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Battery-assisted charging system for simultaneous charging of electric vehicles
Energy 100 (2016) 82e90
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Energy journal homepage: www.elsevier.com/locate/energy
Battery-assisted charging system for simultaneous charging of electric vehicles Muhammad Aziz a, *, Takuya Oda a, Masakazu Ito b a b
Solutions Research Laboratory, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan Advanced Collaborative Research Organization for Smart Society, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2015 Received in revised form 22 January 2016 Accepted 24 January 2016 Available online xxx
A battery-assisted charging system has been developed to improve the charging performance of a quick charger for electric vehicles. The developed system mainly consists of an alternating current-to-direct current inverter, a direct current-to-direct current voltage converter, a stationary battery, and an electric vehicle charger. The difference in charging rates in different seasons (winter and summer) was determined initially to measure the effect of electric vehicle battery temperature (influenced by surrounding temperature) on the charging rate. The charging rate during summer was higher than that during winter. In addition, simultaneous charging experiments were performed in different seasons (winter and summer) and for different contracted power capacities (50, 30, and 15 kW). Compared to a conventional charging system, the developed system can improve the charging performance of electric vehicle chargers in terms of the charging rate, while maintaining the contracted power capacity. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Quick charging Battery assistance Electric vehicles Contracted power capacity Charging rate
1. Introduction The development and deployment of EV (electric vehicle) have been recently accelerated because of fluctuating oil and gas prices, advances in battery technology [1] and lower environmental impact of EVs [2]. EVs have significantly lower running costs than conventional ICEV (internal combustion engine vehicles) because of higher total energy efficiency [3]. In case of Japan, the total fuel cost of EV in a year can be less than 30% of one consumed by ICEV (considering that total driving distance in a year, night time electricity price, gasoline price, fuel consumption of ICEV and EV are 10,000 km, 12 JPY kWh1, 110 JPY l1, 15 km l1, and 6 km kWh1, respectively). Moreover, the dissemination of EVs can improve the integration of intermittent renewable energy resources, GHG (reduce greenhouse gases) emissions, and increase grid efficiency and reliability [4]. In general, EV charging can be categorized as follows: (1) slow charging up to 4 kW, (2) fast charging with power of 10e20 kW, and (3) ultrafast charging which is capable of supplying up to 50 kW or higher [5]. In Japan, the quick chargers follow the CHAdeMO (the abbreviation of “charge de move” which is equivalent to “move by
* Corresponding author. Tel.: þ81 3 5734 3809; fax: þ81 3 5734 3559. E-mail address: [email protected] (M. Aziz). http://dx.doi.org/10.1016/j.energy.2016.01.069 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
charge”) standard, which was established by the CHAdeMO association and offers charging rates of 10e50 kW. Currently, Japan has about 5971 CHAdeMO-standard quick charging units across 5878 sites [6]. This is equivalent to about 38% of total installed EV chargers across the country including slow, fast and ultrafast chargers. As of date, EV charging is still considered to be a simple load, and its effects on the electricity grid can be neglected because of low EV penetration. However, as the number of EVs increases, the electricity demand for their charging would increase accordingly, leading to multiple grid problems. Paul and Aisu calculated the impact of electricity demand for quick charging in the area covered by Tokyo Electric Power Company and showed that overloading would occur because of peak demand for quick charging. As an example, in case 50% of vehicles are transformed to EVs and 50% of them demand quick charging, an additional 7.31 GW of electricity supply is required (considering that the number of vehicles, charging time and available quick charging time are 21,065,219 vehicles, 20 min and 12 h, respectively) [7]. In addition, ClementNyns et al. explained that the demand for EVs charging in Belgium in 2013 may reach about 5% of total electricity consumption leading to the afraid of power loss and overloads in transformers and feeders [8]. To avoid those problems, an intelligent management system is demanded, especially when multiple EVs are to be charged simultaneously. Intelligent charging facilitates operators and EV owners to manage the charging behavior of their
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vehicle to achieve a certain objective function, including minimization of electricity cost, reduction of GHG emission, and avoidance of line and transformer saturation. Moreover, Kavousi-Fard et al. stated the possibility to improve the share of renewable energy through EVs charging management in microgrids [9]. With regard to cost minimization on the charging operator side, simultaneous charging of EVs leads to high electricity demand which may reach to 100 kW for two simultaneous EVs charging (30% SOC). Therefore, operators are required to increase the contracted power capacity, specified in the power purchase agreement, to cover this high demand leading to higher electricity price or penalty. In contrast, with a certain limited contracted-power capacity, simultaneous charging leads to longer charging time owing to the low power distributed to each EV. In addition, the time spent in the queue for charging would increase. Scheduled and coordinated charging during off-peak hours is one method to solve the peak-to-valley equilibrium problem [10] as well as increasing the economic benefit and utilization of renewable energy [11]. To realize it, bidirectional communication between the operators and EVs, and accurate demand and supply forecasting are required. In addition, this method is feasible when EVs are parked for a relatively long period, i.e., at home in the night or in an office parking lot during working hours. It is not suitable in case prompt charging is needed once EVs are plugged in to chargers. In addition, currently, there is lack of a study dealing with the development, especially focusing on experimental analysis, of charging stations integrated with battery systems. To solve the abovementioned problems, we developed a BACS (battery-assisted charging system) and evaluated it, especially considering the simultaneous charging of EVs. By employing BACS, it is expected that simultaneous quick charging can be performed within the contracted power capacity, long queue for charging can be avoided and the quality of electricity grid can be maintained. In addition, as the charging behavior is influenced by some factors including temperature, experimental tests were performed to clarify the effect of temperature on charging behavior.
frequency regulation, (5) store any surplus electricity in the grid, such as renewable energy and excess power, and (6) provide emergency back-up to the surrounding community. Fig. 1 shows a schematic diagram of BACS developed by NEC Corporation, Japan. The solid and dashed lines represent electricity and information flows, respectively. The CEMS (community energy management system) deals with the overall management of energy, including supply and demand across the community. It optimizes energy performance in the community and minimizes both environmental impacts and social cost. CEMS communicates with other EMSs under its authority and negotiates with other CEMS or utilities to maximize benefits for its community. In electricity flow, there are three main modules connected by high-capacity DC lines: AC/DC inverter, battery, and quick charger. The AC/DC converter receives electricity from the grid and converts it to high voltage DC. The server controls the amount of electricity purchased from the grid based on demand, electricity price and grid condition. In addition, it also controls the charging and discharging behaviors of the stationary battery and the charging rate from a quick charger to the corresponding EV. In the battery unit, a bi-directional DC/DC converter and BMU (battery management unit) are installed before the battery to facilitate controllable charging and discharging. In the quick charger module, a DC/DC converter and a CCU (charging control unit) are installed to provide active control during charging. In this study, two quick charger modules are installed. The stationary battery is used to store electricity when there is remaining contracted power capacity or when the electricity price is low and discharge the stored electricity in the case of high electricity demand for charging or during peak hours with high electricity price. Basically, the battery having relatively large capacity is adopted to facilitate simultaneous charging of multiple EVs and hence improve service quality. Based on the charging and discharging conditions of the installed stationary battery and the source of electricity used for charging, quick charging modes of the developed BACS can be categorized as follows:
2. Integrated charging system
1. Battery discharging mode
The US DOE (Department of Energy) has set a relatively high charging rate of 10 miles of range per minute of charging during quick charging [12]. It means that for an EV having a battery capacity of 24 kWh (driving range of about 100 miles), the required charging time is 10 min with the 6 C rate [13]. In case of Japan, it is almost equivalent to the electricity consumed simultaneously by 20 households (100 V, 30 A). Therefore, very high charging rate when thousands of EVs are charged simultaneously causes a significant fluctuation in the electricity grid [7]. The embedment of battery in the charging system to assist the charger improves the performance of quick chargers [14] as well as answers the problems related to electricity demand [15]. In this study, BACS has been developed and evaluated. BACS can control the power input received from the grid (according to contracted power capacity, etc.) and power output from each charger to the corresponding EV. In addition, BACS controls the distribution of electricity inside its own system, including the stationary battery and chargers, to achieve its objective function. Hence, BACS can satisfy both supply side (minimizing grid load through load shifting and electricity cost reduction) and demand side (fascinating EV owners by quick charging, although in peak hours). The purposes of BACS are as follows: (1) reduce the contracted power capacity, (2) avoid high electricity demand in peak hours owing to EV charging, (3) shorten charging and waiting times, (4) participate in grid-ancillary services including spinning reserve and
Battery discharges its electricity in assisting the system. Hence, EV charging is performed using electricity from both the grid and the battery. This mode is applied in case of simultaneous quick
Fig. 1. Schematic diagram of a developed BACS.
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charging of multiple EVs, especially when the price of electricity is high. Electricity flow in this mode can be represented as follows:
Pgrid þ Pbatt ¼ Pqc;1 þ Pqc;2 þ Ploss ;
(1)
Table 1 Specifications of EV and QC used during the experiment. Component
Property
Value
EV
Type Battery type Total battery capacity Maximum voltage Nominal voltage Cathode Anode Cell rated capacity Cell average voltage Cell maximum voltage Type Standard Output voltage Output current Rate power output
Nissan Leaf Laminated lithium-ion battery 24 kWh 403.2 V 360 V LiMn2O4 with LiNiO2 Graphite 33.1 Ah (0.3 C) 3.8 V 4.2 V DC quick charger CHAdeMO DC 50e500 V 0e125 A 50 kW
where Pgrid, Pbatt, Pqc, and Ploss are electricity purchased from the grid, electricity charged (negative value) or discharged (positive value) from stationary battery, electricity for EV quick charging, and electricity loss, respectively. 2. Battery charging mode In case there is still margin in the contracted power capacity or the price of electricity from the grid is relatively low (due to surplus electricity, night time, etc.), the battery is charged to store electricity. Electricity flow in this mode can be expressed as Eq. (2).
Pgrid Pbatt ¼ Pqc;1 þ Pqc;2 þ Ploss
(2)
3. Battery idling mode Battery can be in the idling mode under some conditions: (a) contracted power capacity is sufficient to cover the simultaneous quick charging of multiple EVs (low demand in charging), (b) battery is empty owing to high and continuous quick charging for EVs (battery cannot supply the electricity unless being charged). In the case of (b), BACS controls the charging rate for each quick charger to maintain the contracted power capacity. Hence, any penalty or high electricity price can be avoided. Electricity flow in the idling mode is expressed as follows:
Pgrid ¼ Pqc;1 þ Pqc;2 þ Ploss
QC
(3)
BACS always ensures that the value of Pgrid is lower than or the same as the contracted power capacity. In addition, Ploss is the total loss in the system owing to some factors, including AC/DC and DC/ DC conversions and electricity consumed by the system. Hence, the value of Ploss in each mode is different from that in the other modes.
3. Effect of temperature on EV charging behavior EVs largely use lithium-ion batteries as the power source due to high energy density, stable electrochemical properties [16], longer lifetime, and low environmental impact [17]. Temperature is considered to be one of the important conditions influencing both charging and discharging behaviors of lithium-ion batteries. In general, lower temperature leads to poor charging and discharging performance due to electrolyte limitation [18] and changes in electrolyte/electrode interface properties, including viscosity, density [19], electrolyte components, dielectric strength, and ion diffusion capability [20]. In addition, Liao et al. found that as temperature decreases, charge-transfer resistance increases significantly, and it is higher than bulk resistance and solidestate interface resistance [21]. Unfortunately, to the best of the authors' knowledge, there is a lack of studies clarifying the charging behavior in different temperatures or seasons. In this study, to clarify the effect of temperature, especially ambient temperature, on the charging behavior of EVs, charging was initially conducted in different seasonsdwinter and summer. Table 1 summarizes the EV and QC specifications used in the experiment. Table 2 shows the experimental conditions employed in this study. The ambient temperatures were based on information from
Japan Meteorological Agency's database for the corresponding time in Yokohama area, where the experiments were conducted [22]. It is important to note that the EVs were charged in the cold condition after being parked for a relatively long time (such as one night) before charging. Hence, the EV battery temperature could be assumed to be very similar to the ambient temperature. Fig. 2 shows the relationship among charging rate, charging time, and SOC of an EV battery both in winter (a) and summer (b). In general, although the rated capacity of the charger is 50 kW, the charging power absorbed by the EV battery was relatively lower, especially in winter. Charging in summer led to a higher charging rate and hence a shorter charging time. Numerically, to reach an SOC of 80%, the required charging times in winter and summer were 35 and 20 min, respectively. In summer, a higher charging rate (about 40 kW) was achieved up to SOC of about 50%. The charging rate decreased gradually following an increase in SOC, and the charging rate was 16 kW when the SOC was 80%. In contrast, in winter, the charging rate reached about 35 kW instantaneously in very short time and decreased following further increase in SOC. The charging rate was about 10 kW when SOC reached 80%. Fig. 3 shows the correlation among charging current, voltage, and time in winter and summer, which corresponds to the charging rate in Fig. 2. The curves of the charging current in Fig. 3 are nearly similar to those of the charging rate in Fig. 2. In general, lithium-ion batteries are charged with a CC(constant current )eCV (constant voltage) method [23]. Charging at lower temperatures resulted in a gradual decrease in the charging current, especially at higher SOC, leading to longer charging time, and vice versa. In case of summer, a higher CC of about 105 A was achieved in the initial 5e10 min of charging (SOC up to about 50%). Although there was no significant difference in charging voltage, charging in a warmer temperature (summer) resulted in a slightly higher initial charging voltage before settling at a constant value. Hence, the CV condition was achieved faster. Temperature influences the charging behavior of EVs significantly. Charging in relatively high ambient temperature (such as
Table 2 Experimental conditions for evaluation of ambient temperature effect Season
EV No.
Ambient Temp. ( C)
Starting SOC (%)
Ending SOC (%)
Winter
#1 #2 #3 #1 #2 #3
11.4 11.0 10.3 27.5 27.1 32.0
32 25 29 34 31 31
80
Summer
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summer) led to higher charging rates, especially due to higher charging current and faster increase in the charging voltage leading to shorter charging time. According to [21], a decrease in temperature decreases the diffusivity of lithium ions in the cathode and causes electrode polarization. In addition, charge-transfer resistance at the electrolyteeelectrode interface increases [24].
4. Simultaneous charging performance of developed BACS
Fig. 2. Relationship among charging rate, charging time, and SOC of EV battery: (a) winter (b) summer.
To measure the performance of the developed BACS, simultaneous quick charging tests for EVs were conducted. Basically, the control systems were basically based on the above Eqs. (1)e(3). Table 3 lists the specifications of BACS. The DC line, which connects the modules, has a voltage of 450 V. Therefore, DC/DC converters in the battery and charger modules convert the voltage from 450 V to designated voltages of battery charging and discharging. Two quick chargers, each having maximum output power of 50 kW, were installed. Fig. 4 shows an overview of the developed BACS used for the experimental study. This developed system was installed in the JX Nippon Oil & Energy Factory in Shinkoyasu, Yokohama, Japan. Regarding the battery for BACS, as it is stationary, a broad range of battery types can be used including lithium-ion, nickel hydrogen, nickel cadmium, and potassium-ion batteries. In this study, lithium-ion batteries are adopted in consideration of its high energy density, ability to provide high current, low self-discharge, and low maintenance. In addition, the used batteries detached from EVs, which are almost lithium-ion type, also can be used as stationary battery for BACS. According to [25], EV battery is replaced after its capacity drops to 60e70% of the initial capacity. The utilization of these used batteries could increase the total economic performance of EV as well as lengthen their end-of-life and minimize the environmental impact due to battery recycling. The contracted power capacity is the electricity received by the AC/DC inverter (point A in Fig. 1). Hence, in case no battery is installed inside BACS, the electricity that can be transferred to EVs during charging, points C and D, will be less than the received electricity, point A, owing to the power loss during conversion and system consumption. The stationary battery of BACS was placed together with the other controllers and converters under controlled room temperature (20 C), and hence, the battery performance is considered to be stable and same at different ambient temperatures.
Table 3 Specifications of the developed BACS. Component
Property
Value
AC/DC inverter
Receiving voltage Converter output voltage Converter output power Power at DC line side Max. current at battery side Voltage at battery side Type Capacity (kWh) Nominal voltage Max. charging voltage Discharge cut-off voltage Max. current in continuous discharge SOC threshold in charging SOC threshold in discharging Number Standard Output voltage Output current Rated output power
200 V DC 450 V 50 kW 50 kW 150 A 0e400 V Lithium-ion 64.2 kWh 364.8 V 393.6 V 336.0 V 176 A 90% 10% 2 units CHAdeMO DC 50e500 V 0e125 A 50 kW
DC/DC converter
Battery
Quick charger
Fig. 3. Correlation among charging current, voltage, and time in different seasons: (a) winter (b) summer.
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Fig. 4. Developed BACS used for the experimental study: (a) overall system (b) EV quick charger (c) lithium-ion batteries pack.
To evaluate the performance of BACS, simultaneous charging experiments were performed. For this purpose, the experiments were performed in three steps: (1) simultaneous charging of two EVs to clarify the required charging time in different seasons, (2) BACS behavior under different contracted power capacity, and (3) simultaneous charging of eight EVs to measure the capability of BACS during peak times. As a basic policy for simultaneous charging, the first connected EV was prioritized and supplied with a higher charging rate, while the second EV was charged at a maximum charging rate of 5 kW until the charging rate of the first EV started decreasing. Further, after completion of charging of the first EV, the second EV was charged according to its maximum charging rate. BACS
Table 4 Experimental conditions for simultaneous charging of two EVs using a conventional system and BACS (contracted power capacity of 50 kW). Season
Charging system
Ambient Temp. ( C)
EV No.
Starting SOC (%)
Ending SOC (%)
Winter
Conventional
10.7 11.0
28 29 25 27 32 35 33 27
80
BACS
#1 #2 #1 #2 #1 #2 #1 #2
Summer Conventional BACS
28.4 27.8
maintains the contracted power capacity and controls the charging rate for each EV connected to the corresponding QC. 4.1. Simultaneous charging of two EVs Table 4 summarizes the experimental conditions for simultaneous charging of two EVs in different seasons. Fig. 5 shows the results of simultaneous charging of two EVs during winter with conventional QCs and BACS under a contracted power capacity of 50 kW. In a conventional charging system, owing to the limit on contracted power capacity, the first connected EV will be charged at a higher charging rate, while the second EV will be charged initially at 5 kW. As the charging rate of the first connected EV decreases, the charging rate of the second EV increases gradually so that the total electricity reaches the maximum contracted power capacity. As the charging rate of both EVs decreases owing to an increase in SOC, the total electricity received from the grid decreases. The first and second EVs reached 80% SOC after 40 and 50 min of charging, respectively. In contrast, when charging using BACS, the first and second EVs enjoyed almost the same charging rate, and reached 80% SOC in almost the same time (about 35 min) due to battery assistance supplying the electricity. In addition, electricity from the grid could be maintained below the contracted power capacity, although the total charging rate was larger than the contracted power capacity. Fig. 6 shows the results of simultaneous charging of two EVs during summer with a conventional charging system and BACS. Similar tendency to the case in winter can be observed. The first and
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Fig. 5. Simultaneous charging experiment during winter with a contracted power capacity of 50 kW: (a) conventional system (b) developed BACS.
Fig. 6. Simultaneous charging experiment during summer with a contracted power capacity of 50 kW: (a) conventional system (b) developed BACS.
second EVs reached 80% SOC after charging of about 20 and 30 min, respectively. When charging using BACS, both EVs reached 80% SOC in a relatively short time of about 20 min. In general, it was observed that the developed BACS improved the charging quality through a higher charging rate, especially during simultaneous charging of multiple EVs. The battery discharged its electricity until the total charging rate of two EVs was the same or lesser than the contracted power capacity of electricity.
connected EV decreased. The charging time increased as the contracted power capacity was reduced to 15 kW. In this case, the required charging time was about 38 and 25 min in winter and summer, respectively. Furthermore, the discharging power from the stationary battery was also larger in the case of lower contracted power capacity. Change in the SOC of the stationary battery was greater when the contracted power capacity decreased. Numerically, SOC decreased by about 5%, 10%, and 20% during the simultaneous charging of two EVs for contracted power capacities of 50, 30, and 15 kW, respectively. In this system, because the SOC threshold for battery charging and discharging were 90% and 10%, respectively, the total SOC change that could be used during simultaneous charging was 80%. Therefore, the stationary battery may continuously assist 32, 16, and 8 EVs during simultaneous charging with contracted power capacities of 50, 30, and 15 kW, respectively.
4.2. BACS performance with a different contracted power capacity To measure the performance of BACS in term of different contracted power capacities, experiments under different modified amounts of received power were performed. In addition to the experiments explained in sub-section 4.1 (contracted power capacity of 50 kW), two other conditions were evaluated, namely, contracted power capacities of 30 and 15 kW. Table 5 summarizes the experimental conditions corresponding to each contracted power capacity both in winter and summer. Figs. 7 and 8 show the experimental results of simultaneous charging of two EVs using BACS and contracted power capacities of 30 and 15 kW, respectively. For additional comparison, the ones with the contracted power capacity of 50 kW in winter and summer can be observed in Figs. 5(b) and 6(b), respectively. Basically, in the simultaneous charging of two EVs, there is no significant difference in terms of the charging time in each season. As the contracted power capacity decreased, the total charging rate of the two QCs decreased accordingly. In addition, in terms of charging time in the two seasons, there was no significant difference between the contracted power capacities of 50 and 30 kW. It seems that although the second connected EV received lower power initially, the power increased to its maximum value as the charging rate of the first
4.3. Simultaneous charging during high demand time (peak time) To measure the performance of the developed system during peak demand, simultaneous charging of eight EVs was performed. The EVs were initially prepared and parked near the BACS as queuing EVs waiting their turn for charging. The experimental conditions used in this study are listed in Table 6. As one EV completed its charging (SOC of 80%), the next EV (SOC of about 30%) started using the same charger. Fig. 9 shows the experimental results in winter and summer under the contracted power capacity of 30 kW. In general, simultaneous charging of eight EVs in summer can be performed faster than in winter owing to a higher charging rate. Although the charging time required for simultaneous charging in summer was significantly less, the SOC of the stationary battery decreased
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Table 5 Experimental conditions of simultaneous charging of two EVs with BACS under different contracted power capacities. Contracted power capacity (kW)
Season
Ambient Temp. ( C)
EV No.
Starting SOC (%)
Ending SOC (%)
30
Winter
10.7
#1 #2 #1 #2 #1 #2 #1 #2
28 29 25 27 32 35 33 27
80
Summer 11.0 15
Winter
28.4
Summer 27.8
considerably. This is because of the high discharging rate of the stationary battery when assisting the chargers. The stationary battery cannot be charged during this type of simultaneous charging because no marginal electricity is available to charge it. In contrast, in winter, the discharging rate of the stationary battery is significantly lower because of the slower charging rate of EVs. The total charging rate of two chargers can be lower than the contracted power capacity, leading to marginal electricity, which could be used to charge the stationary battery (Fig. 9(a)). Hence, the SOC of the stationary battery did not decrease by as much in winter as it did in summer. Fig. 10 shows the simultaneous charging of eight EVs during summer under a contracted power capacity of 15 kW. Compared with the case of a higher contracted power capacity (30 kW), almost no significant change was found in the charging rate of EVs, except that of the last connected EV. Unfortunately, the discharging rate of the stationary battery was very high, leading to faster decrease in its SOC. As can be observed, the SOC of the battery dropped rapidly and reached 10% during charging of the last two
Fig. 8. Simultaneous charging of two EVs using BACS with contracted power capacity of 15 kW: (a) winter (b) summer.
EVs. As a result, the last connected EV was charged using electricity received from the grid, without any assistance from the stationary battery. However, because the contracted power capacity was very low, the very last connected EV was not charged until the EV before it was charged completely. The stationary battery could not be
Table 6 Experimental conditions for simultaneous charging of eight EVs with BACS under different contracted power capacities. Contracted power capacity (kW)
Season
Ambient Temp. ( C)
EV No.
Starting SOC (%)
Ending SOC (%)
30
Winter
10.7
#1 #2 #3 #4 #5 #6 #7 #8 #1 #2 #3 #4 #5 #6 #7 #8 #1 #2 #3 #4 #5 #6 #7 #8
29 34 33 33 27 30 29 33 31 20 25 29 31 36 27 32 30 30 30 30 30 30 27 29
80
Summer 29.0
15
Fig. 7. Simultaneous charging of two EVs using BACS with contracted power capacity of 30 kW: (a) winter (b) summer.
Summer 27.8
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Fig. 9. Simultaneous charging of eight EVs using BACS under a contracted power capacity of 30 kW: (a) winter (b) summer.
Fig. 10. Simultaneous charging of eight EVs using BACS under a contracted power capacity of 15 kW during summer.
charged during simultaneous charging owing to the lack of marginal electricity and the high charging rate of EVs. Based on the results of the conducted experiments, the application of BACS can significantly improve the charging performance of QCs, especially during the simultaneous charging of multiple EVs. The balance among EV charging rate, contracted power capacity, and stationary battery SOC is seemingly very important and must be considered in depth. In addition, EVs charging demand should be forecast initially. 5. Conclusions A BACS-based QC was developed and its performance in simultaneous charging was evaluated. First, the charging behavior of EVs in different seasonsdwinter and summerdwas clarified in terms of the influence of temperature on the charging rate. It is believed that battery temperature strongly influences the charging
behavior of a battery, and ambient temperature influences the battery temperature. It was clarified that the charging rate during summer is higher than that during winter. In the simultaneous charging experiments, the application of BACS clearly improved the performance of EV chargers. Charging was performed in a shorter time, while maintaining the contracted power capacity. In future, as the demand for EVs charging increases, grid stress due to charging demand and its fluctuations would increase accordingly. The adoption of BACS would minimize this stress and maintain the quality of grid electricity. In addition, as BACS uses a large stationary battery, it can participate in ancillary service programs dedicated for the electricity grid when the demand for quick charging is relatively low. In this case, a bi-directional AC/DC inverter is required to facilitate both services up (from battery to the grid) and down (from grid to battery). This participation can improve the economic performance of BACS and grid reliability.
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Acknowledgments This research was part of the Yokohama Smart City Project funded by New Energy Promotion Council, Japan (No. 3115105). The authors express their sincere thanks to NEC Corp. and JX Nippon Oil & Energy Corp. for their collaboration. Furthermore, we also offer our thanks to Ms. Yoko Watanabe for her assistance in data reduction and analysis.
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[11] Carrion M, Zarate-Minano R. Operation of renewable-dominated power systems with a significant penetration of plug-in electric vehicles. Energy 2015;90:827e35. [12] Howell D, Faguy P, Cunningham B. US DOE vehicle battery R&D: progress update. US Department of Energy; 2011 [accessed 15.04.15], http://www. hydrogen.energy.gov/pdfs/htac_nov2011_howell.pdf. [13] Chandrasekaran R. Quantification of bottlenecks to fast charging of lithiumion-insertion cells for electric vehicles. J Power Sources 2014;271:622e32. [14] Ishii Y, Shizuno T, Kogure J, Itabashi N, Watanabe H, Nyu T. Development of battery and charger integration system (BCIS). NEC Tech J 2012;7:24e7. [15] Ishii Y, Shizuno T, Kogure J, Itabashi N, Watanabe H, Nyu T. Development of battery and charger integration system (BCIS) for EV charging stations. In: IEEE Int. Conf. Smart Grid Engineering (SGE'12), Oshawa, ON, Canada; August 27-29, 2012. [16] Shih YT, Wu CH, Hung FY, Lui TS, Chen LH. A study at room temperature and 55 C on the chargeedischarge characteristics of Si(100 x)Alx thin film anode for Li-ion batteries. Surf Coat Technol 2013;215:79e84. [17] Yao JW, Wu F, Qiu XP, Li N, Su YF. Effect of CeO2-coating on the electrochemical performances of LiFePO4/C cathode material. Electrochim Acta 2011;56:5587e92. [18] Xiao LF, Cao YL, Ai XP, Yang HX. Optimization of EC-based multi-solvent electrolytes for low temperature applications of lithium-ion batteries. Electrochim Acta 2004;49:4857e63. [19] Ping P, Wang Q, Huang P, Sun J, Chen C. Thermal behaviour analysis of lithium-ion battery at elevated temperature using deconvolution method. Appl Energy 2014;129:261e73. [20] Jansen AN, Dees DW, Abraham DP, Amine K, Henriksen GL. Low-temperature study of lithium-ion cells using a LiySn micro-reference electrode. J Power Sources 2007;174:373e9. [21] Liao L, Zuo P, Ma Y, Chen X, An Y, Gao Y, et al. Effects of temperature on charge/discharge behaviors of LiFePO4 cathode for Li-ion batteries. Electrochim Acta 2012;60:269e73. [22] Japan Meteorological Agency. http://www.jma.go.jp/jma/index.html [accessed 17.04.15]. [23] Seyama Y, Shimozono T, Nishiyama K, Nakamaura H, Sonoda T. Development of large-scale lithium ion batteries “LIM series” for industrial applications. GS News Tech Rep 2003;62:76e81. [24] Zheng H, Zhang H, Fu Y, Abe T, Ogumi Z. Temperature effects on the electrochemical behavior of spinel LiMn2O4 in quaternary ammonium-based ionic liquid electrolyte. J Phys Chem B 2005;109:13676e84. [25] Viswanathan VV, Kintner-Meyer M. Second use of transportation batteries: maximizing the value of batteries for transportation and grid services. IEEE Trans Veh Technol 2011;60:2963e70.
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Energy journal homepage: www.elsevier.com/locate/energy
Battery-assisted charging system for simultaneous charging of electric vehicles Muhammad Aziz a, *, Takuya Oda a, Masakazu Ito b a b
Solutions Research Laboratory, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan Advanced Collaborative Research Organization for Smart Society, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2015 Received in revised form 22 January 2016 Accepted 24 January 2016 Available online xxx
A battery-assisted charging system has been developed to improve the charging performance of a quick charger for electric vehicles. The developed system mainly consists of an alternating current-to-direct current inverter, a direct current-to-direct current voltage converter, a stationary battery, and an electric vehicle charger. The difference in charging rates in different seasons (winter and summer) was determined initially to measure the effect of electric vehicle battery temperature (influenced by surrounding temperature) on the charging rate. The charging rate during summer was higher than that during winter. In addition, simultaneous charging experiments were performed in different seasons (winter and summer) and for different contracted power capacities (50, 30, and 15 kW). Compared to a conventional charging system, the developed system can improve the charging performance of electric vehicle chargers in terms of the charging rate, while maintaining the contracted power capacity. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Quick charging Battery assistance Electric vehicles Contracted power capacity Charging rate
1. Introduction The development and deployment of EV (electric vehicle) have been recently accelerated because of fluctuating oil and gas prices, advances in battery technology [1] and lower environmental impact of EVs [2]. EVs have significantly lower running costs than conventional ICEV (internal combustion engine vehicles) because of higher total energy efficiency [3]. In case of Japan, the total fuel cost of EV in a year can be less than 30% of one consumed by ICEV (considering that total driving distance in a year, night time electricity price, gasoline price, fuel consumption of ICEV and EV are 10,000 km, 12 JPY kWh1, 110 JPY l1, 15 km l1, and 6 km kWh1, respectively). Moreover, the dissemination of EVs can improve the integration of intermittent renewable energy resources, GHG (reduce greenhouse gases) emissions, and increase grid efficiency and reliability [4]. In general, EV charging can be categorized as follows: (1) slow charging up to 4 kW, (2) fast charging with power of 10e20 kW, and (3) ultrafast charging which is capable of supplying up to 50 kW or higher [5]. In Japan, the quick chargers follow the CHAdeMO (the abbreviation of “charge de move” which is equivalent to “move by
* Corresponding author. Tel.: þ81 3 5734 3809; fax: þ81 3 5734 3559. E-mail address: [email protected] (M. Aziz). http://dx.doi.org/10.1016/j.energy.2016.01.069 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
charge”) standard, which was established by the CHAdeMO association and offers charging rates of 10e50 kW. Currently, Japan has about 5971 CHAdeMO-standard quick charging units across 5878 sites [6]. This is equivalent to about 38% of total installed EV chargers across the country including slow, fast and ultrafast chargers. As of date, EV charging is still considered to be a simple load, and its effects on the electricity grid can be neglected because of low EV penetration. However, as the number of EVs increases, the electricity demand for their charging would increase accordingly, leading to multiple grid problems. Paul and Aisu calculated the impact of electricity demand for quick charging in the area covered by Tokyo Electric Power Company and showed that overloading would occur because of peak demand for quick charging. As an example, in case 50% of vehicles are transformed to EVs and 50% of them demand quick charging, an additional 7.31 GW of electricity supply is required (considering that the number of vehicles, charging time and available quick charging time are 21,065,219 vehicles, 20 min and 12 h, respectively) [7]. In addition, ClementNyns et al. explained that the demand for EVs charging in Belgium in 2013 may reach about 5% of total electricity consumption leading to the afraid of power loss and overloads in transformers and feeders [8]. To avoid those problems, an intelligent management system is demanded, especially when multiple EVs are to be charged simultaneously. Intelligent charging facilitates operators and EV owners to manage the charging behavior of their
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vehicle to achieve a certain objective function, including minimization of electricity cost, reduction of GHG emission, and avoidance of line and transformer saturation. Moreover, Kavousi-Fard et al. stated the possibility to improve the share of renewable energy through EVs charging management in microgrids [9]. With regard to cost minimization on the charging operator side, simultaneous charging of EVs leads to high electricity demand which may reach to 100 kW for two simultaneous EVs charging (30% SOC). Therefore, operators are required to increase the contracted power capacity, specified in the power purchase agreement, to cover this high demand leading to higher electricity price or penalty. In contrast, with a certain limited contracted-power capacity, simultaneous charging leads to longer charging time owing to the low power distributed to each EV. In addition, the time spent in the queue for charging would increase. Scheduled and coordinated charging during off-peak hours is one method to solve the peak-to-valley equilibrium problem [10] as well as increasing the economic benefit and utilization of renewable energy [11]. To realize it, bidirectional communication between the operators and EVs, and accurate demand and supply forecasting are required. In addition, this method is feasible when EVs are parked for a relatively long period, i.e., at home in the night or in an office parking lot during working hours. It is not suitable in case prompt charging is needed once EVs are plugged in to chargers. In addition, currently, there is lack of a study dealing with the development, especially focusing on experimental analysis, of charging stations integrated with battery systems. To solve the abovementioned problems, we developed a BACS (battery-assisted charging system) and evaluated it, especially considering the simultaneous charging of EVs. By employing BACS, it is expected that simultaneous quick charging can be performed within the contracted power capacity, long queue for charging can be avoided and the quality of electricity grid can be maintained. In addition, as the charging behavior is influenced by some factors including temperature, experimental tests were performed to clarify the effect of temperature on charging behavior.
frequency regulation, (5) store any surplus electricity in the grid, such as renewable energy and excess power, and (6) provide emergency back-up to the surrounding community. Fig. 1 shows a schematic diagram of BACS developed by NEC Corporation, Japan. The solid and dashed lines represent electricity and information flows, respectively. The CEMS (community energy management system) deals with the overall management of energy, including supply and demand across the community. It optimizes energy performance in the community and minimizes both environmental impacts and social cost. CEMS communicates with other EMSs under its authority and negotiates with other CEMS or utilities to maximize benefits for its community. In electricity flow, there are three main modules connected by high-capacity DC lines: AC/DC inverter, battery, and quick charger. The AC/DC converter receives electricity from the grid and converts it to high voltage DC. The server controls the amount of electricity purchased from the grid based on demand, electricity price and grid condition. In addition, it also controls the charging and discharging behaviors of the stationary battery and the charging rate from a quick charger to the corresponding EV. In the battery unit, a bi-directional DC/DC converter and BMU (battery management unit) are installed before the battery to facilitate controllable charging and discharging. In the quick charger module, a DC/DC converter and a CCU (charging control unit) are installed to provide active control during charging. In this study, two quick charger modules are installed. The stationary battery is used to store electricity when there is remaining contracted power capacity or when the electricity price is low and discharge the stored electricity in the case of high electricity demand for charging or during peak hours with high electricity price. Basically, the battery having relatively large capacity is adopted to facilitate simultaneous charging of multiple EVs and hence improve service quality. Based on the charging and discharging conditions of the installed stationary battery and the source of electricity used for charging, quick charging modes of the developed BACS can be categorized as follows:
2. Integrated charging system
1. Battery discharging mode
The US DOE (Department of Energy) has set a relatively high charging rate of 10 miles of range per minute of charging during quick charging [12]. It means that for an EV having a battery capacity of 24 kWh (driving range of about 100 miles), the required charging time is 10 min with the 6 C rate [13]. In case of Japan, it is almost equivalent to the electricity consumed simultaneously by 20 households (100 V, 30 A). Therefore, very high charging rate when thousands of EVs are charged simultaneously causes a significant fluctuation in the electricity grid [7]. The embedment of battery in the charging system to assist the charger improves the performance of quick chargers [14] as well as answers the problems related to electricity demand [15]. In this study, BACS has been developed and evaluated. BACS can control the power input received from the grid (according to contracted power capacity, etc.) and power output from each charger to the corresponding EV. In addition, BACS controls the distribution of electricity inside its own system, including the stationary battery and chargers, to achieve its objective function. Hence, BACS can satisfy both supply side (minimizing grid load through load shifting and electricity cost reduction) and demand side (fascinating EV owners by quick charging, although in peak hours). The purposes of BACS are as follows: (1) reduce the contracted power capacity, (2) avoid high electricity demand in peak hours owing to EV charging, (3) shorten charging and waiting times, (4) participate in grid-ancillary services including spinning reserve and
Battery discharges its electricity in assisting the system. Hence, EV charging is performed using electricity from both the grid and the battery. This mode is applied in case of simultaneous quick
Fig. 1. Schematic diagram of a developed BACS.
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charging of multiple EVs, especially when the price of electricity is high. Electricity flow in this mode can be represented as follows:
Pgrid þ Pbatt ¼ Pqc;1 þ Pqc;2 þ Ploss ;
(1)
Table 1 Specifications of EV and QC used during the experiment. Component
Property
Value
EV
Type Battery type Total battery capacity Maximum voltage Nominal voltage Cathode Anode Cell rated capacity Cell average voltage Cell maximum voltage Type Standard Output voltage Output current Rate power output
Nissan Leaf Laminated lithium-ion battery 24 kWh 403.2 V 360 V LiMn2O4 with LiNiO2 Graphite 33.1 Ah (0.3 C) 3.8 V 4.2 V DC quick charger CHAdeMO DC 50e500 V 0e125 A 50 kW
where Pgrid, Pbatt, Pqc, and Ploss are electricity purchased from the grid, electricity charged (negative value) or discharged (positive value) from stationary battery, electricity for EV quick charging, and electricity loss, respectively. 2. Battery charging mode In case there is still margin in the contracted power capacity or the price of electricity from the grid is relatively low (due to surplus electricity, night time, etc.), the battery is charged to store electricity. Electricity flow in this mode can be expressed as Eq. (2).
Pgrid Pbatt ¼ Pqc;1 þ Pqc;2 þ Ploss
(2)
3. Battery idling mode Battery can be in the idling mode under some conditions: (a) contracted power capacity is sufficient to cover the simultaneous quick charging of multiple EVs (low demand in charging), (b) battery is empty owing to high and continuous quick charging for EVs (battery cannot supply the electricity unless being charged). In the case of (b), BACS controls the charging rate for each quick charger to maintain the contracted power capacity. Hence, any penalty or high electricity price can be avoided. Electricity flow in the idling mode is expressed as follows:
Pgrid ¼ Pqc;1 þ Pqc;2 þ Ploss
QC
(3)
BACS always ensures that the value of Pgrid is lower than or the same as the contracted power capacity. In addition, Ploss is the total loss in the system owing to some factors, including AC/DC and DC/ DC conversions and electricity consumed by the system. Hence, the value of Ploss in each mode is different from that in the other modes.
3. Effect of temperature on EV charging behavior EVs largely use lithium-ion batteries as the power source due to high energy density, stable electrochemical properties [16], longer lifetime, and low environmental impact [17]. Temperature is considered to be one of the important conditions influencing both charging and discharging behaviors of lithium-ion batteries. In general, lower temperature leads to poor charging and discharging performance due to electrolyte limitation [18] and changes in electrolyte/electrode interface properties, including viscosity, density [19], electrolyte components, dielectric strength, and ion diffusion capability [20]. In addition, Liao et al. found that as temperature decreases, charge-transfer resistance increases significantly, and it is higher than bulk resistance and solidestate interface resistance [21]. Unfortunately, to the best of the authors' knowledge, there is a lack of studies clarifying the charging behavior in different temperatures or seasons. In this study, to clarify the effect of temperature, especially ambient temperature, on the charging behavior of EVs, charging was initially conducted in different seasonsdwinter and summer. Table 1 summarizes the EV and QC specifications used in the experiment. Table 2 shows the experimental conditions employed in this study. The ambient temperatures were based on information from
Japan Meteorological Agency's database for the corresponding time in Yokohama area, where the experiments were conducted [22]. It is important to note that the EVs were charged in the cold condition after being parked for a relatively long time (such as one night) before charging. Hence, the EV battery temperature could be assumed to be very similar to the ambient temperature. Fig. 2 shows the relationship among charging rate, charging time, and SOC of an EV battery both in winter (a) and summer (b). In general, although the rated capacity of the charger is 50 kW, the charging power absorbed by the EV battery was relatively lower, especially in winter. Charging in summer led to a higher charging rate and hence a shorter charging time. Numerically, to reach an SOC of 80%, the required charging times in winter and summer were 35 and 20 min, respectively. In summer, a higher charging rate (about 40 kW) was achieved up to SOC of about 50%. The charging rate decreased gradually following an increase in SOC, and the charging rate was 16 kW when the SOC was 80%. In contrast, in winter, the charging rate reached about 35 kW instantaneously in very short time and decreased following further increase in SOC. The charging rate was about 10 kW when SOC reached 80%. Fig. 3 shows the correlation among charging current, voltage, and time in winter and summer, which corresponds to the charging rate in Fig. 2. The curves of the charging current in Fig. 3 are nearly similar to those of the charging rate in Fig. 2. In general, lithium-ion batteries are charged with a CC(constant current )eCV (constant voltage) method [23]. Charging at lower temperatures resulted in a gradual decrease in the charging current, especially at higher SOC, leading to longer charging time, and vice versa. In case of summer, a higher CC of about 105 A was achieved in the initial 5e10 min of charging (SOC up to about 50%). Although there was no significant difference in charging voltage, charging in a warmer temperature (summer) resulted in a slightly higher initial charging voltage before settling at a constant value. Hence, the CV condition was achieved faster. Temperature influences the charging behavior of EVs significantly. Charging in relatively high ambient temperature (such as
Table 2 Experimental conditions for evaluation of ambient temperature effect Season
EV No.
Ambient Temp. ( C)
Starting SOC (%)
Ending SOC (%)
Winter
#1 #2 #3 #1 #2 #3
11.4 11.0 10.3 27.5 27.1 32.0
32 25 29 34 31 31
80
Summer
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summer) led to higher charging rates, especially due to higher charging current and faster increase in the charging voltage leading to shorter charging time. According to [21], a decrease in temperature decreases the diffusivity of lithium ions in the cathode and causes electrode polarization. In addition, charge-transfer resistance at the electrolyteeelectrode interface increases [24].
4. Simultaneous charging performance of developed BACS
Fig. 2. Relationship among charging rate, charging time, and SOC of EV battery: (a) winter (b) summer.
To measure the performance of the developed BACS, simultaneous quick charging tests for EVs were conducted. Basically, the control systems were basically based on the above Eqs. (1)e(3). Table 3 lists the specifications of BACS. The DC line, which connects the modules, has a voltage of 450 V. Therefore, DC/DC converters in the battery and charger modules convert the voltage from 450 V to designated voltages of battery charging and discharging. Two quick chargers, each having maximum output power of 50 kW, were installed. Fig. 4 shows an overview of the developed BACS used for the experimental study. This developed system was installed in the JX Nippon Oil & Energy Factory in Shinkoyasu, Yokohama, Japan. Regarding the battery for BACS, as it is stationary, a broad range of battery types can be used including lithium-ion, nickel hydrogen, nickel cadmium, and potassium-ion batteries. In this study, lithium-ion batteries are adopted in consideration of its high energy density, ability to provide high current, low self-discharge, and low maintenance. In addition, the used batteries detached from EVs, which are almost lithium-ion type, also can be used as stationary battery for BACS. According to [25], EV battery is replaced after its capacity drops to 60e70% of the initial capacity. The utilization of these used batteries could increase the total economic performance of EV as well as lengthen their end-of-life and minimize the environmental impact due to battery recycling. The contracted power capacity is the electricity received by the AC/DC inverter (point A in Fig. 1). Hence, in case no battery is installed inside BACS, the electricity that can be transferred to EVs during charging, points C and D, will be less than the received electricity, point A, owing to the power loss during conversion and system consumption. The stationary battery of BACS was placed together with the other controllers and converters under controlled room temperature (20 C), and hence, the battery performance is considered to be stable and same at different ambient temperatures.
Table 3 Specifications of the developed BACS. Component
Property
Value
AC/DC inverter
Receiving voltage Converter output voltage Converter output power Power at DC line side Max. current at battery side Voltage at battery side Type Capacity (kWh) Nominal voltage Max. charging voltage Discharge cut-off voltage Max. current in continuous discharge SOC threshold in charging SOC threshold in discharging Number Standard Output voltage Output current Rated output power
200 V DC 450 V 50 kW 50 kW 150 A 0e400 V Lithium-ion 64.2 kWh 364.8 V 393.6 V 336.0 V 176 A 90% 10% 2 units CHAdeMO DC 50e500 V 0e125 A 50 kW
DC/DC converter
Battery
Quick charger
Fig. 3. Correlation among charging current, voltage, and time in different seasons: (a) winter (b) summer.
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Fig. 4. Developed BACS used for the experimental study: (a) overall system (b) EV quick charger (c) lithium-ion batteries pack.
To evaluate the performance of BACS, simultaneous charging experiments were performed. For this purpose, the experiments were performed in three steps: (1) simultaneous charging of two EVs to clarify the required charging time in different seasons, (2) BACS behavior under different contracted power capacity, and (3) simultaneous charging of eight EVs to measure the capability of BACS during peak times. As a basic policy for simultaneous charging, the first connected EV was prioritized and supplied with a higher charging rate, while the second EV was charged at a maximum charging rate of 5 kW until the charging rate of the first EV started decreasing. Further, after completion of charging of the first EV, the second EV was charged according to its maximum charging rate. BACS
Table 4 Experimental conditions for simultaneous charging of two EVs using a conventional system and BACS (contracted power capacity of 50 kW). Season
Charging system
Ambient Temp. ( C)
EV No.
Starting SOC (%)
Ending SOC (%)
Winter
Conventional
10.7 11.0
28 29 25 27 32 35 33 27
80
BACS
#1 #2 #1 #2 #1 #2 #1 #2
Summer Conventional BACS
28.4 27.8
maintains the contracted power capacity and controls the charging rate for each EV connected to the corresponding QC. 4.1. Simultaneous charging of two EVs Table 4 summarizes the experimental conditions for simultaneous charging of two EVs in different seasons. Fig. 5 shows the results of simultaneous charging of two EVs during winter with conventional QCs and BACS under a contracted power capacity of 50 kW. In a conventional charging system, owing to the limit on contracted power capacity, the first connected EV will be charged at a higher charging rate, while the second EV will be charged initially at 5 kW. As the charging rate of the first connected EV decreases, the charging rate of the second EV increases gradually so that the total electricity reaches the maximum contracted power capacity. As the charging rate of both EVs decreases owing to an increase in SOC, the total electricity received from the grid decreases. The first and second EVs reached 80% SOC after 40 and 50 min of charging, respectively. In contrast, when charging using BACS, the first and second EVs enjoyed almost the same charging rate, and reached 80% SOC in almost the same time (about 35 min) due to battery assistance supplying the electricity. In addition, electricity from the grid could be maintained below the contracted power capacity, although the total charging rate was larger than the contracted power capacity. Fig. 6 shows the results of simultaneous charging of two EVs during summer with a conventional charging system and BACS. Similar tendency to the case in winter can be observed. The first and
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Fig. 5. Simultaneous charging experiment during winter with a contracted power capacity of 50 kW: (a) conventional system (b) developed BACS.
Fig. 6. Simultaneous charging experiment during summer with a contracted power capacity of 50 kW: (a) conventional system (b) developed BACS.
second EVs reached 80% SOC after charging of about 20 and 30 min, respectively. When charging using BACS, both EVs reached 80% SOC in a relatively short time of about 20 min. In general, it was observed that the developed BACS improved the charging quality through a higher charging rate, especially during simultaneous charging of multiple EVs. The battery discharged its electricity until the total charging rate of two EVs was the same or lesser than the contracted power capacity of electricity.
connected EV decreased. The charging time increased as the contracted power capacity was reduced to 15 kW. In this case, the required charging time was about 38 and 25 min in winter and summer, respectively. Furthermore, the discharging power from the stationary battery was also larger in the case of lower contracted power capacity. Change in the SOC of the stationary battery was greater when the contracted power capacity decreased. Numerically, SOC decreased by about 5%, 10%, and 20% during the simultaneous charging of two EVs for contracted power capacities of 50, 30, and 15 kW, respectively. In this system, because the SOC threshold for battery charging and discharging were 90% and 10%, respectively, the total SOC change that could be used during simultaneous charging was 80%. Therefore, the stationary battery may continuously assist 32, 16, and 8 EVs during simultaneous charging with contracted power capacities of 50, 30, and 15 kW, respectively.
4.2. BACS performance with a different contracted power capacity To measure the performance of BACS in term of different contracted power capacities, experiments under different modified amounts of received power were performed. In addition to the experiments explained in sub-section 4.1 (contracted power capacity of 50 kW), two other conditions were evaluated, namely, contracted power capacities of 30 and 15 kW. Table 5 summarizes the experimental conditions corresponding to each contracted power capacity both in winter and summer. Figs. 7 and 8 show the experimental results of simultaneous charging of two EVs using BACS and contracted power capacities of 30 and 15 kW, respectively. For additional comparison, the ones with the contracted power capacity of 50 kW in winter and summer can be observed in Figs. 5(b) and 6(b), respectively. Basically, in the simultaneous charging of two EVs, there is no significant difference in terms of the charging time in each season. As the contracted power capacity decreased, the total charging rate of the two QCs decreased accordingly. In addition, in terms of charging time in the two seasons, there was no significant difference between the contracted power capacities of 50 and 30 kW. It seems that although the second connected EV received lower power initially, the power increased to its maximum value as the charging rate of the first
4.3. Simultaneous charging during high demand time (peak time) To measure the performance of the developed system during peak demand, simultaneous charging of eight EVs was performed. The EVs were initially prepared and parked near the BACS as queuing EVs waiting their turn for charging. The experimental conditions used in this study are listed in Table 6. As one EV completed its charging (SOC of 80%), the next EV (SOC of about 30%) started using the same charger. Fig. 9 shows the experimental results in winter and summer under the contracted power capacity of 30 kW. In general, simultaneous charging of eight EVs in summer can be performed faster than in winter owing to a higher charging rate. Although the charging time required for simultaneous charging in summer was significantly less, the SOC of the stationary battery decreased
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Table 5 Experimental conditions of simultaneous charging of two EVs with BACS under different contracted power capacities. Contracted power capacity (kW)
Season
Ambient Temp. ( C)
EV No.
Starting SOC (%)
Ending SOC (%)
30
Winter
10.7
#1 #2 #1 #2 #1 #2 #1 #2
28 29 25 27 32 35 33 27
80
Summer 11.0 15
Winter
28.4
Summer 27.8
considerably. This is because of the high discharging rate of the stationary battery when assisting the chargers. The stationary battery cannot be charged during this type of simultaneous charging because no marginal electricity is available to charge it. In contrast, in winter, the discharging rate of the stationary battery is significantly lower because of the slower charging rate of EVs. The total charging rate of two chargers can be lower than the contracted power capacity, leading to marginal electricity, which could be used to charge the stationary battery (Fig. 9(a)). Hence, the SOC of the stationary battery did not decrease by as much in winter as it did in summer. Fig. 10 shows the simultaneous charging of eight EVs during summer under a contracted power capacity of 15 kW. Compared with the case of a higher contracted power capacity (30 kW), almost no significant change was found in the charging rate of EVs, except that of the last connected EV. Unfortunately, the discharging rate of the stationary battery was very high, leading to faster decrease in its SOC. As can be observed, the SOC of the battery dropped rapidly and reached 10% during charging of the last two
Fig. 8. Simultaneous charging of two EVs using BACS with contracted power capacity of 15 kW: (a) winter (b) summer.
EVs. As a result, the last connected EV was charged using electricity received from the grid, without any assistance from the stationary battery. However, because the contracted power capacity was very low, the very last connected EV was not charged until the EV before it was charged completely. The stationary battery could not be
Table 6 Experimental conditions for simultaneous charging of eight EVs with BACS under different contracted power capacities. Contracted power capacity (kW)
Season
Ambient Temp. ( C)
EV No.
Starting SOC (%)
Ending SOC (%)
30
Winter
10.7
#1 #2 #3 #4 #5 #6 #7 #8 #1 #2 #3 #4 #5 #6 #7 #8 #1 #2 #3 #4 #5 #6 #7 #8
29 34 33 33 27 30 29 33 31 20 25 29 31 36 27 32 30 30 30 30 30 30 27 29
80
Summer 29.0
15
Fig. 7. Simultaneous charging of two EVs using BACS with contracted power capacity of 30 kW: (a) winter (b) summer.
Summer 27.8
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Fig. 9. Simultaneous charging of eight EVs using BACS under a contracted power capacity of 30 kW: (a) winter (b) summer.
Fig. 10. Simultaneous charging of eight EVs using BACS under a contracted power capacity of 15 kW during summer.
charged during simultaneous charging owing to the lack of marginal electricity and the high charging rate of EVs. Based on the results of the conducted experiments, the application of BACS can significantly improve the charging performance of QCs, especially during the simultaneous charging of multiple EVs. The balance among EV charging rate, contracted power capacity, and stationary battery SOC is seemingly very important and must be considered in depth. In addition, EVs charging demand should be forecast initially. 5. Conclusions A BACS-based QC was developed and its performance in simultaneous charging was evaluated. First, the charging behavior of EVs in different seasonsdwinter and summerdwas clarified in terms of the influence of temperature on the charging rate. It is believed that battery temperature strongly influences the charging
behavior of a battery, and ambient temperature influences the battery temperature. It was clarified that the charging rate during summer is higher than that during winter. In the simultaneous charging experiments, the application of BACS clearly improved the performance of EV chargers. Charging was performed in a shorter time, while maintaining the contracted power capacity. In future, as the demand for EVs charging increases, grid stress due to charging demand and its fluctuations would increase accordingly. The adoption of BACS would minimize this stress and maintain the quality of grid electricity. In addition, as BACS uses a large stationary battery, it can participate in ancillary service programs dedicated for the electricity grid when the demand for quick charging is relatively low. In this case, a bi-directional AC/DC inverter is required to facilitate both services up (from battery to the grid) and down (from grid to battery). This participation can improve the economic performance of BACS and grid reliability.
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Acknowledgments This research was part of the Yokohama Smart City Project funded by New Energy Promotion Council, Japan (No. 3115105). The authors express their sincere thanks to NEC Corp. and JX Nippon Oil & Energy Corp. for their collaboration. Furthermore, we also offer our thanks to Ms. Yoko Watanabe for her assistance in data reduction and analysis.
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