SR article

The final changes to the design of our solar dryer

By improving the design of last year by evaluating it and doing some experiments at ASSS, we have come to a final design. This final design of the solar dryer has been built in Jakaba.

Last year’s design was of an indirect, passive solar dryer. It used a separate air heater and drying box. We have kept this basic design. However, we wanted the solar dryer to have a higher efficiency and thus we decided to make the dryer active by installing fans that use electricity from a solar panel, following the advice of solar drying expert Fred van Hessen. Also we increased the size 8 times by simply putting 8 air heaters next to each other. With this design, the capacity of the solar dryer is big enough to dry the crops of the community garden of Jakaba.

 

Figure 1. Our final design of the solar dryer

The design itself

The final design is an indirect, active solar dryer, as explained above. The 8 meter version is the same as the 1 meter version, but then put next to each other 8 times. That is why the small version is elucidated here. Because one solar panel can provide energy for more than one air heater, four air heaters share one solar panel.

Because the design is indirect, the dryer should consist of two separate parts, an air heater and a non-transparent drying box. The air heater consists of a frame that is approximately two meters long for which red timber was used. Two beams are placed parallel with an aluminium corrugated sheet stretched between them at the bottom side and a plastic corrugated sheet stretched between the beams at the upside. The aluminium sheet has been made wider by gluing half a sheet to a full sheet with silicon glue in order to make them as wide as the plastic sheet. This way we use the materials in an optimal way. Between these sheets, a space is created in which air can be heated. The aluminium sheet is painted black in order to absorb heat and the plastic sheets lets sunlight through, as it is transparent. This construction is placed under approximately 10°, which causes a convection flow of the hot air between these two sheets. At the top of the beams, two fans are installed that increase the airflow even more. The fans are placed in a beam that has been chiselled in such a way that the fans fit perfectly. These fans use electricity that is generated by a solar panel. The space between the fans has been closed to improve the quality of the airflow. On the other side of the fans, the frame continues horizontally. Drying boxes can be placed on this horizontal spot. The whole air heater has been raised 8 centimetres from the ground to prevent dust from coming in. Also, an air filter has been placed at the inlet of the air heater to stop dust from entering.

Because the large solar dryer consists of eight air heaters, 16 fans are needed. We have used simple computer fans for the design, which have a power of 0.88 Watt. We used 2 solar panels of 10 W to provide for this. In order to reach this power, the sun has to shine directly on the panel. In days where the sun is not shining directly, less power will be generated and it might be better to connect less than eight dryers to the panel. Each solar panel was connected to a PWM controller. This PWM controller has 4 outputs, so after each output a splitter was inserted in order to connect 8 fans to each controller. With this controller, the speed of the fans can be adjusted. With a high speed, the air has less time to be heated but the air flow is larger. This could be convenient on a hot day with food that is very moist for example. For more details about the wiring system that we used, you can take a look at Appendix B.

Figure 2. The air heater from the back. The mosquito net can be seen, just as the pieces of wood that raise the frame some centimetres.

Figure 3. The air heater from the front. The fans and the black fabric to make the heater air tight are shown. Also, the spot where the drying boxes are placed can be seen clearly.

The drying boxes are made of jerry cans and old bicycle tires. To increase the capacity of the solar dryer, one side of the jerry cans is cut out and two jerry cans are melted together by using the pieces that were cut off. This way, the drying boxes are twice as big. Bicycle tires are glued around the edges to prevent unwanted air flow in the box. Inside the boxes four food trays are placed on strings of metal wire. These trays are made from metal rods, bent in a rectangle and tied with metal wire, with mosquito nets or other gauze sewed on top.

On one side of the box, an air inlet that is shaped as a fan is cut out at the bottom of the centre. There is a piece of foam attached to the box at the outside around this air inlet. Because this is a soft material, it makes sure the drying box fits tightly against the air heater.

On the opposite side two air outlets are cut out, placed at the upper left and right corner. In front of these air outlets, mosquito nets are glued to prevent insects from getting inside the box and spoiling the food. At the side of the air outlet, a large opening is made with a door in front of it. The edges are again closed by bicycle tires and the door is closed with a string of metal wire around a small screw. With this design, two drying boxes can be placed on each air heater.

Figure 4. Drying box. The door and air outlets can be seen clearly

Figure 5. Drying box from the inside. The air inlet and the metal wire strings for the food trays are shown.

Our choices regarding the design

For this design it was very important that people in The Gambia could build and maintain the solar dryer themselves. We have made some choices that might not seem obvious at first sight. That is why some noteworthy choices are explained here.

We have tried to keep the design as cheap as possible and we wanted to make sure one needs simple tools to build it. As you can find in the costs, the largest expenses of the solar dryer are found in the air heater. The drying boxes can be made from local materials that are quite cheap. Since local people cannot afford themselves an air heater but they would be able to buy just a drying box, we had decided to decouple them. This way, people can rent a place on the air heater for a small amount of money. This kind of design also opens opportunities for the future. If people could invest in their own drying box, it would be realistic for them to get access to this facility. We found that in Africa it is very common to divide a product into smaller parts, so people can afford to use them. You can find very small bags of washing powder, salt and sugar sold for only a few Dalasi, which can only be used for a few times. Investing more money in a bigger bag for the future is not common, since people spend most of their money they earn that day immediately on food and basic needs. We wanted to implement this dividing-into-smaller-parts system in our design, not only because it is cheaper and more common to the local people, but also because a smaller drying box is easier to repair or replace than a large drying box. With these arguments in mind, we decided that the air heaters should be sturdy and durable while the drying boxes should be separated from the air heater, easy to make and cheap. This way, the air heater will be working for a long time with little maintenance and the boxes are easily repaired or replaced.

One of our other requirements was that the solar dryer should consist of local materials as much as possible. We wanted to get the local materials really ‘locally’, so as close to Jakaba as possible. However, the availability of materials at the coastal areas is much higher, so we were forced to buy some materials there, like the plastic corrugated sheets and the solar panels. The only materials that we did not buy in The Gambia are the fans and some wiring, because we were not sure if these things were available here. We got some white plastic gauze from Fred van Hessen’s drying factory which we used for the fruit trays as well as some thick fabric that we used to close the gap between the fans and the corrugated sheets. We used this to make the air heater more air tight. It was very difficult to do this with wood.

Since Fred van Hessen told us that fans are absolutely necessary and the previous group also advised us to make the design somehow active, we had decided to install computer fans. These fans can be taken out of a computer which means that one can get them in The Gambia, although it may take some time to find old computers that are not being used anymore. It turned out that one can buy some types of wiring in The Gambia, but we do not know for sure if the right type is available.

Most of the materials, including all drying box materials, that we bought were available at the Monday market in Wassu (see Appendix A) near Jakaba. However, some materials were only available in Bansang, a city across the river as can be seen in Appendix A as well. All materials were much cheaper here anyway. However, this has more transports costs, which is quite inconvenient for most Gambians. For example metal corrugated sheets are widely available and cheap, just as nails, wood glue and mosquito nets. Screws however are difficult to find in a certain size and only available in Bansang. Drill bits are very rare so we have borrowed these from ASSS when working in Jakaba.

The reason we have used relatively expensive plastic corrugated sheets although these are only available at the coast, is that it is the only material available in The Gambia that is transparent and durable. For example, there is also some kind of plastic foil, which is very cheap but will not last longer than a few months. Red timber was used for the frame, which is also quite expensive but it is the only wood available that does not get overflown with termites. However, red timber is also very hard so a drill is absolutely necessary, which might be difficult to get for the average Gambian.

The drying boxes were made from jerry cans because they are very cheap and they can be bought in large amounts in every small town. They are used to keep oil, so they are unfortunately not easy to clean, especially the ones with yellow palm oil. It is easy to get hot charcoal for melting two boxes together, since Gambians use it all day through anyway for cooking and brewing tea. A disadvantage however is that gas masks should be used to protect your lungs. However, they are not available in the inlands, so we borrowed these from ASSS as well. Bicycle tire glue (‘Mastik’) and used bicycle tires are very easy to get. Especially used tires are available at very low prices. All the materials for the food trays are available in Wassu. For sewing the gauze to the metal rod a needle and normal thread were used. Iron thread would have been stronger but that was unavailable. The metal wire, on which the trays are placed, is not extremely strong but it is again cheap and very easy to replace.

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