Warming and Cooling Systems
for Product Development

There are numerous heating and cooling applications that involve small amounts of heat gain
or loss, where it is advantageous for reasons of safety, convenience, etc. to eliminate the need
for electricity. One way that this can be accomplished is by using Phase Change Materials
(PCMs). Examples of existing products that employ phase change technology include:

• Hand Warmers that are used by football fans, snow skiers, ice fishermen, etc.
• Infant Heel Warmers that are used to help maintain the body temperature of newborn infants.
• Baby Bottle Warmers that keep the milk warm prior to and during use.
• Thermal Pads (cold and warm) that are used in hospitals, nursing homes, etc. to
raise or lower the body temperature of patients.
• Carrying Containers (cold or warm) for biomedical and biological materials.

PCMs utilize their latent heat of fusion to absorb, store, and release thermal energy during
phase conversion between solid and liquid phases. Upon melting and freezing, a phase change
material absorbs and releases substantially more energy, per unit weight, than a sensible heat
storage material. The latent heat of fusion of many PCMs is substantial and can be used as
either a heat source or a heat sink.

Of particular interest for many applications is the fact that PCMs release a large quantity of
energy in the vicinity of their melting/freezing point (i.e. phase change temperature), but the
equilibrium temperature of the substance remains the same. As a result, PCMs are essentially
self regulating with respect to temperature and are able to maintain temperature within a very
narrow range without the need for any type of external control.

In the example, shown in Figure 1, the temperature of 500 mL of solution, in two identical, non-insulated
containers was monitored for 4 hours. One of the containers was filled with 100^o F
water and the other container was filled with PCM that has been “activated”. The graph shows
that the temperature of the water decreased quickly and reached ambient temperature in about
4 hours. The “activated” PCM reached a peak temperature of 105^o F after 30 minutes and
remained warm for several hours.


There are several factors to consider in determining if a PCM is a viable option for your
application. They include:

• Temperature – Materials with phase change temperatures from –28^o F to 215^o F are
readily available.
• Cost - The cost of PCMs range from a few cents per pound to hundreds of dollars
per pound.
• Safety – Some PCMs are very safe even for use in food contact or healthcare
applications.
• Toxicity – Some PCMs are hazardous materials that may be difficult to use and can
present disposal problems.
• Stability – Solutions that contain PCM are normally very stable (unlikely to
spontaneously “activate”) at low concentrations and considerably less stable at high
concentrations.
• Reusability – Some products that contain PCMs are intended to be used only once
and then discarded. Other products are “regenerated” and used over and over
again.

One of the most commonly used PCMs is Sodium Acetate. Sodium Acetate is relatively
inexpensive (about $2.50 per pound, at retail), non-toxic, and has a melting point (phase
change temperature of 137^o F). In a typical application, the Sodium Acetate is dissolved in hot
(e.g. 180^o F) water and then cooled to a temperature below the phase change temperature
without allowing the PCM to crystallize, creating a super-cooled (or super-saturated) condition.
The solution can then be “activated”, when needed, to provide heat.

Most applications that employ Sodium Acetate involve concentrations between 1:1 and 2:1
(Sodium Acetate:water). The amount of heat generated depends on the concentration of
Sodium Acetate and lower concentrations are inherently more stable and can be cooled to
lower temperatures before they will spontaneously “activate”. Higher concentrations produce
more heat when they are “activated”, but they are also more prone to spontaneous activation.

There are several different methods that are used to “activate” sodium Acetate solutions.
Examples include: “seeding” the solution with Sodium Acetate crystals; creating nucleation sites
by flexing a metal spring that is immersed in the solution; or rubbing glass or ceramic beads,
that are immersed in the solution, together.

An important factor to consider in choosing an “activation” method, is whether the product will
be used only once, or if it will be “regenerated” and reused multiple times. For example,
“seeding” the solution with Sodium Acetate crystals could be a good choice for a single use
application, but it is probably not very practical for multi use applications, because the user
would need to provide “fresh” seed-crystals each time the product is used.

One of the reasons that products that containing Sodium Acetate are so popular is because the
phase change temperature is close to that of the temperature of the human body (e.g. 98.6^o F).
A 1:1 solution of Sodium Acetate and water, for example, will typically produce a peak
temperature of about 105^o F, which is nearly ideal for applications where the intended use
involves maintaining body temperature in a cold environment or providing a slight increase in
temperature for therapeutic purposes. Highly concentrated Sodium Acetate solutions can reach
temperatures that approach the phase change temperature of Sodium Acetate (137^o F), but
they will never exceed that temperature because the Sodium Acetate re-dissolves at 137^o F and
moderates the reaction.

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