As a starting point, it has to be emphasized that single-phase forced convection BTMS are estimated to reach heat transfer coefficients of the order of 100-500 W/m2K [1], [2], which are deemed as inadequate for keeping the EV battery-pack temperature rise in the order of 5-10°C, as required for next-generation vehicles at high charge/discharge rates.
The vast majority of commercial EVs currently employ BTMSs based on air, indirect liquid cooling with water/glycol and phase-change (either solid/liquid or liquid/vapor) cooling configurations and variations/combinations of them [3], [4]. Air-cooled BTMSs come with different geometric layouts aiming to maximize cooling capacity and flow efficiency [5] with the relevant technology being primarily borrowed by other applications (CPU, integrated circuits, photovoltaics). Liquid BTMSs can be further categorized into direct and indirect cooling layouts. In the former case, a dielectric liquid is in direct contact with the battery, while in the latter one, the working medium, predominantly water or water/glycol mixtures, flows within a jacket or cold plates encompassing the module [6]. Two-phase, liquid/vapor BTMSs comprise primarily heat pipes, with this technology having been commercialized [2]. A range of working media with relatively low boiling points (below 65°C) have been proposed. Since heat pipes constitute indirect cooling systems, the fluid phase-change usually occurs in tubular ducts and, therefore the heat-transfer area with the cells cannot be maximized. The concept of realizing a flow layout resembling a heat pipe within the battery cell by taking advantage of a multi-purpose electrolyte has been demonstrated in [7], representing a cutting-edge, yet highly complex design. A two-phase system nebulizing water droplets into the air stream has been recently proposed by [8]. Few prototype cooling designs employing fluid boiling have also been proposed in the literature, comprising either forced convection of boiling flow in mm-sized ducts embedded in the cell module or immersed-cooling boiling systems [9]. Direct contact between the coolant and cells can be achieved by flooding the cells in a pool of dielectric liquid. The studies of [1], [10] are the only ones available in the open literature that demonstrate the feasibility of developing a direct-cooling boiling BTMS with hydrofluoroether (or its commercial alias Novec7000) as the working medium. Passive-cooling BTMSs are recently gaining momentum, as they do not require equipment to circulate the cooling medium, hence reducing the complexity of the system and avoiding pumping-power losses [11], [12]. Instead, they take advantage of the latent heat of vaporization required for liquid to vapor phase-change in order to extract heat from the battery modules. Within this last category of BTMSs, Phase Change Materials (PCMs), which absorb the surplus heat from the battery cells through melting/solidification processes, are currently being actively investigated. Paraffin mixtures and inorganic salts constitute typically used PCMs due to their low cost and non-hazardous nature, yet their performance is crucially limited by their poor-to-moderate conductivity (up to 0.4-0.5 W/mK) and therefore increased thermal resistance [11]. Hence, PCM-based BTMSs cannot cope with EV high-discharge rates. Several approaches have been attempted to alleviate this drawback through combining PCMs with thermally conductive materials, e.g. carbon fibers and nanotubes, graphene and nanoparticles, porous media and metal foams or meshes, expanded graphite matrix, as well as plate-fin heat sinks coupled to forced air convection [12].
References:
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[2] Smith, J. et al. Int. J. Therm. Sci. 134, 517 (2018).
[3] Al-Zareer, M. et al. Int. J. Energy Res. 42, 3182 (2018).
[4] Siddique, A. R. M. et al. J. Power Sources 401, 224 (2018).
[5] Sefidan, A. M. et al. Int. J. Therm. Sci. 117, 44 (2017).
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[9] An, Z. et al. Appl. Therm. Eng. 117, 534 (2017).
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[11] Bubbico, R. et al. J. Therm. Sci. Eng. Appl. 10, 061009 (2018).
[12] Lv, Y. et al. Int. J. Heat Mass Transf. 128, 392 (2019).