ABSTRACT

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.

CHAPTER 1

INTRODUCTION

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

[1].   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 [2].

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 [3].

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 absorber [4].

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 season.