In today’s evolving landscape of electrified and hybrid power systems, efficient thermal management is no longer optional—it’s essential. Whether it’s for electric buses, trucks, or stationary power systems, cooling is critical to maintaining performance, extending component lifespan, and preventing overheating.
As power demands rise, so does the need for robust cooling systems capable of handling the increased heat generated by electric powertrains, batteries, and power electronics. However, not all cooling systems are the same. Cooling technologies for heavy vehicles and stationary power applications generally fall into two categories: active cooling and passive cooling.
In this Grayson Thermal Academy blog, we will break down what active and passive cooling systems are, how they work, and explore which type you may need for your application.
What is Passive Cooling?
Passive cooling relies on natural heat dissipation with minimal energy input (i.e. the fans blowing air or pumps to circulate water). It operates using the principles of heat transfer through conduction, convection, or radiation, when external conditions allow for it.
However, passive cooling is only viable when the ambient temperature is lower than or close to the target operating temperature. In such cases, heat can be naturally rejected into the surrounding air or environment.
How Passive Cooling Works
A typical passive cooling system includes components like heat exchangers, radiators, pumps, and fans.
Heat generated by an application’s powertrain components is transferred to a coolant fluid (water-glycol, for instance) and circulated through the system. The coolant moves through radiators or heat exchangers, where the heat is dissipated into the air, often with the help of electric fans to improve airflow.
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The benefits of passive cooling
In moderate climates, where the ambient temperature is below the target temperature of the coolant, passive cooling can offer some valuable benefits.
One of the biggest advantages is energy efficiency—since minimal power is required, passive cooling helps conserve energy, making it especially beneficial for electrified vehicles and machinery where reducing power draw can extend range.
Passive cooling systems also have fewer components, resulting in lower upfront costs and reduced maintenance over time. For examples, our Cooler Range uses brazed aluminium heat exchangers combined with electric fans to provide passive cooling for power electronics, traction motors, hydrogen fuel cells, and traditional combustion engines.
However, if the ambient temperature rises above the target operating temperature, passive cooling alone would no longer be sufficient to meet a system’s or space’s cooling requirements, and active cooling would be required.
Active cooling solutions go beyond natural heat dissipation by using external energy and more complex components to actively remove heat, making them essential when ambient temperatures are higher than a system’s target temperature.
These systems rely on powered mechanisms like compressors and refrigerant cycles to transfer heat out of the system, ensuring efficient cooling even in extreme environments or under heavy heat loads. A common example is a BTMS (Battery Thermal Management System), which can cool a high voltage battery pack to the target temperature (the set point) even when the ambient temperature is high.
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How Active Cooling Works
Active cooling typically involves refrigerant-based systems. A compressor circulates refrigerant through a cycle that absorbs heat from components and releases it into the environment. This enables active cooling to maintain a lower coolant temperature than the ambient air, ensuring optimal performance even when outside temperatures are significantly higher.
This is often the case with high-capacity components like batteries or vehicle cabins in hot weather. Heat is transferred to the refrigerant, which absorbs and carries it through a cycle where it is compressed, condensed, and eventually released through a heat exchanger or condenser.
Unlike passive cooling, which is limited by the surrounding ambient temperature, active cooling can maintain a system’s temperature well below the ambient air. This makes it indispensable in hot or tropical environments where the ambient temperature exceeds the system’s target temperature.
In addition to providing air conditioning for drivers and passengers, active cooling systems can also deliver precise, constant temperature control for powertrain components. For example, our Battery Thermal Management Systems (BTMS) constantly monitors and dynamically adjusts cooling requirements in real-time, based on parameters such as coolant flow, refrigerant flow, and fan speed. This ensures that temperature-sensitive batteries operate within their optimal range at all times, regardless of external conditions.
By contrast, passive cooling systems, which rely on natural heat dissipation, do not offer this level of precision and are less capable of managing rapid or significant temperature changes.
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Choosing Between Active and Passive Cooling
The choice between active or passive cooling depends on several factors, including ambient temperature, target coolant temperature, and coolant flow rate.
Passive cooling is often sufficient when the ambient temperature is lower than or close to the target coolant temperature, allowing natural heat dissipation to occur efficiently. However, when external conditions become more extreme—such as when the ambient temperature exceeds the system’s operating temperature—passive cooling alone cannot keep the system within the desired limits.
Coolant flow also plays a key role in this decision. High heat loads require greater coolant flow to transfer heat away from components effectively. If the flow rate is too low, even an active cooling system may not maintain the desired temperature. Conversely, a high flow rate may prevent the heat transfer to the coolant. Balancing flow rate is essential to achieving optimal cooling, especially in high-demand applications.
Ultimately, the choice between active and passive cooling depends on the specific conditions of your application. In milder climates and lower-load scenarios, passive cooling may suffice. However, for high-performance systems, hot environments and conditions, or where precise temperature control is required, active cooling would be more suitable.
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Finding the Right Cooling Balance
In some applications, a combination of both active and passive cooling offers the ideal balance between efficiency and performance. Systems like our BTMS feature multi-stage cooling, where both passive and active cooling modes are available. The system adapts to varying ambient temperatures and heat loads, with passive cooling operating in mild conditions and active cooling taking over when the temperature rises or heat load increases. This ensures the BTMS is always using the most efficient and suitable cooling option available.
Our Complete Thermal Management System (CTMS) takes this approach a step further, delivering battery thermal management, power electronics cooling, and cabin HVAC in a single unit. It uses both active and passive cooling circuits—passive cooling handles heat dissipation for some circuits while active cooling manages others, depending on ambient temperature and the specific requirements of the system. For instance, it can cool the cabin while simultaneously cooling the powertrain and maintaining optimal battery temperature, all thanks to its indirect condensing system.
If you’re interested in learning more on indirect condensing, we’ll be diving deeper into the topic in an upcoming blog post.Â
Ultimately, finding the right balance depends on the specifics of your application—whether it’s the ambient environment, target temperature, or cooling demand. Whether you need a fully active solution, a purely passive system, or a hybrid like the BTMS or CTMS, Grayson offers a broad portfolio to help you achieve the best performance under any condition.
Contact our team today to discuss your specific cooling requirements with our thermal management experts, and don’t forget to sign up for our upcoming newsletter to stay informed on the latest innovations and updates.
Cooling Applied: Battery Thermal Management
Let’s consider a scenario where a vehicle’s battery pack needs to be kept at 25°C using a liquid cooling system, and the vehicle is operating in an environment where the ambient temperature is 15°C. In this case, passive cooling would be sufficient. The surrounding air is cooler than the target coolant temperature, allowing natural heat dissipation.
However, if the ambient temperature rises to 40°C, passive cooling alone would be insufficient. The air is too warm to cool the system effectively, so active cooling is required. A liquid battery chiller, such as our BTMS, would use a refrigerant-based system to actively pump heat out of the coolant and reject it into the hot ambient air.
Cooling Applied: Cabin HVAC
Now, let’s consider the cooling requirements for an operator’s cabin in heavy machinery, such as an excavator. In an ambient temperature of 20°C, passive cooling could be sufficient to maintain a comfortable cabin temperature for the driver, as the external air is close to the desired temperature.
However, in hotter environments, such as when the excavator is operating in an ambient temperature of 35°C or higher, active cooling becomes necessary. A cabin air conditioning system would actively lower the temperature by removing heat from the cabin and expelling it into the external environment via a refrigerant system.
