Advances in Thermal Management of Powertrain Components in Electric Vehicles

At the heart of an EV is the powertrain, which mainly comprises: the battery, the inverter, the motor, and the mechanical transmission system. Each of the aforementioned components of the powertrain system has an efficiency less than 100%. The inefficiencies in these components manifest themselves mainly in the form of thermal losses that need to be removed by a thermal management system (TMS). The main role of the TMS is to maintain optimal or close to optimal operating temperatures for the powertrain and the vehicle cabin. The performance of the TMS is crucial to the efficiency, performance, reliability, safety, and customer acceptance of an EV. This paper reviews some of the main advances in the thermal management of the powertrain components in an EV. This includes the thermal management of: the battery, the motor, the inverter, and the gear box. The paper also investigates the state of the art in oils. The thermal management techniques considered in the paper include: gas cooling, liquid systems (both indirect and direct), refrigerant systems, phase change materials (PCM), heat pipes, thermoelectric systems, and others. In general and as outlined in the paper, each of the different TMS techniques considered has its own challenges. Mainly, air cooling suffers low thermal properties and noise, indirect liquid cooling suffers complexity and added thermal resistance; refrigerant systems suffer highly coupled controls and the growing restrictions on non-natural refrigerants, PCM suffers thermal and mechanical challenges (low thermal conductivity, form stability, leakage, material compatibility, and others), immersion cooling suffers from material compatibility and extra weight, thermoelectrics suffer from low coefficients of performance (COP), and heat pipes suffer integrability and durability challenges. However and based on this review, room exists for hybridization (i.e. combining two or more of the above mentioned techniques and others). Examples are combining liquid cooling with heat pipes, or thermoelectrics. Also, the capability to isolate a part of the power train to be on a standalone cooling system (e.g. heat pipe-assisted air cooled system) will create an extra degree of freedom in the overall TMS system, which opens doors for more overall system optimization. Refrigerant systems will potentially see some limitations in spreading to multiple components (e.g. to the battery or the motor), because of the trend of shifting to natural refrigerants (where the two most obvious candidates are: CO2 which faces high pressure challenges and the consequent need for thicker piping, and propane that has flammability and consequent charge limitation constraints). PCM and immersion cooling will need to overcome their packaging challenges before seeing their way to production.