Solar Energy Techonologies - Solar Drying Systems

Introduction

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

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