Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h, Vo lume 2, Issue 12, Dece mbe r-2011 1

ISS N 2229-5518

Power-Management Strategies for a Grid-Connected PV-FC Hybrid Systems

T.Kranthi kumar , Ratnaraju , Apparao

Abstract - This pa pe r presents a method to opera te a grid connecte d hybrid s ys te m. The hybrid s ys tem compose d of a Photovolta ic (PV) a rray a nd a Proton e xcha nge me mbra ne fue l ce ll (PEMFC) is cons ide re d. Two ope ration modes , the unit -powe r control (UPC) mode a nd the feede r-flow control (FFC) mode , ca n be a pplie d to the hybrid syste m. I n the UPC mode , varia tions of loa d de ma nd are compe nsate d by the ma in grid beca use the hybrid s ource output is re gula te d to re fere nce power. Re ne wa ble e nergy is currently wide ly use d. One of these resources is solar e nergy. The photovolta ic (PV) a rra y norma lly uses a ma ximum powe r point tra cking (MPPT) technique to continuous ly de live r the highes t power to the loa d whe n the re are va ria tions in irra dia tion a nd te mpe ra ture . The dis a dva nta ge of PV e nergy is that the PV output power de pe nds on wea the r conditions a nd ce ll te mpe ra ture, ma king it an uncontrolla ble source. Furthermore , it is not a va ila ble during the night. I n orde r to overcome these inhe re nt dra wbacks , a lterna tive s ources , s uch as PEMFC, s hould be ins ta lle d in the hybrid s ys te m. By cha nging the FC output powe r, the hybrid s ource output be comes controlla ble . The re fore , the refe re nce va lue of the hybrid s ource output mus t be de te rmine d. I n the FFC mode , the fee der flow is re gulate d to a cons ta nt, the e xtra loa d de ma nd is picke d up by the hybrid source , a nd, he nce , the fee de r refe rence powe r m us t be known. He s ys tem ca n ma ximize the ge nera te d powe r whe n loa d is hea vy a nd minimizes the loa d s he dding a rea. W he n loa d is light,

the UPC mode is se lecte d a nd, thus , the hybrid source works more s ta bly. The cha nges in ope rating mode only occur whe n the lo a d de ma nd is a t the boundary of mode cha nge ; otherwise , the ope ra ting mode is e ithe r UPC mode or FFC mode . Bes ides , the va ria tion of
hybrid s ource re fe rence powe r is e limina te d by mea ns of hys te res is . The propose d ope ra ting s tra tegy with a fle xible ope ra tion mode cha nge a lwa ys ope ra tes the PV a rra y a t ma ximum output powe r a nd the PEMFC in its high efficie ncy pe rforma nce ba nd, thus
improving the pe rforma nce of sys te m ope ra tion, e nhancing s ys tem s ta bility, a nd de creas ing the numbe r of ope ra ting mode cha nge s.

INTRODUCT ION:


Re ne wa ble e ne rgy is curre ntly wide ly us e d. One of these re sources is s ola r e nergy. The photovolta ic (PV) a rra y norma lly uses a ma ximum powe r point tra cking (MPPT) te chnique to continuous ly de liver the highes t powe r to the loa d whe n there a re va riations in irra dia tion a nd te mpera ture . The dis a dva nta ge of PV e nergy is tha t the PV output powe r de pe nds on we athe r conditions and ce ll te mpe ra ture, ma king it a n uncontrolla ble s ource . Furthe rmore , it is not ava ila ble during the night. I n orde r to ove rcome these inhe rent dra wbacks , a lterna tive s ources , s uch as PEMFC, s hould be ins ta lle d in the hybrid sys te m. By cha nging the FC output powe r, the hybrid source output be comes controlla ble . Ho we ve r, PEMFC, in its turn high e fficie ncy within a s pecific powe r ra nge .
The hybrid s ys tem ca n e ithe r be connecte d to the ma in grid or work a utonomous ly with res pe ct to the grid-connecte d mode or is la nde d mode , res pective ly. I n the grid-connecte d mode , the hybrid s ource is connecte d to the ma in grid a t the point of common coupling (PCC) to de live r powe r to the loa d. W he n loa d dema nd cha nges, the powe r s upplie d by the ma in grid a nd hybrid s ys tem mus t be prope rly cha nge d.
The powe r de live re d from the ma in grid a nd PV a rray as we ll
as PEMFC mus t be coordina te d to mee t loa d de ma nd. The
hybrid s ource ha s two control modes : 1) unit-po we r control
(UPC) mode a nd fee de r-flow control (FFC) mode . I n the UPC
mode , va ria tions of loa d dema nd are compe nsa te d by the ma in

grid beca use the hybrid s ource output is regula te d to re fere nce
powe r. fore, the refe re nce va lue of the hybrid s ource
output mus t be de termine d. I n the FFC mode , the fee der

flow is re gula te d to a cons ta nt, the e xtra loa d dema nd is picke d
up by brid s ource , a nd, he nce , the fee der re fere nce
powe r mus t be known.The propose d ope rating
s tra tegy is to coordina te the two control modes a nd de termine the re fere nce va lues of the UPC mode a nd FFC mode s o tha t a ll cons tra ints a re sa tis fie d. This ope rating s trate gy will minimize the number of ope ra ting mode cha nges , improve
pe rforma nce of the s ys tem ope ra tion, a nd e nha nce s ys tem s ta bility.

DIST RIBUT ED GENERAT ION:

Dis tribute d e nera tion, a lso ca lle d on-s ite ge nera tion, dis pe rse d ge ne ra tion, embe dde d ge ne ra tion, dece ntra lize d ge nera tion, dece ntra lize d ene rgy or dis tribute d ene rgy ge ne ra tes e lectricity from ma ny sma ll e nergy s ources . Curre ntly, indus tria l countries gene ra te mos t of the ir e lectricity in la rge ce ntra lize d facilitie s , s uch as fos s il fue l (coa l, gas powe re d) nuclea r or hydropowe r pla nts . These pla nts have e xce lle nt e conomie s of sca le , but us ua lly tra ns mit e le ctricity long dis ta nces a nd nega tive ly a ffect the e nvironme nt.
Mos t pla nts a re built this wa y due to a number of e conomic , hea lth & sa fety, logis tica l. For e xample , coa l powe r pla nts a re built a wa y from cities to pre ve nt the ir heavy a ir pollution from a ffecting the populace . I n a ddition, s uch pla nts a re ofte n built near collie rie s to minimize the cost of tra ns porting coa l. Hydroe lectric pla nts a re by the ir nature limite d to ope ra ting a t s ites with s ufficie nt wa ter flow. Mos t powe r pla nts a re ofte n cons ide re d to be too far a wa y for the ir wa s te hea t to be use d for hea ting buildings .Dis tribute d

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ge ne ra tion is a nother a pproa ch. I t re duces the a mount of e nergy los t in tra ns mitting e lectricity beca use the e lectricity is ge ne ra te d very nea r where it is use d, pe rha ps e ve n in the same building. Th is a lso re duces the s ize a nd number of powe r lines tha t mus t be cons tructe d. Typica l dis tribute d powe r s ources in a Fee d-in Ta riff (FI T) s cheme ha ve low ma inte na nce, low pollution a nd high e fficie ncies . I n the pa st, these tra its re quire d de dica te d ope ra ting enginee rs a nd la rge comple x pla nts to re duce pollution. Howe ve r, mode rn embe dde d sys te ms ca n provide these tra its with a utoma te d opera tion a nd rene wa bles , s uch as s unlight, win d a nd ge otherma l. This re duces the s ize of powe r pla nt tha t ca n s how a profit.

DIST RIBUT ED ENERGY RESOURCE

Dis tribute d e nergy resource (DER) sys te ms a re s ma ll-sca le powe r ge nera tion technologies (typica lly in the ra nge of 3 kW to 10,000 kW ) use d to provide a n a lterna tive to or a n e nha nceme nt of the tra ditiona l e lectric powe r s ys tem. The us ua l problems with dis tribute d ge nera tors a re the ir high cos ts .One popula r s ource is sola r pa ne ls on the roofs of buildings . The production cos t is $0.99 to 2.00/W (2007) plus ins ta lla tion a nd s upporting equipme nt unless the ins ta lla tion is Do it yourse lf (DI Y) bringing the cos t to $6.50 to 7.50 (2007). This is compa ra ble to coa l powe r pla nt cos ts of $0.582 to
0.906/W (1979), a djus ting for infla tion. Nuclea r power is highe r at $2.2 to $6.00/W (2007). Some solar ce lls ("thin-film" type ) a lso ha ve was te dis pos a l iss ues , s ince "thin-film" type solar ce lls ofte n conta in hea vy-me ta l e lectronic wa s tes , s uch as Ca dmium te lluride (CdTe ) a nd Copper indium ga llium se le nide (CuI nGaSe ), a nd nee d to be recycle d. As oppose d to s ilicon semi-conductor type s olar ce lls which is ma de from qua rtz. The plus s ide is tha t unlike coa l a nd nuclea r, there are no fue l cos ts , pollution, mining s a fe ty or opera ting sa fety is s ues . Sola r a ls o has a low duty cycle , producing pea k powe r a t loca l noon each da y. Ave rage duty cycle is typica lly 20%. Another source is sma ll wind turbines . These ha ve low ma inte na nce , a nd low pollution. Cons truction cos ts a re higher ($0.80/W , 2007) pe r wa tt tha n la rge powe r pla nts , e xce pt in very windy a reas . W ind towe rs a nd ge nera tors have s ubs ta ntia l ins urable lia bilitie s ca use d by high win ds , but good opera ting sa fety. I n s ome areas of the US the re may a ls o be Property Ta x costs involve d with wind turbine s tha t a re not offset by incentives or a cce le ra te d de pre cia tion. W ind a lso te nds to be compleme nta ry to solar; on da ys there is no s un there te nds to be wind a nd vice versa . Ma ny dis tribute d ge ne ra tion s ites combine win d po we r a nd solar powe r s uch as Slippe ry Rock Unive rs ity, which ca n be monitore d online . Dis tribute d coge ne ra tion sources use na tura l gas -fire d micro turbines or reciproca ting e ngines to turn gene ra tors . The hot e xha us t is the n use d for s pace or wa ter hea ting, or to drive an a bsorptive chille r for a ir-conditioning. The clea n fue l has only low pollut ion. Des igns currently ha ve uneve n re lia bility, with s ome ma ke s ha ving e xce lle nt ma inte na nce cos ts , a nd othe rs be ing unacce ptable .Co-ge nera tors are a lso more e xpe ns ive per wa tt tha n centra l ge nera tors . The y find fa vor be ca use mos t buildings a lrea dy burn fue ls , a nd the cogene ra tion ca n e xtra ct more va lue from the fue l. Some la rger ins ta lla tions utilize combine d cycle ge nera tion. Us ua lly this cons ists of a gas turbine whose e xha us t boils wa te r for a s team turbine in a Ra nkin cycle . The conde nser of the s tea m cycle provides the hea t for s pa ce hea ting or a n a bs orptive chille r. Combine d cycle pla nts with coge nera tion have the highes t known the rma l e fficie ncies , ofte n e xcee ding 85%.I n countries with high pre ss ure gas dis tribution, s ma ll turbines ca n be use d to bring the ga s press ure to domes tic leve ls whils t e xtracting use ful e nergy. I f the UK we re to impleme nt this countrywide
a n a dditiona l 2-4 GWe would be come a va ilable . (Note tha t the e nergy is a lrea dy be ing gene ra te d e lse whe re to provide the high initia l gas press ure - this me thod s imply dis tributes the e nergy via a diffe rent route .)Future ge nera tions of e le ctric ve hicles will ha ve the a bility to de live r power from the ba ttery into the grid whe n nee de d. This could a ls o be a n importa nt dis tribute d ge nera tion resource. Re ce ntly inte rest in Dis tribute d Energy Syste ms (DES) is increas ing, pa rticula rly ons ite gene ra tion. This inte res t is beca use la rge r power pla nts a re economica lly unfeas ible in ma ny re gions due to increas ing s ys tem a nd fue l cos ts , a nd more s trict e nvironme nta l re gula tions . I n a ddition, re cent techno logica l a dva nces in sma ll ge ne ra tors , Power Electronics , a nd e nergy s tora ge de vices ha ve provide d a ne w opportunity for dis tribute d e ne rgy re sources a t the dis tribution leve l, a nd es pecia lly, the ince ntive la ws to utilize re ne wable e ne rgie s has a lso e ncoura ge d a more dece ntra lize d a pproach to powe r de live ry.The re a re ma ny ge ne ra tion sources for DES: conve ntiona l technologies (diese l or na tura l gas e ngines ), emerging technologies (micro turbines or fue l ce lls or e ne rgy s torage de vices ), a nd re ne wa ble technologie s (s ma ll win d turbine s or sola r/photovolta ic’s or s ma ll hydro turbines ). These DES are use d for a pplica tions to a s ta nda lone , a s ta ndby, a grid-inte rconnecte d, a coge nera tion, pea k s ha vings , e tc. a nd ha ve ma ny a dvanta ges s uch as e nvironme nta l-frie ndly a nd modula r e lectric ge nera tion, increase d re lia bility, high powe r qua lity, uninte rruptible se rvice , cos t sa vings , on-s ite ge ne ration, e xpa nda bility, e tc. So ma ny utility compa nies are trying to cons truct sma ll dis tribution s ta tions combine d with s e vera l DES a va ila ble a t the re gions , ins tea d of large powe r pla nts . Bas ica lly, these technologie s a re base d on nota bly a dva nce d Powe r Electronics be ca use a ll DES require Powe r Converters , interconnection techniques , a nd e lectronic control units . Tha t is , a ll powe r ge ne ra te d by DES is ge nera te d a s DC Powe r, a nd the n a ll the powe r fe d to the DC dis tribution bus is a ga in converte d into a n AC powe r with f ixe d ma gnitude a nd fre que ncy by control units us ing Digita l Signa l Process or (DSP). So improve d powe r e lectronic tec hnologies tha t pe rmit grid inte rconnection of as ynchronous ge nera tion s ources are de finite ly require d to s upport dis tribute d gene ra tion resources
The research works in the rece nt pa pe rs about DES focus on be ing utilize d directly to a s ta nda lone AC s ys tem o r fe d back to the utility ma ins . That is , whe n in norma l ope ra tion or ma in fa ilures , DES dire ctly s upply loa ds with powe r (s ta nda lone mode or s ta ndby mode ), while , whe n DES have s urplus powe r or nee d more power, this s ys tem ope rates in pa ra lle l mode to the ma ins . The re fore , in orde r to pe rmit to connect more ge ne ra tors on the network in good conditions , a good technique a bout inte rconnection with the grid a nd voltage re gula tions s hould ove rcome the proble ms due to pa ra lle l opera tion of Powe r Converte r for a pplica tions to DES.

DIST RIBUT ED ENERGY SYST EMS

Toda y, ne w a dva nces in technology a nd ne w dire ctions in e lectricity regula tion e ncoura ge a s ignifica nt increase of dis tribute d ge nera tion resources around the world. As s hown in Fig. the curre ntly compe titive s ma ll ge nera tion units a nd the ince ntive la ws to use rene wa ble e nergies force e le ctric utility compa nies to cons truct a n increas ing number of dis tribute d ge ne ra tion units on its dis tribution ne twork, ins tea d of la rge ce ntra l power pla nts . Moreover, DES ca n offe r improve d se rvice re lia bility, bette r economics a nd a re duce d de pe nde nce on the loca l utility. Dis tribute d Ge nera tion Sys tems have ma inly been use d as a s ta ndby powe r s ource for critica l bus inesses . For e xample , mos t hos pita ls a nd office buildings ha d s ta nd-by diese l ge nera tion as a n e merge ncy powe r source

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for use only during outa ges . Howe ve r, the diese l ge ne ra tors we re not inhe rently cos t-e ffective , a nd produce noise a nd e xha us t tha t would be objectiona ble on a nything e xce pt for a n eme rgency bas is .
Fig. A la rge ce ntra l power pla nt a nd dis tribute d e nergy s ys tems
Mea nwhile , rece ntly, the use of Dis tribute d Ene rgy Sys tems unde r the 500 kW leve l is ra pidly increas ing due to re cent technology improveme nts in s ma ll ge nera tors , powe r e lectronics , a nd e nergy s tora ge de vices . Efficie nt clea n foss il fue ls technologies s uch as micro -turbines a nd fue l ce lls , a nd e nvironme nta lly frie ndly re ne wa ble e ne rgy technologies s uch as solar/photo volta ic , s ma ll win d a nd hydro a re increas ingly use d for ne w dis tribute d gene ra tion s ys tems . These DES are a pplie d to a s ta nda lone, a s ta ndby, a grid-interconnecte d, a cogene ra tion, pea k s ha vings , e tc. a nd ha ve a lot of be ne fits s uch as e nvironme nta l-frie ndly a nd modula r e le ctric ge ne ra tion, increase d re lia bility, high powe r qua lity, uninte rruptible service , cost s avings , on-s ite ge nera tion, Expa nda bility, e tc. The ma jor Dis tribute d Ge nera tion technologie s tha t will be discusse d in this section a re as follows : micro-turbine s , fue l ce lls , sola r/photovolta ic s ys tems , a nd e nergy storage de vices . Micro-turbines , es pe cia lly the s ma ll gas fire d micro turbines in the 25-100 kW tha t ca n be mass -produce d a t low cos t ha ve been more a ttra ctive due to the compe titive price of na tura l gas , low ins ta lla tion a nd ma inte na nce cos ts . I t ta kes very cleve r e ngineering a nd use of innova tive des ign (e .g. a ir bea ring, recupe ra tion) to achieve rea sona ble e fficie ncy a nd costs in machine s of lowe r output, a nd a big a dva nta ge of these s ys tems is s ma ll be ca use these ma inly use high-s pee d turbines (50,000-90,000 RPM) with a ir foil bea rings . The re fore , micro turbines hold the mos t promise of a ny of the DES te chnologies toda y. Fue l ce lls a re a ls o we ll use d for dis tribute d ge nera tion a pplica tions , a nd ca n esse ntia lly be de scribe d as ba tteries which ne ver become dis cha rge d as long as hydroge n a nd oxygen a re continuous ly provide d. The hydrogen ca n be s upplie d directly, or produce d from na tura l gas , or liquid fue ls s uch as a lcohols , or gasoline . Ea ch unit ra nges in s ize from 3 – 250 kW or larger MW s ize . Eve n if they offer high e fficie ncy a nd low e miss ions , toda y’s cos ts a re high. The poss ibility of us ing gas oline a s a fue l for ce lls has res ulte d in a major de ve lopme nt e ffort by the a utomotive compa nies . The recent research work about fue l ce lls is focuse d towa rds the polyme r e le ctrolyte membra ne (PEM) f ue l ce lls . Fue l ce lls in s izes greate r tha n 200 kW , hold promise be yond 2005, but res ide ntia l s ize fue l ce lls a re unlike ly to ha ve a ny s ignifica nt ma rke t impa ct a ny time soon.Mixe d micro-turbine a nd fue l ce ll s ys te ms will a ls o be a va ilable as a dis tribute d ge nera tion s ource . Rece ntly, a solid oxide fue l ce ll has bee n combine d with a gas micro -turbine creating a combine d cycle powe r pla nt. I t has e xpe cte d e lectrica l e fficie ncy of greate r tha n 70 %, a nd the e xpe cte d powe r le ve ls ra nge from 250 kW to 2.5 MW . Sola r/photovolta ic sys te ms ma y be use d in a va rie ty of s izes , but the insta lla tion of la rge numbe rs of photovolta ic syste ms is undes ira ble due to high la nd costs a nd in ma ny geogra phic areas with poor inte ns ity a nd re liability of s unlight.
I n ge nera l, a lmos t one a cre of la nd would be nee de d to provide 150 kW of e lectricity, so solar/photovolta ic s ys tems will continue to ha ve limite d a pplica tions in the future . Ene rgy s tora ge de vices s uch as ultra ca pacitors , ba tteries , a nd flywhe e ls a re one of the mos t critica l technologie s for DES. I n ge ne ra l, the e lectrochemica l ca pacitor ha s high powe r de ns ity as we ll a s good e nergy de ns ity. I n pa rticula r, ultra ca pacitors ha ve seve ra l be ne fits s uch as high puls e powe r ca pacity, long life time , high powe r de ns ity, low ESR, a nd very thin a nd tight.
I n contra st, ba tteries ha ve highe r e ne rgy de ns ity, but lowe r
powe r de ns ity a nd s hort life time re lative to ultra -ca pa citor. So
hybrid Powe r Sys tem, a combina tion of ultra -ca pacitor a nd
ba tte ry, is s trongly recomme nde d to sa tis fy seve ra l
re quire me nts a nd to optimize s ys tem pe rforma nce . Rece ntly
s tora ge s ys tems a re much more e fficie nt, chea pe r, a nd longe r
tha n five years a go. I n particula r, flywhee l s ys tems ca n
ge ne ra te 700 kW for 5 se conds , while 28-ce ll ultra ca pacitors
ca n provide up to 12.5 kW for a fe w seconds . I n the pas t, the
e lectric utility indus try did not offe r va rious options tha t were
s uite d for a wide ra nge of cons umer nee ds , a nd mos t utilitie s
offe re d a t bes t two or three combina tions of re lia bility-price .
Ho we ve r, the types of mode rn DES give comme rcia l e le ctric
cons umers va rious options in a wide r ra nge of re lia bility -price
combina tions . For these reasons , DES will be ve ry like ly to
thrive in the next 20 yea rs , a nd es pecia lly, dis tr ibute d
ge ne ra tion technologies will ha ve a much grea te r ma rke t
pote ntia l in a reas with high e lectricity cos ts a nd low re lia bility
s uch as in de ve loping countries

PROBLEM ST AT EMENT S

DES technologies ha ve very diffe re nt iss ues compa re d with tra ditiona l ce ntra lize d powe r sources . For e xa mple , they are a pplie d to the ma ins or the loa ds with volta ge of 480 volts or less ; a nd require powe r conve rte rs a nd diffe re nt stra te gies of control a nd dis pa tch. All of these ene rgy technologies provide a DC output which requires powe r e lectronic inte rfaces with the dis tribution powe r ne tworks a nd its loa ds . I n mos t cases the convers ion is pe rforme d by us ing a volta ge source inve rter (VSI ) with a pos s ibility of puls e width modula tion (PW M) tha t provides fas t regula tion for volta ge ma gnitude .
Powe r e le ctronic inte rfaces introduce ne w control iss ues , but a t
the same time , ne w poss ibilitie s . For e xample , a s ys tem which
cons ists of micro-ge ne ra tors a nd s tora ge de vices could be
des igne d to opera te in both a n a utonomous mode a nd
connecte d to the powe r grid. One la rge class of problems is
re la te d to the fa ct tha t the powe r sources s uch as microturbines
a nd fue l ce ll ha ve s low res ponse a nd the ir ine rtia is much less .
I t mus t be re membere d tha t the curre nt powe r sys te ms have
s tora ge in ge nera tors ’ ine rtia , a nd this ma y res ult in a s light
re duction in s ys tem freque ncy. As these ge nera tors become
more compact, the nee d to lin k the m to lowe r ne twork voltage
is s ignifica ntly increas ing.
Ho we ve r, without a ny me dium volta ge ne tworks a da pta tion ,
this fas t e xpa ns ion ca n a ffect the qua lity of s upply as we ll a s
the public a nd equipme nt sa fe ty beca use dis tribution ne tworks
ha ve not been des igne d to connect a s ignifica nt a mount of
ge ne ra tion. There fore, a ne w volta ge control s ys tem to
facilita te the connection of dis tribute d ge nera tion resources to
dis tribution ne tworks s hould be de ve lope d.
I n ma ny cases there a re a ls o ma jor technica l ba rriers to
opera ting inde pende ntly in a s ta nda lone AC s ys tem, or to
connecting sma ll ge ne ra tion sys te ms to the e lectr ica l
dis tribution network with lo we r voltage , a nd the re cent
re search iss ues includes :
1. Control s tra te gy to facilita te the connection of dis tribute d ge ne ra tion resources to distribution ne tworks .
2. Efficie nt ba ttery control.
3. Inv e rte r control base d on only loca l informa tion.

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4. Synchroniza tion with the utility ma ins .
5. Compe nsa tion of the reactive power a nd higher ha rmonic
compone nts .
6. Po we r Factor Correction.
7. Syste m protection.
8. Lo ad sharing .
9. Re liability of communica tion.
10. Re quireme nts of the cus tome r.
DES offe rs s ignifica nt research a nd e ngineering cha lle nges in
s olving these problems . Moreover, the e le ctrica l a nd e conomic
re la tions hips be twee n cus tome rs a nd the dis tribution utility
a nd a mong cus tomers ma y ta ke forms quite dis tinct fro m those
we know toda y. For e xa mple , ra the r tha n de vices be ing
indiv idua lly inte rconnecte d in pa ra lle l with the grid, the y ma y
be groupe d with loa ds in a semi-a utonomous ne ighborhood
tha t could be te rme d a micro grid is a clus ter of s ma ll s ources ,
s tora ge s ys tems , a nd loa ds which presents itse lf to the grid as a
le gitima te s ingle e ntity. He nce, future research work will focus
on s olving the a bove iss ues so that DES with more a dva ntages
compare d with tra dition la rge powe r pla nts ca n thrive in
e lectric powe r indus try.

MODELING AND CONTROL OF INVERT ER INT ERFACED DG UNIT S


Bas ica lly each DG unit ma y have DC type or rectifie d ge ne ra tion unit (Fue l ce ll, sola r ce ll, wind turbine , micro turbine …), s tora ge de vices, DC -DC conve rte r, DC-AC inve rter, filte r, a nd tra ns forme r for connecting to loa ds or utility in orde r to e xcha nge powe r. Mode l a nd dynamic of each of this pa rt ma y ha ve influe nce in s ys tem ope ra tion. But he re for s implifica tion it is cons idere d tha t DC s ide of the units has s ufficie nt s tora ge a nd cons ide re d as a cons ta nt DC source . He nce only DC-AC inve rter mode ling a nd control inves tiga te d in this pa pe r.A circuit mode l of a three -phase DC to AC inve rter with LC output f ilte r is furthe r de scribe d in Figure As s hown in the figure , the s ys tem cons is ts of a DC volta ge source (Vdc), a three - phase PW M inve rter, a n output filte r (Lf a nd C with cons idering pa ras itic res is ta nce of filter - Rf). Some times a tra ns former may be use d for s te pping up the output voltage a nd he nce Lf ca n be tra ns former inducta nce .
Figure PW M inve rter dia gram
The re a re two wa ys for controlling a n inverte r in a dis tribute d ge ne ra tion s ys tem

A. PQ Inverter Control

This type of control is a dopte d whe n the DG unit s ys tem is connecte d to a n e xte rna l grid or to a n is la nd of loa ds a nd more

ge ne ra tors . I n this s itua tion, the va ria bles controlle d by the inve rter a re the a ctive a nd reactive powe r inje cte d into the grid, which ha ve to follow the se t points Pre f a nd Qre f, re s pective ly. These se t points ca n be chose n by the cus tome r or by a centra l controlle r. The PQ control of a n inve rter ca n be pe rforme d us ing a curre nt control te chnique in qd re fere nce frame which the inve rter curre nt is controlle d in a mplitude a nd phase to mee t the des ire d set-points of active a nd reactive powe r.W ith the a im of Park tra ns form a nd equa tions be twee n inve rter input a nd output, the inverte r controlle r block dia gra m for s upplying re fe rence va lue of Pre f a nd Qre f is as figures . For the curre nt controlle r, two Proportiona l-I nte gra l (PI ) re gula tors ha ve bee n chose n in orde r to meet the re quire me nts of s tability of the sys te m a nd to ma ke the s tea dy s ta te e rror be ze ro. W ith this control scheme , it is poss ible to control the inve rter in s uch wa y tha t inje cts re fe rence va lue of Pre f, Qre f into othe r pa rt of s ta nd-a lone ne twork. W he n the output volta ge is nee de d to be regula te d, the PV control sche me that is s imila r to PQ mode with fee dback of voltage use d to a djus t Qre f.
Figure : PQ control sche me of inve rter

B. Vf Inverter Control

This controller has to act on the inve rte r whe neve r the s ys tem is in s ta nd-a lone mode of opera tion. I n fa ct in this case it mus t re gula te the volta ge va lue a t a re fere nce bus bar a nd the fre que ncy of the whole grid. A re gula tors work in orde r to kee p the meas ure d volta ges upon the se t points . More ove r the fre que ncy is impose d through the modula ting s igna ls of the inve rter PW M control by mea n of a n os cilla tor. A s imple PI controlle r ca n re gula te bus volta ge in re fe re nce va lue with ge tting fee dback of rea l bus volta ge .
Figure outlines this control s tra tegy. I n this ca se it is obvious
tha t the DG unit s hould ha ve s tora ge de vice in order to
re gula te the powe r a nd voltage .

Figure : Vf control sche me of inve rter

III. FUEL CELL:

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Introduction:

A fue l ce ll is a n e lectrochemica l ce ll tha t converts a source fue l into a n e le ctrica l curre nt. I t ge ne ra tes e lectricity ins ide a ce ll through reactions be twee n a fue l a nd a n oxida nt, triggere d in the presence of a n e lectrolyte. The reacta nts flow into the ce ll, a nd the rea ction products flow out of it, wh ile the e lectrolyte re ma ins within it. Fue l ce lls ca n ope ra te continuous ly as long as the necessary reacta nt a nd oxida nt flows a re ma inta ine d.

Fue l ce lls a re diffe re nt from conve ntiona l e lectrochemica l ce ll ba tte ries in tha t they cons ume rea cta nt from a n e xte rna l s ource , which mus t be re ple nis he d[1 ] – a the rmodyna mica lly ope n s ys tem. By contras t, ba tteries s tore e le ctrica l e ne rgy che mica lly a nd hence re present a the rmodyna mica lly close d s ys tem.

Ma ny combina tions of fue ls a nd oxida nts a re poss ible . A hydroge n fue l ce ll uses hydrogen as its fue l a nd oxyge n (us ua lly from a ir) as its oxida nt. Othe r fue ls include hydrocarbons a nd a lcohols . Other oxida nts include chlorine a nd chlorine dioxide
Fue l ce lls come in ma ny va rie tie s ; howeve r, they a ll work in
the sa me ge ne ra l ma nne r. The y a re ma de up of three se gme nts
which a re sa ndwiche d toge ther: the a node , the e lectrolyte, a nd
the ca thode . Two che mica l reactions occur a t the inte rfaces of
the three differe nt se gme nts . The ne t res ult of the two reactions
is tha t fue l is cons ume d, wa ter or ca rbon dioxide is crea te d,
a nd a n e lectrica l curre nt is crea te d, whic h ca n be use d to
powe r e le ctrica l devices , norma lly re fe rre d to as the loa d.
At the a node a ca ta lys t oxidizes the fue l, us ua lly hydroge n,
turning the fue l into a pos itive ly charge d ion a nd a nega tive ly
cha rge d e le ctron. The e le ctrolyte is a s ubs ta nce s pecifica lly
des igne d s o ions ca n pass through it, but the e lectrons ca nnot.
The free d e le ctrons tra ve l through a wire creating the e lectrica l
curre nt. The ions tra ve l through the e le ctrolyte to the ca thode .
Once reaching the ca thode , the ions are re unite d with the
e lectrons a nd the two react with a third chemica l, us ua lly
oxyge n, to crea te wa ter or ca rbon dioxide .

DESIGN FEAT URES IN A FUEL CELL ARE:

The e lectrolyte s ubs ta nce. The e lectrolyte s ubs ta nce us ua lly de fines the type of fue l ce ll. The fue l tha t is use d. The mos t common fue l is hydrogen.The a node ca ta lyst, which brea ks down the fue l into e lectrons a nd ions . The a node ca ta lys t is us ua lly ma de up of very fine pla tinum powde r. The ca thode ca ta lyst, which turns the ions into the was te chemica ls like wa ter or carbon dioxide . The ca thode ca ta lys t is often ma de up of nicke l. A typica l fue l ce ll produces a volta ge from 0.6 V to
0.7 V a t full ra te d loa d. Volta ge de creases as current increases ,
due to se vera l factors :Activa tion los s ,Ohmic loss (voltage drop
due to res is ta nce of the ce ll compone nts a nd inter connects )
Mas s tra ns port loss (de ple tion of reacta nts a t ca ta lys t s ites
unde r high loa ds , ca us ing ra pid los s of voltage ). To de live r the
des ire d amount of ene rgy, the fue l ce lls ca n be combine d in
se ries a nd para lle l circuits , whe re series yie lds highe r voltage ,
a nd pa ra lle l a llows a highe r curre nt to be s upplie d. Such a
des ign is ca lle d a fue l ce ll s ta ck. The ce ll s urface a rea ca n be
increase d, to a llow s tronge r curre nt from eac h ce ll.

T ypes of fuel ce lls:

Proton e xcha nge fue l ce lls :I n the archetypa l hydrogen–oxyge n proton e xcha nge membra ne fue l ce ll (PEMFC) des ign, a proton-conducting polyme r membra ne , (the e lectrolyte ), se pa ra tes the a node a nd ca thode s ide s . This was ca lle d a "s olid polyme r e le ctrolyte fue l ce ll" (SPEFC) in the ea rly 1970s , be fore
the proton e xcha nge mecha nis m was we ll-unde rs tood. (Notice tha t "polyme r e lectrolyte membra ne" a nd "proton e xcha nge me cha nis m" res ult in the same a cronym.)On the a node s ide , hydroge n diffuses to the a node ca ta lyst whe re it la ter dis s ocia tes into protons a nd e le ctrons . These protons ofte n rea ct with oxida nts ca us ing the m to become wha t is commonly re fe rre d to as multi-fa cilita te d proton membra nes . The protons a re conducte d through the membra ne to the ca thode , but the e lectrons a re force d to tra ve l in a n e xterna l circuit (s upplying powe r) beca use the me mbra ne is e lectrica lly ins ula ting. On the ca thode cata lys t, oxyge n molecules rea ct with the e lectrons (which ha ve tra ve le d through the e xterna l circuit) a nd protons to form wa te r.The ma teria ls use d in fue l ce lls diffe r by type . I n a typica l membra ne e lectrode asse mbly (MEA), the e lectrode – bipola r pla tes a re us ua lly ma de of me ta l, nicke l or carbon na no tubes , a nd are coa te d with a ca ta lys t (like pla tinum, na no iron powde rs or pa lla dium) for highe r e fficie ncy. Ca rbon pa pe r se pa ra tes them from the e lectrolyte . The e le ctrolyte could be ce ramic or a membra ne .Proton e xcha nge membra ne fue l ce ll des ign iss ues :Costs . I n 2002, typica l fue l ce ll s ys tems cos t US$1000 pe r kilowa tt of e le ctric powe r output. I n 2009, the De pa rtment of Ene rgy re porte d tha t 80-kW a utomotive fue l ce ll s ys tem cos ts in volume production (projecte d to 500,000 units pe r yea r) a re $61 per kilo wa tt. The goa l is $35 pe r kilowa tt. I n 2008 UTC Po we r has 400 kW s ta tiona ry fue l ce lls for $1,000,000 per 400 kW ins ta lle d cos ts . The goa l is to re duce the cos t in orde r to compe te with curre nt ma rke t te chnologies including gas oline inte rna l combus tion e ngines . Ma ny compa nies a re working on techniques to re duce cos t in a va rie ty of wa ys including re ducing the a mount of pla tinum nee de d in ea ch individua l ce ll. Ba lla rd Powe r Sys tems have e xpe riments with a ca ta lys t enha nce d with carbon s ilk wh ich a llows a 30% re duction (1 mg/cm² to 0.7 mg/cm²) in pla tinum usa ge without re duction in performa nce. Monas h Unive rs ity, Me lbourne uses PEDOT as a ca thode . The production cos ts of the PEM (proton e xcha nge membra ne ). The Na fion membra ne curre ntly cos ts $566/m². I n 2005 Ba lla rd Power Sys tems a nnounce d tha t its fue l ce lls will us e Shola pur, a porous polye thyle ne film pa te nte d by DSM.
Wa ter a nd a ir ma nage ment (in PEMFCs ). I n this type of fue l
ce ll, the membra ne mus t be hydra te d, requiring wa te r to be
e va pora te d at precise ly the sa me ra te tha t it is produce d. I f
wa ter is e va pora te d too quickly, the membra ne dries ,
re s is ta nce a cross it increases , a nd e ve ntua lly it will crack,
crea ting a ga s "s hort circuit" whe re hydroge n a nd oxyge n
combine directly, ge ne ra ting heat tha t will da ma ge the fue l
ce ll. I f the wa te r is e va pora te d too s lowly, the e le ctrodes will
flood, pre venting the reacta nts from reaching the ca ta lys t a nd
s topping the reaction. Me thods to ma na ge wa te r in ce lls are
be ing de ve lope d like e lectro os motic pumps focus ing on flow
control. J ust as in a combus tion engine , a s tea dy ra tio be twee n
the reacta nt a nd oxygen is necessa ry to kee p the fue l ce ll
opera ting e fficie ntly.
Te mpe rature ma na geme nt. The sa me tempe ra ture mus t be
ma inta ine d throughout the ce ll in orde r to pre vent des truction
of the ce ll through therma l loa ding. This is pa rticula rly
cha lle nging as the 2H2 + O2 -> 2H2 O rea ction is highly
e xothe rmic, s o a la rge qua ntity of hea t is gene ra te d within the
fue l ce ll.
Dura bility, se rvice life , a nd s pecia l re quireme nts for s ome type
of ce lls . Sta tiona ry fue l ce ll a pplica tions typica lly re quire more
tha n 40,000 hours of re lia ble opera tion a t a tempe ra ture of -35
°C to 40 °C (-31 °F to 104 °F), while a utomotive fue l ce lls
re quire a 5,000 hour lifes pa n (the e quiva le nt of 150,000 miles )
unde r e xtreme te mpera tures . Curre nt se rvice life is 7,300 hours

unde r cycling conditions .[1 1 ] Automotive e ngines mus t a lso be

a ble to s tart re lia bly a t -30 °C (-22 °F) a nd ha ve a high powe r to
volume ra tio (typica lly 2.5 kW pe r lite r).
Limite d ca rbon monoxide tole ra nce of the ca thode .

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High temperature fue l ce lls: A solid oxide fue l ce ll:
A s olid oxide fue l ce ll (SOFC) is e xtre me ly a dva nta geous
‚be ca use of a poss ibility of us ing a wide varie ty of fue l‛ [.
Unlike mos t other fue l ce lls which only use hydroge n, SOFCs
ca n run on hydrogen, buta ne , me tha nol, a nd othe r pe trole um
products . The diffe re nt fue ls each ha ve the ir own che mis try.
For metha nol fue l ce lls , on the a node s ide , a ca ta lys t brea ks
me tha nol a nd wa ter down to form ca rbon dioxide , hydroge n
ions , a nd free e le ctrons . The hydroge n ions move across the
e lectrolyte to the ca thode s ide , whe re they react with oxyge n to
crea te wa te r. A loa d connecte d e xterna lly be twee n the a node
a nd ca thode completes the e le ctrica l circuit. Be low a re the
che mica l equa tions for the rea ction:
Anode Reaction: CH3 OH + H2 O CO2 + 6H+ + 6e -
Cathode Reaction: 3/2 O2 + 6H+ + 6e - 3H2 O
Ove ra ll Reaction: CH3 OH + 3/2 O2 CO2 + 2H2 O + e lectrica l
e nergy.
At the a node SOFCs ca n use nicke l or other ca ta lys ts to brea k
a pa rt the me tha nol a nd crea te hydroge n ions a nd CO 2 . A s olid
ca lle d yttrium s tabilize d zirconia (YSZ) is use d as the
e lectrolyte. Like a ll fue l ce ll e lectrolytes YSZ is conductive to
ions , a llowing the m to pass from the a node to ca thode , but is
non-conductive to e lectrons . YSZ is a durable s olid a nd is
a dva nta geous in la rge indus tria l s ys tems . Although YSZ is a
good ion conductor, it only works a t ve ry high te mpera tures .
The s ta nda rd ope ra ting tempe ra ture is about 950oC. Running
the fue l ce ll a t s uch a high tempe ra ture eas ily brea ks down the
me tha ne a nd oxygen into ions . A ma jor disa dva nta ge of the
SOFC, as a res ult of the high hea t, is tha t it ‚pla ces
cons idera ble cons tra ints on the ma te ria ls which ca n be use d for
inte rconnections ‛. Anothe r disa dva nta ge of running the ce ll a t
s uch a high tempe ra ture is tha t othe r unwa nte d reactions ma y
occur ins ide the fue l ce ll. I t is common for ca rbon dus t,
gra phite , to build up on the a node , pre ve nting the fue l from
rea ching the ca ta lys t. Much resea rch is curre ntly be ing done to
find a lte rna tives to YSZ tha t will ca rry ions a t a lowe r
tempe ra ture .

MCFC:

Molte n-ca rbona te fue l ce lls (MCFCs ) a re high-tempe ra ture fue l ce lls , tha t ope ra te at te mpe ra tures of 600°C a nd a bove .Molte n ca rbona te fue l ce lls (MCFCs ) a re currently be ing deve lope d for na tura l gas a nd coa l-base d power pla nts for e lectrica l utility, indus tria l, a nd milita ry a pplica tions . MCFCs are high- tempe ra ture fue l ce lls tha t use a n e lectrolyte compose d of a molte n ca rbona te sa lt mixture s us pe nde d in a porous , che mica lly ine rt ce ramic matrix of be ta -a lumina s olid e lectrolyte (BASE). Since they opera te a t e xtreme ly high tempe ra tures of 650°C (roughly 1,200°F) a nd a bove , non - precious me ta ls ca n be use d as ca ta lys ts at the a node a nd ca thode , re ducing cos ts .I mprove d e fficie ncy is a nothe r reason MCFCs offe r s ignifica nt cos t re ductions ove r phos phoric acid fue l ce lls (PAFCs ). Molte n ca rbona te fue l ce lls ca n reach e fficie ncies a pproaching 60 pe rce nt, cons ide ra bly higher tha n the 37-42 pe rce nt e fficie ncies of a phos phoric acid fue l ce ll pla nt. W he n the wa ste hea t is ca pture d a nd use d, ove ra ll fue l
e fficie ncies ca n be as high as 85 percent.Unlike a lka line , phos phoric acid, a nd polymer e lectrolyte membra ne fue l ce lls , MCFCs don't require a n e xte rna l re former to convert more e nergy-de nse fue ls to hydrogen. Due to the high te mpera tures a t which MCFCs ope ra te , these fue ls a re conve rte d to hydroge n within the fue l ce ll itse lf by a process ca lle d inte rna l re forming, which a lso re duces cos t.Molte n ca rbona te fue l ce lls a re not prone to pois oning by ca rbon monoxide or ca rbon dioxide —they ca n eve n use ca rbon oxides as fue l— ma king them more a ttractive for fue ling with ga ses ma de from coa l. Beca use the y a re more res is ta nt to impurities tha n othe r fue l ce ll type s , s cie ntis ts be lie ve tha t the y could e ven be ca pa ble of inte rna l re forming of coa l, ass uming they ca n be ma de re s is ta nt to impurities s uch as s ulfur a nd particula tes tha t re s ult from conve rting coa l, a dirtie r foss il fue l source tha n ma ny others , into hydrogen.The primary disa dva nta ge of curre nt MCFC te c hnology is dura bility. The high te mpera tures a t which these ce lls ope ra te a nd the corros ive e le ctrolyte use d acce lera te compone nt brea kdown a nd corros ion, decreas ing ce ll life . Scie ntis ts a re curre ntly e xploring corros ion-res is ta nt ma teria ls for compone nts as we ll as fue l ce ll des igns tha t increase ce ll life without de creas ing pe rforma nce .

Fuel ce ll eff iciency:

The e fficie ncy of a fue l ce ll is de pe nde nt on the amount of powe r dra wn from it. Dra wing more powe r mea ns dra wing more curre nt, this increases the losses in the fue l ce ll. As a ge ne ra l rule , the more power (curre nt) dra wn, the lower the e fficie ncy. Mos t losses ma nifes t themse lves as a volta ge drop in the ce ll, s o the efficie ncy of a ce ll is a lmos t proportiona l to its voltage . For this reas on, it is co mmon to s how gra phs of voltage vers us curre nt (s o -ca lle d pola riza tion curves ) for fue l ce lls . A typica l ce ll running a t 0.7 V has a n efficie ncy of about
50%, mea ning tha t 50% of the e ne rgy content of the hydroge n
is converte d into e lectrica l e nergy; the re ma ining 50% will be
converte d into hea t. (De pe nding on the fue l ce ll sys te m des ign,
s ome fue l might lea ve the syste m unreacte d, cons tituting a n
a dditiona l loss .).For a hydrogen ce ll ope ra ting a t s ta nda rd
conditions with no rea cta nt lea ks , the e fficie ncy is e qua l to the
ce ll voltage divide d by 1.48 V, base d on the e ntha lpy, or
hea ting va lue , of the reaction. For the sa me ce ll, the second la w
e fficie ncy is e qua l to ce ll voltage divide d by 1.23 V. (This
voltage va ries with fue l us e d, a nd qua lity a nd te mpe ra ture of
the ce ll.) The differe nce betwee n these numbe rs re prese nts the
diffe re nce be tween the reactio n's e ntha lpy a nd Gibbs free
e nergy. This diffe rence a lwa ys a ppears as hea t, a long with a ny
losses in e lectrica l conve rs ion e fficie ncy.Fue l ce lls do not
opera te on a therma l cycle . As s uch, the y a re not cons tra ine d,
as combustion e ngines a re , in the same wa y by
thermodynamic limits , s uch as Ca rnot cycle e fficie ncy. At
times this is misre prese nte d by sa ying that fue l ce lls are
e xempt from the la ws of the rmodyna mics , beca use mos t
pe ople think of the rmodyna mics in te rms of combus tion
processes (entha lpy of formation). The la ws of
thermodynamics a ls o hold for chemica l processes (Gibbs free
e nergy) like fue l ce lls , but the ma ximum theore tica l e fficie ncy
is higher (83% e fficie nt a t 298K in the case of hydrogen/oxyge n
rea ction) tha n the Otto cycle the rma l e fficie ncy (60% for
compress ion ra tio of 10 a nd s pecific hea t ra tio of 1.4).
Compa ring limits impose d by the rmodyna mics is not a good
pre dictor of pra ctica lly a chie vable e fficie ncies . Als o, if
propuls ion is the goa l, e le ctrica l output of the fue l ce ll has to
s till be converte d into mecha nica l powe r with a nother
e fficie ncy drop. I n re fere nce to the e xe mption cla im, the correct
cla im is tha t the "limita tions impose d by the second la w of
thermodynamics on the opera tion of fue l ce lls a re much less
se vere tha n the limita tions impose d on conventiona l e ne rgy

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convers ion syste ms".[2 3 ] Conse que ntly, they ca n have ve ry high e fficie ncies in converting chemica l e nergy to e lectrica l e ne rgy, es pecia lly whe n they a re ope ra te d a t low powe r de ns ity, a nd us ing pure hydroge n a nd oxygen as reacta nts .I t s hould be unde rline d tha t fue l ce ll (es pecia lly high tempe rature ) ca n be use d as a hea t source in conve ntiona l hea t e ngine (ga s turbine s ys tem). I n this case the ultra high e fficie ncy is pre dicte d (a bove 70%).

In practice:

For a fue l ce ll opera ting on a ir, losses due to the a ir s upply s ys tem mus t a ls o be ta ke n into account. This re fers to the press uriza tion of the a ir a nd de humidifying it. This re duces the efficiency s ignifica ntly a nd brings it nea r to tha t of a compress ion ignition e ngine . Furthe rmore , fue l ce ll e fficie ncy decreases as loa d increases .The ta nk-to-whee l e fficie ncy of a fue l ce ll ve hicle is grea ter tha n 45% at low loa ds a nd s hows a vera ge va lues of a bout 36% whe n a driving cycle like the NEDC (Ne w Europea n Driving Cycle ) is use d as tes t proce dure . The compa ra ble NEDC va lue for a Diese l ve hicle is
22%. I n 2008 Honda re lease d a fue l ce ll e le ctric ve hicle (the
Honda FCX Clarity) with fue l s tack cla iming a 60% ta nk-to-
whe e l e fficie ncy. I t is a ls o importa nt to ta ke losses due to fue l
production, tra ns porta tion, a nd s torage into a ccount. Fue l ce ll
ve hicles running on compresse d hydroge n ma y ha ve a powe r -
pla nt-to-whee l e fficie ncy of 22% if the hydroge n is s tore d as

high-press ure gas , a nd 17% if it is s tore d as liquid hydroge n.[2 9 ]

I n a ddition to the production losses , over 70% of US' e le ctricity
use d for hydroge n production comes from therma l powe r,
which only ha s a n e fficie ncy of 33% to 48%, res ulting in a ne t
increase in ca rbon dioxide production by us ing hydrogen in
ve hicles [Fue l ce lls ca nnot s tore e ne rgy like a ba tte ry, but in
s ome a pplica tions , s uch as s ta nd-a lone power pla nts base d on
dis continuous s ources s uch as s ola r or wind powe r, the y are
combine d with e lectrolyzes a nd s tora ge s ys te ms to form a n
e nergy s torage s ys tem.The ove ra ll e fficie ncy (e lectricity to
hydroge n a nd ba ck to e lectricity) of s uch pla nts (known as
round-trip e fficie ncy) is be twee n 30 a nd 50%, de pe nding on
conditions . W hile a much chea pe r lea d-a cid ba tte ry might
re turn about 90%, the e lectrolyze/fue l ce ll s ys tem ca n s tore
inde finite qua ntities of hydroge n, a nd is the re fore be tter s uite d
for long-te rm s torage .Solid-oxide fue l ce lls produce e xothe rmic
hea t from the recombina tion of the oxyge n a nd hydrogen. The
ce ramic ca n run as hot as 800 de grees Ce ls ius . This hea t ca n be
ca pture d a nd use d to hea t wa ter in a micro combine d hea t a nd
powe r (m-CHP) a pplication. W he n the heat is ca pture d, tota l
e fficie ncy can rea ch 80-90% a t the unit, but doe s no t cons ide r
production a nd dis tribution losses . CHP units a re be ing
de ve lope d today for the Europea n home ma rke t.Sta tionary
fue l ce ll a pplica tions (or s ta tiona ry fue l ce ll powe r s ys tems ) are
s ta tiona ry that are e ithe r connecte d to the e lectric grid
(dis tribute d gene ra tion) to provide s upple me nta l powe r a nd as
eme rgency powe r s ys tem for critica l a reas , or ins ta lle d a s a
grid-inde pe nde nt ge ne ra tor for on-s ite se rvice .
Codes a nd s ta nda rds Sta tiona ry fue l ce ll a pplica tions is a
class ifica tion in FC Hydroge n codes a nd s ta nda rds a nd fue l
ce ll codes a nd s ta nda rds . The othe r ma in s ta nda rds are
Portable fuel ce ll a pplicat ions and Fue l ce ll vehicle.
Fue l ce ll gas a pplia nces up to 70 kW .I ns ta lla tion pe rmitting guida nce for hydroge n a nd fue l ce lls s ta tionary a pplica tions
Sta ndard for the ins ta lla tion of s ta tionary fue l ce ll po we r s ys tems

Emergency power systems:

Emerge ncy powe r sys te ms a re a type fue l ce ll sys te m, whic h may include lighting, ge nera tors and othe r a ppara tus , to provide backup res ources in a cris is or whe n re gula r s ys tems fa il. They find uses in a wide va rie ty of settings from re s ide ntia l homes to hos pita ls , s cie ntific la bora tories , da ta ce nters , te le communica tion e quipme nt a nd mode rn na va l s hips .

Uninterrupted power supply:

An uninte rrupte d power s upply (UPS) provides eme rge ncy powe r and, de pe nding on the topology, provide line re gula tion as we ll to connecte d e quipme nt by s upplying powe r from a se pa ra te s ource whe n utility powe r is not a va ilable . I t diffe rs from a n a uxilia ry powe r s upply or s ta ndby ge ne ra tor, which does not provide ins ta nt protection from a momenta ry powe r inte rruption.

Cogeneration

Cogene ra tion ca n be use d whe n the fue l ce ll is s ite d nea r the point of use , its was te hea t ca n be ca pture d for be neficia l purposes . Micro combine d hea t a nd power (MicroCHP) is us ua lly le ss tha n 5 kWe for a home fue l ce ll or s ma ll bus iness .

POWER:

Fue l ce lls a re very use ful as powe r s ources in remote loca tions , s uch as s pacecra ft, remote wea the r s ta tions , large pa rks , rura l loca tions , a nd in ce rta in milita ry a pplications . A fue l ce ll s ys tem running on hydroge n ca n be compact a nd lightwe ight, a nd ha ve no ma jor moving pa rts . Beca use fue l ce lls ha ve no moving parts a nd do not involve combus tion, in ide a l conditions they ca n a chie ve up to 99.9999% re liability. This equa tes to around one minute of down time in a two year pe riod.Since e lectrolyses s ys tems do not s tore fue l in themse lves , but ra ther re ly on e xterna l s tora ge units , the y ca n be s uccess fully a pplie d in la rge-s ca le e nergy s torage , rura l a reas be ing one e xample . I n this a pplica tion, ba tteries would ha ve to be large ly overs ize d to mee t the s tora ge de ma nd, but fue l ce lls only nee d a la rger s tora ge unit (typica lly chea per tha n a n e lectrochemica l de vice ).

Cogeneration:

Micro combine d hea t a nd powe r (MicroCHP) s ys tems s uch as home fue l ce lls a nd cogene ra tion for office buildings a nd factories are in the mass production phase. The s ys tem ge ne ra tes cons ta nt e le ctric powe r (se lling e xcess power ba ck to the grid whe n it is not cons ume d), a nd a t the sa me time produces hot a ir a nd wa te r from the was te heat. MicroCHP is us ua lly le ss tha n 5 kWe for a home fue l ce ll or s ma ll bus iness .
A lowe r fue l-to-e le ctricity convers ion e fficie ncy is tole ra te d (typica lly 15-20%), beca use mos t of the e ne rgy not converte d into e lectricity is utilize d as hea t. Some hea t is los t with the e xha us t gas jus t a s in a norma l furnace , s o the combine d hea t a nd powe r efficie ncy is s till lowe r tha n 100%, typica lly a round
80%. I n te rms of e ne rgy howeve r, the process is ine fficie nt, a nd one could do be tter by ma ximizing the e lectricity ge nera te d a nd the n us ing the e lectricity to drive a hea t pump. Phos phoric -a cid fue l ce lls (PAFC) comprise the la rges t se gme nt of e xisting CHP products worldwide a nd ca n provide combine d e fficie ncies close to 90% (35-50% e lectric + rema inde r

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as the rma l) Molte n-ca rbona te fue l ce lls have a lso bee n ins ta lle d in these a pplica tions , a nd s olid-oxide fue l ce ll prototypes e xis t.
Othe r a pplica tions :

Pro v iding powe r for base sta tions or ce ll s ites

Off-grid powe r s upply

Dis tribute d gene ra tion

Fork Lifts

Emerge ncy power sys te ms a re a type of fue l ce ll s ys tem, which may include lighting, ge nera tors and othe r a ppara tus , to provide backup res ources in a cris is or whe n re gula r s ys tems fa il. They find uses in a wide va rie ty of settings from re s ide ntia l homes to hos pita ls , s cie ntific la bora tories , da ta ce nters , te le communica tion e quipme nt a nd mode rn na va l s hips .
An uninte rrupte d power s upply (UPS) provides eme rge ncy
powe r and, de pe nding on the to pology, provide line
re gula tion as we ll to connecte d e quipme nt by s upplying
powe r from a se pa ra te s ource whe n utility powe r is not
a va ilable . Unlike a s ta ndby ge nera tor, it ca n provide ins ta nt
prote ction from a mome nta ry power interruption.
Base loa d powe r pla nts
Ele ctric a nd hybrid ve hicles .
Note book compute rs for a pplica tions where AC charging ma y not be ava ila ble for wee ks a t a time.
Sma rtphone with high powe r cons umption due to la rge
dis pla ys a nd a dditiona l fea tures like GPS might be e quippe d
with micro fue l ce lls .

Sma ll hea ting a pplia nces .

Fue l ce lls a re a technology tha t both the public a nd priva te sectors a re increas ingly turning to for both primary a nd back- up po we r nee ds . Although the unde rs ta nding of the chemis try of fue l ce lls goes back more tha n a ce ntury , they a re very much a 21s t ce ntury technology. The bas ic des ign a nd e lectrochemica l principle behind fue l ce lls is s tra ightforwa rd. A fue l ce ll s tack re quires only hydrogen (or a s imila r ene rgy ca rrie r), oxyge n, a nd a n e lectrolytic solution.
Hydroge n a nd a mbie nt a ir flow into the fue l ce ll, wh ich conta ins a n a node a nd a ca thode . At the a node , the hydroge n se pa ra tes into a proton a nd a n e lectron. The proton migra tes to the ca thode , whe re it reacts with the oxyge n to form wa ter. The e lectrons , which ca nnot pass through the membra ne , flow from the ce ll to provide use ful e lectrica l powe r. Fue l ce lls are quie t, have no moving pa rts , a nd produce no pa rticula te emiss ions . The y a re virtua lly ma inte na nce free a nd ca n be both tes te d a nd opera te d remote ly. Beca use they a re modula r, they ca n be configure d for a ny s ize powe r nee ds , from a fe w kilowa tts for a remote te lecommunica tions tower to mega wa tt - sca le for hos pita ls a nd a irports . Hydroge n is sa fe ly s tore d on - s ite or produce d within the fue l ce ll itse lf.

SOLID OXIDE FUEL CELLS:

Solid oxide fue l ce lls (SOFCs ) offer a clea n, low-pollut ion technology to e lectroche mica lly ge nera te e lectricity a t high e fficie ncies ; s ince the ir e fficie ncie s a re not limite d the wa y conventiona l hea t e ngine's is . These fue l ce lls provide ma ny a dva nta ges over tra ditiona l e ne rgy convers ion s ys tems including high e fficie ncy, re lia bility, modula rity, fue l a da pta bility, a nd very low le ve ls of polluting e miss ions . Quie t, vibra tion-free opera tion of SOFCs a ls o e limina tes noise us ua lly ass ociate d with conve ntiona l powe r ge nera tion s ys tems .
Up until a bout s ix years a go, SOFCs we re be ing de ve lope d for
opera tion prima rily in the tempe ra ture ra nge of 900 to 1000oC
(1692 to 1832oF); in a ddition to the ca pa bility of inte rna lly
re forming hydroca rbon fue ls (for e xa mple , na tura l gas ), s uch
high te mpe rature SOFCs provide high qua lity e xha us t hea t for
cogene ra tion, a nd whe n press urize d, ca n be inte gra te d with a
gas turbine to further increase the overa ll e fficie ncy of the
powe r s ys tem. Howe ve r, re duction of the SOFC ope ra ting
tempe ra ture by 200oC (392oF) or more a llows use of a broa de r
se t of ma te ria ls , is less de ma nding on the sea ls a nd the
ba la nce -of-pla nt components , s implifies the rma l ma na ge ment,
a ids in fas ter s ta rt up a nd cool down, a nd res ults in less
de gra da tion of ce ll a nd s ta ck compone nts . Beca use of these
adva nta ges , a ctivity in the de ve lopme nt of SOFCs ca pa ble of
opera ting in the te mpe ra ture ra nge of 650 to 800oC (1202 to
1472oF) has increase d drama tica lly in the las t fe w yea rs .
Ho we ve r, a t lowe r te mpera tures , e lectrolyte conductivity a nd
e lectrode kine tics decrease s ignifica ntly; to overcome these
dra wbacks , a lte rna tive ce ll ma te ria ls a nd des igns are be ing
e xte ns ive ly inves tiga te d.
An SOFC esse ntia lly cons is ts of two porous e le ctrodes
se pa ra te d by a de nse, oxide ion conducting e lectrolyte . The
opera ting principle of s uch a ce ll is illus tra te d in Figure .

Oxyge n s upplie d a t the ca thode (a ir e lectrode ) reacts with
Fig. Ope ra ting principle of a s olid oxide fue l ce ll. incoming e le ctrons from the e xterna l circuit to form oxide ions ,
which migra te to the a node (fue l e le ctrode ) through the oxide
ion conducting e lectrolyte . At the a node , oxide ions combine
with hydroge n (a nd/or ca rbon monoxide ) in the fue l to form
wa ter (a nd/or ca rbon dioxide ), libe ra ting e lectrons . Electrons
(e lectricity) flow from the a node through the e xte rna l circuit to
the ca thode . The ma teria ls for the ce ll components are se lecte d
base d on s uita ble e lectrica l conducting properties require d of
these compone nts to pe rform the ir inte nde d ce ll functions ;

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a de qua te che mica l a nd s tructura l s tability a t high tempe ra tures encounte re d during ce ll ope ration as we ll as during ce ll fa brica tion; minima l reactivity a nd inte r diffus ion a mong differe nt compone nts ; a nd matching the rma l e xpa ns ion a mong diffe re nt compone nts .

MAT ERIALS AND CELL DESIGNS ELECTROLYT E

Yttrium-dope d zirconium oxide (YSZ) re ma ins the mos t wide ly use d ma teria l for the e lectrolyte in SOFCs beca use of its s ufficie nt ionic conductivity, che mica l s tability, a nd me cha nica l s trength. The only dra wback of s ta bilize d YSZ is the low ionic conductivity in the lowe r ce ll ope ra tion tempe ra ture regime , be low a bout 750oC (1382oF). Two s olutions tha t ha ve been trie d to resolve this proble m a re to decrease the thicknes s of the YSZ e lectrolyte a nd to find othe r ma teria ls to re pla ce the yttrium. Sca ndium -dope d zirconium oxide has highe r conductivity tha n YSZ but high cos t of sca ndium a nd de trime nta l age ing e ffects in sca ndium dope d zirconium oxide ma ke it less a ttra ctive in commercia lizing SOFCs . Ga dolinium- or sa ma rium-dope d ce rium oxide ma teria ls possess highe r oxide ion conductivity compa re d to zirconium base d ma te ria ls . Howe ver, cerium oxide base d ma teria ls , unde r re ducing conditions a t high te mpera tures , e xhibit s ignifica nt e lectronic conductivity a nd dime ns iona l cha nge . Opera tion a t te mpera tures be low about 600oC (1112oF) ove rcomes this proble m, a nd ce rium oxide base d ma teria ls are s uccess fully be ing use d as e le ctrolyte in SOFCs by Ceres Powe r Limite d (UK). I n a ddition to the tra ditiona lly use d oxides of zirconium a nd cerium, othe r mixe d oxide s a lso provide a n opportunity to deve lop oxide ion conducting e lectrolytes. One mixture , conta ining among othe rs ga llium oxide , has a ttracte d a tte ntion as a n e le ctrolyte . Howe ve r, it has two dra wba cks : unce rta in cos t of ga llium, a nd unce rta in che mica l a nd mecha nica l s ta bility of the oxide . I n s pite of these dra wbacks , Mits ubis hi Ma teria ls Corpora tion (Ja pa n) is us ing this as the e le ctrolyte in its SOFCs a nd has s uccess fully built a nd tes te d up to 10-kW s ize SOFC power s ys tems .

CAT HODE

The oxida nt gas is a ir or oxyge n a t the SOFC ca thode , a nd the e lectrochemica l re duction of oxyge n requires a series of e le menta ry reactions a nd involves the tra ns fer of multiple e lectrons . The SOFC ca thode mus t meet the re quire ments of high cata lytic a ctivity for oxyge n molecule diss ocia tion a nd oxyge n re duction, high e le ctronic conductivity, che mica l a nd dime ns iona l s tability in e nvironme nts encounte re d during ce ll fa brica tion a nd ce ll ope ra tion, therma l e xpa ns ion ma tch with othe r ce ll compone nts , a nd compa tibility a nd minimum rea ctivity with the e lectrolyte a nd the interconnection.
Fina lly, the ca thode mus t ha ve a s table , porous micros tructure s o tha t gase ous oxyge n ca n rea dily diffus e through the ca thode to the ca thode /e lectrolyte interfa ce. These s tringe nt e lectrochemica l a nd mecha nica l require me nts greatly res trict the number of s uita ble candida te ma teria ls . La ntha num ma nga nite , which, whe n s ubstitute d with lo w va le nce e le ments s uch as ca lcium or s trontium, has good e lectronic conduction. More ove r, it possesses a de qua te e lectroca ta lytic
activity, a reasona ble therma l e xpa ns ion ma tch to YSZ, a nd s ta bility in the SOFC ca thode ope rating e nvironme nt. For SOFCs ope ra ting a t s ubsta ntia lly lowe r tempe ra tures , s uch as
650 to 800oC (1202 to 1472oF), a lte rna tive ca thode ma teria ls , typica lly conta ining tra ns ition meta ls s uch as coba lt, iron, a nd/or nicke l, ha ve been de ve lope d a nd optimize d for be tter pe rforma nce . I n gene ra l, these ma teria ls offer higher oxide ion diffus ion ra tes a nd e xhibit fas te r oxyge n re duction kine tics a t the ca thode /e lectrolyte inte rface compa re d with la ntha num ma nga nite . Howe ve r, the therma l e xpa ns ion coe fficie nt of coba ltites is much higher tha n tha t of the YSZ e le ctrolyte , a nd the e lectrica l conductivitie s of fe rrite s a nd nicke lites are low. Ne ve rthe less , promis ing res ults ha ve bee n re porte d us ing these ma teria ls , though in ma ny cases the improve d cathodic pe rforma nce is found to decrease during the ce ll life time as a re s ult of chemica l or micro s tructura l ins ta bility.
Minimiza tion of ca thodic pola riza tion losses is one of the bigges t cha lle nges to be ove rcome in obta ining high, s ta ble powe r de ns ities from lower tempe ra ture SOFCs . Howe ver, these ma teria ls are ve ry reactive towa rd YSZ. The re fore , a thin la ye r, gene ra lly of a cerium oxide base d ma teria l, is use d to re duce the che mica l reaction betwee n the ca thode a nd YSZ. Microstructure a ls o plays a ma jor role in the ca thode pola rization; this is pa rticula rly true whe n a compos ite ca thode , which s hows a be tter performa nce compa re d to a s ingle compos ition ca thode , is use d. I t has bee n s hown tha t pola rization res is ta nce de pe nds upon the gra in s ize of the ionic conductor in the compos ite e le ctrode a nd the volume fraction of poros ity.

ANODE

The a node mus t be a n e xce lle nt ca ta lys t for the oxidation of fue l (hydroge n, ca rbon dioxide ), sta ble in the re ducing e nvironme nt of the fue l, e lectronica lly conducting, a nd mus t ha ve s ufficie nt poros ity to a llow the tra ns port of the fue l to a nd the tra ns port of the products of fue l oxida tion a way from the e lectrolyte /a node inte rface where the fue l oxida tion rea ction ta kes place . The other requireme nts include ma tching of its therma l e xpa ns ion coe fficie nt with tha t of the e lectrolyte a nd interconnect; inte grity of poros ity for gas pe rmea tion; che mica l s tability with the e lectrolyte a nd inte rconnect; a nd a pplicability to use with ve rsa tile fue ls a nd impurities . I n a ddition, cos t e ffective ness is a lwa ys a factor for comme rcia liza tion. Nicke l-YSZ compos ites a re the mos t commonly use d a node mate ria ls fo r SOFCs . Nicke l is a n e xce lle nt ca ta lys t for fue l oxida tion; howeve r, it pos sesses a high the rma l e xpa ns ion coe fficie nt, a nd e xhibits coa rsening of micros tructure due to me ta l aggrega tion through gra in growth a t ce ll ope ra tion tempe ra tures . YSZ in the a node cons tra ins nicke l a ggrega tion a nd preve nts s inte ring of the nicke l pa rticles , de creases the e ffective the rma l e xpa ns ion coe fficie nt bringing it close r to tha t of the e lectrolyte , a nd provides be tter a dhes ion of the a node with the e lectrolyte. I n these a nodes , nicke l has dua l roles of the ca ta lys t for hydroge n oxida tion a nd the e lectrica l curre nt conductor. I n a ddition, it is a ls o highly active for the s tea m re forming of me tha ne . This ca ta lytic prope rty is e xploite d in the so -ca lle d inte rna l re forming SOFCs tha t ca n opera te on fue ls compose d of mixtures of metha ne a nd wa ter. Although nicke l is a n e xce lle nt hydroge n oxida tion a nd me tha ne -s team re forming ca ta lys t, it a ls o ca ta lyzes the forma tion of ca rbon from hydroca rbons unde r re ducing conditions . Unless s ufficie nt a mounts of s team a re present a long with the hydroca rbon to re move ca rbon from the nicke l s urface , the a node may be des troye d. As a res ult, eve n whe n us ing me tha ne as the fue l, re la tive ly high s team -to-ca rbon ra tios are nee de d to s uppress this de le terious reaction. Unfortuna te ly, due to the high ca ta lytic activity of nicke l for hydrocarbon cra cking, this a pproach does not work for higher hydrocarbons , a nd it is ge nera lly not poss ible to opera te nicke l-base d a nodes on highe r hydroca rbon-conta ining fue ls without pre -re forming with s tea m or oxygen. I n s pite of this dra wback, a nicke l-YSZ compos ite rema ins the mos t

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commonly utilize d a node ma teria l for SOFCs a nd is sa tis factory for ce lls ope rating on clea n, re forme d fue l.
Ho we ve r, a dva nce d SOFC des igns place a dditiona l cons tra ints on the a node , s uch as to lera nce of oxidizing e nvironme nts
a nd/or the a bility to tolera te s ignifica nt qua ntities of s ulphur a nd/or hydroca rbon s pecies in the fue l s tream. Alte rna tive ma teria ls , s uch as ce rium oxide or s trontium titana te /ce rium oxide mixtures , ha ve yie lde d s ome pro mis ing res ults in these des igns , but the be ne fits obta ine d in terms of s ulphur, hydrocarbon a nd/or re dox tole ra nce are counterba la nce d by othe r limita tions (s uch as the difficulty of inte gra tin g s uch ma teria ls with e xis ting ce ll a nd s tack fa brica tion processes a nd ma teria ls ). Copper base d a nodes have a lso bee n propose d for inte rme dia te tempe ra ture (<800oC; <1472oF) SOFCs inte nde d to opera te dire ctly on hydroca rbon fue ls without prior re forma tion, but the lack of ca ta lytic activity for oxida tion of fue l in copper a nd s inte ring of coppe r a t the ce ll ope ra ting tempe ra tures have limite d the ir use in practica l SOFCs .

Interconnect

Since a s ingle ce ll only produces volta ge le ss tha n 1 V a nd powe r a round 1 W /cm2 , ma ny ce lls a re e lectrica lly connecte d toge ther in a ce ll s tack to obta in highe r volta ge a nd powe r. To connect multiple ce lls toge ther, a n inte rconnection is use d in SOFC sta cks . The re quire ments of the inte rconnection are the mos t se vere of a ll ce ll compone nts a nd include : nea rly 100 pe rce nt e lectronic conductivity; s tability in both oxidizing a nd re ducing a tmos phe res at the ce ll ope ra ting tempe ra ture s ince it is e xpose d to a ir (or oxygen) on the ca thode s ide a nd fue l on the a node s ide ; low pe rmeability for oxyge n a nd hydroge n to minimize direct combina tion of oxida nt a nd fue l during ce ll opera tion; a the rma l e xpans ion coe fficie nt close to tha t of the ca thode a nd the e le ctrolyte ; a nd non-reactivity with othe r ce ll ma teria ls . To sa tis fy these require me nts , dope d la ntha num chromite is use d as the inte rconnection for ce lls inte nde d for
opera tion a t a bout 1000oC (1832oF). I n ce lls inte nde d for opera tion a t lowe r tempe ra tures (<800oC; <1412oF), it is poss ible to use oxida tion-res is ta nt meta llic ma teria ls for the inte rconnection. Compa re d to la ntha num chromite ce ramic inte rconnects , meta llic a lloys offer a dva nta ges s uch as improve d ma nufactura bility, s ignifica ntly lowe r ra w ma teria l a nd fabrica tion cos ts , a nd highe r e le ctrica l a nd the rma l conductivity. But to be useful for the inte rconnect a pplica tion, the meta llic a lloys mus t sa tis fy a dditiona l re quireme nts , including res is ta nce to s urfa ce oxida tion a nd corros ion in a dua l a tmos phere (s imulta ne ous e xpos ure to oxidizing a nd re ducing atmos pheres ), the rma l e xpa ns ion ma tching to other s tack compone nts (pa rticularly for s tacks us ing a rigid sea l des ign), che mica l compa tibility with other ma teria ls in contact with the inte rconnect, s uch as sea ls a nd ce ll ma teria ls , high e lectrica l conductivity not only through the bulk ma te ria l but a lso in in-s itu-forme d oxide sca les , mecha nica l re lia bility a nd dura bility a t the ce ll ope ra ting tempe ra ture , a nd s trong a dhes ion be twee n the as -forme d oxide sca le a nd the unde rlying a lloy s ubs tra te . Fe rritic s ta inless s tee ls a re the mos t promis ing ca ndida tes , owing to the fa ct tha t some a lloys in this fa mily offer a protective a nd conductive chromium-base d oxide sca le , a ppropria te the rma l e xpa ns ion be ha vior, ease of ma nufacturing a nd low cos t. Seve ra l ne w fe rritic s ta inless s tee ls ha ve bee n de ve lope d s pecifica lly for SOFC inte rconnects . Although these a lloys demons tra te improve d pe rforma nce ove r tra ditiona l compos itions , se vera l critica l iss ues re ma in; a mong these are chromium oxide sca le e va pora tion a nd s ubse que nt poisoning of ca thodes ; sca le e le ctrica l res is tivity in the long term; corros ion a nd s pa lling unde r interconnect e xpos ure conditions ; a nd compatibility with the a dja cent compone nts s uch as sea ls a nd e le ctrica l conta ct la yers . To ove rcome some of these problems , s ome s urface coa tings ca n be a pplie d onto me ta llic inte rconnects to minimize sca le growth, e lectrica l res is ta nce a nd chromium vola tility.

HYBRID POWER SYST EMS:

INTRODUCT ION

Ele ctrica l e nergy requireme nts for ma ny re mote a pplica tions a re too large to a llow the cos t-e ffective use of s ta nd-a lone or a utonomous PV s ys tems . I n these cases , it may prove more feas ible to combine se vera l diffe rent types of powe r s ources to form wha t is known a s a "hybrid" s ys tem. To da te , PV has been e ffective ly combine d with othe r types of power gene ra tors s uch as wind, hydro, the rmoe le ctric, pe trole um -fue le d a nd e ven hydroge n. The se le ction process for hybrid powe r source types a t a give n s ite ca n include a combina tion of ma ny factors including s ite topogra phy, seasona l a va ilability of e ne rgy s ources , cos t of source impleme nta tion, cos t of ene rgy storage a nd de live ry, tota l s ite e ne rgy requireme nts , e tc.
•Hybrid powe r sys te ms use loca l re ne wable resource to
provide powe r.
•Villa ge hybrid powe r s ys tems ca n ra nge in s ize from sma ll
house hold s ys tems (100 W h/da y) to ones s upplying a whole
a rea (10’s MW h/da y).
•The y combine ma ny technologies to provide re lia ble powe r
tha t is ta ilore d to the loca l res ources a nd community.
•Pote ntia l compone nts include : PV, wind, micro -hydro, rive r- run hydro, biomass , ba tte ries a nd conve ntiona l ge nera tors .

A. Configuration of hybr id system

Figure s hows the bas ic configura tion of hybrid s ys tem dis cusse d in this s tudy. The hybrid s ys tem wa s cons is te d of re duction gea r, ma in-motor (EM1), s ub- motor (EM2), e ngine , powe r controller a nd ba tte ry. I t was s uppose d tha t a double - motor s ys tem was pre pa re d for the driving s ys tem dis cusse d in this s tudy. At f irs t, acce lera tion wa s ass is te d by was a pplie d only by ma in motor whe n the driving s pee d was low, wh ile the corpora tion by two motors was often achieve d to drive the s ys tem.
I f the SOC (s ta te of cha rge) of ba tte ry wa s de crease d be low the s pecific thres hold, the ba ttery was cha rge d by s ub -motor. This opera tion was priority to ove r other actions . Figure 2 s hows the modifie d configura tion of hybrid s ys te m propose d in this s tudy.

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I n the modifie d s ys tem, CVT was utilize d to kee p cons ta nt re volution numbe rs of the s ub -motor whe n the s ub-motor contribute d to as s is t the syste m.

Schema tic vie w of double motor hybrid sys te m with CVT

Petroleum-fueled eng ine generators (Genset s)

Pe trole um-fue le d ge nse ts (ope ra ting continuous ly in ma ny cases ) are prese ntly the mos t common me thod of s upplying powe r a t s ites re mote from the utility grid s uch as villa ges , lodges , resorts , cotta ges a nd a va rie ty of indus tria l s ites including te lecommunica tions , mining a nd logging camps , a nd milita ry a nd other government ope ra te d loca tions . Although ge nsets a re re lative ly ine xpe ns ive in initia l cos t, they a re not ine xpe ns ive to ope ra te . Cos ts for fue l a nd ma inte na nce ca n increase e xpone ntia lly whe n these nee ds mus t be me t in a re mote loca tion. Environme nta l fa ctors s uch as noise , ca rbon oxide emiss ions , tra ns port a nd s tora ge of fue l mus t a ls o be cons idere d.

Figure Hybrid PV/Ge nera tor Sys tem Example ; Courtesy Photron Ca na da I nc., Loca tion: Sheep Mounta in I nte rpre tive Ce ntre , Pa rks Ca na da Kluone Na tiona l Pa rk, Yukon Te rritories , Ca na da , 63° North La titude ; Compone nts s hown include : ge ne ra tor (120/240 V), ba ttery (dee p cycle indus tria l ra te d @ ± 10 kW h ca pacity), DC to AC s ta nd-a lone inve rter (2500 W @ 120 V output), misce lla neous sa fe ty + control equipme nt including PV a rray disconnect, PV control/re gula tor, a utoma tic ge nera tor s ta rt/ -s top control, DC/AC s ys tem me te ring e tc.; -Components not s hown: PV a rra y (800 W pea k).

Figure Genset fue l e fficie ncy vs . ca pacity utilize d.
Fue l to power convers ion e fficie ncies ma y be as high as 25% (for a diese l fue le d unit ope ra ting at ra te d ca pacity). Unde r pa rt loa d conditions , howeve r, efficiencies may decline to a fe w pe rce nt. Cons ide ra ble was te hea t is there fore a va ilable a nd may be utilize d for othe r re quire me nts s uch as s pace a nd/or wa ter hea ting.

Why a PV/Genset hybrid?

PV a nd ge nset s ys tems do not have much in common. I t is precise ly for this reason tha t they ca n be ma te d to form a hybrid s ys tem tha t goes far in ove rcoming the dra wbacks to each te chnology. Ta ble 10.1 lis ts the res pective a dva nta ges a nd dis a dva nta ges. As the s un is a va riable e nergy s ource , PV s ys tem des igns a re increase d in s ize (a nd there fore cos t) to a llow for a degree of s ys tem a utonomy. Autonomy is re quire d to a llow for provis ion of re lia ble power during "wors t case" s itua tions , which a re us ua lly pe riods of a dverse wea ther, seasona lly lo w s ola r insola tion va lues or a n unpre dicte d increase d de ma nd for powe r. The a ddition of a utonomy to the s ys tem is accomplis he d by increas ing the s ize of the PV a rra y a nd its requis ite e ne rgy storage s ys te m (the ba tte ry).W he n a ge nset is a dde d, a dditiona l ba ttery charging a nd direct AC loa d s upply ca pa bilitie s are pro vide d. The nee d to build in s ys tem a utonomy is the re fore grea tly re duce d. W hen e ne rgy de ma nds ca nnot be me t by the PV portion of the s ys tem for a ny reason, the ge nse t is brought on line to provide the re quire d ba ckup powe r. Substa ntia l cos t sa vings ca n be achieve d a nd overa ll s ys tem re lia bility is e nha nce d.PV/gense t hybrid s ys tems ha ve bee n utilize d a t s ites with da ily e ne rgy re quire me nts ra nging from as low a s 1 kW h pe r day to as high as 1 MW h pe r da y, which illus tra tes the ir e xtreme fle xibility. The y are a prove n a nd re liable me thod for e fficie nt a nd cos t e ffective powe r s upply a t remote s ite s .

PV/genset hybrid system de scr ipt ion

The PV/genset hybrid utilize s two dive rse e nergy sources to powe r a s ite's loa ds . The PV arra y is e mploye d to ge nera te DC e nergy that is cons ume d by a ny e xis ting DC loa ds , with the ba la nce (if a ny) be ing use d to charge the s ys tem's DC e ne rgy s tora ge ba tte ry.
The PV a rra y is a utoma tica lly on line a nd fee ding powe r into the s ys tem whe neve r solar insola tion is ava ila ble a nd continues to produce s ys tem power during daylight hours until its ra te of production e xcee ds wha t a ll e xis ting DC loa ds a nd the s torage ba ttery ca n a bs orb. Should this occur, the a rra y is inhibite d by the sys te m controller from fee ding a ny further e nergy into the loa ds or ba tte ry. A gense t is employe d to ge ne ra te AC ene rgy tha t is cons ume d by a ny e xis ting AC loa ds , with the ba la nce (if a ny) be ing use d by the ba ttery

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cha rger to ge nera te DC e nergy tha t is use d in the ide ntica l fas hion to tha t de scribe d for the PV arra y a bove .

Figure Block dia gra m of a hybrid PV -Ge nse t s ys tem.
At times whe n the gense t is not running, a ll s ite AC power is de rive d from the syste m's powe r conditione r or inve rter, which a utoma tica lly converts s ys tem DC e nergy into AC e nergy whe neve r AC loa ds a re be ing ope rate d. The ge nse t is opera te d cyclica lly in dire ct res ponse to the nee d for ma inta ining a s uitable s ta te of cha rge leve l in the sys te m's ba tte ry s tora ge ba nk.

Other PV/hybrid types

Ce rta in s pecific s ite loca tions may offer a ccess to othe r forms of powe r ge nera tion. Access to flowing wa te r prese nts the pote ntia l for hydro powe r. Access to cons is te nt wind a t s ufficie nt ve locity prese nts the pote ntia l for wind po we r. PV/hydro a nd PV/wind hybrid s ys tems ha ve been utilize d a t s ites with da ily e ne rgy re quireme nt ra nges s imilar to those des cribe d for PV/gense t hybrids . The ir use , howeve r, is much more s ite de pe nde nt, as the ir e nergy source is a factor of tha t loca tions ' topogra phy.
PV/The rmoe lectric ge ne ra tor hybrid s ys tems have bee n use d e ffective ly a t s ites whose da ily e ne rgy re quireme nt is re la tive ly
low, ra nging from 1 to 20 kW h pe r da y. Propa ne is the fue l s ource for the the rmoe lectric process , a nd convers ion e fficie ncies of up to 8% ca n be achie ve d. Cons ide rable was te hea t is there fore a va ila ble which ma y be utilize d for other re quire me nts . I n cold clima tes , this heat is ofte n use d to ma inta in the ba tte ry storage s ys tem a t des ire d te mpe rature le ve ls .

Architectura l Integration

Motivation

The las t two deca des ha ve brought s ignifica nt cha nge s to the des ign profess ion. I n the wa ke of tra uma tic esca la tions in e nergy prices , s horta ges , emba rgoes a nd wa r a long with he ighte ne d concerns over pollution, e nvironme nta l
de gra da tion a nd resource de ple tion, a wa re ness of the e nvironme nta l impact of our wo rk as des ign profess iona ls has dra ma tica lly increase d. I n the process , the s hortcomings of ye ste rda y's buildings ha ve a ls o become increas ingly clea r: ine fficie nt e lectrica l a nd clima te conditioning s ys tems s qua nde r grea t a mounts of e ne rgy. Combus tion of foss il fue ls on-s ite a nd a t powe r pla nts a dd greenhouse gases , acid ra in a nd other polluta nts to the e nvironment. I ns ide , ma ny building mate ria ls , furnis hings a nd finis hes give off toxic by - products contributing to indoor a ir pollution. Poorly des igne d lighting a nd ve ntila tion sys te ms ca n induce hea da ches a nd fa tigue .Architects with vis ion ha ve come to unde rs ta nd it is no longe r the goa l of good des ign to s imply crea te a building tha t is aesthe tica lly pleas ing - buildings of the future mus t be e nvironme nta lly res pons ive as we ll. The y have res ponde d by s pecifying increase d le ve ls of the rma l ins ulation, hea lthie r inte riors , highe r-e fficie ncy lighting, be tter gla zing a nd HVAC (hea ting, ve ntila tion a nd a ir conditioning) e quipme nt, a ir -to- a ir hea t e xcha nge rs a nd heat-recovery ve ntila tion s ys tems . Significa nt a dva nces ha ve been ma de a nd this progress is a very importa nt firs t s te p in the right dire ction. Howe ver, it is not e nough. For the de ve lope d countries to continue to e njoy the comforts of the la te twe ntieth ce ntury a nd for the de ve loping world to e ver hope to a tta in them, s us ta ina bility mus t become the corners tone of our des ign philos ophy. Ra ther tha n me re ly us ing less non-re ne wable fue ls a nd crea ting less pollution, we mus t come to des ign s us ta inable buildings tha t re ly on re ne wa ble res ources to produce some or a ll of the ir own e ne rgy a nd crea te no pollution. One of the mos t promis ing re ne wa ble e ne rgy technologies is photovolta ic ’s . Photovolta ics (PV) is a truly e le ga nt mea ns of producing e lectricity on s ite , directly from the s un, without conce rn for e nergy s upply or e nvironme nta l ha rm. These solid-s ta te de vices s imply ma ke e lectricity out of s unlight, s ile ntly with no ma inte na nce , no pollution a nd no de ple tion of ma teria ls . Photovolta ics a re a ls o e xcee dingly ve rsa tile - the same technology tha t ca n pump wa ter, grind gra in a nd provide communica tions a nd villa ge e le ctrifica tion in the de ve loping world ca n produce e lectricity for the buildings a nd dis tribution grids of the indus tria lize d countries .The re is a growin g conse ns us tha t dis tribute d photovolta ic s ys tems which provide e lectricity a t the point of use will be the firs t to reach wide s prea d commercia liza tion. Chie f among these dis tribute d a pplica tions a re PV powe r s ys tems for individua l buildings .
I nteres t in the building inte gra tion of photovolta ics , whe re the
PV e le me nts actua lly become a n integra l pa rt of the building,
ofte n serving as the e xterior wea thering s kin, is growing
world- wide . PV s pe cia lis ts from s ome 15 countries are
working within the I nterna tiona l Ene rgy Age ncy's Tas k 16 on a
5-yea r e ffort to optimize these s ys tems a nd architects a re now
be ginning to e xplore innova tive ways of incorpora ting s ola r
e lectricity into the ir building de s igns .

Planning conte xt of an energy conscious design project

The poss ibilitie s of a n active a nd pas s ive sola r ene rgy use in buildings is the fa ca de or into diffe rent building compone nts , s uch as a photovolta ic rooftile . Such a n inte gra tion ma ke s se nse for
va rious reas ons : ·The s ola r irra diation is a dis tribute d ene rgy
s ource ; the e ne rgy dema nd is dis tribute d as we ll.·The building
e nve lopes s upply s ufficie nt a rea for PV ge nera tors a nd
there fore
·.

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Active a nd Pass ive Solar Des ign Principles
I n orde r to use PV toge the r with othe r a va ilable techniques of active a nd pass ive s ola r e nergy, it mus t be cons idere d tha t s ome techniques fit we ll togethe r a nd others e xclude each othe r. For e xa mple : As a kind of a "pass ive cooling sys te m", cree pers a re use d for covering the s outh fa ca de of building. The leaves e va pora te wa ter a nd provide s ha de on the faca de . This he lps to a void pe ne tration of dire ct s unlight a nd re duces the tempe ra ture in the rooms be hind the faca de . At the same time the lea ves crea te s ha ding on PV modules tha t ma y be mounte d on the fa ca de res ulting in a fa r lower e le ctricity production. To a void s uch des ign fa ults it is necessa ry to compare a nd eva lua te the diffe re nt technique s tha t are a va ilable for crea ting a n e ne rgy conscious building. An ove ra ll e nergy conce pt for a building s hould be ma de a t the be ginning of the des ign process . The re fore , the a rchitect a nd the other e xpe rts involve d in the de s ign a nd pla nning process nee d to work toge the r right from the beginning of the des ign a nd pla nning process . All toge the r they ha ve to search right from the be ginning for the bes t des ign for a building project.

Photovoltaics and Archite cture

Photovolta ic’s a nd Archite cture are a cha lle nge for a ne w ge ne ra tion of buildings . I ns ta lla tions fulfilling a numbe r of technica l a pproaches do not a utoma tica lly re present aes the tica l s olutions . Collabora tion be tween e ngineers a nd a rchite cts is esse ntia l to crea te outs ta nding overa ll des igns . This aga in will s upport the wide use of PV. These s ys tems will a cquire a ne w ima ge , ceas ing to be a toy or a sola r module reserve d for a mounta in cha le t but becoming a mode rn building unit, inte grate d into the des ign of roofs a nd fa ca des . The architects , toge ther with the e nginee rs involve d a re as ke d to integra te PV a t leas t on four le ve ls during the pla nning a nd rea lisa tion of a building:
· Des ign of a building (s ha pe , s ize , orie nta tion, colour)
· Mecha nica l inte gra tion (multi functiona lity of a PV e leme nt)
· Ele ctrica l integra tion (grid connection a nd/or direct use of the
powe r)
· Ma inte na nce a nd ope ra tion control of the PV sys te m mus t be
inte grate d into the us ua l building ma inte na nce a nd control.

Pla nning Res pons ibilities a nd La y Down of Ene rgy
Cons umption.

MICROGRID CONCEPT

To rea lize the e merging pote ntia l of dis tribute d ge nera tion one mus t ta ke a s ys tem a pproa ch which vie ws ge nera tion a nd ass ociate d loa ds as a s ubs ys tem or a ‚microgrid‛. During dis turba nces , the ge nera tion a nd corres ponding loa ds ca n se pa ra te from the dis tribution s ys tem to isola te the microgrid’s loa d from the dis turba nce (a nd the reby ma inta ining se rvice ) without ha rming the tra ns miss ion grid’s inte grity.
The difficult tas k is to achie ve this functiona lity without e xte ns ive cus tom e ngineering a nd s till ha ve high s ys tem re lia bility a nd gene ra tion place ment fle xibility. To achie ve this we promote a peer-to-peer a nd plug-a nd-pla y mode l for each compone nt of the microgrid. The pee r -to-pee r conce pt ins ures tha t there a re no compone nts , s uch as a mas ter controlle r or ce ntra l s tora ge unit tha t is critica l for o pe ra tion of the microgrid. This implie s tha t the microgrid ca n continue opera ting with los s of a ny compone nt or ge ne ra tor. W ith one a dditiona l s ource (N+1) we ca n ins ure comple te functiona lity with the loss of a ny source .
Plug-a nd-pla y implie s tha t a unit ca n be place d a t a ny point on the e lectrica l s ys tem without ree nginee ring the controls . Plug- a nd-pla y functiona lity is much a kin to the fle xibility one has whe n us ing a home a pplia nce .
Tha t is it ca n be a tta che d to the e lectrica l s ys tem a t the loca tion whe re it is nee de d. The tra ditiona l mode l is to clus ter ge ne ra tion a t a s ingle point tha t ma kes the e lectrica l a pplica tion s imple r. The plug -a nd-pla y mode l facilita tes
pla cing ge nera tors nea r the hea t loa ds the reby a llowing more e ffective use of was te hea t without comple x hea t dis tribution s ys tems s uch as s team a nd chille d wa te r pipes .
This ability to is la nd ge ne ration a nd loa ds togethe r has the pote ntia l to provide a highe r loca l re lia bility tha n tha t provide d by the power s ys tem as a whole . Sma ller units , ha ving powe r ra tings in thousa nds of wa tts , ca n provide eve n highe r re lia bility a nd fue l e fficie ncy. These units ca n create microgrid se rvices a t cus tomer s ites s uch as office buildings , indus tria l parks a nd homes . Since the sma lle r units are modula r, s ite ma na geme nt could decide to ha ve more units (N+) tha n re quire d by the e lectrica l/hea t loa d, providing loca l, online backup if one or more of the ope ra ting units fa ile d. It is

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Als o much eas ie r to place s ma ll ge nera tors near the hea t loa ds there by a llowing more e ffective use of wa ste hea t. Bas ic Microgrid a rchite cture is s hown in figure 2. This cons is ts of a group of ra dia l fee de rs , which could be pa rt of a dis tribution s ys tem or a building’s e le ctrica l s ys tem. The re is a s ingle point of connection to the utility ca lle d point of common coupling. Some fee de rs , (Fee ders A-C) ha ve se ns itive loa ds , which re quire loca l ge nera tion.
The noncritica l loa d fee ders do not ha ve a ny loca l ge nera tion. I n our e xample this is Fee der D. Fee de rs A-C ca n is la nd from
the grid us ing the s ta tic s witch which ca n se pa ra te in less tha n a cycle . I n this e xample there a re four micro s ources a t nodes 8,
11, 16 a nd 22, which control the ope ra tion us ing only loca l
voltages a nd curre nts meas urme nts . W he n the re is a problem
with the utility s upply the s ta tic s witch will ope n, is ola ting the
se ns itive loa ds from the powe r grid. Fee de r D loa ds ride
through the e ve nt. I t is ass ume d tha t the re is s ufficie nt
ge ne ra tion to meet the loa ds ’ de ma nd. W he n the Microgrid is
grid-connecte d powe r from the loca l ge nera tion ca n be
dire cte d to fee der D.

PHOT OVOLT AIC T ECHNOLOGY

Photovolta ic’s is the fie ld of technology and research re la te d to the de vices which dire ctly convert s unlight into e le ctricity us ing semiconductors tha t e xhibit the photovolta ic e ffec t. Photovolta ic effect involves the crea tion of voltage in a ma teria l upon e xpos ure to e lectro magne tic ra dia tion..The photovolta ic effect was firs t note d by a Fre nch phys icis t, Edmund Be cque re l, in 1839, who found tha t certa in ma teria ls would produce sma ll a mounts of e le ctric curre nt whe n e xpose d to light. I n 1905, Albe rt Eins te in des cribe d the nature of light a nd the photoe lectric e ffect on which photovolta ic technology is base d, for which he la te r won a Nobe l prize in phys ics . The first photovolta ic module wa s built by Be ll La bora tories in 1954. I t was bille d as a solar batte ry a nd was mos tly jus t a curios ity as it wa s too e xpe ns ive to ga in wide s prea d use. I n the 1960s , the s pace indus try bega n to ma ke the firs t se rious use of the technology to provide po we r a boa rd s pacecra ft. Through the s pace programs , the technology a dva nce d, its re lia bility wa s e sta blis he d, a nd the cos t bega n to decline . During the e ne rgy cris is in the 1970s , photovolta ic technology ga ine d re cognition as a source of powe r for non-s pace a pplica tions .The solar ce ll is the e le menta ry building block of the photovolta ic technology. Sola r ce lls
a re ma de of semiconductor ma te ria ls , s uch as s ilicon. One of the prope rties of semiconductors tha t ma kes them mos t useful is tha t the ir conductivity ma y eas ily be modifie d by introducing impuritie s into the ir crys ta l la ttice . For ins ta nce, in the fabrica tion of a photovolta ic solar ce ll, s ilicon, which has four va le nce e lectrons , is trea te d to increase its conductivity.
On one s ide of the ce ll, the impurities , which a re phos phorus a toms with five va le nce e lectrons (n-donor), dona te wea kly bound va le nce e lectrons to the s ilicon ma te ria l, creating e xcess nega tive charge ca rriers . On the othe r s ide , a toms of boron with three va le nce e lectrons (p-donor) create a grea te r a ffinity tha n s ilicon to a ttract e lectrons . Beca use the p-type s ilicon is in intima te conta ct with the n-type s ilicon a p-n junction is es tablis he d a nd a diffus ion of e lectrons occurs from the region of high e le ctron concentra tion (the n-type s ide ) into the region of low e lectron conce ntra tion (p-type s ide ).
W he n the e lectrons diffus e across the p-n junction, they re combine with holes on the p-type s ide . Howe ve r, the diffus ion of ca rriers does not occur inde finite ly, beca use the imba la nce of cha rge imme dia te ly on e ithe r s ide s of the junction origina tes a n e le ctric fie ld. This e lectric fie ld forms a diode tha t promotes curre nt to flow in only one dire ction.
Ohmic me ta l-se miconductor conta cts a re ma de to both the n- type a nd p-type s ides of the solar ce ll, a nd the e le ctrodes are rea dy to be connecte d to a n e xterna l loa d. W he n photons of light fa ll on the ce ll, the y tra ns fer the ir e nergy to the cha rge ca rrie rs . The e le ctric fie ld acros s the junction se pa ra tes photo - ge ne ra te d pos itive cha rge ca rrie rs (holes ) from the ir nega tive counte rpart (e le ctrons ). I n this way a n e lectrica l curre nt is e xtracte d once the circuit is close d on a n e xte rna l loa d.

SOLAR CELL

The photovolta ic e lect was re porte d by Edmund Be que re l in
1839 whe n he obse rve d tha t the a ction of light on a s ilve r
coa te d pla tinum e lectrode imme rse d in e lectrolyte produce d
a n e le ctric curre nt. Forty yea rs la ter the res t s olid s ta te
photovolta ic de vices we re cons tructe d by worke rs
inves tiga ting the rece ntly discovere d photoconductivity of
se le nium. I n 1876 W illia m Adams a nd Richard Day found tha t
a photocurre nt could be produce d in a sa mple of se le nium
whe n contacte d by two hea te d platinum contacts . The
photovolta ic action of the se le nium diverte d from its
photoconductive a ction in tha t a curre nt was produce d
s ponta neous ly by the action of light. No e xterna l powe r s upply
wa s nee de d. I n this early photovolta ic de vice, a rectifying
junction ha d bee n forme d be twee n the semiconductor a nd the
me ta l conta ct. I n 1894, Cha rles Fritts pre pa re d wha t was
proba bly the rus t la rge area s ola r ce ll by pres s ing a la ye r of
se le nium betwee n gold a nd a nother meta l. I n the following
yea rs photovolta ic e le cts were obse rve d in copper{coppe r
oxide thin _lm s tructures , in lea d s ulphide a nd tha llium
s ulphide . These ea rly ce lls we re thin _lm Schottky barrie r
de vices , where a semitra ns pa rent la yer of me ta l de pos ite d on
top of the semiconductor provide d both the as ymme tric
e lectronic junction, which is necessa ry for photovolta ic action,
a nd a ccess to the junction for the incide nt light. The
photovolta ic e le ct of s tructures like this wa s re la te d to the
e xiste nce of a ba rrier to curre nt ow a t one of the se miconductor
{me ta l inte rfaces (i.e ., rectifying a ction) by Goldma n a nd
Brods ky in 1914. La te r, during the 1930s , the the ory of
me ta l{semiconductor ba rrie r la yers was de ve lope d by Wa lte r
Schottky, Ne ville Mott a nd others .Howe ver, it wa s not the
photovolta ic prope rties of ma teria ls like se le nium which

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e xcite d researchers , but the photoconductivity. The fa ct tha t the curre nt produce d was proportiona l to the inte ns ity of the incide nt light, a nd re la te d to the wa ve le ngth in a de nite wa y mea nt tha t photoconductive ma te ria ls we re idea l for photogra phic light me ters . The photovolta ic e lectin barrie r s tructures was a n a dde d be net, mea ning tha t the light me ter could ope ra te without a powe r s upply. I t was not until the
1950s , with the de ve lopme nt of good qua lity s ilicon wa fers for a pplica tions in the ne w s olid s ta te e lectronics , tha t pote ntia lly use ful qua ntities of powe r we re produce d by photovolta ic de vices in crys ta lline s ilicon.I n the 1950s , the de ve lopme nt of s ilicon e le ctronics followe d the discove ry of a wa y to ma nufacture p{n junctions in s ilicon. Na tura lly n type s ilicon wa fe rs de ve lope d a p type s kin whe n e xpose d to the gas boron trichloride . Pa rt of the s kin could be e tche d a wa y to give access to the n type la ye r be neath. These p{n junction s tructures produce d much bette r rectifying a ction tha n Schottky barrie rs , a nd be tter photovolta ic be ha viour. The rus t s ilicon s ola r ce ll wa s re porte d by Cha pin, Fulle r a nd Pea rs on in 1954 a nd converte d s unlight with a n e fficie ncy of 6%, s ix times higher tha n the bes t pre vious a ttempt. Tha t figure wa s to rise s ignifica ntly over the following yea rs a nd de ca des but, a t a n es tima te d production cos t of s ome $200 pe r Wa tt, these ce lls we re not se rious ly cons ide re d for power ge ne ra tion for seve ra l deca des . Neve rthe less , the ea rly s ilicon s olar ce ll did introduce the poss ibility of powe r ge ne ra tion in re mote loca tions where fue l could not eas ily be de live re d. The obvious a pplica tion was to sa te llites where the re quireme nt of re liability a nd low we ight ma de the cos t of the ce lls unimporta nt a nd during the
1950s a nd 60s , s ilicon sola r ce lls we re wide ly de ve lope d for a pplica tions in s pace .Also in 1954, a ca dmium s ulphide p{n junction wa s produce d with a n e fficie ncy of 6%, a nd in the following yea rs s tudies of p{n junction photovolta ic devices in ga llium a rsenide , indium phos phide a nd ca dmium te lluride we re s timula te d by the ore tica l work indica ting tha t these ma teria ls would ove r a highe r e fficie ncy. Howe ve r, s ilicon re ma ine d a nd re ma ins the foremos t photovolta ic ma teria l, be ne ting from the a dva nces of s ilicon technology for the microe lectronics indus try. Short his tories of the s ola r ce ll are give n e lse whe re. I n the 1970s the cris is in e ne rgy s upply e xpe rie nce d by the oil-de pe nde nt we s tern world le d to a s udde n growth of inte res t in a lte rna tive s ources of e nergy, a nd funding for research a nd de ve lopme nt in those areas . Photovolta ics was a s ubject of inte nse interes t during this pe riod, a nd a ra nge of s tra tegies for producing photovolta ic de vices a nd ma te ria ls more chea ply a nd for improving de v ice e fficie ncy were e xplore d. Routes to lowe r cos t include d photo e lectrochemica l junctions , a nd a lterna tive mate ria ls s uc h as polycrys ta lline s ilicon, amorphous s ilicon, other `thin _lm' ma teria ls a nd orga nic conductors . Stra te gies for higher e fficie ncy include d ta nde m a nd other multiple ba nd ga p des igns . Although none of these le d to wide s prea d comme rcia l de ve lopme nt, our unde rs ta nding of the scie nce of photovolta ics is ma inly roote d in this period..During the 1990s , inte res t in photovolta ics e xpa nde d, a long with growing a wa reness of the nee d to secure sources of e le ctricity a lte rna tive to foss il fue ls . The tre nd coincides with the wide s prea d de re gula tion of the e lectricity ma rke ts a nd growing re cognition of the viability of de centra lize d powe r. During this pe riod, the e conomics of photovolta ics improve d prima rily through economies of sca le . I n the la te 1990s the photovolta ic production e xpa nde d a t a ra te of 15{25% pe r a nnum, driving a re duction in cos t. Photovolta ics rus t became compe titive in conte xts whe re conventiona l e lectricity s upply is mos t e xpe ns ive , for ins ta nce , for remote low powe r a pplica tions s uch as na viga tion, te lecommunica tions , a nd rura l e lectrica tion a nd for e nhanceme nt of s upply in gr id-connecte d loa ds a t pea k use as prices fa ll, ne w ma rke ts a re ope ne d up. An importa nt e xa mple is building inte gra te d photovolta ic a pplica tions , whe re the cost of the photovolta ic sys te m is onse t
by the savings in building ma te ria ls .The re a re seve ra l types of s ola r ce lls . Howe ve r, more tha n 90 % of the s ola r ce lls curre ntly ma de worldwide cons is t of wa fer -base d s ilicon ce lls . The y a re e ither cut from a s ingle crys ta l rod or from a block compose d of ma ny crys ta ls a nd a re corres pondingly ca lle d mono-crysta lline or multi-crys ta lline s ilicon solar ce lls . Wa fer- base d s ilicon sola r ce lls a re a pproxima te ly 200 μm thick. Another importa nt family of sola r ce lls is base d on thin-films , which a re a pproxima te ly 1-2 μm thick a nd the re fore re quire s ignifica ntly less active , se miconducting ma te ria l. Thin -film s ola r ce lls ca n be ma nufa cture d a t lowe r cos t in la rge production qua ntities ; he nce the ir ma rket s hare will like ly increase in the future . Howe ver, they indica te lowe r e fficie ncies tha n wa fer-base d s ilicon s ola r ce lls , which mea n tha t more e xpos ure s urfa ce a nd ma te ria l for the ins ta llation is re quire d for a s imila r pe rforma nce . A numbe r of solar ce lls e lectrica lly connecte d to each other a nd mounte d in a s ingle s upport s tructure or fra me is ca lle d a ‘photovolta ic module ’. Module s a re des igne d to s upply e le ctricity at a certa in voltage , s uch as a common 12 volt s ys tem. The curre nt produce d is dire ctly de pe nde nt on the inte ns ity of light reachin g the module . Se vera l modules ca n be wire d toge the r to form a n a rra y. Photovolta ic modules a nd a rra ys produce dire ct -current e lectricity. The y ca n be connecte d in both series a nd pa ra lle l e lectrica l a rra ngeme nts to produce a ny re quire d volta ge a nd curre nt combina tion.

ELECTRICAL CONNECTION OF T HE CELLS


The e le ctrica l output of a s ingle ce ll is de pe nde nt on the des ign of the de vice a nd the Semi-conductor mate ria l(s ) chose n, but is us ua lly ins ufficie nt for mos t a pplica tions . I n orde r to provide the a ppropria te qua ntity of e lectrica l powe r, a numbe r of ce lls mus t be e lectrica lly connecte d. The re a re two bas ic connection me thods : se ries connection, in which the top contact of each ce ll is connecte d to the back conta ct of the ne xt ce ll in the se que nce , a nd para lle l connection, in which a ll the top contacts a re connecte d toge ther, as are a ll the bottom conta cts . I n both cases , this res ults in jus t two e le ctrica l connection points for the group of ce lls .Se ries connection:Figure s hows the se ries connection of three individua l ce lls as a n e xample a nd the re s ulta nt group of connecte d ce lls is commonly re fe rre d to as a se ries s tring. The curre nt output of the s tring is e quiva le nt to the curre nt of a s ingle ce ll, but the voltage output is increase d, be ing a n a ddition of the volta ges from a ll the ce lls in the s tring (i.e . in this case, the voltage output is equa l to 3Vce ll).

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Fig. Se ries connection of ce lls , with re s ulting curre nt–volta ge cha racte ris tic.
I t is importa nt to have we ll ma tche d ce lls in the se ries s tring, pa rticula rly with res pe ct to curre nt. I f one ce ll produces a s ignifica ntly lowe r curre nt tha n the othe r ce lls (unde r the same illumina tion conditions ), then the string will opera te a t tha t lowe r curre nt le ve l a nd the re ma ining ce lls will not be opera ting a t the ir ma ximum powe r points .
Figure s hows the pa ra lle l connection of three individua l ce lls as a n e xample . I n this case , the curre nt from the ce ll group is
equiva le nt to the a ddition of the curre nt from each ce ll (in this case , 3 I ce ll), but the voltage rema ins equiva le nt to tha t of a s ingle ce ll.As be fore, it is importa nt to have the ce lls we ll ma tche d in orde r to ga in ma ximum output, but this time the voltage is the importa nt pa rame ter s ince a ll ce lls mus t be a t the sa me ope ra ting volta ge . I f the volta ge a t the ma ximum powe r point is s ubs ta ntia lly diffe re nt for one of the ce lls , the n this will force a ll the ce lls to opera te off the ir ma ximum power point, with the poore r ce ll be ing pus he d towa rds its ope n -circuit voltage va lue a nd the bette r ce lls to volta ges be low the ma ximum powe r point volta ge . I n a ll cases , the powe r le ve l will be re duce d be low the optimum.

T HE PHOT OVOLT AIC ARRAY

A PV a rray cons is ts of a numbe r of PV module s , mounte d in the same pla ne a nd e lectrica lly connecte d to give the re quire d e lectrica l output for the a pplica tion. The PV a rra y ca n be of a ny s ize from a fe w hundre d wa tts to hundre ds of kilowa tts , a lthough the larger s ys tems a re ofte n divide d into seve ra l e lectrica lly inde pe nde nt s ub arra ys each fee ding into the ir own powe r conditioning s ys tem.

MOUNT ING ST RUCT URE

The ma in purpose of the mounting s tructure is to hold the modules in the require d pos ition without undue s tress . The s tructure ma y a ls o provide a route for the e lectrica l wiring a nd may be free s ta nding or pa rt of a nothe r s tructure (e .g. a building). At its s imples t, the mounting s tructure is a me ta l frame work, secure ly fixe d into the ground. I t must be ca pa ble of withs ta nding a ppropria te environme nta l s tresses , s uch as wind loa ding, for the location. As we ll as the mecha nica l is s ues , the mounting has a n influe nce on the ope ra ting tempe ra ture of the s ys tem, de pending on how eas ily hea t ca n be diss ipa te d by the module .

T ILT ANGLE AND ORIENT AT ION

The orie nta tion of the module with re s pect to the direction of the Sun dete rmines the inte ns ity of the s unlight fa lling on the module s urface . Two ma in pa rame te rs are de fine d to des cribe this . The first is the tilt a ngle , which is the a ngle be twee n the pla ne of the module a nd the horizonta l. The se cond para me ter is the a zimuth a ngle , which is the a ngle betwee n the pla ne of the module a nd due s outh (or some times due north de pe nding on the de finition use d). Correction of the direct norma l irra dia nce to tha t on a ny s urface ca n be de te rmine d us ing the cos ine of the a ngle betwee n the norma l to the Sun a nd the module pla ne .The optimum a rra y orie nta tion will de pe nd on the latitude of the s ite , pre va iling we a ther conditions a nd the loa ds to be me t. I t is ge nera lly a cce pte d tha t, for low la titudes , the ma ximum a nnua l output is obta ine d whe n the a rra y tilt a ngle is roughly e qua l to the la titude a ngle a nd the a rray faces
due south (in the northe rn he mis phe re ) or due north (for the s outhern he mis phere ). For higher la titudes , s uch as those in northe rn Europe , the ma ximum output is us ua lly obta ine d for tilt a ngles of a pproxima te ly the la titude a ngle minus 10 –15 de grees. The optimum tilt a ngle is a lso a ffecte d by the proportion of diffuse ra dia tion in the s unlight, s ince diffuse light is only wea kly dire ctiona l. The refore, for loca tions with a high proportion of diffuse s unlight, the e ffect of tilt a ngle is re duce d.
Howe ve r, a lthough this condition will g ive the ma ximum
output over the year, there ca n be cons ide rable va ria tion in
output with s eason. This is pa rticula r ly true in high-la titude
loca tions whe re the da y le ngth va rie s s ignifica ntly be twee n
s ummer a nd winte r. There fore, if a cons ta nt or reasona bly
cons ta nt loa d is to be me t or, pa rticula rly, if the winte r loa d is
highe r tha n the s umme r loa d, the n the bes t tilt a ngle may be
highe r in orde r to boos t winte r output. Preva iling we a ther
conditions ca n influe nce the optimisa tion of the a rra y
orie nta tion if the y a ffect the s unlight le ve ls a va ilable a t ce rta in
times of the day. Alte rna tive ly, the loa d to be me t ma y a ls o
va ry during the day a nd the a rray ca n be des igne d to ma tch
the output with th is va ria ble de ma nd by va rying the a zimuth
a ngle . Notwiths ta nding the a bility to ta ilor the output profile
by a lte ring the tilt a nd a zimuth a ngles , the ove ra ll a rra y
pe rforma nce does not va ry s ubs ta ntia lly for sma ll diffe re nces
in a rray orie nta tion. Figure s hows the pe rce ntage varia tion in
a nnua l insola tion leve ls for the location of London as tilt a ngle
is va rie d be twee n 0 a nd 90 de grees a nd a zimuth a ngle is va rie d
be twee n –45o (south eas t) a nd +45o (s outh wes t). The
ma ximum ins ola tion le ve l is obta ine d for a s outh-facing
s urface a t a tilt a ngle of about 35 de grees, as would be
e xpecte d for a la titude of a bout 51oN. Howe ve r, the ins ola tion
le ve l va rie s by less tha n 10% with cha ng ing a zimuth a ngle a t
this tilt a ngle . A s imila rly low va ria tion is obse rve d for s outh
facing s urfaces for a va ria tion of +/- 30 de grees from the

optimum tilt a ngle .
Fig. Pe rce ntage va ria tion of a nnua l s unlight le ve ls as a function of tilt a ngle a nd a zimuth a ngle .
The ca lcula tions we re carrie d out for the loca tion of London us ing Me teonorm Ve rs ion 3.0. The fina l as pect to cons ide r whe n deciding on a rra y orie nta tion is the incorpora tion in the s upport s tructure. For building -inte gra te d a pplica tions , the s ys tem orie nta tion is a ls o dicta te d by the nature of the roof or faça de in which it is to be incorpora te d. It may be necessary to tra de off the a dditiona l output from the optimum orie nta tion a ga ins t a ny a dditiona l cos ts tha t might be incurre d to accomplis h this . The aesthe tic iss ues mus t a ls o be cons idere d.

SUN-T RACKING/CONCENT RAT OR SYST EMS

The pre vious section has ass ume d a fixe d a rray with no cha nge

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of orienta tion during ope ra tion. This is the us ua l configura tion for a fla t-pla te arra y. Howe ve r, some a rra ys a re des igne d to track the pa th of the Sun. This ca n a ccount fully for the s un’s moveme nts by tracking in t wo a xes or ca n account partia lly by tracking only in one a xis , from eas t to we st. For a fla t -pla te a rra y, s ingle -a xis tracking, whe re the a rray follo ws the eas t- we s t move ment of the Sun, has been s hown to increase the output by up to 30% for a loca tion with pre domina ntly clea r s ky conditions .
Two-a xis tracking, whe re the a rray follows both the da ily eas t -
we s t a nd north-s outh moveme nt of the s un, could provide a
further increase of a bout 20% (Le ple y, 1990). For loca tions
whe re there are fre que nt overcas t conditions , s uch as northern
Europe , the be ne fits of tracking a re cons ide ra bly less . I t is
us ua lly more economica l to ins ta ll a la rge r pa ne l for loca tions
with less tha n about 3000 hours of direct s uns hine pe r a nnum.
For each case , the a dditiona l output from the s ys tem mus t be
compare d to the a dditiona l cos t of including the tracking
s ys tem, which include s both the control sys te m a nd the
me cha nis m for moving the a rray. For concentra tor s ys tems , the
s ys tem must track the Sun to ma inta in the concentra te d light
fa lling on the ce ll. The accura cy of tracking, a nd he nce the cos t
of the tracking s ys te m, increases as the concentra tion ra tio
increases .

SHADING

Sha ding of a ny pa rt of the a rra y will re duce its output, but this re duction will va ry in ma gnitude de pe nding on the e lectrica l configura tion of the arra y. Clea rly, the output of a ny ce ll or module which is s ha de d will be re duce d a ccording to the re duction of light inte ns ity fa lling on it. Ho we ve r, if this s ha de d ce ll or module is e lectrica lly connecte d to othe r ce lls a nd modules which a re uns ha de d, the ir pe rforma nce ma y a lso be re duce d s ince this is esse ntia lly a misma tch s itua tion. For e xa mple , if a s ingle module of a se ries s tring is pa rtia lly s ha de d, its curre nt output will be re duce d a nd this will the n dicta te the ope ra ting point of the whole s tring.
I f se vera l modules a re s ha de d, the s tring volta ge ma y be re duce d to the point whe re the ope n-circuit voltage of tha t s tring is be low the ope ra ting point of the res t of the a rray, a nd the n tha t s tring will not contribute to the a rray output. I f this is like ly to occur, it is ofte n use ful to include a blocking diode for s tring protection, as dis cusse d ea rlie r.
Thus , the re duction in output from s ha ding of a n a rra y ca n be s ignifica ntly grea ter tha n the re duction in illumina te d area , s ince it res ults from• the loss of output from s ha de d ce lls a nd modules ;• the loss of output from illumina te d modules in a ny se vere ly s ha de d s trings that ca nnot ma inta in ope ra ting voltage ; a nd• the loss of output from the rema inder of the a rra y beca use the s trings a re not ope ra ting a t the ir individua l ma ximum power points .For s ome syste ms , s uch as those in a city e nvironment, it ma y be imposs ible to avoid a ll s ha ding without se vere ly restricting the s ize of the a rray a nd he nce los ing output a t othe r times . I n these cases, good s ys tem des ign, including the optimum inte rconnection of modules , the use of s tring or module inve rters a nd, whe re a ppropria te, the use of protection de vices s uch as blocking diodes , ca n minimize the re duction in s ys tem output for the mos t preva le nt s ha ding conditions .

T HE PHOT OVOLT AIC SYST EM

wa ter from a we ll, to powe r a s ma ll ca lcula tor or one of ma ny more poss ible uses of solar-ge nera te d e lectricity. The des ign of the syste m de pe nds on the ta s k it mus t perform a nd the loca tion a nd other s ite conditions unde r which it mus t opera te . This section will cons ide r the compone nts of a PV s ys tem, va ria tions in des ign a ccording to the purpose of the s ys tem, s ys tem s izing a nd a s pects of s ys tem ope ra tion a nd ma inte na nce .

System design


The re a re two ma in s ys tem configura tions – s ta nd-a lone a nd grid-connecte d. As its name implies , the s ta nd-a lone PV s ys tem ope ra tes inde pe nde ntly of a ny othe r power s upply a nd it us ua lly s upplies e le ctricity to a de dica te d loa d or loa ds . I t may include a storage facility (e .g. ba tte ry ba nk) to a llow e lectricity to be provide d during the night or a t times of poor s unlight le ve ls . Sta nd-a lone s ys tems a re a lso ofte n re ferre d to as a utonomous sys te ms s ince the ir ope ration is inde pe nde nt of othe r powe r s ources . By contras t, the grid-connecte d PV s ys tem opera tes in para lle l with the conve ntiona l e le ctricity dis tribution sys te m. I t ca n be use d to fee d e le ctricity into the grid dis tribution s ys tem or to powe r loa ds which ca n a lso be fe d from the grid.I t is a lso poss ible to a dd one or more a lte rna tive power s upplies (e .g. diese l gene ra tor, wind t urbine ) to the sys te m to mee t some of the loa d requireme nts . These s ys tems a re the n known as ‘hybrid’ syste ms . Hybrid s ys tems ca n be use d in both s ta nd-a lone a nd grid-connecte d a pplica tions but a re more common in the forme r beca use , provide d the power s upplies ha ve been chose n to be comple menta ry, they a llow re duction of the storage re quire me nt without increa se d loss of loa d probability. Figures be low illus tra te the s chema tic diagra ms of the three ma in s ys tem types .
Fig. Sche ma tic dia gram of a s ta nd-a lone photovolta ic s ys tem.

Fig. Sche ma tic dia gram of grid-connecte d photovolta ic s ys tem.
A PV s ys tem cons ists of a numbe r of inte rconnecte d compone nts des igne d to accomplis h a des ire d tas k, which ma y be to fee d e lectricity into the ma in dis tribution grid, to pump

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Fig. Sche ma tic dia gram of hybrid s ys tem incorpora ting a photovolta ic a rra y a nd a motor ge ne rator (e .g. diese l or wind).

POWER MANAGEMENT

Powe r mana ge ment is a fea ture of s ome e lectrica l a pplia nces , es pecia lly copiers , compute rs a nd computer pe riphe ra ls s uch as monitors a nd printe rs , tha t turns off the powe r or s witches the s ys tem to a low-po we r sta te whe n inactive . I n computing this is known as PC power ma na geme nt a nd is built a round a s ta nda rd ca lle d ACPI . This s uperse des APM. All re cent (cons umer) compute rs have ACPI s upport.

Motivation:

PC powe r ma nage ment for compute r s ys tems is des ire d for ma ny reasons , particula rly:
Re duce overa ll e nergy cons umption
Prolong ba ttery life for porta ble a nd e mbe dde d s ys tems
Re duce cooling re quire ments
Re duce noise.
Re duce ope ra ting cos ts for e nergy a nd cooling.
Lowe r power cons umption a lso mea ns lowe r hea t diss ipa tion,
which increases sys te m s ta bility, a nd less e nergy use , which
sa ves money a nd re duces the impact on the e nvironme nt.

Processor level technique s:

The powe r ma na geme nt for microprocess ors ca n be done over the whole process or, or in s pe cific a reas .
W ith dyna mic volta ge sca ling a nd dyna mic fre que ncy s ca ling,
the CPU core volta ge , clock ra te , or both, ca n be a ltere d to
decrease power cons umption a t the price of pote ntia lly lowe r
pe rforma nce . This is s ome times done in rea l time to optimize
the powe r-pe rforma nce tra de off.
Exa mples :

AMD Cool'n'Quie t AMD Powe rNow! [1 ] I BM Ene rgySca le [2 ]

I nte l Spee d Ste p
Tra ns me ta Long Run a nd LongRun2
VI A Long Ha ul (Powe rSa ver)
Addit iona lly, process ors ca n se lective ly powe r off inte rna l
circuitry (powe r ga ting). For e xa mple :
Ne we r I nte l Core processors s upport ultra -fine powe r control
ove r the functiona l units within the processors .
AMD Cool Core technology ge t more e fficie nt pe rforma nce by

dyna mica lly activa ting or turning off pa rts of the processor.[3 ]

I nte l VRT technology s plit the chip into a 3.3V I /O section a nd
a 2.9V core section. The lowe r core voltage re duces powe r
cons umption.

Power Management System helps to:

 Avoid Bla ck-outs
I n case of a la ck of powe r, Loa d She dding secures the e lectrica l powe r to critica l loa ds by s witching off non-critica l loa ds according to dynamic priority tables .Re duce Ene rgy Cos ts / Pea k Sha ving.W he n a ll on-s ite powe r ge nera tion is ma ximize d a nd the powe r de ma nd s till te nds to e xcee d the contracte d ma ximum e lectricity import, the sys te m will a utoma tica lly s he d s ome of the low priority loa ds .Enha nce d Ope ra tor Support..At s ite s whe re e le ctricity is produce d by seve ra l ge ne ra tors , the de ma nds with res pect to control activities by opera tors a re much highe r. Adva nce d functions s uch as inte llige nt a la rm filte ring, cons is te ncy a na lys is , ope ra tor guida nce , a nd a we ll orga nize d s ingle -windo w inte rface s upport the ope ra tor a nd pre ve nt incorre ct interve ntions . Achieve Sta ble Opera tion
The Powe r Control function s ha res the active a nd reactive powe r betwee n the diffe rent ge nera tors a nd tie -lines in s uch a wa y tha t the working points of the machines are as fa r as poss ible a wa y from the border of the individua l PQ-ca pa bility dia gra ms s o tha t the pla nt ca n withs ta nd bigge r dis turba nces. Optimize Ne twork Des ign Be ca use the se t points for the ge ne ra tors , turbines a nd tra ns forme rs a re ca lcula te d in s uch a wa y tha t no compone nt will be overloa de d a nd the e lectrica l ne twork ca n be use d up to its limits , over -dime ns ioning of the ne twork is no longe r nee de d.
 Minimize Cabling a nd Engineering
All the s igna ls a nd informa tion which a re ava ila ble in prote ction/control re la ys , governor/e xcita tion controlle rs a nd othe r mic roprocess or base d equipme nt ca n be eas ily tra nsmitte d to the I ndus tria l PMS via seria l communica tion lin ks . This a voids ma rs ha lling cubicles , inte rpos ing re la ys , ca ble ducts , s pa ghetti wir ing, cabling e ngineering a nd provides e xtra functiona lity s uch as para me ter setting/rea ding, s tore d e ve nts , dis turba nce data a na lys is a nd a s ingle win dow to a ll e le ctrica l re la te d da ta .

MODELLING OF CASE ST UDY: SYST EM DESCRIPTION

A. Structure of Grid-Connected Hybrid Power System

The s ys tem cons is ts of a PV-FC hybrid s ource with the ma in grid connecting to loa ds a t the PCC as s hown in Fig. 1. The photovolta ic a nd the PEMFC a re mode le d as nonlinea r voltage s ources . These s ources a re connecte d to dc–dc converte rs which a re couple d a t the dc s ide of a dc/a c inve rter. The dc/dc connecte d to the PV a rray works as a n MPPT controlle r. Ma ny MPPT a lgorithms ha ve bee n propose d in the lite ra ture , s uch as incre me nta l conducta nce (I NC), cons ta nt voltage (CV), a nd pe rturba tion a nd obse rva tion (P&O). The P&O me thod has been wide ly use d beca use of its s imple

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fee dba ck s tructure a nd fe wer meas ure d pa ra mete rs . The P&O a lgorithm with po we r fee dba ck control is s hown in Fig. 2. As PV volta ge a nd curre nt a re dete rmine d, the power is ca lcula te d. At the ma ximum powe r point, the deriva tive is e qua l to ze ro. The ma ximum powe r point ca n be achie ve d by cha nging the re fere nce voltage by the amount

a lso de pe nds on solar irra dia tion a nd ce ll te mpera ture a nd ca n be ma thema tica lly e xpresse d as follows :

of

B. PV Array Model


The ma thema tica l mode l ca n be e xpresse d as

Equa tion (1) s hows tha t the output cha ra cte ristic of a solar ce ll is nonlinear a nd vita lly a ffecte d by s ola r ra dia tion,
tempe ra ture , a nd loa d condition. Photocurre nt is directly proportiona l to s ola r ra dia tion
The s hort-circuit curre nt of s ola r ce ll de pe nds linea rly on ce ll tempe ra ture


Thus , de pe nds on s ola r irra dia nce a nd ce ll te mpe rature

C. PEMFC Model

The PEMFC stea dy-s ta te fea ture of a PEMFC source is assesse d by mea ns of a polariza tion curve , which s hows the nonlinea r re la tions hip be twee n the volta ge a nd current de ns ity. The PEMFC output volta ge is as follows :

W here is the ‚thermodynamic potentia l‛ of Ne rs t, which re prese nts the re vers ible (or ope n-circuit) volta ge of the fue l


ce ll. Activa tion voltage drop is give n in the Ta fe l equa tion as
whe re are the cons ta nt te rms in the Ta fe l e qua tion (in volts pe r
Ke lvin)
The ove ra ll ohmic voltage drop ca n be e xpresse d as

The ohmic res is ta nce of PEMFC cons is ts of the re s is ta nce of the polyme r me mbra ne a nd e le ctrodes , a nd the re s is ta nces of the e le ctrodes .
The conce ntra tion voltage drop is e xpresse d as

D. MPPT Control

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Ma ny MPPT a lgorithms have been propose d in the lite rature , s uch as incre me nta l conducta nce (I NC), cons ta nt volta ge (CV), a nd perturba tion a nd obse rvation (P&O). The two a lgorithms ofte n use d to achieve ma ximum powe r point tra cking are the P&O a nd I NC me thods . The I NC me thod offers good pe rforma nce unde r ra pidly cha nging a tmos phe ric conditions . Ho we ve r, four sens ors a re re quire d to pe rform the computa tions . I f the se nsors re quire more conve rs ion time , the n the MPPT process will ta ke longe r to tra ck the ma ximum powe r point. During tracking time , the PV output is less tha n its ma ximum powe r. This mea ns tha t the longer the convers ion time is , the la rger amount of powe r loss will be on the contrary, if the e xecution s pee d of the P&O me thod increases , the n the s ys tem los s will de crease. More ove r, this method only re quire s two se ns ors , which res ults in a re duction of hardwa re re quire me nts a nd cost. The refore, the P&O me thod is use d to control the MPPT process . In orde r to achie ve ma ximum powe r, two diffe re nt a pplie d control me thods tha t a re ofte n chose n a re volta ge -fee dba ck control a nd powe r-fee dback control. Volta ge -fee dba ck control uses the s ola r-a rray te rmina l voltage to control a nd kee p the a rra y opera ting nea r its ma ximum powe r point by re gula ting the arra y’s voltage a nd ma tching the voltage of the a rra y to a des ire d volta ge. The dra wback of the voltage -fee dback control is its negle ct of the e ffect of irra dia tion a nd ce ll te mpe rature . The re fore , the powe r-fee dback control is use d to achie ve ma ximum powe r. The P&O MPPT a lgorithm with a po we r -fee dba ck control is s hown in Fig. 2. As PV volta ge a nd curre nt a re de termine d, the powe r is ca lcula te d. At the ma ximum powe r point, the de riva tive ( ) is e qua l to zero. The ma ximum powe r point ca n be achie ve d by cha nging the re fere nce voltage by the a mount of .
I n orde r to impleme nt the MPPT a lgorithm, a buck- boos t dc/dc converte r is use d as de picte d in Fig. 3. The pa ra mete rs L a nd C in the buck-boos t converte r mus t sa tis fy the following conditions :

The buck-boos t converte r cons is ts of one s witching device (GTO) tha t ena bles it to turn on a nd off de pe nding on the a pplie d ga te s igna l D. The ga te s igna l for the GTO ca n be obta ine d by comparing the s a w tooth wa veform with the control volta ge .
The cha nge of the re fere nce volta ge obta ine d by
MPPT a lgorithm becomes the input of the pulse width modula tion (PW M). The PW M ge ne ra tes a ga te s igna l to control the buck-boos t conve rter a nd, thus , ma ximum power is tracke d a nd de livere d to the ac s ide via a dc/a c inve rte r .

CONTROL OF T HE HYBRID SYST EM

The control modes in the microgrid inclu de unit po we r control, fee de r flow control, a nd mixe d control mode . The two control modes we re firs t propose d by Las serter. I n the UPC mode , the DGs (the hybrid s ource in this s ys tem) regula te the voltage magnitude a t the connection point a nd the powe r tha t s ource is injecting. I n this mode if a loa d increases a nywhe re in the microgrid, the e xtra powe r comes from the grid, s ince the hybrid source re gula tes to a cons ta nt powe r. I n the FFC mode , the DGs re gula te the volta ge ma gnitude a t the connection point a nd the powe r tha t is flowing in the fee der a t connection point. W ith this control mode , e xtra loa d dema nds a re picke d up by the DGs , which ma inta in a cons ta nt loa d from the utility vie wpoint.. I n the mixe d control mode , the sa me DG could control e ither its output powe r or the fee der flow powe r. I n othe r words , the mixe d control mode is a coordina tion of the UPC mode a nd the FFC mode . Both of these conce pts were cons idere d. I n this pa pe r, a coordination of the UPC mode a nd the FFC mode was inves tiga te d to de termine whe n each of the two control mode s was a pplie d a nd to de termine a re fere nce va lue for each mode . Moreover, in the hybrid s ys tem, the PV a nd PEMFC s ources have the ir cons tra ints . There fore , the re fe re nce power mus t be se t a t a n a ppropria te va lue so tha t the cons tra ints of these s ources a re satis fie d.
The propose d ope ra tion s trate gy prese nte d in the ne xt section
is a lso base d on the minimiza tion of mode cha nge . This
propose d opera ting s tra tegy will be a ble to improve pe rforma nce of the sys te m’s opera tion a nd e nha nce s ys tem
s ta bility.

OPERATING STRAT EGY OF T HE HYBRID SYST EM

As me ntione d before, the purpose of the ope ra ting a lgorithm is to de termine the control mode of the hybrid s ource a nd the re fe re nce va lue for each control mode so tha t the PV is a ble to work a t ma ximum output powe r a nd the cons tra ints a re fulfille d.
Once the cons tra ints ( a nd ) a re known, the control mode of the
hybrid s ource (UPC mode a nd FFC mode ) de pe nds on loa d va ria tions a nd the PV output. The control mode is decide d by the a lgorithm s hown in Fig. 7, Subsection B. I n the UPC mode , the re fere nce output power of the hybrid source de pe nds on the PV output a nd the cons tra ints of the FC output. The a lgorithm de termining is prese nte d in Subsection A a nd is de picte d in Fig. 4.

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Operating Strategy for the Hybrid System in the UPC Mode

I n this s ubsection, the prese nte d a lgorithm de te rmine s the hybrid s ource works in the UPC mode . This a lgorithm a llows the PV to work a t its ma ximum power point, a nd the FC to work within its high e fficie ncy ba nd. I n the UPC mode , the hybrid s ource re gula tes the output to the re fere nce va lue . The n

Equa tion (11) s hows tha t the va ria tions of the PV output will be compe nsa te d for by the FC power a nd, thus , the tota l powe r will be re gula te d to the refe re nce va lue .
Ho we ve r, the FC output mus t sa tis fy its c ons tra ints a nd, he nce , mus t se t a t a n a ppropria te va lue . Fig. 4 s hows the ope ra tion s tra tegy of the hybrid source in UPC mode to de termine . The a lgorithm includes two a reas : Area 1 a nd Area 2.
I n Area 1, is less tha n , a nd the n the re fere nce
Powe r is se t a t whe re



I f PV output is zero, the n (11) de duces to be e qua l to
. I f the PV output increases to , the n from (11)

a nd (12), we obta in equa l to . I n othe r words , whe n the PV output va ries from ze ro to , the FC output
will cha nge from to . As a res ult, the cons tra ints
for the FC output a lwa ys reach Area 1. I t is note d tha t the
re fe re nce powe r of the hybrid s ource during the UPC mode is fixe d a t a cons ta nt . Area 2 is for the case in which PV

output powe r is grea ter Tha n . As e xa mine d ea rlie r, whe n the PV output increases . To , the FC output will
decrease to its lower limit . I f PV output kee ps increas ing, the FC output will de crease be low its limit .
I n this ca se, to ope ra te the PV a t its ma ximum powe r
point a nd the FC within its limit, the re fe rence power mus t be


increase d. As de picte d in Fig. 4, if PV output is la rge r tha n
, the re fe re
nce power will be increase d by the amount of , a nd we obta in
Simila rly, if is greate r tha n , the FC output
be comes less tha n its lowe r limit a nd the re fere nce powe r will be thus increase d by the a mount of . I n othe r words , the re fe rence
powe r re ma ins uncha nge d a nd e qua l to if is le ss tha n a nd greate r tha n .

whe re
it is note d tha t is limite d s o that with the ne w

re fe re nce powe r, the FC output mus t be le ss tha n its uppe r limit . The n, we ha ve
I n gene ra l, if the PV output is betwee n a nd , the n we ha ve
Equa tions (17) a nd (18) s how the me thod of finding the re fe re nce powe r whe n the PV output is in Area 2. The re la tions hip be tween a nd is obta ine d by us ing (12), (13), a nd

(18) in (17), a nd the n

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The de te rmina tion of in Are a 1 and Area 2 ca n be
ge ne ra lize d by s ta rting the inde x from 1. There fore , if the PV
output


it is note d tha t whe n is give n in (12), a nd

I n brie f, the refe re nce powe r of the hybrid source is de termine d according to the PV output powe r. I f the PV output is in Area 1, the re fere nce powe r will a lwa ys be cons ta nt a nd se t at . Othe rwise , the re fe rence va lue will be cha nge d by the amount of , according to the cha nge of PV powe r.

The re fere nce powe r of the hybrid s ource in Area 1 a nd Area 2 is de te rmine d by (20) a nd (21). , a nd a re s hown in (22), (12), a nd (16), res pective ly. Fig. 5. s hows the control a lgorithm dia gra m for de termining the re fe rence powe r a utoma tica lly.

The
cons ta nt mus t
sa tis fy (16). I f increases the numbe r of cha nge of will de crease a nd thus the pe rformance of s ys tem ope ra tion will be improve d. Howe ve r, C s hould be s ma ll e nough s o tha t the fre que ncy does not cha nge over its limits 5%). I n order to improve the pe rforma nce of the a lgorithm, a hys teres is is include d in the s imula tion mode l. The hys teres is is use d to
preve nt oscilla tion of the se tting va lue of the hybrid s ys tem re fe re nce powe r . At the bo unda ry of cha nge in , the re fere nce va lue will be cha nge d continuous ly due to the oscilla tions in PV ma ximum power tracking. To a void the os cilla tions a round the bounda ry, a hys teres is is include d a nd its control scheme to control is de picte d in Fig.6.

B. Overall Operating Stra tegy for the Grid-Connected Hybrid

System

I t is we ll kno wn tha t in the microgrid, ea ch DG as we ll a s the hybrid s ource has two control modes : 1) the UPC mode a nd 2) the FFC mode . I n the a fore mentione d s ubsection, a me thod to de te rmine in the UPC mode is propose d. I n this s ubsection, a n ope ra ting s trate gy is prese nte d to coordina te the two control modes .
The purpose of the a lgorithm is to decide whe n each control mode is a pplie d a nd to de te rmine the refe re nce va lue of the fee der flow whe n the FFC mode is use d. This ope ra ting s tra tegy mus t ena ble the PV to work a t its ma ximum powe r point, FC output, a nd fee de r flow to s a tis fy the ir constra ints . I f the hybrid source works in the UPC mode , the hybrid output is re gula te d to a re fere nce va lue a nd the va ria tions in loa d are ma tche d by fee der powe r. W ith the re fere nce powe r propose d in Subsection A, the cons tra ints of FC a nd PV a re a lwa ys sa tis fie d. The re fore , only the cons tra int of fee der flow is cons idere d. On the othe r ha nd, whe n the hybrid works in the FFC mode , the fee der flow is controlle d to a re fere nce va lue

And, thus , the hybrid s ource will compe nsa te for the loa d va ria tions . I n this case , a ll cons tra ints mus t be cons ide re d in the ope rating a lgorithm. Base d on those a na lyses , the opera ting s tra te gy of the s ys tem is propose d as de monstra te d in Fig. 7. The ope ra tion a lgorithm in Fig. 7 involves two areas (Area I a nd Area I I ) a nd the control mode de pe nds on the loa d powe r. I f loa d is in Area I , the UPC mode is se lecte d. Othe rwise , the FFC mode is a pplie d with res pect to Area I I . I n the UPC a rea, the hybrid source output.
I f the loa d is lowe r tha n , the re dunda nt powe r will be tra nsmitte d to the ma in grid. Othe rwise , the ma in grid will se nd powe r to the loa d s ide to ma tch loa d dema nd. W he n loa d increases , the fee der flow will increase corres pondingly. I f

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fee der flow increases to its ma ximum , the n the fee de r flow ca nnot meet loa d dema nd if the loa d kee ps increas ing. I n orde r to compe nsa te for the loa d de ma nd, the control mode mus t be cha nge d to FFC with res pect to Area II . Thus , the boundary be twee n Area I a nd Area II is

W he n the mode cha nges to FFC, the feede r flow re fe re nce mus t be de termine d. I n order for the s ys tem opera tion to be seamless , the fee der flow s hould be uncha nge d during control mode tra ns ition. Accordingly, whe n the fee der flow re fe re nce is set a t , the n we have

I n the FFC area , the varia tion in loa d is ma tche d by the hybrid source . I n other words , the cha nges in loa d a nd PV output a re compe nsa te d for by PEMFC powe r. I f the FC output increases to its upper limit a nd the loa d is highe r tha n the tota l ge ne ra ting power, the n loa d s he dding will occur. The limit
tha t loa d s he dding will be rea che d is

Equa tion (25) s hows tha t is minima l whe n PV output is a t 0 kW . The n

Equa tion (26) mea ns tha t if loa d de ma nd is le ss tha n , loa d s he dding will ne ve r occur.
loa d s he dding is e ns ure d not to occur. Howe ve r, in se vere conditions , FC s hould mobilize its a va ila bility, to s upply the loa d. Thus , the loa d ca n be higher a nd the la rges t loa d is

I f FC powe r a nd loa d de ma nd sa tis fy (27), loa d s he dding will ne ver occur. Accordingly, base d on loa d forecas t, the ins ta lle d powe r of FC ca n be de termine d by following (27) to a void loa d s he dding. Corres ponding to the FC insta lle d power, the width of Area II is ca lcula te d as follows :


I n order for the s ys tem to work more s tably, the number of mode cha nges s hould be de crease d. As seen in Fig. 7, the limit cha nging the mode from UPC to FFC is , which is ca lcula te d in (23). Equa tion (23) s hows tha t de pe nds on a nd . is a cons ta nt.Thus de pe nds on Fig. 4 s hows
tha t in Area 2 de pe nds on . The refore , to decrease the numbe r of mode cha nges , cha nges mus t be re duce d. Thus , mus t be increase d. Howe ve r

mus t sa tis fy condition (16) a nd, thus , the minimize d numbe r of mode cha nge is reache d whe n is
ma ximize d

I n s umma ry, in a light-loa d condition, the hybrid s ource works in UPC mode , the hybrid s ource re gula tes output powe r to the re fe re nce va lue , a nd the ma in grid compe nsa tes for

load variations. is determined by the algorithm shown in Fig. 4 and, thus, the PV always works at its maximum power point and the PEMFC always works within the high efficiency band . In heavy load conditions, the control mode changes to FFC, and the variation of load will be matched by the hybrid source. In this mode, PV still works with the MPPT control, and PEMFC operates within its efficiency band until load increases to a very high point. Hence, FC only works outside the high efficiency band

in severe conditions. With an

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Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h, Vo lume 2, Issue 12, Dece mbe r-2011 24

ISS N 2229-5518

installed power of FC and load demand satisfying (27), load shedding will not occur. Besides, to reduce the number of mode changes, must be increased and, hence, the number of mode changes is minimized when is maximized, as shown in (29). In addition, in order for system operation to be seamless, the reference value of feeder flow must be set at .

MAT LAB DESIGN OF CASE ST UDY AND RESULT S: Case I: Fig. 8a

Fig. Grid connecte d PV-FC hybrid system

Case II: Fig. 8b

Fig. Grid connecte d PV-FC hybrid system

Case III: Fig. 9a

Fig. Grid connecte d PV-FC hybrid system

Case IV: Fig. 9b

Fig. Grid connecte d PV-FC hybrid system

CONCLUSION:

This pa pe r has prese nte d a n a va ila ble method to opera te a hybrid grid-connecte d syste m. The hybrid s ys tem, compose d of a PV a rra y a nd PEMFC, wa s cons ide re d. The opera ting s tra te gy of the sys te m is base d on the UPC mode a nd FFC mode . The purposes of the propose d ope ra ting s tra tegy presente d in this pa pe r a re to de termine the control mode , to minimize the numbe r of mo de cha nges , to ope rate PV a t the ma ximum powe r point, a nd to opera te the FC output in its high-e fficie ncy pe rforma nce ba nd.
The ma in ope rating s tra tegy, s hown in Fig. 7, is to s pecify the control mode ; the a lgorithm s hown in Fig. 4 is to de termine in the UPC mode . W ith the ope ra ting a lgorithm, PV a lwa ys ope ra tes a t ma ximum output powe r,

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Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h, Vo lume 2, Issue 12, Dece mbe r-2011 25

ISS N 2229-5518

PEMFC opera tes within the high-e fficie ncy ra nge , a nd feede r powe r flow is a lwa ys le ss tha n its ma ximum va lue . The cha nge of the ope ra ting mode de pe nds on the curre nt loa d de ma nd, the PV output, a nd the cons tra ints of PEMFC a nd fee der powe r. W ith the propose d ope ra ting a lgorithm, the s ys tem works fle xibly, e xploiting ma ximum s ola r ene rgy; PEMFC works within a high-e fficie ncy ba nd a nd, he nce , improves the pe rforma nce of the sys te m’s ope ra tion. The sys te m ca n ma ximize the ge ne ra te d powe r whe n loa d is hea vy a nd minimizes the loa d s he dding a rea . Whe n loa d is light, the UPC mode is se lecte d a nd, thus , the hybrid s ource works more s tably.
The cha nges in opera ting mode only occ ur whe n the loa d de ma nd is a t the bounda ry of mode cha nge ; othe rwise , the opera ting mode is e ither UPC mode or FFC mode . Bes ides , the varia tion of hybrid s ource re fere nce powe r is e limina te d by mea ns of hys teres is . I n a ddition, the numbe r of mode cha nges is re duce d. As a conseque nce, the s ys tem works more s ta bly due to the minimiza tion of mode
cha nges a nd re fe rence va lue va ria tion. I n brie f, the propose d
opera ting a lgorithm is a s implifie d a nd fle xible method to opera te a hybrid s ource in a grid-connecte d microgrid. I t ca n improve the pe rforma nce of the s ys tem’s ope ra tion; the s ys tem works more s tably while ma ximizing the PV output powe r. For furthe r research, the ope ra ting a lgorithm, ta king the opera tion of the ba ttery into account to enha nce ope ra tion pe rforma nce of the s ys tem, will be cons idere d. More over, the a pplica tion of the ope rating a lgorithm to a microgrid with multiple fee de rs a nd DGs will a ls o be s tudie d in de ta il.

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