Simultion nd Design of Fst Chrging Bttery Sttion in Prking Lot of n e-crshring System Konstntinos N. Genikomskis DeustoTech Energy Deusto Institute of Technology, University of Deusto Bilbo, Spin kosts.genikomskis@deusto.es Igncio Angulo Gutierrez Engineering Design KTH Royl Institute of Technology Stockholm, Sweden ncho.ngulog@gmil.com Dimitrios Thoms nd Christos S. Iokimidis ERA Chir 'Net-Zero Energy Efficiency on City Districts' Reserch Institute for Energy, University of Mons Mons, Belgium {dimitrios.thoms, christos.iokeimidis}@umons.c.be Abstrct Crshring hs the potentil to reduce the totl number of crs on the rod, with significnt benefits to the society nd the environment, while t the sme time relevnt studies show tht university communities re often more receptive to lterntive trnsporttion services compred to the generl popultion. With the growing interest in electromobility, s mens of decrbonizing the trnsporttion sector, this pper considers the cse of combining crshring with electric vehicles (EVs) to serve the commuting needs of students, employees nd fculty of university in Bilbo, Spin. The im of the present work is to conceptulize the design of the chrging infrstructure of the e-crshring system under fst chrging scheme nd define its components, their ttributes nd interctions. To this end, MATLAB/Simulink bsed simultor is developed incorporting the dynmics of rel-world scenrio bsed on rrivl nd deprture dt from the university prking lot. Keywords electric vehicle; cr shring; university; chrging post; bttery; simultion I. INTRODUCTION Admittedly, the incresing humn needs for mobility combined with the use of privte vehicles re primry cuses of serious environmentl nd socil problems worldwide, hving negtive impct on the qulity of life [1-2]. The high use of privte crs is known to be source of high levels of ir nd noise pollution, s well s prking nd trffic congestion problems, minly in urbn res [2-3]. Relevnt studies indicte tht conditions of trffic congestion re typiclly observed t pek hours when people commute to work [4-5]. In this context, crshring serves s substitute for privte cr ownership tht hs lso positive contribution to urbn mobility, minly becuse the shred crs re used more efficiently [6-8], while reducing t the sme time the totl number of crs on the rod [9-11]. In this direction, university dministrtions nd members re often considered to be more receptive to lterntive trnsporttion services such s crshring [12], compred to the generl popultion [13]. The present pper considers the cse of providing crshring services with electric vehicles (EVs) in university setting to serve the mobility needs of students, employees nd fculty from their residentil point to the cmpus. In prticulr, the focus is on the design of the chrging infrstructure to support the effective opertion of the e- crshring system under fst chrging scheme. To this end, MATLAB/Simulink model is developed for properly dimensioning the electronic components of the chrging sttion to meet the energy requirements of fleet of 8 EVs. II. DESCRIPTION OF E-CARSHARING SYSTEM In the frme of this work, it is considered tht the bse of the envisged e-crshring system is prking lot t the university cmpus with n EV sttion with fst chrging cpbilities (Fig. 1), while there re 7 prking spots (A-G) round the city of Bilbo, Spin, where the users cn pick-up nd drop-off the EVs in order to commute to the university. Moreover, the chrcteristics of the Nissn LEAF 2011 model in [14] re employed in order to estimte the energy consumption of trip. Although the specific EV is equipped with lithium-ion bttery rted t 24 kwh, it is further ssumed tht the usble bttery cpcity C btev is 21 kwh (57.6 Ah), resulting in rnge R EV of 116.8 km per complete chrge. When the user mkes reservtion for n EV, the system must ensure tht the stte of chrge (SOC) of the EV bttery is sufficient to complete roundtrip, i.e. it cn rech its destintion nd return in single chrge. To this end, the minimum requirements in terms of energy stored in the EV bttery E btrndtrip to complete trip re expressed in (1), where D is the roundtrip distnce. Specificlly, the clcultions tke into ccount the energy consumption of the trip, incremented by 10% of bttery cpcity s sfety fctor nd by 15% of
bttery cpcity tht represents the low chrge zone of the bttery, which should not be reched in order to preserve the bttery s life expectncy. Tble I shows the corresponding minimum bttery energy nd SOC requirements for ech roundtrip to the 7 pick-up/drop-off points A-G, hving the university cmpus s strting point [15]. *( E = C D/ R + 10 / 100 + 15 / 100). (1) btrndtrip btev EV A. Dtsets of Prking Lot Occupncy To nlyze the prking behvior of the potentil users, two sources of dt re exmined: (i) dtset provided by the university services covering three prking lots for nerly 1- yer period, nd (ii) dtset collected nd processed from n experiment performed t specific prking lot of the university cmpus (s potentil loction of instlling the EV chrging sttion), where tem of prticipnts recorded the prking spot (plce) nd time of crs over the course of dy, including both weekdys nd weekends. The first dtset (obtined from the university services) regrding the entrnce nd exit of crs during 12-month period is compred with the dt of the field experiment in order to exmine whether the ltter is good representtion of n verge dy or not. The purpose is to use the dt from the experiment if possible, s it comprises exct recordings from the prking lot, wheres the dt provided by the university hd some flws resulting from the non exct recording of the rrivls or deprtures of the crs. Specificlly, the dt provided by the university hd to be filtered for consistency, since the originl rw dt contined entries of crs without registered entrnce or exit, e.g. due to filure of the gte or they were moving behind nother cr tht opened the gte. Thus, these entries were excluded from further nlysis, given tht no useful informtion could be extrcted bout the prking durtion. Fig. 1. The envisged fst chrging sttion of the e-crshring system. TABLE I. MINIMUM BATTERY ENERGY AND SOC REQUIREMENTS FOR ROUNDTRIPS FROM UNIVERSITY CAMPUS TO DESTINATION POINTS Destintion point D (km) Minimum bttery energy (kwh) Minimum SOC A 23.0 9.39 44.69 B 24.0 9.57 45.55 C 29.6 10.57 50.34 D 19.4 8.74 41.61 E 5.6 6.26 29.79 F 8.0 6.69 31.85 G 5.2 6.18 29.45 B. Distribution of Cr Prking Durtion The dt of the cr prking durtion provide useful insight on whether there will be enough time to chrge the EV bttery of potentil user of the e-crshring system using the fst chrging option. Indictively, Figs. 2 nd 3 present the cr rrivls nd deprtures respectively during dy ccording to the field experiment, while Fig. 4 shows the distribution of verge prking durtions on Mondys bsed on the yerly dt. The nlysis of the vilble dt reveled tht the commuters using cr follow similr pttern during the weekdys, except for the cse of Fridys, where the frequency of short durtion prking (up to 30 min) is roughly doubled. The similrity (in terms of correltion) of the vilble dt from the two sources indictes tht those collected from the field experiment cn be used s good verge estimte of those provided by the university for the 1-yer period. The dt of rrivls, prking durtion nd deprture comprise the bsis for representing the behvior of the potentil users of the e-crshring system in the simultion model developed for properly dimensioning the components of the EV chrging sttion. Fig. 2. Fig. 3. Cr rrivls during dy ccording to the field experiment. Cr deprtures during dy ccording to the field experiment.
V k = 1 s V. (3) L c 2 (1 k) * k* R =. (4) 2* f s Fig. 4. Averge prking durtion on Mondys ccording to the filtered university dt. C min = I * k f * ΔV. (5) s III. CHARGING CIRCUIT DESIGN The electronic circuit of the EV chrging sttion includes full wve rectifier nd boost converter in order to chrge its bttery t the proper voltge nd current. The focus is on clculting the design prmeters of these components to provide the required electricity to ech chrging post (of the EV chrging sttion), strting from the rectifier nd then proceeding with the electronic elements of the boost converter. It is noted though tht the efficiency of this prt of the system is out of the scope of the present pper. IV. CONTROL CIRCUIT DESIGN The bttery to bttery (B2B) chrging simultion includes control circuit in order to decide whether chrging post should chrge n EV rriving to the prking or not, built s simultion model tht combines Simulink logicl blocks with MATLAB code (Fig. 7). Bsed on the current SOC nd the destintion ech EV is heding to, it evlutes the required SOC nd decides to chrge it, if necessry. A. Rectifier The bttery of chrging post is considered to be chrged during the nighttime from the grid, when the electricity price is lower, nd be used during the dytime to chrge the EV btteries. A prcticl nd efficient wy to trnsform the threephse electricl (AC) input into usble stte for the system (DC) is by using three-phse bridge rectifier consisting of 6 diodes, s shown in Fig. 5. B. Boost Converter The role of the boost converter is to step up the voltge to the required vlue, being the output voltge higher thn the input voltge (Fig. 6). In the cse of the present work, it is ssumed tht the bttery of chrging post needs nominl voltge of 420 V to chrge the EV bttery, while voltge boost to 483 V is needed to chrge the bttery of chrging post. Given tht the input voltge V s is 345 V, the output voltge V is 483 V, the switching frequency f s is 10 khz nd the output current I is 60 A, the resistnce R, the duty cycle k, the criticl inductnce L c nd the minimum cpcitnce C min re clculted in (2) to (5) respectively, where V is the desired output voltge ripple. The vlues of the design prmeters re selected to meet the ppliction requirements, enbling the chrge of 120 Ah bttery in 6 hours, which is sufficient for the cse of night chrging from the grid. Fig. 5. Three-phse bridge rectifier. R. (2) I = V Fig. 6. Boost converter.
Fig. 8 shows tht the chrging process is controlled by opening or closing the Idel Switch1 upon system request, where Bttery2 represents the bttery of chrging post nd Bttery3 represents n EV bttery. The logicl blocks in Fig. 7 provide binry input to indicte whether the system needs to strt or stop chrging bsed on the SOC of the EV nd the destintion generted by the code. The EV destintions re generted with the subsystem in Fig. 9, which comprises component of the control circuit (Fig. 7). As strting point, it is further ssumed tht the btteries of the chrging posts hve cpcity of 120 Ah (50.4 kwh) to supply energy to the 8 EVs. Tking lso into ccount tht the mximum number of rrivls in single time step is 22 crs, while the cpcity of the prking lot is 65 crs, it follows tht 22 *8 / 65 = 3 chrging posts re required to supply energy to the EV fleet in the worst cse scenrio. Fig. 7. Control circuit of the chrging posts. Specificlly, the dytime is divided into 64 timeslots of 15 min, corresponding to the hours from 6 m to 10 pm. The time step durtion ws chosen on the bsis of enbling fst chrging of EVs. The underlying ssumptions nd steps of the simultion re the following: 1) The initil SOC of ech chrging post is set to 100% (chrged overnight from the grid). 2) The initil SOC of the 8 EVs of the e-crshring system is generted rndomly in the rnge between 50% nd 60%. 3) The rrivls of the EVs re generted ccording to the probbility estblished by the dt from the field experiment. 4) The destintions of the EVs re generted ccording to the distribution of popultion living in the surroundings of ech one of the 7 destintion points A-G, bsed on the dt vilble in [16-17]. 5) The necessry energy to rech the destintion is compred with the energy stored in the EV bttery nd the post initites the chrging process if needed. 6) A while loop repets the process from step 3 nd onwrds, until 64 itertions re completed. Then, n evlution report shows the number of chrged EVs, s well s if nd when there hs been ny unsuccessful chrging. 7) After the system is run for time step, the SOC of the chrging posts nd EVs is evluted in order to determine if the system behved properly. An indictor is employed to check whether the chrging hs been completed successfully or not nd determine the efficcy of the system. Fig. 8. Fig. 9. System for controlling the chrging process of the EV bttery. Subsystem for generting the EV destintions.
V. SIMULATION AND RESULTS As lredy pointed out, the simultion model depends on number of vribles tht tke vlues from probbility distribution, thus 5 simultion runs re performed to obtin sufficient output results over the working dys of week with respect to the energy needed by the system. The btteries of the chrging posts hve initilly cpcity of 120 Ah (50.4 kwh) nd re fully chrged t the beginning of ech dy (chrged overnight from the grid). The im of the simultion is to optimize the cpcity of the chrging post btteries, while ensuring tht the whole system opertes effectively. The objective is to chieve 100% success rte for the proposed EV chrging system, i.e. chrge the EVs of the e-crshring system when needed in order to successfully complete their roundtrips. The results obtined for ech simulted dy re presented in Tble II, clerly showing tht there is excess of energy remining t the posts fter chrging between 20 nd 30 EVs. Hence, there is significnt mrgin to reduce the cpcity of the chrging post btteries, nd thus improve the cost of the proposed system. Fig. 10 shows the totl mount of energy provided by the three chrging posts ech weekdy. Although the verge energy provided by the three chrging posts (47.22 kwh) is key indictor for determining the optiml size of their bttery cpcity, sfer solution is to consider the cpcity needed in the worst cse scenrio (58.7 kwh), i.e. dy 2 in Fig. 10. Similrly to the cse of determining the minimum SOC needed by n EV to complete roundtrip to ech destintion point A-G, the totl cpcity of the chrging post btteries C totl is determined by tking into ccount 10% sfety fctor nd the fct tht the bttery opertion in the low chrge zone (below 15% of cpcity) cn significntly shorten its life expectncy, s follows: C totl = 58.7 *1.1 / 0.85 = 75.97 kwh. (6) It follows tht the cpcity of the bttery in ech chrging post C cpost is third of the totl cpcity clculted for the system: Hving determined the bttery cpcity of ech chrging post, nother set of 5 simultions is run in order to check the performnce of the system in supplying the energy required by the EVs. The results obtined in this cse re presented in Tble III, while comprison between the initil nd finl simultion with respect to the totl remining energy in the chrging posts in ech dy is given Fig. 11. The results confirm tht the system successfully covers the energy demnd by the EVs nd t the sme time the chrging post btteries operte out of the low chrge zone. Fig. 10. TABLE III. Dy Totl energy provided by the chrging posts ech dy (initil simultion). REMAINING ENERGY AT THE END OF EACH DAY (FINAL SIMULATION) Chrging post 1 Chrging post 2 Chrging post 3 1 68.07 18.48 36.58 2 68.86 63.32 50.56 3 40.00 22.83 30.80 4 45.46 54.31 55.74 5 32.46 53.02 62.22 totl C = = 25.32 kwh. (7) cpost C 3 TABLE II. REMAINING ENERGY AT THE END OF EACH DAY (INITIAL SIMULATION) Dy Chrging post 1 Chrging post 2 Chrging post 3 1 78.21 64.31 61.93 2 63.24 61.00 59.30 3 63.92 86.28 79.54 4 62.54 68.69 73.08 5 67.97 66.61 74.89 Fig. 11. Totl remining energy in the chrging posts in ech dy between initil (blue) nd finl (red) simultion.
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