Solar drying systems

Solar energy can be used to increase temperatures needed for most of the drying applications. Solar dryers use air heated through solar energy collectors, which can be installed in modules according to the requirements of hot air.

Types of solar dryers

Integrated solar dryers

An integrated solar dryer is one in 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 of solar dryer

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 agro-industries. Solar driers have the following 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 40-50^oC.

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. The MNRE provides interest subsidy in the form of soft loans available through IREDA and banks.

Source : MNRE

Low cost domestic solar dryer

How to fabricate a solar dryer:

Material required:

1. TV or Computer packing box - (Computer 14 x 11 Inches Packed paper box, of size Length: 17.5, Breadth: 19, Height: 14.5 )
2. Axa blade
3. Transparent plastic.
4. Needle& Thread.
5. Mosquito net
6. Card Board.
7. Black paint.
8. Painting brush
9. Scissor.

Technical Specifications:

1. Sun Light hitting area (Open mouth area): 2.7 Sft Area: 20.5 X 19=390 Sqin = 2.7 Sft.
2. Drying area (Drying material placing area):2.3 Sft Area: 17.5 x 19 = 333 Sq in=2.3 Sqft.
3. Heating volume inside dryer: 0.96 Cft

Methodology:

The box is cut slant top to down leaving 10 cm from the bottom. After cutting, the inside is painted with black enamel paint and dried for 6 hours. The top is covered with transparent plastic sheet and made air tight with gum tap around the open side.

Holes are made on sides, bottom and back side to flow out the moisture of drying products. The legs are fixed with thermocoal. A tray is made with mosquito net and card board to place the drying products in the dryer. A door is cut at the back side to place and remove the products inside with hand.

This unit can be used to dry red chillies, tomato, brinjal, ladies finger, potato, and carrot, green leafy vegetables, medicinal plants parts, fish, mutton and pickles. The dried products can be packed in plastic cover to preserve the products to avail round the year. The cost of the unit is Rs 200/- and is easy to fabricate.

Preparation of materials for drying

Mango/ Banana:

• Wash well in clean water.
• Peel or cut or slice to dry faster.
• Dip in lemon juice and water (2:1) for less vitamins loss.
• Spread on tray
• Cover the dryer in night and if rains or take it inside.
• After drying, pack in plastic bags.
• Hang it inside to protect from rats.

Guavas & Papaya:

• Lemon juice is mixed with two parts of water. This is heated gently and double amount of sugar is gently dissolved by stirring.
• Continue stirring until it cools
• Wash the ripe fruits
• Remove seeds and threads and cut thin slice
• Immediately after cutting dip slice in the syrup and hold for 15 minutes.
• Remove the slice and drain the syrup.
• Keep in dryer and pack after drying.

Preservation of tomato

• The tray capacity of Tomato is 4 Kgs. The Tomato was dried for 4 days from 9 to 15 hours.
• The temperature variation inside and outside of the dryer and corresponding weight loss was recorded.
• The drying rate is 200g/day and maximum drying period is 6 days depends on the clouds.
• The number of days for drying depends on moisture content and the quantity, if the moisture and quantity is less the drying period decreases.
• The size decreases considerably and colour and odour remains same as fresh Tomato.

Advantages of solar dryer:

The solar dryer helps to retain the nutrients, taste, colour, odour, prevent infection and to preserve the agro produces. Solar energy is a free and clean source of energy.

Source : Rural Development Organisation, Tirupati

Solar dryer for fish and vegetables

A forced flow type dryer, which consisted of air heater, fan and drying chamber has been developed by MCRC, Chennai for drying food products.

This unit or system consists of drying chamber and 60W PV panel for converting solar radiation into electricity. This panel has been integrated with dryer for powering the 12V DC fan to circulate the air inside the dryer. In this case the drying unit is a single unit of black box which traps most of the solar radiation or heat. The incoming solar light fall on the PV module, which converts solar radiation into electricity. When the solar rays fall on the dryer its surface gets heated up, the air inside the dryer is in circulation by fan, so the air which passes over the surface of the dryer, gets heated due to convection. This hot air passes through the food product to be dried which is placed on trays absorbs the moisture present in the drying product.

Advantages of the dryer

• Both shade drying and open drying is possible.
• Reduces drying time.
• Open direct sunlight heating is possible by removing the top cover of the dryer and providing transparent thin polyethylene film sheet. This will transmit 99% of direct solar radiation.
• Works throughout the year. It can work at night and during rainy season by burning biomass under the dryer.
• Easily assembling and disassembling and light weight for transportation purpose.
• Low maintenance

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Foldable solar dryer

Foldable solar dryer is a rectangular box with triangular top. The solar radiations are absorbed by black mat metallic outer surface of dryer. Heat is absorbed by inside air through conducting metallic plates. The geometry of the solar dryer ensures that the resultant hot air moves up and heats the product evenly and dries it by evaporating trapped moisture. Finally, this air escapes through vent at top. The dryer is of modular type and can be easily dismantled into a thin rectangular box for easy transportation & storage.

Applications

The foldable solar dryer in the capacity of 10, 25 & 100 kg is used for the preparation of raisin, drying of grapes, onion, mango pulp, green leafy vegetables, Amla, jack fruit pulp, ginger, green pepper, herbal medicines etc.

Advantages

By use of this dryer, the drying process is made 5 times faster than normal sun drying procedure resulting in substantial weight reduction. 100 kg of grapes can be converted to 25 kg of raisin. It has no adverse effect to environment.

Head, Technology Transfer & Collaboration Division,
Bhabha Atomic Research Centre,
Trombay, Mumbai - 400 085.
Fax: +91-22-25505151/25519613
Phone: +91-22-25593897
E-mail: headttcd@barc.gov.in

Source : BARC