From 2GreenEnergy Intern Fabio Porcu: An Overview of Energy Storage
Energy storage consists of a series of techniques and processes that allow one to concentrate different forms of energy to be used later; thus devices that store energy are sometimes called “accumulators.”
Consider for a moment the concepts of potential, kinetic and chemical energy. A clock spring stores potential energy (mechanical in this case, in the tension of the spring), a battery accumulates chemical energy, and a dam of a hydroelectric installation stores potential energy by exploiting the height. The tanks of ice accumulation store ice (thermal energy) at night to meet the requirements of maximum cooling. Fossil fuels such as coal and gasoline preserve ancient energy from the sun. Even the food we eat (which is made with the same process by which fossil fuels were derived) is a type of energy stored in chemical form.
The storage of energy in the electricity grid allows power producers to transfer excess electrical energy from the point of power generation to places of temporary storage. In this way they become energy providers when the electricity demand is greater. Energy storage networks are particularly important to harmonize the supply and demand during 24-hour time intervals.
There are many types of energy storage, each taking advantage of different physical principles; consequently they have very different characteristics. The best known use the following storage principles:
Chemical
The main storage system that uses chemical principles are the production of hydrogen, bio-coal and methane.
Hydrogen can be used to produce electrical energy in fuel cells, or it can be used as a traditional gaseous fuel, for direct combustion. Hydrogen is in fact an excellent fuel and burns with oxygen. The product of this combustion is water, in the form of water vapor. This methodology of exploitation allows the use of hydrogen in thermal power plants virtually identical to those fueled by methane, whether they are traditional thermoelectric power plants or combined cycle.
The production of hydrogen requires large amounts of energy. You can use the energy excess in the hours of low demand to produce hydrogen, and then use the hydrogen for energy production when demand is high.
Various fuels such as biodiesel, vegetable oil, petrol-methane mixture, or biomass can be used to replace hydrocarbons. Various chemical processes are able to convert the bonds between the carbon and hydrogen atoms in coal. Natural gas, biomass plants and animals, and organic waste can be transformed into short-chain hydrocarbons that are suitable to replace the existing hydrocarbons. Examples are Fischer-Tropsch diesel, methane, dimethylether, or syngas. This source of diesel was used extensively in World War II in Germany, which had limited access to sources of crude oil, since the Allies were bombing German oil refineries, largely with B-17s piloted by Craig’s father, J Craig Shields, Jr. (pictured above with wife Patricia) and thousands of other brave soldiers acting in the same capacity.
Methane is the simplest hydrocarbon with molecular formula CH4, and can be produced with electricity from renewable sources. Methane is easier to store than hydrogen and to transport; in addition, storage and combustion are mature infrastructure (pipelines, gas tanks, power plants).
While hydrogen and oxygen are produced in the electrolysis of water 2H2O → 2H2 + O2, hydrogen can then be reacted with carbon dioxide in the process Sabatier, producing methane and water. Methane can then be stored and subsequently used to produce electricity.
Mechanical
The potential energy in the water can be pumped to a higher elevation using the methods of accumulation of energy by pumping, in the compressed air, or in the flywheels.
The mass of a kilogram raised to 1000 meters can store energy in an amount of 9.81 kJ. This is equivalent to 1 kg mass moving at a velocity of 140 m/s. The temperature of 1 kg of water may be increased by 2.34 degrees Celsius using the same quantity of energy. Certainly, this is a strange comparison, but that makes it easy to see how it is possible to store more energy in 1 m³ of rock or sand than in 1 m³ of lead acid battery.
Compressed air technology in an underground reservoir takes advantage of the accumulation of energy at low cost in hours of limited demand. This is then released during the hours of peak demand and heated with the exhaust gases of a combustion turbine. This air heated is converted into electrical energy by expansion turbines to produce electricity.
An energy storage power plant based on compressed air in McIntosh, Alabama has been working successfully since 1991. Other similar applications of compressed gases are possible as well. Walker Architects study published in October 2008 the first application of gas CO2 and proposed for use for energy storage. Several companies have carried out preliminary work to design vehicles employing energy from compressed air.
Thermal
Heat accumulators store excess heat energy created by a generator and is unable to return it to the user at any time is requested.
The easiest way to accumulate thermal energy is through the heating (or cooling) of a solid body or a liquid, or, rarely, of a gas. The capacity of accumulation of thermal energy by a material is expressed by its specific heat; in case the substance used to store the heat is affected by a phase change (for example, evaporating water, passing from the liquid state to the gaseous state), the capacity of accumulation increases for the contribution given by the latent heat. The most common substance used in plant engineering for accumulating thermal energy is water. Among the latent heat accumulators are lithium fluoride, but for economic reasons their use is generally limited to navigation in space.
Electrochemistry
A battery is a device that converts chemical energy into electrical energy.
Batteries that are not rechargeable are called primary batteries, distinguished from rechargeable batteries, or secondary batteries. A set of more batteries arranged in series is called a battery pack.
The chemical-physical principle of operation of a battery is a redox reaction that takes place inside it, in which a certain substance undergoes a process of oxidation, losing electrons, while another substance undergoes a process of reduction, gaining them. Given its configuration, batteries exploit the flow of electrons between two different substances. Such flow generates a continuous electrical current, whose electric potential is a function of the reactions of oxidation and reduction occurring there. A battery is fully discharged when these chemical reactions reach the equilibrium state. Generally, batteries considered as systems with high energy density, but low power densities, contrary to the supercapacitors.
Every kind of storage system has is strengths and is weaknesses and those will be analyzed individually in future articles, in such a way to give the reader the possibility of a more detailed point of view about new technology being developed in this field.
