Wednesday, October 21, 2009

Energy storage

Energy storage is the storing of some form of energy that can be drawn upon at a later time to perform some useful operation.
Energy storage media are matter that store some form of energy that can be drawn upon at a later time to perform some useful operation. A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (eg. chemical, gravitational or electrical energy) or kinetic energy (eg. thermal energy). A wind up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to keep a clock chip in a computer running (electrically) even when the computer is turned off, and a hydroelectric dam stores power in a reservoir as gravitational potential energy. Ice storage tanks store ice (thermal energy)at night to meet peak demand for cooling. Fossil fuels such as coal and gasoline store ancient energy from sunlight. Even food (which is made by the same process as was fossil fuel) is a form of energy stored in chemical form.

Types of energy storage

History of energy storage

 

Energy storage as a natural process is as old as the universe itself - the energy present at the initial creation of the Universe has been stored in stars such as the Sun, and is now being used by humans directly (e.g. through solar heating), or indirectly (e.g. by growing crops or conversion into electricity in solar cells). Storing energy allows humans to balance the supply and demand of energy. Energy storage systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, biological, thermal and nuclear. As a purposeful activity, energy storage has existed since pre-history, though it was often not explicitly recognized as such. An example of deliberate mechanical energy storage is the use of logs or boulders as defensive measures in ancient forts - the logs or boulders were collected at the top of a hill or wall, and the energy thus stored used to attack invaders who came within range. A more recent application is the control of waterways to drive water mills for processing grain or powering machinery. Complex systems of reservoirs and dams were constructed to store and release water (and the potential energy it contained) when required. Energy storage became a dominant factor in economic development with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural gas in the late 1800s. Unlike other common energy storage used in prior use, such as wood or coal, electricity has been used as it has been generated. It has not been stored on a major scale but that may soon change. In the U.S, the 2009 Stimulus plan is researching energy storage and how it may be used with the new plans for a Smart Grid. [1]. Electricity is transmitted in a closed circuit, and for essentially any practical purpose cannot be stored as electrical energy. This means that changes in demand could not be accommodated without either cutting supplies (as by brownouts or blackouts) or by storing the electric energy in another medium. Even renewable energy must be stored in order to make it reliable. Wind blows intermittently and so some form of storage is required to compensate for calm periods, and solar energy is not effective on cloudy days so stored energy must be available to compensate for the loss of sun energy. An early solution to the problem of storing energy for electrical purposes was the development of the battery, an electrochemical storage device. It has been of limited use in electric power systems due to small capacity and high cost. A similar possible solution with the same type of problems is the capacitor. In the 1980s, a small number of manufacturers carefully researched thermal energy storage (TES) to meet the growing demand for air-conditioning during peak hours. Today a few companies continue to manufacture TES. [2] The most popular form of thermal energy storage for cooling is ice storage, since it can store more energy in less space than water storage and it is also cheaper than fuel cells & flywheels. Thermal storage has shifted jiggawatts of power away from daytime peaks, cost-effectively, and is used in over 3,300 buildings in over 35 countries. It works by storing ice at night when electricity is cheap, and then using the ice to cool the air in the building the next day. Chemical fuels have become the dominant form of energy storage, both in electrical generation and energy transportation. Chemical fuels in common use are processed coal, gasoline, diesel fuel, natural gas, liquefied petroleum gas (LPG), propane, butane, ethanol, biodiesel and hydrogen. All of these materials are readily converted to mechanical energy and then to electrical energy using heat engines (turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical power generation. Heat-engine-powered generators are nearly universal, ranging from small engines producing only a few kilowatts to utility-scale generators with ratings up to 800 megawatts. Electrochemical devices called fuel cells were invented about the same time as the battery. However, for many reasons, fuel cells were not well-developed until the advent of manned spaceflight (the Gemini Program) when lightweight, non-thermal (and therefore efficient) sources of electricity were required in spacecraft. Fuel cell development has increased in recent years due to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity. At this time, liquid hydrocarbon fuels are the dominant forms of energy storage for use in transportation. However, these produce greenhouse gases when used to power cars, trucks, trains, ships and aircraft. Carbon-free energy carriers, such as hydrogen, or carbon-neutral energy carriers, such as some forms of ethanol or biodiesel, are being sought in response to concerns about the possible consequences of greenhouse gas emissions. Some areas of the world (Washington and Oregon in the USA, and Wales in the United Kingdom are examples) have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up to the reservoirs, then letting the water fall through turbine generators to retrieve the energy when demand peaks. Several other technologies have also been investigated, such as flywheels or compressed air storage in underground caverns, but to date no widely available solution to the challenge of mass energy storage has been deployed commercially.

General energy storage concepts

1. Using a high density magnetic field. Energy E in terms of magnetic field strength B is
E = \frac{1}{2}B^2 = \frac{1}{2}Li^2
where L is the inductance of the electromagnet and i is the current.
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