Many hybrid cars use an interesting system called "regenerative braking" to recapture some of the energy which is wasted with conventional braking systems. On a typical car, each wheel has a rotor disk, and braking is accomplished by causing the brake pads to squeeze the rotor and create friction which slows the car, converting the forward momentum into waste heat. But a hybrid doesn't use the brake pads at all unless you hit the brakes hard… Instead, the car's momentum is used to crank its electric motors, which slows the car while recharging the onboard batteries.
This brilliantly simple system is part of why hybrid cars are so fuel efficient in stop-and-go traffic. But hybrids have their downsides… For one, a modern hybrid's batteries only last 8-10 years on average, and they are extremely expensive to replace, on the order of $3000-7000. Battery disposal is also a sticky problem, since Nickel-Metal Hydride batteries contain hazardous chemicals. In addition, although hybrids save some weight by including a smaller gas engine, they add it all back by including the heavy electrical components: two electric motors and the batteries. Could there be a way of usefully recapturing a car's kinetic energy on deceleration without adding so much weight, and without the expense and environmental impact of batteries? What about a wind-up automobile?
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