All engineering design is based in physics, so we will start off with a reminder of some fundamental thermal management principles. For those responsible for cooling systems who don’t have a mechanical engineering background, this will get you started. Why would any system need to have the added cost and power burden of a cooling system? When is a cooling system needed? When it is needed, how do I explain to my product manager or lead engineer that it is necessary?
When adding cost and complexity to any project the decisions need to be defended, usually in the simplest of terms. It helps to keep these conversations short. It also helps to know that an active cooling system is needed at the onset of the project. It is much harder to add cooling after the design is 90% done than it does at the beginning. This means that waiting, and putting off the inevitable need for a cooling system, could lead to a situation in which a kilowatt of heat needs to be dissipated through something the size of a thimble. Redesigns are more expensive, in both schedule and dollars than designing in the cooling system at the beginning of the project. Entire papers at electronics packaging symposiums have been dedicated to this idea.
Active cooling, for the purposes of this discussion, is a system that will allow the elements to be cooled to be kept at or below ambient temperature. Typically, either a vapor compression (refrigeration cycle) or a thermoelectric (Peltier) device is used to accomplish this.
Passive cooling in this nomenclature means a system that by its design cannot maintain a temperature that is below the ultimate heat sink, typically ambient air. For the purposes of these discussions, ambient air will be the ultimate heat sink of choice.
In every application that consumes power, essentially every laser, electronics, human, battery, and system in existence, the cooling path needs to be determined so that the limits on the operating temperature can be maintained. In a passive system such as the one shown in the figure, the application draws power in and produces its signal, energy conversion to provide a useful output. The inefficiencies innate to all systems dictate that there will also be waste heat produced. Without active cooling, the temperature of the device will increase until the energy balance of Power = Power Out + Waste Heat is satisfied.
This blog and all cooling systems are designed to control the temperature of the device in question to sustainable and desirable levels with a minimum of cost, size, weight, and maximum efficiency. If the required temperature is lower than the ambient temperature, then an active cooling system is required.
Have you ever wondered why you can only ride an exercise bike indoors for a short period of time without overheating when you can ride for hours on the open road? As an endurance cyclist, I learned that indoors is not the place to practice endurance rides. Hint, it’s not boredom. David Gordon Wilson, in his book “Bicycling Science” explains it. The human body is essentially a heat engine wherein the power output is determined by the difference in temperature between the hot and cold sides of the system. On the open road, there is an airstream flowing over the entire body dissipating the heat while the sweat provides evaporative cooling from the body’s surface. When riding indoors, even with a fan, the cooling effect is far more limited than it is on the road. There is simply not enough delta T to sustain a high-power output. Indoors, the sweat builds up and drips onto the bicycle, because there is not enough airflow to evaporate the sweat. The body overheats. This is what happens to our devices when they are not cooled properly. Next time, we will cover some of the nuances and limitations of passive cooling in hot environments.