Vegetables are typically high moisture content foods. The high levels of water activity makes them vulnerable to spoilage due to microbiological or biochemical reactions.
Drying (dehydration) is a simple, cost-effective way of reducing spoilage of vegetables hence prolonging their shelf lives.
Dehydration lowers the water activity of the vegetable, which reduces the amount of water available for microbiological and biochemical reactions, and mold growth (Mohammed, 2013).
The video below (all rights to Strumble) shows a dehydration process. It uses an electric dryer, though.
For ages, people have solved the problem by spreading their vegetables on open surfaces or mats and letting them dry off in direct sunlight.
This, however, has been shown to be less hygienic since the product is vulnerable to contamination by dust, dirt, air-borne fungi, rodents, insects, and other sources of contamination available in the immediate environment. Furthermore, direct drying is not efficient under high levels of humidity.
Welcome solar dryers
Solar dryers present a simple solution to averting the challenges of direct dehydration under the sun. They produce better quality dehydrated product in all aspects of organoleptic and nutritional quality as well as enhanced shelf life.
The enclosure of a solar dryer eliminates the risks of contamination and over dehydration of the product since the process is controlled (Ibid).
Again, given the design of the dryer, it is more efficient at drying the vegetables even under high humidity since it concentrates dry air into the drying chamber to dehydrate the vegetable.
Passive solar dryers involve an initial cost and needs a professional designer who understands the dynamics of dehydration to fabricate.
However, they have minimal maintenance costs since they rely only on solar energy to dehydrate the product. There are no other operational expenses involved in drying the product.
This makes vegetable solar dryers a better alternative drying method in developing countries like Kenya, which have abundance of solar radiation all the year round.
This article shows how to design a simple low-cost solar vegetable dryer. It will highlight the important factors to consider during the process to ensure the dryer meets all the basic requirements for safety and quality of dehydrated vegetables.
Problem with conventional solar vegetable dryers.
The efficiency of a solar dryer relies on its solar collector. Most conventional solar dryers have unidirectional solar concentrators, which mean that they have to be oriented to the direction of the sun for maximum impact (Hegde et al., 2015).
This may limit the efficiency of the dryer to some extent given that the position of the sun will shift at different times of the day. This may mean that the dryer will require shifting to trap more solar energy at different times of the day.
Solution to the conventional solar vegetable dryer problem
To solve this problem, there is need to design a solar vegetable dryer that will maximize solar concentration at any time of the day without having to move the dryer to trap as the sun rays shift.
In this article, we’ll design a solar vegetable dryer with a 360° solar collector. This will enable the user to ensure maximum solar concentration throughout the day without having to shift the dryer.
The solar dryer has a conical shaped solar concentrator at the base and the vegetable drying chamber on top. The solar collector has a black surface to absorb solar energy. It is sealed at the bottom to insulate the dryer from unnecessary heat loss.
A secondary transparent layer is placed at 7 cm above the black surface (suspended with a frame) to trap and direct the heated air into the drying chamber. It has an allowance to facilitate air flow in the system.
The drying chamber is painted white on the inside walls to avoid loss of energy.
There are a series of drying trays are arranged in the drying chamber to hold the vegetable to be dried. Moisture laden air from the drying chamber vents at the top.
Considerations for the design
a) Angle of tilt of the solar collector
The solar collector is designed at a tilted angle for two major reasons:
- To maximize solar energy concentration
- To facilitate air flow in the system
Since the system is passive, there is need to use the convectional currents created by the hot air to actuate flow in the system.
The angle of tilt, β, is derived by β = 10° + Ф (Alamu, Nwaokocha & Adunola 2010). Where, Ф is the latitude of the collector’s location.
b) Surface area of the collector
The surface area of the solar collector will contribute to the efficiency of the dryer.
The air gap between the black surface and the transparent air trap will determine the volumetric air flow into the dryer, which can accelerate the drying process. This can also help in determining the load to put in the drying chamber for any particular drying session.
The volumetric flow rate of hot air is determined by the volume of air contained in the air gap at any particular time.
c) Insulation of the dryer
The dryer needs to be properly insulated to avoid unnecessary loss of heat energy.
Plywood has been chosen at the best option for this design due to”
- its low cost,
- ease of manipulation during design,
- poor heat conductivity, and
- polished finish to avoid loss of heat through conduction.
d) Cost of constructing and maintaining the dryer
The dryer is to be made from locally available materials that will cost less. In fact, a greater part of the materials used to construct this dryer are recycled from used items.
The dryer should be affordable to make and cheap to maintain while delivering the best efficiency attainable.
Once finished, the dryer should be rugged enough to withstand the elements of nature while delivering value to the user with minimal maintenance costs.
e) Efficiency of the dryer
Efficiency of a system is derived by dividing the work output by the work input. The value is expressed as a percentage (Onigbogi, Sobowale, & Ezekoma, 2012).
In this case, one can derive the efficiency of this dryer by determining the mass of the vegetable loaded into the system and the mass of the vegetable obtained after drying.
The former is divided by the latter and the value obtained is then multiplied by 100.
An efficient dryer should remove as much moisture from the load as possible. To increase the efficiency, the incoming hot air from the solar collector should not reach the saturation point before venting.
The capacity of the drying chamber is derived by getting the volume of the chamber. That is, circumference multiplied by the height because the drying chamber of this system is cylindrical.
The vegetable solar dryer
Figure 1: Solar dryer for vegetables
Key advantages of this solar vegetable dryer
- The solar collector is 360° hence can collect solar energy in any orientation regardless of the position of the sun. Once set, you don’t have to worry as to whether the solar collector is shielded from the sun or not.
- The dryer uses locally available materials, which are cheap and easy to maintain. For instance, the solar concentrator uses a black polythene paper and the air trap is a clear polythene paper. These are cheap locally available materials. The drying chamber consists of a bucket and the drying racks are sieves of wire mesh.
- The dryer is easy to use. You can remove the rack for additional loading once the first batch has dried.
- This dryer is environmentally friendly because it relies on solar energy alone to dry the vegetables loaded in the drying rack. It is also portable; you can move it around with ease. The solar collector folds like an umbrella and can fit in the drying chamber when you need to store it away.
- Since it is completely sealed from the environmental elements, the product is cleaner, fresher, and safer. The sealing protects the vegetables from contaminants and pests. Controlled dehydration also ensures that you never over-dehydrate the product.
Further Reading (Download citation)
- Hegde, V. N., Hosur, V. S., Rathod, S. K., Harsoor, P. A., & Narayana, K. B. (2015). Design, fabrication and performance evaluation of solar dryer for banana. Energy, Sustainability and Society, 5(23), 1-12. doi:10.1186/s13705-015-0052-x
- Mohammed, A. (2013). Design and construction of a Vegetable Drier. The International Journal Of Engineering And Science (IJES), 2(9), 88-94.
- Onigbogi, I., Sobowale, S. S., & Ezekoma, O. (2012). Design, Construction, and Evaluation of a Small-Scale Solar Dryer. Journal of Engineering and Applied Science, 4, 8-21. doi:10.13140/RG.2.1.3370.2886