Past refrigeration work

Report overview
the project was carried out by Odelia Ghodsizadeh, Scott Kasper, Sean McGuire, and Brian Robinson for the BME262 design course taught by Dr. Robert Malkin

• the team is designing for a 1-2L cavity at 2-8degC for less than $300
• shaker/solar-powered fridge to deliver 6W with an energy storage mechanism
  • testing was performed on a small volume "soda-fridge" ($40) that employs a fan to suck air over a heatsink and out the device's back
  • initial testing indicates that it takes about ten hours at 16W or five hours at 23W to reach 8degC
  • purchased 28W solar panel ($150) able to generate 23W on sunny days; clouds reduced output to 1W
• ran a 19hr test after powering the fridge for 5hrs
  • utilized a phase change material from "TCP Reliable, Inc." to (presumably) solidify during the fridge's cooling, and absorb latent heat energy as the box warmed. Melting point was 5degC
  • fridge remained below 8degC for 12hrs; ambient temp was 21degC
  • team stated desire to add additional insulation to the fridge
• a test involving simulated ambient temperatures of 44degC (via a hair dryer) saw the fridge hold at about 15degC with constant grid power
• team hypothesized that the box could fall below 2degC (their lower temperature bound) on cold days
  • sealed the exhaust ducts to bring the temperature back up; the ducts normally carry away latent heat from the box
• report includes extensive list of fridge-related patents

Comments
It was surprising to learn the device only worked with a fan and heatsink. We could easily build one of these devices for a few dollars. The solar decision does not seem like a good idea; the device becomes far more fragile and too susceptible to the weather. They discussed including shaker-flux generators but did not pursue the idea.

Report overview
The report was signed by Cynthia, Brittany, Elise, Josh, and Graham Gipson; their team website is here and has extensive documentation.

• four nested styrofoam cups were covered in various layers of bubble tape, more sytrofoam, and insulative tape were filled with an ammonium-nitrate, water mixture to create a container that remained below 10degC for 10hrs at a cost of $2.
• a good deal of heat-transfer modeling work was also listed
• compiles other potential endothermic reactions on page six of the final report
• did some thinking about zeer pots
• mentioned the GreenBox, a product made by Entropy Solutions, that can maintain a temperature from -20degC to 50degC for two to four days. The customer specifies their target temperature and ES customizes an insulation layer that will correspond to that temperature. All the materials are supposed to be eco-friendly, hence the "Green." The product is designed for shipping needs.

Comments
There was a passing comment in the paper about ammonium-nitrate being readily available but this was not expanded further. There's also no indication about the lifetime of these devices. I would presume they're one-time use; this isn't too much of a problem if they're so cheap though. There's a noticeable lack of information on the amounts of reagent used in the tests.

Report overview
The report was compiled by Ayaa Maggay, Kathryn Olson, Bindi Patel, and Aakash Datt.

• the device consists of two chambers separated by an aqueous chemical solution of sodium acetate trihydrate
  • claim that this device may be reused once water evaporates; the sodium acetate may not stay in its trihydrate state after the water dries up
• some thermo calculations indicate they will add nearly 500g of reagent ($30) to keep the box cold for 12hrs in 38degC ambient temperature
• 6mL of water would need to be added to the system every 20 minutes in their worst case scenario
• the device would be heavily insulated ($20 worth of sytrofoam blocks for each box)
• thermo calculations completed to determine cooling and insulative effects

Comments
The large quantity of reagent required and the need to rehydrate the device frequently are certainly drawbacks. The device has not been built yet but it seems quite feasible and well thought out. It would be worth contacting the group to see if they ended up building a prototype.