Solar Drying: Fundamentals,Applications and Innovations - National ...

Bala and Hossain - Solar Drying of Fishery Products
op forced convection solar driers such as (i) solar tunnel drier (Esper and Mühlbauer,
1993), (ii) indirect forced convection solar drier (Oosthuizen, 1996), (iii) greenhouse
type solar drier (Janjai et al., 2008a), (iv) roof integrated solar drier (Janjai et al., 2008b)
and (v) solar assisted drier (Smitabhindu et al., 2008). Numerous tests in the different
regions of the tropics and subtropics have shown that fruits, vegetables, cereals, grain,
legumes, oil seeds, spices, fish and even meat can be dried properly in the solar tunnel
drier (Esper and Mühlbauer, 1993; Mühlbauer et al., 1993; El-Shiatry et al., 1991;
Schirmer et al., 1996; Esper and Mühlbauer, 1994 & 1996; Bala, 1997b; Bala et al., 1997
& 1999; Bala et al., 2002; Bala et al., 2003; Bala and Mondol, 2001).
The purpose of this chapter is to present the developments and potentials of solar
drying technologies for drying fishery products
4.2. SOLAR DRYING SYSTEMS
The major two categories of solar driers are natural and forced convection solar
driers. In the natural convection solar driers the airflow is established by buoyancy induced
airflow while in forced convection solar driers the airflow is provided by using
fan either operated by electricity/solar module or fossil fuel.
Natural convection solar drying has advantages over forced convection solar drying
as it requires lower investment though it is difficult to control the drying temperature
and drying rate. Due to low cost and simple operation and maintenance, natural convection
solar drier appears to be the obvious option and popular choice for drying of agricultural
products. Natural convection solar drier can be classified as (i) indirect natural
convection solar drier, (ii) direct natural convection solar drier and (iii) mixed mode
natural convection solar drier.
The limitations of natural convection solar driers prompted researchers to develop
forced convection solar driers. Mühlbauer and his associates at the Institute of Agricultural
Engineering in Tropics and Subtropics, University of Hohenheim, Germany developed
the solar tunnel drier. Solar tunnel drier consists of a flat plate air heating collector
and a tunnel drying unit with a small fan to provide the required airflow over the product
to be dried. Solar tunnel drier can be operated by a 40 Watt photovoltaic module independent
of electrical grid. The collector and drying chamber can be made of plain
metal sheets and wooden frames in a number of small sections and joined together in
series. These sections can be dismantled easily for transportation from one place to
another. Glass wool is used between the two metal sheets at the bottom of the drier as
an insulation material to reduce the heat loss from the bottom. Collector size of the drier
is 10 m x 2 m. The photovoltaic system has the advantage that temperature of the drying
air could be automatically controlled by the solar radiation.
Numerous tests in the different regions of the tropics and subtropics have shown
that fruits, vegetables, cereals, legumes, oil seeds, spices, fish and even meat can be
properly dried in the solar tunnel drier. Bala (2000) has widely demonstrated the potentiality
of the solar tunnel drier in Bangladesh for small scale industrial production of
high quality dried fish which has a wide acceptance in the supermarkets in Bangladesh.
98 Drying of Foods, Vegetables and Fruits