Analysis of heat losses from the cabin of a battery electric vehicle

Abstract

Thermal comfort and air quality are crucial for the efficient operation of Battery Electric Vehicles (BEVs). Unlike Internal Combustion Engine (ICE) vehicles, which use the engine waste heat energy in their heating and cooling systems, BEVs rely solely on electrical energy stored in traction batteries, making thermal management essential for their driving range. Achieving suitable thermal comfort and air quality in a BEV cabin has therefore a greater impact on driving range compared to ICE vehicles. This study aimed to understand heat transfer effects in a car cabin and their impact in energy-efficient climate control in BEVs. Computer Fluid Dynamics (CFD) simulations were conducted to investigate heat transfer within a BEV cabin during a cool-down scenario. The study focused on a simplified geometry resembling the boundaries of an actual car cabin, considering factors such as the value of total air mass flow rate, conduction, convection, solar loads, air mass flow distribution, and driver presence. Results showed that during a summer night drive (no solar loads) at 32 km/h and 43ºC ambient temperature, cooling the cabin with a total air mass flow rate of 68.5 g/s and 7ºC inlet air temperature lead to average air temperature variations of ±2ºC inside the cabin, considering different levels of insulation. On a summer day, considering moderate and intense solar loads, variations of ±5ºC on the average air temperature were observed. Applying the total air mass flow rate only at the top air inlets increased the average temperature by 1ºC but improved passengers’ local temperature distribution. In order to achieve thermal comfort inside the car cabin, at least 120 g/s total air mass flow rate was required in an intense solar load scenario, whereas only 95 g/s was required in a moderate solar load scenario. At least 15 g/s total air mass flow was required to achieve proper ventilation of carbon dioxide in the presence of the driver. A time-step of 1 s was sufficient to capture the overall time evolution of temperature at probe points of interest. The study concluded that achieving thermal comfort and proper air quality inside a simplified car cabin required careful consideration of the total air mass flow rate. The analysis of heat fluxes at cabin boundaries and the energy consumption of the HVAC unit further contributed to understanding the system. By gaining insights into heat transfer mechanisms and improving thermal management strategies, BEV manufacturers can enhance cabin comfort, air quality, and overall energy efficiency, ultimately maximizing the driving range of these vehicles.