Thermal Management of Electronic Equipment using Phase Change Materials (PCMs)

Background

The cooling of electronics and telecom equipment is essential to the proper operation of all applications. Removal of generated heat from these systems has been traditionally carried out via heat conduction/convection techniques. Phase Change Materials (PCMs) are a relatively new concept for the cooling of electronic systems. A PCM is used to absorb peak energy loads during power-on operation and to then reject that heat load at another time. PCM materials typically have high heats of fusion (energy adsorption required to change the PCM from a solid to liquid), which allows small volumes of material to absorb/store large amounts of energy when it undergoes phase change. The addition of a PCM to electronic devices can prevent the use of assisted systems or fully active systems to maintain the electronics at the desired conditions. With a PCM, the melting point is selected such that the energy from the electronics is absorbed while it undergoes a phase change or melts, and it can then be solidified (recharged) once the ambient temperature goes below its melting point.

All type of equipment can be cooled using PCMs, however, the most applicability is found in transient applications. Most designers like to use passive methods for their simplicity and lack of maintenance. Passive methods rely primarily on natural (free) convection in conjunction with PCMs. Natural convection is the transport of heat by buoyancy-induced fluid flows.

PCMs are substances that change phase, most often from solid to liquid, as they absorb heat.  PCMs are selected for the temperature at which they change phase and for the latent heat associated with phase change.  PCMs are sometimes used in conjunction with thermosiphons.  Typical PCMs are waxes, salts, paraffins, etc. for high temperature applications and water (ice) for low temperature applications.

The PCM is kept inside or attached to an enclosure or a component in appropriately designed and sealed reservoirs.  The use of PCMs takes advantage of thermal inertia and phase change effects.  For example, an outdoor enclosure with PCMs during daylight hours will absorb heat through the cabinet walls and not allow the enclosure electronics to overheat.  The heat absorbed during the day will be released to the outside at night when it is cooler.

PCMs can also be incorporated into assisted systems for the cooling of enclosures.  In order to enhance cooling, PCMs can be incorporated into a heat exchanger structure in which two fluids that are at different temperatures are separated by a PCM (possibly in encapsulated form).  Hotter internal air is first circulated through the heat exchanger and cooled by transferring its energy to the PCM, which slowly changes phase.  This will occur during the hottest part of the day when external air temperatures prevent the use of outside air to cool the enclosure. Later in the day, when outside air temperatures drop, outside air is brought in to remove the heat stored in the PCM.

The PCM material can also be used to absorb energy to prevent an enclosure from overheating in the summer also functions as thermal mass during the winter to retain as much generated heat as possible.

 

Design Issues for Using PCMs

There are three general classes of PCMs that can be effectively used for a PCM cooling device. These three classes are salt-hydrates, n-alkanes, and non-paraffin organics (although there are other types of PCMs but we will concentrate on these three.)  The five most important factors to consider in the selection of a PCM are

• The thermal characteristics of PCM’s
• The cost of PCM,
• The safety, toxicity, and environmental characteristics of the PCM, and
• The useful life of the PCM
• The proper packaging of the PCM

The thermal properties of salt-hydrates can deteriorate as a result of undergoing thermal cycling. The impact of the cycling can be decreased through the addition of additives (nucleators) to the salt-hydrates and mechanical agitation or mixing. Glauber’s salt (sodium sulfate, 10-hydrate) for example is a very popular PCM material but may not be adequate for some applications that require a high number of thermal cycles, unless properly encapsulated.  Data obtained from material safety data sheets (MSDSs) indicate that it is not a significant health hazard.  In addition, the cost for Glauber’s salt is low. Not all hydrated salts are inexpensive or are relatively harmless.  Each hydrated salt must be evaluated for each application.

The use of n-alkanes or paraffins or other organic materials (such as fatty acids) is also possible and may be advantageous because the phase change temperature can be selected over a wide range (0 to 120^o C). Review of MSDSs for candidate materials also indicate that paraffins pose no significant health or safety hazards.  Rather than use a single pure chemical species, the actual paraffin or organic can be a mixture of hydrocarbons. The phase change for these mixtures will occur over a small temperature range.

Non-paraffin organics are more expensive than the other two PCM’s, and sometime might be necessary to use them if they meet the thermal and safety requirements.

 

Thermal Design Issues

When designing a passive PCM cooling system there are several issues that must be addressed. These include

• heat adsorption during power on (to include solar load, if any);
• solar load and reflector design, if outdoors;
• heat rejection during off-peak times (night or down (power off) time);
• stress levels within the reservoir;
• container compatibility with the PCM

The amount of energy that is to be absorbed during the power-on period directly determines the volume of PCM that must be used in the heat exchanger. The energy absorbed depends on the total thermal load (to include solar load) and the energy generated by the electronics within the system. To determine the design load, a simple steady-state analysis can be performed of the complete system to determine how much heat can be rejected for a specified internal temperature. The difference between the heat rejected and the total thermal load (electronics and solar, etc) then determines the maximum thermal load the PCM must be able to absorb.

Another factor that influences the calculation of the PCM volume is the number of thermal cycles the system will experience over its useful life which leads to the degradation of the thermal properties. Knowledge of the degradation then leads to the application of a safety factor in the calculation of the PCM volume. The process is transient and the rate of heat absorption (and melting of the material) is not uniform.

The melting of the PCM is typically a lesser issue than the resolidification of the PCM. The driving force to melt the PCM is the difference between the phase change temperature of the PCM and the enclosure air temperature. Solidification of the PCM after it has been melted can be much more difficult.  The temperatures overnight act as the heat sink for PCM solidification. Furthermore, issues such as supercooling should be monitored.

 

PCM Uses for the Thermal Management of Equipment (Electronic/Telecom.)

The ability of phase change material to absorb and release energy for a long time makes ideal for indoor and outdoor equipment that is exposed to transient operation cycles. However, the benefits of using PCM’s do not end there.  The following are most of the possible applications of PCM’s are being developed or are already in the market:

• PCM at the component/chip level
• PCM at board/subcomponent level
• PCM at the system/enclosure level
• PCM for battery backup comportments or systems
• PCM for auxiliary or emergency cooling

What follows is a brief description of each application in which PCM’s are incorporated.

 

PCM at the Component/Chip Level

The incorporation of PCMs at the chip level has been extensively investigated and the scientific literature has a good number of sources. The key for the use of PCMs here is that the heat generation is either transient or the PCM is used to take care of peak heat generation (typical situation in telecom systems when traffic is at peak levels). A variety of PCMs can be used, however, paraffins and hydrated salts appear to be the best candidates. Primarily, this involves the attachment of PCM reservoirs next to heat generation loci. Another design would involve the use of reservoirs that are connected to the chip/component via high conductivity devices such as heat pipes.

 

PCM at Board/Subcomponent Level

Thermal management of high-power electronic components (chips) with high heat dissipation ratings using air as the cooling fluid clearly demands non-traditional means to be successful. Many different approaches have been attempted in the past with varying degrees of success.  One method is by using heat sinks made of channels or pin fins. Multi-Chip Modules (MCMs) have benefited from the use of microchannels for the effective removal of heat. For the case of air cooled microchannels, substantial increases in heat removal rates were made by designing for laminar conditions, which results in lower pressure drops.

Heat sinking capabilities and requirements of high-power dissipation IC’s and microprocessors is well understood for steady operation. However, many electronics components operate transiently, that is most of their useful like is spent in on-off operation. One way to reduce the cooling load requirement or the convective flow requirement for on-off operation is to incorporate PCM (Phase Change Materials) to traditional heat sinks such as the standard pin fin and the longitudinal plate fins. In addition, wherever chips must be cooled by passive means only, which involves the use of natural convection and radiation as the only heat transfer modes available to the designers, PCM’s are the ideal candidates.

Systems that incorporate PCM’s such as passive heat moderators (such as a reservoir of wax-like materials) can be designed that incorporate the PCM into the board structure or into heat sinks that are cooling the chips.

 

PCM at the System/Enclosure Level (PCM incorporated in the Enclosure/shell Structure)

One way is to incorporate a PCM (phase change material) into the passive cooling scheme which might rely on fins. This scheme might include placing PCM into walls or roofs of outdoor enclosures or in discrete PCM reservoirs that are attached to walls or ceiling.

The PCM material is used to absorb energy to prevent the enclosure from overheating. During the winter, the PCM allows the system to retain as much heat as possible. Please note that the need for resistance heating during the coldest periods is not eliminated if PCM’s are used. The PCM materials for this system must be selected such that they can absorb sufficient amounts of energy to maintain the enclosure as required, and to then reject that energy during cooler periods. For this to occur, the phase change temperature of the PCM will have to be below the maximum allowable chamber temperature and above the overnight low temperature expected for the hottest day the system is designed for.

It should be pointed out that each application, i.e. enclosure design, will require a unique PCM type and configuration that reflects the maximum allowable internal temperature and external/internal heat load conditions.  Furthermore, stress analyses will be carried out as part of any design work that incorporates PCM. The stress analyses are required to ensure that the PCM enclosure has the necessary structural integrity. The design should also take into account: a) thermal capacity of a system when exposed to outdoor environment, b) analysis on cycling that will occur during a 24-hour period, c) winter performance, and d) maximum number or melt/freeze cycles that the PCM may undergo without degradation of its thermal properties.

 

PCM Heat Exchanger (with or without Forced Cooling) 

These materials are kept inside the cabinets in appropriately sealed enclosures and take advantage of thermal inertia and phase change effects.  For example, an enclosure with PCM’s during the daylight hours will absorb heat and not allowed heating up the cabinet air; the heat absorbed during the day will be released to the outside at night when it is cooler.  During all these processes take place, heat will continue to be transferred in/out through the cabinet walls.

We can, in order to enhance cooling, incorporate PCM into a heat exchanger structure, in which two fluids that are at different temperatures are separated by a PCM (possibly in encapsulated form). The difference lies (when compared with a standard air-to-air heat exchanger) in when the exchange of heat takes place.  The two air streams do not flow at the same time (and may not need to flow through the same passages).  Hotter internal air is first circulated in the heat exchanger and allowed to cool by transferring its heat to the PCM that will slowly change phase.  This will occur during the hottest part of the day when outside temperatures cannot be brought in.  Later in the day cycle when outside temperatures are low outside air is brought in to remove the heat stored in the PCM and therefore it will increase its temperature.

  

4.4 PCM for Battery Comportments or Systems

Outdoor enclosures (remote cabinets and sheds) are designed to house various equipment configurations and are installed in all type of weather conditions, from the coldest to the hottest. Almost all these cabinets are fitted with back-up batteries, which are housed in separate but attached compartments or in stand-alone enclosures. Battery compartments can be cooled by passive means. Passive methods include natural (free) convection and phase-change materials (PCMs). Natural convection is the transport of heat by buoyancy-induced fluid flows. Fluid heated by a hot wall (exposed to the sun) rises and is displaced by colder fluid. As batteries are added in a compartment, natural convection loops become more difficult. The designer must analyze different likely scenarios, such as battery placement and external temperatures, in order to ascertain the likely thermal behavior of the compartment. These studies can be extensive. Furthermore, the designer should be aware that the body of scientific literature on natural convection within enclosures is vast and that buoyancy induces flows are complex

 

PCM for Auxiliary or Emergency Cooling

In many outdoor enclosures, their thermal management is carried out using a variety of methods such as air conditioners, air-to-air heat exchangers, or other methods. However, when these cooling systems fail to operate due to a malfunction or loss of power, especially for systems that dissipate larger amounts of heat, one needs an emergency cooling system that can be operated without power or mechanical. This system needs to maintain temperatures at or near design conditions for a reasonable length of time until work crews can come in to repair the regular cooling equipment or restore electric power. This is very important when enclosures are to be installed in very remote areas where travel times are longer than insulation and battery back up can hold up.

PCM’s are ideal for this type of application. Here the key is to use PCM’s that are low in cost since the amount to be used is large.  One system would be composed of a stand-by PCM heat exchanger that incorporates PCM that melts at a temperature above the design cabinet temperature.  For example, if the enclosure is to be kept at 65 C, then the PCM to be used in the heat exchanger should melt at least 10 C above the cabinet temperature (75 C) or near the maximum temperatures the equipment can tolerate for a short length of time.  Thus, for the design to be successful the design engineers needs to take into account that the PCM needs to absorb full solar radiation and the heat generated by the equipment for an extended length of time (based on how long the estimated repair times will be).  If the enclosure will be installed primarily in cold environments, then the PCM to be used can one that melts/freezes at a temperature 10 C below the minimum temperature.

 

Conclusion

Phase Change Materials are coming to age for the thermal management of outdoor and indoor telecom systems.  The major criteria that the designer needs to take into account when designing with PCM are the cost of PCM, temperatures involved, packaging of the PCM, and the type of cooling mode desired (fan-driven or fully passive).  The technology and the range of PCM’s available in the market place are growing every year, and good designers need to begin incorporating PCM’s into their designs.