Energy storage is having energy in reserve for future needs. While it takes millions of years to create huge energy stores in the form of coal, oil, natural gas, and uranium, humans are able to quickly manufacture energy stores such as batteries, and to build dams to store the gravitational energy of water. Although batteries and dams are very useful and important storage systems, the amount stored is very small compared to natural stores. These natural energy stores are large, but finite, and it is important to realize that they will not last forever.
The food a person eats is a personal energy store banking on photosynthesis. In photosynthesis, gaseous carbon dioxide, liquid water, and solar energy interact to produce solid carbohydrates, such as sugar and starch, and gaseous oxygen.
Solar energy for photosynthesis is converted and stored in the molecular bonds of carbohydrate molecules. When the process is reversed by combining carbohydrates with oxygen, stored energy is released. We literally see and feel the results when wood is burned to produce heat. A flame is not produced when carbohydrates and oxygen interact in the human body, but the process is similar and appropriately called "burning." The energy we derive from food for performing our daily activities comes from burning carbohydrates.
About 80 percent of the electric energy used in the United States is derived from stored energy in coal. The stored energy has its origin in photosynthesis. Coal is the end product of the accumulation of plant matter in an oxygen-deficient environment where burning is thwarted. Formation takes millions of years. Proven reserves of coal in the United States are upwards of 500 billion tons, a reserve so great that even if coal continues to be burned at a rate of over one billion tons per year, the reserves will last for hundreds of years.
Stores of petroleum, from which gasoline is derived, are vital to industry and the transportation sector. Stores of natural gas are important to industry and the residential and commercial sectors. Both petroleum and natural gas result from the decay of animal and plant life in an oxygen-deficient environment. Solar radiation is the source of the energy stored in the molecules making up all the fossil fuels (i.e., coal, petroleum, and natural gas). The energy is released when petroleum and natural gas are burned. Economic considerations figure prominently in determining proven reserves of petroleum and natural gas. To be proven, a reserve must provide an economically competitive product. There may be a substantial quantity of an energy product in the ground, but if it cannot be recovered and sold at a profit, it is not proven. There are substantive quantities of energy stored as oil in shale deposits in the western United States, but, to date, extracting them are not economical. Proven reserves of oil and natural gas for the United States are around thirty-five billion barrels and 250 trillion cubic feet, respectively. Consumption rates of about three billion barrels per year and twenty trillion cubic feet per year suggest that they will be depleted in the early part of the twenty-first century. But economic conditions can change and new proven reserves will emerge.
For several decades, the United States has relied on foreign sources of petroleum energy. The availability of petroleum from foreign sources is subject to political instability and changes in the economies in these countries. To guard against losses in petroleum supplies, the United States has established the Strategic Petroleum Reserve. The reserve is an emergency supply of crude oil stored in huge underground salt caverns along the coastline of the Gulf of Mexico. Upwards of one billion barrels of petroleum are stored in six sites along the coasts of Texas and Louisiana.
A battery stores electric energy. Although the concentration of energy is small compared, for example to gasoline, we see a myriad of uses of batteries in radios, cellular phones, flashlights, computers, watches, and so on. The public's demand for these portable products is ever increasing, and scientists strive to develop lighter and better batteries.
An electric power plant generates huge amounts of electric energy, but it does this only on demand by consumers because there is no economical way to store electric energy. Electric power plants are massive units that are difficult to shut down and expensive to start up. At night when consumer demand is low, electric utilities are willing to reduce the cost of electric energy in order to keep the plants operating. During the night, it is often economical to use electricity to run pumps that move water to an elevated position. Then when demand for electricity is high, the water is released to run turbogenerators at a lower level. Such a system is called a pumped storage unit. There are fourteen of these in the United States, each having an electrical output of more than 240 megawatts, which is comparable to a large electric power plant.
A nuclear power plant converts energy stored in uranium and plutonium. Uranium occurs naturally; plutonium is made through nuclear transmutations. The nucleus of a uranium atom consists of protons and neutrons that are bound together by a nuclear force. The stored energy in a uranium nucleus is associated with nuclear forces holding the nucleus together. Energy is released in a nuclear reactor by nuclear (fission) reactions initiated by interaction of neutrons with uranium nuclei. Typically, a single reaction produces two other nuclei and three neutrons. Schematically
Importantly, the neutrons and protons in the reacting products (n + U) are rearranged into lower energy configurations in the reaction products (X + Y + 3n), and stored energy is released. A single nuclear fission reaction releases upwards of 10,000,000 times more energy than a chemical reaction in the burning of coal. Whereas a coal-burning electric power plant producing 1,000 megawatts of electric power must burn some 10,000 tons of coal per day, a nuclear power plant producing comparable power will need an initial loading of around a hundred tons of uranium of which about twenty-five tons are changed each year.
Water freezes and ice melts at 0°C (32°F). Melting requires 334 kilojoules (kJ) of energy for each kilogram (kg) of ice turned to water. The 334 kJ of energy are stored in the one kg of water. The 334 kj of energy must be removed from the one kg of water in order to convert the water back to ice. If heat is added to water at temperatures between the freezing point (0°C ) and boiling point (100°C), the temperature of the water increases 1°C for every 4200 J added to a kilogram of water. The 4,200 kJ/kg °C is called "the specific heat" of water. The specific heat is a property of all materials. Importantly, the specific heat is substantially larger for water than for nearly all other substances in nature. Heat transferred to a substance to increase the temperature is called "sensible heat." The energy added stays within the substance making the substance a reservoir of thermal energy. Because of its high specific heat, water is a superb coolant for an automobile engine or steam turbine. Fortunately, water is cheap and usually available. In a climate where the coolant in an automobile engine might freeze, antifreezes keep this from happening by lowering the freezing point of the coolant.
Solar energy systems are very popular among many for environmental reasons, and people are even willing to pay more for this "clean" energy. The biggest problem with these sources is that the time when energy is available does not necessarily match when it is needed. One way that solar system designers address this problem is by incorporating a thermal storage system. A solar-heated house or commercial building must have a thermal energy store to provide heat at night or on days when clouds block solar radiation. Solar heating systems using roof-top collectors often use water or rocks to store thermal energy. Some solar houses employ large, south-facing windows to maximize the amount of solar energy entering the house. The solar energy may warm a brick or concrete wall or floor so that it becomes a thermal energy store that provides heat when needed at night.
Priest, J. (2000). Energy: Principles, Problems, Alternatives, 5th ed. Dubuque, IA: Kendall/Hunt Publishing.
stor·age / ˈstôrij/ • n. the action or method of storing something for future use: the chair can be folded flat for easy storage | [as adj.] the room lacked storage space. ∎ the retention of retrievable data on a computer or other electronic system; memory. ∎ space available for storing something, esp. allocated space in a warehouse: Cooper had put much of the furniture into storage. ∎ the cost of storing something in a warehouse.