Heat Sink

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Phase Change Materials

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Why PCM Heat Exchanger/Cooling Devices

Phase-Change Materials (PCM) have long been identified as prime candidates for thermal energy storage systems. Electronic enclosures installed in environments where solar loads are significant benefit greatly from a system which can absorb excess thermal energy during peak heating periods and discharge the excess heat at other times. A system to meet this need is a PCM heat exchanger. The heat exchanger can handle peak heating loads and maintain the enclosure at safe operating temperatures.

System Designs

PCM Heat Exchangers for Large Enclosures

There are several ways a PCM heat exchanger can be incorporated into the design of an existing electronics enclosure. A PCM heat exchanger can be placed in the side or bottom of the enclosure (see drawings). There are slight differences in system performance - but both will assist in handling peak heat loads when conventional systems are inadequate to remove the heat generated by the electronics.

The forced flow systems shown here are ideal for handling peak heating loads during the summer months when an enclosure is exposed to solar heating. Passive systems could also be designed for when power supplied to the enclosure is a premium.

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The heat exchanger consists of multiple aluminum tubes filled with one or more PCMs. The system is economical and easily modified for different operating conditions by either changing the tube diameter, the PCM within the tubes, or both. System performance can be enhanced by using two or more PCMs within different tubes.

Another option presently being analyzed is the use of a smaller profile system that relies on internal and external fins that sandwich a PCM region within the wall of the enclosure.

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System design calculations have been performed and indicate the system will be able to handle peak heating loads for 4 to 5 hours at a nominal heating load of 500 W. At the end of this period of time, the heat exchanger will shut down until the atmospheric temperature decreases to the point the PCMs can be solidified.

Heat removal for different tube configurations. Sufficient PCM exists within the tubes for over 4 hrs of operation at a 500 W load.

Heat removal characteristics were analyzed and are shown in the PCM Heat Exchanger Performance Figure. These results are for a simplified analysis of the system. A lower initial peek would be expected for actual system performance. More uniform removal can be obtained through flow control based on enclosure temperature, the use of multiple PCMs, or the use of different tube diameters.


Small Outdoor Enclosures

In many telecommunications applications, switching/signal processing equipment is continuously increasing in capabilities, and more importantly, heat dissipation. The equipment in enclosures are often than not will be placed outdoors. The housed equipment generates heat that must dissipated while keeping the air temperature inside the box within prescribed limits for optimum performance. Furthermore, the outdoor enclosure receives full solar irradiation, creating an extra heat that must be handled.

The problem of thermal management is acerbated when the enclosure becomes smaller. For example, for enclosures that are mounted on telephone poles or rooftops, solar loads account for roughly one-half of the total cooling loads. In addition, the small enclosure must be cooled using passive means only, which involves the use of natural convection and radiation as the only heat transfer modes available to the designers.

Storage of thermal energy can be accomplished by either raising the temperature of a working fluid (sensible heat) or by having a working material change phase (latent heat). Most systems typically use liquid to solid phase change for its ease of implementation. We offer ready-made solutions for the thermal management of outdoor enclosures using PCM's . One need not be reminded that power dissipation loads in the enclosures are continually increasing and that, in many instances, air-to-air heat exchangers and air conditioners alone will not be able to maintain the temperature and humidity levels demanded at reasonable costs (both first and operating/maintenance). Systems that incorporate PCM's such as passive heat moderators (such as a reservoir of wax-like materials)

PCMs can then be incorporated into the walls of an outdoor telecommunication enclosure. This method is particularly suited to smaller systems that rely on passive thermal control (natural convection through external fins, for example). PCMs also allow for thermal management in very cold weather since they can be used to retain heat to prevent below-freezing temperatures.

Battery Compartments and Systems

Telephone operating companies have a long history of powering switching systems with batteries. Instead of large central offices, current trends are towards remote switches, closer to the customer. This poses new problems, since manpower is now required at multiple locations. Further, the ambient temperature within the remote terminals is heavily influenced by the outside environment, described by a -40� C to +55� C temperature range and solar loading, which are telephony industry standards. The life of a battery is directly related to the load applied, proper recharging and most of all, the ambient temperature of the battery which must be kept at optimum conditions.

Batteries for back up service normally are stored in compartments attached to or inside outdoor cabinets. These compartments are exposed to solar loads, and must be kept at optimum temperatures, 77 �F (25 �C) as prescribed by the manufacturers. Furthermore, these compartments must allow for the proper venting of fumes that may be given off by the batteries in the course of their operating life.

Maintaining the battery at as small a temperature variation as possible is a unique challenge. The battery over the course of a year can see temperature swings from -40oC to 55oC. During the summer months, the solar thermal load must be prevented from overheating the battery. Overheating can be prevented by active cooling of the battery, or through the storage of the energy. Storage of thermal energy can be accomplished by either raising the temperature of a working fluid (sensible heat) or by having a working material change phase (latent heat). Typically most systems use liquid to solid phase change for its ease of implementation. We can offer you ready-made solutions for the thermal management of battery compartments using PCM's.

We can provide solutions for the thermal management of individual batteries or whole battery systems. The PCM devices are inexpensive and simple, and can be used to passively decrease the maximum temperatures during the summer months. During the winter months, heating elements are required, but the PCM can act as insulation during this time period to decrease the operating costs. Overall, the use of PCMs leads to smaller temperature variations and lower operating costs.


MJM Engineering Co
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