Thermal management system issues for smart electric vehicles

introduction

In a car, since the ambient temperature faced by the car is in a very wide range, the thermal management of the car is a big issue. The starting point of automotive thermal management is more inclined to "no thermal damage risk to each component". The performance, cost and development trend of each component are different. In the design, it is necessary to consider the thermal damage risk of components under harsh working conditions and consider low load conditions.The efficiency of the thermal management system , energy dissipation in both hot and cold conditions and improved thermal efficiency. This article discusses the design and material issues from the perspective of components and materials.

01   New demands raised by    smart electric vehicles

                       
1) Application of lithium batteries in the automotive industry

Lithium battery is an important energy source. As mentioned above, it can be divided into 12V battery, 48V battery, HEV high voltage battery, PHEV battery and BEV battery in the car. These batteries gradually increase from 3.6Ah to more than 200Ah. From the perspective of thermal management, it can be divided into passive heat dissipation, liquid cooling and refrigerant direct cooling.

12V battery: auxiliary power supply, high current, requires passive cooling 

48V and HEV batteries: Energy recovery, large current, passive cooling and air cooling can be used in the design, and some companies are considering liquid cooling 

Power battery: drives the vehicle, power output continuity, the current mainstream choice is to use active cooling 

Figure 1 Application of lithium batteries in automobiles 

In a car, the operating temperature range of lithium batteries is limited, as shown in the figure below. In both low and high temperature zones, temperature will affect the operating characteristics of the battery, especially affecting battery life in the high temperature zone, and working in the low temperature zone may even affect safety. 

Figure 2 Working area of ​​lithium battery

Therefore, the thermal design of the electric vehicle's power lithium battery pack is a key technology to ensure the reliable operation of the battery, and how to conduct the heat of the battery cell is the core design consideration. Usually, the liquid cooling heat dissipation part of the power battery pack is composed of a liquid cooling tube and a thermal conductive adhesive. The liquid cooling tube contains the internal coolant, which is important for dissipating the heat generated by the lithium-ion battery pack. The thermal conductive adhesive completes the heat conduction between the battery cell and the liquid cooling tube. Thermal conductive adhesive is currently widely used in new energy electric vehicles.Thermally conductive material that can form a soft, elastic and surface-adhesive silicone elastomer through an addition-curing reaction at room temperature, while also having excellent electrical insulation properties.

Figure 3: Conductivity of thermal conductive adhesive 

2) Use of power electronics 

In the process of developing high power density electronic technology for electric vehicles, the core challenge is to manage the heat generated by power electronics (inverter DC-AC, DC transformer DCDC, charger ACDC) under high load. Through practice, the use of thermally conductive potting compounds in this regard is a fast and effective way to conduct heat from power components to the heat sink. For power electronic devices, the operating temperature of semiconductors also has a ceiling, and the high temperature area will affect the life span.

Figure 4 Heat conduction in power electronics 

3) The core computing computer of smart cars 

In the design of vehicle computing platforms, in order to ensure the real-time requirements of autonomous driving, it is necessary to ensure that the maximum delay of software response is within an acceptable range, and the requirements for computing resources have become extremely high. The computing platform of the future will be heterogeneous, and will use a CPU+GPU+FPGA computing platform, which requires a lot of power for computing. Therefore, in the future, we will start to weigh computing power and power consumption, and we will need to use thermal conductivity technology to remove heat as quickly as possible.
 

Figure 5 Computing requirements for future onboard computers

02 Thermally conductive materials play a role in these applications

In this design, it is necessary to select good thermal management interface materials according to different requirements.

1) What materials can be used?

At present, the following materials are available: 

a) Silicone is one of the most versatile chemicals that maintains its inherent elasticity over a wide temperature range (-75°C to +200°C). Silicone products are the first choice when considering flame retardancy, high temperature resistance and permanent elasticity in design. 

b) Epoxy resin has the advantages of high strength, wide application, high durability, strong adhesion, chemical corrosion resistance and high temperature resistance. 

c) Polyurethane is used in applications where high temperature resistance is not required. Polyurethane works best in low temperature applications, protecting sensitive electronic devices and being waterproof. In fact, these different materials were first used in different fields and then slowly expanded to automobiles.

Here we first look at silicone materials. In this regard, we have to mention Dow, which is a world leader in the field of silicone materials and provides more than 7,000 silicone products and related services to more than 25,000 customers worldwide. Dow's thermally conductive silicone materials have a wide range of viscosities, curing speeds and thermal conductivities, which can meet the growing demand for thermal management in the design of electronic products in almost all industries. The growth of the thermal conductive materials business is accompanied by the growth of customers and the industry. In recent years, with the rapid development of smart cars and new energy vehicles, vehicles have become increasingly dependent on power batteries and electronic devices to achieve various functions. In response to the growing demand for thermal conductivity of automotive power batteries and electronic devices, Dow has developed a series of thermal conductive solutions with higher performance, reliability, service life and more cost-effectiveness.

Dow Silicones' expertise lies in more than 70 years of accumulated technology and industry experience. High-quality upstream raw materials ensure the stability and reliability of downstream products. The product line covers different types of thermal conductive products such as thermal conductive fillers, thermal conductive adhesives, thermal conductive potting glues and gels, thermal conductive greases, etc. In addition, Dow Silicones can quickly adjust the performance of thermal conductive products such as viscosity, curing speed and thermal conductivity according to the specific needs of customers, thereby developing innovative products to meet the needs of different customers. 

Taking 2.0/m·kW thermal conductive gap filler as an example, previously, thermal conductive gap fillers with the same thermal conductivity often required a density of 2.7g/cm3. Now, with the support of the localized technical team and global technical experts, the newly developed 2.0/m·kW thermal conductive gap filler TC-5515 LT has a density significantly reduced by 28% to 1.95 g/cm3, thus meeting the lightweight needs of industries such as new energy vehicles. In addition, the TC-5515 LT product can currently meet the 3000-hour reliability aging test requirements, which is three times the basic industry requirements.

2) Application of materials to vehicles

In fact, the original division of labor between automobile, parts and materials companies is gradually improved through product-level requirements. The vehicle level is mainly through pre-defined requirements, perfect test requirements and after-sales iterative feedback to gradually meet the requirements. Parts companies, by breaking down the requirements, improve the requirements step by step, while material companies conduct matrix characteristic evaluation of these requirements in terms of performance, and finally the iterative chain gradually improves a subsystem. This is a process of collaboration and improvement. Due to the long service life of automobiles, the content related to quality, reliability and life will be difficult to handle if it is discovered in the later stage, and the overall processing cost is relatively high, so this set of division of labor and collaboration methods has been widely used.

Figure 6: Gradually improving the subsystems of the car

Summary: As the industry sets a battery life target of 1 million kilometers in 10 years or even 2 million kilometers in 15 years, battery thermal management will play an increasingly important role in the future.