Solar Food Dryer.
Bacteria cannot survive without moisture so, we can preserve foods by removing all moisture from them. Electric dehydrators exist, but can be expensive to run. Food can be dried directly in the sun but it takes several days and there is always the danger that the food could spoil in the process. To speed up the drying we need to use higher temperatures, typically 40 - 60 degree's C. The aim of this project is to produce an efficient solar powered food dryer.
This design is based on the original "Brilliant Scandinavian beer can, hot air heater", that has been copied by many people, simply because it works. Basically imagine a vertical five foot long tube made up of aluminum beer cans. The cans have all been holed top and bottom so air can pass up the tubular column. The cans are exposed to sunlight which heats up the cans. This in turn heats up the air inside the cans. Hot air rises towards the top of the column, causing colder air to be sucked into the bottom of the column, ie it works by convection. Typically, if the air entering the bottom of the column is 21ºC, then it will typically end up being about 40ºC+ at the top of the column. To conserve heat, the beer can columns are contained in an insulating box, with a window to let the sun heat up the cans. We then feed the hot air into a box containing net trays of food to be dried, the air and moisture then escapes via a vent at the top of the dryer box.
Beer and soft drink cans.
Beer or drink cans are made from either steel or aluminum. Steel would rust, so we need to use aluminum cans which can be tested with a magnet. Cans come in a range of lengths and it makes a neater job, if all of the cans used are of the same length. I personally do not drink beer but I had two friends who were prepared to empty full cans ... in the interests of science !. First the cans need to be washed and dried. Next the holes need to be made in both ends of the can, without crushing it. I did the earlier cans with a hole saw, but it was sometimes a bit dodgy because occasionally the saw teeth picked up on the can and bounced them around the workshop at high speed !. In the end I decided that I did not really want nice clean round holes, because irregular shaped holes would cause better turbulence and make sure that the air touched every part of the inside of the beer cans. So instead I mounted a 32.0mm spade bit in a drill press and used the press to punch a series of radial slots in both ends of the can. It does not look very neat, but will never be seen and is probably much more efficient than the 'pretty' method.
I think that before I make the next panel I will make up a V shaped punch to use in the press, mainly because it will present less of a crushing pressure on the can, than the square edge. The cans are very flimsy and will crush easily, if too much pressure is applied. .
Making the can columns.
To make up the tubular columns the cans have to be glued together. The job is made a lot easier if you make up a simple V shaped gluing jig out of two pieces of wood, nailed together. If the jig is made long enough, a weight can be applied to the top can to keep the cans together while the adhesive sets.
The adhesive I used was a 'high grab' adhesive such as Gripfill or Hardasnails. Until I get the sheet materials for the case, I am not 100% sure of how many cans I need, so I have initially made up eight columns of six 'long tom' sized cans. If I decide I need them longer, I can always add some extra cans. The cans incidentally, are each 170mm long by 68mm wide.
The can stack.
I am not going to bother trying to measure it all out, but instead will built the case around the can stack, that way I will not be able to over-look anything. Never having built one of these before I want to try and build in as many options as possible. One is that the original 'Brilliant Scandinavian heater' was a full sheet size, ie 2.4 x 1.2 metres. I think that the output temperature is proportional to the stack height and the total airflow is proportional to the stack width. The can stack (heat exchanger) I have made is approximately quarter sheet size ie 1.2 x 0.6 meters. My gut feeling is that the width is going to be OK because a one cubic foot drier cabinet, does not require a high volume airflow. On the other hand, I want to know what the temperature difference would be between a 1.2 meter and a 2.4 meter long can stack (for future projects). So I plan to build two of the quarter sheet size heat exchangers, so that I can try cascading them (feeding the output of one into the input of the other).
Another thought is that the above stack may well work well on an exceptional day, but how many of those do we get in UK ?. One must also consider the seasonal temperature, at the time when food is ready to be harvested. So a part of my project will be trying to increase the potential output temperature as high as possible.
Each tube acts as an airlift, and the efficiency of an airlift is proportional to the height difference between the input and the output of the lift tube. Against this, maximum transfer of the suns heat to the cans takes place when the heat exchanger is perpendicular to the sun, so there are several variables to play with here.
With the quarter sheet heat exchanger on it's own and UK's weather drying might be a slow process. One way of speeding things up is to increase the temperature of the air passing through the heat exchanger. One method of doing this would be to fit a solar concentrator to the heat exchanger box. Commercial domestic dehydrators appear to work around the 130ºF - 140ºF or 55ºC - 60º C range.
Whoops ! .....
I completely forgot that I had started this page !. However I did continue with the basic hot air project and details can be found by clicking on this link ....
Solar DIY .