Avestia Publising International Journal of Mecanical Engineering and Mecatronics Volume 1, Issue 1, Year 1 ISSN: 199-74 Article ID: 011, DOI: 10.11159/ijmem.1.011 88 Termal Management Analysis of a Litium-Ion Battery Pack using Flow Network Approac G. Karimi, A.R. Degan Department of emical Engineering, Siraz University Siraz, Iran gkarimi@sirazu.ac.ir; adb068@gmail.com Abstract- Among various issues tat ig power application litium-ion (Li-ion) batteries are encountered, termal issues ave received more attention because of teir potential to degrade battery performance. In te present work, a lumped capacitance eat transfer model was developed in conjunction wit a flow network approac to investigate termal performance of a commercial-size Li-ion battery pack under various operating conditions. Air and silicon oil were cosen as cooling media in te battery pack for two conventional flow arrangements, U- and Z-configurations. Numerical results ave revealed tat te temperature distributions inside te battery pack can be significantly affected by te coolant type and te flow configuration. Te calculated parasitic losses due to te fan/pump power demand as well as te temperature dispersion witin te battery pack can be used to devise and optimize te battery termal management system (BTMS). Keywords: Li-ion Battery Pack, Modeling, Lumped apacitance, Battery Termal Management System, Flow Network Approac opyrigt 1 Autors - Tis is an Open Access article publised under te reative ommons Attribution License terms (ttp://creativecommons.org/licenses/by/.0). Unrestricted use, distribution, and reproduction in any medium are permitted, provided te original work is properly cited. 1. Introduction Because of increasing demand on new reliable power source for ybrid electric veicles, litium-ion (Li-ion) batteries ave received muc attention in te last decade. Higer power density and efficiency, lower self-discarge rate, longer life and te lack of memory effect are te special features of tis new tecnology tat makes it attractive for researcers and manufacturers. Problem free Li-ion batteries are already in use for low power demand applications suc as cell pone and laptop battery packs, owever; for ig power applications suc as in automotive propulsion drives, tere are serious issues wic need to be addressed. To generate large power density required for automotive applications normally a number of battery units needs to be assembled in parallel or series (or combination) in a battery pack to acieve te required output specifications. In te absence of an effective battery termal management system (BTMS), significant temperature gradients can be developed inside te battery pack due to te battery internal resistance and eat generation resulting from te electrocemical reactions inside te individual cells. Te performance of an electric veicle depends strongly on its battery pack performance. Te temperature variations inside te battery pack will lead to different cell-to-cell internal resistance and voltage and can degrade te pack performance and sortening batteries life cycle (Pesaran, 01). In order to maintain temperature uniformity inside te pack and prevent unfavourable voltage distribution in different cells, application of a BTMS is inevitable. A BTMS must be able to regulate temperature and voltage distributions witin te pack in order to keep battery performance witin an acceptable range and also bring te cooling cost to te lowest possible value (Karimi and Li, 13). It is evident tat lower cooling cost and better battery performance will make electric cars a better competitor in comparison wit internal combustion engine automobiles. Due to inerent restrictions and limited operating conditions associated wit experimental measurements, numerical modeling is very efficient for design and performance analysis of BTMS under battery transient operations. Somasundaram et al. developed a two-dimensional transient termal-electrocemical model for passive termal Nomenclature A cross sectional area [m ] Bi Biot number c eat capacity [J/(kg.K)] f friction coefficient
89 D F ydraulic diameter [m] Faraday constant; 96485 [/mole] eat transfer coefficient [W/(m.K)] i discarge current per unit volume [A/m 3 ] k L m N p termal conductivity [W/(m.K)] distance/lengt [m] mass flow rate [kg/s] number of cells/loops/brances/flow cannels pressure [Pa] P perimeter [m] q eat generation rate per unit volume [W/m 3 ] R i internal equivalent resistance [Ω.m 3 ] Re S t Reynolds number entropy [J/(mole.K)] time [s] T temperature [, K] v fluid velocity [m/s] x distance/spatial coordinate [m] y spatial coordinate [m] Greek Letters termal diffusivity [m /s] Difference viscosity [Pa.s] Density [kg/m 3 ] Subscripts c oolant f Friction management of a spiral-wound Li-ion battery. Termal and electrocemical equations are coupled troug eat generation terms and temperature-dependent pysical properties. It was sown tat passive termal management troug a pase cange material (PM) can lower te overall cell temperature at a discarge rate of 5, if te PM tickness is large enoug to stand trougout te discarge time (Somasundaram et al., 1). en and Evans were first to develop a two-dimensional model to study te effect of various cell components, stack size and cooling conditions on te performance of Li-polymer electrolyte batteries under different discarge rates. Based on te temperature profiles obtained, tey provided useful insigts suc as ow to maintain operating temperature by designing proper cell stacks and coosing proper cooling/insulating systems from eat transfer point of view (en and Evans, 1994). Al-Hallaj et al. studied te termal beavior of commercial cylindrical and prismatic Li-ion cells using electrocemical-calorimetric metods. Tey measured te cell eat generation during te cell discarge and eat consumption during te cell carge wit significant dependence on te state of carge at te end of discarge due to concentration polarization (Al-Hallaj et al., 00). Maleki and Samsuri evaluated numerically te termal performance of a notebook computer Li-ion batterypack under various operating conditions. Tey found tat te battery temperature rise during carge is dominated by te power dissipation from te control electronics and during discarge by te eat dissipation from Li-ion cells (Maleki and Samsuri, 03). Forgez et al. studied te termal caracteristics of a cylindrical LiFePO 4/grapite Li-ion battery and a lumped termal model was developed based on experimentally acieved parameters (Forgez et al., 10). Sabba et al. and Kizilel et al. compared te effectiveness of passive cooling by pase cange materials wit tat of active (forced air) cooling for termal management of ig power Li-ion battery packs. Numerical simulations were performed at different discarge rates, operating temperatures and ambient temperatures and compared wit te experimental results. Te results sowed tat at ig discarge rates and/or at ig operating or ambient temperatures, aircooling is not a proper termal management system to keep te temperature of te cell in te desirable operating range witout expending significant fan power (Sabba et al., 08; Kizilel et al., 09). Inui et al. developed two- and treedimensional simulation codes of te transient response of te temperature distribution in te cylindrical and prismatic Liion battery during a discarge cycle. Te numerical results for te cylindrical battery were in good agreement wit te experimental data. Teir results indicated tat battery wit te laminated cross section as a remarkable effect on te suppression of te temperature rise in comparison wit te battery wit square cross section (Inui et al., 07). Fang et al. used an electrocemical termal coupled model to predict performance of a Li-ion cell and its individual electrodes at various operating temperatures. Tey validated te model against te experimental data for constant current and pulsing conditions, caracteristic of ybrid electric veicle (HEV) applications. Te prediction of individual electrode potential compared wit 3 electrode cell experimental data wit good agreement (Fang et al., 10). Duan and Naterer studied experimentally termal management of battery modules wit pase cange materials. Tey examined te effectiveness of PM termal management under variable simulated battery discarge conditions and variable ambient temperatures, as well as te effects of buoyancy during PM melting (Duan and Naterer, 10). Performance analysis of a commercial size Li-ion battery pack under various discarge conditions was recently conducted by Karimi and Li. Various cooling strategies were examined and te resulting cell-to-cell temperature, internal resistance, state of carge (SO) and voltage distributions were obtained. It was found tat a distributed cooling
90 strategy based on forced air convection would be an effective metod in a BTMS (Karimi and Li, 13). Te present researc is an extension to Karimi and Li s work in wic a combination of lumped capacitance metod and flow network approac are used to assess a battery pack performance under various flow configurations and discarge rates. Te performance of te BTMS is evaluated based on te temperature uniformity inside te battery pack and te parasitic power requirements to circulate te coolant witin te pack. Te results of tis work can be used for optimization of te BTMS.. Model Formulation Li ion batteries are fabricated in many different sapes and configurations, for example, cylindrical, coin, prismatic (rectangular), and tin and flat. Altoug all four battery configurations are suited for portable and low power demand electronic applications, te prismatic and flat type batteries seems to be te preferred power source for ybrid electric (HE), plug in ybrid electric (PHE) or plug in electric veicular applications. In tis work, combinations of te tin film flat type battery is considered for te analysis. Typical Li ion cell consists of a negative electrode formed from a tin layer of powdered grapite, or certain oter carbons, mounted on a copper foil and a positive electrode composed of a tin layer of powdered metal oxide (eg., LioO) mounted on aluminum foil. Te two electrodes are separated by a porous plastic film soaked typically in LiPF6 dissolved in a mixture of organic solvents suc as etylene carbonate, etyl metyl carbonate, or dietyl carbonate. Te stacked cells are generally eld togeter by pressure from te battery container. A diagram of a typical Li-ion battery pack is sown in Figure 1. Individual battery units comprised of several cell units are located inside te pack. Eac cell unit is consisted of seven tin layers formed into anode, electrolyte and catode. Individual cells in a battery unit are electrically insulated from eac oter using separators. Te material types, ticknesses and termo-pysical properties of cell layers are listed in Table 1. During te carge/discarge process, litium ions are inserted or extracted from te interstitial space between atomic layers witin te active materials. Te cemical reactions are expressed as: Positive reaction oo Li Negative reaction 6 - + Li + e 6 + Li Total reaction 6 LioO (1) + e - () oo + Li LioO + 6 (3) Individual battery units are termally managed by direct eat excange wit a dielectric cooling medium, suc as air or silicon oil wic is circulated troug te pack. As illustrated in Figure 1, te cooling medium enters te pack via an inlet port, travels troug a main entrance manifold, distributes into te cooling ducts and eventually collected in te exit manifold and leaves te pack via an exit port. Two configurations namely U- and Z-configurations are considered in te present study. Te battery pack model presented ere consists of two parts: te single battery termal model and te battery pack flow network model. Te battery termal model determines transient temperature distribution in eac battery unit and te surrounding cooling medium as te battery discarges into a fictitious load. Te flow conditions around eac battery unit are determined using te flow network model. Bot models are described in te following sections.. 1. Te Battery Termal Model Te structure and assembly of te battery pack under consideration is te same as tat used in a previous work by Karimi and Li (Karimi and Li, 13). Heat generation in te cells is assumed to be caused by omic internal resistances and entropy cange resulting from electrocemical reactions and can be calculated from: i q Rii TS F (4) were q is te rate of internal eat generation per unit volume, R i is te internal equivalent resistance and i is te rate of discarge per unit volume. Different kinds of Li-ion batteries ave different values of te internal resistances and te entropy canges. A termal management strategy requires tat tese data be determined accurately. Since te internal resistances depend on bot te temperature and state of carge of te battery, it sould be measured under various battery operating conditions. Te battery internal resistance as been determined experimentally (Karimi and Li, 13). From te pysical dimensions and termal properties of cell layers and coolant flow conditions around te battery units, te dimensionless Biot number (Bi) is estimated to be less tan 0.1. Tis indicates tat te temperature gradients across individual battery units are negligible. On te oter and, te temperature variations would be dominant only along te cell/battery eigts. Hence, te battery governing equation is as follow: In tese cemical formulas, te reactions proceed from te left side to te rigt side during te discarge and in te reverse direction during te carge cycle. T y p T T ka q 1 T k t (5)
1 3 N-1 N opper LioO Aluminum LioO Insulating case 91 were y is spatial coordinate along te battery eigt, is local eat transfer coefficient, P and A are te battery perimeter and cross sectional area, respectively. Te average termal conductivity and termal diffusivity of te battery are denoted by k and, respectively. temperature. T is te local coolant c Te frictional ead loss Weisbac equation: p can be expressed by Darcy- L v p f (8) D Property m. ρ g m3 g. Table 1. Unit cell specifications. Al LioO Electrolyte 0.35 37 4 0.59 1400 710 700 13 1551 90 715 1375 1.04 1347 1437 u 398 8930 386 were L and D are te lengt and ydraulic diameter of brances in te loops and and v are te associated averaged values of te coolant s density and velocity, respectively. Te friction coefficient, can be determined from: f Tickness of cell layers [μm]: u Electrolyte LioO Al LioO Total tic ness of a unit cell μm : 1 1 40 180 180 50 730 16 Re 0.079 0.5 Re Re 00 Re 4000 A linear relationsip is used to interpolate f in te transient region and all te surfaces are considered to be smoot (Karimi et al., 05). Hardy ross metod is implemented to obtain te flow rate distributions in te coolant flow loops surrounding te battery units (ross, 1936). (9) Equation 5 is subject to te initial and boundary conditions. It is assumed tat te wole battery pack including all te battery cells and te cooling medium are initially at a specified temperature, and during te battery discarge, eac battery unit excanges eat wit te adjacent cooling media by convection. 3. Results and Discussion Table lists te battery pack termal management system parameters used in te present study. Te battery pack consists of a total of battery units connected in series. Eac battery unit considered to be16 cm wide and 3 cm tall Battery pack.. Flow Network Model Te coolant flow pat consisting of te inlet, outlet, supplying and collecting manifolds and te coolant flow cannels forms a series of flow loops. Te flow distribution in eac loop is governed by te conservation laws. Te conservation of mass principle requires tat total mass flow into and out of eac junction in te flow network must be equal, or: Outlet (U-onf.) oolant inlet y x Termal management mdeia Outlet (Z-onf.) min m out (6) In order to satisfy te conservation of energy, te sum of pressure canges around eac of te flow loops sould be zero: p 0 (7) LOOP A single battery unit A single cell unit Fig. 1. Scematic of a typical Li-ion battery pack termal management system.
9 and comprised of 10 cell units connected in parallel. Hence, te electric capacity of eac battery unit is ma/cm and te associated current density is estimated to be 5.4 ma. Te overall magnitude of te battery voltage depends on te arrangement of all battery packs in te system owever, for te present battery pack configuration; it varies between 7 V (fully carged) and a lower value depending on te SO. As illustrated in Figure 1, two conventional flow configurations are considered for te coolant (a) Z- configuration in wic te coolant enters te pack from te lower left inlet and leaves te pack at te upper rigt outlet and (b) U-configuration in wic te coolant enters te pack from lower left inlet and leaves te pack at te upper left outlet. Silicon oil and air are selected as possible coolant media. Litium-ion battery cells ave demonstrated excellent performance wen operated at ambient temperature conditions. Te adverse effect of temperature excursions on Li-ion cell performance is reported wen te cell is operated at temperatures below 0 or beyond 65 (Karimi and Li, 13). Despite te relatively wide operating temperature range (0-65 ) for battery cells, a non-uniform temperature distribution in a battery pack will lead to non-uniform voltage distribution in te pack wic adversely affect te performance durability. Terefore, te main objective of te present termal management system is (a) to keep te battery cell temperatures witin an acceptable range for wic eat generation data are available (eg. -40 ), and (b) to ensure tat a relatively uniform voltage distribution prevails witin te pack. Te wole computational domain (including te battery container and termal management ducts) are divided into segments of appropriate sizes and te governing equations are discretized for various regions. Te implicit alternating-direction tecnique, ADI, is applied to solve model Equations along wit te boundary conditions. Te performance of te termal management system is assessed based on te temperature uniformity witin te pack and te parasitic power consumption (pump or blower) for te coolant circulation. In te following section, numerical results for various termal management strategies are presented. Since, te objective is to examine te termal beavior of te Li-ion battery pack during discarge, wen eat is generated witin te battery, only cooling strategies are considered. However, te concluding remarks are still valid for battery carging. Figure sows te temperature distributions inside te battery pack for various rates of discarges (-rate) at te end of discarge wen SO = %. Silicon oil in a Z- configuration is used to manage te temperature. Te numerical results indicate tat a pretty uniform temperature is prevailed inside te pack even at te ig rate of discarge (e.g. 5 ) and te maximum temperature rise in te battery is less tan 1. Tis is due to te large eat capacitance of te silicon oil wic can absorbs almost all of te eat generated witin te battery cells. Figure 3 compares temperature distributions in te battery pack at te end of discarge at a rate of. Te comparison is made for silicon oil and air as coolant under U- and Z-configurations. As evident from tese figures, wit air as te coolant, te battery (and coolant) temperatures can rise to as ig as 3 and te associated temperature dispersion in te pack is large. However, wit silicon oil te maximum tem erature rise in te ac is less tan and te temperature dispersion is very small. Detailed examination of tese figures reveals tat te temperature dispersion wit U-configuration is muc larger tan tat of te Z-configurations for bot coolants. Hence te Z- configuration outperforms U-configuration for bot coolant fluids. Battery: Number of battery units in te pack Number of cells per battery unit ell surface area [cm ] Table. System parameters. Electric capacity of eac battery unit [A. ] Open-circuit voltage of eac cell [V ] ut-off state of carge [%] nitial ac tem erature Total tic ness of a unit cell μm ooling System based on air and silicon oil: m ient tem erature nlet tem erature of cooling media Inlet fluid velocity [m/s] Heat transfer coefficient for te cooling media [W/m. 10 368 3.6 730 0. 0 00 50 It is very important for a termal management system to warrant uniformity in temperature at minimum power consumption. A compreensive study is conducted to assess te performance of te termal management system at various rates of discarge. Table 3 compares te numerical results for pressure drop, power requirement and temperature dispersion for silicon oil and air flowing wit U- and Z-configurations. As indicated in tis table, wen air circulates in te pack (bot U- and Z-configurations), altoug te power demand is very small te temperature dispersion is large. Te maximum standard deviation (SD) for temperature witin te pack is about 11 for U- configuration and 7.3 for Z-configuration. However, wit silicon oil as coolant, te temperature dispersion is very small
93 and a uniform temperature prevails trougout te pack even at ig rates and at te end of te discarge period. For bot coolants, te Z-configuration results in a more uniform flow and temperature distributions. Te calculated pressure drops trougout te pack and te associated power requirements are also listed in tis table. As expected, a larger power is required to circulate silicon oil inside te pack. It is wortwile to mention tat minor losses are not included in te power calculations. From te information in Table 3 one can conclude tat a BTMS based on silicon oil in Z-configuration would be very efficient. Fig. 3. Temperature distributions in te battery pack at te end of discarge for various flow configurations and coolant fluids (a) Silicon oil wit U-configuration (b) Air wit U-configuration (c) silicon oil wit Z-configuration and (d) Air wit Z- configuration [ooling medium for (a) and (c): silicon oil, inlet flow rate = 0.0001168 m 3 /sec, = 100 W/m., inlet velocity = 0.1 m/sec, ooling medium for (b) and (d): Air, inlet flow rate = 0.01168 m 3 /sec, = 50 W/m., inlet velocity = 10 m/sec]. Table 3. Performance analysis of te presented BTMS. Fig.. Effect of rate on te battery pack temperature distributions (a) 1, (b) and (c) 5 [ooling medium: silicon oil, Z-configuration, inlet flow rate = 0.0001168 m 3 /sec, = 100 W/m., inlet velocity = 0.1 m/sec] 4. onclusions Te performance of an electric veicle is affected by te operating temperature and te degree of temperature gradient in its battery pack. In tis work, termal analysis of a Li-ion battery pack is carried out to examine te relationsip between battery termal beavior and design parameters. A lumped capacitance eat transfer model was developed in conjunction wit a flow network approac to investigate termal performance of a commercial-size Li-ion battery pack for constant discarge rates of up to 5. Air and silicon oil were cosen as cooling media in te battery pack for two conventional flow arrangements, U- and Z- configurations. From te modeling results, te following conclusions can be made. (a) Te temperature distribution inside te battery pack is significantly affected by te coolant type and te flow configuration. A uniform temperature distribution can be acieved inside te pack wen silicon oil is used as a coolant. (b) Te parasitic power demand for distribution of te coolant is largely dependent on te coolant oolant configuration U configuration Z configuration Fluid Air Silicon oil Air Silicon oil Discarge rate 0.5 Q (m 3 /sec ) P (kpa) Power (Watt) Temp.SD ( ) 1.6 1 0.01168 0.0095 0.11 4. 11.06 0.5 0.06 1 0.0001168 5.75.95 0.17 0.53 0.5 1.06 1 0.01168 0.0155 0.18.84 7.33 0.5 0.0 1 0.0001168 7.556 3. 0.05 0.15 (c) Numerical results ave indicated tat wit Z- configuration (for bot air and silicon oil) a more uniform temperature distribution develops inside te pack at te expense of a sligtly larger power demand. (d) Te results of te present model can be used to design and optimize Li-ion battery termal management systems type. ompared to air,
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