Compressed Hydrogen – Pure hydrogen can be reacted with oxygen in the air to produce energy and water, making this method very clean. If compressed, hydrogen can also achieve a similar energy density to gasoline, as well as fueling quickly. However, as with CNG, infrastructure is severely lacking. Although the use of hydrogen is carbon-neutral, the production of pure hydrogen is very energy-intensive, and as of now, much of that energy is produced by coal burning power plants and other heavy polluters.
Compressed Air or Fluid – If sufficiently compressed, gas or fluid of any kind can be used to store energy as a result of its pressure, which can be used to drive machinery. This “fuel” could be produced anywhere, in any sufficiently powerful compressor, not just at specialized plants. Compressing a tank would be a rapid process. However, even heavily compressed gases and fluids do not contain nearly as much energy per kilogram as gasoline. Increasing energy density is a simple matter of increasing pressure, but this results in more expense, as increasingly resilient containers, compressors, tubing, and machinery must be constructed. It is also dangerous in case of an explosion, although all hydrocarbons carry a risk of fire and explosion as well.
Flywheels – Much like compressed gas and fluid, energy can also be stored mechanically by spinning a heavy wheel, known as a flywheel. This method achieves moderate energy density, much like compressed air or fluid, and offers similarly rapid energy storage and release, as well as the ability to charge anywhere. Flywheels lose energy over time as a result of friction between moving parts and with air. This can be partially alleviated by using magnetic bearings and having the flywheel spin in a partial vacuum, but this significantly adds expense.
A flywheel used in Formula 1 cars.
Capacitors – Unlike conventional batteries, capacitors have extremely high charge and discharge rates; these are the type of “batteries” used in computer components operating at high speeds. They do not, however, offer a high energy density. Even conventional batteries store more energy per kilogram than do capacitors.
A diagram of a basic capacitor. Sufficiently powerful capacitors are much more complex.
Solar – Experimental vehicles have been built that run entirely from solar panels on the vehicle, but even if it is completely covered in solar panels, the amount of energy generated is not enough to power the vehicle at any reasonable speed. The cost of producing large solar panels is high for the amount of energy that would be obtained. Solar energy is free but highly unpredictable, and would need to be used alongside other methods in order to produce a practical vehicle.
An experimental vehicle that runs on solar power.
