PCM Heat Sink FAQ

With the exception of thermal storage heat sinks, the term heat sink is a misnomer.  Standard heat sinks for electronics cooling are actually heat exchangers, taking the heat from the electronics, and transferring it to a fluid, either air or a coolant.  Phase Change Material (PCM) heat sinks are the only heat sink that actually acts as a (temporary) sink for heat.   If you have been wondering, “How do thermal storage (PCM) heat sinks work?” you first need to understand the different types of phase change materials and how they fit into the PCM heat sink process. What follows is a quick introduction to everything you need to know about phase change material heat sinks.  More detail can be found on ACT’s Thermal Storage Resource Page.

What is PCM?

A Phase Change Material (PCM) is a material that absorbs a large amount of energy when undergoing a phase change. These materials have a high latent heat of fusion, which means they will absorb a large amount of energy while maintaining a constant temperature. Typically PCM refers to materials using the solid-liquid phase change to absorb energy.

What is a PCM Heat Sink?

How does PCM fit in with heat sink technology? A PCM heat sink put simply is a hermetically enclosed fin stack with PCM filling the void space. Most PCMs have very low thermal conductivities and require features to enhance heat transport to effectively utilize all of the latent heat potential. This can be achieved easily by incorporating fins or even heat pipes into the PCM to better distribute the heat and evenly melt or freeze the PCM.

Figure 8. A Phase Change Material (PCM) PCM absorbs thermal energy and stores it during a solid to liquid phase transition, allowing the temperature to be maintained near the melting point of the PCM

Figure 1. A Phase Change Material (PCM) PCM absorbs thermal energy and stores it during a solid to liquid phase transition, allowing the temperature to be maintained near the melting point of the PCM

 

How a PCM Heat Sink Works

See Figure 9 below for a visual representation of the standard operation of the PCM heat sink.

Figure 9. As heat is applied to the heat sink, the temperature rise is dictated by the sensible heat (specific heat) of the system, until the PCM starts to melt. The temperature stays nearly constant while the PCM is melting. Once all of the PCM material is melted, the system temperature will begin rising again until the heat source is turned off.

Figure 2. As heat is applied to the heat sink, the temperature rise is dictated by the sensible heat (specific heat) of the system, until the PCM starts to melt. The temperature stays nearly constant while the PCM is melting. Once all of the PCM material is melted, the system temperature will begin rising again until the heat source is turned off.

When are PCM Heat Sinks Used?

What types of applications would benefit from phase change material heat sinks? PCM heat sinks are used for pulsed operation, allowing the heat removal system to be sized for the average load, rather than the peak.  A second application is short term thermal storage, when there is no ultimate heat sink available.  Finally, they can be used for protection from failure when the cooling system is temporarily unavailable.

Typical applications include:

  • Cooling electronics with a known duty cycle
  • Dampening of heat loads in pulse mode operation
  • Protection from momentary failure
  • Storage for one-time use applications
  • Thermal energy storage for renewable energy applications

If you are not sure whether a PCM heat sink is appropriate for your situation, please contact Advanced Cooling Technologies right away.  One of our cooling technology experts will help you determine the solution you need.

What are the Benefits of a PCM Heat Sink?

There are some universal benefits of a PCM heat sink across almost all applications.  First, PCM heat sinks are passive, with no moving parts to maintain, and no power required to operate.  Second, they offer long life with no maintenance.  The vacuum seal prevents liquid losses, and protective coatings can give each device a long-lasting guard against corrosion.  A PCM heat sink can also lower costs by lowering the operating temperature, these devices can increase the Mean Time Between Failure (MTBF) for electronic assemblies.  In turn, this lowers the maintenance required, and the replacement costs.

Answering All of your PCM Heat Sink Common Questions

Now that you have the basics, we’re sure you have more complex questions. While some answers are specific to your needs and system requirements, these responses to standard questions will give you a better understanding as to how these devices operate.

What is the Operating Temperature Range?

The operating temperature range depends on the application. Many paraffins are appropriate for electronics applications where the temperature is typically maintained below 100°C; see Table 1 below. For higher temperature applications, such as energy storage for power plants, salts and metals can be used.

What is the Storage Temperature Range?

The storage temperature of a PCM heat sink will depend on the type of PCM selected. Most PCM heat sinks can be stored at any temperature below its maximum operating temperature. Because most PCMs become less dense, expanding, as their temperature increases, PCM heat sinks are filled in an environment above their maximum operating point. If the heat sink were heated beyond its fill temperature it could become over pressurized and deform or open up a leak path. If a PCM heat sink is designed to keep a source below 90°C, it should not be stored in an environment above 90°C. (Note that in this case, 90°C is the maximum allowable design temperature; it is always above the melting point of the PCM).

Will the PCM Heat Sink Operate in Any Orientation?

A  PCM heat sink will operate in any orientation

What Constrains the Maximum PCM Heat Sink Size?

The PCM heat sink size will be limited by the ability to effectively move the heat into the PCM. If the volume of PCM required is large and/or the heat flux is very large, the manufacturability of the internal fin structure will restrict the PCM heat sink size and storage capabilities. In certain cases heat pipes can be used to spread the heat to multiple PCM modules.

How is the low thermal conductivity of PCM overcome?

In most cases fins are used to overcome the low thermal conductivity of PCM. Their increased surface area and high conductivity allows the heat to penetrate the PCM, resulting in an even melt front and lower temperature gradient within the PCM. Heat pipes can also be used with or without fins to better distribute heat to the PCM in high heat flux applications.

What Materials are Used for the Envelope and Fins?

A variety of materials can be used for the envelope and fins. Aluminum is the most common material used for PCM heat sinks, but ACT has also worked with copper, steel, and magnesium.

When Should I use Paraffin Wax, Hydrated Salts, or Metallic Phase Change Materials?

As shown in Table 1, the PCM materials include paraffin wax, non-paraffin organics, hydrated salts, and metals.  Paraffin wax is ideal for most electronics applications.  There is a large selection of pure hydrocarbon materials and paraffin blends available, with melting temperatures ranging from roughly -20 to 100°C, every few degree.  Hydrated salts are generally used for larger energy storage applications, since they are much cheaper.  However, the design using hydrated salts must account for both corrosion, and the limited number of cycles available.  Finally, the metallics are generally used for high temperature applications.

Table 1.  Properties of Typical Phase Change Materials.

Property or Characteristic

Paraffin Wax Non-Paraffin Organics Hydrated Salts Metallics

Heat of Fusion

High High High Med

Thermal Conductivity

Very Low Low High Very High

Melt Temperature (°C)

-20 to 100+ 5 to 120+ 0 to 100+ 150 to 800+

Latent Heat (kJ/kg)

200 to 280 90 to 250 60 to 300 25 to 100

Corrosive

Non-Corrosive Mildly Corrosive Corrosive Varies

Economics

$$ $$$ to $$$$ $ $$ to $$$

Thermal Cycling

Stable Elevated Temperature Can Cause Decomposition Unstable over Repeated Cycles Stable

Weight

Medium Medium Light Heavy

I Have a Question Not Answered Here

If you still have questions about phase change materials, heat sinks or any of the other issues that have been covered here, Advanced Cooling Technologies is happy to help. While some of these concepts are complex, we have experienced professionals standing by that will be able to answer any questions or clarify any issues you may have. Customer service is our top priority, so please don’t hesitate to contact us with your questions today!

Get Your Quote Today

Now that you’ve learned what a PCM heat sink is and how they are used, it’s time to contact ACT for more information and a quote for including a PCM heat sink with your equipment. We will help you decide how best to meet your needs with equipment including:

  • Thermal management
  • PCM Heat Sinks
  • Heat pipe assemblies
  • HiK™ plates
  • Vapor chamber assemblies
  • Single and Two Phase Pumped Loop Cooling
  • Cold plates
  • And much, much more.

We’ll provide you with everything you need to understand cost and installation of PCM heat sinks, as well as related, single-use options.

Reduce your costs and improve the life of your equipment with a simple conversation designed to make your operations easier and affordable. Contact ACT for more information about how we can help meet all your thermal management and energy storage needs.

 

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