Monday, January 16, 2012

Soda can solar collectors

One of the major projects for the trip to Guatemala was to built two soda can solar collectors to show the coffee producers an environmentally friendly way to dry their coffee beans and other fruits and vegetables.  Rufino had purchased the building materials and I brought down the needed tools and the fan and fan temperature controls. It is difficult to check a 4x8 sheet of plywood on to an airplane especially now, what with the baggage fees.  I also brought down some high temperature silicone caulk and can openers.  One of the first problems we had was the can openers that worked so well at cutting off the tops of soda cans in the  USA did not work on the same Coke cans in Guatemala. I don't really know what the difference was or why but it was enough that the can openers did not work.  That problem did slow down the project.  We used tin snips and the old fashion Swiss Army knife type of can openers.  Once we got the tops off the cans we drilled the three holes in the bottom of the soda cans.
While one group was working on the soda cans another group was drilling holes in the header that held the soda can in place.  We had to measure, mark, and drill the 2 1/2 inch holes used to hold the cans in place.  I had brought down several hole saws for this task.  We found the Bosch hole saw worked the best.  It cost more but was included in a set of four and actually was the better value.  The Bosch hole saws would also cut metal.
 The cooler air enters the bottom of the collector and as the air is heated by the sun it will rise and empty out the top.  We added a fan and a temperature control to aid the coffee bean drying process.
We built the collector frame out of a 4x8 sheet of plywood and 1x6's.   The lower header is for the cool air and the top header was covered with sheet metal and then we added a clear plastic over the whole collector to keep out the rain.
Some local visitors heard about this and stopped by to discuss their solar ideas.  Andre, the camera man for this project, filmed everything we did.  Once everything was complete we moved the collectors outside to apply the black paint.  You can see the lower header that is used to allow the cooler air to the enter the collector and the wider top section where the warm air exits the collector.
You can see the colors of the unpainted soda cans at the right hand upper corner.  It is surprising how well this simple system works.  The collector with the fan had a heat sensor in it that would start the fan once the air temperature in the collector was 110 degrees and the fan would not shut off until the air temperature dropped to 90 degrees.  The collectors we built are actually designed for heating homes in colder climates but they do work OK in Guatemala, as we found out.  A little bit of time out in the sun and the fan will start up.  The fan will stay on even when the sun drops behind a few clouds.
This is just a simple eight inch duct booster fan wired to a temperature control.  After talking to the coffee coop people we may just add a simple fan and a speed control and run the fan all the time.  The day the high winds knocked out the power and water we went up the mountain to learn about the coffee drying process but that is a story for a different blog.  The finished soda can collectors were loaded on the truck and moved up the mountain to the test site.

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