Energy harvesting is increasingly used in the tiniest electronic components right up to multiple KW lift and swingarm braking on huge construction vehicles where 1.5 KW thermoelectric harvesting is planned next. However, electrodynamics and photovoltaics are most popular so far, increasingly in combination, and application in vehicles is of increasing importance.
The recently updated IDTechEx report Energy Harvesting/Regeneration for Electric Vehicles Land, Water & Air 2014-2024 finds that 90% of heavy industrial electric vehicles will use regenerative braking by 2024 to charge traction batteries.
Regenerative braking is an efficient process at speed by which a vehicle brakes primarily by making the traction motor working in reverse. It takes the kinetic energy from the traction wheels or propeller and converts it into electricity which is then stored for future use. It is widely used in electric and hybrid electric vehicles that already have batteries to store the recaptured energy - and used in air, water and land vehicles, though not universally and never without a backup such as the flaps on an aircraft and disc brakes in its wheels.
Simplest scheme for vehicle regenerative braking
Source Clemson University
Regenerative brakes have minimal impact on fuel economy during highway driving, but can significantly improve the fuel economy of vehicles that are driven primarily in city traffic. In heavy vehicles that make frequent stops (e.g. garbage trucks) regenerative braking systems can improve fuel economy substantially.
Land vehicles particularly can have a problem with lead-acid batteries being damaged by the surge of power from regenerative braking, lithium-ion batteries being acceptable though supercapacitors have the highest power density. Kinetic Energy Recovery Systems such as low-mass, very high revolution flywheels may charge the traction battery in a 10-15% efficient controllable system or have their rotational energy directly tapped for subsequent acceleration, this being less controllable but up to 20% efficient.
Some regenerative braking systems store the recaptured energy mechanically, typically by pumping hydraulic fluid into an accumulator where the energy is stored in a compressed gas. When the accelerator is pressed, the direction of fluid flow is reversed, and the pressure is used to aid in acceleration. These systems boast higher efficiency than electronic regenerative braking systems, but are primarily implemented only in commercial vehicles due to their size and noise of operation.
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Top image: Evo Charge