frequently than freight trains, and so have more opportunities to reclaim kinetic energy through braking. Freight trains rarely stop altogether, but do decrease speed before going around curves. When regenerative braking techniques are applied, this corresponds to a 30% increase in efficiency in city based passenger trains, but only a 6% increase for long distance freight trains. Though the variability is wide, on average regenerative braking in trains has been shown to decrease energy usage by 15%. [4] Since railway activity accounted for 2.1 × 10¹⁸ Joules yr^-1 of energy consumption worldwide, if this regenerative braking technique were applied to all trains the world energy consumption would decrease by 3.15 × 10 ¹⁷ Joules yr^-1 (neglecting energy costs associated with implementing the techniques). [3]

Conclusion

When considering its impact on both automobile and locomotives, regenerative braking has the potential to decrease world energy usage by about 6 × 10¹⁸ Joules yr^-1. Since the world energy budget is about 6 × 10²⁰ Joules yr^-1, universal adoption of regenerative braking could reduce the world energy consumption by 1%. While appreciable, this number alone would not be extremly impactful on the fate of the planet. However, with current technology, only electric vehicles are well situated to take advantage of regenerative braking, since their power sources are easily reconfigured into generators. If this technology is able to make electric vehicles more cost effective than gasoline alternatives by reducing the energy consumption requirements, then more people will buy them, reducing direct emissions from their cars. Since these vehicles rely on a central power plant, an eventual conversion of the power plant to greener energy sources will necessarily also convert the electric vehicles and reduce global emissions sooner. To aid in this effort, regenerative braking should be further studied and improved.

© Dylan Drescher. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] B. Cao, Z. Bai. and W. Zhang, "Research on Control For Regenerative Braking of Electric Vehicle," IEEE International Conference on Vehicular Electronics and Safety, IEEE 1563620, 14 Oct 05.

[2] S. Van Sterkenburg et al., "Analysis of Regenerative Braking Efficiency - A Case Study of Two Electric Vehicles Operating in the Rotterdam Area," 2011 IEEE Vehicle Power and Propulsion Conference, IEEE 6043109, 6 Sep 11.

[3] "Railway Handbook 2017: Energy Consumption and CO₂ Emissions," International Energy Agency, 2017.

[4] A. Fayad et al., "Energy Recovering Using Regenerative Braking in Diesel-Electric Passenger Trains: Economical and Technical Analysis of Fuel Savings and GHG Emission Reductions," Energies, 15, 37 (2022).