36 January/February 2024 | E-Mobility Engineering PCM materials There are a number of phase change materials that are used in battery pack systems, from paraffin as a solid that changes to a liquid, to refrigerant liquids that change into a gas. The choice of the PCM of course leads to different design requirements. A solid PCM is easier to manage, as shown in the example with the thermal interface to a heatsink, but this makes use of the specific heat of the material. A liquid turning into a gas uses the latent heat of the material, which can absorb more thermal energy but a liquid and a gas need more complex engineering. Paraffin wax is seen as a suitable phase change material for charging and discharging of a battery cell with a melting point of around 67 C, which is low enough to keep battery cells in a pack at a low temperature. One investigation into the use of PCMs used a battery pack with cylindrical cells. The paraffin PCM is packaged in cylindrical cells that are the same size as the battery cells and placed in the pack in various patterns. This demonstrated the PCM’s capacity to regulate temperature as well as its practical use as a passive thermal management system for cells with no external power supply. Employing a PCM has also been shown to provide a longer battery life than without PCM. It improves the energy capacity as 90% of the nominal battery capacity is accessible, rather than 60% with air cooling. One experiment used three different placements of the PCM cylinders in between twenty 18650 battery cells, and the system was tested based on the measured temperature profiles inside the module under discharge rates of 1, 2, and 4C with charging currents of 50, 100, and 200 A, respectively. In general, all three configurations showed an average temperature of no more than 40 C and 55 C for 1 and 2C, respectively. In contrast, the 4C discharge test shows only two out of these three configurations could still maintain an acceptable temperature profile below 70 C. The first configuration, which offers the most straightforward design among the three, would not withstand any constant discharge rates higher than 1C. In such a case, it could be recommended to have a spreadout placement of the PCM tubes in the battery module. That is as in the third configuration, this to prevent the accumulation of dissipated heat around the battery cells, despite a possibly more complex wiring system and installation. In terms of the degree of temperature uniformity, the minimum values of all three configurations lie within the range of 0.60–0.79. With the discharge rate of 1C, all configurations could maintain the average temperature below 40 C. However, in a higher discharge rate, the end temperature between the three configurations is different. The average configuration temperature in A reaches around 45 C for 2C discharge. For the 4C discharge the cut-off temperature was reached quickly so the measurement was stopped on multiple occasions. This also happened during repeatability tests, showing configuration A could not be efficiently used at the discharge rate of 4C or higher without exceeding the maximum temperature threshold. Composite cooling Due to the possible mixture of different carbon chains, paraffin tends to have a wide melting spectrum, which makes it more difficult to target the precise phase change temperature. A composite container for a battery module filled with a PCM wax material was also experimentally tested at various discharge rates. The average cell temperatures at 1C, 2C, and 4C discharge rates, respectively, might reach 38 C, 50 C, and 70 C in the absence of any heat-absorbing material. The temperature was noticeably lower with PCM present than with a conventional battery module. For instance, at 4C discharge rates, none of the battery cells inside the PCM-filled module were able to reach 70 C. However the PCM addition also Focus | Phase change materials Solid phase change material for thermal management (Image courtesy of Henkel)
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