Food Engineering Reviews

, Volume 4, Issue 1, pp 16–54

Solar Drying Technology

Review Article

DOI: 10.1007/s12393-011-9044-6

Cite this article as:
Janjai, S. & Bala, B.K. Food Eng Rev (2012) 4: 16. doi:10.1007/s12393-011-9044-6

Abstract

Drying is the oldest preservation technique of agricultural products, and sun drying is still widely used for preservation of agricultural products in the tropics and subtropics. Previous efforts on solar drying of cereal grains, fruits, vegetables, spices, medicinal plants, and fish are critically examined. Recent developments of solar dryers such as solar tunnel dryer, improved version of solar tunnel dryer, roof-integrated solar dryer, and greenhouse-type solar dryer for drying of fruits, vegetables, spices, medicinal plants, and fish are also critically examined in terms of drying performance and product quality, and economics in the rural areas of the tropics and subtropics. Experimental performances of different types of solar dryers, which have demonstrated their potentialities for drying of fruits, vegetables, spices, medicinal plants, and fish in the tropics and subtropics, are addressed, and also the simulated performances of the different types of solar dryers are discussed. The agreement between the simulated and experimental results was very good, and it is within the acceptable limit (10%). The simulation models developed can be used to provide design data and also for optimal design of the dryer components. A multilayer neural network approach was used to predict the performance of the solar tunnel dryer. Using solar drying data of jackfruit and jackfruit leather, the model was trained using back propagation algorithm. The prediction of the performance of the dryer was found to be excellent after it was adequately trained and can be used to predict the potential of the dryer for different locations and can also be used in a predictive optimal control algorithm. Finally, prospects of solar dryers for drying of fruits, vegetables, spices, medicinal plants, and fish in the tropics and subtropics are discussed.

Keywords

Solar dryingNatural convectionForced convectionPhotovoltaic (PV) ventilatedMathematical modelingNeural networkAgricultural products

List of Symbols

Cp

Specific heat, kJ/kg °K

E

Solar radiation, W/m2

Ga

Mass flow rate of air, kg/m2s

H

Humidity ratio, kg/kg

K

Drying constant, min−1

Lg

Latent heat of chili, kJ/kg

M

Moisture content, % or ratio (db) or (wb)

Me

Equilibrium moisture content, % or ratio (db) or (wb)

Mo

Initial moisture content, % or ratio (db) or (wb)

T

Temperature,  °C

V

Air velocity, m/s

b

Depth of collector/dryerB

hc

Convective heat transfer coefficient, W/m2°K

hr

Radiative heat transfer coefficient, W/m2°K

t

Time, min

z

Thickness, m

Greek

α

Absorbance

ρ

Density, kg/m3

ρ

Reflectance

τ

Transmittance

Subscripts

a

Air

am

Ambient

c

Collector

e

Equilibrium moisture content

g

Product

l

Liquid

L

Long wave

p

Absorber plate

s

Sky

s

Short wave

w

Water

v

Water vapor

Copyright information

© Springer Science + Business Media, LLC 2011

Authors and Affiliations

  1. 1.Department of Physics, Faculty of Science, Solar Energy Research LaboratorySilpakorn UniversityNakhon PathomThailand
  2. 2.Department of Farm Power and MachineryBangladesh Agricultural UniversityMymensinghBangladesh