Unfortunately I made my comments before I looked at John’s attachment. If the 4°C PCM is frozen in a refrigerator compartment which is at 2°C then no conditioning would be necessary. To safely cool these ice pack sin a refrigerator a compartment would be needed which is thermally isolated from the vaccine storage area. It would not be advisable to place warm PCM packs adjacent to stored vaccines. These isolated compartments are available on very few refrigerators, which is problematic.
If a 4°C PCM is being solidified in a compartment which is at 2°C then the freezing process would be very slow. Perhaps some tests have been done to measure the freezing rate
Claire Frija Madsen suggested placing an insulating material between the vaccines and the ice packs to keep vaccines from freezing. This technique will increase the temperature of the vaccine storage area in the carrier. What would be more effective is a layer of water between the sub cooled ice packs and the vaccines. The sub cooled ice packs will then freeze some of the water in the barrier and protect the vaccines from freezing. The thickness of the water layer would depend on the quantity and temperature of the sub cooled ice packs. If the entire layer of water in the barrier does not freeze it is thick enough. With support we could design a simple phase change barrier based on this principle.
After doing some back of the envelope calculations I came to the conclusion that the freezing of vaccines by sub cooled ice packs increases geometrically as the temperature of the ice packs decreases. In other words, an ice pack at -20°C will be 4x more problematic than an ice pack at -10°C.
One simple method to minimize the problem is to set the temperature on the freezer no colder than necessary. For example, if you have 2 days to freeze your ice packs you may be able to set your thermostat to a much warmer setting. This will also save energy and reduce the run time of the compressor. One of the barriers to setting the thermostat warmer than -6°C is that the ice packs will sometimes sub cool, which is to say that they won’t start to freeze until the temperatures of the water pack is close to -5°C. Elimination of sub cooling would allow the temperature of the freezer to be set much closer to 0°C.
We are currently developing an essentially no cost method to accomplish this. Eliminating sub cooling would then allow ice packs to be frozen at a higher temperature which would reduce the risk of freezing vaccines and the time it takes to condition ice packs.