into heat and blown out of the radiator!) And what about the goal of
getting off fossil fuels?
In this section, we’ll discuss five technologies: regenerative braking;
hybrid cars; electric cars; hydrogen-powered cars; and compressed-air cars.
There are four ways to capture energy as a vehicle slows down.
- An electric generator coupled to the wheels can charge up an electric
battery or supercapacitor.
- Hydraulic motors driven by the wheels can make compressed air,
stored in a small canister.
- Energy can be stored in a flywheel.
- Braking energy can be stored as gravitational energy by driving the
vehicle up a ramp whenever you want to slow down. This gravitational
energy storage option is rather inflexible, since there must
be a ramp in the right place. It’s an option that’s most useful for
trains, and it is illustrated by the London Underground’s Victoria
line, which has hump-back stations. Each station is at the top of a
hill in the track. Arriving trains are automatically slowed down by
the hill, and departing trains are accelerated as they go down the far
side of the hill. The hump-back-station design provides an energy
saving of 5% and makes the trains run 9% faster.
Electric regenerative braking (using a battery to store the energy) salvages
roughly 50% of the car’s energy in a braking event, leading to perhaps
a 20% reduction in the energy cost of city driving.