Since there was a lot of interest in our solar dryer, I will describe how I built it.
Everything started with a nice find by the road. One day I spotted a broken double glazing window by the rubbish containers. One glass was cracked but still in place and the other one was still complete. Since one man’s trash is another man’s treasure, I took it home in hope that one day I might build something nice from it. The day came and I had the idea to build a solar dryer to preserve the harvest from our food forest garden. I took inspiration from different models of solar dryer online and started creating my own design along the measures of my found glass window.
The design of the heat collector
I knew, I wanted to build a box, that would have the glass on top and that should have a dark background, which would heat up nicely in the sun. This way I would trap the sun’s heat in the box. Since I estimated the background to get quite hot on a full sunny day, I chose a metal sheet, which is usually used for roofing as a background. I just cut it in shape and lay it in my box. This kind of sheets also have a heat resistant paint and comes with prefabricated ripples, which allow an exchange between the metal sheet and the surrounding air both on top and underneath the sheet.
If I tilt my box 45°, the hot air would travel up. Just because of the basic physics concept, that hot air always rises. To improve the flow of air in my box I would need an in- and outlet. Inlets where drilled into the lower sidewall of my box with a 50mm diameter whole cutter. To prevent insect to enter my box I also attached a fine metal net over them. This way the air can flow nicely, but the pests are kept out. You wanna make sure, that you staple the net nicely all around and that you net is fine enough that even the smallest ants can not pass through. The outlet was created by just removing the upper sidewall.
Trays for the fruits and veggies
Now I had my heat collector. All I needed now was the trays to hold my harvest. I chose thong and groove boards and built flat tray boxes out of them. Since I was very careful to make them all the same size, I could now easily stack them one on top of the other. The thong and groove connectors would make sure that the trays sit nicely and no air would escape. I built only 3 so far, but the more trays you stack on top of each other, the higher your chimney gets. This will increase your airflow with each tray. Inside the trays I attached the same fine metal net, that I used for the inlet openings. On top of the last tray I put a lid, which I made from the same OSB sheet. Again I cut an opening with 50mm drill, which I covered with metal net.
How to connect the solar dryer together
To join my chimney, stack of trays and my heat collector I build one more tray. This one was more like a drawer but had only 3 sides. The one missing side joined with the upper outlet of my heat collector.
A few legs and handles an the whole thing was ready to work. Later I figured I would also like to dry herbs in my solar dryer. That should happen in dark conditions. So I added a simple cardboard lid on top of my chimney, which blocked sunlight from shining through the outlet wholes and onto my tray nets.
Performance and materials
The drier is put in the sun and your goods are being spread out on the different tray’s nets. Naturally the lowest tray dries the fastest, since the air can still absorb the most humidity. While the air rises through your stack of trays, it will more and more saturated and eventually escape through the outlet wholes in the top lid.
With nice weather I can dry 3 trays of apple rings, tomato slices, grapes, peaches or apricots in 1-3 days. If my product is not drying fast enough or regains too much humidity over night, I can also take my trays off and place them over my oven or radiator.
The material costs for my model where super low.
- one window – ideally tempered, clear and insulating glass (for free in my case, but probably the main cost)
- sheet of 9mm OSB,
- sheet of roofing metal,
- pack of floor boards with thong and groove
- metal net.
You could also use plastic net, which is cheaper, but it might get moldy over time, since there is often a bit of juice coming from your fruit slices plus with the high temperature of the air flowing through I prefer metal over plastic. With all materials you use, you wanna make sure to not use any chemical treatments or paints that might evaporate fumes.
With every project, I love to inspire myself by permaculture principles and you will certainly find many in this one. I hope you could follow my explanations and are eager to build your own solar drier, soon.