Design Analysis and Studies on Some Solar Drying Systems

Abstract

Solar drying is an unsophisticated, yet very ancient technology. It is one of the most accessible and, hence, the most common preservation techniques applied to agricultural products, mainly in tropical regions of the world where the intensity of solar radiation justifies its use. Different types of solar dryers exist and find suitable applications in the preservation of different agro-produce as a proven means of reducing postharvest losses. The present chapter discusses extensively the design procedures for solar drying systems, including the use of rules of thumb, psychrometric charts and design equations. Economic aspects of solar drying systems are also considered. Various case studies, relating to the application of direct-mode, indirect-mode and mixed-mode types of solar drying systems, have been presented. Although direct-mode dryers are generally cheaper to construct, a major drawback in their application is the fact that drying temperatures cannot be regulated, leading to over- or under-drying. It is concluded that well-designed solar crop drying systems are capable of drastically reducing losses of agricultural products occasioned by postharvest spoilage and preservation challenges.

Nomenclature

Av :

Area of air vent, m ²

a _w :

Water activity

C _1/2 :

Cost of equipment

E :

Total useful heat energy, KJ,

ERH :

Equilibrium relative humidity, %

g :

Acceleration due to gravity (9.81 m/s ² )

H :

Height of air column, m

h :

Drying bed thickness, m

hi/f :

Initial/final enthalpy, kJ/kg

I :

Total global solar radiation, kJ/m ²

L_dc :

Length of the drying chamber, m

M _1/2 :

Plant output

M _A :

Mass of air, kg

MC :

Moisture content, %

Mi/f :

Initial/final mass, kg

Mp :

Initial mass of product, kg

Mw :

Mass of moisture to be expelled, kg

mdr :

Drying rate, kg/hr

P :

Atmospheric pressure, kPa

Δp _b :

Pressure drop across crop bed, Pa

ΔP _T :

Pressure drop through the dryer, Pa

R :

Universal gas constant, 0.291 kPa m ³ /kg K

t _d :

Total drying time, hrs

U :

Superficial air velocity, m/s

V :

Volume of air, m ³

Vw :

Wind speed, m/s

Va :

Volumetric airflow rate, m ³ /h

Wa :

Quantity of air required, m ³

η :

Efficiency