ASHRAE Journal - February 2022 - 22

TECHNICAL FEATURE
FIGURE 8 Melting-solidification process.
Vaccine Temperature
FIGURE 9 Phase diagram of carbon dioxide.
Solid + Gas Curve
Solid
D
C
B
A
Cooling Surface Temperature
Enthalpy
Fleisher11 provides a very good overview of fundamentals
and applications of TES using PCM. The melting
point is the primary property in selecting PCM material.
In general, use a PCM with the highest possible melting
point that is still below the required vaccine temperature.
This will enable using the cooling source with the
highest possible temperature. The next important criterion
is the heat of fusion, and the higher the latent heat,
the better.
The other criteria are:
* Single-phase specific heat (high specific heat is
preferred);
* Thermal conductivity (the higher conductivity is
preferred);
* Solid and liquid density difference (has an impact
on the containment structure due to contraction upon
solidification;
* Chemical and physical stability over repeated thermal
cycling with repeatable and consistent melting and
solidification cycles;
* Compatibility with the casing material;
* Environmental safety;
* Nonflammable;
* Nontoxic;
* Cost-effective.
The PCM market is dominated by paraffin products.
The Department of Agriculture and the National Science
Foundation have sponsored research to investigate the
potential for vegetable-derived compounds to become
a significant factor in the PCM market.12-14 The melting
temperature of about 300 different fat- and vegetableoil-based
PCM ranges from minus -90°C to 150°C
22
ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2022
Sublimation
Point
Solid + Vapor Curve
Triple
Point
Liquid
Critical
Point
Liquid +
Vapor Curve
Temperature
(-130°F to 302°F) with latent heats of fusion between
150 kJ/kg and 220 kJ/kg (64 Btu/lb and 95 Btu/lb) .
Dry Ice
Dry ice is the solid form of carbon dioxide (CO2). Dry
ice is used to passively cool frozen foods in portable
transportation devices when the use of refrigeration
systems is not cost-effective. Large quantities of dry ice
are used for transportation of the COVID-19 vaccine.
Packing the vaccine container with dry ice is the current
passive cooling technology used in portable transportation
devices and is applicable to the Pfizer vaccine.
Dry ice is colorless, odorless and nonflammable. It is
not toxic, but is considered a hazardous material in the
U.S. The U.S. Department of Transportation issued a
Safety Alert for Operators in 2020 for dry ice.15 High levels
of gaseous CO2 can degrade cognitive functions and
present an asphyxiation hazard to persons subjected to
it. It is extremely cold; skin contact with dry ice can lead
to severe frostbite.
A phase diagram of CO2 is shown in Figure 9. The figure
demonstrates three thermodynamics phases: solid,
liquid and gas. The critical point of carbon dioxide is
7,377 kPa (1070 psi) and 31°C (88°F). Usually, carbon
dioxide states above the critical pressure and critical
temperature are referred as to supercritical fluid; and
the gaseous states below the critical pressure on the left
of the liquid phase are referred as to vapor states. The
phases are separated by the saturated curves: solid +
liquid, solid + vapor, liquid + vapor, and solid + gas (top
right). The dry ice triple point is 519.8 kPa (75.4 psi) and
-56.4°C (-69.5°F). Heating dry ice at the states below the
triple point turns it into vapor and this process is called
Gas
Solid + Gas Curve
Supercritical
Temperature
Pressure
Melting Process
Solidification Process
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