Comparison of air cooling vs liquid cooling in electric vehicle battery.

Why EV Batteries Need to Be Cooled

EV Batteries have specific operating ranges, which are critical for the battery life and performance. They are designed to operate at ambient temperature, which is between 68°F and 77°F (20°C and 25°C). A better control over the battery temperature improves their performance and life.

• During operation, they can withstand temperature between -22°F and 140°F (-30°C and 50°C)
• During recharges, they can withstand temperatures between 32°F and 122°F (0°C and 50°C)

Batteries generate a lot of heat during operation and their temperature must be brought down within operating ranges. At high temperatures (between 158°F and 212°F, or 70°C and 100°C), thermal runaways can occur, causing a chain reaction that destroys the battery pack.

During fast charges, batteries must be cooled down. This is because the high current going into the battery produces excess heat that must be extracted to preserve the high charging rate and not overheat the battery.

They sometimes also need to be heated up when the temperature is too low or to boost performances. For example, cells cannot be charged below 32°F (0°C). Or, companies like Tesla offer battery preheating in some models to reach high performances, going from 0 to 60 mph in less than 2 seconds.

Thermal Management Challenges

The most common thermal management challenges for EV batteries are leaks, corrosion, clogging, the climate, and aging. As you will see, liquid cooling systems present challenges that are inexistent for air cooling systems.

• Leaks can only occur in liquid cooling systems, whose pipe connections have risks of leaks as the battery ages. Any leaks will rapidly degrade the battery performance and life. They can even cause the EV to stop operating if humidity attacks the battery’s electrical insulation. Battery modules, interconnections, pumps, and valves must all remain intact.
• Corrosion can only occur in liquid cooling systems, whose cold plates can corrode as the liquid glycol gets older. Therefore, the cooling liquid must be replaced as part of the vehicle’s maintenance.
• Clogging is a risk that is present in the hundreds of small channels where liquid travels in the battery.
• Climates around the globe pose different thermal challenges for batteries. Examples include leaving the car under heavy sun for a long time, or living in a place where there are extremely low temperatures in winter. Batteries must be able to tolerate wide temperature ranges at all times. To achieve this, the battery cooling system must be active even when the vehicle is not in use.
• Aging causes thermal management problems that must be planned for. As batteries get older, a larger portion of energy is loss as heat. The thermal management system must be built for these tougher conditions that occur later in the battery life, not just for typical conditions during the first years.

Examples of Battery Thermal Management Systems

Nissan

Air Cooling

Air cooling uses air to cool the battery and exists in the passive and active forms.

Passive air cooling uses air from the outdoor or from the cabin to cool or heat the battery. It is usually limited to a few hundred watts of heat dissipation.

Active air cooling gets its air intake from an air conditioner, which includes an evaporator and a heater to control the air’s temperature. It is usually limited to 1kW of cooling and can be used to cool or heat the cabin.

Active cooling is more complex and expensive but provides better performances such as propulsion and charging power. It is also more effective at removing heat from the battery, but it requires more energy to control the battery’s temperature. The difference between active and passive cooling is that passive cooling does not require any external system to operate, whereas active cooling involves the use of external devices or systems to cool the battery, such as fans, heat sinks, and cooling fluids (in the case of liquid cooling).

Tesla

Liquid Cooling

Liquid cooling is the most popular cooling technology. It uses a liquid coolant such as water, a refrigerant, or ethylene glycol to cool the battery. The liquid goes through tubes, cold plates, or other components that surround the cells and carry heat to another location, such as a radiator or a heat exchanger. Components carrying the liquid prevent direct electrical contact between the cells and the liquid coolant.

Because liquid cooling involves pumps, fans, and other devices to actively extract and redirect the heat, it is an active form of cooling.

Some thermal management systems use a direct-contact medium such as oil or other dielectric liquids that are directly in contact with the cells. This is mostly used in non-consumer EVs, as they are less safe and provide a less effective insulation between the cells and the surrounding environment.

Peltier cooling

Peltier cooling is a method of cooling that utilizes the Peltier effect, which occurs when an electrical current flows through two dissimilar conductors, causing one side to become hot and the other side to become cold. This method of cooling has been used in various applications, including electronic devices, medical equipment, and even electric vehicles. In this blog, we will explore the use of Peltier cooling in an electric vehicle and its potential benefits and drawbacks.

How Peltier Cooling Works:

Peltier cooling utilizes a thermoelectric module, which is made up of two different semiconductor materials that are sandwiched together. When an electrical current flows through the module, one side becomes hot, and the other side becomes cold. The cold side of the module is then used to cool the object or component that requires cooling.

Benefits of Peltier Cooling in an Electric Vehicle:

Peltier cooling can provide several benefits when used in an electric vehicle. Firstly, it is a solid-state cooling solution, meaning that it doesn’t require any moving parts, such as a compressor or fan. This makes it more reliable and requires less maintenance than other cooling systems.

Secondly, Peltier cooling is more energy-efficient than other cooling systems, such as air cooling or liquid cooling. This is because it utilizes the electrical energy from the vehicle’s battery to power the thermoelectric module, which is much more efficient than converting mechanical energy into cooling power.

Thirdly, Peltier cooling is compact and lightweight, making it ideal for use in electric vehicles where space and weight are critical considerations.

Drawbacks of Peltier Cooling in an Electric Vehicle:

Despite its potential benefits, Peltier cooling does have some drawbacks when used in an electric vehicle. Firstly, it is less efficient at dissipating heat than other cooling systems, such as liquid cooling. This means that it may not be suitable for use in high-performance electric vehicles or in hot climates.

Secondly, Peltier cooling can be expensive compared to other cooling systems, making it less accessible for some electric vehicle manufacturers.

Lastly, Peltier cooling can generate a significant amount of heat on the hot side of the thermoelectric module, which can lead to increased temperatures inside the electric vehicle. This can be mitigated by using additional cooling systems or by designing the electric vehicle’s layout to maximize airflow.

What are Phase Change Materials?

PCMs are materials that can absorb and release heat as they change from one phase to another, such as from solid to liquid or liquid to gas. During this phase change, the material absorbs or releases a large amount of latent heat, which can be used to regulate temperature in various applications, including battery cooling.

How do Phase Change Materials Work for Battery Cooling?

In a battery cooling system that uses PCMs, the material is typically located in a container adjacent to the battery cells. As the battery cells heat up during operation, the PCM absorbs the excess heat and undergoes a phase change from solid to liquid. This process of absorbing heat is known as the endothermic phase transition.

Once the PCM reaches its melting point and becomes a liquid, it can absorb additional heat without significantly increasing in temperature. This is known as the sensible heat absorption phase. The PCM then releases heat as it solidifies back into a solid state, which is known as the exothermic phase transition. The solid PCM can then absorb more heat again and repeat the process.

The best phase change material (PCM) for a battery pack depends on the specific requirements of the application. Some common PCMs used for battery cooling applications include:

Paraffins: Paraffins are hydrocarbons that have a high latent heat of fusion and a wide range of melting temperatures. They are commonly used in battery packs because they have good thermal stability and are chemically inert.

Fatty Acids: Fatty acids are organic compounds that have a high latent heat of fusion and a melting point close to room temperature. They are commonly used in battery packs because they have good thermal conductivity and are biodegradable.

Salt Hydrates: Salt hydrates are inorganic compounds that have a high latent heat of fusion and a melting temperature close to room temperature. They are commonly used in battery packs because they have a high heat absorption capacity and are non-flammable.

Glycerol: Glycerol is a viscous liquid that has a high latent heat of fusion and a melting point close to room temperature. It is commonly used in battery packs because it is non-toxic, non-flammable, and has good thermal conductivity.

Phase Change Emulsions: Phase change emulsions are mixtures of water and a PCM that have a high latent heat of fusion and good thermal conductivity. They are commonly used in battery packs because they are cost-effective and have a wide range of melting temperatures.

Ultimately, the best PCM for a battery pack depends on the specific requirements of the application, such as the desired temperature range and cooling efficiency, as well as the cost and availability of the PCM. It is important to carefully consider all of these factors and evaluate different PCM options to determine the best choice for the battery pack.

Benefits of Using Phase Change Materials for Battery Cooling

There are several benefits to using PCMs for battery cooling:

High Heat Absorption Capacity: PCMs have a high latent heat of fusion, which means they can absorb a large amount of heat during the phase transition without significantly increasing in temperature.

Passive Cooling: PCM-based cooling systems require no additional energy input to operate, making them a passive cooling solution.

Compact Design: PCM-based cooling systems can be designed to be compact and take up less space than other cooling solutions.

Safe: PCMs are non-toxic and do not pose a safety risk, making them a safe cooling solution.

While phase change materials (PCMs) offer many benefits for battery cooling, there are also some disadvantages to consider:

Limited Temperature Range: PCMs have a specific melting and solidification temperature, which limits their effectiveness in applications where the temperature varies widely. If the temperature exceeds the melting point of the PCM, the material will no longer absorb heat, and the cooling system will lose effectiveness.

Thermal Conductivity: PCMs generally have lower thermal conductivity than traditional cooling fluids, such as water or oil. This can reduce the efficiency of heat transfer and require larger PCM containers to achieve the same cooling effect.

Limited Lifetime: Over time, PCMs can degrade or leak from their container, reducing their effectiveness as a cooling solution. This requires careful design and maintenance to ensure the PCM remains functional throughout the life of the battery.

Cost: PCMs can be more expensive than traditional cooling fluids, which can increase the overall cost of the cooling system.

Complexity: The use of PCMs in a cooling system requires careful consideration of the materials and design of the container and heat exchangers to ensure efficient heat transfer and prevent leakage.

Blog by —

Shreyash Karekar , Tejas Khandekar , Viraj Killedar ,Akshay Koli , Kshitij Kularni .

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