Miodrag Mesarovi}

Energoprojekt, ENTEL, Belgrade, Serbia

Energy Storage Technologies andRenewable Energy Sources

Re view pa perUDC: 502.171:531.62:697.329BIBLID: 0350-218X, 34 (2008), 1, 31–44

Re new able en ergy sources are ex pected to play an im por tant role in pre vent ing cli -mate change, but they are con fronted with a num ber of bar ri ers, such as theirintermittency and high costs. En ergy stor age pres ents an ex cel lent op por tu nity forthem to over come intermittency ob sta cle. Con ven tional ther mal, com pressed air,pumped hy dro, bat ter ies, fly-wheels, as well as new hy dro gen fuel cell, wind, so lar,and dif fer ent hy brid en ergy stor age tech nol o gies are de scribed and a num ber ofprom is ing ex am ples high lighted.

Key words: re new able en ergy sources, en ergy stor age tech nol o gies, intermittency

En ergy stor age – a chal lenge for renewables

Oil short age and cli mate change is sues

The re cent spike in the price of crude-oil brings most con sum ers view gas o lineprices as the prob lem, but, in re al ity, they are only a symp tom of a more se ri ous prob lem.The prob lem is that oil soon will be come scarce so that de mand will out strip sup ply, andthe mere threat of oil short age drive the price of oil up. Oil pro duc tion will peak world -wide some time soon and be gin to de cline. Mean while, oil de mand will con tinue to growun less some very ag gres sive fuel con ser va tion and im ple men ta tion of al ter na tives to oiltake place. Vir tu ally all ac cept this as the most se ri ous is sue fac ing the world, and this is -sue opens the door for more re new able and other en ergy sources.

An other is sue in fa vour of re new able sources is cli mate change. Hu man ac tiv i -ties have changed the cli mate of the Earth, with sig nif i cant im pacts on eco sys tems and hu -man so ci ety, and the pace of change is in creas ing. The larg est of all of the hu man andnat u ral in flu ences on cli mate over the past 250 years has been the in crease in the at mo -spheric con cen tra tion of car bon di ox ide re sult ing from de for es ta tion and fos sil-fuel

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burn ing (in re cent de cades, which have been re spon si ble for the larg est part of thisbuildup, have come 75 to 85% from fos sil fu els (largely in the in dus tri al ized coun tries)and 15 to 25% from de for es ta tion and other land-cover change (largely from de vel op ingcoun tries in the trop ics). Even if hu man emis sions could be in stan ta neously stopped, theworld would not es cape fur ther cli ma tic change. The slow equil i bra tion of the oceans with changes in at mo spheric com po si tion means that a fur ther 0.4 to 0.5 °C rise in global-av er -age sur face tem per a ture will take place as a re sult of the cur rent at mo spheric con cen tra -tions of green house gases and par ti cles. If con cen tra tions con tinue to grow, the globalav er age sur face tem per a ture is ex pected to rise by 0.2 to 0.4 °C per de cade through out the 21st cen tury and would con tinue to rise there af ter. The cu mu la tive warm ing by the year2100 would be ap prox i mately 3 to 5 °C over preindustrial con di tions [1]. There is nodoubt that to com bat cli mate change, so ci ety must switch to car bon-free, re new able en -ergy sources as quickly as pos si ble.

Intermittency and pen e tra tion of re new able en ergy sources

Ma jor re new able en ergy sources are in ter mit tent in na ture. The intermittencyof re new able en ergy sources such as the sun and wind can not be con trolled to pro videpower when it is needed. Hy dro power can also be in ter mit tent de pend ing on the con fig u -ra tion of hy dro elec tric plant (plants in the dam con fig u ra tion may be con sid ereddispatchable, but run of the river plants will typ i cally have lim ited or no stor age ca pac ity,and may be in ter mit tent on a sea sonal or an nual ba sis). As a mat ter of fact, all en ergysources have some de gree of un pre dict abil ity, and de mand pat terns (while rel a tively pre -dict able) drive large swings in the amount of en ergy re quired, but the sources are de -signed to match sup ply with de mand, and the in tro duc tion of an in ter mit tent en ergysource may not be well-matched to de mand cy cles.

Intermittency there fore may in tro duce ad di tional costs that are dis tinct from orof a dif fer ent mag ni tude than for tra di tional gen er a tion types. Us ing a va ri ety of en ergysources in com bi na tion can help to over come intermittency. For ex am ple, stormyweather, bad for di rect so lar col lec tion, is gen er ally good for wind power. Elec tric ity pro -duced from so lar en ergy could be a coun ter bal ance to the fluc tu at ing sup plies gen er atedfrom wind. In some lo ca tions, it tends to be wind ier at night and dur ing cloudy or stormyweather, so there is likely to be more sun shine when there is less wind, while in other lo ca -tions elec tric ity de mand may have a high cor re la tion with wind out put, par tic u larly in lo -ca tions where cold tem per a tures drive elec tric con sump tion.

Intermittency in her ently af fects so lar en ergy, as the pro duc tion of elec tric ityfrom so lar sources de pends on the amount of light en ergy in a given lo ca tion. The ex tentto which the intermittency of so lar-gen er ated en ergy is an is sue will de pend to some ex -tent on the de gree to which the gen er a tion pro file of so lar cor re sponds to de mand cy cles.In ar eas with high so lar pro duc tion pos si bil i ties, and where air con di tion ing is a driver ofde mand and cor re sponds to pe ri ods of high sun light, vari abil ity may ac tu ally be ben e fi -cial. For ex am ple, so lar power plants are ide ally matched to sum mer peak loads in ar easwith sig nif i cant cool ing de mands. Us ing ther mal en ergy stor age sys tems, so lar ther malop er at ing pe ri ods can even be ex tended to meet base-load needs.

Wind-gen er ated power is also a typ i cal vari able re source, and the amount ofwind-gen er ated elec tric ity will de pend on wind speeds, tur bine char ac ter is tics, and otherfac tors. Wind en ergy stor age may also be used to ar bi trage be tween pe ri ods of low and

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high de mand. As the frac tion of en ergy pro duced by wind (pen e tra tion) in creases, dif fer -ent tech ni cal and eco nomic fac tors af fect the need for grid en ergy stor age fa cil i ties. Asad di tional op er at ing re serve is needed if ad di tional wind does not cor re spond to de mandpat terns, at high pen e tra tions (more than 30%), so lu tions for deal ing with high out put ofwind dur ing pe ri ods of low de mand may be re quired.

There is no gen er ally ac cepted max i mum level of pen e tra tion, as each sys tem’sca pac ity to com pen sate for intermittency dif fers, and the sys tems them selves will changeover time. Large net works, con nected to mul ti ple wind plants at widely sep a rated geo -graphic lo ca tions, may ac cept a higher pen e tra tion of wind than small net works or thosewith out stor age sys tems or eco nom i cal meth ods of com pen sat ing for the vari abil ity ofwind. In sys tems with sig nif i cant amounts of ex ist ing pumped stor age, hy dro power orother peak ing power plants, this pro por tion may be higher. Iso lated, rel a tively small sys -tems with only a few wind plants may only be sta ble and eco nomic with a lower frac tion ofwind en ergy al though mixed wind/die sel sys tems have been used in iso lated com mu ni tieswith suc cess at rel a tively high pen e tra tion lev els. The max i mum pro por tion of windpower al low able in a power sys tem will thus de pend on many fac tors, in clud ing the size ofthe sys tem, the at tain able geo graph ical di ver sity of wind, the con ven tional plant mix, andsea sonal load fac tors (heat ing in win ter, air-con di tion ing in sum mer) and their sta tis ti calcor re la tion with wind out put. Wind power gen er a tion tends to be higher in the win ter andat night (due to higher air den sity), so the ap pro pri ate ness of wind power may be weak estin the hot sum mer months, and par tic u larly dur ing the day when air con di tion ing de mandis high est. Con versely, sys tems where heat is elec tri cal may be well-suited to higher pen e -tra tion of wind power [2].

Sys tems with ex ist ing high lev els of hy dro elec tric gen er a tion may be able to in -cor po rate sub stan tial amounts of wind, al though high hy dro pen e tra tion may in di catethat hy dro is al ready a low-cost source of elec tric ity. Stor age ca pac ity in hy dro power fa -cil i ties will be lim ited by size of res er voir, and en vi ron men tal and other con sid er ations.In creased wind pen e tra tion may raise the value of ex ist ing peak ing or stor age fa cil i tiesand par tic u larly hy dro elec tric plants, as their abil ity to com pen sate for wind’s vari abil itywill be un der greater de mand. Pumped stor age hy dro power is the most prev a lent ex ist ing tech nol ogy used, and can sub stan tially im prove the eco nom ics of wind power. The cost ofcom pen sat ing for the vari abil ity of wind is ex pected to rise with higher pen e tra tion lev els,par tic u larly so if stor age needs to be pur pose-built for wind.

En ergy stor age

En ergy stor age be came a ma jor fac tor with the wide spread in tro duc tion of elec -tric ity, which, un like the other com mon en ergy car ri ers, has to be used as it is gen er ated.Stor ing of some form of en ergy that can be drawn upon at a later time to per form someuse ful op er a tion re quires spe cial equip ment to be added. The en ergy stor age tech nol o -gies can be based on the fol low ing stor age meth ods: elec tro chem i cal (bat ter ies, flow bat -ter ies, fuel cells), elec tri cal (ca pac i tor, supercapacitor, super con duct ing mag netic en ergystor age), me chan i cal (com pressed air en ergy stor age, fly-wheel en ergy stor age, hy drau lic ac cu mu la tor, hy dro elec tric en ergy stor age), and ther mal (mol ten salt, cryo genic liq uid

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air or ni tro gen, sea sonal ther mal stor age, so lar pond, hot bricks, steam ac cu mu la tor). Anearly so lu tion to the prob lem of stor ing elec tric ity was the de vel op ment of the bat tery. Asim i lar so lu tion with the same type of prob lems is the ca pac i tor. Elec tro chem i cal de vicescalled fuel cells were also in vented at the same time as the bat tery, but were not de vel -oped un til the ad vent of manned space-flight and are now be ing com mer cial ized to al lowthe ef fi cient con ver sion of chem i cal en ergy stored in re fined hy dro car bon or hy dro genfu els di rectly into elec tric ity. En ergy can be stored as wa ter pumped to a higher el e va tion,com pressed air, and spin ning fly-wheels, but me chan i cal meth ods of stor ing en ergy havelim ited ca pac ity or ef fi ciency.

En ergy stor age sys tems com ple ment re new able re sources with sit ing flex i bil ityand min i mal en vi ron men tal im pacts. They can be used to re duce the stress on trans mis -sion lines that are near peak rat ing by re duc ing sub sta tion peak load. En ergy stor age canserve cus tom ers as a con trol la ble de mand-side man age ment op tion that can also pro videpre mium ser vices. Fi nally, en ergy stor age is also used in stand-alone ap pli ca tions, whereit can serve as an uninterruptible power sup ply units used for back-up power and only ac -ti vate in cases of power out ages un like the en ergy stor age sys tems dis cussed herein thatper form a num ber of on-line ap pli ca tions. This is par tic u larly the case in iso lated, re motelo ca tions, with out con nec tion to elec tric ity grids, where some type of back-up power must be con sid ered if an in ter mit tent re new able en ergy source is used.

So lar or wind en ergy gen er ated at the home, in ad di tion to be ing re new able,would avoid the dis tri bu tion loses be tween power plants and homes. Wind en ergy is a fastgrow ing en ergy sec tor and has re corded con sis tent growth of over 20% over the last tenyears. The world’s larg est unit size of wind tur bine is 5 MW (with a three-bladed ro tor at adi am e ter of 126 me ters), while pro jects to build units in the 10 MW size range are con sid -ered. An en ergy stor age sys tem linked with a wind farm could guar an tee an un in ter -rupted sup ply of green power to the grid, im prove ef fi ciency of the en ergy and re movefi nan cial risk. The en ergy stor age tech nol ogy may al low wind en ergy gen er ated dur ingoff-peak pe ri ods to be stored and sup plied to the na tional grid at a sched uled time.

Over view of en ergy stor age tech nol o gies

Pumped wa ter stor age

Pumped stor age hy dro elec tric ity is a method of stor ing and pro duc ing elec tric ity to sup ply high peak de mands by mov ing wa ter be tween res er voirs at dif fer ent el e va tions.At times of low elec tri cal de mand, ex cess gen er a tion ca pac ity is used to pump wa ter intothe higher res er voir, where it can be stored as po ten tial en ergy. Upon de mand, wa ter isre leased back into the lower res er voir, pass ing through hy drau lic tur bines which gen er ate elec tri cal power.Re vers ible tur bine/gen er a tor as sem blies act as pump and tur bine (usu -ally a Fran cis tur bine de sign) by the use of the height dif fer ence be tween two nat u ral bod -ies of wa ter or ar ti fi cial res er voirs. Pure pumped-stor age plants just shift the wa terbe tween res er voirs, but com bined pump-stor age plants also gen er ate their own elec tric -ity like con ven tional hy dro elec tric plants through nat u ral stream-flow, fig. 1.

Pumped wa ter stor age has been in use since 1929, mak ing it the old est of the cen -tral sta tion en ergy stor age tech nol o gies. In fact, un til 1970 it was the only com mer ciallyavail able stor age op tion for gen er a tion ap pli ca tions. In many places, pumped stor age hy -

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dro elec tric ity is used to even out the daily gen er at ing load, by pump ing wa ter to a highstor age res er voir dur ing off-peak hours. Dur ing peak hours, this wa ter can be used for hy -dro elec tric gen er a tion, of ten as a high value rapid-re sponse re serve to cover tran sientpeaks in de mand. Tak ing into ac count evap o ra tion losses from the ex posed wa ter sur faceand con ver sion losses, ap prox i mately 70 to 85% of the elec tri cal en ergy used to pump thewa ter into the el e vated res er voir can be re gained. There is over 90 GW of pumped stor age in op er a tion, which is about 3% of global gen er a tion ca pac ity. The tech nique is cur rentlythe most cost-ef fec tive means of stor ing large amounts of elec tri cal en ergy on an op er at -ing ba sis, but cap i tal costs and the pres ence of ap pro pri ate ge og ra phy are crit i cal de ci sionfac tors. The rel a tively low en ergy den sity of pumped stor age sys tems re quires ei ther avery large body of wa ter or a large vari a tion in height.

A new use for pumped stor age is to level the fluc tu at ing out put of in ter mit tentpower sources. The pumped stor age ab sorbs load at times of high out put and low de -mand, while pro vid ing ad di tional peak ca pac ity. It is par tic u larly likely that pumped stor -age will be come es pe cially im por tant as a bal ance for large scale wind or pho to vol taicgen er a tion. The new con cept in pumped-stor age by uti liz ing wind en ergy to pump wa tercan make this a more ef fi cient pro cess. For ex am ple, the Vlasina Wind Power pro ject inSer bia makes use of the ex ist ing Lisina (lower) and Vlasina (up per) lakes and Vrla I to IVhy dro elec tric plants to achieve flex i bil ity with an op ti mum stor age of the wind en ergywith out ex tra in vest ments [3].

Com pressed air stor age

An other grid en ergy stor age method is Com pressed Air En ergy Stor age(CAES) that uses off-peak elec tric ity to com press air, which is usu ally stored in an

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Fig ure 1. Sche matic of a pumped wa ter stor age plant

air-tight un der ground stor age cav ern (an old mine or some other kind of geo log i cal fea -ture). When elec tric ity de mand is high, the com pressed air is re leased from the cav ern,heated with a small amount of nat u ral gas and ex panded and ex panded through a com -bus tion tur bine to gen er ate elec tric ity [4]. In ter mit tent wind en ergy may re place the gridoff-peak elec tri cal en ergy for use to com press and store the air, fig. 2.

Com pressed air stor age is safe, non-toxic and en ergy dense at high pres sures.Round-trip ef fi ciency is fair due to heat re jec tion dur ing com pres sion, but some of thisheat re claimed dur ing gas ex pan sion. The CAES tech nol ogy re quires an un used emptysalt dome, aqui fer or aban doned mine for com pressed-air stor age and is not self-con -tained, as it de pends on sup ply nat u ral gas for the com bus tion cham ber. For these andother rea sons, CAES has been at tempted in util ity-scale gen er at ing plants, typ i cally over100 MW. The break through in com bin ing en ergy stor age tech nol o gies was de cid ing totake el e ments of both fly-wheel and CAES tech nol ogy to cre ate a self-con tained en ergystor age sys tem. Com pressed air and ther mal en ergy drive the ex pan sion tur bine forlong-du ra tion out ages, while a small fly-wheel sys tem gives in stan ta neous re sponse tostep loads and short out ages.

A con ven tional tur bine en gine has three stages: a com pres sion stage for pres sur -iz ing in com ing air; an ig ni tion stage, where the fuel and com pressed air are in jected into acom bus tion cham ber and ig nited, and an ex pan sion stage, where the ex pand ing ex haustgas drives the tur bine blades con nected to the out put shaft of the en gine [5]. Since thenew hy brid prod uct stores com pressed air in gas cyl in ders and heats the air, only the thirdstage of the tur bine (the ex pan sion stage) is re quired. The heated com pressed air is usedto spin a sim ple sin gle-stage ex pan sion tur bine. The low in er tia en ables it to reach full op -er at ing speed (70,000 rpm) in less than two sec onds.

To elim i nate the dis ad van tages as so ci ated with fos sil fuel used by the CAES tur -bine and to re duce op po si tion to trans mis sion line sit ing in ag ri cul tural ar eas, al ter na tive

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Fig ure 2. Wind/CAES en ergy stor age ar range ment and its en ergy bal ance

com pletely re new able baseload wind sys tem is re plac ing nat u ral gas with farm-de rivedbiofuel [6]. This al ter na tive sys tem also pro vides unique op por tu ni ties for the use ofbiofuels that might oth er wise be un avail able. Un like fos sil fu els, bio mass crops are a re -new able en ergy source, and largely car bon neu tral over the en tire cy cle of har vest, com -bus tion, and re gen er a tion. As a re sult, the use of biofuels in the CAES tur bines re ducesthe net green house gas emis sions, while in creas ing over all price sta bil ity [7]. High trans -por ta tion costs, and other land re stric tions are ex pected to limit the size of stan dardpower plants to be low 200 MW, which is typ i cally much smaller than de sir able for the de -vel op ment of ded i cated long dis tance trans mis sion.

Ther mal en ergy stor age

Ther mal en ergy stor age can re fer to a num ber of tech nol o gies that store en ergyin a ther mal res er voir for bal ance en ergy de mand be tween day time and night time. Thether mal res er voir may be main tained at a tem per a ture above (hot ter) or be low (colder)than that of the am bi ent en vi ron ment. The prin ci pal ap pli ca tion to day is the pro duc tionof ice, chilled wa ter, or eutectic so lu tion at night, which is then used to cool en vi ron mentsdur ing the day. Off-peak elec tric ity can be used to make ice from wa ter, and the ice can bestored un til the next day, when it is used to cool ei ther the air in a large build ing, therebyshift ing that de mand off-peak, or the in take air of a gas tur bine gen er a tor, thereby in -creas ing the on-peak gen er a tion ca pac ity.

Ther mal en ergy stor age tech nol o gies may in clude ac tive so lar col lec tors to pro -duce and store heat in an insulated re pos i tory for later use in space heat ing, do mes tic orpro cess hot wa ter, or to gen er ate elec tric ity. Most prac ti cal ac tive so lar heat ing sys temshave stor age for a few hours to a day’s worth of heat col lected. There are also a small butgrow ing num ber of sea sonal ther mal stores, used to store sum mer heat for use dur ingwin ter. Mol ten salt has been pro posed as a means to re tain a high tem per a ture ther malstore for later use in elec tric ity gen er a tion.

The most widely used form of this tech nol ogy is in large build ing or cam pus-wide air con di tion ing or chilled wa ter sys tems. Air con di tion ing sys tems, es pe cially in com mer -cial build ings, are the most sig nif i cant con tri bu tors to the peak elec tri cal loads seen onhot sum mer days. In this ap pli ca tion a rel a tively stan dard chiller is run at night to pro ducea pile of ice [8]. Wa ter is cir cu lated through the pile dur ing the day to pro duce chilled wa -ter that would nor mally be the day time out put of the chill ers. A par tial stor age sys temmin i mizes cap i tal in vest ment by run ning the chill ers 24 hours a day. At night they pro duce ice for stor age, and dur ing the day they chill wa ter for the air con di tion ing sys tem, theirpro duc tion aug mented by wa ter cir cu lat ing through the melt ing ice. A full stor age sys temmin i mizes the cost of en ergy to run the sys tem by shut ting off the chill ers en tirely dur ingpeak load hours, but re quires chill ers some what larger than a par tial stor age sys tem, anda larger ice stor age sys tem, so that the cap i tal cost is higher.

Ther mal en ergy stor age is also used for com bus tion gas tur bine air in let cool ing.In stead of shift ing elec tri cal de mand to the night, this tech nique shifts gen er a tion ca pac -ity to the day. To gen er ate the ice at night, the tur bine is of ten me chan i cally con nected tothe com pres sor of a large chiller. Dur ing peak day time loads, wa ter is cir cu lated be tween

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the ice pile and a heat exchanger in front of the tur bine air in take, cool ing the in take air tonear freez ing tem per a tures. Be cause the air is colder, the tur bine can com press more airwith a given amount of com pres sor power. Typ i cally, both the gen er ated elec tri cal powerand tur bine ef fi ciency rise when the in let cool ing sys tem is ac ti vated.

Bat tery stor age

Bat tery stor age was used in the very early days of elec tric power net works, but isno lon ger com mon, be cause bat ter ies are gen er ally ex pen sive, have main te nance prob -lems, and have lim ited life-spans. In a bat tery, charg ing causes re ac tions in elec tro chem i -cal com pounds to store en ergy from a gen er a tor in a chem i cal form. Upon de mand,re verse chem i cal re ac tions cause elec tric ity to flow out of the bat tery and back to the grid.Bat ter ies are man u fac tured in a wide va ri ety of ca pac i ties rang ing from less than 100 W to mod u lar con fig u ra tions of sev eral mega watts. Bat tery stor age has rel a tively high ef fi -ciency, as high as 90% or even better.

Bat tery stor age de vices are cur rently avail able in a wide va ri ety of chem i cal mod -els. One pos si ble tech nol ogy for large-scale stor age are large-scale flow bat ter ies. So -dium-sul phur (NaS) bat ter ies could also be in ex pen sive to im ple ment on a large scale.Va na dium re dox (re duc tion-ox i da tion) bat ter ies and other types of flow bat ter ies are also be gin ning to be used for en ergy stor age in clud ing the av er ag ing of gen er a tion from windtur bines. A flow bat tery is a form of re charge able bat tery in which elec tro lyte con tain ingone or more dis solved electroactive spe cies flows through a power cell in which chem i calen ergy is con verted to elec tric ity. Var i ous classes of flow bat ter ies ex ist in clud ing the re -dox flow bat tery, in which all electroactive com po nents are dis solved in the elec tro lyte.

Be cause flow bat ter ies can be rap idly “re charged” by re plac ing the elec tro lyte,they have been pro posed for elec tric ve hi cles, and the use of va na dium re dox flow bat -teries for load lev el ling in wind farm ap pli ca tions is al ready show ing prom ise. The va na -dium re dox bat tery en ergy stor age sys tem is de signed to al low wind en ergy gen er ateddur ing off-peak pe ri ods to be stored and sup plied to the na tional grid at a sched uled time.The stor age sys tem has the po ten tial to in crease the sup ply re li abil ity of wind en ergy andre duce the cost of the re serve re quire ments from gen er a tion plants. Bat ter ies can be used to max i mize wind in ten sity in ru ral re gions by pro vid ing high qual ity power out put andstor ing ex cess wind power for later re lease. In this way, in creased wind pen e tra tion can be achieved more rap idly by de fer ring the cost of grid up grades.

The util i ties are in stall ing the NaS bat ter ies at sub sta tions to meet peak loadsand de lay costly sub sta tion up grades. De spite the fact that NaS bat ter ies are costly, theyhave a great ad van tage over power gen er a tion in side a city, pre cisely at the right place and the right time the power is needed. Hence util ity scale bat ter ies that can store so lar pho to -vol taic en ergy and sup port de mand side man age ment are also ex tremely valu able re -sources to ur ban util i ties. NaS bat ter ies have the ad van tage of ex tremely quick re sponsetime, mak ing them more suit able for power qual ity ap pli ca tions (smooth ing short termspikes in de mand). This, along with their better ef fi ciency, is why NaS bat ter ies cur rentlyare used by util i ties seek ing to de lay trans mis sion and dis tri bu tion up grades.

In ap pli ca tions for elec tri cal en ergy stor age sys tems, hy brid elec tric ve hi cles andgrid-en hanced man age ment ar chi tec tures, it is es sen tial to achieve larger bat ter ies us ingcheaper, less toxic ma te ri als, de liv er ing higher en ergy den si ties at higher rates and withbetter charge/dis charge cy cling. How ever, cur rently, no sin gle tech nol ogy is able of ful fill -

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ing all the re quire ments for en ergy stor age, and next gen er a tion of en ergy-stor age cost ef -fi cient, long-life, and high-power ad vanced lith ium en ergy stor age sys tems are be ingde vel oped, based on the use of nano-pow der and nano-com pos ite elec trodes and elec tro -lytes, thus en sur ing high qual ity elec tric ity and en abling the de vel op ment of grid-en -hance ment man age ment ar chi tec ture in cor po rat ing re new able en ergy sources andad vanced bat tery sys tems, while con trol ling en vi ron men tal pol lu tion.

Fly-wheel stor age

A fly-wheel stor age de vice con sists of a fly-wheel that spins at a very high ve loc ityand an in te grated elec tri cal ap pa ra tus that can op er ate ei ther as a mo tor to turn thefly-wheel and store en ergy or as a gen er a tor to pro duce elec tric ity on de mand us ing theen ergy stored in the fly-wheel. Me chan i cal in er tia is the ba sis of this stor age method. Aheavy ro tat ing disc is ac cel er ated by an elec tric mo tor, which acts as a gen er a tor on re ver -sal, slow ing down the disc and pro duc ing elec tric ity. The use of mag netic bear ings and avac uum cham ber helps re duce en ergy losses. A proper match be tween ge om e try and ma -te rial char ac ter is tics in flu ences op ti mal wheel de sign. Fly-wheels are cur rently be ingused for a num ber of non-util ity re lated ap pli ca tions, but re cently, how ever, util ity en ergy stor age ap pli ca tions are con sid ered as well. Fly-wheel power stor age sys tems have stor -age ca pac i ties com pa ra ble to bat ter ies and faster dis charge rates. They are mainly used to pro vide load lev el ing for large bat tery sys tems, such as an uninterruptible power sup plyand for main tain ing power qual ity in renewable en ergy sys tems.

Fly-wheel energy stor age (FES) works by ac cel er at ing a ro tor (fly-wheel) to avery high speed and main tain ing the en ergy in the sys tem as ro ta tional en ergy. Elec tric ityis thus stored as the ki netic en ergy of the disc. Fric tion must be kept to a min i mum to pro -long the stor age time. The en ergy is con verted back by slow ing down the fly-wheel. MostFES sys tems use elec tric ity to ac cel er ate and de cel er ate the fly-wheel, but de vices that di -rectly use me chan i cal en ergy are also be ing de vel oped. Ad vanced FES sys tems have ro -tors made of high strength car bon-com pos ite fil a ments that spin at speeds from 20,000 toover 50,000 rpm in a vac uum en clo sure and use mag netic bear ings. Com pared with otherways of stor ing elec tric ity, FES sys tems have long life times, high en ergy den sities andlarge max i mum power out puts. The en ergy ef fi ciency of fly-wheels can be as high as 90%.

Elec tro chem i cal ca pac i tors

Elec tro chem i cal ca pac i tors (also known as ultracapacitors or supercapacitors)are de vel oped as an en ergy stor age tech nol ogy for elec tric util ity ap pli ca tions. An elec -tro chem i cal ca pac i tor has com po nents re lated to both a bat tery and a ca pac i tor. Thecharge is stored by ions as in a bat tery, but, as in a con ven tional ca pac i tor, no chem i cal re -ac tion takes place in en ergy de liv ery. An elec tro chem i cal ca pac i tor con sists of two op po -sitely charged elec trodes, a sep a ra tor, elec tro lyte and cur rent col lec tors. Pres ently, verysmall supercapacitors in the range of seven to ten watts are widely avail able com mer ciallyfor con sumer power qual ity ap pli ca tions and are com monly found in house hold elec tri cal de vices. De vel op ment of larger-scale ca pac i tors has been fo cused on elec tric ve hi cles.

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Cur rently, smal-scale power qual ity (<250 kW) is con sid ered to be the most prom is ingutil ity use for ad vanced ca pac i tors.

Con ven tional ca pac i tors have enor mous power, but store only tiny amounts ofen ergy, while bat ter ies can store lots of en ergy but have low power and they take a longtime to be charged or dis charged. Supercapacitors of fer a unique com bi na tion of highpower and high en ergy. Bat ter ies are charged when they un dergo an in ter nal chem i cal re -ac tion, and de liver the ab sorbed en ergy, or dis charge, when they re verse the chem i cal re -ac tion. In con trast, when a supercapacitor is charged, there is no chem i cal re ac tion.In stead, the en ergy is stored as a charge or con cen tra tion of elec trons on the sur face of ama te rial. Supercapacitors are ca pa ble of very fast charges and dis charges, and ap par entlyare able to go through a large num ber of cy cles with out deg ra da tion.

Small scale hy brid stor age sys tems

In a nor mal res i den tial ap pli ca tion, a house hold is served si mul ta neously by a lo -cal power source (the wind tur bine, for ex am ple) and the util ity elec tri cal sys tem. Usu allythere are no bat ter ies in a mod ern res i den tial wind sys tem, but small wind sys tems for re -mote ap pli ca tions must op er ate by the use of bat ter ies for en ergy stor age. A com bi na tionof the wind and so lar sys tems may take the ad van tages of both. They are con nected to theutil ity grid in case the weather is in suf fi cient for the so lar or wind sys tem, but they alsohave bat ter ies to store elec tric ity in case the util ity grid goes down as well, fig. 3.

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Fig ure 3. Small scale wind and wind/so lar hy brid sys tems with bat tery stor age

A wind tur bine col lects ki netic en ergy from the wind and con verts it to elec tric itythat is com pat i ble with util ity sys tem. If the wind speeds are be low cut-in speed there willbe no out put from the tur bine and all of the needed power is pur chased from the util ity.As wind speeds in crease, tur bine out put in creases and the amount of power pur chasedfrom the util ity is pro por tion ately de creased, and when the tur bine pro duces more powerthan needed, the ex tra elec tric ity is sold to the util ity. The de sign and in stal la tion of thesesys tems is rather com pli cated and ex pen sive, but they are the most ef fec tive in pro vid ingcon stant, re li able elec tric ity sup ply.

Fuel cells

A fuel cell is an elec tro chem is try en ergy con ver sion de vice that pro duces elec -tric ity from ex ter nal sup plies of fuel (on the an ode side) and ox i dant (on the cath odeside). These re act in the pres ence of an elec tro lyte. Gen er ally, the re ac tants flow in andre ac tion prod ucts flow out while the elec tro lyte re mains in the cell. Fuel cells dif fer frombat teries in that they con sume re ac tant, which must be re plen ished, while bat ter ies storeelec tri cal en ergy chem i cally in a closed sys tem. Ad di tion ally, while the elec trodes within a bat tery re act and change as a bat tery is charged or dis charged, a fuel cell’s elec trodes arecat a lytic and rel a tively sta ble. How ever, the fuel cells can not store en ergy like a bat tery,but in some ap pli ca tions, such as stand-alone power plants based on dis con tin u oussources such as so lar power or wind power, they are com bined with electolysis and stor -age sys tems to form an en ergy stor age sys tem.

Fel cells are very use ful as power sources in re mote lo ca tions, such as space craft,re mote weather sta tions, and in cer tain mil i tary ap pli ca tions. The over all ef fi ciency (elec -tric ity to hy dro gen and back to elec tric ity) of such plants is be tween 30 and 50%. While amuch cheaper lead-acid bat tery might re turn about 90%, the electrolyzer/fuel cell sys temcan store in def i nite quan ti ties of hy dro gen, and is there fore better suited for long-termstor age. With in ter mit tent renewables such as so lar, small hy dro and wind, the out putmay be fed di rectly into an elec tric ity grid. At pen e tra tions be low 20% of the grid de -mand, this does not se verely change the eco nom ics; but be yond about 20% of the to tal de -mand, ex ter nal stor age will be come im por tant. If these sources are used for elec tric ity tomake hy dro gen, then they can be uti lized fully when ever they are avail able, fig. 4.

Hy dro gen is a por ta ble en ergy stor age method, be cause it must first be man u fac -tured by other en ergy sources in or der to be used. How ever, as a stor age me dium, it maybe a sig nif i cant fac tor in us ing re new able en er gies. Hy dro gen may be used in con ven -tional in ter nal com bus tion en gines, or in fuel cells which con vert chem i cal en ergy di rectly to elec tric ity with out flames, sim i lar to the way the hu man body burns fuel. Mak ing hy -dro gen re quires ei ther re form ing nat u ral gas with steam, or, for a pos si bly re new able andmore en vi ron men tal friendly source, the elec trol y sis of wa ter into hy dro gen and ox y gen.The for mer pro cess has car bon di ox ide as a by-prod uct [8].

Solid-ox ide fuel cells pro duce exo ther mic heat from the re com bi na tion of theox y gen and hy dro gen. This heat can be cap tured and used to heat wa ter in a com binedheat and power (CHP) ap pli ca tion. When the heat is cap tured, to tal ef fi ciency can reach80-90%. A new ap pli ca tion is mi cro CHP, which is cogeneration for fam ily homes, of fice

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build ings and fac to ries, with com bined heat and power ef fi ciency is typ i cally around 80%. Be cause fuel cells have no mov ing parts, and do not in volve com bus tion, in ideal con di -tions they can achieve up to 99.9999% re li abil ity.

Super con duct ing mag netic en ergy stor age

Super con duct ing mag netic en ergy stor age sys tems store en ergy in the mag neticfield cre ated by the flow of di rect cur rent in a superconducting coil which has been cryo -genically cooled to a tem per a ture be low its super con duct ing crit i cal tem per a ture. Oncethe super con duct ing coil is charged, the cur rent will not de cay and the mag netic en ergycan be stored in def i nitely. The stored en ergy can be re leased back to the net work by dis -charg ing the coil. Super con duct ing mag netic en ergy stor age sys tems are highly ef fi cient;the ef fi ciency is greater than 95%, but the high cost of su per con duc tors is the pri mary lim -i ta tion for com mer cial use of this en ergy stor age method.

The en ergy out put of a super con duct ing mag netic en ergy stor age sys tem is much less de pend ent on the dis charge rate than bat ter ies. These sys tems also have a high cy clelife and, as a re sult, are suit able for ap pli ca tions that re quire con stant, full cy cling and acon tin u ous op er a tion mode. Al though re search is be ing con ducted on larger super con -duct ing mag netic en ergy stor age sys tems in the range of 10 to 100 MW, re cent fo cus hasbeen on the smaller mi cro de vices in the range of 1 to 10 MW.

Sum mary and con clu sion

Global warm ing, fi nite fos sil-fuel sup plies and en vi ron men tal pol lu tion con spire to make re new able en ergy an im per a tive. This makes it nec es sary for re new able en ergy

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Fig ure 4. Re new able en ergy stor age by the use of fuel cell

sources to over come pres ent bur dens of their intermittency by en ergy stor age and thus be more com pet i tive with the con ven tional en ergy sources. A fu ture ”green en ergy” econ -omy must be based on small, dis trib uted gen er a tors that ex ploit a di ver sity of tech nol o -gies such as wind, so lar, fuel cells, etc.[9]. For ex am ple, as wind does n’t al ways blow, abat tery or other stor age tech nol ogy is re quired to pre serve en ergy gen er ated un tilneeded.

A press ing need will there fore arise for en ergy stor age sys tems to bal ance sup plywith de mand. In their con tri bu tion to com bat global warm ing, the ma jor is sues to be con -sid ered in eval u at ing en ergy stor age op tions is the amount of en ergy that is lost in thestor age pro cess. The es ti mates of the typ i cal en ergy ef fi ciency of sev eral avail able oremerg ing en ergy stor age tech nol o gies that are gen er ally rec og nized as hav ing com mer -cial po ten tial reached or ex pected in the near fu ture is pre sented in tab. 1, which also sum -ma rizes par tic u lar sys tem size ranges (ex ist ing and po ten tial) and de vel op ment sta tus.

Ta ble 1. Com par i son of ma jor en ergy stor age tech nol o gies

Energy storagetechnology

Status ofdevelopment

Unit sizes ”Round trip”energy

efficiencyExisting Potential

Compressed air Commercial 100-300 MW <30 MW ~80%

Pumped hydro Commercial 5-2100 MW <2000 MW 75-80%

Batteries (conv.) Commercial >1 kW <10 MW 50-90%

Flow batteries Limited commercial <20 MW <1000 MW 70-75%

Magnetic Limited demo. <10 MW <100 MW ~90%

Fly-wheels Limited demo. <1 MW <1-1000 MW ~80%

Fuel cells Commercial 50 kW-1 MW >100 MW 40-80%

Also, with trans por ta tion ac count ing for a great deal of emis sions, hy brid elec -tric ve hi cles have a well-de fined part to play; hence re charge able bat ter ies are needed.Fur ther more, the de vel op ment of tech no log i cal ad vance and mo bil ity makes it in ev i ta blefor elec tric ity pro vid ers to mod ify their pres ent dis trib uted elec tric ity net works, and de -velop grid-en hanced man age ment ar chi tec tures com bin ing re new able en ergy sourceswith bat ter ies and other en ergy stor age tech nol o gies for meet ing peak-power de mandswhile pro vid ing high-power qual ity.

Ref er ences

[1] Mesarovi}, M., Elec tric ity Gen er a tion and Cli mate Change Is sues, Pro ceed ings on CD, 2nd In -ter na tional Con fer ence “Power Plants 2006”, Vrnja~ka Banja, Ser bia, 2006

[2] Cavalo, A. J., En ergy Stor age Tech nol o gies for Util ity Scale for In ter mit tent Re new able En -ergy Sys tems, Jour nal on So lar En ergy En gi neer ing, 7 (2001), 123, 387-389

M. Mesarovi}: Energy Storage Technologies and Renewable Energy SourcesTERMOTEHNIKA broj 1 ‡ godina XXXIV, 31‡44 (2008)

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[3] Rakovi}, R., Liljergren, C., Lindahl, F., To wards Ap pli ca tion of Wind En ergy So lu tions forRe gional Power Sup ply within Moun tain Re gion of Ser bia, Pro ceed ings, In ter na tional Con -gress on En ergy Ef fi ciency and Re new able En ergy Sources in In dus try and Build ings, Plovdiv, Bul garia, 2005, 126-135

[4] Mesarovi}, M., Martinoli, A., Com pressed Air En ergy Stor age for Load Man age ment, Pro -ceed ings, In ter na tional Sym po sium, “En ergy Sys tems in South-East ern Eu rope”, Ohrid, Mac -e do nia, 1995, 45-54

[5] Djordjevi}, B., In stal la tion de tur bine a gaz pour l’accumulation d’energie, Pat ent deYougoslavie, No. 17374/1950

[6] Denholm, D., Wind Sys tem In cor po rat ing Biofueled CAES, Re new able En ergy, 31 (2006), 9,1355-1370

[7] Mesarovi}, M., Sus tain able En ergy from Bio mass, Ther mal Sci ence, 15 (2001), 2, 4-27[8] Mesarovi}, M., HVAC&R Tech nol o gies and En ergy Sup ply at a Cross roads, Jour nal

Klimatizacija, Grejanje, Hla|enje – KGH, 35 (2006), 1, 53-64[9] Mesarovi}, M., Re la tion ship of Power Gen er a tion and Cli mate Change – Fa mous Dis cov er ies

of Nikola Tesla and Milutin Milankovi}, Pro ceed ings, 6th In ter na tional Sym po sium NikolaTesla, 2006, Bel grade, 271-274

Apstrakt

Miodrag MESAROVI]

Energoprojekt ‡ Entel, Beograd, Srbija

Tehnologije akumulacije energijei obnovqivi izvori energije

Obnovqivi izvori treba da igraju va`nu ulogu u spre~avawu promene klime,

ali su konfrontirani sa jednim brojem barijera, kao {to su prekidi i visoke cene.

Akumulacija energije im pru`a odli~nu mogu}nost da prevazi|u manu prekidnosti.

Opisane su konvencionalne tehnologije akumulacije toplote, komprimovawem

vazduha, pumpawem vode, baterija, zamajaca, kao i nove akumulacione tehnologije u

vidu gorivnih }elija, solarne i energije vetra i razne hibridne verzije, uz nagla-

{avawe perspektivnih primera.

Kqu~ne re~i: obnovqivi izvori energije, tehnologije akumulacije energije,prekidnost

Au thor's e-mail:m-mesarovic@ep-entel.com

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