Energy. It's a simple word, but it is big science—and it is a science that relates very much to road-bound vehicles, and especially to hybrids and all-electrics. Simply defined, energy is the capacity to do work. And according to the First Law of Thermodynamics, the total amount of energy in the entire Universe is constant—none can be added and none can be taken away. The law states that energy can be neither created nor destroyed, it can only change from one form to another (e.g. heat to light, chemical energy to electrical energy, chemical energy to mechanical motion).
There are two categories of energy: potential and kinetic. While potential energy is untapped energy-in-waiting that is yet to be deployed (chemicals in a battery, a stretched rubber band, gravity), kinetic energy is the energy of full bore motion (waves of heat from a burning fire, a vibrating speaker and the sound waves emanating from it, a car in full acceleration zooming down the highway). Let’s take a look at how energy can change form.
Potential to Kinetic: Something Spent, Something Lost
When an object at rest (say a regular car, for this example) is put into motion, potential energy in the form of liquid gasoline is converted into kinetic energy as the fuel is ignited in the engine’s combustion chamber, and the vehicle accelerates and then cruises happily along. Conversely, when said car starts to come to a halt with the application of the brake pedal, its kinetic energy of mechanical motion undergoes a conversion to different kind of kinetic energy—that of heat waves emanating from the friction brake pads (and for all intents and purposes, it is lost to the vehicle for any future use).
Potential to Kinetic to Potential: Something Spent, Something Gained
An electric car (or a hybrid, to a slightly lesser extent) that is driving down the highway has used the potential chemical energy that is stored in the battery, converted it into electrical energy that drives the traction motor which in turn is converted into mechanical kinetic energy that sets the wheels of the car in motion. When this car is brought to a stop, a small portion of its kinetic energy is converted to heat (through friction in brake pads) and is lost to the atmosphere, but almost all of it is recaptured as electricity via regenerative braking and is stored for future use as potential chemical energy in the battery.
It is quite apparent that recapturing energy that is normally lost from slowing and stopping a vehicle is the next best thing to a free lunch. And this energy does not necessarily need to be harnessed via electricity and batteries. Other innovative methods (like flywheel power units) exist, or are in development, that will make this proposition a relatively simple no-brainer. Why throw away (or more accurately, convert to difficult to manage heat) a perfectly good energy form when a few hundred-bucks-worth of hardware can package and re-release it on an as needed basis. Talk about "free fuel."