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
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
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.
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
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.
OTHER COOLING SOLUTIONS
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
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
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
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.