Solar energy, wind energy and storage for the electricity grid of today and tomorrow

. Photovoltaic (PV) generators and wind turbines (WT) are, nowadays, the most used technologies for power generation. This paper has the primary objective to explain the reasons of this success, in the present and in the future. Secondly, in this paper, an estimation of PV-WT percentage share of the electrical consumption is provided, according to a case study of power generation in southern Italy. The simulation results demonstrate that the PV-WT share can exceed the 40% threshold, considering about 10% of the national consumption located in a region where sun shines and wind blows generously.


Reasons to adopt the PV and WT technologies in the electricity mix
The first ad wew reas f the curret ad future assive devepet f PV ad WT techgies is the abudat aut f sar ad wid resurces i ast a the wrd's cutries The sar eergy received by the Earth ver a year is abut 1000 ties re tha a hua eergy csupti 1 The sar resurce is re equay distributed acrss cutries tha the wid as a resurce which i tur has higher pwer desity Ideed wid pwer desity is abut 5 W2 whe the wid bws at 20 s (a rare cditi) whereas sar irradiace ca reach 12 W2 i particuary cear sy cditis i utai catis Ather reas t chse the PV techgy is represeted by stabe ad rearabe vaues f eergy retur  eergy ivested (ERI esewhere used as EREI) i cparis with the fssi fues ad ucear techgies which ted t decrease their ERI Tw artices give detaied ERI vaues fr PV techgies The first study 2 prvides a systeatic review f eergy paybac tie (EPBT) ad ERI etrics The ea harised EPBT varied fr 10 t 41 years the due types raed i the fwig rder fr west t highest cadiu teuride (CdTe) cpper idiu gaiu diseeide (CIGS) arphus siic (aSi) pycrystaie siic (pySi) ad crystaie siic (Si) The ea harised ERI varied fr 87 t 342 The secd study 3 gives fast ad sipe ethds fr estiatig the eergy required fr prducig the PV array T this ed tw sipe apprxiatis wi be itrduced e thught t uderestiate the eergy cst (ebdied eergy) ad the ther thught t verestiate it Usig the upper budary fr the required iput eergy ad awig fr due degradati suggests that the ERI f a PV due wud have a wer budary f 5 r 6 I shrt the wrd's future geerati f PV eectricity appears t t be iited by ERI The depeti f raw aterias i utipe idustria sectrs causes serius ccer abut future grwth but the raw ateria used fr bth WT bades ad PV ces 4 siica ad purified arphuscrystaie prducts is ast iexhaustibe with egigibe csts BEF (Bberg Fiace) cacuates the eveised cst f eectricity (CE) fr each techgy taig it accut everythig fr equipet cstructi ad fiacig csts t peratig ad aiteace expeses ad average ruig hurs 5 I 2018 the bechar gba CE was 55 $Wh fr shre wid ad 70 $Wh fr PV withut tracig systes ther studies 6 cai that the CE f phtvtaic ad wid turbie techgies is tday wer tha fssi fue based techgies BEF gathers ifrati abut the cst f ivestet fr eectrcheica batteries (ithiui) Accrdig t the experiece curve techique aready used fr PV techgies the curret cst f eergy capacity is 300 $Wh 7 Fiay techgica prgress ad csteffectiveess are the drivig frces f curret widespread istaati f PV ad WT I 2017 gba istaed wid pwer geerati was ≈540 GW (bth adbased ad ff shre) ew aua istaati has bee reativey cstat i recet years (≈50 GWyear) ew PV istaatis are draaticay icreasig hwever the wrd tta PV pwer reached 400 GW i 2017 f which 100 GW was istaed i 2017 (re tha the cbied et capacity additis f fssi fues ad ucear pwer) I that year the wrd eectricity prducti was 1430 TWh fr WT ad 416 TWh fr PV 8 2 Determination of the optimum PV-WT share without grid reinforcement I 9 a prcedure is preseted t cacuate the ptia prtfi f PV geeratrs WT pars ad strage systes which suppy the aggregati f severa ads The ai ga was t shw the advisabe sefsufficiecy fr ca reewabe eergy surces (RES) that ca be achieved i grid cecti As shw i Fig 1 withut strage the sefsufficiecy frua is the rati betwee the eergy cay geerated ad iediatey csued Egc cpared t the tta ad Ead 10 The eergy Ead is thus the su f the quta Egc ad absrpti fr the grid Eabs If strage is istaed the grid iecti Ei ca be stred ad used ater t icrease sefsufficiecy  I rder t avid expesive grid upgrades RES peetrati was iited as a cstrait I additi y csteffective ivestets were csidered fr the pwer ad eergy resuts The resuts btaied thrugh a cparis betwee ads ad reewabe geerati i suther Itay shwed that the axiu sef sufficiecy cud reach ≈55% f the ads thas t the use f strage systes Batteries hep the itegrati f iterittet reewabes evertheess strage csts are sti high ad fr this reas the retur  ivestet is axiised if strage is t used but the sefsufficiecy decreases t ≈40% These resuts referred t a aggregati f ads with Eyear 530 GWh which is 016% f Itaia csupti (Eyear≈320 TWh) i 2017 11 Siiar wr is prpsed i the preset paper i which the aggregati f ads crrespds t ≈7% f the Itaia csupti prfies btaied fr the trasissi syste peratr (TS)

Load profiles and accurate measurements of irradiance and temperature
The Itaia TS prvides hury csupti prfies fr the third Wedesday f each th 12 I Itay i 2017 the axiu csupti was ≈54 GW i auary uy ad i Deceber Csupti decreases draaticay durig August I this th due t suer hidays ad the csequet cessati f ay wrig activities the pea ad is ≈35 GW ccurrig at 9 p Fr the sae f sipicity the siuatis are perfred csiderig ad prfies siiar t thse f the whe cutry i the preset wr These prfies are weighted accrdig t the ppuati ivig i Apuia which is ≈7% f the Itaia ppuati As shw i Fig 2 the Itaia aua ad (≈320 TWh) is thus scaed dw t ≈23 TWh ad the ad prfies are t dified Furtherre each e f the tweve daiy prfies fr the TS is used as represetative f the whe th I additi t the ad prfies the ther iputs are btaied fr etergica statis cated i suther Itay at atitudes withi 3941 prvidig data with 1i tie steps Irradiaces are easured usig pyraeters with easureet ucertaity i the rage 1525 W 2 12 Wid speeds are easured usig cup aeeters (gd accuracy) at 3  height fr the grud Ther hygreters ad bareters are used fr air teperature huidity ad pressure

Presentation of the system and simulation procedure
Fig 3 shws a sipified schee f the syste uder study It cprises PV geeratrs ad WT used t suppy ca ads with the hep f a battery eergy strage syste (BESS) The geeratrs are cected t DCAC cverters t feed AC ads (they are t shw i the schee) The battery aageet syste (BS) icuded i the BESS aages the chargedischarge cyces ad guaratees the crrect perati f the batteries The ptia share i ters f PV wid turbies ad strage is deteried by siuatig pwer geerati prfies with respect t csupti prfies This prcedure wrs as fws Firsty rad sies are defied fr PV WT ad strage where the ter sie stads fr the rated pwer fr PV ad WT whie sie eas the eergy capacity fr BESS The secd step is the cacuati f pwer baaces perfred with 1h tie steps fr a etire year The baaces csist f cpariss betwee reewabe geerati ad ads If the RES prducti is t eugh r is t high fr ca ads the BESS is used If batteries cat aage a the deficit r surpus pwer is exchaged with the grid A reated eergy ad ecic resuts are stred i a database The prcedure is repeated fr each pssibe cbiati f geeratrs ad strage sies The sies a start fr er ad are icreased after each siuati At the ed f a the prcess a data set is aaysed t excude uacceptabe cbiatis f geeratrs ad strage accrdig t ecic ad techica cstraits

Simulation constraints
The first cstrait is ecessary t avid grid upgrades it iits the axiu pwer that ca be iected it the grid As expaied i 131415 the high peetrati eves f upredictabe reewabes ay cause techica issues such as vtage rises harics ad ubaace Fr this reas i rder t avid gridupgrades the axiu acceptabe pwer iected it the grid has t be wer tha the axiu csued pwer ver the whe year I this way curret ad pwer are aways uder the iits f the grid ies ad the aua eergy iecti cat be t high The secd cstrait is ecic y cst effective ivestets are accepted Ivestets ust have a psitive et preset vaue (PV0) ad the itera rate f retur (IRR) which aws the yieds f differet ivestets t be cpared ust be higher tha 6% 1617 These assuptis are ade regardig csts • A iterest rate i3% this w vaue ca be used fr g ter ivestet i reewabes with w ris such as PV ad wid turbies • A aicusive istaati cst f 1000 W fr PV geeratrs ad 1200 W fr wid turbies • Yeary perati ad aiteace (&) csts crrespdig t 08% ad 2% f the istaati csts fr PV ad WT respectivey • A iitia cst fr the BESS with ii batteries equa t 300 Wh 18 • A average cst f csued eectricity equa t ≈15 cWh • A average price paid fr the eectricity iecti it the grid equa t ≈4 cWh 19

Models for PV generators, WT parks and storage
The pwer fw is siuated thas t apprpriate des fr PV ad wid geeratrs ad strage The eergy prducti fr wid turbies is cacuated startig fr the wid speed easureets The wid speed is trasferred at the height f the WT hub usig the frua i 20 This data is used i the pwer utputwid speed" curve fr the datasheet f wid turbies ad the resut is a prfie f pwer geerati fr the etire year 21 The PV geerati prfie is cacuated startig fr sar irradiace ad abiet teperature PV prducti is cacuated accrdig t a prprtia de i which pwer utput depeds  the tw pieces f evireta data ted abve ad the PV rated pwer 22 The strage perati is cacuated by ctiuusy ctrig its state f charge (SC) ad the iits i the axiu exchaged pwer The quatity f eergy that ca be charged r discharged is defied accrdig t iits ipsed t preserve battery ife strage is fu whe SCSCax epty whe SCSCi ad these iits cat be exceeded The iit f axiu pwer ust as be respected If it cat hade the pwer i the ca syste due these pwer ad eergy restrictis the extera grid is used t feed the ads

Simulation results
The siuatis f RES geerati ad csupti prfies aws the ptia sies f geeratrs ad strage t be deteried As a exape f the effective itegrati betwee PV ad WT prducti hury prfies f geerati are shw i Fig 4 fr a suer day where sar ad wid resurces are cpeetary Actuay the wid starts t strgy bw ust whe the su is decreasig i after As a resut the ad is fed by RES fr re tha 15 h per day axiisati f sefsufficiecy (SS) reaches ≈31% f the ad It is advisabe t ista 4 GW f PV geeratrs 12 GW f WT pars ad a BESS eergy capacity f 8 GWh The cputed IRR is ≈11% The sie f wid fars is wer tha PV geerati because the PV prfies better atch the csupti  the ther had i case f the cstptia suti (CS) this sefsufficiecy decreases t 26% The PV sie is the sae fr the previus suti ad WT ad BESS have rated pwers f 04 GW ad 4 GWh respectivey The reducti i the strage istaati is due t its high cst as a csequece a high WT sie cat be istaed t avid high iectis The abve resuts ivve ad prfies ad the August ad eve is ast 50% ess tha the ther ths due t the wr iterrupti (typica Itaia hidays) This w ad des t atch the high PV prducti i a adequate way resutig i ipsed iits t the PV ad WT sies t prevet high pwer iecti it the grid evertheess i the ear future there wi be a better atch betwee PV prducti ad csupti fr the widespread use f heat pups durig August I this sceari the previus siuatis are repeated withut icudig the August ad prfie I the case f SS sefsufficiecy reaches ≈44% f the ad A high strage capacity (20 GWh) supprts the istaati f 6 GW f PV geeratrs ad 17 GW f WT pars I the case f CS the sefsufficiecy is ≈37% btaied with a BESS eergy capacity f 12 GWh I bth cases eergy iecti it the grid is egigibe Regardig the csteffectiveess f the tw ivestets the IRR f the SS is 7% ad 11% fr CS

Conclusions
The curret techica iprveets ad the ecies f scae tday aw i catis where the su shies ad wid bws the csts f eectricity fr PV ad WT techgies are wer tha fssi fue ad ucear pwer pats The strage techgies eeded fr cpesatig fr RES iterittecy as destrate decreasig csts i eergy capacity dw t 300 $Wh I se cutries the PVWT share has w exceeded 20% f the tta ads hwever the siuatis i the iterature ad thse preseted i this paper agree that higher shares ca be achieved I the prpsed case study the axiu sefsufficiecy is ≈30% This eve is reativey pr because it icudes w ad prfies crrespdig t the cessati f wr activity fr the suer hidays I a sceari i which the ad eve reais siiar acrss the suer seas sefsufficiecy icreases t 4050% icudig battery supprt The crrespdig ecic perfrace is aways rearabe with IRR vaues i the rage f 6-11%