VariChill
VariChill is a portable, solar powered, vaccine storage refrigerator for the developing world.
I headed the mechanical design, thermodynamic analysis, and industrial design on a four person interdisciplinary team for our year-long senior design project. We were one of three finalists in the Electrical and Systems Engineering department at Penn.
April 2017
Key Features
Thermoelectric Cooling
Unlike typical compressor refrigerators, the core technology behind VariChill is thermoelectric cooling, a cheaper and more environmentally friendly alternative. When electricity is run through the thermoelectric cooler (TEC), one side gets extremely hot, and the other side gets extremely cold. In order to effectively take advantage of this, I devised a TEC assembly that dissipates heat from the hot side of the TEC out of the fridge, while also cooling a cylindrical aluminum via. The via then cools internal fins which have air circulated over them, effectively removing heat from the inside of the fridge.
Dual Chamber Design
A drawback of thermoelectric coolers (TEC) is that they always need to be turned on to be effective. Because we were designing for places with limited to no grid power, we needed a high efficiency fridge that could run for at least a day without solar power.
This is where our dual chamber design comes in. There are two chambers: a cooling chamber and a vaccine chamber. When the vaccines need to be cooled, the chambers are connected and cold air flows throughout the fridge. Once the vaccine chamber has reached the desired temperature range, the chambers are separated and individually insulated. At this point the TEC is turned off, but because the two chambers are isolated, only the cooling chamber heats up, while the vaccine chamber maintains its cold temperature.
Actuated Chamber Plug
In order to actualize the two chamber concept, I designed a servo actuated rack and pinion mechanism that opens and closes the plug between the two chambers.
Testing and Results
The two chamber design enables us to keep the vaccine chamber in the required 35-46°F temperature range while introducing a 60% duty cycle for the TEC.
This corresponds to an increased battery cycle time of 1.6 times while only consuming 65 Watts, comparable to the power point of typical refrigeration compressors.
Remote Monitoring and Control
Another important part of vaccine storage is maintaining a record of the temperature the vaccines have been stored at, known as the Cold Chain. This is to ensure they have not expired from overheating. Our remote monitoring system logs the historical temperature data of the fridge so clinicians can be confident that they are administering valid vaccines.
Additionally, the web page has controls to remotely actuate the cooling and plugging systems. We never created a control algorithm to automatically manage the temperature, however, this could be easily implemented given our system architecture.
PROCESS
© Julian Mickelson 2021