Power Electronics

Arguments Against Liquid Cooling Are All Wet

Are customers asking you to put 10 pounds of electricals/electronics into a 5-pound bag? Do they want you to double the functionality and halve the volume each time you turn around? And when you do that, is the resulting heat buildup greatly shortening the lives of your components or even killing them? If you are typical, you don't really have an electrical/electronic problem, you have a cooling problem.

If you limit your thinking to common forced-air cooling, you don't have much wiggle room. That's because air cooling is already mature and, as you well know, flat up against its heat ceiling. Lots of time and money are required to gain even a little room. And even after you succeed in wringing out a few more watts of cooling, you are still just barely getting by. You have no margin, even for aging degradation and contamination. I have heard air cooling called cooling with dirt. Don't forget that customers' demands follow the Chicago method: heat early and heat often.

Why not use a cooling system that can add orders of magnitude of cooling capability rather than a few extra percent? Why not consider liquid cooling? “Liquid cooling!” you say in horror. “What about the extra complexity, cost, weight, space and power consumption of the pump and coolant? And what about the reliability? If the pump breaks down, that equipment will overheat.”

Well, if you are as old as I am, all this naysaying will have a familiar ring. Remember when forced air was first introduced to the wonderful world of convective air cooling? What did people say in horror? “Those fans are going to add extra complexity, cost, weight and space and will consume power. And if the fan breaks down, that equipment is going to overheat!” Well, we have learned how to deal with the disadvantages of forced-air cooling in order to use its advantages. And I have no doubt that we will learn how to deal with the disadvantages of liquid cooling in order to use its huge advantages.

Even though air is plentiful and cheap, when I think back over my engineering career, I have made many more designs with air as an insulator than with air as a heat-transfer medium.

Instead of air, consider another common and inexpensive fluid: water. For starters, water is about 1000 times denser than air. That gives it a huge size advantage for cooling systems by reducing the cooling passages, the amount of fluid being used and the size of the motive driver (i.e., small pumps versus large fans). And it's kept in a clean, closed system. No more dust bunnies, hair balls and salt air wafting through the chassis.

Now add the fact that water can absorb more than four times as much heat as the same weight of air. Compared to air, that's a cool 4000-fold advantage per pint of coolant. But wait, there's more! Water conducts heat about 35 times better than air, so you can move heat in and out of it much more easily. If water is a possible electrical problem in a given application, then dielectric fluids are readily available, although at an additional cost, of course.

If you really want to think out of the box, consider letting the water boil in your cooling system. Don't like the 100°C required to do this? Just lower the system pressure to lower the boiling point to anything you want. Boiling supersizes the cooling to a whopping 2 million times as much as the cooling capacity of air. In my book, a 2-million-fold improvement is worth considering something different. Walk around the wall instead of clamoring over it.

In power electronics, there is somewhat of a chicken-and-egg problem. Electrical power component and system designers have limited themselves by accepting and designing around the limitations of air cooling. And, while it's trivial to use liquid cooling to allow more performance from existing devices, the real leaps in performance will come when some forward-looking component designer makes parts specifically for liquid cooling. Such parts will be much smaller and more powerful, efficient and long-lived, which will give the designer's company a huge leg up on the rest of the industry. Will that designer be you? Think about it.

Rex J. Harvey has worked for the last 20 of his 38-year engineering career directly in the aerospace-related spray atomization field, most recently in the liquid-cooling application field. Previous experience includes eight years as an engineer and administrator in the electrical and electronic controls field. He currently serves as a principal engineer in the Advanced Cooling Systems interdivisional team at Parker Hannifin, where he has worked for 14 years. He holds a BSME from Iowa State University and a master's degree in systems management from the University of Southern California.

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