Advantages of Single-Phase Liquid Immersion Cooling Technology

More technological progress and the widespread use of lithium-ion (Li-ion) batteries for storing electricity will depend on the creation of effective thermal management systems. The demand for quicker charging and discharging rates, higher amp-hour capacities, longer service lives, and higher levels of safety in both fixed battery arrays and electric vehicle batteries is growing.

Better and more secure thermal management systems are essential for all of these. There are efficiency constraints associated with either conventional air cooling or indirect liquid cooling, which are discussed in detail in this article. Recent trials conducted by Engineered Fluids show how effective single-phase liquid immersion cooling (SLIC) technology is for Li-ion battery temperature controls.

Ⅰ. What is air cooling?

By directing cool or room-temperature air through the battery cell or module's outer layer — which may have cooling fins or other ways to make the module's surface area bigger — the heat generated inside can be released into the surrounding air mass.

Ⅱ. Challenges in Air Cooling

Heat capacity of air (Cp) = 1.006 kJ/kg K at standard temperature

Due to the extremely low heat capacity, a significant amount of air is required, especially in situations where there is a small temperature differential between the battery module casing and surrounding air.

Another problem with air cooling is that batteries can only be physically placed in areas with available airflow or where air can be directed since the heat transfer surface of the battery needs to be exposed to air.

This can necessitate adding more infrastructure and volume to the system to guarantee that the entire mass of air required to cool the system is available.

New application requirements for batteries and evolving battery chemistries pose challenges to this technique. The following are some battery application needs that call for more effective thermal management:

1. Quicker cycles of charge and discharge

2. Shorter intervals of rest between cycles

3. Longer lifespan

4. Higher voltages for the batteries

5. Upon discharge and throughout the battery's service life, a more stable, consistent voltage output must be maintained.

These are the requirements for thermal management that modern electric vehicle battery systems impose, which are difficult to accomplish using air cooling.

Ⅲ. What is water cooling?

To reduce the temperature of the battery cells, a water/glycol solution is circulated through a heat sink or "cold plate" located inside the battery or battery module.

Ⅳ. Challenges of Water Cooling

Water/glycol heat capacity (Cp) = 3.777 kJ/kg K at 26.7 °C

Water and glycol coolants are effective in removing heat from the heat sink and have a very high capacity. This efficiency is lessened, though, because heat must be transferred through the fins or the jacket or container's walls.

Also, for the heat sink to properly transfer heat to the cold plate or jacket, it must be thermally attached to the full outermost layer of the battery cell walls and tabs. Any remaining air spaces within the battery cells function as heat insulation, quickly reducing the system's cooling effectiveness.

This is because many battery modules are made up of a lot of pouch or cylindrical cells. So, very precise aluminum frames and cold plates are often used, along with epoxy bonding agents or dielectric thermal grease, to make sure there are no air gaps between the framework and the battery cells' surfaces. Because of the system's total complexity, these systems might have high failure rates, low yield rates, and high fabrication costs.

A lot of manufacturers and users have stressed the fact that in aqueous indirect cooling systems, no aqueous solution gets on the batteries, wiring, or other electrical parts and causes them to short. This could result in thermal runaway in the affected cells or harm to the user because of the high amperage and voltages involved.

Aqueous indirect cooling systems for Li-ion batteries must be reliable so that no aqueous solution comes into contact with the batteries, wiring, or other electrical parts.

The speed at which heat moves from the battery module walls to the water jacket, heat sink framework, and cold plate, which does not conduct electricity, depends on the difference in temperatures and how well the different materials in each layer conduct heat.

Ⅴ. What is direct immersion cooling?

Single-phase liquid immersion cooling (SLIC) refers to the process of cooling battery cells by completely submerging them in a dielectric coolant. In most designs, a pump is used to recirculate the dielectric coolant throughout the system, keeping it in continual contact with the cell walls, tabs, and wiring of the battery module.

Ⅵ. Advantages of Direct Immersion Cooling

The thermal conductivity, density, viscosity, and velocity of the dielectric coolant are the variables that affect how quickly heat is transferred across the walls of the battery cells and the coolant.

Dielectric coolants heat capacity =1.3–1.4 kJ/kg K

For the same volumetric flow rate, heat transfer is much more efficient with dielectric coolants than with air. This is because the boundary layer is smaller and the thermal conductivity of the liquid is higher. Compared to air, a dielectric coolant can have a substantially lower flow rate because of its high heat capacity. Because liquid coolants are more thermally conductive and have a larger heat density than air, they are excellent cooling media.

Direct immersion in liquid single-phase dielectric coolants (SLIC) is supposed to provide the best results among these options in terms of keeping battery cells and modules within the proper temperature range with the least amount of temperature variation, as well as the lowest cost and system complexity.

Since the dielectric liquid coolant comes in direct contact with every part of the system, there is no longer a need for a heat sink framework in the middle, complicated manufacturing processes to keep the cells in place, or thermal bonding.

Dielectric SLIC coolants are safer than other conductive coolants and water glycol in a number of ways. For example, they can be in direct contact with all system parts without increasing the risk of cell thermal runaway, electronic short circuits, or user shock.

Using food-grade, non-toxic, biodegradable dielectric liquid coolants can also help with the safety and environmental problems that come up when dangerous aqueous glycol coolants leak.

Dielectric fluids were not accessible until now with an appropriate combination of properties such as compatibility profile, environmental impact, and fire safety. AmpCool Dielectric Coolants were developed in 2017 as a secure and trustworthy way to enhance and improve the value of today's battery systems.

Direct immersion in a single-phase, nonconductive cooling fluid is the most efficient cooling method for batteries. A comparative analysis of different types of cooling systems was summarized as shown in Fig. 1.

Fig. 1. Comparative analysis of different types of cooling system Source: IEEE Open Journal of Vehicular Technology

Ⅶ. Summarizing the Key Points

● Effective thermal management systems are crucial for optimizing the performance and safety of lithium-ion batteries.

● Conventional air cooling and indirect liquid cooling methods have limitations in meeting the increasing demand for faster charging and discharging rates, higher capacities, longer service lives, and improved safety.

● Single-phase liquid immersion cooling technology offers several advantages over traditional cooling methods, including better temperature control, lower cost, and reduced system complexity.

● The latest advancements in thermal management techniques have the potential to revolutionize battery cooling and improve the overall efficiency and reliability of battery systems.

● The adoption of advanced thermal management systems is essential for the widespread use of lithium-ion batteries in various applications, including electric vehicles, renewable energy storage, and consumer electronics.

Ⅷ. Reference

Sundin, David W., and Sebastian Sponholtz. “Thermal Management of Li-Ion Batteries With Single-Phase Liquid Immersion Cooling.” IEEE Open Journal of Vehicular Technology 1 (2020): 82–92. https://doi.org/10.1109/ojvt.2020.2972541.