Why are capacitors not used to store electricity?

Answer 1

Short Answer:

If capacitor technology permitted capacitors to be a large scale source of energy, it would transform the way energy is produced and used. Capacitors are not used because they can not conveniently store large quantities of energy. Batteries, as weak and heavy and inefficient as they are, remain far more practical at this time. Hundreds of millions of dollars are being invested in various battery and capacitor research areas to mitigate this significant technological barrier. The future technology is currently expected to be "super capacitors" and the goal is that these will viable as a source/supply/store of electrical energy.

Long Answer:

Capacitors are, of course, used as a source of electricity billions of times every day in millions of electronic devices. (Every time a capacitor is charged, it stores energy and every time it discharges it provides electrical energy.) What the real question is has to do with the quantity of electrical energy they can store and the relative benefits to batteries.

Lets us address some of the issues of capacitors and batteries and energy.

1. You don't make energy with capacitors and batteries. Capacitors, like batteries, are capable of storing energy, not creating it or generating it from another source. Once one has some source (e.g. hydro electric, solar panel, peddling a stationary bicycle) one can use the source to store energy in a battery or capacitor. After that, they become the source for some other energy use.
2. Batteries and capacitors are DC, i.e. direct current devices. Most electrical energy use is from AC devices. Fortunately, many energy sources, are either naturally, e.g. solar power, or easily adapted to supply DC for storage applications. But, DC is often converted to AC by an inverter, to supply power to a device and there is energy loss in the process.
3. There are natural ranges to voltage and current for various technologies. Batteries naturally operate at voltage levels determined by chemistry, i.e. a few volts. Getting large voltages from batteries requires a design where batteries are connected in series; not a problem to get 12 volts, but if one needed 12,000 volts, that becomes a design issue. Capacitors easily achieve thousand or tens of thousands of volts because this is electrostatics, not chemistry.
4. Batteries are slow (not withstanding the sparks one gets hooking up the car battery) and capacitors are fast. Depending on the battery, times range from minutes to hours for a charge and discharge cycle. Capacitors charge and discharge is a fraction of a second.
5. Safety is an issue. Since capacitors tend to have high voltages and can discharge quickly, damage due to heating is easier with capacitors than batteries. More importantly, the danger of electrical shock with potentially fatal results exists with high voltage and high energy and capacitors are widely known to be able to provide a serious electrical shock.
6. The main issue is energy density. Oil or gasoline contains just over 40 million Joules of energy per kilogram (or 30 million Joules per liter). Good batteries hold under 2 million Joules per kilogram. Lead acid batteries are ten times worse. The standard capacitor one can easily purchase holds a tiny amount of energy, but a Joule of energy at 100 volts is not difficult to obtain. There is a huge range, but only exotic capacitors can approach a tenth of the energy density of a lead acid battery.
7. Batteries wear out and capacitors do not. The typical computer batter can be recharged about a thousand times, then you buy a new one. Capacitors can be charged billions of times with no apparent degradation. (Batteries have a fixed maximum voltage and capacitors have a maximum as well called the breakdown voltage. The breakdown voltage results in a charge leak and is the typical avenue of failure. It is normal for capacitors to last for decades.)
8. Super capacitors are evolving. It is hoped that super capacitors can far exceed batteries in electrical energy storage density. Currently, they are many times better than traditional capacitors but barely even as good as a lead acid battery when compared to batteries. There are many candidate types of super capacitors. This is a new technology (part of that 100 million dollar investment referenced above). These are more like capacitors than batteries, but they find ways to store charge other than merely sitting on a conducting surface. Unlike simple metal capacitors, super capacitors of some types do degrade with operation and so have a limited lifetime. There are many such technologies and some are more battery like than capacitor like and so there is no easy term other than "super capacitor."