Proceedings of 3 rd IRF International Conference, 18 th May-2014, Hyderabad, India, ISBN: 978-93-84209-18-6 99 NUMERICAL ANALYSIS OF PCM BASED THERMAL MANAGEMENT SYSTEM FOR LI-ION BATTERY USED IN HYBRID AND ELECTRICAL VEHICLES 1 RAVICHANDRA RANGAPPA, 2 SRITHAR RAJOO 1 Nilai University, 2 UTM Centre for Low Carbon Transport, University Technology Malaysia Abstract- Increasing demand for Hybrid and Electrical vehicles due to environmental and sustainable issues has imposed more challenges on the energy storage system used for automotive applications. Lithium based batteries are showing an extended performance with high cycle life, thus creating hopes on the energy storage system challenges. Although li-ion battery technology has the most market penetration, they are comparatively more sensitive in operation. Operating temperature is one of the major factors that can affect the performance of li-ion batteries. Many research works have been done on thermal management system to maintain the working temperature for Li-ion batteries. Active air cooled system is the most common, however passive Phase Change Material based thermal management system is getting more attention due to its many advantages. Index Terms- Hybrid and electrical vehicles, Li-Ion batteries, thermal management, phase change material I. INTRODUCTION Li-Ion batteries are the most common energy storage devices in hybrids and electrical vehicles. They are highly sensitive to the operating temperature, which can affect its life and performance. To obtain the maximum efficiency and extended life, battery thermal management system (BTMS) is most essential. It is necessary to maintain the operating temperature within 20°C to 40°C and the temperature variation between the battery modules should be within 5°C [1] Al-Hallaj et al. conducted an extensive experimental study on battery thermal management, and they have set up a prototype system to test its suitability for scaled-up Li-ion batteries inEVand HEVapplications. They proposed the PCM-based battery cooling system, operating as a passive thermal management system, is relatively simple, therebypromising appreciable cost reduction compared to activecooling systems. They concluded that PCM based thermal management systems are most effective under cold ambient condition and in space applications [2]. Sabbah R et al. conducted a comparative study between active and passive thermal management systems during normal and stressful discharge protocols for high power Li-Ion battery packs. They have found that in a high discharge rates and high ambient temperatures, adequate active cooling system requires air flow rates close to or within turbulent range, which is not practical for vehicle applications. The parasitic fan power required to run the active cooling system and failing to maintain uniform temperature across the battery pack seems to be the drawbacks compared to PCM based cooling with uniform heat distribution [3]. II. OBJECTIVE AND METHODOLOGY The authors have previously built a thermal management system for Li-Ion battery pack for an electrical vehicle (Fig 1). The study was focused on numerical study to analyze the active cooling system [4]. Tan later extended the work by experimental investigation to design an efficient active air cooled thermal management system (Fig. 2) [5]. In the present work a numerical analysis is carried to investigate the possibilities of passive cooling system using PCM. The main objective of the current work is to identify the suitable PCM or PCM composite which can be adapted to be used in a passive thermal management system, replacing the active (forced air) thermal management system. Fig 1. Battery module arrangement within the EV boot [4] Fig 2. Active thermal management system by Tan L. W. [5]
Transcript
Proceedings of 3rd IRF International Conference, 18th May-2014, Hyderabad, India, ISBN: 978-93-84209-18-6
99
NUMERICAL ANALYSIS OF PCM BASED THERMAL MANAGEMENT SYSTEM FOR LI-ION BATTERY USED IN
HYBRID AND ELECTRICAL VEHICLES
1RAVICHANDRA RANGAPPA, 2SRITHAR RAJOO
1Nilai University, 2UTM Centre for Low Carbon Transport, University Technology Malaysia
Abstract- Increasing demand for Hybrid and Electrical vehicles due to environmental and sustainable issues has imposed more challenges on the energy storage system used for automotive applications. Lithium based batteries are showing an extended performance with high cycle life, thus creating hopes on the energy storage system challenges. Although li-ion battery technology has the most market penetration, they are comparatively more sensitive in operation. Operating temperature is one of the major factors that can affect the performance of li-ion batteries. Many research works have been done on thermal management system to maintain the working temperature for Li-ion batteries. Active air cooled system is the most common, however passive Phase Change Material based thermal management system is getting more attention due to its many advantages. Index Terms- Hybrid and electrical vehicles, Li-Ion batteries, thermal management, phase change material I. INTRODUCTION Li-Ion batteries are the most common energy storage devices in hybrids and electrical vehicles. They are highly sensitive to the operating temperature, which can affect its life and performance. To obtain the maximum efficiency and extended life, battery thermal management system (BTMS) is most essential. It is necessary to maintain the operating temperature within 20°C to 40°C and the temperature variation between the battery modules should be within 5°C [1] Al-Hallaj et al. conducted an extensive experimental study on battery thermal management, and they have set up a prototype system to test its suitability for scaled-up Li-ion batteries inEVand HEVapplications. They proposed the PCM-based battery cooling system, operating as a passive thermal management system, is relatively simple, therebypromising appreciable cost reduction compared to activecooling systems. They concluded that PCM based thermal management systems are most effective under cold ambient condition and in space applications [2]. Sabbah R et al. conducted a comparative study between active and passive thermal management systems during normal and stressful discharge protocols for high power Li-Ion battery packs. They have found that in a high discharge rates and high ambient temperatures, adequate active cooling system requires air flow rates close to or within turbulent range, which is not practical for vehicle applications. The parasitic fan power required to run the active cooling system and failing to maintain uniform temperature across the battery pack seems to be the drawbacks compared to PCM based cooling with uniform heat distribution [3].
II. OBJECTIVE AND METHODOLOGY The authors have previously built a thermal management system for Li-Ion battery pack for an electrical vehicle (Fig 1). The study was focused on numerical study to analyze the active cooling system [4]. Tan later extended the work by experimental investigation to design an efficient active air cooled thermal management system (Fig. 2) [5]. In the present work a numerical analysis is carried to investigate the possibilities of passive cooling system using PCM. The main objective of the current work is to identify the suitable PCM or PCM composite which can be adapted to be used in a passive thermal management system, replacing the active (forced air) thermal management system.
Fig 1. Battery module arrangement within the EV boot [4]
Fig 2. Active thermal management system by Tan L. W. [5]
Numerical Analysis of Pcm Based Thermal Management System For Li-Ion Battery Used In Hybrid And Electrical Vehicles
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A. Active air cooling During the previous study [4,5], it was noticed that to keep the battery modules within its operating temperature and to maintain the uniform distribution of heat within the battery pack, it is necessary to keep higher flow rate or air with more ventilation and fans. It was also required to apply two-channel system for more effective and evenly distributed heat, as seen in Fig. 3.
Fig 3. Two dedicated channel flow for each row of battery
modules for improved thermal management [5]
Fig 4. Temperature profile of battery pack with (a) 3
fans@30CFM and (b) 4 fans@90CFM [4] B. Pure PCM Phase change materials (PCM) have high latent heat of fusion, capable of removing large quantity of heat. It acts like a heat sink to absorb the heat generated by the
li-ion battery during high discharge conditions. PCM block can be molded with the shape and size of the battery cell/module and each cell/module are inserted into the cavity so that the battery body has physical contact with the PCM as show in Fig 5 [6].
When temperature of the battery module exceeds the melting point of the PCM, it starts to melt and high latent heat of PCM prevents the battery temperature from rising sharply. This kind of thermal management eliminates the use of manifolds, fan or pumpwhich is essential for active air thermal management system [7]. General requirements of well selected PCM would be nontoxic, non-corrosive, stable, large latent heat and low cost [8]. Thermo-physical properties of PCM considered by various researchers are tabulatedin Table 1.
Table1: Thermo-physical properties of PCM (Paraffin)
Properties
Values [10
]
Values[3
]
Values [11
]
Values [12
]
Values [13
]
Values [9]
Density (kg m-3)
880
866 804.3
810
866
866
Specific Heat, Cp (J (kg K)-1)
2000
1980
2384
2250
1770
1770
Thermal Conductivity
0.236 (W/m K)
16.6 (W/m2 K)
0.15
(W/m K)
0.2 (W/m K)
0.2 (W/m K)
0.25
(W/m K)
Latent heat (J kg-1)
134200
181000
184480
270700
270700
195000
In order to select a suitable PCM with the range of melting point from 42°C to 49°C, various composition of paraffin are available which are listed in Table 1. One drawback of paraffin PCM is its low thermal conductivity, due to which the absorbed heat accumulates within the PCM causing high temperature buildup during heavy duty applications like hybrid and
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electrical vehicle. Many research works were conducted to enhance the thermal conductivity of the commercial PCM [6]. It can be achieved by introducing PCM composites with extended graphite, nano-metal particles, or by using combination of PCM with metal foam or porous metal structure or by metal fins and honeycomb structures [13]. C. PCM Composites Many researchers worked on PCM/Graphite composite to enhance the thermal conductivity of PCM. Thermo-physical properties of the PCM and extended graphite (EG) composite are listed in Table 2. Andrew Mills and Al-Hallaj proposed an ideal value for the PCM/EG composite with desired properties that could be most efficient. D. PCM/Metal foam structure Some researchers worked on metals combined with PCM for thermal conductivity enhancement. Most commonly used metals were Aluminum and Copper, as shown in Table 3. In some cases metal fins and honeycomb structures were inserted into the PCM bulk to provide better thermal conductivity. In other cases, metal foams or porous metal structure filled with PCM (most commonly paraffin) at its molten stage. A mixture of nano particles with PCM in an appropriate mass fraction has also been tried as PCM/Metal composite.
Density (kg m-3) 1623 1232 Specific Heat, Cp(J (kg K)-1)
1980 2500
Thermal Conductivity
11.33 (W/(m2 K))
43.8 (W/(m K))
Latent heat (J kg-1) 123000 62700
Effective Thermal conductivity and density of PCM with solid and liquid values can be calculated using the expressions (1) and (2) [9]: (1)
(2) For metal forms with porosity coefficient ε, the effective specific heat, thermal conductivity and density of metal foam (eg. Aluminum) and PCM (eg. Paraffin) is calculated using equations (3) – (5).[2c]
(3)
(4)
(5)
Considering the density of Copper as 8940 kg/m3[18], the calculated value of effective density of copper porous and PCM is 1623 kg/m3. III. CFD SIMULATION In the present work, computational fluid dynamics (CFD) simulations were conducted to analyze the various PCM and its composites. ANSYS 14 FLUENT was used as the CFD tool. Mesh was generated using ANSYS meshing tool with 81540 elements.. The heat generation rate was considered to be 25000 j/m3 with 3A discharge rate [3]. Four Li-Ion cells per each module were considered, similar to the authors’ previous work [4].. There are in total thirteen modules to support the complete electrical drive. For the current work only six modules are considered for analysis. Four sets of simulation were conducted based on (i) pure PCM, (ii) PCM/Graphite composite, (iii) PCM/Copper composite and (iv) PCM/Aluminum composite. For an initial testing, only one row of battery modules were considered, and successful results can be applied to the second row of battery modules without any specific conditions. Some of the assumptions made during the simulation are stated bellow. These assumptions were considered based on similar research works [8,15]. a. Density change due to solid-phase change is negligible b. The specific heat and thermal conductivity of the PCM, Composite and Battery module are constant c. The heat dissipation through radiation is neglected d. The viscosity of the composite CPM is assumed to be infinity since there is no liquid motion e. PCM is homogeneous and isentropic f. Melting point of the PCM is a constant value rather than a range
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Fig 6. Temperature profile for PCM composites
Fig 7. Liquid fraction of PCM, in various PCM composites
IV. RESULTS AND DISCUSSION Initially the temperature profiles were compared to analyze the mechanism of heat absorption and dissipation. In the Fig 6, it can be noticed that the Aluminum and Copper based PCM composites are showing better temperature distribution. Analysis of the liquid fraction of PCM depends on the thermal conductivity of the composite. Lower thermal conductivity will result in higher thermal resistance, thus heat builds up within the battery kit causing increase in PCM melting.
Fig 8.Static Temperature for each battery module under
various PCM composites. Graphite based PCM also demonstrated better results compared to pure PCM but temperature distribution is not even across the module. Due to the lack of thermal conductivity, heat accumulates in the center core causing more melting of PCM. From Fig 8 it can be noticed that the pure PCM has highest temperature for each battery module, the main reason is due to accumulation of heat within the PCM causing it to over-heat. PCM can absorb the heat until it melts, after which if the heat is not dissipated, it will work like thermal storage causing increase in temperature. PCM composites have shown much more reduction in the temperature of battery module, mainly due to the increase in conductivity. PCM/Aluminum composition demonstrated the lowest temperature among the composite rivals. CONCLUSION In the current work, numerical simulations were conducted to analyze the performance of various PCM
Numerical Analysis of Pcm Based Thermal Management System For Li-Ion Battery Used In Hybrid And Electrical Vehicles
Proceedings of 3rd IRF International Conference, 18th May-2014, Hyderabad, India, ISBN: 978-93-84209-18-6
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Composites in comparison with pure PCM. Paraffin PCM seems to be one of the best alternative passive cooling media for battery thermal management, which is simple and cost effective. The only known drawback of Paraffin is its low thermal conductivity to disperse the heat absorbed from the battery modules. Thermal conductivity of PCM needs to be improved by means of adding metal, graphite foam or nano-metal particles. It was noticed that the metal composites are performing better than the graphite composites due its higher thermal properties. Appropriate mass fraction for the composition of PCM with either metal or graphite is more specific to the type of battery and cell/module layout. An extensive study is to be carried out to have better efficiency in the thermal management of specific battery system. REFERENCES
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