Sunday 29 September 2019

Material Selection Considerations for Electric Vehicles (EV’s) - Thermal Management Systems

In this post, we will have a closer look at the key considerations for thermal management systems used in electric and hybrid cars. This can support an optimal selection of plastic materials.
Traditionally, in internal combustion engine cars a powertrain thermal management and a passenger cabin thermal management system are present. Here, it well understood which type of plastic materials can be used depending on the application requirements.

In EV’s, thermal management systems support additional systems such as:
• Lithium battery: Thermal management systems need to ensure operating temperatures of 40-45°C to maximize battery service life. Composing parts need to retain material properties after 6.000-10.000 hours to ensure safe handling. Thus, precise control of temperature deltas is crucial.
• Traction motor: Thermal management systems need to ensure operating temperature of coils up to 190°C to allow high torque at small size.
• Power electronics: For high power electronic controllers, liquid cooled systems are favored and plastic materials used in housing need to have a thermal conductive role.
• Hybrid EVs: Downsizing of the combustion engine leads to local hot spots, which thermal management systems need to be able to handle.

What are the market trends and emerging needs?

For internal combustion engines increased temperatures due to downsizing of engines are expected. Simultaneously, more and more turbocharging systems need to be used to compensate the missing engine performance. EV’s have a system temperature ranging from 70°C to 80°C. The latter makes polyolefins accessible for thermal management applications.
Furthermore, in battery EV’s an increased aging exposure to water glycol coolant from 3.000 up to 6.000 till 10.000 hours is expected. Thermal management systems need to be active during charging time as well as when the surrounding temperatures are extremely low. Also, when the EV is not operating, temperature is monitored and in case extreme temperatures are reached, thermal management systems have to be activated too.
All this leads to ongoing discussions at the OEM and Tier-1 level on the requirements, which can be summarized as follows:
1) Coolant fluid temperature: ranging from 80°C to 110°C
2) System pressures: can reach up to 3 bar
3) Increased lifetime: up to 10.000 hours
4) Use of dielectric conductive coolant fluids

New design challenges

As mentioned in the beginning, having additional systems such as the battery, traction motor and power electronics for monitoring tasks require higher complexity thermostat valves which need to fulfill a more precise control. Temperature deltas between one battery module and the next module can be as stringent as 1°C.
Since available space for battery module placement is limited, the design of thermal management systems must be compact. Long operation times (during driving, charging and parking) and chemical resistance of water glycol coolant fluids represent another design challenge. One of the most critical challenges is the material strength which includes the weldline strength, especially after long term aging exposure.
Weldlines represent always the weak point of the plastic application, since the connection is weakened due to random orientation of glass fibers in the connection area. In addition, weldline strength is further weakened by water glycol aging. Since we will be dealing with more complex parts, weldlines are unavoidable and need to be taken care of.

Can all this be handled by plastics? - Yes, but only with the proper material selection

Let us summarize the key considerations for our material selection and give suitable polymer examples:
1. Aging temperature: 120-150°C: Polyphenylene sulfide (PPS); 120-135°C: Polyphthalamide (PPA)
2. Aging time: 1.000 – 3.000 hours: PPS and PPA; > 6.000 hours: PPS or PPA based on required temperature.
3. Increased chemical degradation due to different coolants: PPS has best in class chemical resistance.
4. Dimensional stability for sealing tasks: PPS and PPA.
5. Secondary operations such as laser welding: PPA has good laser welding capabilities.
PPS and PPA show promising mechanical behavior even when exposed to high temperatures, water-glycol coolant and long aging time.
Figure 1 shows mechanical data of PPS after exposure to water-glycol coolant (Ryton® R-4-220BL, Solvay). Figure 2 shows the mechanical data of a suitable PPA for water-glycol applications (Amodel® A-1933 HSL, Solvay).

Figure 1: Mechanical data of PPS after exposure to water-glycol coolant (Ryton® R-4-220BL, Solvay).

Figure 2: Mechanical data of a suitable PPA for water-glycol applications (Amodel® A-1933 HSL, Solvay).

PPS and PPA: not all grades are equal

PPS has unbeatable chemical performance due to the benzene sulfide group in the backbone. When exposed to water-glycol coolant, the PPS polymer matrix can withstand long aging times. However, attention needs to be payed to the glass fiber sizing. The same applies to PPA grades as well.
Interfacial adhesion of the polymer to the glass fiber is achieved over the sizing which is coated onto the glass fiber. Standard glass fiber sizings are cleaved when exposed to glycol and thus the mechanical values drop. Therefore, special sizings are used when the final compound is exposed to glycol. When your application is exposed to coolants, using of glycol resistant glass fillers is a must. The compounds shown in Figure 1 and 2 use such glycol resistant glass fiber fillers.
Material recommendations
Table 1 shows a recommendation concerning material selection for EV’s thermal management system based on existing data.

Table 1: Comparison of PPS and PPA for EV thermal management systems
Comparison PPA and PPS for thermal management systems

Overarching, high performance and engineering plastics will find more and more applications in EV thermal management systems.
If you would like support in the material selection of thermal management systems (from polymer to supplier) for ICE and/or EV feel free to get in contact with me. We can discuss your project.

Thanks for reading & till next time!

Herwig Juster

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Thursday 19 September 2019

Plastics Part Design: Coefficient of Linear Thermal Expansion (CLTE) of 136 Polymers

Topic of this blog post is the coefficient of linear thermal expansion (CLTE).
CLTE is often presented with the letter “α” and is calculated using the following equation:

α = ΔL / (L0 * ΔT).

L0 is the length of the part at room temperature; ΔL is the length variation of the specimen when it is heated up, and ΔT is the temperature difference between start and end. More details can be found in the standard ASTM D696.

Polymers in applications such as bus bars, which are used in traction motors, battery modules and power electronics, need to pass thermal shock tests. In such tests, metal bars are overmoulded. Thermal cycles between -40°C (1 hour) and +150°C (1 hour) of the overmoulded bars are done. The cycles are counted until cracking of the polymer layer occurs. The similar the CTLE value of both materials and the better the elongation at break of the overmoulded polymer are, the easier the selected material will pass such tests.

In the table below you can find the maximum CLTE of 136 polymers. Furthermore, I added a factor which shows how similar the polymer is to copper in terms of thermal expansion. This is useful for overmoulding of copper elements.

You can add this table to your part design library. Here, you can find some more part design related data: continuous use temperature and thermal conductivity.

Thanks for reading & till next time!
Herwig Juster