The VacPac is, in the simplest sense, a backpack refrigerator. Unlike regular refrigerators, however, it utilizes a stirling cooler to generate cold. It also has a temperature monitor/control circuit that keeps the interior temperature of the VacPac between 2-8 degrees Celsius, the suitable temperature range for vaccine storage. The temperature is also displayed to the user. The cold generated by the Stirling Cooler is circulated within the vaccine cavity using a convection system on the bottom of the cavity.
The stirling cooler, temperature monitoring circuit, and convection system are electrically powered and the VacPac supports a wide variety of energy sources for operation, making it truly a versatile off grid refrigerator. The VacPac can be powered by batteries, a car cigarette lighter, solar panels, a portable generator, or directly from an electrical grid. In addition, the battery can be chosen based on the user's needs. Anything from lead-acid to lithium ion batteries can be used as long as it can supply 12V and source up to 5A.
Vial Arrangement within the VacPac
Convection System in VacPac
In addition to the rugged and sturdy hollow aircraft aluminum, the VacPac was built to withstand drops from 1 meter, satsifying World Health Organization specifications for vaccine carriers. The vaccine vial racks were also designed to fit the most common of vaccine vial sizes and to miniize wasted space and keep the vials upright while mimizing their movement in transit. The VacPac can hold 1200 doses of vaccine. Current vaccine carriers can hold, on average, about 100 doses. Racks can also be replaced with thermal buffers or ice packs if necessary to prolong cold life.
On a full charge, the VacPac can last 18 to 24 hours. This, combined with its large vaccine capacity and multiple charging/operating options, can extend vaccine outreach sessions immensely. Current outreach sessions are limited due to low vaccine capacity and the inability to keep vaccines at the suitable temperature range for long periods of time. The longest lasting off-grid vaccine carrier can last for about 21 hours. However, once that time has been reached, there is no means of recharging. In addition, it has a very limited vaccine capacity.
To minimize the power necesary to keep the necessary temperature range, thus increasing the VacPac's life on a battery, there are about 1.5 inches of insulating foam around the vaccine storage cavity to minimize cold leakage from the VacPac.
The following are thermal images taken of the VacPac to detect cold leakage of a previous prototype of the VacPac. Insulation has been improved on the current prototype. The bluer colors are the regions of lower temperature. As expected, there are regions of higher cold leakage than others, but by using insulating foam, the amount of cold leakage is kept as low as possible.
A thermal image of the back of the VacPac
A thermal image of the joint between the Stirling Cooler and the Vaccine Cavity
Vial Arrangement within the VacPac 
Convection System in VacPac 
A thermal image of the back of the VacPac 
A thermal image of the joint between the Stirling Cooler and the Vaccine Cavity 
