Background
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Ideas
The Zeer Pot (pictured) makes use of evaporative cooling. It's difficult to find a figure for the the extent of cooling possible; it likely varies with pot and filler types.
Contact
if interested, contact Matt Ball (matt.ball.2 a gmail)
Zeer Pot Experimentation
Test One, 14/9/08
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Setup The refrigeration team constructed two zeer pots on September 14, 2008. A large and small pot was created with ceramics from Home Depot; there were concerns that the pots may not have been porous enough in their walls. The bottoms, on the other hand, had quarter-sized holes in them for water drainage. The team did their best to seal the holes with painters tape and duct tape. Some water was still leaking as the 14-hour temperature testing began. The tests were performed in the Teer basement labs overnight. These labs are kept fairly cool so the team decided to shine two 100W, incandescent, reflector-equipped lamps on them through the testing to encourage evaporation. 20mL of water were placed in each pot and their temperature measured every five minutes via a LoggerPro device. A control water sample of ~200mL was placed under the lamps as well as a probe to measure the air temperature. Preliminary Results At hour four, it was observed that water leaking from the large pot had soaked the dangling t-shirt covering completely. There were also yellowish rings forming above the sand on each pot; possible proof of some evaporation. Data Just after shaking off the screensaver the morning after data-collection, the laptop taking the temperature data went to a blue screen. The attached LoggerPro file, zeer_test1.cmbl, shows the initial data point (which was autosaved) and the final temperature data, roughly 17 hours after the test began. The actual collected data can be visualized with an asymptotic rise in temperature over a four hour period from the initial to the final data point (the data was briefly seen before the computer crashed).
need info from Toni as to which Ti belongs to which probe Conclusions The pots in test one showed no cooling effect. T1 seemed to be somewhat insulated from the heating effects but none of the probes showed a drop in temperatures. In all likelihood, this is due to the loss of water through the bottom of the pot. Incidentally, the leaking water didn't really do much for cooling the pot. Sand in both pots was dry to the touch. By the test's end, the leaking water was gone and the cloth coverings had dried. It was qualitatively observed to be somewhat warm in the large pot, difficult to tell in the small one. Improvements
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Zeer pot experimentation
The design team spent the first two weeks working with zeer pots, refrigeration devices that use evaporative cooling. Click the above link for the documentation and results.
Insulated box testing
The design team is currently experimenting with insulated containers. The idea is to put the 5mL sample that we're trying to keep cold in an insulated box and surround it with other cold items like cold packs or ice. Click the above link for more information and test results.
Thermoelectric Cooler
Numerous portable refrigerators make use of a thermoelectric cooler. Wikipedia does a good job defining the Peltier Cooler: "The effectiveness of the pump at moving the heat away from the cold side is totally dependent upon the amount of current provided and how well the heat from the hot side can be removed." A very crude equation describes this, given here (http://en.wikipedia.org/wiki/Thermoelectric_effect#Peltier_effect).
Unfortunately, as engineers, we know this isn't always the case, and so a better (but much more complicated) description of the cooler's effectiveness is given by multiple factors, many of which depend on the environment in which they operate. This may prove difficult in trying to optimize a system.
The equation (http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=6325) has many unknowns, many of which may be difficult in trying to elucidate from manufacturers we try to contact.
As a last resort of trying to understand these heat sinks, we can try and test several out, by using a method described in this paper (PeltierConstantDetermination.pdf), which has been uploaded. But based on experience, it seems that most commercial coolers are very broad in their ability to cool in different environments. Understanding all this (something I don't have enough knowledge to do...yet), will be the next step in trying to build a fridge from scratch.
Ideas
- The zeer pot (pictured) makes use of evaporative cooling. It's difficult to find a figure for the the extent of cooling possible; it likely varies with pot and filler types.
- another zeer pot link: http://solarcooking.wikia.com/wiki/Pot-in-pot_cooler with experimentation results
- Cold Packs consist of ammonium nitrate (fertilizer) mixed with water. They can allegedly drop to 35F for 10-15 minutes. Combining the packs with a thermos or some type of insulator could allow us to maintain the required low temperature. Consuming a lot of the cold packs would be an immediate problem; ammonium nitrate would need to be produced on site.
- Traditional small-scale refrigerators could be modified to run on solar. A Duke design team has worked on this concept in the past.
- Thermoacoustic refrigeration would provide for some interesting experiments but would probably be less effective than a traditional design. Thermoacoustic and traditional methods both require electricity and the latter approach has higher efficiencies.
- Another possible design would be using a readymade portable USB mini fridge and powering it with AA batteries . The device costs about $35 and the AA battery power source can be built ourselves. The thing that we need to figure out is how much total power (C) the USB fridge needs, and how much the AA could provide. We could modify the design to incorporate larger C batteries or even 9V batteries.
- The author of the Power device says there is only a 60% efficiency problem we will need to fix. Better find some EE kids!
Past work
Click the link above for a page with reports and summaries from "legacy" work with this project.
Contact
if interested, contact Matt Ball (matt.ball.2 at gmail)