Review of Experiments Passive Thermal Management Using Phase Change Materials

Groundwork

The cooling of electronics and telecom equipment is essential to the proper operation of all applications. Removal of generated oestrus from these systems has been traditionally carried out via heat conduction/convection techniques. Stage 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 performance and to then reject that estrus load at another time. PCM materials typically take high heats of fusion (energy adsorption required to change the PCM from a solid to liquid), which allows pocket-sized volumes of material to absorb/store large amounts of energy when it undergoes phase alter. The improver 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) in one case the ambience temperature goes below its melting point.

All blazon of equipment tin can be cooled using PCMs, nonetheless, the most applicability is institute in transient applications. Virtually designers similar 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, every bit they absorb estrus.  PCMs are selected for the temperature at which they change phase and for the latent estrus 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 depression temperature applications.

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

PCMs tin can also be incorporated into assisted systems for the cooling of enclosures.  In order to enhance cooling, PCMs tin can be incorporated into a heat exchanger structure in which 2 fluids that are at different temperatures are separated by a PCM (possibly in encapsulated form).  Hotter internal air is showtime circulated through the heat exchanger and cooled by transferring its free energy to the PCM, which slowly changes phase.  This will occur during the hottest office of the day when external air temperatures forestall the use of outside air to cool the enclosure. Afterwards 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 as well exist used to blot energy to prevent an enclosure from overheating in the summer too functions equally thermal mass during the wintertime to retain every bit much generated heat equally possible.

Design Bug for Using PCMs

There are three general classes of PCMs that can exist finer used for a PCM cooling device. These iii classes are table salt-hydrates, due north-alkanes, and non-paraffin organics (although there are other types of PCMs just we volition concentrate on these three.)  The five nearly 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'south salt (sodium sulfate, 10-hydrate) for case is a very popular PCM material just may not be adequate for some applications that require a high number of thermal cycles, unless properly encapsulated.  Data obtained from material safe data sheets (MSDSs) betoken that it is not a significant health gamble.  In addition, the toll for Glauber'south common salt is depression. Not all hydrated salts are inexpensive or are relatively harmless.  Each hydrated salt must be evaluated for each awarding.

The use of n-alkanes or paraffins or other organic materials (such as fatty acids) is too possible and may exist 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 alkane or organic tin can be a mixture of hydrocarbons. The phase change for these mixtures will occur over a small temperature range.

Non-methane series organics are more expensive than the other 2 PCM'southward, and sometime might be necessary to use them if they meet the thermal and rubber requirements.

Thermal Design Issues

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

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

The amount of energy that is to be absorbed during the ability-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 past the electronics within the system. To determine the pattern load, a simple steady-state analysis can be performed of the complete system to determine how much heat tin can exist rejected for a specified internal temperature. The divergence between the heat rejected and the total thermal load (electronics and solar, etc) and then determines the maximum thermal load the PCM must be able to absorb.

Another cistron that influences the calculation of the PCM book is the number of thermal cycles the system volition feel over its useful life which leads to the degradation of the thermal properties. Noesis 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 assimilation (and melting of the textile) is not compatible.

The melting of the PCM is typically a lesser issue than the resolidification of the PCM. The driving strength to melt the PCM is the deviation betwixt the phase change temperature of the PCM and the enclosure air temperature. Solidification of the PCM later it has been melted can be much more hard.  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 stage change material to absorb and release energy for a long fourth dimension makes ideal for indoor and outdoor equipment that is exposed to transient performance cycles. Nonetheless, the benefits of using PCM'southward exercise not end in that location.  The post-obit are nigh of the possible applications of PCM's are being adult or are already in the marketplace:

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

What follows is a brief description of each application in which PCM'due south 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 skilful number of sources. The primal for the use of PCMs here is that the heat generation is either transient or the PCM is used to have care of peak heat generation (typical situation in telecom systems when traffic is at top levels). A multifariousness of PCMs can be used, however, paraffins and hydrated salts announced to exist 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 scrap/component via high conductivity devices such equally heat pipes.

PCM at Board/Subcomponent Level

Thermal management of high-power electronic components (chips) with loftier rut dissipation ratings using air as the cooling fluid clearly demands non-traditional means to exist 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-Flake Modules (MCMs) take benefited from the use of microchannels for the effective removal of heat. For the instance of air cooled microchannels, substantial increases in heat removal rates were made by designing for laminar weather, which results in lower pressure drops.

Rut sinking capabilities and requirements of high-power dissipation IC'due south and microprocessors is well understood for steady operation. However, many electronics components operate transiently, that is well-nigh of their useful like is spent in on-off performance. Ane fashion to reduce the cooling load requirement or the convective catamenia requirement for on-off operation is to incorporate PCM (Phase Alter Materials) to traditional heat sinks such every bit the standard pin fin and the longitudinal plate fins. In add-on, wherever chips must be cooled by passive means only, which involves the use of natural convection and radiation as the simply estrus transfer modes available to the designers, PCM'due south are the ideal candidates.

Systems that comprise PCM's such equally passive heat moderators (such as a reservoir of wax-similar materials) tin be designed that contain the PCM into the lath construction 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 modify 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 non eliminated if PCM'southward are used. The PCM materials for this organization must exist selected such that they tin absorb sufficient amounts of energy to maintain the enclosure as required, and to then refuse that energy during cooler periods. For this to occur, the phase alter temperature of the PCM will have to be beneath the maximum allowable chamber temperature and in a higher place the overnight depression temperature expected for the hottest day the system is designed for.

It should be pointed out that each application, i.e. enclosure pattern, will require a unique PCM blazon and configuration that reflects the maximum allowable internal temperature and external/internal estrus load weather condition.  Furthermore, stress analyses will be carried out as function of whatsoever design work that incorporates PCM. The stress analyses are required to ensure that the PCM enclosure has the necessary structural integrity. The pattern should also take into business relationship: a) thermal chapters of a system when exposed to outdoor environment, b) analysis on cycling that volition occur during a 24-60 minutes period, c) wintertime performance, and d) maximum number or melt/freeze cycles that the PCM may undergo without degradation of its thermal properties.

PCM Oestrus Exchanger (with or without Forced Cooling)

These materials are kept inside the cabinets in appropriately sealed enclosures and have advantage of thermal inertia and phase change effects.  For case, an enclosure with PCM'southward during the daylight hours will blot heat and not immune heating up the cabinet air; the heat absorbed during the mean solar day will exist released to the outside at dark when information technology is cooler.  During all these processes have place, oestrus volition continue to be transferred in/out through the chiffonier walls.

We tin, in order to enhance cooling, contain PCM into a heat exchanger structure, in which two fluids that are at unlike temperatures are separated by a PCM (possibly in encapsulated form). The deviation lies (when compared with a standard air-to-air heat exchanger) in when the exchange of oestrus takes place.  The two air streams exercise not menstruum at the same time (and may non demand to menses through the same passages).  Hotter internal air is first circulated in the heat exchanger and immune to absurd by transferring its rut to the PCM that will slowly change phase.  This will occur during the hottest office of the day when outside temperatures cannot exist brought in.  Later in the solar day bicycle when exterior temperatures are low outside air is brought in to remove the heat stored in the PCM and therefore it will increment its temperature.

4.4 PCM for Battery Comportments or Systems

Outdoor enclosures (remote cabinets and sheds) are designed to firm diverse equipment configurations and are installed in all type of weather weather condition, from the coldest to the hottest. Well-nigh all these cabinets are fitted with back-upwardly batteries, which are housed in split but attached compartments or in stand up-alone enclosures. Battery compartments tin can be cooled by passive means. Passive methods include natural (costless) convection and phase-change materials (PCMs). Natural convection is the transport of heat by buoyancy-induced fluid flows. Fluid heated past a hot wall (exposed to the sun) rises and is displaced by colder fluid. Every bit batteries are added in a compartment, natural convection loops get more difficult. The designer must analyze dissimilar likely scenarios, such as bombardment placement and external temperatures, in order to define the likely thermal behavior of the compartment. These studies tin be all-encompassing. 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 circuitous

PCM for Auxiliary or Emergency Cooling

In many outdoor enclosures, their thermal direction is carried out using a diverseness of methods such as air conditioners, air-to-air rut 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 rut, i needs an emergency cooling system that can be operated without power or mechanical. This organization needs to maintain temperatures at or about design conditions for a reasonable length of time until work crews tin can come in to repair the regular cooling equipment or restore electric power. This is very important when enclosures are to exist installed in very remote areas where travel times are longer than insulation and battery dorsum up tin hold up.

PCM'southward are ideal for this blazon of application. Here the key is to use PCM's that are low in toll since the corporeality 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 instance, if the enclosure is to be kept at 65 C, then the PCM to be used in the heat exchanger should cook at least x C in a higher place the cabinet temperature (75 C) or near the maximum temperatures the equipment tin tolerate for a short length of fourth dimension.  Thus, for the design to be successful the pattern engineers needs to take into account that the PCM needs to absorb total 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 common cold environments, then the PCM to exist used tin one that melts/freezes at a temperature x C beneath the minimum temperature.

Conclusion

Stage Change Materials are coming to age for the thermal direction of outdoor and indoor telecom systems.  The major criteria that the designer needs to have into account when designing with PCM are the toll of PCM, temperatures involved, packaging of the PCM, and the blazon of cooling mode desired (fan-driven or fully passive).  The engineering and the range of PCM'southward bachelor in the market place identify are growing every yr, and good designers demand to begin incorporating PCM's into their designs.

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Source: https://www.electronics-cooling.com/2019/03/thermal-management-of-electronic-equipment-using-phase-change-materials-pcms/

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