Vehicle Thermal Management: Keeping Cool on the Road

The Crucial Role of Vehicle Thermal Management

In the ever-evolving landscape of automotive technology, particularly with the energetic development of new energy vehicles (NEVs), the focus on module integration and maximising battery energy density is paramount. However, this drive for innovation presents significant challenges, not least of which is the effective management of thermal conditions within the battery pack. As battery energy density increases, so too does the potential threat to safety. Lithium-ion batteries, prized for their superior energy density, power load, and cycle life, are the backbone of electric vehicles (EVs). Yet, during charging and discharging cycles, internal chemical reactions and ohmic resistance generate heat. This accumulation of heat can lead to a dangerous rise in battery temperature, and in severe cases, thermal runaway – a catastrophic event. The consequences of inadequate thermal management are starkly illustrated by incidents such as the overheating batteries in the Boeing 787 in 2013, a Tesla Model S fire in Norway in 2016, and an explosion in Florida in 2018. While mechanical failures or sealing issues can contribute to accidents, those stemming from overcharging, overdischarging, short circuits, or localised overheating are largely preventable through robust thermal management systems. Therefore, understanding and implementing effective battery thermal management technology (BTMS) is absolutely critical for high-power battery packs.

Traditional Approaches to Thermal Management

Historically, several methods have been employed to keep vehicle components, especially batteries, within their optimal operating temperature ranges. The primary goal is to dissipate excess heat or, at the very least, provide sufficient time for the system to react to overheating. Let's delve into some of the most common strategies:

Air Cooling: The Simplicity Advantage

Due to its inherent low cost and straightforward design, air cooling has been a prevalent method for battery thermal management. The system typically involves forcing ambient air over the battery modules, carrying away heat. However, the effectiveness of air cooling is significantly hampered by the relatively low thermal conductivity and specific heat capacity of air. This limitation becomes particularly apparent under strenuous operating conditions or in high ambient temperatures. To illustrate the difference, consider a comparative study where, under an 8C discharge/charge rate, an air-cooled battery pack experienced a 10°C temperature rise, whereas a liquid-cooled system under identical conditions only saw a 3°C increase. Furthermore, liquid cooling also demonstrated superior temperature uniformity across the battery pack.

Liquid Cooling: Efficiency Through Immersion or Circulation

While more complex than air cooling, liquid cooling offers significantly better thermal performance. Its efficacy stems from the high thermal conductivity of liquids, making it a widely adopted and efficient thermal management technology. There are two main configurations for liquid cooling:

• Indirect Cooling: In this approach, the coolant is kept separate from the battery cells, typically circulating through a jacket or a dedicated cooling plate. Heat is transferred from the battery to the coolant via convection or, in some advanced systems, boiling.
• Direct Immersion Cooling: Here, battery modules are submerged directly into a dielectric fluid, such as specialized cooling oil. This allows for heat exchange through conduction, offering an even more direct and efficient heat transfer path.

Phase Change Materials (PCMs): Harnessing Latent Heat

The potential of latent heat during phase transitions has also attracted considerable research interest for thermal management. Phase Change Materials (PCMs) are designed to absorb a substantial amount of heat during their melting process while maintaining a relatively constant temperature. Researchers have explored various PCM-based BTMS, including those utilising pure PCMs, nanosilica-enhanced PCMs, and other composite materials. The findings consistently indicate that PCMs can effectively control battery temperatures and improve temperature uniformity between individual cells. However, a significant limitation for long-duration or long-distance operation is the finite capacity of PCMs; once fully melted, their passive cooling effect diminishes. This makes them more suitable for applications with intermittent high heat loads rather than continuous, high-demand scenarios.

Heat Pipes: An Alternative Latent Heat Solution

Similar to PCMs, heat pipes leverage the large latent heat associated with the evaporation and condensation of a working fluid. They offer an efficient way to transfer heat away from a source. Heat pipes are often integrated with other cooling methods, such as air, liquid, or PCM systems, to enhance overall thermal management performance. While effective, the incorporation of heat pipes can add complexity to the overall system design.

Refrigerant-Based Systems: Direct and Active Control

A more compact and sophisticated approach to thermal management involves the use of refrigerants, similar to those found in air conditioning systems. In these refrigerant-based BTMS, the refrigerant is circulated directly through cooling plates integrated within the battery pack. As the refrigerant absorbs heat from the battery, it undergoes a phase change (evaporation), effectively removing thermal energy. This method offers several key advantages over traditional systems:

• Reduced Losses: By eliminating the need for a secondary coolant loop (as in some liquid cooling systems), heat exchange losses and exergy losses are minimised.
• Proactive Control: The process is more initiative and controllable, allowing for precise management of battery temperatures.
• System Simplification: Integrating the cooling directly can lead to a more streamlined and compact overall system.
• Enhanced Safety: Refrigerants typically have lower electrical conductivity than water-based coolants, reducing the risk of short circuits in the event of a leak.

Investigating Refrigerant-Based BTMS Performance

Numerous studies have explored the capabilities of refrigerant-based BTMS. For instance, a numerical model developed by Park et al. [14] analysed the impact of refrigerant temperature and mass flow rate on battery cooling performance. Their findings indicated that refrigerant temperature had a more significant influence on battery temperature characteristics than mass flow rate. Furthermore, when compared to passive PCM-based systems, refrigerant-based BTMS demonstrated superior cooling performance, especially under cyclic operating conditions.

Brotz et al. [15] simulated a refrigerant-based BTMS with varying cooling plate thicknesses. Their research showed that even a 0.5 mm thick cooling plate could maintain temperature differences across a stack of 40 flat cells within 3 Kelvin, while a 1 mm plate resulted in only a 1 Kelvin rise. Thinner plates offer benefits in terms of space, refrigerant usage, and compressor power, while also simplifying the overall structure by potentially eliminating the need for intricate cooling channels.

Cen et al. [16] investigated the performance of these systems with cylindrical battery modules under various driving conditions. Under extreme ambient temperatures (40°C) and discharge rates of 0.5–1.5C, temperature differences remained within 4°C. During standard driving cycles, this difference reduced to 1.5°C. The study also highlighted the importance of refrigerant circuit configuration, noting that inlet and outlet placement on different sides of the battery pack led to a 6 K worse temperature difference compared to placement on the same side.

Krueger et al. [17] modelled a refrigerant-based BTMS integrated with an air-conditioning cycle. They analysed the behaviour of different refrigerants, R134a and R1234yf, under varying weather conditions. R1234yf showed performance advantages in milder weather, while R134a excelled in hotter conditions, attributed to differences in their Coefficient of Performance (COP) at lower compressor speeds. Their work also qualitatively suggested that the BTMS operation can influence cabin temperature, particularly in high ambient temperatures.

Rahman et al. [18] developed an intelligent fuzzy control method for refrigerant-based BTMS, focusing on energy consumption. Their results demonstrated effective temperature control, maintaining average battery temperatures between 20–45°C under high-speed conditions. Crucially, the intelligent control strategy showed potential for significant energy savings, with the refrigerant-based BTMS consuming approximately 23% less energy than an air-cooled system under similar operating parameters.

Integrating Thermal Management with Vehicle Systems

It is important to note that many studies focus solely on the battery component and its immediate thermal environment. However, a comprehensive understanding requires considering the impact of the BTMS, including the energy demands of its components like compressors, on other vehicle systems, such as the traditional air conditioning (AC). The novelty of some advanced work lies in the systematic and dynamic study of refrigerant-based BTMS performance, examining its interaction and optimisation with the AC system. This involves simulating BTMS performance under diverse driving conditions, ambient temperatures, and AC operating modes. By coupling battery thermal management and air conditioning models within the context of an electric vehicle system, researchers can analyse the integrated management effects during scenarios like hot soaking and high-speed cycling. The ultimate aim is to refine control strategies and system configurations to mitigate the coupling effects, optimise overall system performance, and enhance the holistic vehicle thermal management.

Why a 3D Thermal Management Model?

Tools like GT-SUITE are at the forefront of thermal management analysis, enabling the assessment of crucial parameters such as pressure drop, flow distribution, and overall system performance. Its advanced solvers can adeptly handle various fluid types, including coolants and refrigerants, within a single, integrated model. The powerful 3D modelling capabilities allow for the rapid creation of both 1D flow networks and underhood airflow simulations directly from CAD geometry. These thermal management models can be seamlessly integrated with vehicle and powertrain simulations, facilitating comprehensive energy management optimisation. Using 3D models provides a more accurate and detailed representation of complex geometries and flow paths, leading to more precise predictions and optimised designs.

Grayson Thermal Systems: Pioneers in Net-Zero Solutions

Companies like Grayson Thermal Systems are instrumental in driving the transition towards a net-zero future. With over 45 years of experience, they design, test, manufacture, supply, and service innovative thermal management systems for a global clientele across diverse sectors, including bus and coach, commercial vehicles, defence, off-highway, rail, special vehicles, and stationary power. Their dedication to innovation, precision engineering, collaboration, and customer service ensures that they deliver advanced solutions capable of performing in the most demanding environments, supporting the development of next-generation zero-emission applications.

Frequently Asked Questions (FAQs)

What is the primary function of a vehicle thermal management system?

The primary function is to maintain critical components, particularly the battery pack in electric vehicles, within their optimal operating temperature range to ensure safety, efficiency, and longevity.

What are the main types of cooling systems used in EVs?

The main types include air cooling, liquid cooling (indirect and direct immersion), phase change material (PCM) cooling, and refrigerant-based cooling.

Why is liquid cooling generally preferred over air cooling for EV batteries?

Liquid cooling offers significantly higher heat transfer coefficients and specific heat capacities compared to air, resulting in more effective cooling and better temperature uniformity, especially under high load conditions.

What are the advantages of refrigerant-based thermal management systems?

Refrigerant-based systems offer direct and active temperature control, reduced system complexity, potential for higher efficiency by eliminating secondary coolant loops, and enhanced safety due to lower electrical conductivity of refrigerants.

Can thermal management systems impact vehicle range or performance?

Yes, an inefficient thermal management system can lead to reduced battery performance (e.g., lower power output at high temperatures) and potentially reduced range if the battery operates outside its optimal window. Conversely, an optimised system contributes to better efficiency and sustained performance.

What is the role of phase change materials (PCMs) in thermal management?

PCMs absorb significant amounts of heat during their melting process at a relatively constant temperature, providing passive cooling and helping to maintain temperature uniformity within battery packs. However, their effectiveness is limited once fully melted.

In conclusion, the sophisticated management of thermal conditions within vehicles, especially electric ones, is not merely a feature but a fundamental requirement. As technology advances, the integration of efficient, reliable, and intelligent thermal management systems will continue to be a cornerstone of automotive innovation, ensuring safety, optimising performance, and paving the way for a sustainable automotive future.

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