Dehydrator

Problem Solutions

The idea was to turn the existing solar dehydrator at ISK into an all weather dehydrator. In Nairobi the weather is not always consistent, this means that a solar dehydrator doesn't always work as well as it should, if the sun isn't strong enough we wont really be able to use the dehydrator. So I came up with the idea to turn it into an all weather dehydrator. First, I had to make it more effective when the sun is out. I did this by taking an extremely reflective plastic curtain (instead of more mirrors for cost), cutting it into the walls of the dehydrator, so that it could generate more heat. After this I started to come up with the Idea of adding an external heat source that could be used instead or with solar.

Research & Design

The planning phase was the most difficult. I came up with the idea of using the biogas system i created before, to generate the heat for the dehydrator. I would have done this by attaching a stove to the biogas system like usual, but use it only for heat instead of cooking. After many ideas and models, I came up with an efficient way of pumping heat through the system. I settled upon using a heat plate along with a Peltier element (Thermo Electric Generator). It would work by having the heatplate right on top of the flame and then having the peltier's hot side be right above that, all enclosed in metal. The heat sink and cold side would be outside the system to create the heat differential so that power could be generated. We needed the TEG to power a fan so that the heat could not only flow smoothy, we could also control the flow. When I started sketching, taking measurements and modelling, I realised that it would be really big. Way bigger than I had anticipated. The system was higher than the actual vents at the bottom of the dehydrator.

Biolite

My supervisor, Mr Sudra, suggested that we use a product with the same technology as my idea, something that is already being sold and used, but from there, adapt it so that it can work for the dehydrator. This way, the design would be a lot sleeker and the dehydrator would be more effective. We decided to use the BioLite CampStove. It works in a very similar way to my original design, having a TEG (thermo-electric generator), a heatsink and a flame. However, the difference is that it measures the heat of the flame and charges a battery, which allows for charging. This, in turn, allows for a much more precise way of regulating air flow, and overall is more efficient. The only downside is the size; the dehydrator is huge, and the BioLite machine is tiny. You need two people to carry the dehydrator, while you can carry the biolite product in one hand. This presents an issue of whether the CampStove would be able to generate enough heat for the dehydrator.

Modification

Stats;

TEG
Heat sink
Oxygen fan
Heat measure

3200Mah Battery
3 or more watts of power via USB based on heat
Boils 1L of water in ~4.5mins

My supervisor, Mr Brown, suggested that we keep everything the same and only replace the TEG with the BioLite one. This way, we wouldn't struggle to provide heat to the dehydrator. Another thing we decided to do was keep the stove inside the dehydrator itself, which would not make it more efficient by preventing heat loss, but it would help reduce the cost of materials. The TEG would sit outside the dehydrator, while being connected to the heat plate inside, to allow the heat differential to work.

Materials

A lot of materials were needed to bring the project to life. We needed a fan that could connect to the TEG, a heat plate made out of a highly conductive material and a way to transfer that heat as effectively as possible to the TEG. I decided to use aluminium as the heat plate and as a sleeve to encase the BioLite hot end. The sleeve would be screwed onto the heat plate to prevent any gaps, and the heat plate would rest on stands sitting on top of the stove. This would not only allow for minimal heat loss it would keep the design sleek and durable.

Fan

The fan was the most difficult part of the whole process; the school only had PWM fans in stock. These fans have a mole connector, which means they can only run when connected to an Arduino board or a Raspberry Pi. Connecting the fan to an Arduino and then connecting it to the fan would be way too complicated. I decided to try and solder the PWM fan to an old USB cord, and that way it would be able to run on the TEG. The issue was, the fan has three cords, and the USB has two; I needed to figure out which sides were positive and negative and which side controlled the fan speed and then connect it all correctly so that it could work without killing the fan. I tried to do this at least three twice with no luck. I watched YouTube videos, asked my supervisors and my friends who have some knowledge on how to do this. Finally, I managed when one of my supervisors helped me and taught me how to solder correctly. By identifying the correct sides to connect the USB to and cutting off the speed controller, the fan was connected successfully, and this time actually turned on when connected to a power outlet.

Soldering Process

Soldered Fan

Fan Spinning

Sleeve

A sleeve was needed to be able to provide as much heat as possible from the heat plate to the hot end of the TEG. The idea was that the sleeve would need to fit on the TEG as snug as possible while being bolted to the heat plate. I needed to first design and model the sleeve to get a better sense of how it would work and fit. I first took the measurements of the hot end, and then, on Fusion, designed a rectangle with a hole in the middle for the hot end rod to sit in and then printed them out (3d printed). I ended up printing two sleeves. The first one was pretty good, but the problem was that it did not have a way to connect to the side of the dehydrator. So the second one was designed with a flange that would help it be screwed on. The aluminium pieces that were available were not only too small to be cut with a flange, but also the machine we had was not really able to cut them out, and finally we realised the side of the dehydrator is strong enough to hold the heat plate and the TEG up. So I reverted to the first sleeve design, but this time I was going to cut the aluminium, when the hot end rod was measured again, there was a difference in size from the base of the rod to the end, this was an issue. Because no air was allowed between the rod and the aluminium the sleeve needed to incase it perfectly, my supervisor helped me use the machine and cut out the aluminium so that there was a cone shape on the inside that perfectly matched the shape of the rod. After this was done, the last step was putting it all together.

Sleeve Designs

Measurements

Stands

There needed to be stands between the heat plate and the stove so that when the heat plate would not be able to burn the stove. The problem was, I needed a material that was heat proof, which is not easy to find or buy, especially in Nairobi. However, the school had polyurethane, which is a rubber like substance, similar to silicone, that does not burn and needs a temperature of around 180 degrees Celsius to melt. Molds were created to set the polyurethane overnight. Unfortunately the stands were not finished in time and the gap between the sleeve and the plate is because of no stands.

Putting It All Together

TEG Attached Outside

TEG connected to Sleeve and Fan

System Inside Above Heat Plate

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