Energy Storage
A creative Introduction about energy storage which is inevitable in the present world,one must know this
Content
ENERGY STORAGE
An example of artificial energy storage and conversion.
Energy storage is accomplished by devices or physical media that store energy to perform useful operation at a later time. A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (e.g. Chemical, gravitational, electrical energy, temperature differential, latent heat, etc.) or kinetic energy (e.g. momentum). Some technologies provide only short-term energy storage, and others can be very long-term such as po er to gas using hydrogen or methane and the storage of heat or cold bet een opposing seasons in deep a!uifers or bedrock. A ind-up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to operate a mobile phone, and a hydroelectric dam stores energy in a reservoir as gravitational potential energy. "ce storage tanks store ice (thermal energy in the form of latent heat) at night to meet peak demand for cooling. #ossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time ere then converted into these fuels. $ven food( hich is made by the same process as fossil fuels) is a form of energy stored in chemical form. $nergy storage as a natural process is as old as the universe itself - the energy present at the initial formation of the universe has been stored in stars such as the Sun, and is no being used by humans directly (e.g. through solar heating), or indirectly (e.g. by gro ing crops or conversion into electricity in solar cells).
As a purposeful activity, energy storage has existed since pre-history, though it as often not explicitly recogni%ed 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 ere collected at the top of a hill or all, and the energy thus stored used to attack invaders ho came ithin range.
A more recent application is the control of ater ays to drive ater mills for processing grain or po ering machinery. Complex systems of reservoirs and dams ere constructed to store and release ater (and the potential energy it contained) hen re!uired. Storing energy allo s humans to balance the supply and demand of energy. $nergy storage systems in commercial use today can be broadly categori%ed as mechanical, electrical, chemical, biological and thermal.
Storage for electricity
$nergy storage became a dominant factor in economic development ith the
idespread introduction of electricity. 'nlike other common energy storage in prior use such as ood or coal, electricity must be used as it is being generated, or converted immediately into another form of energy such as potential, kinetic or chemical. A very traditional ay doing this on large scales is pumped-storage hydroelectricity. #or example the pumped-storage hydroelectricity in (or ay has a capacity of )* +,, hich could be expanded to -* +,, hich ould be enough to be the battery of $urope../0 An early solution to the problem of storing energy for electrical purposes as the development of the battery as an electrochemical storage device. 1atteries have previously been of limited use in electric po er systems due to their relatively small capacity and high cost. 2o ever, since about the middle of the first decade of the 3/st century, ne er battery technologies have been developed that can no significant utility scale load-leveling capabilities4 .30 some of hich, as of 3*/), sho ed promise of being competitive ith alternative methods .)0(see also rechargeable battery). provide
Another solution to deal found in the capacitor.
ith the intermittency issue of solar and ind energy is
Some areas of the orld such as ,ashington and 5regon in the 'nited States, and ,ales in the 'nited 6ingdom, have used geographic features to store large !uantities of ater in elevated reservoirs, using excess electricity at times of lo demand to pump ater up to the reservoirs, then letting the ater pass through turbine generators to retrieve the energy hen electrical demands peak. .30 5ther possibilities to store electricity are7 fly heel, compressed air energy storage, hydrogen storage, thermal energy storage, po er to gas.
Short-term thermal storage, as heat or cold
"n the /89*s, a number of manufacturers carefully researched thermal energy storage (:$S) to meet the gro ing demand for air conditioning during peak hours. :oday, several companies manufacture :$S systems. :he most popular form of thermal energy storage for cooling is ice storage, since it can store more energy in less space than ater storage and it is also less costly than energy recovered via fuel cells or fly heels. :hermal storage has cost-effectively shifted giga atts of po er a ay from daytime peak usage periods, and in 3**8 as used in over ),)** buildings in over ); countries. "t orks by creating ice at night hen electricity is usually less costly, and then using the ice to cool the air in buildings during the hotter daytime periods. <atent heat can also be stored in technical phase change materials (=C>s), besides ice. :hese can for example be encapsulated in all and ceiling panels, to moderate room temperatures bet een daytime and nighttime.
Inter seasonal thermal storage, as heat or cold
Another class of thermal storage that has been developed since the /8?*s that is no fre!uently employed is seasonal thermal energy storage (S:$S). "t allo s heat or cold to be used even months after it as collected from aste energy or natural sources, even in an opposing season. :he thermal storage may be accomplished in
contained a!uifers, clusters of boreholes in geological substrates as diverse as sand or crystalline bedrock, in lined pits filled ith gravel and ater, or ater-filled mines. An example is Alberta, Canada@s Arake <anding Solar Community, for hich 8?B of the year-round heat is provided by solar-thermal collectors on the garage roofs, ith a borehole thermal energy store (1:$S) being the enabling technology. S:$S proCects often have paybacks in the D-- year range.
Energy storage in chemical fuels
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, li!uefied petroleum gas (<=+), propane, butane, ethanol and biodiesel. All of these materials are readily converted to mechanical energy and then to electrical energy using heat engines (via turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical po er generation. 2eat-engine-po ered generators are nearly universal, ranging from small engines producing only a fe kilo atts to utility-scale generators ith ratings up to 9** mega atts. A key disadvantage to hydrocarbon fuels are their significant emissions of greenhouse gases that contribute to global arming, as ell as other significant pollutants emitted by the dirtier fuel sources such as coal and gasoline. <i!uid hydrocarbon fuels are the most commonly used forms of energy storage for use in transportation, but because the byproducts of the reaction that utili%es these li!uid fuels@ energy (combustion) produce greenhouse gases other energy carriers like hydrogen can be used to avoid production of greenhouse gases.
Advanced systems
$lectrochemical devices called fuel cells ere invented about the same time as the battery in the /8th Century. 2o ever, for many reasons, fuel cells ere not elldeveloped until the advent of manned spaceflight (such as the +emini =rogram in the '.S.) hen light eight, non-thermal (and therefore efficient) sources of electricity ere re!uired in spacecraft. #uel 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. Several other technologies have also been investigated, such as fly heels, hich can store kinetic energy, and compressed air storage that can be pumped into underground caverns and abandoned mines..30.90 Another method used at the Solar =roCect and the Solar :res =o er :o er uses molten salt to store solar po er and then dispatch that po er as needed. :he system pumps molten salt through a to er heated by the sun@s rays. "nsulated containers store the hot salt solution, and hen needed ater is then used to create steam that is fed to turbines to generate electricity. Eesearch is being conducted on harnessing the !uantum effects of nano scale capacitors to create digital !uantum batteries. Although this technology is still in the experimental stage, it theoretically has the potential to provide dramatic increases in energy storage capacity..80./*0
+rid energy storage
+rid energy storage (or large-scale energy storage) lets energy producers send excess electricity over the electricity transmission grid to temporary electricity storage sites that subse!uently become energy suppliers hen electricity demand is greater. +rid energy storage is particularly important in matching supply and demand over a 3D hour period of time. A proposed variant of grid energy storage is called vehicle-to-grid energy storage system, here modern electric vehicles that are plugged into the energy grid can release the stored electrical energy in their batteries back into the grid hen needed.
Storage methods
Chemical 1io fuels
2ydrated salts 2ydrogen 2ydrogen peroxide <i!uid nitrogen =o er to gas Fanadium pent oxide 1iological +lycogen Starch $lectrochemical #lo battery
Eechargeable battery $lectrical Capacitor Super capacitor Superconducting magnetic energy storage (S>$S) >echanical Compressed air energy storage (CA$S) #ireless locomotive #ly heel energy storage
+ravitational potential energy (device) 2ydraulic accumulator 2ydroelectric energy storage Spring :hermal 1rick storage heater Cryogenic li!uid air or nitrogen $utectic system "ce Storage >olten salt Seasonal thermal energy storage Solar pond Steam accumulator :hermal energy storage (general)
Hydrogen
A chart depicting the durations and po er capabilities of various energy storage technologies, including po er to gas
2ydrogen is also being developed as an electrical po er storage medium. 2ydrogen is not a primary energy source, but a portable energy storage method, because it must first be manufactured by other energy sources in order to be used. 2o ever, as a storage medium, it may be a significant factor in using rene able energies. See hydrogen storage. ,ith intermittent rene able such as solar and into an electricity grid. At penetrations belo
needed0
ind, the output may be fed directly
3*B of the grid demand, this does not
severely change the economics4 but beyond about 3*B of the total demand .citation , external storage ill become important..//0 "f these sources are used for hen they cut in or electricity to make hydrogen, then they can be utili%ed fully henever they are available, opportunistically. 1roadly speaking, it does not matter out, the hydrogen is simply stored and used as re!uired. $nergy losses are involved in the hydrogen storage cycle of hydrogen production for vehicle applications ith electrolysis of ater, li!uification or compression, and conversion back to electricity../)0 and the hydrogen storage cycle of production for the stationary fuel cell applications like >icroC2= at 8) B./D0 ith biohydrogen or biological hydrogen production, and conversion to electricity. About ;* k,·h (/9* >G) of solar energy is re!uired to produce a kilogram of hydrogen, so the cost of the electricity clearly is crucial, even for hydrogen uses other than storage for electrical generation. At H*.*)Ik,h, common off-peak high-
voltage line rate in the 'nited States, this means hydrogen costs H/.;* a kilogram for the electricity, e!uivalent to H/.;* a '.S. gallon for gasoline if used in a fuel cell vehicle. 5ther costs ould include the electroly%er plant, hydrogen compressors or li!uefaction, storage and transportation, hich ill be significant.4 nderground hydrogen storage 'nderground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. <arge !uantities of gaseous hydrogen have been stored in underground caverns by "mperial Chemical "ndustries ("C") for many years ithout any difficulties =o er to gas is a technology hich converts electrical po er to a gas fuel. :here are 3 methods, the first is to use the electricity for ater splitting and inCect the resulting hydrogen into the natural gas grid. :he second less efficient method is used to convert carbon dioxide and ater to methane, (see natural gas) using electrolysis and the Sabatier reaction. :he excess po er or off peak po er generated by ind generators or solar arrays is then used for load balancing in the energy grid. 'sing the existing natural gas system for hydrogen #uel cell maker 2ydro genics and natural gas distributor $nbridge have teamed up to develop such a po er to gas system in Canada. =ipeline storage of hydrogen here a natural gas net ork is used for the storage of
hydrogen. 1efore s itching to natural gas, the +erman gas net orks ere operated using to n gas, hich for the most part consisted of hydrogen. :he storage capacity of the +erman natural gas net ork, hich contains also many so called caverns (artificial caves produced by mining), is more than 3**,*** +, ·h hich is enough for several months of energy re!uirement. 1y comparison, the capacity of all +erman pumped storage po er plants amounts to only about D* +, ·h. :he transport of energy through a gas net ork is done ith much less loss (J*./B) than in a po er net ork (9B) (besides 2igh-voltage direct current).
!iofuels
Farious bio fuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to replace hydrocarbon fuels. Farious chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal biomass, and organic astes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. $xamples are #ischer-:ropsch diesel, methanol, dimethyl ether, or syngas. :his diesel source as used extensively in ,orld ,ar "" in +ermany, ith limited access to crude oil supplies. :oday South Africa produces most of the country@s diesel from coal for similar reasons..330 A long term oil price above 'SH);Ibbl may make such synthetic li!uid fuels economical on a large scale (see coal). Some of the energy in the original source is lost in the conversion process. 2istorically, coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. Additionally, compressed natural gas is also used as fuel, for instance for buses ith some mass transit agencies. "ethane #SNG Synthetic Natural Gas$ >ethane is the simplest hydrocarbon ith the molecular formula C2 D. >ethane could be produced from electricity of rene able energies using po er to gas technologies.
.3)0
>ethane can be stored more easily than hydrogen and the transportation, storage
and combustion infrastructure are mature (pipelines, gaso meters, po er plants). As hydrogen and oxygen are produced in the electrolysis of ater, 3235 → 323 K 53 2ydrogen ould then be reacted ith carbon dioxide in Sabatier process, producing methane and ater. C53 K D23 → C2D K 3235 >ethane ould be stored and used to produce electricity later. =roduced ater pure
ould be recycled back to the electrolysis stage, reducing the need for ne
ater. "n the electrolysis stage oxygen ould also be stored for methane combustion in a pure oxygen environment in an adCacent po er plant, eliminating e.g. nitrogen oxides. "n the combustion of methane, carbon dioxide and ater are produced.
C2D K 353 → C53 K 3235 =roduced carbon dioxide ould be recycled back to boost the Sabatier process and ater ould be recycled back to the electrolysis stage. :he carbon dioxide produced ould by methane combustion ould be turned back to methane, thus producing no greenhouse gases. >ethane production, storage and adCacent combustion recycle all the reaction products, creating a lo carbon cycle.
:he C53 ould be a resource having economic value as a component of an energy storage vector, not a cost as in CCS (Carbon capture and storage).
Aluminum, %oron, silicon, and &inc
Aluminum 1oron silicon lithium, and %inc have been proposed as energy storage solutions.
"echanical storage
$nergy can be stored in ater pumped to a higher elevation using pumped storage methods, by moving solid matter to a higher location by compressing air, or by storing it in spinning fly heels. A mass of / kg, elevated to a height of /,*** m stores 8.9 kG of gravitational energy, hich is e!uivalent to / kg mass accelerated to /D* mIs. :o store the same mass of ater, if increased in temperature by 3.)D Celsius, re!uires the same amount of energy. Compressed air energy storage (CA$S) technology stores lo during peak load hours and heated cost off-peak energy,
in the form of compressed air in an underground reservoir. :he air is then released ith the exhaust heat of a standard combustion turbine. :his heated air is converted to energy through expansion turbines to produce electricity. A CA$S plant has been in operation in >c"ntosh, Alabama since /88/ and has run successfully. 5ther applications are possible. ,alker Architects published the first C53 gas CA$S application, proposing the use of se!uestered C53 for $nergy Storage. :he paper as Submitted to the Conoco =hilips $nergy
=ri%e April 3**9. :his paper precedes A<< others and in that paper ,alker defined the C53 energy storage cycle. Several proCects sponsored by the A5$ are no under ay to develop the body of technology pioneered by :erry <. ,alker in 3**9. Several companies have done preliminary design compressed air po er. ork for vehicles using
Thermal storage,
>ain articles7 :hermal energy storage and Seasonal thermal energy storage
Aistrict heating accumulation to er from :hesis near 6rems an der Aonauin <o er Austria ith a thermal capacity of 3 +,h
:hermal storage is the temporary storage or removal of heat for later use. An example of thermal storage is the storage of solar heat energy during the day to be used at a later time for heating at night. "n the 2FACIE field, this type of application using thermal storage for heating is less common than using thermal storage for cooling. An example of the storage of LcoldL heat removal for later use is ice made during the cooler night time hours for use during the hot daylight hours. :his ice storage is produced Loff-peakL cooling. ,hen used for the proper application ith the appropriate design, off-peak cooling hen electrical utility rates are lo er. :his is often referred to as
systems can lo er energy costs. :he '.S. +reen 1uilding Council has developed the <eadership in $nergy and $nvironmental Aesign (<$$A) program to encourage the design of high-performance buildings that ill help protect our environment. :he
increased levels of energy performance by utili%ing off-peak cooling may !ualify of credits to ard <$$A Certification. :he advantages of thermal storage are7 Commercial electrical rates are lo er at night4 it takes less energy to make ice hen the ambient temperature is cool at night. Source energy (energy from the po er plant) is saved. a smaller, less costly system can do the Cob of a much larger unit by running for more hours.
Rene'a%le energy storage
>any rene able energy sources (most notably solar and ind) produce intermittent po er. ,herever intermittent po er sources reach high levels of grid penetration, energy storage becomes one option to provide reliable energy supplies. "ndividual energy storage proCects augment electrical grids by capturing excess electrical energy during periods of lo demand and storing it in other forms until needed on an electrical grid. :he energy is later converted back to its electrical form and returned to the grid as needed. Common forms of rene able energy storage include pumped-storage hydroelectricity, hich has long maintained the largest total capacity of stored energy orld ide, as ell as rechargeable battery systems, thermal energy storage including molten salts hich can efficiently store and release very large !uantities of heat energy, and compressed air energy storage. <ess common, speciali%ed forms of storage include fly heel energy storage systems, the use of cryogenic stored energy, and even superconducting magnetic coils. 5ther options include recourse to peaking po er plants that utili%e a po er-togas methane creation and storage process ( here excess electricity is converted to hydrogen via electrolysis, combined ith C53 (lo to neutral C53 system) to produce methane (synthetic natural gas via the Sabatier process) ith stockage in the natural gas net ork) and smart grids ith advanced energy demand management. :he
latter involves bringing Lprices to devicesL, i.e. making electrical e!uipment and appliances able to adCust their operation to seek the lo est spot price of electricity. 5n a grid ith a high penetration of rene able, lo times of high availability of ind andIor sunshine. Another energy storage method is the consumption of surplus or lo -cost energy (typically during night time) for conversion into resources such as hot ater, cool ater or ice, hich is then used for heating or cooling at other times hen electricity is in higher demand and at greater cost per kilo att hour (6,h). Such thermal energy storage is often employed at end-user sites such as large buildings, and also as part of district heating, thus @shifting@ energy consumption to other times for better balancing of supply and demand. Seasonal thermal energy storage (S:$S) stores heat deep in the ground via a cluster of boreholes. :he Arake <anding Solar Community in Alberta, Canada has achieved a 8?B solar fraction for year-round heating, ith solar collectors on the garage roofs as the heat source..)?0 "n 1raestrup, Aenmark, the community@s solar district heating system also utili%es S:$S, at a storage temperature of -;MC (/D8M#). A heat pump, hich is run only hen there is surplus ind po er available on the national grid, is used hen extracting heat from the storage to raise the temperature to 9*MC (/?-M#) for distribution. :his helps stabili%e the national grid, as ell as contributing to maximal use of ind po er. ,hen surplus ind generated electricity is not available, a gas-fired boiler is used. =resently, 3*B of 1raestrup heat is solar, but expansion of the facility is planned to raise the fraction to ;*B. spot prices ould correspond to
$conomic evaluation
:he economic valuation of large-scale applications (including pumped hydro storage and compressed air) must evaluate benefits including7 ind curtailment avoidance, grid congestion avoidance, price arbitrage, and carbon free energy delivery.
An example of artificial energy storage and conversion.
Energy storage is accomplished by devices or physical media that store energy to perform useful operation at a later time. A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (e.g. Chemical, gravitational, electrical energy, temperature differential, latent heat, etc.) or kinetic energy (e.g. momentum). Some technologies provide only short-term energy storage, and others can be very long-term such as po er to gas using hydrogen or methane and the storage of heat or cold bet een opposing seasons in deep a!uifers or bedrock. A ind-up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to operate a mobile phone, and a hydroelectric dam stores energy in a reservoir as gravitational potential energy. "ce storage tanks store ice (thermal energy in the form of latent heat) at night to meet peak demand for cooling. #ossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time ere then converted into these fuels. $ven food( hich is made by the same process as fossil fuels) is a form of energy stored in chemical form. $nergy storage as a natural process is as old as the universe itself - the energy present at the initial formation of the universe has been stored in stars such as the Sun, and is no being used by humans directly (e.g. through solar heating), or indirectly (e.g. by gro ing crops or conversion into electricity in solar cells).
As a purposeful activity, energy storage has existed since pre-history, though it as often not explicitly recogni%ed 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 ere collected at the top of a hill or all, and the energy thus stored used to attack invaders ho came ithin range.
A more recent application is the control of ater ays to drive ater mills for processing grain or po ering machinery. Complex systems of reservoirs and dams ere constructed to store and release ater (and the potential energy it contained) hen re!uired. Storing energy allo s humans to balance the supply and demand of energy. $nergy storage systems in commercial use today can be broadly categori%ed as mechanical, electrical, chemical, biological and thermal.
Storage for electricity
$nergy storage became a dominant factor in economic development ith the
idespread introduction of electricity. 'nlike other common energy storage in prior use such as ood or coal, electricity must be used as it is being generated, or converted immediately into another form of energy such as potential, kinetic or chemical. A very traditional ay doing this on large scales is pumped-storage hydroelectricity. #or example the pumped-storage hydroelectricity in (or ay has a capacity of )* +,, hich could be expanded to -* +,, hich ould be enough to be the battery of $urope../0 An early solution to the problem of storing energy for electrical purposes as the development of the battery as an electrochemical storage device. 1atteries have previously been of limited use in electric po er systems due to their relatively small capacity and high cost. 2o ever, since about the middle of the first decade of the 3/st century, ne er battery technologies have been developed that can no significant utility scale load-leveling capabilities4 .30 some of hich, as of 3*/), sho ed promise of being competitive ith alternative methods .)0(see also rechargeable battery). provide
Another solution to deal found in the capacitor.
ith the intermittency issue of solar and ind energy is
Some areas of the orld such as ,ashington and 5regon in the 'nited States, and ,ales in the 'nited 6ingdom, have used geographic features to store large !uantities of ater in elevated reservoirs, using excess electricity at times of lo demand to pump ater up to the reservoirs, then letting the ater pass through turbine generators to retrieve the energy hen electrical demands peak. .30 5ther possibilities to store electricity are7 fly heel, compressed air energy storage, hydrogen storage, thermal energy storage, po er to gas.
Short-term thermal storage, as heat or cold
"n the /89*s, a number of manufacturers carefully researched thermal energy storage (:$S) to meet the gro ing demand for air conditioning during peak hours. :oday, several companies manufacture :$S systems. :he most popular form of thermal energy storage for cooling is ice storage, since it can store more energy in less space than ater storage and it is also less costly than energy recovered via fuel cells or fly heels. :hermal storage has cost-effectively shifted giga atts of po er a ay from daytime peak usage periods, and in 3**8 as used in over ),)** buildings in over ); countries. "t orks by creating ice at night hen electricity is usually less costly, and then using the ice to cool the air in buildings during the hotter daytime periods. <atent heat can also be stored in technical phase change materials (=C>s), besides ice. :hese can for example be encapsulated in all and ceiling panels, to moderate room temperatures bet een daytime and nighttime.
Inter seasonal thermal storage, as heat or cold
Another class of thermal storage that has been developed since the /8?*s that is no fre!uently employed is seasonal thermal energy storage (S:$S). "t allo s heat or cold to be used even months after it as collected from aste energy or natural sources, even in an opposing season. :he thermal storage may be accomplished in
contained a!uifers, clusters of boreholes in geological substrates as diverse as sand or crystalline bedrock, in lined pits filled ith gravel and ater, or ater-filled mines. An example is Alberta, Canada@s Arake <anding Solar Community, for hich 8?B of the year-round heat is provided by solar-thermal collectors on the garage roofs, ith a borehole thermal energy store (1:$S) being the enabling technology. S:$S proCects often have paybacks in the D-- year range.
Energy storage in chemical fuels
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, li!uefied petroleum gas (<=+), propane, butane, ethanol and biodiesel. All of these materials are readily converted to mechanical energy and then to electrical energy using heat engines (via turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical po er generation. 2eat-engine-po ered generators are nearly universal, ranging from small engines producing only a fe kilo atts to utility-scale generators ith ratings up to 9** mega atts. A key disadvantage to hydrocarbon fuels are their significant emissions of greenhouse gases that contribute to global arming, as ell as other significant pollutants emitted by the dirtier fuel sources such as coal and gasoline. <i!uid hydrocarbon fuels are the most commonly used forms of energy storage for use in transportation, but because the byproducts of the reaction that utili%es these li!uid fuels@ energy (combustion) produce greenhouse gases other energy carriers like hydrogen can be used to avoid production of greenhouse gases.
Advanced systems
$lectrochemical devices called fuel cells ere invented about the same time as the battery in the /8th Century. 2o ever, for many reasons, fuel cells ere not elldeveloped until the advent of manned spaceflight (such as the +emini =rogram in the '.S.) hen light eight, non-thermal (and therefore efficient) sources of electricity ere re!uired in spacecraft. #uel 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. Several other technologies have also been investigated, such as fly heels, hich can store kinetic energy, and compressed air storage that can be pumped into underground caverns and abandoned mines..30.90 Another method used at the Solar =roCect and the Solar :res =o er :o er uses molten salt to store solar po er and then dispatch that po er as needed. :he system pumps molten salt through a to er heated by the sun@s rays. "nsulated containers store the hot salt solution, and hen needed ater is then used to create steam that is fed to turbines to generate electricity. Eesearch is being conducted on harnessing the !uantum effects of nano scale capacitors to create digital !uantum batteries. Although this technology is still in the experimental stage, it theoretically has the potential to provide dramatic increases in energy storage capacity..80./*0
+rid energy storage
+rid energy storage (or large-scale energy storage) lets energy producers send excess electricity over the electricity transmission grid to temporary electricity storage sites that subse!uently become energy suppliers hen electricity demand is greater. +rid energy storage is particularly important in matching supply and demand over a 3D hour period of time. A proposed variant of grid energy storage is called vehicle-to-grid energy storage system, here modern electric vehicles that are plugged into the energy grid can release the stored electrical energy in their batteries back into the grid hen needed.
Storage methods
Chemical 1io fuels
2ydrated salts 2ydrogen 2ydrogen peroxide <i!uid nitrogen =o er to gas Fanadium pent oxide 1iological +lycogen Starch $lectrochemical #lo battery
Eechargeable battery $lectrical Capacitor Super capacitor Superconducting magnetic energy storage (S>$S) >echanical Compressed air energy storage (CA$S) #ireless locomotive #ly heel energy storage
+ravitational potential energy (device) 2ydraulic accumulator 2ydroelectric energy storage Spring :hermal 1rick storage heater Cryogenic li!uid air or nitrogen $utectic system "ce Storage >olten salt Seasonal thermal energy storage Solar pond Steam accumulator :hermal energy storage (general)
Hydrogen
A chart depicting the durations and po er capabilities of various energy storage technologies, including po er to gas
2ydrogen is also being developed as an electrical po er storage medium. 2ydrogen is not a primary energy source, but a portable energy storage method, because it must first be manufactured by other energy sources in order to be used. 2o ever, as a storage medium, it may be a significant factor in using rene able energies. See hydrogen storage. ,ith intermittent rene able such as solar and into an electricity grid. At penetrations belo
needed0
ind, the output may be fed directly
3*B of the grid demand, this does not
severely change the economics4 but beyond about 3*B of the total demand .citation , external storage ill become important..//0 "f these sources are used for hen they cut in or electricity to make hydrogen, then they can be utili%ed fully henever they are available, opportunistically. 1roadly speaking, it does not matter out, the hydrogen is simply stored and used as re!uired. $nergy losses are involved in the hydrogen storage cycle of hydrogen production for vehicle applications ith electrolysis of ater, li!uification or compression, and conversion back to electricity../)0 and the hydrogen storage cycle of production for the stationary fuel cell applications like >icroC2= at 8) B./D0 ith biohydrogen or biological hydrogen production, and conversion to electricity. About ;* k,·h (/9* >G) of solar energy is re!uired to produce a kilogram of hydrogen, so the cost of the electricity clearly is crucial, even for hydrogen uses other than storage for electrical generation. At H*.*)Ik,h, common off-peak high-
voltage line rate in the 'nited States, this means hydrogen costs H/.;* a kilogram for the electricity, e!uivalent to H/.;* a '.S. gallon for gasoline if used in a fuel cell vehicle. 5ther costs ould include the electroly%er plant, hydrogen compressors or li!uefaction, storage and transportation, hich ill be significant.4 nderground hydrogen storage 'nderground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. <arge !uantities of gaseous hydrogen have been stored in underground caverns by "mperial Chemical "ndustries ("C") for many years ithout any difficulties =o er to gas is a technology hich converts electrical po er to a gas fuel. :here are 3 methods, the first is to use the electricity for ater splitting and inCect the resulting hydrogen into the natural gas grid. :he second less efficient method is used to convert carbon dioxide and ater to methane, (see natural gas) using electrolysis and the Sabatier reaction. :he excess po er or off peak po er generated by ind generators or solar arrays is then used for load balancing in the energy grid. 'sing the existing natural gas system for hydrogen #uel cell maker 2ydro genics and natural gas distributor $nbridge have teamed up to develop such a po er to gas system in Canada. =ipeline storage of hydrogen here a natural gas net ork is used for the storage of
hydrogen. 1efore s itching to natural gas, the +erman gas net orks ere operated using to n gas, hich for the most part consisted of hydrogen. :he storage capacity of the +erman natural gas net ork, hich contains also many so called caverns (artificial caves produced by mining), is more than 3**,*** +, ·h hich is enough for several months of energy re!uirement. 1y comparison, the capacity of all +erman pumped storage po er plants amounts to only about D* +, ·h. :he transport of energy through a gas net ork is done ith much less loss (J*./B) than in a po er net ork (9B) (besides 2igh-voltage direct current).
!iofuels
Farious bio fuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to replace hydrocarbon fuels. Farious chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal biomass, and organic astes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. $xamples are #ischer-:ropsch diesel, methanol, dimethyl ether, or syngas. :his diesel source as used extensively in ,orld ,ar "" in +ermany, ith limited access to crude oil supplies. :oday South Africa produces most of the country@s diesel from coal for similar reasons..330 A long term oil price above 'SH);Ibbl may make such synthetic li!uid fuels economical on a large scale (see coal). Some of the energy in the original source is lost in the conversion process. 2istorically, coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. Additionally, compressed natural gas is also used as fuel, for instance for buses ith some mass transit agencies. "ethane #SNG Synthetic Natural Gas$ >ethane is the simplest hydrocarbon ith the molecular formula C2 D. >ethane could be produced from electricity of rene able energies using po er to gas technologies.
.3)0
>ethane can be stored more easily than hydrogen and the transportation, storage
and combustion infrastructure are mature (pipelines, gaso meters, po er plants). As hydrogen and oxygen are produced in the electrolysis of ater, 3235 → 323 K 53 2ydrogen ould then be reacted ith carbon dioxide in Sabatier process, producing methane and ater. C53 K D23 → C2D K 3235 >ethane ould be stored and used to produce electricity later. =roduced ater pure
ould be recycled back to the electrolysis stage, reducing the need for ne
ater. "n the electrolysis stage oxygen ould also be stored for methane combustion in a pure oxygen environment in an adCacent po er plant, eliminating e.g. nitrogen oxides. "n the combustion of methane, carbon dioxide and ater are produced.
C2D K 353 → C53 K 3235 =roduced carbon dioxide ould be recycled back to boost the Sabatier process and ater ould be recycled back to the electrolysis stage. :he carbon dioxide produced ould by methane combustion ould be turned back to methane, thus producing no greenhouse gases. >ethane production, storage and adCacent combustion recycle all the reaction products, creating a lo carbon cycle.
:he C53 ould be a resource having economic value as a component of an energy storage vector, not a cost as in CCS (Carbon capture and storage).
Aluminum, %oron, silicon, and &inc
Aluminum 1oron silicon lithium, and %inc have been proposed as energy storage solutions.
"echanical storage
$nergy can be stored in ater pumped to a higher elevation using pumped storage methods, by moving solid matter to a higher location by compressing air, or by storing it in spinning fly heels. A mass of / kg, elevated to a height of /,*** m stores 8.9 kG of gravitational energy, hich is e!uivalent to / kg mass accelerated to /D* mIs. :o store the same mass of ater, if increased in temperature by 3.)D Celsius, re!uires the same amount of energy. Compressed air energy storage (CA$S) technology stores lo during peak load hours and heated cost off-peak energy,
in the form of compressed air in an underground reservoir. :he air is then released ith the exhaust heat of a standard combustion turbine. :his heated air is converted to energy through expansion turbines to produce electricity. A CA$S plant has been in operation in >c"ntosh, Alabama since /88/ and has run successfully. 5ther applications are possible. ,alker Architects published the first C53 gas CA$S application, proposing the use of se!uestered C53 for $nergy Storage. :he paper as Submitted to the Conoco =hilips $nergy
=ri%e April 3**9. :his paper precedes A<< others and in that paper ,alker defined the C53 energy storage cycle. Several proCects sponsored by the A5$ are no under ay to develop the body of technology pioneered by :erry <. ,alker in 3**9. Several companies have done preliminary design compressed air po er. ork for vehicles using
Thermal storage,
>ain articles7 :hermal energy storage and Seasonal thermal energy storage
Aistrict heating accumulation to er from :hesis near 6rems an der Aonauin <o er Austria ith a thermal capacity of 3 +,h
:hermal storage is the temporary storage or removal of heat for later use. An example of thermal storage is the storage of solar heat energy during the day to be used at a later time for heating at night. "n the 2FACIE field, this type of application using thermal storage for heating is less common than using thermal storage for cooling. An example of the storage of LcoldL heat removal for later use is ice made during the cooler night time hours for use during the hot daylight hours. :his ice storage is produced Loff-peakL cooling. ,hen used for the proper application ith the appropriate design, off-peak cooling hen electrical utility rates are lo er. :his is often referred to as
systems can lo er energy costs. :he '.S. +reen 1uilding Council has developed the <eadership in $nergy and $nvironmental Aesign (<$$A) program to encourage the design of high-performance buildings that ill help protect our environment. :he
increased levels of energy performance by utili%ing off-peak cooling may !ualify of credits to ard <$$A Certification. :he advantages of thermal storage are7 Commercial electrical rates are lo er at night4 it takes less energy to make ice hen the ambient temperature is cool at night. Source energy (energy from the po er plant) is saved. a smaller, less costly system can do the Cob of a much larger unit by running for more hours.
Rene'a%le energy storage
>any rene able energy sources (most notably solar and ind) produce intermittent po er. ,herever intermittent po er sources reach high levels of grid penetration, energy storage becomes one option to provide reliable energy supplies. "ndividual energy storage proCects augment electrical grids by capturing excess electrical energy during periods of lo demand and storing it in other forms until needed on an electrical grid. :he energy is later converted back to its electrical form and returned to the grid as needed. Common forms of rene able energy storage include pumped-storage hydroelectricity, hich has long maintained the largest total capacity of stored energy orld ide, as ell as rechargeable battery systems, thermal energy storage including molten salts hich can efficiently store and release very large !uantities of heat energy, and compressed air energy storage. <ess common, speciali%ed forms of storage include fly heel energy storage systems, the use of cryogenic stored energy, and even superconducting magnetic coils. 5ther options include recourse to peaking po er plants that utili%e a po er-togas methane creation and storage process ( here excess electricity is converted to hydrogen via electrolysis, combined ith C53 (lo to neutral C53 system) to produce methane (synthetic natural gas via the Sabatier process) ith stockage in the natural gas net ork) and smart grids ith advanced energy demand management. :he
latter involves bringing Lprices to devicesL, i.e. making electrical e!uipment and appliances able to adCust their operation to seek the lo est spot price of electricity. 5n a grid ith a high penetration of rene able, lo times of high availability of ind andIor sunshine. Another energy storage method is the consumption of surplus or lo -cost energy (typically during night time) for conversion into resources such as hot ater, cool ater or ice, hich is then used for heating or cooling at other times hen electricity is in higher demand and at greater cost per kilo att hour (6,h). Such thermal energy storage is often employed at end-user sites such as large buildings, and also as part of district heating, thus @shifting@ energy consumption to other times for better balancing of supply and demand. Seasonal thermal energy storage (S:$S) stores heat deep in the ground via a cluster of boreholes. :he Arake <anding Solar Community in Alberta, Canada has achieved a 8?B solar fraction for year-round heating, ith solar collectors on the garage roofs as the heat source..)?0 "n 1raestrup, Aenmark, the community@s solar district heating system also utili%es S:$S, at a storage temperature of -;MC (/D8M#). A heat pump, hich is run only hen there is surplus ind po er available on the national grid, is used hen extracting heat from the storage to raise the temperature to 9*MC (/?-M#) for distribution. :his helps stabili%e the national grid, as ell as contributing to maximal use of ind po er. ,hen surplus ind generated electricity is not available, a gas-fired boiler is used. =resently, 3*B of 1raestrup heat is solar, but expansion of the facility is planned to raise the fraction to ;*B. spot prices ould correspond to
$conomic evaluation
:he economic valuation of large-scale applications (including pumped hydro storage and compressed air) must evaluate benefits including7 ind curtailment avoidance, grid congestion avoidance, price arbitrage, and carbon free energy delivery.
