Sustainable Energy and Economic Evaluation in Stand-Alone Photovoltaic Systems

Transcription

1 Sustinble Energy nd Economic Evlution in Stnd-Alone Photovoltic Systems SONIA LEVA Diprtimento di Elettrotecnic Politecnico di Milno Pizz Leonrdo d Vinci, 32 MILANO ITALY DARIO ZANINELLI Diprtimento di Elettrotecnic Politecnico di Milno Pizz Leonrdo d Vinci, 32 MILANO ITALY Abstrct: - The pper introduces hybrid photovoltic/diesel genertion systems for supplying remote power plnt tking into ccount the enhncement of the sustinble energy on the economic point of view. In prticulr, cost evlution is performed on rel plnt showing the effect nd weight of the sustinbility on the economicl sving. Key-Words: - Renewble source of energy, photovoltic systems, sustinble energy. 1 Introduction The continuous decresing of the costs of technologies ssocited to renewble energy sources together with consolidted know-how of the relevnt industry, bring to n incresing ppliction of these renewble genertion units in isolted electric plnts. In prticulr, the combintion of renewble electric genertion systems tht include btteries for energy storge with diesel genertors cn reduce the bttery size nd increse the relibility of the supply. These systems so-clled hybrid permit to decrese the fuel consumption of the diesel-mchine in the rnge of 70 90% s regrds to only btterydiesel supply rchitecture. Therefore, the primry source of electric energy for the plnt becomes the renewble one, while the diesel mchine constitutes the uxiliry source in cse of emergency, loss of the min supply or for bttery chrge. The hybrid systems cn be very effective in terms of sustinble energy development, permitting genertion of electric energy with minor environmentl interference. After discussion on the updte topic of sustinble energy progrms, the pper presents n economicl evlution of photovoltic (PV) ppliction for stnd-lone plnts where sustinbility ssumes n importnt role. In fct, politicl decisions nd people sensibility re going towrds the recognition of the economicl burden ssocited to environmentl nd socil costs tht cn rnsom renewble energy lso from the economic point of view. Nowdys, some countries consider the socil costs using the crbon tx. The pper introduces hybrid photovoltic/diesel genertion systems for supplying remote power plnt - mountin hut - tking into ccount the enhncement of the sustinble energy on the economic point of view. In prticulr, cost investment evlution is performed on rel plnt showing the effect nd the weight of the sustinbility economicl sving. 2 Sustinble energy for the development The world energy scenrio is deeply chnged in these lst yers nd the ttention hs been moved on the environmentl effects of the energy cycle trying to estblish connection between energy, development nd sustinbility. The sustinbility concept hs its roots in correct use of sources nd-mong they prticulrly importnt is the energy, tht is prt of ll humn ctivities. The vilbility of energy, the cost nd the impct of energy choices on the environment, re fctors tht influence the community development. Sustinbility nd environmentl blnce re rel problems. The proposed solutions, s lrge spred of lterntive energy sources to ingrte the clssicl ones, represent the deep consciousness of the need of culturl chnges with the support of the technology, essentil tool mking possible new strtegic solution for energy production.

2 Fig.1. Internl nd externl (environmentl) costs of electricity production. For ech energy crrier two cses re shown - one representing the lower rnge of vlues nd the other representing the upper rnge. Nowdys, for determining the cost of energy source the min fctors re its vilbility nd the chrges for its introduction on the mrket. The costs for the environmentl impct due to the energy use re missing t the present. This fct permits tht the energy sources from gs or oil or fossil result tody economiclly convenient nd widely used [1]. Renewble energy sources, nd in prticulr solr technology, re considered in the ctions to promote sustinble development both for the environmentl dvntges tht they offer, nd for the trend of their costs tht continue to decrese. In spite of this, from the point of view of the possible spred of the photovoltic in the energetic mrket, the cost of the systems nd consequently of the energy tht they produce constitutes still tody the more importnt limit. In fct, from the point of view of the customer who cn choose between vrious sources of energy the photovoltic offers some indirect dvntges. Fig. 1 shows internl nd externl (environmentl) costs of electricity production. In the context of internl costs some dditionl outliers on the upper rnge side re here excluded in the cse of hydropower. The photovoltic (PV) plnts, on the cost point of view, re penlized in respect to other conventionl energy resources [2]. In fct, without specific public incentives which clculte the socil dvntges offered by PV technology, the photovoltic is not yet competitive with other resources. The therml conventionl technologies re ctully more expensive in term of socil costs, but users no directly pys this socil costs tht re in chrge on the society. Rel resource costs should include both the privte costs incurred to provide power nd the externl costs of dmge (or deteriortion in qulity) to the environment cused by power genertion. Even if there is not stndrd ppliction of the externl costs, their progressive introduction in the energy price evlution should improve the possibility to use renewble sources nd their efficiency on the economicl point of view. A typicl pproch for the evlution of the externl environmentl costs pplied to electric energy production is methodology so clled impct pthwy [2] tht evlutes the dispersion, deposition nd effect of the pollution. This methodology summrizes the impct of the system in money indictor of the globl performnce. Anywy, this method does not tke into ccount socil fctors neither environmentl spect tht is difficult to quntify in costs. Considering the limittions of tht methodology, the evlution of the totl costs remins useful method tht still needs further improvement by mens of reserch nd development of models [3]. Alredy in the lst twenty-yers of the pst century thnks to study performed by the Frunhover Gesellschft - Institut fur Solrenergiesysteme (FhG-ISE) of Friburg, on behlf of the Europen Union some vlues hve been supplied [4]. The job of the FhG-ISE mkes reference to the existing sitution in the Federl Republic of Germny in 1984 nd tkes in considertion the production of thermo-electricl, conventionl or nucler systems. The conclusions of the study demonstrte tht, lso using the prudentil criteri the medium indirect lbor costs ssocite to

3 the fossil fuel use (converted in euro) re of the order of 0.04 /kwh, while those ssocite to the nucler re of 0.06 /kwh. This study, for its innovtive chrcter nd its importnce, hs been tken to model in successive nlyses. The Europen project THERMIE [5] leds to cost for producing energy with fossil sources tht cn be estimted in 0.20 /kwh. The increse of this vlue cn be correlted to n increse of the emissions in gs tmosphere of crbonic greenhouse gs s crbon dioxide (Fig.2). It is possible to ssume tht the CO 2 emission into the tmosphere will continue to increse, in the next yers, with liner shpe. Since the gs emissions of greenhouse gses in the tmosphere re directly tied to the monetry vlue of the sustinbility, it cn be supposed tht lso these costs linerly increse. An esteem of the economic vlue of the sustinbility for the next yers cn be mde, strting from the economic dt previously indictes, in the following wy 1 : S = t (1) The proposed eqution is bsed on the hypothesis tht the crbon dioxide emissions will grow with the sme behvior of the lst twenty yers. This is conservtive hypothesis, while considering the emissions of the numerous Underdeveloped Countries, the growth of CO 2 in the tmosphere could become t lest of prbolic type. This would surely influence the economic vlue of the environmentl spects, mking them to remrkbly increse their vlue. The evluted sustinbility costs re pplied to the economicl evlution of rel hybrid plnt in the following sections. Fig.2. CO 2 emissions over pst fifty yers 1 The proposed eqution ws derived by liner interpoltion of two economic vlues for sustinbility, relted to two different yers. The used vlues re respectively 0.04 /kwh for 1995 nd 0.20 /kwh for Hybrid photovoltic systems The Fig.3 reports the schemtic digrm of the hybrid photovoltic system. These systems re profitble in isolted res where the lod demnd is strongly vrible. Recent studies [6] nd simultions demonstrte tht the dvntge of hybrid genertion becomes interesting, on the economic point of view, when the rtio between the pek power demnd nd the minimum vlue of the bsorbed power is equl or greter thn 3. However, this rtio of convenience cn become lower thn 3 tking into ccount possible difficulties due to the isoltion of the plnt (gret distnce, nturl obstcles; etc.) nd/or some costs ssocited to sustinbility prmeters. In ddition, the presence of two different energy sources increses the relibility of the supply. Fig.3. Schemtic digrm of hybrid photovoltic system. The min configurtions of hybrid systems re: ) the series type; b) the switching type; c) the prllel type. The series configurtion is chrcterized by the presence of the dc bus-br only. The mjor prt of the energy flow psses through the btteries with consequent increment of the chrge-dischrge cycles, reducing the bttery efficiency nd its expected lifetime. The control system voids bttery overchrges when the produced energy is greter thn the lod demnd, cting both on the diesel genertor turn-on nd on the dc-dc converter tht opertes s power conditioner. The dvntges of this configurtion re the continuity of the supply nd the possibility to supply lods with the required wveforms. The disdvntges re reduced efficiency of the power produced by the diesel mchine (tht flows through two conversion stges) nd loss of supply in cse of fult in the inverter.

4 This ltter must be designed for the pek power of the lod. The switching configurtion permits, through breker, to supply the lod from the dc busbr by mens of the inverter or from the c busbr directly connected to the diesel genertor. The utomtic control cts on the breker, monitoring, on the bsis of the lod demnd, the solr power vilble, the stte of the bttery chrge nd the diesel mchine production. The dvntges of this configurtion re the possible use of the inverter for supplying lods with different wveforms nd the good efficiency of the power generted by the diesel mchine tht flows directly to the lods. The disdvntge is momentry interruption in the lod supply during the breker switching. In this cse lso, the inverter must be sized for stisfying the pek power of the lod. The prllel configurtion provides the supply by both the sources in n independent (for low-medium vlue of the power bsorbed by the lod) nd contemporneous (for high vlue of the power bsorbed by the lod) wy. The system control optimizes the c-dc converter opertion synchronizing the voltge on the c bus-br with the one generted by the diesel mchine. This configurtion presents the min dvntge tht the lod power demnd cn be greter thn the rted power of the photovoltic nd diesel genertor. Then, this solution requires lower component sizing thn the previous reclled configurtions, in prticulr for btteries, converter nd diesel mchine. On the contrry, more sophisticted control nd the condition of fixed voltge wveform for the lod supply t the c-dc converter output, represent the burden of this solution. 4 Exmple of ppliction In the following the problem of design rel plnt for electric power genertion is summrized [3]. This design concerns the fesibility nd the reliztion study of the photovoltic hybrid plnt of mountin hut in the Lombrdi Alps (Itly). This hybrid system hs to meet the following requirement: - reduce the run-time, nd then the cost regrding fuel nd mintennce, of the diesel genertor (Gen-Set) currently used; - hve n electric power genertion system tht respects nd protects, s fr s possible, the mountin environment where the hut is locted; hve system ble to control nd monitoring, from remote sttion, the hybrid photovoltic plnt (nd/or more plnts positioned in different plces). 4.1 Rdition dt The rdition dt of the hut site re reported in Tble I. In light grey re indicted the hut opening months. The first column shows the irrdition for squre meter evluted on horizontl surfce for typicl dy in ny month of yer. Strting from this dt it is possible to evlute the dily irrdition vlue on n inclined PV rry [3]. The optimum inclintion for the PV rry in the site corresponds to tilt ngle of 40 nd n zimuth ngle of 0. The rel inclintion used, imposed by the provisions on the environments impct, is 20 of tilt ngle nd 70 degree of zimuth ngle tht corresponds to the instlltion of the photovoltic modules on the hut roofing; the corresponding irrdition is indicted in the second column of Tble 1. Tble 1. Dily Irrdition Of The Hut Site, Evluted On The Horizontl Surfce And In The Rel Condition. month RADIATION ON THE HORIZONTAL SURFACE [KWh / m² /dy] RADIATION TILT 20 AZIMUTH 70 [KWh / m² /dy] Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Lod dt The lods present in the hut, optimised from the energy point of view, re subdivided in two different groups: Btteries lods, powered by the btteries directly (dc lods V dc ), or by the inverter (low power c lods V c ), s lights, refrigertor, lrms, etc.. The required totl energy per dy is 15.2kWh/d; Direct lods powered directly by the diesel mchine, including dishwsher, hot plte nd microwve oven, for totl of 13.6kWh/d. Furthermore the dc lods re subdivided in primry e secondry lods, tking into ccount the importnce of them ssocited to the plnt work nd the sfety. 4.3 The photovoltic hybrid system The photovoltic rry is designed, strting from irrdition, lod nd PV electric chrcteristic dt, for mximum power of 4kW p. This power is

5 Fig.4. Schemtic digrm tht represents n hybrid system with the control nd monitoring system. obtined using 72 PV modules fixed on the hut roof, nd 8 PV modules of 50W p ech one, fixed on the verticl wll of the south front of the house. The 8 PV modules fixed on the front serve to keep chrged the btteries in the wintertime, when the primry 72 PV modules on the roof could be covered with snow. The bttery rnk is designed to power ll the dc nd c lods for 4 dys during low-irrdition or no-sun period. Insted, the diesel genertor llows both to chrge the bttery nd to supply directly the c users for longer no-sun periods (more then 4 dys). Tking into ccount the totl consumption of energy relted to bttery lods (631Ah/dy), considering the energy received during the month with the lowest sunshine (4.87kWh/m²/dy) nd considering reduction coefficient of the 20% on the estimte dt, the photovoltic system is be ble to produce n energy contribution of 69% (434Ah/dy) of the btteries energy consumption. The inverter for the power supply of the bttery lods hve nominl power of 5kW, 150% overchrge for 3s., nd efficiency of 90%. Using the hybrid system it is possible to reduce the diesel genertor run-time from bout 16h/dy to 2h/dy. This mens tht we cn sve bout 14 h/dy of diesel genertor work tht correspond to bout 1400 h/yer nd to pproximtely 1400 liter of diesel fuel sved per opening seson. From the environmentl impct point of view this mens tht the emission of crbon dioxide (CO 2 ) in the tmosphere is cut down of bout 3 tons/yer. 4.4 Control nd Monitoring System The configurtion of this HIHS system is mix of ll the previous described configurtions. The schemtic digrm of the system is reported in Fig. 5. It is possible to note tht the interction between the genertors (photovoltic nd diesel), the btteries nd the lods is regulted by the dt cquisition nd control system tht optimizes the use of the energy sources nd the opertion of the btteries. The lods re divided on the bsis of their importnce for the customer needs nd the sfety. In cse of energy deficit nd bttery low chrge level, the control device provides scheduled lod shedding for ssuring the supply of the min lods only. Moreover, it is possible to trnsfer ll the dt to personl computer (PC) in remote control sttion. Other importnt chrcteristics of the control system described in [3] - re summrized in the following: The possibility to mesure electric quntities (voltges, currents, frequency of the c supply) on the plnt nd externl quntities useful for the efficiency optimiztion. The regultion of the bttery power flow to void life time reduction or operting conditions tht cn decrese the bttery relibility. The definition of protection nd wrning system ble to prevent fult conditions. The possibility to check the present opertion of the plnt nd the previous dt recorded in the memory, together with the possibility to set the wrning thresholds. These ctions cn be crried out both on site nd on remote sttion by mens of communiction protocol. The cpbility of the system to trnsmit wrning to remote sttion, pointing out the nomlous opertion of the plnt. The presence of locl memory crd for environmentl nd electricl dt collection nd processing. Thnks to these properties on the collected dt, it is possible to mnge the HIHS nd to modify, if necessry, the control lgorithm in order to optimize the system work with respect to the locl conditions. 5 Economic nlysis Scope of this section is to estimte the cost nd therefore the economic convenience to instll photovoltic system in plce of electricitygenerting groups for n isolted lod. The nlysis

6 is performed compring the hybrid plnt (HIHS) shown in Section 4 nd the sme plnt, which uses s energy source stnd-lone Genertor Set (GS). The produced kwh cost is clculted using the Unit Electricity Cost (UEC) method referring to the two plnts solution nlyzed. 5.1 Evlution of the hrdwre plnt s cost The first step for the economic nlysis consists in the evlution of the overll costs of the two plnts under exmintion: GS type nd HIHS type. Diesel genertor, fuel nd mintennce costs. The used GS hs nd indictive cost of bout Focusing on the fuel costs, in the GS plnt the genertor works 16 hours per dy for 110 dys. The fuel consumption is bout 1 l/h nd its cost is bout 1.02 /l. The yerly fuel cost mounts to In the HIHS type plnt the diesel genertor works only 2 hours per dy with n yerly fuel cost of The diesel genertor mintennce costs re clculted on the number of mintennce events per yer. Mintennce hs to be done every 150 working hours for the diesel genertor. As result the GS type plnt needs 11 mintennces per yer with n overll cost of 5500, while HIHS type plnt needs only 2 mintennces per yer with n overll cost of Photovoltic modules. The system under nlysis is built of 80 modules of 50Wp ech mde up of 36 solr cells. The men price of ech module is 171. So the PV modules cost is bout The PV hve to be instlled on support structures in order to obtin the design inclintion, for totl price of 878. Btteries. The used btteries hs cpcity of 3100 Ah nd the cost is bout The chemicl protection system price is ssumed equl to The inverter hs n overll cost of The bttery chrger on the bsis of the mrket nlysis is evluted in bout 850. Instlltion. The evlution of the instlltion costs is done with the lbor price tken from Miln s Chmber of Commerce in It is ssumed tht three persons compose the tem involved in the instlltion of the plnts. The instlltion of the GS type plnt cn be done in bout five working dys, three of them in which is present the whole tem nd two of them in which the most qulified worker is not present. The HIHS type plnt needs, for instlltion, the presence of the whole tem for twelve working dys. In Tbles 2 nd 3 the costs of the two considered plnts re summrized. A comprison of the two tbles shows tht the instlltion of HIHS plnt needs sum of money lrger thn the GS one. This is due substntilly to the PV modules costs. This mount, despite its vlue, will be reduced in the next yers becuse of the increse of the number of PV producer compny. This led to greter mrket competition, which involves costs reduction. 5.2 Produced energy cost The nlysis is completed with the evlution of the cost of the produced energy by the rel plnt, using the Life Cycle-Costing (LCC). The prmeters for LCC evlution re [6]: - period n: the period of time, n, during which the system is under nlysis, tht is, the gretest expected lifetime of the component; - infltion i; - discount rte d: the pplied ctive rte on d=2-5%; - cost of money nd the overll hrdwre nd instlltion costs; - benefits: in prticulr the economic vlue due to the reduction of greenhouse gses emitted in the tmosphere; - opertion nd mintennce costs; - fuel costs; - substitution costs of every prt t the end of its opertive life. Tble 2. GS plnt costs Instlltion [ ] 1871 Hrdwre [ ] 6138 Opertion nd minteinnce [ ] 7295 Totl [ ] Tble 3. HIHS plnt costs Instlltion [ ] 5227 Hrdwre [ ] Opertion nd mintinnce [ ] 1224 Totl [ ] 39290

7 The LCC evlution needs tht ll costs nd benefits, present nd future, re brought bck to present vlue or Present Worth (PW). The present vlue of the mounts is clculte, s function of the period of the pyment, using two different updting fctors: N - single pyment 1 + i (3) - nnul pyment PW PWr = C r 1+ d = C P N 1 + i i i (4) 1 = C P = C 1 1+ d d d where C r is the cost t yer r nd N is the expected life-time of the prt. The C is the recurrent nnul cost. The sum of ll PW gives the LCC. The produced energy cost is so clculble by mens of the Annulized life-cycle costing (ALCC): ALCC = LCC P = yer (5) which leds to the Unit Electricity Cost (UEC) by mens of the nnul produced energy given by the rtio between the ALCC nd the nnul energy E produced by the plnt: ALCC UEC = = E kwh (6) Tble 4 nd Fig.5 show the UEC evluted in different conditions. These conditions differs ech other only for the wy of evlution of the sustinbility. Three different sustinbility vlues re proposed: null vlue of the environmentl spects, constnt vlue [5] nd liner incresing vlue s proposed by eqution (1). The UEC reported in Tble 4 is very significnt indictor for performnce evlution of energy genertion systems nd gives, prcticlly, the kwh generted cost. In prticulr, the UEC for different plnt type with rnge of ±20% round the norml produced energy, nd with different evlution of the sustinbility costs re reported in Fig.5. 6 Conclusions The pper present n economic study of possible photovoltic hybrid systems studied for improving sustinble energy development. The result is the significnce nd vlidity of hybrid systems in isolted plnts where solr energy is effective nd the respect of the environment is importnt. The proposed method represents vlid reference determining the economic evlution of stndlone PV system where the following fctors cn be highlighted: - the socil nd environmentl benefits due to non polluting nd no noise opertion of the PV r r system, together with the possibility to integrte the PV modules in the building structure; - the independence form the fuel use. Tking into ccount these costs, menly relted to the sustinble development, the economicl fctor s UEC become of interest in PV ppliction for isolted users. Opening period [dys] Tble 4. HIHS unit electricity cost UEC with Energy UEC without constnt consumption sustinbility sustinbility [kwh/yer] [ /kwh] [ /kwh] UEC with liner sustinbility [ /kwh] Genset HIHS (constnt sustinbility) HIHS HIHS (liner sustinbility) Fig.5. Energy costs. References: [1] Lndu Network-Centre Volt School Interntionl UNESCO Science for the Pece. Energetic politicl nd Strtegies for the future: Scientific, Technologicl nd Economic spects Vol.1 Como - Vill Olmo Settembre 1998 (In Itlin) [2] Europen Commission, ExternE-Externlities of Energy, Vol. 10: Ntionl Implementtion, EUR 18528, Directorte-Generl XII, Science, Reserch nd Development, Luxembourg (1999). [3] W. Dlbon, S. Lev, M. Rosci, D. Zninelli, Hybrid Photovoltic System Control for Enhncing Sustinble Energy, Proc. IEEE PES Summer Meeting, Chicgo (IL), USA, July 21-25, 2002, n.pp. 6 [4] _fot.html [5] [6] T. Mrkvrt, Solr Electricity, Englnd: John Wiley & Sons, 1997