This research presents the design, construction, and analysis of the
performance of a mixed-mode solar dryer for crops. A mixed-mode solar
dryer utilizes direct solar radiation from the sun as well as input heat
ducted from the solar collector inlet which is directly connected to
the dryer. Such dryers have been shown to outperform passive solar
dryers as it was also shown in this work using drying kinetics.
Tomatoes were dried in the drying chamber under the mixed-mode condition. The maximum dryer temperature obtained was 39.2,
while the lowest relative humidity in the dryer was 32%. These
conditions are shown to be only fair for drying of tomatoes as it
prolongs drying time. The system’s performance was largely affected by
poor insolation and high heat losses during chosen drying days. Based on
drying kinetics, a drying rate of 2.88units/day was obtained during the chosen drying days. With this rate, the dryer can dry 2kg of tomatoes within three days.
The dryer can reduce the moisture content of 1kg of tomatoes from 95% to 14% within 45 hours of drying time. The capacity of the dryer is 1kg of products per tray.
A simplified 2D transient heat transfer model of the temperature
distribution in the drying chamber is presented. Results obtained show
that material selection, insolation, and inlet temperature play a
crucial role in the solar dryer performance.
The use of solar drying systems for agricultural products to preserve
fruits, vegetables and other crops has been proven to be cheap,
reliable, and environmentally friendly
. These solar dryers offer another option for processing
vegetables and fruits under safe conditions that conform to standards.
Some of the good qualities of these solar dryers are minimal maintenance
cost, no fuel costs, time saving, occupying less area, improvement of
the product quality, environmental protection, and control of required
air condition .
The availability of satisfactory information regarding efficient
solar dryers is lacking in many countries where the food processing
methods like indirect solar drying are needed. To eliminate the risk of
spoilage during drying and quality production of the products, indirect
drying with forced convection air flow is one of the best options .
Although the solar air collector is a very important component in the
solar drying system, much attention has not been drawn to it during
dryer design. In principle, the performance of the solar dryer depends
on several operating conditions such as the climatic condition,
collector orientation, the thickness of the cover material, wind speed,
length and depth of the collector, and the type of material used for the
For this reason, this research has dealt with the optimization of the
design, material selection, and required parameters to enhance the
efficiency of the designed solar dryer. Drying is one of the methods
used to preserve food products for longer periods. Drying helps in the
preservation of food, fruits, and vegetables for a long time with good
quality. It is a process of moisture removal due to simultaneous heat
and mass transfer.
Drying of most agriculture products, especially fruits and vegetables
requires hot air in the temperature range of 40 – 60 degrees for safe
drying. Direct solar energy coupled with the wind has been used to dry
food for years. Sun drying of crops is the widest spread method of food
preservation in a lot of African countries due to solar irradiation
being very high for the most part of the year. There are some drawbacks
relating to the traditional method of drying i.e. placing the crops on
mats, trays or paved grounds and exposing the product to the sun and
wind. These include poorer quality food caused by contamination by dust,
insect attack, enzymatic reactions, and infection by micro-organisms.
This system is labor and time intensive as crops have to be covered at
night and during bad weather, and the crops continually have to be
protected from attack by domestic animals. Non-uniform and insufficient
drying is noted, resulting in deterioration of the crop during storage
in this method. Fierce drying problems occur especially in humid
tropical regions where some crops have to be dried during the rainy