Many agricultural and industrial products
need drying in order to reduce their moisture content as part
of processing or for preservation. While open sun drying may
be the most inexpensive and extensively used option for many
of the products, the process is unhygienic and time-consuming.
One option is to use a conventional fuel such as biomass,
oil, or electricity for drying applications. However, such
fuels are expensive, and their use causes pollution. With
the rising costs of conventional fuels and increasing awareness
of the dangers of pollution, solar dryers are becoming a technically
and economically viable option in many industrial and agricultural
applications.
Technology
Solar energy can be used to temperatures
needed for most of the drying applications. Solar dryers use
are heated through solar energy collectors, which can be installed
in modules according to the requirements of hot air. Drying
is basically a heat and mass transfer process: moisture from
the surface and inside of the product is vaporized, and the
vapour is removed by flowing hot air. Important factors affecting
the drying process are listed below.
Relative humidity and temperature of
air
Airflow rate
Initial moisture content of the product
Final desired moisture content of the
product
Types of solar dryers
Integrated solar dryers
An integrated solar dryer is one which solar
energy collection and drying take place in a single unit.
Cabinet dryers, rack dryers, tunnel dryers, greenhouse dryers,
and multi-rack dryers fall under this category. Normally,
these dryers are small in size and are stand-alone units.
Distributed solar dryers
A solar dryer in which solar energy collection
and drying take place in separate units is known as a distributed
solar dryer. This type of solar dryer has two parts: (1) a
flat-plate air heater and
(2) a drying chamber, Air is heated in the flat-plate heater
placed on the roof of the building or on the ground. Hot air
from the air heater is circulated in the drying chamber with
the help of a blower. These dryers can be designed in different
sizes with various configurations, depending upon the temperature
of hot air, airflow rate, types of products to be dried, etc.
Mixed-mode solar dryers
A solar dryer in which solar energy collection
takes place in both air heater and drying unit, and drying
takes place only in the drying unit, is known as a mixed-mode
solar dryer. In this dryer, solar energy is collected through
flat-plate solar collectors and also by the roof of the drying
chamber. In large industrial drying systems, the solar-heated
air is combined with air heated by conventional energy; this
adds to the reliability of the system and at the same time
helps in significantly reducing conventional energy consumption.
Uses
Solar dryers can be utilized for various
domestic purposes. They also find numerous applications in
industries such as textiles, wood, fruit and food processing,
paper, pharmaceutical, and agor-industries.
Advantages
Solar dryers are more economical compared
to dryers that run on conventional fuel/electricity.
The drying process is completed in the
most hygienic and eco-friendly way.
Solar drying systems have low operation
and maintenance costs.
Solar dryers last longer. A typical dryer
can last 15-20 years with minimum maintenance.
Limitations
Drying can be performed only during sunny
days, unless the system is integrated with a conventional
energy-based system.
Due to limitations is solar energy collection,
the solar drying process is slow in comparison with dryers
that use conventional fuels.
Normally, solar dryers can be utilized
only for drying at 40-50oC.
Cost
The cost of a solar dryer of 50 kg capacity
ranges form about Rs 30 000 to Rs 50 000. Larger industrial
- scale systems may cost between Rs 4 lakhs and Rs 10 lakhs.
Subsidies
The MNES is implementing a national programme
on solar thermal energy, which provides interest subsidy in
the form of soft loans available through IREDA and banks.
The MNES also provides capital subsidy of up to 50% of the
project cost for a specific demonstration project that is
based on new technology and or is located in a new area.
There are a few manufacturers in India who
are capable of supplying solar dryers for specific purposes
and in specific sizes. Larger systems for industrial establishment
have to be designed and installed on a project-by-project
basis.
Potential for solar dryers
There is vast potential for solar dryers
in the country: for drying agro-products and also for use
in the fruit and food processing industries. Solar drying
systems can also be installed for drying a variety of industrial
products such as chemicals, leather, salt, plywood, and textiles.
Success stories
Solar drying systems have been installed
in many places in the country. Listed below are some locations
where the systems are being used.
Main Application:Solar
Contact:
Ministry of Non-Conventonal Energy Sources,
Govt. of India
Block-14, CGO Complex, Lodi Road
New Delhi – 110 003
Tel: 91 11 2436 0707/ 2436 0404
Fax: 91 11 2436 1298
