PCM Thermal Storage

Phase Change Materials (PCMs) are incorporated into heat sink designs when thermal engineers need to manage substantial yet transient spikes in either ambient temperature or component thermal load. For example, RF amplifiers for Wi-Fi systems used by commercial airline passengers onboard need to stay cool when airborne (not a problem at 30k feet) and while on the hot tarmac during flight changeover (for a short period of time).

As with most things aeronautical, a thermal solution’s weight and size play important roles in device selection. Traditional RF heat sinks (non-PCM) designed for use only while airborne or on winter tarmac will be smaller and lighter than ones designed for use on the ground at elevated ambient temperatures. The latter will require some combination of more base area, more fin area, additional airflow (fans), or a more thermally conductive material (copper vs aluminum). The result is likely a heat sink that is heavier, bigger, and considerably more costly.

The challenge then becomes one of designing the lightest and smallest heat sink for a duty cycle where 90% of its operational time is at very moderate or even cold ambient temperatures with only 10% of its time at elevated ambient temperatures – generally less than 90 minutes for this application. This is where phase change material comes into play.

Types of Phase Change Material (PCM)

There are 4 broad categories of PCM materials highlighted in the table below.

Attribute / PCM Type	Metallics	Salt Hydrate	Plant-Based	Paraffin Wax
Latent Heat (KJ/Kg)	25 to 100	60 to 3000	90-250	150-280
Thermal Conductivity	High (~20)	Low (0.6-1.2)	Very Low (0.20)	Very Low (~0.25)
Corrosiveness	Varies	Corrosive	Mildly Corrosive	Non-Corrosive
Toxicity	Some Are	Highly	Some Are	No
Melt Temp (C)	30 to 125	0 to 120	5 to 120+	-20 to 100+
Thermal Cycling	Stable	Unstable over repeated cycles	Elevated temp can cause decomposition	Stable
Weight	Heavy	Light	Medium	Medium
Price	Close to that of Plant Based	Least Expense	Most Expensive	Close to that of Salt Hydrate

 

Of these, paraffin wax is the most widely used PCM for electronics cooling applications because of its high latent heat without the corrosivity, toxicity, and potentially unstable thermal cycling characteristics of other materials. Further, it’s available in melt temperatures that align nicely with electronics applications (20-100^oC) while it scores well on weight and price attributes. The biggest drawback of paraffin wax is its very low thermal conductivity. A proper heat sink design must incorporate features to uniformly spread the heat into the paraffin.

Phase Change Material Heat Sink Design Elements

A heat sink that incorporates paraffin wax PCM for thermal storage in its simplest form looks very much like a standard heat sink. As seen in the image below, a fin structure optimized to maximize wax volume while providing enough fin area to adequately heat the low-conductivity paraffin is mounted to an aluminum base plate with six embedded heat pipes (optional for improved heat spreading). Next, the paraffin is added to an enclosure lid (not shown) and the fin side is pressed into it. Lastly, the enclosure is diffusion bonded to the base sealing the wax inside.

 

How Do PCM Heat Sinks Work?

Using our earlier example of an RF amplifier used to cool a commercial jet’s Wi-fi system, we can see just how these devices work. While airborne, ambient temperatures are quite cold resulting in a rather simple thermal challenge. The real challenge is typical during the 60 to 90 minutes aircraft are sitting at the gate, on the hot tarmac, in ambient temperatures up to 50°C. During this phase of the flight cycle the heat generated substantially exceeds the ability of the heat sink to expel the heat to air. By incorporating a paraffin PCM into the heat sink design, as the heat sink temperature rises to the melting point of the wax the phase change of the paraffin absorbs the heat.

This chart exhibits a typical prediction of the temperature profile of a heat sink that incorporates a PCM. The heat sink temperature rise is linear up until the point of the phase change and flat during the time it takes to fully melt the PCM. This particular PCM has a melting point of 72°C. The result of this type of heat sink design is a significantly smaller and lighter cooling solution than if the design was simply an air-cooled design.