One of the main problems with food security is distribution and storage of food. Most agricultural produce is lost to pests, rodents, and contamination by the elements all before it can reach the storing phase. The reason is that many countries lack adequate storage facilities. For example, 10-20% of Africa’s grains are lost to scavengers each year (Royte). Implementing solar dryers in countries can help prevent food from spoiling even in sub-optimal storage conditions. Once storage is no longer an issue, the government can help distribute these crops to areas in the country where they are needed.
Solar drying has two phases. Phase one is when the outermost layer of the crop is dried out; moisture is released via evaporation. Phase two is when the innermost part of the crop’s moisture travels to the surface to be evaporated. As time goes on, less and less moisture leaves the crop, and this is where problems can occur with solar drying. The crop can overheat since temperatures are really high and there is little moisture on the surface, so the surface of the crop hardens, trapping moisture inside, leading to deterioration of the crop (Gregorie).
Solar dryers are structures with a rectangular base and a tilted top in order to expose solar radiation to a transparent glass or plastic. Near the base of the dryer, the crops lay upon a thin wire tray or mesh. The purpose of the mesh is the holes it has in its structure. When solar radiation penetrates the translucent material, and the gases and molecules inside the structure trap heat in between them, thus increasing the temperature inside the solar dryer (“The Greenhouse”). The mesh allows airflow to enter the solar dryer so moisture that reaches the surface can be evaporated away. In order to prevent overheating, which can happen during solar drying, farmers can use two different types of solar dryers: passive and active.
Passive vs. Active
Active dryers require external means in order to turn solar energy to heated air. Fans and pumps usually operate active dryers. These type of dryers usually require lots of energy to be put into it, but it allows the user to control temperature more easily. Passive dryers utilize solar radiation without the use of extra machinery or extra resources to power their system. They are also easier to build, maintain, and use, and, for this reason, Mission 2019 recommends that passive dryers should be used. Passive solar dryers can be further divided into two types: direct and indirect (Gregoire).
Direct vs. Indirect
Direct models expose crops directly to solar radiation. They consist of a structure tilted at an angle that puts the crops in the lines of the sun’s rays. The structure of a direct solar dryer uses a transparent or translucent material to cover the dryer to convert the solar radiation to heat (Figure 1). The food being dried lies on a coarse mesh or wire tray. The hole size in the tray determines the airflow in the structure to take away excess moisture (Gregoire).
Figure 1: A diagram of a direct solar dryer (Gregoire, 1984)
Indirect dryers do not expose crops to direct solar radiation. The incident solar radiation is absorbed by a material, known as the solar collector, where it is converted to heat. Air flows over the solar collector and is heated (Gregoire). The air then moves up the trays inside the solar dryer hold the crops to dry them (Figure 2). Indirect dryers use the solar collector to allow the user to control temperatures, but are much more expensive. However using them improves the quality of goods if the crop cannot handle high temperatures (Espiard).
Figure 2: A diagram of an indirect solar dryer (Espiard, 1983)
Solar drying has many benefits, such as allowing goods to be available year round, especially when many crops are seasonal. Storage also can help keep a consistent supply for demand. With a continuous source of food or crop, the government can release the crop into the market when there is short supply, this decreases the prices in the market for consumers. With lower prices, consumers might be more willing to buy the crops, thus giving farmers a more consistent revenue throughout the year. Livestock are also regularly fed when there are consistent amounts of stored goods, and planting materials are available all year round.
The temperatures reached inside solar dryers are high above the ambient temperature, making it faster to dry crops than sun drying. Sun drying involves laying a product or crop outside to dry, which makes the crop susceptible to pests, insects, rodents, and contaminable by dirt. Sun drying is also very difficult in humid climates, where the air is filled with moisture.
The main disadvantage to using direct solar dryers is the difficulty in controlling the temperature inside them. The only way to decrease this temperature is to increase the size of the ventilation holes or the holes in the mesh, to allow more airflow. Since the final crop quality after solar drying is dependent on the temperature within the dryer, a farmer would have to monitor the drying process carefully in case the crop overheats and traps moisture inside the crop, which leads to it spoiling. These limitations are countered by the fact that solar dryers are convenient, easy, and cheap to make.
The Right Solar Dryer
There are two different types of solar dryers, and which is more effective depends on four factors of the surrounding environment:
- Crops that will be grown
- Climate conditions (rainfall, temperature, humidity, etc)
- Long term vs short term shipment
- Materials available to make the solar dryer
Figure 3 shows how the type of solar dryer can depend on climate. Countries that have high percentages of sunshine and are very warm benefit greatly using direct solar dryers. Countries that are humid or exposed to colder air would be better off with indirect solar dryers where the user can control the temperature of air that is heating the crop. Figure 4 reveals that the type of crop can be the deciding factor for which dryer to use also. Crops that lose vitamins or nutrients at high temperatures or sun exposure, like fruits and vegetables, should stay away from direct solar dryers. If a product is set to be shipped for a long time, it is better for the crop to be dried in a direct solar dryer for a longer amount of time to make sure all the moisture has evaporated (Gregoire).
Figure 3: A flowchart for farmers to determine the best solar dryer in their climate (Gregoire, 1984)
Figure 4: A flowchart for farmers to determine the best solar dryer for their crops (Gregoire, 1984)
The goal of utilizing solar dryers is to make storage of crops and goods more effective so they can be used at a later time. Countries that have very poor storage facilities should implement direct solar dryers as soon as possible to save produce from disease and contamination. Over the course of the next five years, governments would supply farmers cooperatives with the materials needed to build solar dryers. At the farmers cooperatives, farmers would be educated on how to choose, build, and operate the correct type of solar dryer for their farm. A solar dryer would not be expense free for the farmer, thought, but the farmer would be given private ownership of the dryer after the farmer paid back in small increments. Basically the government would offer to lease it to the farmer. Each individual farmer, however, would be responsible for repairs and maintenance, but they would be allowed to decide what crops to make and how much to store. The farmers thus could send their dried goods to warehouses. Then during the next 5-10 years, governments would continue to distribute solar dryers to every farmer in their country After a span of 1-10 years of using direct solar dryers, farmers should be more comfortable with solar dryers, and government and private funding would help farmers cooperatives with more innovations in solar drying to make it more effective. As technology increases, the government would invest more and more money into the farmers cooperatives when they see the increase in profit from leasing their solar dryers. As more money is invested, the farmers cooperatives will have more opportunity to research new strategies for solar drying.
0- 5 years:
- Government buys solar dryer materials in bulk
- Distribute materials to build direct solar dryers to farmers
- Teach farmers how to build direct solar dryers with government or private funding
5- 10 years:
- Continue the distribution of direct solar dryers
- Farmers should be comfortable with direct solar dryers by now
- Government continues this distribution
- Low Income farmers can increasingly conserve their supply, and eat healthily
- Increase solar dryer innovation
Espiard, E., & Mihailov, S. (1983, August 1). Expert consultation on planning … in Africa – 4.2 Report of the second preparatory mission on sundrying techniques in Africa. Retrieved November 25, 2015, from http://www.fao.org/docrep/x5018e/x5018E07.htm
Espiard, E., & Mihailov, S. (Photographer). (1983). Indirect Solar Dryer [digital image]. Retrieved http://www.fao.org/docrep/x5018e/x5018E07.htm
Royte, E. (2014, October 13). One-Third of Food Is Lost or Wasted: What Can Be Done. Retrieved November 22, 2015, from http://news.nationalgeographic.com/news/2014/10/141013-food-waste-national-security-environment-science-ngfood/
The Greenhouse Effect. (n.d.). Retrieved November 25, 2015, from https://www.ucar.edu/learn/1_3_1.htm
Gregoire, R. (1984). Understanding Solar Food Dryers (pp. 1-27). Arlington, Virginia: Volunteers in Technical Assistance (VITA). http://pdf.usaid.gov/pdf_docs/PNABC941.pdf
Gregoire, R. (Photographer). (1984). Direct Solar Dryer [digital image]. Retrieved from http://pdf.usaid.gov/pdf_docs/PNABC941.pdf