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Heat Pipes, Pumped Two-phase Cooling, and Phase Change Material Heat Sinks

Now more than ever, devices with increasing power and smaller footprints are demanding a good stable cost effective thermal management solution. Fortunately many options are available today. However, for device packaging designers not familiar with these technologies, determining which thermal management technology is appropriate for their application can be a challenging task. To help sort out the many options, here is an overview of three very different thermal technologies: passive heat pipes, pumped two-phase cooling systems and temporary thermal storage/ phase change materials (PCM) heat sinks. In each case we’ll provide an overview of each of these technologies, explain how they function and provide some examples of when they should be considered.

Heat Pipes

Figure 1. Heat Pipe Schematic

Heat pipes are super conductors of heat. They are passive, quiet, rugged and reliable; yet, they can have effective thermal conductivities of more than an order of magnitude greater than copper. Heat pipes are two-phase heat transfer devices that operate in a closed vacuum system. To work, a heat pipe must be in contact with a hot end or evaporator and a cold end or condenser, as can be seen in Figure 1. The applied ΔT is the driving force for the heat transfer. The heat from the evaporator causes the working fluid to vaporize. The vapor then flows to the cooler end where it condenses to a liquid. The condensed liquid then returns to the evaporator, driven by the capillary force generated by the wick structure. This process is continuous, causing the heat pipe to become isothermal.

The key benefits of heat pipes are heat spreading and heat transport. Heat pipes can transfer heat away from a heat source, either to spread to a nearby area that is less sensitive to heat or to external sinks. Heat pipes can transfer heat 8-10" against gravity (evaporator located above the condenser), and even longer if operated with the condenser above the evaporator (gravity aided). They can be bent and flattened to meet a myriad of packaging requirements. You can see in Figure 2, heat pipes which are bent in 3 dimensions to transport heat away from an electronic component to a cold rail.

Heat pipes can also be used to move heat away from the inside of an enclosure to exterior air cooling without subjecting the components to the outside environment.

Pumped Two-Phase Cooling (P2P)

Figure 2. Two bent heat pipes removing waste heat from a device to a cold rail.

Pumped two-phase cooling refers to an actively pumped cooling system that uses boiling and condensation to provide enhanced thermal management for high heat flux applications. As can be seen in Figure 3, P2P uses the same core components as a traditional single phase pumped liquid system: a pump, an evaporator, a heat exchanger and a working fluid reservoir. The key difference between pumped single-phase and pumped two-phase cooling is that the working fluid is close to its boiling temperature when it comes in contact with the heat source. The heat causes the working fluid to boil and heat is removed through the latent heat of vaporization.

P2P systems have a significantly higher heat flux capability, with pumped liquid systems typically in the 10-20 W/cm2 range and pumped two-phase from 300 – 1,000 W/cm2. A real advantage for weight and space confined applications is the significantly reduced pumping power and flow rate requirements for pumped two-phase systems. A comparison is seen in Table 1.

Table 1. Comparison of single and two-phase cooling

In order to dissipate 80 kW of heat, a pumped liquid system using PAO as the working fluid would require a flow rate of 35 gallons per minute and approximately 5.3 kw of power. A pumped two-phase system using R245fa would require only 6 gallons/minute and 250W of power, which is an 83% reduction in flow rate and a 95% reduction in power, compared to a pumped liquid system.

One area P2P cooling is being used in is high heat flux laser cooling. In these applications, high heat fluxes must be dissipated while maintaining tight temperature limits across the diode surface. It can be challenging for a single-phase system to meet small ΔT requirements across very large cold plate areas. Another common area is power electronics where high operational reliability of the electronics is required. In these high-voltage applications, in addition to high heat fluxes, dielectric refrigerants can be used to improve reliability in the case of a leak.

Phase Change Material Heat Sinks (PCM)

Figure 3. Components for pumped two-phase and pumped single-phase cooling systems are the same, but P2P offers higher performance capability.

Solutions providing temporary thermal storage are in increasing demand for reducing heat sink size, providing a temporary fail safe mechanism for active cooling systems and other military and pulsed applications. Heat sinks incorporating phase change materials offer a solution that minimizes the mass and volume in these transient applications.

Phase change materials (PCMs) operate using a phase change, in most cases a solid to liquid phase transition. During the phase transition, the material’s latent heat provides a dramatic increase in the thermal energy storage capability material near its melting point.

Figure 4. Typical PCM Curve. In the green area, during the solid to liquid phase transition, increased energy does not result in increased temperature.

This can be seen in Figure 4. The green area is the solid to liquid phase transition. In this regime an external increase in temperature does not cause the PCM temperature to rise due to the high specific heat, as evidenced by the horizontal line showing essentially no temperature rise with increasing energy input. A major benefit of integrating PCM is that it will maintain a more constant temperature during the phase transition. Depending on how much PCM is in your system and the power it is required to store, this storage capability can be a couple seconds to several hours.

Common problems that PCM solutions are being used for include temporary heat storage and thermal management of pulsed devices. For one-time use applications, such as a missile, the PCM can be used as a standalone heat sink. When designed properly it can maintain temperature below electronics failure temperature.

Often times, a PCM solution can greatly reduce complexity and increase reliability by eliminating the need for active cooling. PCM heat sinks can eliminate the need for a cold plate, pumps, valves, etc. PCM can also be incorporated into liquid-cooled heat exchangers that dampen high peak loads, allowing the ultimate heat sink to be designed for average heat load. This application provides significant weight and volume reductions for pulsed-mode operating systems.

Summary

We reviewed three very different thermal management technologies: heat pipes, pumped two-phase cooling and PCM thermal storage. Each provides excellent thermal management solutions for selected applications.

Heat pipes are super conductors of heat. They provide completely passive heat transfer, and heat spreading. Used mostly for electronics cooling applications, heat pipes are rugged and reliable devices that are ubiquitous from laptop computers to satellites.

For demanding heat flux requirements, a technology that is finding increasing commercial adoption is pumped two-phase cooling. Pumped two-phase cooling takes advantage of the latent heat of vaporization in an actively pumped system. Compared to the more conventional pumped liquid solutions, P2P offers significant power and weight savings. P2P systems also provide a higher heat flux dissipation capability while offering tighter isothermal performance.

Figure 5. PCM Heatsinks can be used as the thermal management solution for some missiles

PCM heat sinks can temporarily absorb high amounts of waste heat without increasing their temperature. These solutions are being used to reduce overall heat sink size and provide temporary thermal management when a capable steady state solution would be overkill.

We hope that by providing overviews of these thermal management solutions, we have given some insight as to when they may be applicable for your thermal management challenges.

This article was written by Pete Ritt, VP Sales and Marketing and Dr. Richard Bonner, Manager, Customs Products Group, Advanced Cooling Technologies (Lancaster, PA). For more information, Click Here .