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Switching from an air-cooled system to liquid is not a decision to be made quickly or lightly; there are many factors and possibilities to consider when improving your thermal management to handle higher heat loads. Although market trends indicate that full liquid cooling systems will eventually be the industry standard for cooling power electronics, there are many options and hybrid solutions that can apply the benefits of both as your system evolves or upgrades. If budget or timeline constrictions are such that a direct switch to liquid is unrealistic, optimizing your forced convection solution either through design improvements or by introducing two-phase cooling or liquid components are viable interim solutions.
Engineers have been developing liquid systems that are complimentary to existing air cooled solutions that can be expanded to fully replace the air cooled systems over time. This is done by focusing on the electronic devices that can gain immediate benefit with liquid cooling. Utilizing fluid couplings, reliable pump systems, and compact heat exchangers, the system removes heat from the air flow to the liquid where it is transferred and managed elsewhere. In other cases, engineers are opting to fully replace their air cooled systems with liquid cooled to immediately enable higher power outputs and optimize thermal performance.
As you consider the switch to liquid cooling in order to improve the performance of your power electronics devices andfacilities, there are several key determining factors:
What are your size, weight, and thermal performance requirements?
Can you further optimize your air cooled system?
How much longer will air cooled systems be a viable thermal solution for your application?
Are there any limitations on liquid or volume availability?
How long will it take for investment in liquid cooling to make a return on performance and efficiency?
How can liquid cooling be implemented or designed into your application? What will be the effect on application/facility down time?
How and when do you begin?
Air cooled systems are significantly less expensive than liquid systems. They do not require regulated or specialized fluids and they are comprised of fewer components that are more economical than components for liquid systems. As they have no liquids to leak and less components to break, they also have less modes of failure. In addition to having higher reliability and lower cost, air cooled systems are also easier to modify or upgrade.
Both natural and forced convection have limitations.
Natural convection is limited by the total surface area needed to dissipate heat, this necessitates large, heavy solutions that are often impractical.
Forced Convection solutions are limited by pressure drop.
Heat sinks with large surface areas in feasible volumes create a high amount of air resistance that hinder the amount of flow and therefore heat transfer that a fan can produce. Larger forced convection solutions also require larger or more fans, increasing the amount of noise generated by the solution.
However, the biggest limitation of air cooled solutions is thermal performance. Air does not have the same capacity asliquid to absorb and transfer heat. At a certain threshold, air cooling becomes an insufficient solution and liquid coolingis necessary.
There are three common methods of improving your air cooled system.
The first is to optimize your heat sink design and fan selection. Generating more air flow, optimizing your fin geometry, or increasing your heat sink volume are ways to improve upon your air cooled solution without introducing additional technologies.
The second is to introduce two phase cooling into your design. Heat pipes may be integrated to spread higher power densities or move the heat to an area where it can be more easily dissipated.
The third most common method of increasing the performance of an air cooled solution is to introduce elements of a liquid system such as a passive thermosiphon.
A liquid cooling system is a hydraulic circuit that typically consists of a coldplate that interfaces with the heat source and device, a pump that circulates the fluidthrough the system, and a heat exchanger that rejects the heat absorbed by the liquidfrom the device.
Liquid cold plates have a much smaller working envelope than a heatsink that would be used in air cooling for the same application. Additionally, multiplecold plates can be connected to the same exchanger with minimal impact onperformance. Liquid cooling grants an additional level of control over the coolingsystem because it controls inlet temperature to the cold plate as well as flow rate.
Some have been reticent to adopt liquid cooling because of the additional complexity and the fear of leakage.Complexity often increases the cost of the solution and the amount of maintenance required to keep the systemrunning. However the additional costs are mitigated in that the improved cooling performance will increase the lifetimeand reliability of your device.
Because of its complexity, liquid cooling requires better planning and design to incorporate into your power electronics. Although the cold plate is much smaller than an extrusion or heat sink, the overall solutions tends to occupy more volume once the heat exchangers, tubes, reservoir, and pumps are all taken into account. Engineers must take all of this into account during the initial design phase in order to avoid complications later on. With proper foresight, the complexity of the systems can be beneficial as there is more flexibility in system design.