Electric vehicles thermal cooling systems

Electric vehicle (EV) car sales increased by 60% in 2022, surpassing 10 million for the first time, reports Quartz. Meanwhile, sales of conventional cars with internal combustion engines went down by a quarter in the past five years. As a result, one in every seven passenger cars bought globally in 2022 was an EV, according to a recent report by the International Energy Agency (IEA). In 2017, just one in every 70 cars sold was an EV.

This transition, and the current trend in the automotive market, show that consumers’ initial doubts regarding EVs systems and maintenance are lessening. Now, the most concerning problem that persists is the thermal management of electric vehicles. In conventional vehicles, thermal management is required to cool down the internal combustion engine. The thermostat, coolant, and radiator system remove heat from the engine. In electric vehicles, thermal management involves the cooling of batteries, power electronic systems, and the motor. But why is it so critical in electric vehicles?

Thermal Management in EVs

The thermal management in electric vehicles powered by batteries is important in that it affects the performance, reliability, and robustness of these vehicles. Electric vehicles need optimal temperatures (neither warm nor cold) to run efficiently. The optimum temperature is essential for the proper working of the battery pack, power electronic systems, and motor in the electric vehicle. When maintained at an optimal temperature, the battery charge, health, and capacity are preserved. Power electronic systems and motors showcase their best working profile under optimal temperatures.

Let’s take a look at a few reasons why thermal management is so important for electric vehicles.

Battery thermal management

The performance, service life, and cost of the battery packs and electric vehicles have a direct dependency. The availability of discharge power for starting and acceleration, charge acceptance during regenerative braking, and the health of the battery are at their best at optimal temperatures. As the temperature increases, the battery life, electric vehicle driveability, and fuel economy degrade. Considering the overall thermal effect of the battery on electric vehicles, battery thermal management is critical.

Thermal management of power electronic systems

Power electronic systems are responsible for controlling electric motors. Power electronic systems operate in line with the electric vehicle control system and drive the electric motor according to the control instructions. DC-DC converters, inverters, and control circuits in the power electronic system are vulnerable to thermal effects. While working, the power electronic circuits generate heat loss, and proper thermal management is essential to release the heat from the circuit and associated systems. If the thermal management is improper, it can result in control glitches, component failures, and vehicular mal-operations. Usually, the power electronic system is connected to the electric vehicle’s cooling system to maintain optimal temperatures.

Thermal management of electric motors

Since the wheel movement of electric vehicles is motor-driven, the working temperature of the electric motor is critical to the performance of the vehicle. With increasing load, the motor draws more power from the battery and heats up. The cooling of the motor is necessary for its full performance in electric vehicles.

Cooling loop in EVs

For a high level of efficiency in electric vehicles, optimal temperature maintenance is essential. The optimal temperature is regulated by the cooling system of the electric vehicle. Usually, the cooling system regulates the vehicle temperature, which includes the battery pack temperature, power electronic-based drive temperature, and motor temperature. In the cooling loop, a coolant is circulated using an electric pump to cool the batteries, electronics, motor, and related systems. In electric vehicles, radiators are used in the cooling loop to release heat to the ambient air. The air conditioning system is used in electric vehicles to cool down the systems within the cooling loop and evaporators are incorporated to remove heat from the cooling loop.

The influence of thermal resistance on heat transfer and thermal management in electronic packages

In an attempt to improve the performance, reliability, and cost of electronics, it is advisable to incorporate a greater number of components or circuits into a single enclosure or package. As circuits are limited to smaller spaces with higher power density, heat generation and dissipation are major concerns for designers. The integration of several circuits into a single package such as a PCB challenges thermal management techniques and heat transfer in electronic devices.

For the thermal analysis or heat transfer analysis of integrated electronic packages, thermal resistance is an important parameter, as it plays a significant role in implementing a cooling mechanism. One key criterion that must be satisfied during the cooling of electronic devices is that temperatures must be kept below the maximum allowable limits in worst-case operating conditions such as thermal resistances, coolant flow rate, and module power.

The different levels of thermal resistance

In electronic packages, heat gets transferred from the component junction to the ultimate heat sink. Total thermal resistance determines the junction temperatures in the heat transfer path. The total thermal resistance of an electronic package can be classified into three levels according to the heat transfer path.

Classification levels are:

  • Component level - At the component level, there is an internal thermal resistance, denoted by Rint. It is the resistance to the flow of heat from the junction or any other circuit element to the outside surface of the component case.
  • Package level - External thermal resistance, expressed by Rext, is at the package level and offers resistance to the heat transfer from the surface of the case to some reference point. This reference point can be ambient temperature, the edge of a PCB, or a liquid-cooled cold plate.
  • System level - System level thermal resistance is the final stage of thermal resistance. This stage focuses on heat transfer from the coolant to the heat sink.

PCB thermal management techniques

PCB thermal management is a complex undertaking involving the analysis of components, materials, layouts, spacing, and electrical interference. There is no single answer for how to manage heat in PCBs, and as new types of PCBs are created, it’s reasonable to assume that new guidelines for thermal management may be necessary. However, here are a few ways used to manage heat in PCB.

  • Carefully select and distribute components
  • Test for thermal shock resilience
  • Active cooling with water-glycol or oil type coolants

The transition to oil in EV thermal management

While the optimal operating temperature for the battery is similar to a human's (~15-30°C), the motors and power electronics are higher, with operation often above 60°C. This generally means that the motors and inverter are on a separate cooling circuit to the battery, although these can interact to transfer heat between them for optimal vehicle efficiency. How the heat is dealt with within the motor varies between manufacturers, with the options generally being segmented into water-glycol and/or oil-cooled motors.

A water jacket is a commonly used method where water-glycol coolant flows in a jacket around the outside of the stator. This helps cool the copper windings in the stator that generate the electric fields used to drive the rotor. Some have adopted alternate water-cooling geometries. However, the major limitation to water glycol is its electrical conductivity; this limits its use such that it cannot be used in direct contact with electrical components. This is where oil cooling comes in.

Traditional combustion vehicles are very accustomed to being lubricated with oils in the transmission. This can also be true in an EV, but the oil can also be used within the electric motor to directly cool the rotor or stator windings. This can be done in a few geometries and the water jacket may remain, but the overarching benefit is that the direct contact means that heat can be removed from the internal components of the motor more effectively, and the oil also provides lubrication. If the water jacket is eliminated, this can also lead to a smaller, and hence, more power-dense motor. In the first half of 2022, motors with oil cooling became the dominant form in the electric car market, taking 50% market share.

The downside of oil cooling is the addition of extra components and typically, the water-glycol circuit still exists to remove the heat from the oil and interact with the rest of the vehicle's thermal system. Despite this, the performance benefits are outweighing the complexity.

While oil cooling is now the dominant thermal strategy for electric motors, the inverter Si IGBTs or SiC MOSFETs are almost always cooled by water-glycol cold plates on one or both sides of the inverter modules. However, there has been some interest in directly oil cooling the inverter. As the inverter is typically packaged alongside the motor in a drive unit, one could imagine that removing the need for a water-glycol loop within the drive unit would simplify the drive system and still provide the benefits of direct oil cooling within the motor and inverter.

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