Analysis of the cooling system influence on the temperature non-

The influence of the cooling system on the temperature non-uniformity was analysed in a virtual test for the LTO pouch cell designs where the terminals were placed on the same side of the cell. In the cooling system, metal plates were used for the heat dissipation from the cell surfaces. The virtual module is shown in Fig. 4.11.

Fig. 4.11: Battery module used in a virtual test where the terminals are at the same end of the pouch cells on the top of the module.

The virtual battery module consists of ten LTO pouch cells, where the terminals are placed at the same end of the LTO pouch cell. The metal plates in the virtual module are placed between each two cells for the heat dissipation from the LTO pouch cell surface.

The metal plates are connected to the metal bottom of the battery module, to which the cooling systems can be attached. The LTO pouch cells in the virtual module are thermally insulated from the metal bottom plates to prevent direct connection between the cells and the bottom of the battery module. The use of the thermal insulation material decreased the temperature non-uniformity by 20%, according to the calculation in COMSOL Multiphysics 4.4. However, it also led to a 1% increase in the operating temperature. The other sides of the module can be covered by a plastic box for instance with a 10 mm wall thickness, which can be regarded as the thermal insulator in the modelling. The structures of the virtual battery module can be assumed to be close to commercial modules available in the market. In addition, because of symmetry, the module can be simplified so that only one cell and half of the metal plate are considered in the temperature calculation.

The temperature distributions were calculated by selecting aluminium and copper as the materials for the bottom plate and plates, which are placed between cells. An aluminium heat sink was considered in the calculation. The thickness of the plates that were placed between each two cells was varied from 1 mm to 6 mm. The temperature calculation was performed in COMSOL Multiphysics 4.4 by using the Heat Transfer in Solids interface.

Vacuum rubber was selected as the thermal insulation material, and the approximated heat losses were used in the LTO pouch cell thermal model. The thermal parameters of the elements used in the thermal model are given in Table 4.4.

Table 4.4: Thermal parameters of the virtual battery model elements.

Element of the model Density, kg/m³

Specific heat capacity, J/(kg∙K)

Thermal conductivity, W/(m∙K)

Aluminium plate 2700 870 205

Copper plate 8700 385 400

Vacuum rubber 910 2010 0.13

LTO pouch cell 2357.6 1024.6 37 (along-plane)

0.7 (through-plane)

The initial temperature of the battery module with the cooling system was 296.9 K. In the modelling, the heat dissipation from the cooling system can be provided by a forced air or forced liquid cooling system. The convection coefficient was chosen to be 100 W/(m∙K), which is typical for a forced air cooling system (Incropera, 2013). The result of the temperature distribution analysis, when the 2 mm aluminium and copper plates are used, is shown in Fig. 4.12.

Fig. 4.12: Pouch cell surface temperatures with aluminium (a) and copper (b) plates placed between two cells for heat transfer towards the cold base plate.

The maximum temperatures, maximum temperature difference and the standard temperature deviation (STD) in the sectional plane, which was shown in Fig. 4.7, are given in Table 4.5 for different thicknesses of the aluminium plates.

Table 4.5: Maximum temperature, temperature difference and STD for different thicknesses of the aluminium cooling plates.

Aluminium cooling plates thickness, m

Maximum temperature, K

Maximum temperature difference, K

STD, K

0.001 332.1 16.8 5.4

0.002 325.4 13.8 4.4

0.004 319.1 10.5 3.3

0.006 316.0 8.8 2.7

The maximum temperatures, maximum temperature difference and the STD in the sectional plane, which was shown in Fig. 4.7, are given in Table 4.6 for different thicknesses of the copper plates.

Table 4.6: Maximum temperature, temperature difference and STD for different thicknesses of the copper cooling plates.

Copper cooling plates thickness, m

Maximum temperature, K

Maximum temperature difference, K

STD, K

0.001 326.5 14.6 4.7

0.002 320.0 11.3 3.6

0.004 3.14 8.3 2.6

0.006 312.3 6.9 2.1

The analysis of the temperature distribution in the LTO pouch cell showed a decrease in the maximum temperature difference and standard temperature deviation when the aluminium cooling plates were replaced by copper cooling plates in the battery module.

In addition, it was shown that an increase in the cooling plate thickness has a positive effect on the characteristics of the cooling system. However, an increase in the plate thickness increases the mass of the module. Therefore, this method to improve the cooling system is recommended for heavy applications, where the light weight of the battery system is not a decisive factor when considering the total mass of the working machine.

On the contrary, a heavy battery system may be beneficial, for example in heavy forklift trucks, where the battery system is also used to keep the balance of the truck when lifting loads.

When a low mass of the battery system is important, it is not possible to use thick cooling plates. In this case, even if copper is used as the material of the cooling system, the temperature non-uniformity is still very high for high C-rate currents. Therefore, a different method of heat dissipation from the pouch surface may be required.

4.2