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| Fig. 1: Tesla Model X. (Source: Wikimedia Commons.) |
As the electric vehicle revolution continues to progress, the general population remains optimistic about reduced carbon emissions. As exciting as that is, the efficiency of the batteries within these electric cars remains as an area to seek improvement. Electric vehicles are significantly more efficient than gas powered vehicles when it comes to wasted energy. A study done by SAE International defines the vehicle specific fuel energy conversion efficiency as "the ratio of (powertrain provided) tractive energy required to move the vehicle over a drive cycle, to the fuel or electrical energy consumes over that cycle." [1] However, electric vehicles are still only about 85% efficient, producing waste heat up to 15% of its nominal power. [2] The SAE International study calculates that battery electric powertrains have tractive efficiencies ranging from 82% to 93%. With the remaining energy being wasted as trash heat, there is a constant struggle to find ways to reduce the wasted energy and instead utilizing it as much as possible. Electric Vehicles have what is called an HVAC system that helps keep the engine and battery at correct operating temperatures. The key to minimizing wasted energy and maximizing efficiency is finding ways to re-use that wasted heat to continue operating the vehicle.
Over the past few years, there have been a few impactful strategies that have been implemented within electric vehicles in order to increase efficiency. One of those being regenerative braking. Which is a process in which the vehicle's kinetic energy is converted and stored as chemical energy within the battery, then to be used again. It is impossible to recover all of the kinetic energy produced by the vehicle, with tire friction, aerodynamics of the car, and most significantly, heat from the brake pads. However, the regenerative braking process improves the overall efficiency of electric vehicle batteries. The SAE International study corroborate this by explaining that high vehicle fuel energy conversion efficiency could be achieved when a "relatively large amount of regenerative braking energy is captured and reused for tractive work over the cycle." [1]
The recently released Tesla roadster has seemingly perfected the regenerative braking process, adopting new strategies and methods in order to recover as much energy as possible that is normally lost to braking. Tesla has created a single moving part for the AC induction motor, which combined with the advanced software algorithms, allows for complete control of motor torque both for driving and braking. Tesla is the leading electric vehicle producer in the world, and the fact that they are constantly finding ways to improve the overall efficiency of their batteries along with perfecting their sleek, futuristic design as shown in Tesla Model X (Fig. 1), contributes to their elite status.
Recently, the Turkish Automotive Factory, has conducted a study on how to reduce the amount of waste heat produced by electric vehicle batteries. The most energy-consuming part of an HVAC system is its heating function. The factory was able to save 2.1 kWh with a new heating system which they have shown could increase the vehicles range by 7.5%. [2] The main change was that they used waste energy that wasn't utilized for charging the battery during braking instead of taking the energy from the battery to supply power for the water heater. This patent pending redesign of electric vehicles increases vehicle range which has potential to transform EVs for the future.
Electric vehicles have already come a long way, but there is still room to increase energy efficiency. The energy put to use in electric cars versus the amount of energy in the battery isn't equal due to power from the battery getting "wasted" as trash heat. Regenerative braking and new heating systems offer a potential future where the entire battery within electric vehicles is utilized.
© Lexie Hull. 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.
[1] G. Pannone et al., "Decomposing Fuel Economy and Greenhouse Gas Regulatory Standards in the Energy Conversion Efficiency and Tractive Energy Domain," SAE Int. J. Fuels Lubr. 10, 202 (2017).
[2] H. Ayartürk et al., "New Heating System Development Working with Waste Heat for Electric Vehicles," Transp. Res. Proc. 14, 1080 (2016).
