Ram Pumps are a great example of compressed air storage in action. Though it only compresses the air for a few moments at a time, it makes great use of this energy by pushing water up a potentially large distance. Since they use the energy of water running downhill to pressurize the tank, it doesn't need electricity to run.
Gravitricity works with a similar and much older principle that you may be familiar with that used to be used in Grandfather Clocks and old tower clocks known as a weighted drive mechanism. It's where a weight is pulled up in order to act as a winding mechanism to spin gears. As the gears spin, the weight slowly falls and the spinning of the weights turns turbines generating electricity (or in the case of grandfather clocks just moving the hands of the clock).
Thermal storage, usually molten salt
Thermal storage takes heat and stores it in a very well insulated area for use at a later time. Molten salt is a great conductor of heat and is the chosen medium in many cases these days.
Compressed Air/Gas Storage
Compressed air storage uses the pressure built up in a tank to propel turbine blades to create electricity. Some systems use the pressure of water to increase the pressure and the more force is the more energy that can be created. If a substance such as hydrogen is used in conjuncture with compressed air storage, then the pressure itself can be used to turn a turbine while the gas can be pushed into another chamber and lit to turn yet another turbine increasing the overall productivity of the compressed gas.
Hydro (water) Storage
There are two ways Hydro Storage is utilized. The first one is one that you see nearly every day and have probably never realized. That is in your local water towers. The reason for this is that the demand for water has "peak" times of usage, such as right after dinner time, and other times where the water is used very little, such as late at night. What happens is, they keep a smaller number of water pumps going more often than having to have enough pumps to meet the maximum need. In this system, if everyone used the peak time amount of water all the time it would run out. Since this is only the amount for part of the day, it's more efficient to use less numbers of pumps and store the water at a higher level so gravity will help do part of the work during these times.
The second way Hydro Storage is utilized is in direct generation of electrical energy. What happens here is that the water flows down (usually from a hill) and turns a generator creating electricity. In fact, in recent times, some water companies have been combining these methods in order to get electricity back out while they are delivering water.
Flywheel Energy Storage
Flywheel energy storage uses kinetic energy for short term storage. It utilizes minimal friction with maximum revolutions per minute (upward of thousands per minute), often within a vacuum, and uses a heavy cylinder, and is suspended by very strong magnets. It uses an electric motor to generate these speeds, then when the energy is needed (minus the amount wasted over time), turns the motor into a generator which slows down the cylinder over time depending on the resistance from the motor/generator.
Though this is an interesting concept, it's probably something better used in the zero gravity of space in order to minimize the energy loss.
I'm sure you know what a battery is, but did you know that most modern day batteries are toxic? Of these commonly used batteries, lithium batteries (the kind used in cell phones and most rechargeable batteries) are the most dangerous as they can potentially explode! While on the safer side and far less toxic than the other batteries is standard alkaline rechargeable batteries, though they are rather expensive and still not something you should just toss in the trash.
Below are a few new types of batteries we are looking into.
Salt Water Batteries
Iron-Nickel or "Edison" Battery
Iron Flow Batteries
Similar to the iron flow or edison batteries, but smaller is the "Water Battery" which simply makes use of 2 kinds of metal with plain water as the electrolyte.
Zinc Bromine Battery
Super capacitors are made with graphene and can hold a much higher charge than typical capacitors. Like traditional capacitors, they can only hold a charge for a limited time. Another big difference between a typical capacitor and a super capacitor is that while a regular capacitor loses its charge after only a few minutes, a super capacitor can hold its charge for up to a few weeks.
A super capacitor can't compete with the energy storage capacity or life span of a battery, but can last for decades and doesn't diminish in capacity no matter how many times it's charged and depleted, making it a great energy storage device for renewable energies where storage is only needed for a few hours to a few days at a time.
When we think of energy storage we tend to think of batteries, but there are several types of storage other than just batteries. There are methods of storing energy from short term storage only lasting a few hours to long term storage lasting for years. There are methods of storing energy that utilize gravity while other methods utilize heat storage and more. There are even some methods that use air pressure!
Not all energy storage used is storing electrical energy. In fact, many of these forms of storage are storing mechanical energy that later gets converted back into the electrical energy that was used to store it in the first place.
Below are several methods of energy storage with only a few of them utilizing electrical energy.
Weighted Drive Mechanism