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.

Regenerative braking

There are four ways to capture energy as a vehicle slows down.

  1. An electric generator coupled to the wheels can charge up an electric
    battery or supercapacitor.
  2. Hydraulic motors driven by the wheels can make compressed air,
    stored in a small canister.
  3. Energy can be stored in a flywheel.
  4. 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.

Figure 20.17. A BMW 530i modified by Artemis Intelligent Power to use digital hydraulics. Lower left: A 6-litre accumulator (the red canister), capable of storing about 0.05 kWh of energy in compressed nitrogen. Lower right: Two 200 kW hydraulic motors, one for each rear wheel, which both accelerate and decelerate the car. The car is still powered by its standard 190 kW petrol engine, but thanks to the digital hydraulic transmission and regenerative braking, it uses 30% less fuel.