Cooling Performance Characteristics of the Stack Thermal Management System for Fuel Cell Electric Vehicles under Actual Driving Conditions

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1 energies Article Cooling Performance Characteristics Stack Thermal Management System for Fuel Cell Electric Vehicles under Actual Driving Conditions Ho-Seong Lee 1, Choong-Won Cho 1, Jae-Hyeong Seo 2 and Moo-Yeon Lee 3, * 1 Thermal Management Research Center, Korea Automotive Technology Institute, 74 Younjung-Ri, Pungse-Myun, Chonan 31214, Korea; hslee@katech.re.kr (H.-S.L.); cwcho@katech.re.kr (C.-W.C.) 2 Research and Development Division, Nano Thermal Fusion Technology Company (NTF TECH), Hadan 8, Saha-gu, Busan 49315, Korea; cheonchw@naver.com 3 Department Mechanical Engineering, Dong-A University, Busan 49315, Korea * Correspondence: mylee@dau.ac.kr; Tel.: Academic Editor: Vladimir Gurau Received: 12 February 16; Accepted: 18 April 16; Published: 25 April 16 Abstract: The cooling performance radiator a fuel cell electric vehicle was evaluated under various actual road driving conditions, such as highway and uphill travel. The rmal stability was n optimized, reby ensuring stable operation rmal management. The coolant inlet temperature radiator in highway mode was lower than that associated with uphill mode because corresponding frontal air velocity was higher than obtained in uphill mode. In both highway and uphill modes, coolant temperatures radiator, operated under actual road driving conditions, were lower than allowable limit (8 C); this is maximum temperature at which stable operation rmal management fuel cell electric vehicle could be maintained. Furrmore, under actual road driving conditions in uphill mode, initial temperature difference (ITD) between coolant temperature and air temperature was higher than that associated with highway mode; this higher ITD occurred even though rmal load in uphill mode was greater than that corresponding to highway mode. Since coolant inlet temperature is expected to exceed allowable limit (8 C) in uphill mode under higher ambient temperature with air conditioning operation, FEM design layout should be modified to improve heat capacity. In addition, overall volume cooling radiator is 52.2% higher than that present model and coolant inlet temperature improved radiator is 22.7% lower than that present model. Keywords: fuel cell electric vehicle; initial temperature difference; radiator; ; rmal management 1. Introduction Owing to limited resources and carbon dioxide (CO 2 ) emissions during combustion, reduction fossil fuel consumption is essential for automotive industry. Concerns about environmental impact this consumption and consequent global warming have increased significantly in recent years. As such, extensive global research has focused on realizing zero-emission vehicles, which do not use internal combustion engines, as an alternative to conventional vehicles [1]. The most prominent zero-emission vehicles are battery electric vehicles, range-extended hybrid electric vehicles, and fuel-cell electric vehicles (FCEVs). Their batteries operate on a battery power source only, with a battery only or a hybrid battery that is charged by an internal combustion engine, and electricity generated by a fuel cells, respectively. All three types vehicles are considered especially Energies 16, 9, 3; doi:.339/en953

2 Energies 16, 9, promising, as evidenced by development undertaken by major automotive companies [2 4]. However, lab and on-road tests have revealed various engineering shortcomings FCEVs that must be overcome in order to achieve similar performance to that internal combustion engine vehicles. These shortcomings include efficiency, performance, rmal-management technology, as well as compatibility among or components under high-voltage conditions [5]. Several papers have addressed issues associated with development and commercialization FCEVs. The effective discharge heat generated from constitutes one main technical challenges for FCEV. In fact, from rmal management viewpoints, since fuel cell electric vehicles have favorable working temperature ranges from C to 8 C, liquid cooling is considered to be optimum to achieve allowable coolant temperatures under 8 C [6 8]. Islam et al. introduced fuel cell cooling s by size fuel cells and reviewed potential applying nanluids in cooling s for fuel cells used in FCEVs. As a result, y reported that coolant using nanluid has a high heat rejection effect, refore size radiator for fuel cells can be reduced relatively [6]. Zhang and Kandlikar reviewed various cooling techniques cooling with heat spreaders, cooling with separate air flow, cooling with liquid, and cooling with phase change for fuel cell s on basis technical research publications and patents [7]. Kandlikar and Lu reviewed fundamental heat transfer mechanisms at component level a fuel cell. The current status PEMFC cooling technology was also reviewed and research needs were identified. They explained importance intimate relation between fuel cell rmal and water transport mechanisms which has an effect on fuel cell performance [8]. Furrmore, Shabani et al. reported that ratio cooling load and power generation increased with power generation and operating temperature increment [9]. In particular, corresponding initial temperature difference (ITD) between coolant and cooling air radiator is almost 5% lower than that an ICE due to different coolant operating conditions. Therefore, cooling capacity radiator, which is approximately proportional to ITD, must be twice as high as that a conventional radiator []. A few articles have focused on rmal management FCEVS. For example, Kim et al. investigated heating-performance enhancement a CO 2 heat-pump that recovered exhaust rmal energy in FCEVs. They reported on heat-pump that heats passenger room, by using waste heat from [11]. Lee et al. evaluated cooling performance an electric air-conditioning (A/C), for use in an FCEV [12]; this A/C used R744 (Carbon dioxide) as coolant. They suggested a modified electrical A/C (using R744) for FCEV and analyzed cooling performance this modified under various operating conditions. In addition, y reported that cooling COP A/C was, on average, 24.3% higher than that that uses R-134a. Kim et al. evaluated performance a cooling in FCEVs that use a CO 2 A/C [13]. In addition, studies conducted on s FCEVs have revealed that various approaches are required to realize effective cooling management. These approaches outnumber those required for an ICE radiator; for example, more frontal area, an optimum layout, and assembling must be considered for FCEVs. Hager and Schickmair found that, for optimum frontal area, radiator must be 7% larger than that an ICE radiator [14]. The components and layout cooling for FCEVs were described by Ngy-Srun [15]. They concluded that extended heat transfer region and low pressure drop, via radiator grille, should be considered in order to obtain an optimum design that improves cooling capacity. However, cooling s rmal management FCEVs have barely been studied. Therefore, to address aforementioned issues, cooling performance rmal management an FCEV is evaluated in this work. Kim et al. previously evaluated performance a supplementary cooling for FCEVs that use a CO 2 A/C unit [16]. As such, objective this work is to evaluate, via experiments, cooling performance rmal management FCEVs under various actual driving conditions and investigate how to improve cooling performance within layout limitations vehicle. The rmal stability required for stable operation FCEV and suitability experimentally determined -cooling capacity

3 Energies 16, 9, under actual driving conditions, were analytically evaluated via one-dimensional modeling Energies 9, from which was 16, built as an installed cooling module in tested vehicle and used data obtained operation FCEV under various actual road driving conditions. The simulations were performed were analytically evaluated via one dimensional modeling which was built as an installed cooling undermodule difficult conditions due to environmental factors with respect and actual way to in tested vehicle and used data obtained from operation to cooling FCEV under various improve cooling performance with minimum alterations was analyzed. road driving conditions. The simulations were performed under difficult conditions due to environmental factors with respect to cooling and way to improve cooling performance 2. Experimental Method with minimum alterations was analyzed Experimental Setup 2. Experimental Method Cooling performance characteristics rmal management tested fuel cell 2.1. Experimental Setup electric vehicle were investigated to understand variations cooling load under various driving Cooling characteristics management tested fuel conditions. Also,performance accumulated results obtained byrmal experiments were used toverify an analytical cell electric vehicle were investigated to understand variations cooling load under various model for simulating cooling performances considered under virtual driving driving conditions. Also, accumulated results obtained by experiments were used to verify an conditions. An experimental apparatus was installed in order to evaluate cooling performance analytical model for simulating cooling performances considered under virtual rmal management FCEV. The frontal air properties, such as temperature driving conditions. An experimental apparatus was installed in order to evaluate cooling and velocity, on radiator measured during actual road driving vehicle.such Figure 1 performance rmalwas management FCEV. The frontal air properties, shows schematicand apparatus that is used was to estimate cooling capacity as a temperature velocity, on radiator measured during actual road driving rmal management underaactual roaddriving conditions. indicates measurement locations vehicle. Figure 1 shows schematic apparatus that isitused to estimate cooling capacity for temperature, pressure, and volume flow rateroad coolant-side andit temperature air-side, rmal management under actual driving conditions. indicates measurement locations for pressure, volume flow coolant side The temperature and positions temperature, anemometers (12and probes) used torate calculate air flowand rate. radiator inlet air side, and were positions anemometers (12 probes) used C, to calculate air flow rate. To and outlet temperatures measured, with an accuracy.1 by using rmocouples. The radiator inlet and outlet temperatures were measured, with anpressure accuracysensors ±.1 with C, byanusing calculate pressure difference coolant-side radiator, accuracy rmocouples. To calculate pressure difference coolant side radiator, pressure.1% were installed at inlet and outlet coolant-side radiator as shown in Figure 1a. sensors with an accuracy ±.1% were installed at inlet and outlet coolant side radiator The mass flow rate coolant was measured by installing a coolant volume flow meter at front as shown in Figure 1a. The mass flow rate coolant was measured by installing a coolant volume flow radiator. flowradiator. rate The turbine-type flowmeter was measured to an was accuracy meter at The frontvolume volume flow rate turbine type flow meter.25%. measured to an accuracy ±.25%. (a) (b) Figure 1. Cont. Figure 1. Cont.

4 Energies 16, 9, Energies 16, 9, Air Velocity (m/s) Sensing position on radiator Low - bumper hole Middle - bumper hole High - radiator grille Average air velocity Vehicle driving velocity (km/h) (c) Air conditioning on Air conditioning f Frontal air velocity (m/s) Vehicle driving velocity (km/h) (d) Figure 1. Schematic test set up and frontal air velocity radiator. (a) Test set up; Figure 1. Schematic test set-up and frontal air velocity radiator. (a) Test set-up; (b) (b) Photograph Photograph radiator; radiator; (c) (c) Air Air velocity velocity sensors sensors radiator radiator with with measuring measuring locations; locations; (d) (d) Averaged Averaged frontal frontal air air velocity velocity radiator. radiator. Furrmore, a portable data logger (Gartner Instruments GmbH, Schruns, Austria) was used to Furrmore, a portable data logger (Gartner Instruments GmbH, Schruns, Austria) was used to store data. The frontal air velocity radiator, was measured by installing an store data. The frontal air velocity radiator, was measured by installing an anemometer anemometer on cooling module that consists radiator and condenser. Figure 1b shows on cooling module that consists radiator and condenser. Figure 1b shows a photograph a photograph measurement locations on radiator anemometer, which was measurement locations on radiator anemometer, which was mounted to mounted to rear face core, between radiator and fan shroud. Figure 1c shows air rear-face core, between radiator and fan shroud. Figure 1c shows air velocity velocity radiator with measuring locations as a function vehicle driving velocity. radiator with measuring locations as a function vehicle driving velocity. This result indicates This result indicates that layout radiator, including radiator grille and bumper hole that layout radiator, including radiator grille and bumper hole radiator, has radiator, has a significant effect on air inlet radiator. Figure 1d shows frontal air a significant effect on air inlet radiator. Figure 1d shows frontal air velocity velocity radiator as a function vehicle driving velocity, with and without operation radiator as a function vehicle driving velocity, with and without operation A/C A/C. In fact, layout plays an important role in realizing optimum heat transfer. In fact, layout plays an important role in realizing optimum heat transfer rate rate radiator. This heat transfer rate is approximately two times higher than that conventional internal combustion engine, owing to lower temperature difference between coolant inlet and air inlet radiator [17]. A frontal air velocity 2 3 m/s is refore

5 Energies 16, 9, radiator. This heat transfer rate is approximately two times higher than that conventional internal combustion engine, owing to lower temperature difference between coolant inlet and air inlet radiator [17]. A frontal air velocity 2 3 m/s is refore estimated for radiator, with fan in idle condition. Frontal air velocities 5 m/s and 6 m/s without and with operation fan, respectively, are estimated in case high vehicle driving velocity. The cooling performance rmal management was evaluated by determining volume flow rate coolant side, inlet and outlet temperatures, and frontal flow rates air-side radiator. Table 1 shows specifications cooling FCEV. The FCEV investigated in this work had a capacity 8 kw. The radiator has a 69 mm (wide) ˆ 435 mm (height) ˆ 26 mm (depth) multi-flow-type core, and a heat-rejection rate 36 kw at a coolant flow rate L/min. Furrmore, motor power and diameter fan radiator are 1 W and 3 mm, respectively. Table 1. Component specifications cooling. Components Stack capacity fuel-cell electric vehicle Radiator Radiator Fan Condenser Pump Specifications Capacity (kw) 8. Capacity (kw) Type Core size (mm) 36. at L/min Multi-flow type W 69 ˆ H 435 ˆ D 26 Motor power (W) 1 Fan diameter (mm) 3 Capacity (kw) Type Core size (mm) Type Flow rate (L/min) 5.2 at 2 m/s Multi-flow type W 59 ˆ H 3 ˆ D 16 Electric-driven pump at 5 rpm, 2.6 kpa The heat-rejection rate condenser, which has a 59 mm (width) ˆ 3 mm (height) ˆ 16 mm (depth) multi-flow-type core, is 5.2 kw at an air velocity 2 m/s. An electric-driven pump was used to circulate coolant cooling at 5 rpm and 2.6 kpa, with a volume flow rate L/min. Table 2 shows test parameters used in this study. During experiments, driving velocity vehicle was set to 1 km/h, in order to evaluate highway driving condition at higher -cooling loads; uphill driving condition at both high -cooling load and low cooling air-flow rate (resulting from low velocity), is evaluated at velocities 3 5 km/h. The heat-transfer rate at coolant side rmal management was determined from measured data, by using temperature difference method [18]. This rate is given as follows:. Q coolant ṁ coolantc p,coolant T coolant (1) Parameters Table 2. Test conditions. Highway Mode Conditions Uphill Mode Vehicle speed (km/h) Ambient temperature ( C) Inclination angle road ( ) 8

6 Energies 16, 9, Analytical Method The cooling performance rmal management operated under actual driving conditions was investigated, by using analytical method embedded within commercial automotive rmal management stware KULI; core components tested cooling were aligned according to cooling air-flow direction [19,]. Figure 2 shows components used in one-dimensional modeling cooling FCEV under various driving and Energies 16, 9, actual road conditions. The considered simulation model was set up based on cooling is and layout same as air flow considered resistance vehicle. at all Aslocations, Figure 2 except shows, for front room end module resistance (FEM). is The same builtin resistance, air-flow resistance including atthat all locations, radiator exceptgrille for and front end bumper module hole, (FEM). is considered The built-in-resistance, obstacle to as including air flow that for cooling. radiator Values grille and CPs (Characteristic bumper hole, parameters) is considered anair obstacle inlet and to outlet airwere flow used for cooling. to reflect Values vehicular speeds CPs (Characteristic and air properties parameters) under simulation air conditions. inlet and The outlet aforementioned were used to analytical reflect vehicular method speeds is used andto air investigate properties under cooling simulation performance conditions. characteristics The aforementioned analytical rmal management method is used to investigate FCEV cooling under performance various driving characteristics conditions and A/C rmal management operation. Driving conditions FCEVreferred under various to as highway driving mode conditions and uphill and A/C mode, which operation. correspond Driving to high conditions cooling load referred with to assufficient highway mode and insufficient and uphill mode, air flow which cooling, correspond respectively, to high cooling were load investigated. with sufficient The temperature and insufficient air-flow cooling cooling, air respectively, radiator were investigated. increases, owing The temperature to heat rejection cooling from air condenser. radiator As such, increases, performance owing to heat rejection cooling from condenser. was evaluated As such, by performance varying operating conditions cooling A/C was. evaluated Table by3 varying shows simulation operating conditions parameters used in A/C this. study. Table From 3 shows viewpoint simulation suitability parameters for used cooling this, study. From coolant viewpoint inlet temperature suitability for radiator cooling, coolant inlet temperature radiator must be maintained values lower than 8 C. must be maintained at values lower than 8 C. This is maximum temperature at which stable and efficient This is operation maximum temperature FCEV can at which be maintained, stable and efficient although operation engine radiator FCEV be maintained, although engine radiator can perform reliably at temperatures up to C can perform reliably at temperatures up C [21]. [21]. Figure 2. 1 D modeling, as performed by using KULI program, cooling. Figure 2. 1-D modeling, as performed by using KULI program, cooling. Table 3. Parameters used during simulation cooling. Parameters Conditions Mode 1 Mode 2 Mode 3 Mode 4 Vehicle speed (km/h) Ambient temperature ( C) Heat load fuel cell (kw) Volume flow rate (L/min) Inclination angle road ( ) 8 8

7 Energies 16, 9, Table 3. Parameters used during simulation cooling. Parameters Conditions Mode 1 Mode 2 Mode 3 Mode 4 Vehicle speed (km/h) Ambient temperature ( C) Heat load fuel cell (kw) Volume flow rate (L/min) Inclination angle road ( ) 8 8 Air-conditioning (A/C) Off On Off On 3. Results and Discussion 3.1. Cooling Performance Stack Thermal Management System under Actual Driving Conditions Figure 3 shows cooling performance rmal management fuel-cell electric vehicle under highway-driving road conditions. The coolant temperature and volume flow rate radiator used in were determined at an average ambient temperature and vehicle velocity 3 C and km/h, respectively. The maximum driving velocity during testing was 1 km/h, and A/C was turned on. These conditions were used to evaluate actual driving conditions under high ambient temperature and high rmal load radiator, owing to condenser. The low ITD between temperatures coolant and air was also considered in this evaluation; ITD is essential for exchanging heat radiator at required heat capacity. As shown in Figure 3, at low flow rate coolant and initial driving conditions, temperature gap through radiator was distinctly higher than that high flow rate coolant, which seemed to indicate that coolant flowed through a bypass line because controlled setting temperature for cooling fuel-cell was not reached. However, at high coolant flow rate and inlet temperature, temperatures differ by only 5 7 C, indicative high power generation that leads to acceleration. The coolant pump radiator was operated at a maximum flow rate 7 L/min, under stable and fast driving conditions. In or words, coolant inlet temperature radiator was maintained at temperatures lower than ~75 C for stable operation rmal management ; coolant supply pump was also controlled. Fuel-cell s for PEMFC are efficiently operated at temperatures ranging from 7 8 C [22]. The coolant temperature radiator was maintained at temperatures below allowable limit 8 C, reby ensuring stable operation rmal management fuel-cell electric vehicle. Figure 3 shows cooling performances this under highway-driving road conditions. Under se conditions, heat-rejection rate radiator is (on average). kw, which remains relatively steady owing to sufficiently high air-flow rate and constant heat load. The maximum heat-rejection rate at maximum temperature difference (7.3 C) between coolant inlet and outlet, was 35. kw. As Figure 3b shows, at initial stages driving, coolant temperature gap through radiator from inlet to outlet measured somewhat high by C with a low coolant flow rate. L/min. These results were caused by bypassed coolant flow path due to lower temperature controlled setting condition to radiator. Hence, installed temperature sensor at outlet radiator had constant value because clogged flow line, while that at inlet was variable. In addition, present rmal management was operated at a maximum temperature 8 C, under highway-driving road conditions with an A/C on.

8 Energies 16, 9, Energies 16, 9, Heat rejection rate (kw) Test mode highway 2. Ambient temperature 3 o C 3. Air conditioning on (Rec-Vent) Heat rejection rate for cooling radiator(kw) Vehicle driving velocity (km/h) Vehicle driving velocity (km/hr) Elapsed time (min) (a) Temperature ( o C) Test mode highway 2. Ambient temperature 3 o C 3. Air conditioning on (Rec-Vent) 9 Coolant inlet temperature ( o C) Coolant outlet temperature ( o C) 8 Coolant flow rate (l/min) Coolant flowrate (l/min) Elapsed time (min) (b) Figure 3. Cooling performance rmal management fuel cell electric vehicle, Figure 3. Cooling performance rmal management fuel-cell electric vehicle, under highway driving road conditions. (a) Heat rejection rate; (b) Temperature and coolant flow rate. under highway-driving road conditions. (a) Heat-rejection rate; (b) Temperature and coolant flow rate. Figure 4 shows cooling performance rmal management under uphill road conditions. The coolant temperature and volume flow rate radiator rmal management were determined. These tests were performed at inclination angles 8 and an average ambient temperature and vehicular speed C and km/h, respectively, with A/C on. The was operated at 1 m above sea level in order to maintain this temperature and uphill driving conditions during test. The rmal load under uphill road conditions is expected to be greater than that associated with highway driving road conditions. The ITD between

9 Energies 16, 9, coolant and air was higher under uphill road conditions than under highway driving road conditions. The inlet coolant temperature radiator, as previously stated, was lower than maximum operating temperature (8 C) at which stable operation rmal management is maintained. As a result, coolant pump radiator yielded a maximum volume flow rate ~ L/min; furrmore, maximum heat-rejection rate, was 3. kw at maximum temperature Energies 16, difference 9, 3 (7.6 C) between coolant inlet and outlet Heat rejection rate (kw) Temperature ( o C) Test mode 8 ~ o uphill 2.Ambient temperature o C 3. Air conditioning on Heat rejection rate (kw) 5 3 Vehicle driving velocity (km/h) Elapsed time (min) (a) Elapsed time (min) (b) Figure 4. Cooling performance rmal management fuel cell electric vehicle, Figure under 4. Cooling uphill road performance conditions. (a) Heat rejection rmal rate; management (b) Temperature and coolant fuel-cell flow rate. electric vehicle, under uphill road conditions. (a) Heat-rejection rate; (b) Temperature and coolant flow rate Cooling Performance Simulations Stack Thermal Management System under Actual Driving Condition The cooling performance rmal management fuel cell electric vehicle was analyzed by using KULI, a commercial simulation program; rmal stability required for stable operation cooling vehicle was analytically determined. Based on results shown in Figures 3 and 4, cooling performance characteristics this were described; cooling exhibited a stable performance. When A/C vehicle is operated, rmal load radiator increases (in general) with decreasing ITD, owing to 5 3 Vehicle driving velocity (km/h) 1. Test mode 8 ~ o uphill 2.Ambient temperature o C 3. Air conditioning on 9 Coolant inlet temperature ( o C) Coolant outlet temperature ( o C) Coolant flow rate (l /min) 8 Coolant flow rate (l /min)

10 Energies 16, 9, Cooling Performance Simulations Stack Thermal Management System under Actual Driving Condition The cooling performance rmal management fuel-cell electric vehicle was analyzed by using KULI, a commercial simulation program; rmal stability required for stable operation cooling vehicle was analytically determined. Based on results shown in Figures 3 and 4 cooling performance characteristics this were described; cooling exhibited a stable performance. When A/C vehicle is operated, rmal load radiator increases (in general) with decreasing ITD, owing to additional heat released from condenser. The radiator and condenser associated with rmal management and A/C, respectively, were designed for front end module (FEM) vehicle. Therefore, based on this layout, increased rmal load from condenser should be taken into consideration. The accuracy simulation analysis was determined by comparing simulation data obtained from KULI with data obtained from actual road-driving tests. The performance radiator ( rmal management ), as determined from experimental and simulation data, is shown in Figure 5. The performance was determined with A/C on, under ambient temperature C, vehicle driving speed 5 km/h, and uphill mode 8.. Under se conditions, heat load fuel-cell for vehicle and heat-rejection rate from condenser are 27. kw and 5.2 kw, respectively. Temperature differences 6.1 C and 5.1 C, between inlet and outlet temperature radiator, were determined from experimental and simulation data, respectively. The predicted inlet temperature differed by only 1.5% from experimentally determined value. Figure 6 shows cooling performance rmal management, with and without operation A/C, under highway-driving conditions. The coolant inlet temperature and frontal air radiator were determined; tests were performed at an ambient temperature 35 C, and a vehicle driving velocity, coolant volume flow rate, and heat load 1 km/h, 1 L/min, and 27. kw, respectively, fuel-cell. Owing to heat gain from condenser, coolant inlet temperature radiator was 3.1 C higher than that obtained when A/C was f. The coolant inlet temperature was, however, lower than maximum temperature (8 C) at which stable operation rmal management is maintained. This resulted from sufficiently high cooling capacity, and correspondingly high air-flow rate, which occur at high vehicle driving velocity. Figure 7 shows cooling performance rmal management under highway mode and uphill mode driving conditions. The cooling performance radiator was determined at an ambient temperature 35 C, a coolant volume flow rate 1 L/min, and heat loads 27. kw and 24. kw, fuel-cell. Furrmore, an inclination angle and a vehicle driving velocity 1 km/h were considered for highway mode; values 8. and 5 km/h were considered for uphill mode. The coolant inlet temperature radiator under highway mode was 13.7 C lower than that obtained under uphill mode. This resulted from higher (by 58.7%) air-flow rate associated with higher vehicular speed under highway mode, compared to that associated with uphill mode; a higher air-flow rate occurs in spite 12.5% higher heat load. Consider uphill mode, where radiator operates under a heat load 24. kw and at an ambient temperature 35 C, with A/C on. In this case, coolant inlet temperature would exceed allowable limit (8 C) for stable operation rmal management. This results from low air-flow rate, low cooling capacity, and low ITD between cooling air and coolant, which all stem from low vehicle driving velocity, with A/C on [23]. Based on results shown in Figure 7, design improved radiator should take design layout FEM fuel-cell electric vehicle into consideration; this improved radiator enhances cooling performance rmal management.

11 a coolant volume flow rate 1 L/min, and heat loads 27. kw and 24. kw, fuel cell. Furrmore, an inclination angle and a vehicle driving velocity 1 km/h were considered for highway mode; values 8. and 5 km/h were considered for uphill mode. The coolant inlet temperature radiator under highway mode was 13.7 C lower than that obtained under uphill mode. This resulted from higher (by 58.7%) air flow rate associated with higher Energiesvehicular 16, 9, 3 speed under highway mode, compared to that associated with uphill mode; a higher air flow rate occurs in spite 12.5% higher heat load. Temperature ( o C) 1. Test mode 8% uphill 2. Ambient temperature o C 3. Vehicle speed 5 km/h 4. Air conditioning on Coolant temperature ( o C) 9 Inlet 8 Outlet 7 o C o C 5 3 Experiment Simulation Figure 5. Comparison cooling performance, as determined under actual road driving conditions Figure 5. Comparison cooling performance, as determined under actual road-driving conditions and from simulations. and from simulations. Energies 16, 9, Test mode 1 km/h highway 2. Ambient temperature 35 o C 3. Heat load 24 kw 4. Coolant flow rate 1 l /min 8 6. Coolant inlet temperature ( o C) Air conditioning f Frontal air velocity (m/s) 2. Air conditioning on Figure 6. Cooling performance rmal management with and without operation Figure 6. Cooling performance rmal management with and without operation air conditioning. air-conditioning. 1. Ambient temperature 35 o C 2. Air conditioning on 3. Coolant flow rate 1 (l /min) The FEM design layout was taken into consideration, in order to improve 6. cooling performance rmal management 9 ; tests were performed under an ambient temperature Allowable limit temperature 8 o C 35 C, and a coolant volume flow 5. 8 rate, heat load, and inclination angle 1 L/min, 24. kw, and 8., respectively. The volume mm (width) ˆ 451 mm (height) ˆ 38 mm (depth) improved 4. radiator is 52.2% higher than that present 69 mm (width) ˆ 435 mm (height) ˆ 26 mm (depth) radiator. Figure 8 shows coolant 5 inlet temperatures present and 3. improved radiators. Owing to its higher heat capacity, coolant inlet temperature improved radiator is 22.7% lower than that present model. This 2. 3improved radiator operates at reasonable temperature levels that ensure stable operation rmal management. Figure 8b shows heat-rejection 1. rate consisting present and improved radiator models, at coolant flow rates L/min and 1 L/min. At se flow rates, heat-rejection rates. equipped with Highway ( o, 1 km/h) Uphill (8 o, 5 km/h) improved radiator were 52% and 62% higher, respectively, than those with present radiator. In addition, Figure compared 7. Cooling to performance current radiator, rmal improved management radiator under highway remained and uphill stable for longer driving modes. durations under more severe conditions; se conditions include extremely high heat load, and hence greater power Consider generation, uphill with mode, an A/C where radiator on. operates under a heat load 24. kw and at an ambient temperature 35 C, with A/C on. In this case, coolant inlet temperature would exceed allowable limit (8 C) for stable operation rmal management. This results from low air flow rate, low cooling capacity, and low ITD between cooling air and coolant, which all stem from low vehicle driving velocity, with A/C on [23]. Based on results shown in Figure 7, design improved radiator should take design layout FEM fuel cell electric vehicle into consideration; this improved radiator enhances cooling performance rmal management. Coolant inlet temperature ( o C) Frontal air velocity (m/s)

12 Coola Air conditioning f 3. Fro 2. Air conditioning on Energies 16, 9, Figure 6. Cooling performance rmal management with and without operation air conditioning. 1. Ambient temperature 35 o C 2. Air conditioning on 3. Coolant flow rate 1 (l /min) 6. 9 Allowable limit temperature 8 o C Energies 16, 9, is 22.7% lower than that present model. This improved radiator operates at reasonable temperature levels that ensure stable operation rmal management. Figure 8b. shows heat rejection rate Highway ( o, 1 km/h) consisting Uphill (8 o, present 5 km/h) and improved radiator models, at coolant flow rates L/min and 1 L/min. At se flow rates, heat rejection rates Figure 7. Cooling performance rmal management under highway and uphill Figure 7. Cooling performance equipped with improved rmal radiator management were 52% and 62% higher, under respectively, highway and than uphill those driving with modes. present radiator. In addition, compared to current radiator, improved driving modes. radiator remained stable for longer durations under more severe conditions; se conditions include Consider extremely uphill high mode, heat load, where and hence radiator greater operates power generation, under a heat with load an A/C 24. kw on. and at an ambient temperature 35 C, with A/C on. In this case, coolant inlet temperature would exceed allowable 1. Ambient limit (8 temperature C) for stable 35 o C operation 2. Air conditioning on rmal management. This results from 3. low Coolant air flow rate, low 1 cooling (l /min) capacity, and low ITD between cooling air and coolant, which all stem from low vehicle driving velocity, with A/C on [23]. Based on results shown 9in Figure 7, design Allowable limit improved temperature 8radiator o C should take design layout FEM 8 fuel cell electric vehicle into consideration; this improved radiator enhances cooling performance 7 rmal management. The FEM design layout was taken into consideration, in order to improve cooling performance rmal management ; tests were performed under an ambient temperature 35 C, and 5 a coolant volume flow rate, heat load, and inclination angle 1 L/min, 24. kw, and 8., respectively. The volume 693 mm (width) 451 mm (height) 38 mm (depth) improved radiator is % higher than that present 69 mm (width) 435 mm (height) 26 mm (depth) radiator. Figure 8 shows coolant inlet temperatures present and improved radiators. Owing to its higher heat capacity, coolant inlet temperature improved radiator Coolant inlet temperature ( o C) Coolant inlet temperature ( o C) Heat rejection rate (kw) Present radiator (a) Frontal air velocity (m/s) Improved radiator 1. Ambient temperature 35 o C 2. Air conditioning on 8 Heat rejection rate present model at l /min Heat rejection rate present model at 1 l /min Heat rejection rate improved model at l /min Heat rejection rate improved model at 1 l /min On average 62% On average 52% Frontal air velocity (m/s) 11 (b) Figure 8. Comparison on cooling performance rmal management Figure 8. equipped Comparison with onimproved cooling and present performance radiator models. (a) Coolant rmal inlet management temperature; equipped (b) with Heat rejection improved rate. and present radiator models. (a) Coolant inlet temperature; (b) Heat-rejection rate.

13 Energies 16, 9, Conclusions This study evaluated cooling performance and analyzed rmal stability a rmal management a FCEV, under various experimental conditions (including actual road driving conditions). An experimental apparatus for actual road driving tests was constructed in order to evaluate cooling performance under different driving conditions. The coolant temperatures radiator, subjected to highway and uphill travel modes during actual road driving conditions, were maintained at values below allowable limit temperature 8 C. In this way, stable operation rmal management FCEV was ensured. In addition, under uphill mode and actual road driving conditions, ITD between coolant inlet and air inlet was higher than that associated with highway mode; this higher IDT occurred in spite greater rmal load under uphill mode, compared to that occurring under highway mode. The cooling performance analysis rmal management FCEV was also investigated by using commercial simulation program, KULI. Furrmore, rmal stability required for stable operation cooling was analytically evaluated. The predicted coolant inlet temperature radiator differed by only 1.5% from experimentally determined value. When A/C was turned on, coolant inlet temperature radiator was 3.1 C higher than that obtained when was turned f; this higher value is attributed to heat gain from condenser. The coolant inlet temperature remained below allowable limit (8 C) at which stable operation rmal management is maintained. This results from sufficiently high cooling capacity, and correspondingly high air-flow rate, that occur at high vehicular driving velocity. In highway mode, inlet temperature was 13.7 C lower than that associated with uphill mode. This is attributed to higher (by 58.7%) air-flow rate that occurs at higher vehicular speed associated with highway mode, compared to that associated with uphill mode; higher air-flow rate occurs in spite a 12.5%-higher heat load. The coolant inlet temperature may exceed desired limit in case uphill mode at high ambient temperature, with A/C turned on. Therefore, FEM design layout was taken into consideration while designing improved radiator rmal management FCEV. The heat-rejection rates equipped with improved radiator were 52% and 62% higher, at coolant flow rates L/min and 1 L/min, respectively, than those present radiator. Owing to its higher heat capacity, coolant inlet temperature higher-volume improved radiator was 22.7% lower than that present model. This improved radiator operates at reasonable temperature levels that are suitable for stable operation rmal management FCEV. In future works, methods for improvement heat transfer rate, such as application nanluids [6] and variation heat exchanger specifications, as well as enlargement radiator size in cooling need to be investigated Acknowledgments: This research was supported by Ministry Trade, Industry and Energy, and Basic Science Research Program through National Research Foundation Korea (NRF), funded by Ministry Science, ICT & Future Planning (13R1A1A62152). This work was supported by Industrial Technology Innovation Program (Advanced Technology Center, 5189) and Korea institute energy technology evaluation and planning (Development Core Components for Variable Pressure PEMFC System, ) funded By Ministry Trade, Industry & Energy. Author Contributions: Ho-Seong Lee played a leading role in writing paper as a first author. Moo-Yeon Lee is corresponding author. He designed paper and proposed critical results and discussions. Choong-Won Cho and Jae-Hyeong Seo are co-authors and performed tests and review published articles in main part in paper. Conflicts Interest: The authors declare no conflict interest. References 1. Lim, D.H.; Lee, M.-Y.; Lee, H.-S.; Kim, S.C. Performance evaluation an in-wheel motor cooling in an electric vehicle/hybrid electric vehicle. Energies 14, 7, [CrossRef]

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