In this post in describing how I built my first solar dehydrator in Portugal. This was done in Quinta dos Sete Nomes, which is a lovely and lively co-op eco farm in Sintra. Building it took me in total a full week. To build one you need basic woodworking skills and tools.
The concept is very simple: To build an insulated box that heats up air with direct sunlight. Air starts moving up and by convection dries up everything that is laid inside. The box is perforated from the bottom and top part to circulate the air. Temperature doesn’t really need to get over 60° Celsius (140F), so that keeps things simple.
I’ve had only built few box solar ovens before, so this was a nice challenge. I was lucky with the materials, as I could use everything I found from the shed: There was plenty of wood, and most importantly, double-layered glass! There were three windows of equal size, plus one larger and one slimmer.
Windows are the most important part in my designs, so I wanted to use all of them. I dreamed about building a hybrid model, that works as a solar oven and as a dehydrator (by simply covering some parts of it when lower temperatures are needed). This seemed like a nice idea, but it would be a fire hazard, as it has wood and fabric inside.
The basis of the design was to cover sun from East, South and West by using the mirror to form three sides and have the door on the north side. The bigger window would work as a lid on top and the slimmer window would be the top part of the lower “sled”, where air is heated.
It also had to be high enough to be easy to use, so I decided to build a stand underneath to hold the windows on a level that you can use the dehydrator while standing up.
From this experience I learned that you need to have good tools, whatever you are building.
Good tools are essential
Tools that I used the most to build the dehydrator:
- Drill / Electric screwdriver
These two things need to be of good quality. When I started, I had a cheap saw and no drill. The first day I realised there’s no way to build the dehydrator in just few days if I need to spend minutes with every screw and tire my arms in two hours. Next day I went to AKI in Lisbon and bought myself a battery drill, BOSCH Easydrill 1200. It came with a salty price tag compared to the competition, but the ergonomy, the performance and ease of screwing and drilling were well worth the money. And it will work for several years, hopefully.
Now when the dehydrator is more or less finished, I wish I also had a better saw. The 2,5 € saw I used is ok for cutting, but not for cutting straight. The bottom part of the top box and the sled are fiberboard, also the door on the back. Fiberboard is difficult to saw and takes a lot of effort. For long cuts, I ended up drilling over 20 pilot holes to keep the saw in the line and then did a lot of sanding.
Rough sand paper was the fourth most important tool in this project. I also used a box cutter, hammer, caulk gun, pen, scissors and pliers. If I had a car, I could also get a miter saw, spirit level, plane, chisel and vise, but as for now I’m travelling with nothing more than my backpack and bicycle, I can only carry the most basic tools.
The design for solar dehydrator
I measured the dimensions of the three windows and designed the rest based on that (38 x 50 cm). The dehydrator has two parts: the top part to hold the trays and the lower part to heat more air. This lower part I call a sled, because the mitered ends make it look like an old sled. Mother Earth News has a very nice design on the same basic idea, if you have the right tools and materials.
For my dehydrator, I was limited by the available recyclable material and my rudimentary tools. I wanted to make separate top and bottom parts, so it’s easier to build and easier to move. Some dehydrators use also solar panels and fans, but I wanted this model to work only with convection of hot air. The parts are connected with a short stretch of garden hose.
I first drew a fast sketch to outline the frame around the windows, the stand and the sled.
I cut a fiberboard to be the base for the three windows. On the sides I added pieces of wood for support and made a small hole for the hot air pipe (garden hose).
The legs I made from various pieces of wood I found from the shed and cut them to 65 cm. This made the dehydrator tall enough to be used while standing. To keep it stable, I added four supports to the legs.
The long cuts for the fiberboard base were anything but straight, so I applied brown caulk to all cracks. You want to make the design as airtight as possible. This will help to create the desired convection in the dehydrator.
For the support of the windows, I built inside a small frame, and around it another frame to secure the windows in place. This outer frame formed also the frame for the door on the backside.
Using the electric drill made building much faster so I was able to do a lot during the day, but most of the second day was spent cycling, travelling with train and shopping for more tools and screws.
Door and sled
On fourth day I made the door on the back and glued on an empty Lays bag for a reflective surface (great excuse to eat chips). I was working on uneven ground and uneven table, so the top window was not sitting flush on top of the lower windows, so I employed my old trick and lined the top frame with weather-stripping tape for sliding doors. It’s not perfect, but it forms quite uniform insulation and the window sits nicely on it.
I also rushed to build the sled on the same day. This was not a good idea and I had to use a lot of caulk to fix my mediocre woodwork. For the the bottom, I cut another piece of fiberboard and painted some tiles black to absorb and store heat from the sun. I also found some more tiles from the shed which I decided to use in the upper part as a heat storage. The top tile fit perfectly inside my frame by chance and I gave it also a coat of black paint.
Heat storage for the dehydrator
Ideally I wanted to use solid bricks, painted black, but I was happy with the tiles. The idea behind this is to have a heat storage that regulates the hot air. When the sun is shining, it heats up the black parts, which store the heat in the dense mass. This heat is released steadily on the passing air, so it doesn’t matter if the sun is occasionally behind the clouds as the dehydrator keeps working all the same.
Under the top tile I added another large tile and four piles of smaller tiles in the corners. They add to the thermal mass, as the heat is conducted to the lower tiles from the top.
I drilled some holes in the sled for the incoming air and found a fitting piece of garden hose to connect the parts together. I added a layer of insulating tape on the pipe to minimize heat loss and finished it with aluminum tape. With the black tile on the bottom, I measured temperatures around 65° C (149F).
I realised that I’m also losing valuable heat through the lower layer of the top window, as it is hanging over on the backside of the frame. I gave it the same treatment of insulation and aluminum tape.
Improving the sled
I wasn’t yet happy with the performance of the sled, so I painted it black with spray paint. There was also no insulation on the backside and it got really warm during the day. This meant heat loss, so I cut a piece of styrofoam and plastic to cover the backside and glued and nailed them in place. I covered the styrofoam with some plastic to protect it and prevent individual beads from falling off. Both were found from trash bins.
To add more functionality, I screwed in three spokes to the lower legs to form a level to keep some stuff off the ground, like the frames I was going to build next. Most of the time, while building this dehydrator, I just did one step at the time without detailed plans and adjusted accordingly.
Trays and gluing to glass
Using the three windows limited the space inside to roughly 33 x 33 cm. I looked for possible materials, but as I didn’t have band saw or table saw, I was not going even to try to make trays with my crappy cheap saw. I went to Cascais to another AKI store and found bolsa wood and some natural fabric for the net material.
On the way I found a huge pile of kitchen cabinet wood behind large bins. If I had a better transport and better tools, I could have made 5 dehydrators with that wood and more.
I also bought smaller saw for finer cuts and cutting for metal, too. Also some thumb tacks, scraping bit for the drill and epoxy glue. I chose the glue, because it was advertised on the packaging to hold up to 150° C temperatures and work well with glass. Some bits of bolsa came off when I started experimenting with the dehydrator. The glue was ok, but some pieces of bolsa bent in the warm oven and didn’t create good bond with the glass.
I got back to the farm and glued in pieces of bolsa to hold up the trays in the dehydrator and left them to dry. This glue takes hours to dry properly. I also made some trays of the bolsa. Bolsa is not optimal material, because it’s very soft, very light and has a low ignition temperature. It will work for testing, however. In future I hope to use something else.
There was a rainy day in between, so I couldn’t do much with the dehydrator. I just finished the frames.
First try to dry
After a full week of working on the dehydrator, I was able to try the oven! The temperatures I got from inside ranged from 38° to 60° C (100, 140 F) with the trays inside. Anything above 38° is
great up to 65 is good. Above 38 degrees most bacteria stops multiplying, so the food will not rot.
Around 10 o’clock, I sliced up tomato, apple, orange and peeled so
me broad beans. Unfortunately some of the bolsa holders were not bonded properly to the glass, so some of them came off.
After a while I saw some condensation on the lower corner of the western side. I was expecting to see it and drilled the vent bigger on top the back door to improve the aeration.
I went to hike during the day and when I came back in the evening, the apples on the top had dried very nicely and also there was some noticeable drying going on with the other stuff.
It was very experimental at this point, as the top part is the hottest and the lower parts cooler and I had practically zero experience of drying foods.
The price of building a solar dehydrator
Luckily for me, I had most of the materials for free. Wood can be found free around waste collection points and glass from window factory or renovation sites, for example. Same goes for bricks or tiles.
Tools, screws, glue, caulk, paint, bolsa, fabric and some miscellaneous things I had to buy, which cost me easily over hundred euros. Biggest expense was the electric drill, but that was a one-time cost. Only for the materials I used maybe 30 euros.
If you find the materials for free and have most of the tools already at hand, you can build one for almost nothing.
Possible improvements and other remarks
- The condensation you see in the windows are between the glass layers. The front glasses had cracks. This is ok.
- Bigger airholes improve the circulation and reduce condensation inside.
- The corners and sides of the glass on top are sharp. It would be good to cover them with some padding like thick tape.
- Having a band saw or table saw would make building of the trays much easier.
- The frame should be built in a way that the holders for trays could be glued in to the frame, not in the glass.
- The inner frame and trays could be made from square aluminum pipe to make them stronger and more heat resistant.
- If you are using one in colder climate or during winter, add mirrors or aluminum reflectors in 45° angles.
- Build an adjustable vent to control the airflow.
- Slightly tilted roof would run off rainwater.
- Be creative! These things do not have to be perfect. If it works, it’s good enough.