‘Energy storage‘ can refer to either the process of saving some form of energy for later use, or the physical infrastructure required to do so. Energy is critical to every accomplishment of humans. This energy can take many forms. If you are interested in only specific forms of energy, you can click on a section of this document by viewing the table of contents to the left.
Energy storage is very useful for the stable function of a power grid. Inevitably, power stations have to be serviced or have failures. Even failures that are a fraction of a second can be disastrous for certain applications, such as monitoring systems for heavy machinery, computers, and other sensitive electronics. Stability of power is a key factor in the design of an effective power grid. Stability requires that there be backup systems in place in case of a baseload power failure. Energy storage is required for most backup systems to be effective.
It is not feasible to ramp up a coal or nuclear power plant to meet demands in a matter of minutes if the situation arises. They are designed for constant output, not for meeting the exact current demand. Hydroelectric power is much better suited for quick adjustment to energy demands. The energy storage ability of hydroelectric dams depends on the amount and height of water contained on the reservoir side of the dam. The water has stored energy because gravity is trying to pull it down. As the water falls, we can extract energy out of it by having it spin a turbine to create power. This is the same concept as water falling on one side of a water wheel, causing it to turn.
Notable Forms of Energy Storage
Water raised to a height is known as hydrostatic energy storage. It is the most common form of mass energy storage in the world. This is due to the fact that compared to other forms of energy storage, it is widely available and easily utilized. This is one of the major reasons that humans construct many hydroelectric dams so that large reservoirs form on one side of them. The reservoir represents a huge amount of water that is waiting to be used for energy production. Unfortunately, hydrostatic energy storage is only feasible in areas that have changes in ground elevation coupled with large flows of water. It is thus difficult to build large-scale hydrostatic energy storage on the prairies.
Pumped Hydro Storage
Especially useful are natural ground formations that can hold water at a higher altitude than another nearby body of water. During times of high demand, water can be run down to create electricity. During times of low demand, when electricity is being cheaply produced from baseload sources, water can be pumped from the lower body of water to the higher body of water. This action would store energy by raising the water to a higher height.
Compressed air is a form of energy storage that is starting to receive more notice. It is beginning to look feasible for both large scale and small scale systems. On the small scale, a house could power itself off a compressed air tank for a certain amount of time. On a large scale, deep wells into the ocean or geological cavities can provide ways to create enormous pressures of air in a stable environment. This technology has existed for a while but has never been used for large scale energy storage. Drawbacks include the possible remoteness of these well pipes, the newness of the technology on a large scale, and construction cost.
This is currently the most widely used form of energy storage for small to medium scale systems. This includes gasoline, jet fuel, hydrogen and food. Chemical storage in general has extremely high energy density. This means that a large amount of energy can be stored in a small amount of space, and with a small amount of mass. Despite their drawbacks, hydrocarbons are still incontestably the best mobile energy storage we have right now. We know how to store and use them well, as well as having the infrastructure in place to do these things. The fact that they release carbon dioxide when burnt would not be important if they were part of a carbon-neutral cycle. For instance, creating hydrocarbons out of plants that have just been grown would in theory be a net zero in terms of CO2.
Nuclear energy is naturally stored. We need to bring nuclear active materials close together in order to cause an increase in the rate of their natural energy emission. The more densely they are packed, the more ‘active’ they become. Incredible safety measures are evident in nuclear power plants because of this.
Some current and future reactor designs have the ability to operate from 50-100% capacity, effectively being able to operate as peaking generators.
Anything that is in motion is mechanically stored energy. This is technically kinetic energy storage. A flywheel is a form of mechanical energy storage. Also the stretching of a solid is a common form of energy storage. Certain types of catapults in the middle ages, as well as slingshots and bows demonstrate this type of energy storage.
Hydrocarbons are the dominant method of small-scale energy storage for several reasons. First of all they have a very high energy density. This means that they can store the most energy per volume of all the storage methods other than nuclear. A general comparison is that one litre of gasoline can do more work than 50 pairs of human hands working for 24 hours. Hydrocarbons are easy to utilize because we historically have used them extensively.
Current hydrogren storage systems have a low energy density so far. This means that it is currently less suitable for transportation needs than hydrocarbons, but will function well for static use. The main problem with hydrogen is low conversion efficiencies, leading to expense. Hydrogen is at a disadvantage when compared to hydrocarbons because it is energy intensive to create, problematic to store, and sports a low energy density per volume. An enormous advantage to hydrogen is the fact that its combustion with oxygen produces only water. This means that hydrogen might be the ultimate fuel in many respects since its by-products are non-existent or extremely benign. The Hydrogen economy ideas have been worked on for many years, but no suitable method for storing hydrogen has been presented. If we cannot store hydrogen effectively, then we cannot use it for small-scale applications extensively because many of these applications involve mobility. If we need to haul a huge hydrogen tank everywhere we go with our car, it might not be as environmentally friendly as we would like.
Batteries are also a very popular form of small scale energy storage for low-energy demand applications. Many forms of consumer electronics rely on batteries. Laptops, cell phones, PDAs, MP3 players and automobiles all use batteries. In general batteries are good for systems that need well controlled but small amounts of electrical energy. They do not have the energy density of hydrocarbons, but their energy is designed to be converted directly into electricity, which is not usually the case with hydrocarbons (which are usually used mainly for mechanical energy in small scale applications).
Capacitors are very fast charging, but their energy density is generally accepted as being too low for widespread use. New capacitor designs are being built that have energy density on the same order of magnitude as batteries. This is a significant technical advancement since capacitors can usually be charged and discharged millions of times with no damage to thier function, unlike batteries.