3.1. Impact of Fill Thickness on Thermal Management Performance
In the composite phase change material battery thermal management module, the phase change material is filled around the battery monomer, and the thickness of the filling determines the quality of the phase change material used for the thermal management system, thus affecting the performance of the thermal management system. Too little phase change material will cause the heat generated by the battery pack to be unable to be fully absorbed, which will reduce the cooling performance; too much filling thickness will take up the space of the battery pack, which will reduce the overall energy density of the battery pack. In order to investigate the most suitable composite phase change material thickness, after a preliminary calculation of the phase change material filling amount, phase change cooling thermal management structures with 2 mm, 4 mm, and 6 mm phase change material filling thickness were designed for the simulation study.
Figure 9 shows the maximum temperature change curve of the battery pack under different phase change material filling thicknesses. From the figure, it can be seen that the maximum temperature of the battery pack is increasing in the pre-discharge period, and with the increase of the filler thickness, the rate of increase of the temperature of the battery pack decreases and the time for the phase change material to start the phase change process is delayed. The increase in the filling mass of the phase change material will use sensible heat to absorb more heat from the battery pack in the early stage, delaying the time to reach the melting point of phase change. At different filling thicknesses, the cell temperature reaches the phase transition melting point at 400 s, 550 s, and 680 s, respectively. Since the melting points of the phase change materials are the same, the latent heat of phase change continues to be used to absorb the heat generated by the battery pack after the phase change occurs, maintaining the maximum temperature of the battery pack unchanged. However, at the end of the discharge period, when the thickness of the phase change material is 2 mm, due to the insufficient filling quality of the phase change material, the phase change material completely melts and the latent heat of phase change is exhausted, so it cannot continue to absorb the heat from the battery pack, resulting in a rapid increase in the temperature of the battery pack. When the thickness of phase change material is 2 mm, 4 mm, and 6 mm, the maximum temperatures of the battery pack are 46.26 °C, 40.98 °C, and 40.67 °C, respectively.
Figure 10 and
Figure 11 show the cloud diagrams of the maximum temperature difference of the battery pack and the overall temperature distribution of the battery module, respectively. From
Figure 10, it can be seen that in the pre-discharge period of the battery pack, the maximum temperature difference increases continuously with the discharge time, and when the phase change melting point is reached, the phase change process makes the maximum temperature difference start to decrease rapidly. As the filling thickness of the phase change material decreases, the time to reach the melting point of the phase change is also accelerated and the start time of the phase change process is advanced, so the maximum temperature difference also decreases. When the filling thickness is 2 mm, 4 mm, and 6 mm, the maximum temperature difference is 0.57 °C, 1.39 °C, and 2.19 °C. As can be seen in
Figure 11, the overall temperature distribution of the battery pack is basically the same under the different filling thicknesses of the phase change materials, with the highest temperature in the middle of the battery pack, and then decreasing to the surrounding area. With the increase of the filling thickness of the phase change material, the overall temperature of the battery pack also decreases; the maximum temperature decreases by 5.28 °C when the filling thickness is increased from 2 mm to 4 mm; the maximum temperature decreases by only 0.69 °C when the filling thickness is increased from 4 mm to 6 mm.
Figure 12 shows the liquid phase rate distribution of the phase change module at the end of the discharge, from which it can be seen that the liquid phase rate of the phase change module with a filling thickness of 2 mm is already all 1, indicating that at this time, the phase change material has been completely melted and cannot continue to use latent heat to absorb the heat of the battery pack and loses its cooling performance, which leads to a rapid increase in the temperature of the battery pack at the end of the discharge. When the thickness of the phase change material is 4 mm, only part of the phase change material around the battery is completely melted and the phase change material far away from the battery pack has not melted, and at this time, there is still a large amount of latent heat of phase change to continue to absorb the heat of the battery pack. When the thickness is 6 mm, the overall distribution of the liquid phase rate is more uniform, only a small amount of the phase change material is melting, and the phase change latent heat utilisation is low, although the subsequent thermal management performance is strong. However, if the phase change material filling amount is too high, it will lead to an increase in the overall mass of the battery pack and a decrease in energy density. Comprehensive research shows that in the actual battery module placement space, a phase change material filling thickness of 4 mm results in the better overall performance of thermal management.
3.2. Thermal Conductivity Impact on Thermal Management Performance
The thermal conductivity of the phase change material determines the speed of heat diffusion from the battery pack to the surrounding area, which has a great influence on the phase change cooling performance. A single paraffin wax has a very low thermal conductivity, and the thermal conductivity of the phase change material can be improved by adding expanded graphite to the paraffin wax to prepare a composite phase change material, but the increase in the mass fraction of expanded graphite will also inevitably reduce the latent heat of the composite phase change material. Therefore, in order to investigate the effect of the thermal conductivity of composite phase change materials on the thermal performance and select the optimal range of thermal conductivity, six groups of composite phase change materials with thermal conductivities of 0.2 W/(m·K), 0.5 W/(m·K), 1 W/(m·K), 3 W/(m·K), 5 W/(m·K), and 10 W/(m·K) are selected in this paper for the simulation and calculation of phase change cooling performance. The evaluation and monitoring indexes of the thermal management performance are mainly the maximum temperature of the battery pack, the maximum temperature difference, and the average liquid phase rate of the phase change material, in which the liquid phase rate of the phase change material is mainly used to evaluate the subsequent thermal management performance.
Figure 13 shows the maximum temperature change of the battery pack under different thermal conductivities. From
Figure 13a, it can be seen that before the discharge time of 500 s, the maximum temperature of the battery pack gradually increases with the discharge time: with the increase of thermal conductivity, the rate of increase of the maximum temperature decreases. When the discharge time is about 500 s, the maximum temperature of the battery pack reaches 42 °C and basically remains unchanged, indicating that the phase change material mainly relies on the phase change sensible heat to absorb part of the heat of the battery pack in the early stage to make the temperature of the battery rise slowly; when the phase change material reaches the melting point of the phase change, the phase change process absorbs all the heat of the battery pack, and the temperature of the battery pack stays unchanged. However, in the late stage of discharge, the degree of consumption of the latent heat of phase change is different, and the maximum temperature of the battery pack under different thermal conductivities changes more obviously. When the thermal conductivity is 0.2 W/(m·K), the maximum temperature of the battery pack increases slowly. This is due to the low thermal conductivity of the phase change material, resulting in the slow diffusion of heat generated by the battery pack; the heat is all gathered around the battery pack, so that the phase change material around the battery pack is in a state of complete melting and cannot continue to cool the battery. As the temperature of the battery pack continues to rise, the phase change material away from the battery pack is in an unmelted state, unable to play a cooling effect, resulting in the low utilisation of the phase change material. When the thermal conductivity is 1 W/(m·K) and above, the maximum temperature of the battery pack remains basically unchanged after reaching the melting point of the phase change, and the maximum temperature decreases slightly as the thermal conductivity increases. As shown in
Figure 13b, when the thermal conductivity of the phase change material is increased from 0.2 W/(m·K) to 3 W/(m·K), the maximum temperature of the battery pack decreases by 2.8 °C, and when the thermal conductivity is increased from 3 W/(m·K) to 10 W/(m·K), the maximum temperature decreases by only 0.2 °C. Phase change materials with a high thermal conductivity can quickly diffuse heat generated by the battery pack to the surrounding area, thus effectively reducing the temperature of the battery pack. The higher thermal conductivity of the phase change material can quickly diffuse the heat generated by the battery pack to the surrounding area, thus effectively reducing the temperature of the battery pack and improving the cooling performance. However, too large a thermal conductivity has little effect on improving the performance of the thermal management system, and for composite phase change materials, increasing the thermal conductivity will inevitably lead to an increase in the cost of the material and a decrease in the latent heat of the phase change, so it is not necessary to increase the thermal conductivity to too high a level in the actual production of composite phase change materials.
Figure 14 shows the liquid phase rate distribution of the phase change material at different thermal conductivities at the end of the discharge. From the figure, it can be seen that with the increase of thermal conductivity, the phase change material liquid phase rate distribution is more uniform. When the thermal conductivity is 0.2 W/(m·K), all the phase change materials around the battery pack have completed the phase change process, and the liquid phase rate is 1. The liquid phase rate of the phase change materials in the periphery can all be 0, and only the phase change materials around the battery play a role. This is due to the low thermal conductivity of the phase change material, which leads to the heat generated by the battery pack only being accumulated around the battery pack and not being transferred to the peripheral phase change material. The PCM around the battery pack quickly completes the phase change process, and the phase change material thermal management system fails. This is also the reason why the temperature of the battery pack rises significantly at the end of discharge when the thermal conductivity is 0.2 W/(m·K). Although the overall liquid phase ratio of the phase change material is only about 0.4, the low thermal conductivity of the phase change material prevents the remaining latent heat of phase change from being utilised. At a thermal conductivity of 10 W/(m·K), the melting degree of the phase change material is very uniformly distributed and the overall liquid phase rate is around 0.4, with all phase change materials in a semi-molten state, none of them completing the phase change process, and still having good thermal management performance. With the increase of thermal conductivity, the liquid phase rate of the phase change materials gradually increases, and the melting rate is gradually accelerated, so that the heat generated by the battery pack can be quickly transferred to the peripheral phase change materials to reduce the temperature of the battery pack. It can also be seen from the figure, in the thermal conductivity from 3 W/(m·K) to 10 W/(m·K), that the phase change module liquid phase rate distribution is uniform, at this time continues to increase the thermal conductivity of the battery pack, and the heat diffusion impact is small.
The maximum temperature difference of the battery pack is also an important indicator for evaluating the cooling performance of the thermal management system, and the temperature difference is too large to make the performance difference between the battery monomers obvious, thus affecting the discharge performance of the whole battery pack.
Figure 15 shows the maximum temperature difference of the battery pack under different thermal conductivities. As can be seen from the figure, in the pre-discharge period of the battery pack, the maximum temperature difference gradually increases with the increase of the discharge time, at which time the phase change material absorbs the heat of the battery pack using sensible heat. In the discharge time of 500 s or so, the maximum temperature difference is suddenly decreased; at this time, it is the phase change material that reaches the melting point and starts the phase change process. The phase change material quickly absorbs the heat generated by the battery so as to improve the temperature uniformity of the battery pack. When the thermal conductivity of the phase change material is too low, the heat of the battery pack cannot be exported quickly and the maximum temperature difference of the battery pack increases. As the thermal conductivity of the phase change material increases from 0.2 W/(m·K) to 10 W/(m·K), the maximum temperature difference of the battery pack decreases from 2.1 °C to 0.5 °C. The temperature difference of the phase change material decreases from 2.1 °C to 0.5 °C and the thermal conductivity of the phase change material increases from 0.2 W/(m·K) to 10 W/(m·K).
Figure 16 shows the temperature distribution of the phase change material cooling module, and it can be seen that when the thermal conductivity is 0.2 W/(m·K), the heat generated by the battery pack cannot be absorbed by the peripheral phase change material due to the thermal conductivity being too low, and the temperature difference between the temperature of the battery pack and the external phase change material reaches 5 °C. The lower the thermal conductivity of the phase change material, the slower the heat transfer between the battery and the phase change material and inside the phase change material, which leads to the accumulation of heat in the battery pack and more uneven temperature distribution. As the thermal conductivity increases, the temperature difference between the battery pack and the phase change material decreases. At a thermal conductivity of 3.0 W/(m·K), the temperature difference between the battery pack and the phase change material decreases to 0.5 °C, at which time the heat from the battery pack can be quickly transferred to the phase change material, giving rise to thermal management performance.
Through the above study, it can be seen that the thermal conductivity of the phase change material in the range of 0.2~10 W/(m·K) can control the maximum temperature difference of the battery pack within 2.5 °C, and the cooling of the phase change material has a significant advantage in improving the temperature uniformity of the battery pack. However, too low a thermal conductivity will limit the thermal management performance of phase change materials, and too high a thermal conductivity is not very effective in improving thermal management performance. Therefore, considering the cooling performance of phase change materials, a composite phase change material with a thermal conductivity of about 3.0 W/(m·K) is selected as the best thermal management system.
3.3. Effect of Phase Change Melting Point on Thermal Management Performance
The melting point of composite phase change materials is an important parameter in determining the thermal management performance. In order to investigate the effect of the melting point of phase change materials on the thermal management performance, a group of composite phase change materials with different melting points were designed by selecting different grades of paraffin waxes for the simulation study. The thermal conductivity of this group of composite phase change materials was set to 3.0 W/(m·K) and the melting points were 32 °C, 34 °C, 36 °C, 38 °C, 40 °C, 42 °C, and 44 °C.
The melting point of the phase change material determines the time at which the phase change cooling takes effect and the maximum temperature of the battery pack: the lower the melting point of the phase change material, the earlier the phase change begins to occur.
Figure 17 shows the variation curve of the maximum temperature of the battery pack with the discharge time under different melting points. As shown in the figure, with the increase of discharge time, the maximum temperature of the battery pack increases continuously, and when the maximum temperature reaches the melting point of the phase change, the temperature of the battery pack stays near the melting point to maintain the same. The higher the phase change melting point, the longer it takes for the phase change process to begin, and the corresponding maximum battery temperature increases. At an ambient temperature of 25 °C, the melting point of the phase change material increases from 32 °C to 44 °C and the maximum temperature of the battery pack increases from 31.7 °C to 43.3 °C. Therefore, the lower the melting point of the phase change material, the lower the maximum temperature of the battery pack, and the maximum temperature remains essentially near the melting point of the phase change.
Figure 18 demonstrates the variation curves of the maximum temperature difference of the battery pack with discharge time at different melting points. As can be seen from the figure, with the increase of discharge time, the maximum temperature difference of battery packs under different phase change melting points firstly increases gradually at the same rate. Then, the temperature difference of the battery packs that first reach the phase change melting point decreases rapidly and basically remains unchanged, and the maximum temperature difference of the battery packs that have not reached the phase change melting point is still increasing. Therefore, the phase change materials with low melting points start the phase change process earlier, which will reduce the maximum temperature difference of the battery pack and improve the temperature uniformity of the battery pack. When the melting point of the phase change material is increased from 32 °C to 44 °C, the start of the phase change process is delayed by 200 s, and the maximum temperature difference of the battery pack increases from 0.39 °C to 1.15 °C. The thermal management system requires that the maximum temperature difference of the battery pack should not be more than 5 °C, and in the melting point ranges of the phase change materials studied above, the cooling of the phase change materials shows good temperature uniformity.
The melting point of phase change materials can significantly affect the maximum temperature of the battery pack. With the increase of the melting point of phase change materials, the maximum temperature of the battery pack increases, so the melting point of the phase change materials should not be higher than 45 °C. However, it is not true that the lower the melting point of the phase change materials, the better the performance of the phase change cooling, as when the melting point is too low, it will lead to the complete melting of the phase change materials prematurely and the loss of the thermal management performance in the late stage of the discharge of the battery pack. The lower limit of the melting point of the phase change material is also subject to the ambient temperature; in order to verify the thermal management performance of the phase change material in a high temperature environment, this simulation study selects a high temperature environment of 38 °C to explore the influence of different melting points on the thermal management performance.
As can be seen from
Figure 19, when the ambient temperature is 38 °C, phase change materials with melting points of 38 °C and below have lost their thermal management performance, and the maximum temperature of the battery pack continues to rise with a consistent upward trend, with the maximum temperature at the end of the discharge reaching 72.5 °C. Phase change materials with melting points higher than 38 °C continue to utilise the latent heat of the phase change to play a role in thermal management performance after the temperature rises to the melting point of the phase change in the early stage of discharge, and the maximum temperature remains at the melting point of the phase change. The maximum temperature remains near the melting point.
Figure 20 shows the liquid phase rate change curve of the phase change module, from which it can be seen that the liquid phase rate of phase change materials with a melting point of 38 °C and below has already reached 1 before the battery pack begins to discharge, indicating that at this time, due to the ambient temperature being higher than the melting point of the phase change, the phase change material has already completely melted and cannot absorb the heat of the battery pack, and also verifies the trend of the maximum temperature of the battery pack. When the melting point is 40 °C~44 °C, the liquid phase rate of the phase change material increases after reaching the melting point of the phase change. The higher the melting point of the phase change, the later the phase change starts, and the lower the liquid phase rate at the end of the discharge. Therefore, in a high temperature environment, when the ambient temperature exceeds the phase change melting point, the phase change material will be in a completely melted state and completely lose its thermal management performance. In summary, the melting point of phase change materials should not only consider the demand of battery pack temperature control but also need to be higher than the ambient temperature in order to give full play to the thermal management performance of the phase change material. So, when the melting point of the phase change material is higher than the ambient temperature of 40 °C, the thermal management performance is better.
3.4. Influence of Latent Heat of Phase Change on Thermal Management Performance
The latent heat of the phase change material is the key to absorb the heat from the battery pack and keep the temperature constant after it reaches the melting point of the phase change, and the size of the latent heat also determines the amount of heat that can be absorbed. In order to investigate the influence of the latent heat of phase change on the performance of thermal management, a set of composite phase change materials with different latent heats of phase change were set up, with latent heat values of 150 J/g, 170 J/g, 190 J/g, 210 J/g, 230 J/g, and 250 J/g, thermal conductivity of 3.0 W/(m·K), and a melting point of 40 °C. The maximum temperature of the battery pack in the process of discharging was monitored, as were the temperature differences and the liquid phase rates of the phase change module.
As can be seen from
Figure 21, in the pre-discharge period of the battery pack, the trend of the maximum temperature rise is consistent because the phase change materials in this group have the same thermal conductivity and melting point. When the phase change melting point is reached, the phase change material uses latent heat to continue to absorb the heat of the battery pack, and at this time there is a difference in the maximum temperature change of the battery pack. When the latent heat of phase change increases from 150 J/g to 250 J/g, the maximum temperature of the battery pack decreases by 0.5 °C. The higher the latent heat of phase change, the longer the time to maintain the temperature of the battery pack near the melting point of the phase change, which improves the cooling performance of the phase change material.
Figure 22 shows the variation of the maximum temperature difference of the battery pack. As can be seen from the figure, phase change materials with different latent heats of phase change in the pre-discharge period are basically the same in terms of their performance in improving the temperature uniformity of the battery pack. When the phase transition process starts, the maximum temperature difference of the battery pack decreases rapidly. The maximum temperature difference is 1.18 °C at the moment before the phase change occurs, and after the phase change starts, the maximum temperature difference of the battery pack decreases rapidly to about 0.3 °C, and then remains in a lower range. The thermal management performance of phase change materials with different latent heats of phase change is basically the same for the maximum temperature and maximum temperature difference of the battery pack.
Figure 23 shows the liquid phase rate changes of phase change material modules with different phase change latent heats during the discharge process. From the figure, it can be seen that phase change materials with different phase change latent heats reach the phase change melting point at the same time around 400 s, and thereafter the liquid phase rate of the phase change module increases continuously. The higher the phase transition latent heat, the more the phase change material absorbs the heat of the battery pack, and the growth rate of the liquid phase rate decreases. At the end of the discharge moment, when the phase change latent heat increases from 150 J/g to 250 J/g, the liquid phase rate decreases from 0.84 to 0.51, at which time the subsequent cooling performance of the thermal management system greatly increases.
Figure 24 demonstrates the cloud diagram of the liquid phase rate of the phase change material at the end moment of discharge under different phase change latent heats. It can be seen that with the increase of the phase change latent heat, the liquid phase rate of the phase change material decreases continuously, and the distribution is more uniform. When the phase change latent heat is lower at 150 J/g, the liquid phase rate of the phase change module is basically 1, indicating that at this time, most of the phase change module is in a completely melted state and the phase change latent heat is basically exhausted. At this time, the phase change material can no longer continue to absorb the heat of the battery pack to play a thermal management role. If the battery pack continues to be discharged, the maximum temperature will rise rapidly, which will easily lead to the risk of thermal runaway. As the latent heat of phase change continues to increase, the subsequent thermal management performance of the phase change material will also increase. When the latent heat of phase transition is 210 J/g, the liquid phase rate of the phase change material is basically at a low level and the liquid phase rate is further reduced by continuing to increase the latent heat of phase transition. In the expanded graphite–paraffin composite phase change material, the mass fraction of expanded graphite and paraffin determines the thermal conductivity and the latent heat of phase change. So, the latent heat of phase change should be increased as much as possible to improve the subsequent thermal management performance under the circumstance of ensuring that the thermal conductivity of the phase change material meets the requirements.