Phase change materials | Focus 39 calorimetry (DSC) to compare the exchanged heat or temperature development up to a reference sample during a test with the evolution of a preconfigured temperature. In most cases, highly conductive materials are made of metals or carbon-based compounds. Metal fins inserted inside a PCM casing, graphite foam, or metal foam might be the major approach. The technique is straightforward, and thermal conductivity may be significantly increased, up to 4500 percent of the PCM’s original thermal conductivity. Another approach is to add powder, nanotubes or nanoparticles to the PCM. Various nanostructures have shown different results, ranging from 7 to 1000% increase in the initial PCM thermal conductivity. However, producing the composites is difficult as it can be difficult to get a homogenous carbon nanostructure dispersion in the PCM. Open cell metal foam with improved thermal conductivity for the continuous skeletal structure, lower apparent density, homogeneously dispersed pores, and stronger structural strength are used to increase the thermal conductivity of the PCMs. One approach is to use pure paraffin with a copper or nickel foam to carry away the heat energy. For a copper foam with a pore size of 25 PPI (pores per inch) and a porosity of 88.89%, 92.31% and 96.95% showed thermal conductivity 44, 31 and 13 times greater than that of pure paraffin. A copper foam with 97% porosity could enhance the PCM thermal conductivity up to 5 W/m/K. An Expanded Graphite Matrix (EGM) is similar to a metal foam and has a similar high thermal conductivity, stable shape, low apparent density and porous internal structure and is easier to produce, although it is harder to produce a homogeneous structure. Safety A patented phase change composite (PCC) material using graphite has been tested in a thermal runaway of a 450 Wh lithium-ion battery pack. This has good heat rejection due to its high thermal conductivity (20 W m−1 K−1) compared to air (0.024 W m−1 K−1) or potting compounds (∼2.5 W m−1 K−1) typically used in packs of cylindrical cells. Experimental nail penetration studies on a Li-ion pack for small electric vehicles, designed with and without PCC, show the effectiveness of PCM thermal management for preventing propagation when a single cell enters thermal runaway. When parallel cells short-circuit through the penetrated cell, the packs without PCC propagate fully while those equipped with PCC show no propagation. In cases where no external short circuits occur, packs without PCC sometimes propagate, but not consistently. In all test conditions, the use of PCC lowers the maximum temperature experienced by neighbouring cells by 60 C or more. Adding PCM to the EV battery module or pack comes with its own challenges. The use of porous materials such as metal foams to store the PCM has been extensively studied but faces a challenge from the way the material is inserted into the pack and the volumetric change during phase change. Another possibility is to submerge the battery module or pack in a tight container filled with a PCM. Although this design likely produces a greater temperature uniformity, the wiring complexity and huge addition of weight are the main drawbacks. Solid to solid phase change There are also phase changing materials with phase change during solid to solid phase. Alteration in the crystal lattice of the material is the characteristic of such phase change. However, because the phase change temperature ranges from 80 to 180 C, the application regions are limited. Liquid phase change Liquid PCMs can also be used to carry the heat away from the battery cells in a pack, but this can require more complex engineering to gain the advantages of the phase change. E-Mobility Engineering | January/February 2024 The layout of battery cells and paraffin cells for cooling (Image courtesy of Research Center for Transportation Technology)
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