Experimental study of hybrid interface cooling system

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N S Kamarrudin1,g) and Nazih A Bin-Abdun1,h). 1School of Mechatronic Engineering, Universiti Malaysia Perlis, Kampus Tetap Pauh Putra, 02600 Arau, Perlis,.
Experimental study of hybrid interface cooling system using air ventilation and nanofluid M. F. H. Rani, Z. M. Razlan, S. A. Bakar, H. Desa, W. K. Wan, I. Ibrahim, N. S. Kamarrudin, and Nazih A. BinAbdun

Citation: AIP Conference Proceedings 1885, 020073 (2017); doi: 10.1063/1.5002267 View online: http://dx.doi.org/10.1063/1.5002267 View Table of Contents: http://aip.scitation.org/toc/apc/1885/1 Published by the American Institute of Physics

Experimental Study of Hybrid Interface Cooling System Using Air Ventilation and Nanofluid M F H Rani1,b), Z M Razlan1,a), S A Bakar1,c), H Desa1,d), W K Wan1,e), I Ibrahim1,f), N S Kamarrudin1,g) and Nazih A Bin-Abdun1,h) 1

School of Mechatronic Engineering, Universiti Malaysia Perlis, Kampus Tetap Pauh Putra, 02600 Arau, Perlis, Malaysia. Corresponding author: a)[email protected] b) [email protected] c) [email protected] d) [email protected] e) [email protected] f) [email protected] g) [email protected] h) [email protected]

Abstract. The hybrid interface cooling system needs to be established to chill the battery compartment of electric car and maintained its ambient temperature inside the compartment between 25°C to 35°C. The air cooling experiment has been conducted to verify the cooling capacity, compressor displacement volume, dehumidifying value and mass flow rate of refrigerant (R-410A). At the same time, liquid cooling system is analysed theoretically by comparing the performance of two types of nanofluid, i.e., CuO + Water and Al2O3 + Water, based on the heat load generated inside the compartment. In order for the result obtained to be valid and reliable, several assumptions are considered during the experimental and theoretical analysis. Results show that the efficiency of the hybrid interface cooling system is improved as compared to the individual cooling system.

INTRODUCTION Generally, all automobiles move due to the usage of fossil fuels for engine combustion. Malaysia is one of the top 50 countries that have the highest dependency on fossil fuels for energy. This dependence leads to an increase in the amount of carbon dioxide to the atmosphere [1]. As the amount of carbon dioxide accumulates, global warming problem arises around the globe. The introduction of electric vehicles is one of the relevant solutions in reducing this problem. However, engineers in research and development of electric car are still struggling to maximize the coefficient of the performance (COP) of the battery. This challenge has delayed the electric vehicles to be deployed commercially. Universiti Malaysia Perlis (UniMAP) has been requested by PROTON Holdings Berhad to conduct a research on maximizing the performance of lithium-ion polymer battery for electric cars. UniMAP Motorsports Technology Research Unit (MoTECH) has converted a Proton BLM Model to an electric car where the battery compartment is patched in the car boot to power an electric motor. The battery compartment of electric vehicle is not only to be cooled down, but also the ambient temperature inside the compartment must be controlled between 25°C to 35°C. A well-designed cooling system should be applied to the battery compartment to ensure that the car can be performed not only at optimum operation but also at the best safety level. Sufficient cooling capacity will reduce the heat load produced by the battery itself. Different cooling methods can be combined

3rd Electronic and Green Materials International Conference 2017 (EGM 2017) AIP Conf. Proc. 1885, 020073-1–020073-9; doi: 10.1063/1.5002267 Published by AIP Publishing. 978-0-7354-1565-2/$30.00

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together as a hybrid interface cooling system to improve the heat dissipation rate rather than single exterior thermal management system [2]. In this project, air and liquid cooling are combined together to verify the performance of the battery after several life cycles. The primary of cooling system will focus on the air vents that using refrigerant R-410A, while the secondary cooling system will act like a radiator which utilising nanofluid, i.e., CuO+Water, Al2O3+Water. Hence, this project will mainly focus on analysing the experimental data for the air cooling interface. Besides, a study on the airflow characteristics of the primary cooling system and cooling coil design for the secondary cooling system are also conducted in this project.

THERMAL MANAGEMENT STRATEGIES Due to the thermal effects such as over-charging, over-discharging and short circuit [3], [4], thermal management strategies have been established to avoid thermal runaway conditions in lithium-ion batteries. Thermal runaway may cause an explosion or fire not only in the battery system but also for the whole electric vehicle (EV) [5]. The thermal and safety performances of lithium-ion batteries can be supported in two ways, either by interior or exterior thermal management systems. For the interior, the heat generation is targeted to be reduced, but for the exterior, the heat dissipation from cells is enhanced. In this study, the exterior thermal management system is chosen as the battery pack operates under abusive charging and discharging conditions. This is due to electric cars are using a large amount of energy that might cause overheating of the batteries. So, exterior thermal management system (TMS) is preferable to secure the performance of the batteries [2]. This type of TMS will keep the battery pack at an optimum and uniform temperature during the charging and discharging processes at different climate conditions. There are two traditional TMS operations that have been extensively used in the electric vehicles (EVs) in recent years to overcome the thermal effects, which are air cooling and liquid cooling [5]. Air cooling can be classified into natural convection and forced convection. In most studies, the forced convection is chosen because of its high convective heat transfer coefficient. On the other hand, forced convection provides better cooling efficiency. These cooling strategies are widely used in electronics and vehicles. The development air based TMS is mainly concentrated on the optimization of the airflow rate, battery cell layout and flow path [2]. It was discovered that higher Reynold (Re) number will reduce the maximum temperature, however there will be uneven temperature distribution. Besides, the cooling performance of TMS also depends on flow rate and cooling channel size. In case of electric vehicles (EVs) and hybrid electric vehicles (HEVs), both natural and forced air convection are found to be lack in controlling the battery temperature within optimal range. Liquid based thermal management is a better approach to handle battery packs that discharge at high rates [3]. The heat transfer through convection of liquid is much more effective compared to air in diminishing the overheating problem of battery packs as liquid works in a conducted manner and has a higher specific heat capacity.

HYBRID INTERFACE COOLING SYSTEM APPROACH Figure 1 illustrates the schematic diagram for the hybrid interface cooling system in order to control the battery compartment, desired temperature between 25°C to 35°C. As the previous study [6], the hybrid interface cooling system is using one compressor to provide suffient pressure and mass flow rate of refrigerant to two evaporators. Air ventilation is used as the primary cooling system while nanofluid is used as the secondary cooling system. The cooling system starts when the compressor compresses the refrigerant, i.e., R-410a to be high pressurized working fluid. The refrigerant R-410a will pass through the condenser to release heat, reach the evaporator 1 and absorb the heat generated by the battery pack in the battery compartment via air ventilating system. While the nanofluid, i.e., secondary working fluid, from tank that has evaporator 2 in it will then flow and reach the battery compartment through the cooling coil to absorb heat. Heat exchange will occur between the low temperature of nanofluid and the high temperature of heat load (battery pack) in the battery compartment. In short, the hybrid interface cooling system happens when there is heat dissipation from the air cooling and liquid cooling simultaneously in the battery compartment.

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FIGURE 1. Schematic Diagram of Hybrid Interface Cooling System

FIGURE 2. Design of Air Cooling System

DESIGN OF AIR VENTILATION FLOW Before implementing the forced air cooling system into the battery compartment, the air ventilation flow must be designed based on the cycle of refrigeration system. The adiabatic efficiency of air ventilation system must be in an acceptable condition, i.e., all walls of the duct was sealed and the thickness was maximized, to obtain reliable results of the forced air cooling experiment. In our study, the heat transfer between battery compartment and outside surrounding throughout experiment has being kept as minimum as it can be. Based on Fig. 2, several dimensions of air ventilation flow must be constrained to match with the dimensions of the existed battery compartment. The air ventilation flow at path (2) must provide sufficient cut-face area for the indoor unit of air conditioner or evaporator (1) to discharge desired low temperature of air into the battery compartment (3). The air ventilation flow at path (5) is also designed with the same criteria to match the cut-face area for hot air suction into the evaporator (1). Generally, the airflow starts from the indoor unit of air conditioner (1), passing through a path (2) to enter the battery compartment (3). The forced air cooling is undergoing at this stage. Then the hot air is pushed out via route (4) and (5). Finally, heat exchange occurs in the evaporator. The cycle continues until desired temperature is obtained.

ALTERNATIVE HEAT SOURCE OF THE BATTERY AND DATA COLLECTION Different heat source of the battery is used in this research to verify the reliability of the forced air cooling in the battery compartment. Since the battery is the most expensive component of electric vehicles (EVs), an alternative heat source of the battery is significant to avoid the cost competitiveness in the battery replacement. The battery replacement during experiment might give inaccurate results of air cooling since each battery has its own thermal behaviour. However, due to the safety factors during the experiment, to prevent the thermal runaway conditions in lithium-ion batteries is necessary. If the battery fails to last its life span, triggering events of failure might occur during the experiment [5]. Thus, it is safer to use an alternative heat source of the battery at this time before obtaining the reliability of the forced air cooling later. The alternative heat source can be supplied either by a heat pump or by a heater. In this experiment, a circulating temperature bath will supply a heat source of distilled water into the compartment through copper coils. The hot water passing through the copper coils will dissipate heat into the battery compartment. The normal range operation of a circulating bath for distilled water is between 10°C – 90°C [7], the same heat load as the lithium-ion battery was supplied into the battery compartment by the heater. Based on the previous studies [8], the lithium-ion battery operating temperature should not be exceeded at most 60°C. Therefore, a 60°C heat load of water from the circulating bath was supplied into the battery compartment. The reliability of the air cooling will be verified experimentally in this case whether the heat load can be removed out from the battery compartment or not by taking several data collection such as inlet temperature of hot water, outlet temperature of water and air temperature throughout the air ventilation flow.

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The dry bulb and wet bulb temperature of air in the air ventilation flow are vital in order to obtain the properties of discharged and sucked air such as their enthalpy, specific volume as well as humidity ratio. These properties will be used to obtain the values in Mollier diagram of air cooling and the next results analysed from the Mollier diagram will be compared based on type of refrigerant as summarized in Table 5. A temperature data logger is used to record the temperature changes in the air ventilation flow and in the battery compartment as well as on the copper coil surface. Based on Fig. 3, a set of thermocouples is used to sense the temperature at multiple places in experimental rigs (air ventilation flow, battery compartment and copper coils). To obtain accurate and reliable results, the data logger and thermocouples must be calibrated first before starting the experiment [9], [10]. Hot test and cold test must be conducted for at least half an hour before an experiment of air cooling is carried out. The calibration tests will determine whether the data logger and thermocouples able to read the temperature changes or not in a set of period.

FIGURE 3. Location of Thermocouple as Temperature Sensor in Air Cooling System

RESULTS AND DISCUSSION The summary of analysis result of the air cooling experiment is compared with previous analytical study and will be shown in the next paragraph. The liquid cooling is analysed theoretically by determining the nanofluid’s performance between CuO + Water and Al2O3 + Water based on the heat load generated by circulating bath with water in the battery compartment. For air ventilation interface cooling system, eight channels of thermocouple were set in the air cooling system to monitor temperature changes inside the battery compartment. Figure 4 shows the experimental result of forced air cooling for each channel. Based on Table 1, the average temperature of air cooling for each thermocouple is obtained. Channel 1 (CH001) is the first thermocouple to sense and read the temperature at the inlet of heat source. Although the circulating bath is set to supply a 60°C heat load of water into the compartment, only 47.6°C heat load is sensed at the inlet of the copper coil. It is found that there is a heat loss occurs along the tube between the heater in the circulating bath and the inlet of the copper coil. Channel 2 (CH002) is the second thermocouple that senses and reads the temperature at the outlet of the copper coil. After the air cooling process, almost 18°C is reduced from the hot water and CH002 only senses 29.6°C of water at the outlet of the copper coil. Channel 3 (CH003) and channel 4 (CH004) reads the dry bulb and wet bulb at inlet temperature of air from the evaporator into the battery compartment respectively. While channel 5 (CH005) and channel 6 (CH006) reads the dry bulb and wet bulb at outlet temperature of air from air ventilation flow into the evaporator respectively. The temperature on the coil surface (CH007) is found to maintain at 34.0°C after about 40 minutes (2400 seconds) of an air cooling process. This finding shows that the forced air cooling that supplied around 15.1°C of air inside the compartment (CH008) is reliable to maintain the temperature inside the battery compartment between 25°C to 35°C. Further analysis of the primary cooling system is conducted once the properties of air and evaporator dimensions have been acquired.

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Graph of Temperature vs Time 60

Temperature (°C)

50 CH001

40

CH002 CH003

30

CH004 CH005

20

CH006 CH007

10

CH008

0 0

500

1000

1500

2000

2500

3000

Time (s) FIGURE 4. Experimental Result of Air Cooling for Each Channel TABLE 1. Average temperature of air cooling for each thermocouple CH003 CH004 CH005 CH006 CH007

CH001

CH002

47.6 °C Hot water (Inlet)

29.6 °C Hot water (Outlet)

16.4 °C ୢୠ (Inlet)

12.3 °C ୵ୠ (Inlet)

17.6 °C ୢୠ (Outlet)

15.1 °C ୵ୠ (Outlet)

34.0 °C On coil surface

CH008

15.1 °C Inside compartment

Based on Table 2, the properties of discharged and sucked air are obtained by using ASHRAE Psychometric Chart No.1. Meanwhile, the airflow rate is obtained by measuring the airflow speed and the surface area of the evaporator. The result of the calculation is summarized in the Table 3. Next, the parameters acquired from these two tables will be utilized to obtain the values based on a Mollier diagram as summarized in Table 4. Figure 5 illustrates the Mollier diagram of air cooling at each stage and at different enthalpy and pressure for refrigerant R-410a. Stage 1-4 indicates the stages of refrigeration cycle system. At point 1-2, the compressor compresses refrigerant at gas state from low pressure line to high pressure line at almost isentropic condition. Stage 2-3 indicates the stage at the condenser, where it changes the state of the refrigerant from gas to liquid state. Heat from the refrigerant R-410a is released through the condenser. Besides, it can be observed that the pressure of refrigerant R-410a, 3.0 MPa is maintained throughout the condenser stage, although its state has been changed from gas to liquid. Stage 3-4 indicates the stage at the expansion valve where the pressure was reduced and the refrigerant becomes easy to evaporate. The last stage 4-1, indicates the stage at the evaporator. It changes the refrigerant’s state from liquid to gas. Heat from the battery compartment is absorbed through the evaporator into the refrigerant R-410a. A fan circulates the cooled air throughout the battery compartment at desired temperature. The refrigerant R-410a goes constantly through the four stages to move out the heat until optimum temperature in the battery compartment is obtained.

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TABLE 2. Properties of discharged and sucked air Discharged air

Dry bulb and wet bulb temperature

ୢୠ = 16.4 °C ୵ୠ = 12.3 °C

Dry bulb and wet bulb temperature

ୢୠ = 17.6 °C ୵ୠ = 15.1 °C

Enthalpy, Šୢ୧ୱୡ୦ୟ୰୥ୣୢୟ୧୰ ൌ ͵ͶǤͺ Ȁ‰ Specific volume, ˜ୢ୧ୱୡ୦ୟ୰୥ୣୢୟ୧୰ ൌ ͲǤͺʹͻଷ Ȁ‰ Humidity ratio, ɘୢ୧ୱୡ୦ୟ୰୥ୣୢୟ୧୰ ൌ ͲǤͲͲ͹Ͳ‰Ȁ‰Ԣ Sucked air

Enthalpy, Šୢ୧ୱୡ୦ୟ୰୥ୣୢୟ୧୰ ൌ ͵ͶǤͺ Ȁ‰ Specific volume, ˜ୢ୧ୱୡ୦ୟ୰୥ୣୢୟ୧୰ ൌ ͲǤͺʹͻଷ Ȁ‰ Humidity ratio, ɘୢ୧ୱୡ୦ୟ୰୥ୣୢୟ୧୰ ൌ ͲǤͲͲ͹Ͳ‰Ȁ‰Ԣ

TABLE 3. Evaporator dimension and parameter Parameter/Dimension Value

Surface Area Airflow speed during experiment Airflow rate, ሶ

ͲǤ͸ʹ͵ ൈ ͲǤͲͺ ൌ ͲǤͲͶͻͺͶଶ ͵ǤͷͷͻȀ• 0.04984 m2 ×3.559 mΤs =0.177 m3 /s

TABLE 4. Values based on Mollier diagram

FIGURE 5. Mollier Diagram of R-410a

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The experimental result have been compared with theoretical result of air cooling to verify the effectiveness of refrigerant R-410a. Study have been conducted to analyse the refrigeration system of refrigerant R-410a for small battery compartment of electric vehicle [11]. Table 5 shows the comparison of forced air cooling based on theoretical analysis and experimental analysis by refrigerant R-410a. Theoretically, about 1700 W of cooling capacity is required to remove the heat load generated in the battery compartment. Meanwhile, the experimental air cooling by refrigerant R-410a reaches only 1578 W of cooling capacity. However, this cooling capacity is acceptable as more than 50% of the generated heat load can be dissipated. TABLE 4. Theoretical and experimental result of air cooling Properties/Characteristics

Theoretical air cooling by R-410A [11]

Experimental air cooling by R-410A

Heat load, ሶ Cooling capacity, ሶ Compressor displacement,  Mass flow rate of refrigerant, ሶ Secondary cooling system is focusing on liquid cooling which utilizing nanofluid for heat dissipation in the battery compartment. Nanofluid - a base fluid with nano-sized particles ranging from 1 to 100 nm is used as the working fluid in liquid cooling for heat dissipation in the secondary cooling system. When the nanoparticles, which are usually in the form of metal or metal oxide combine with a suitable carrier fluid such as distilled water, the conduction and convection coefficients will increase thus allowing more heat to be removed from the coolant [12]. Two types of nanofluid, Copper (II) Oxide and Aluminium Oxide have been used in this case of study to verify the reliability of liquid cooling in the battery compartment. Apart from Zinc Oxide, these two materials are common metal oxide that has the highest thermal conductivity [12] that are suitable to be used as a nanofluid in the secondary cooling system. Before analysing the performance of nanofluid in the liquid cooling, several properties must be determined first such as nanofluid’s density, thermal conductivity and its thermal diffusivity at desired cooling temperature [13]. Since the analysis of the liquid cooling is still at an early stage, the desired cooling temperature by nanofluid is set to be 10°C which is lower than the temperature supplied by the forced air cooling into the battery compartment as stated in Table 1. This is one of the methods in determining the reliability of liquid cooling whether it is able to dissipate the heat load generated in the battery compartment or not. After obtaining all the required properties of nanofluid, the nanofluid’s heat transfer rate can be analysed starting from the mass flow rate of nanofluid until its actual heat transfer rate. The actual heat transfer rate is approximately will reduce by 20 % from the ideal heat transfer rate due to several considerations such as an environmental factor and material resistance. Based on Table 6, it can be observed that nanofluid of 50 grams Copper (II) Oxide possess better heat transfer rate rather than nanofluid of 50 grams Aluminium Oxide. Overall, nanofluid of Copper (II) Oxide is more suitable to be adopted as the working fluid in the secondary cooling system.

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TABLE 5. Comparison of liquid cooling result based on type of nanofluid Properties/Characteristics

CuO + Water

Density, ɏ୬୤ Thermal conductivity,  ୬୤ Specific heat, …୮Ǥ୬୤ Dynamic viscosity, ρ୬୤ Thermal diffusivity, Ƚ୬୤ Prandtl number, ”୬୤ Peclet number, ‡୬୤ Mass flow rate, ሶ୬୤ Reynold number, ‡୬୤ Nusselt number, —୬୤ Heat transfer coefficient, Š୬୤ Estimated exit temperature, ୣ Log mean temperature difference, ο୪୫ Ideal heat transfer rate, ሶ ୬୤ Actual heat transfer rate, ሶ ୟୡ୲Ǥ୬୤

Al2O3 + Water ଷ

ͳͲͳͳǤͻ͵‰Ȁ ͲǤͷͺͲͲʹʹͶʹ͵ ԏ Ǥ  ͶͳͶͳǤ͸ͺ ԍ‰ Ǥ  ͳǤ͵ͳͷ ൈ ͳͲିଷ ‰Τ Ǥ • ͳǤ͵ͺͶ ൈ ͳͲି଻ ଶ Ȁ• ͻǤ͵ͻ ͳ͵ͳ͵ͺ ͲǤͲͳ͵‰Ȁ• ͳ͵ͻͻ ʹʹǤ͸ͷ ͵ͲʹͲǤͳʹ ԏଶ Ǥ  ͵͵ǤͻͻͻͻͶͲͳʹι ͳǤͺ͸Ͳι ͳʹͻʹǤͲͲ͹ ͳͲ͵͵Ǥ͸Ͳ͸

ͳͲͳͲǤ͵ͻ‰Ȁଷ ͲǤͷͺͲͲʹͶͷ͵͸ ԏ Ǥ  ͶͳͶͷǤͷͳ ԍ‰ Ǥ  ͳǤ͵ͳͻ ൈ ͳͲିଷ ‰Τ Ǥ • ͳǤ͵ͺͷ ൈ ͳͲି଻ ଶ Ȁ• ͻǤͶ͵ ͳ͵ͳʹͻ ͲǤͲͳ͵‰Ȁ• ͳ͵ͻ͵ ʹ͹ǤͲͲ ͵͸ͲͲǤͳͷ ԏଶ Ǥ  ͵͵ǤͻͻͻͻͻͻͶͻι ͳǤ͵ͷͺι ͳͳʹͶǤͶ͹ͳ ͺͻͻǤͷ͹͹

CONCLUSION This research is mainly focused on experimental studies of hybrid interface cooling systems by using one compressor to provide sufficient pressure to two evaporators. The air cooling requires about 1580 W of cooling capacity while the liquid cooling requires about 1100 W of cooling capacity. The establishment of a hybrid interface cooling system has been possible since the combined ratio of cooling capacity of air cooling and liquid cooling are in the range of satisfaction ratio (1100 W + 1580 W < 3500 W). In case of the secondary cooling system, theoretical analysis of liquid cooling was carried out to verify the performance of both nanofluid (CuO + Water, Al2O3 + Water) as the next step of this experimental study. Nanofluid of 50 grams Copper (II) Oxide achieves better heat transfer rate of 12.97% than nanofluid of Alumina. Results show that the efficiency of hybrid interface cooling system is able to transfer out 82.83 % of heat load generated by the circulating bath into the battery compartment. Thus, it is concluded that the combination ratio of air and liquid cooling is satisfied based on the experimental and theoretical data. ACKNOWLEDGMENTS The authors acknowledge School of Mechatronic Engineering, Universiti Malaysia Perlis for the lab facilities. Special thanks to those who contributed to this project directly or indirectly. REFERENCES 1. 2. 3. 4. 5. 6. 7.

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T. Yuksel, S. Litster, V. Viswanathan, and J. J. Michalek, “Plug-in hybrid electric vehicle LiFePO4 battery life implications of thermal management, driving conditions, and regional climate,” J. Power Sources, vol. 338, pp. 49–64, 2017. Information on https://blog.lesman.com/2012/05/30/wiring-industrial-thermocouples-basic-tips.html H. M. T. Meter, “Handheld Multi-channel Temperature Meter User ’ s Guide” (Applent Instruments Inc., 2012). A. R. Tajudin, “Refrigeration system analysis (R-410a) for small battery compartment of electric vehicle” (Universiti Malaysia Perlis, 2014). R. Saidur, S. N. Kazi, M. S. Hossain, M. M. Rahman, and H. A. Mohammed, “A review on the performance of nanoparticles suspended with refrigerants and lubricating oils in refrigeration systems,” Renew. Sustain. Energy Rev., vol. 15, no. 1, pp. 310–323, 2011. N. A. Bin-abdun et al., “The Performance of a Heat Exchanger Designed for Cooling Electric Vehicle Car Battery System by Use Base Fluid and Nano-Fluid,” J. Power Sources, pp. 4–8, 2014.

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