Experimental investigation of subcooled flow boiling in annuli with reference to thermal management of ultra-fast electric vehicle charging cables

Abstract

Transportation industry is presently in fast track to transition from Internal Combustion Engine Vehicles (ICEVs) to Electrical Vehicles (EVs). One of the most pressing challenges to full adoption of EVs is very slow charging at the networks of charging stations proposed worldwide. Despite many recent so-called ‘ultra-fast’ charging methods, which capitalize on a variety of single-phase liquid schemes to cool the charging cable, thermal constraints limit the electrical current carrying capacity of the fastest commercial chargers to about 500 A. Achieving the faster charging time required for the anticipated proliferation of EVs will require increasing this current capacity to at least 2000 A, which poses formidable thermal challenges in design of the charging cable. This study explores the development of a vastly more powerful charging cable thermal management scheme to achieve this higher current threshold. Subcooled flow boiling is proposed as the primary means to dissipating the larger amounts of heat generated at higher currents. Experiments are performed by pumping highly subcooled dielectric liquid HFE-7100 though a concentric circular annulus mimicking a segment of an actual cable, with a uniformly heated 6.35-mm-diameter inner surface representing the electrical conductor and adiabatic 23.62-mm-diameter outer surface the external conduit. All experimental cases considered are conFig.d to ensure subcooled fluid conditions throughout the test module. It is shown the proposed cooling scheme is capable of tackling currents up to 2438 A, around four times higher than the present-day commercial maximum. With appropriate batteries and other ancillary components, this technology is expected to bring EV charging times down to less than 5 minutes. Aside from demonstrating this potential, an assessment of available subcooled boiling heat transfer coefficient correlations identified Moles and Shaw's to predict the new experimental data with an overall mean absolute error of only 11.68%. The flow and heat transfer physics are also explained in detail.