Features

High-Temperature Phase Change Materials (Salts and Metals)

Phase change material melting from -10 to 100°C is used for thermal management in a variety of commercial and military applications, e.g., building thermal management, electronic cooling, and supplemental cooling for energy weapons, to name a few. Salts, and metals, which are phase change materials with higher melting temperatures, are also attractive. One application is storing high quality heat from concentrated solar radiation and using it to generate electricity. Inorganic salts have been intensively studied, and precursor developments [6] have identified promising storage medium for the high temperature applications. Pilot-scale units have demonstrated its technical and cost feasibility.

PCM Challenges

Figure 4. PCM Heat Sink Schematic. Folded fins are commonly used to increase the effective thermal conductivity. Heat pipes can also be added for larger systems, or systems operating at higher heat flux.

Most PCMs used for electronics cooling have a very low thermal conductivity, effectively insulating a heat source in high heat flux applications. If there is a large temperature gradient through the PCM, the surface in contact with the heat source may reach its maximum temperature before all of the PCM latent heat is utilized (melted). In order to fully utilize the latent heat of the material, thermal enhancements may be required. Aluminum or copper fins are commonly used to improve the heat transfer through the PCM. Other thermal enhancements such as nanoparticle impregnation have been experimented with [7] , but the nanoparticles have not been shown to increase the thermal conductivity significantly enough to make the material viable for most applications. Further, the nanoparticles settle out after very few cycles without a stabilizer [8] . For this reason, finned structures and heat pipes are most commonly used in industry to better distribute the heat into the PCM (Figure 4).

During phase change, the density of a material changes. Depending on the material, different features are used to compensate for the volumetric change. Typically, the PCM volume is controlled during filling so that there is still some void space at the highest expected temperature. If the higher pressure can occur, a pressure relief feature may be required.

PCM Heat Sink Design

Figure 5. Simplified Thermal Resistance Network.

Advanced Cooling Technologies, Inc. (ACT) has developed a simple figure that can indicate whether PCM is suitable for an application and whether thermal enhancements are required. Average PCM heat sink properties were used to generate this figure, based on ACT's experience with PCM heat sink design. The PCM material properties are an average of several paraffins used in practice and the fin material is aluminum. The model also assumes that no more than a 10°C temperature rise to overcome thermal conduction resistance is allowed. A simple energy balance and resistance network, shown in Figure 5, can be iterated to find a goal temperature gradient from the base of the heat sink to the PCM.

Figure 6. PCM heat sink selection guide, showing the PCM/Thermal-Enhancement mass ratio as a function of time and heat flux.

For applications with a known storage time and heat flux, Figure 6 will indicate whether a PCM heat sink solution is a suitable thermal solution. The PCM ratio represents the amount of PCM required relative to the volume of aluminum in the fin enhancement structure. PCM ratios near zero indicate an all metal solution would be required to remove the flux. PCM ratios near unity indicate a PCM structure without any metal enhancement structure would be required.

A closer estimation of the mass, volume, and thermal performance of a PCM heat sink can be obtained using ACT's heat pipe calculator here. The expected transient performance for three different PCM options suitable for your application is plotted. This calculator assumes a generally conservative fin design. High performance custom solutions with heat pipes can be designed upon request.

Conclusions

PCM heat sinks are used in several electronics cooling applications including temperature stabilization during pulsed operation, short term thermal storage when a suitable heat sink is not available, and protection from failure during loss of coolant scenarios, to name a few. If the thermal storage capacity of PCM is suitable for an application, a PCM heat sink can reduce system size, cost, maintenance, and power requirements.

The specific phase change material and enclosure selected is dependent on the specific application requirements. Paraffin waxes and non-paraffin organics are most frequently used, but hydrated salts, non-hydrated salts, and metals can be used in some specialized applications.

PCM heat sink design challenges include the low thermal conductivity of most PCMs. Thermal enhancement such as fin or heat pipes are generally used to improve the thermal conductivity.

This article was written by Rebecca Weigand, Product Development Engineer; Ying Zheng, R&D Engineer II; William G. Anderson, Chief Engineer; and Richard W. Bonner III, Vice President, R&D; Advanced Cooling Technologies, Inc. (Lancaster, PA). For more information, visit here.