Sun, the biggest available carbon free energy source offers the living beings with more energy in 1 hour than is consumed on the earth in an entire year. Achieving sufficient supplies of clean energy for the next generation is a prodigious societal challenge.
The International Energy Outlook 2006 (EIA, 2006) reviewed World marketed energy consumption on total grows on an average by 2.0% per year from 421 quadrillion Btu in 2003 to 563 quadrillion Btu in 2015 and 722 quadrillion Btu in 2030 and India becomes the fifth leading energy consumer as per British petroleum.
Good sunshine is bless to India. Most of all parts of the India receive solar radiation in the range of 5'7 kWh/m2 on average, and delivers more than 275 sunny days in a twelve months (Bal et al, 2010). Hence, solar energy as drying medium has a great prospective of distribution throughout the country, and offers a viable opportunity in the domestic sector. It is identified as an appropriate technology for Indian agricultural proceedings and possesses numerous advantages such as high durability, high dietetic value of food, less recurring costs and potential to reduce drudgery, etc.
2.2. Types of solar dryers
In the traditional use of solar drying, the product to be dried is exposed directly to the sun in open environment. This process is known as open sun drying. This process possesses so many drawbacks like degradation by windblown rubbishes, rain and pest infestation. Humanoid and animal interference will results in rottenness of the product. There is no control over drying temperature, speed of drying and quality of dried product will be reduced. Intermittent sunshine, interruption and wetting by rain also take part into reducing the quality (Ong, 1999). Numerous mechanical solar dryers using natural convection or forced circulation have been studied to overcome these difficulties.
Solar dryers have been classified as follows:
Figure 1. Classification of solar dryers
2.2.1. Direct solar dryer: In this direct type of solar dryer, solar radiation influence directly on the sample material which is to be dry. A part of radiation replicates back and some pass through glass glazing which come in contact with sample. Sample absorbs some of radiation and part of it reflects back from material surface. Due to this phenomena of radiation, temperature inside dryer rises such that material will emanates long wavelength radiation but due to glass cover they are not allowed to escape to environment. But, this method faces some limitations like small capacity, discoloration due to direct exposure, higher moisture content inside product reduces transitivity etc.
2.2.2. Indirect solar dryer: In this case, a separate unit for heating of air is connected to main drying chamber. This unit is known as solar collector comprises of parabolic troughs. Cold air pass through solar collector, gets heated and feed to drying chamber. Convective heat transfer take place in during this operation. A better control over drying is achieved in indirect type of solar drying systems and the product obtained is good quality. Air velocity, dryer temperature, solid loading can be control easily compared to direct type dryer.
2.2.3. Passive type solar dryer: Due to the effect of wind pressure variance or buoyancy forces, ambient cold air draw inside the dryer. Due to direct or indirect effect of solar radiation, cold air heated to a reasonable temperature and then convey out the moisture from the feed samples. This type of dryers are often known as natural convection solar dryers. It seems the most attractive possibility for use in remote rural location.
2.2.4. Active type solar dryer: This type of systems depend only partly on solar energy. They employ solar energy and/or electrical or fossil fuel based heating systems and motorized fans and/or pumps for air circulation. All active solar dyers are thus known as forced convection dryers.
2.2.5. Tunnel type solar dryer: A multipurpose solar tunnel dryer consists of a small centrifugal blower, solar collector and tunnel drying chamber. Sometimes it is integrated with biomass furnace or heat exchanger to heat the drying air during cloudy and rainy days. It is useful for drying in tropical regions. Drying of cocoa, coffee and coconut dried using tunnel solar dryer and reduction in time up to 40 % reported (Vijayavenkataraman et al, 2012).
2.2.6. Distributed type natural circulation solar energy dryers: This type of solar dryer contains trays to place sample material inside the opaque solar drying chamber. Due to the effect of thermosyphonic solar collector, pressure drop generated between ambient air which is at lower temperature and heated air inside the collector. This heated air passes through the trays, come in contact with the sample and moisture get evaporate. Advantage of this solar dryer is that air is continuously moving from different part of drying chamber so hotspot generate and caramelization diminished. Fruits contains vitamins and mineral, at higher heating temperature in case of direct exposure to the sun cause evaporation and color and texture change occur. In this type of condition distributed type natural circulation solar dryer are recommended (Ekechukwu, 1999). Operational difficulties during loading- unloading and variation in inlet air temperature to drying chamber are some drawbacks associated with this dryer.
2.2.7. Mixed mode natural circulation type solar dryer: By combining principles of direct and indirect heating solar dryer, mixed mode solar dryer is designed. Dryer arrangement contains solar air heaters with solar collectors arrangement for preheat the air with combination of direct adsorption of solar radiation. Very much higher temperature than ambient can be attained such that dryer has ability to reach reasonable moisture level in food within short instant of time compared with other solar dryer.
2.2.8. Forced convection greenhouse solar dryer: This type of dryer contains hut type of drying chamber with motorized pump for circulation of heated air. It is an active mode type solar dryer. For large scale of commercial drying purpose solar air heating systems with solar collectors combines with fossil fuel driven pumps such that overall consumption of fossil fuel is being reduced (El-Sebaii, 2012).
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