If cars are linked to electric power systems and run by frequent recharges, capacitors will gain in importance. Unlike trains, when cars are driven by electricity, they cannot be charged by using overhead wires stretched along roads. And they cannot carry around enough energy source to run 400-500km, unlike gasoline-fueled vehicles.
So, we have to build an infrastructure that can frequently recharge cars with a small amount of energy at a time. As for cars, all they need are devices for quick recharge and discharge.
Then, which is a better energy source for plug-in hybrid vehicles, secondary battery or capacitor? I would go with capacitor because it has a high current capability and can be quickly charged. The discharging rates of secondary batteries are high, but it takes time to charge them.
For example, in the case of plug-in hybrid vehicles with secondary batteries, they need to be charged for one to two hours at home. Though some people say the charging time will be shortened to about 15 to 20 minutes in the near future, it's still long. In this respect, capacitors can be charged in about 30 seconds.
However, we should mount a small number of capacitors because it would take long to charge many of them.
There are more useful characteristics of capacitors. First, it has a long operating life because no chemical reaction occurs when it is being charged or discharged. Second, the remaining energy in a capacitor can be accurately known by checking its terminal voltage. Third, containing no heavy metal, it is more environmentally friendly. And there are more advantages of capacitor.
The energy density of capacitor is still lower than, for example, that of Li-ion secondary battery but may catch up with it due to technological advances in the future.
In my laboratory, we are developing one-seater electric vehicle "C-COMS," which runs only with electric double layer capacitors, as part of our research on future cars.