The improvement of a battery thermal management system for an electric vehicle

Vosloo, Christoffel Jacobus (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: The increasing cost of fossil fuels, along with the growing concern over climate change, has sparked renewed interest in the research and development of Electric Vehicles (EVs). This thesis focusses on the utilization of Thermosyphon Heat Pipes (THPs) for the purpose of improving an existing Battery Thermal Management System (BTMS) of an EV. The current battery pack utilizes aluminium plates that are compressed between the Lithium-ion (Li-ion) cells, to serve as the BTMS. These plates have protrusions extending into a cooling channel, to allow heat transfer to the cooling air via forced convection. Problems with heat transfer and temperature uniformity arise, since these protrusions (or fins) are orientated perpendicular to the flow direction. When air is forced over these fins, a low pressure wake is created after the first set of fins, compromising the heat transfer from the fins behind it. It was decided to implement a THP BTMS that could replace the aluminium plate BTMS without requiring a reconfiguration of the battery pack. Copper fins were brazed to the condenser section in order to increase the heat transfer surface area. These fins were orientated parallel to the flow in order to improve the uniformity of the heat transfer from the fins. It was decided to use methanol as the working fluid as it vaporises easily at low temperatures. The filling ratio of the methanol was calculated to be 30 %. To compare the new BTMS with the old version, an analytical model was developed of the two BTMSs. These models were improved by incorporating the results of CFD simulations and verified experimentally. To assist in the numerical modelling of the THPs, the THPs were modelled as solid superconductors. The thermal conductivity of these superconductors was calculated to be 3032.9 W/m·K. To conduct the experiments an existing test bench, similar to the existing battery pack, was modified to accommodate four Li-ion cells along with three THP sections. This small scale experiment was used to determine the thermal characterization of the Li-ion cells and the two different BTMSs, as well as verify the results of the CFD simulations. The experiments were performed for two cases. For the first case the cells were cycled through charging and discharging cycles, in order to increase the cell temperature. After 4 hours the cooling fan was switched on in order to cool down the cells. For the second case the cooling fan was switched on from the start of the first charging cycle. This allowed the cell temperatures to reach a steady state throughout the subsequent charging and discharging cycles. The utilization of the THP BTMS resulted in a 16 % increase in heat transfer coefficient with a 589 % increase in total heat transfer rate. This resulted in a 5 % decrease in steady state cell temperature. This shows the potential of using heat pipe technology for the cooling of EV battery packs.

AFRIKAANSE OPSOMMING: Die toenemende koste van fossielbrandstowwe, tesame met kommer oor klimaatsverandering, het hernude belangstelling in die navorsing en ontwikkeling van Elektriese Voertuie (EV's) teweeg gebring. Die fokus van hierdie tesis is beperk tot die gebruik van Termohewel Hittepype (THPe) om ‘n bestaande Battery Termiese Bestuur Sisteem (BTBS) te verbeter. Die huidige batterypak gebruik aluminiumplate wat saamgedruk is tussen die litium-ioon (Li-ion) selle, as die BTBS. Hierdie plate het vinne wat in 'n verkoelingskanaal uitsteek om hitte-oordrag via geforseerde konveksie moontlik te maak. Probleme met hitte-oordrag en temperatuur eenvormigheid ontstaan, aangesien hierdie vinne loodreg op die vloei gerig is. As lug oor hierdie vinne geforseer word, vorm 'n lae druk zone na die eerste stel vinne wat die hitte-oordrag van die agterste vinne negatief beïnvloed. Daar was besluit om 'n THP BTBS te implementeer wat die aluminiumplaat BTBS kan vervang sonder om die uitleg van die batterypak te herontwerp. Kopervinne is op die kondensor seksie gesoldeer om die hitte-oordrag oppervlakte te vergroot. Hierdie vinne is parallel met die vloei gerig om die eenvormigheid van die hitte-oordrag vanaf die vinne te verbeter. Daar is besluit om metanol as werkvloeistof te gebruik, aangesien dit maklik verdamp by lae temperature. Die vulverhouding van die metanol was bereken as 30 %. Om die nuwe BTBS met die ou weergawe te vergelyk, was 'n analitiese model van die twee BTBS'e ontwikkel. Hierdie modelle is verbeter deur die resultate van CFD-simulasies in die modelle te inkorporeer en eksperimenteel geverifieer. Om die numeriese modellering van die THPe te vergemaklik, was die THPe as soliede supergeleiers gemodelleer. Die termiese geleiding van hierdie supergeleiers was bereken as 3032,9 W/m·K. Om die eksperimente uit te voer, was 'n bestaande toetsbank, soortgelyk aan die bestaande batterypak, aangepas om 4 Li-ioon-selle te akkommodeer tesame met 3 hittepyp seksies. Hierdie kleinskaalse eksperiment was gebruik om die termiese karakterisering van die Li-ion-selle en die twee BTBSe te bepaal, asook om die resultate van die CFD-simulasies te verifieer. Die eksperimente was vir twee gevalle uitgevoer. Vir die eerste geval was die selle deur laai- en ontladingsiklusse gesirkuleer om die seltemperatuur te verhoog. Na 4 uur was die verkoelings waaier aangeskakel om die selle af te koel. In die tweede geval is die verkoelings waaier van die begin van die eerste laaisiklus af aangeskakel sodat die temperatuur 'n konstante toestand kon bereik regdeur die laai- en ontlaaisiklusse. Die gebruik van die THP BTMS het gelei tot 'n 16 % toename in hitte-oordragskoëffisiënt met 'n 589 % toename in die totale hitte-oordragstempo. Gevolglik lei dit tot 'n 5 % afname in die stabiele batterytemperatuur. Die resultate dui op die potensiële voordele in die gebruik van hittepype vir die verkoeling van EV batterypakke.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108179
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