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Solar Crop Dryer

1 Project Report-2011

AbstractA multi-purpose solar crop dryer was developed for drying various agricultural products such as fruits, vegetables, medicinal plants etc. The newly developed system consists of a small fan, a solar air heater and a tunnel dryer. The simple design allows production either by farmers themselves, using cheap and locally available materials, or by small scale industries. Due to the low investment required, the solar dryer is predestined for application on small farms in developing countries. Depending on the crop to be dried and the size of the dryer 1001000 kg of fresh material can be dried within 17 days to safe storage conditions. The solar dryer was successfully tested in Greece, Yugoslavia, Egypt, Ethiopia and Saudi Arabia drying grapes, dates, onions, peppers and several medicinal plants. Compared to traditional sun drying methods, the use of the solar dryer reduces drying time significantly and prevents mass losses. Furthermore, product quality can be improved essentially. During drying, the crop is protected completely from rain, dust, insects and animals. All these features contribute to the desired high product quality. The energy cost required for operating the fan features contribute to the the additional earnings from reduced mass losses and improved quality. On-farm tests also showed that the dryer can be easily operated by farmers. However, at present the dissemination of the solar dryer is limited to electrified areas.

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Chapter 1 IntroductionDrying is an excellent way to preserve food and solar food dryers are appropriate food preservation technology for sustainable development . Drying was probably the first ever food preserving method used by man, even before cooking. It involves the removal of moisture from agricultural produce so as to provide a product that can be safely stored for longer period of time. Sun drying is the earliest method of drying farm products ever known to man and it involves simply laying the agricultural products in the sun on mats, roofs or drying floors. This has several disadvantages since the farm products are laid in the open sky and there is greater risk of spoilage due to adverse climatic conditions like rain, wind, moist and dust, loss of products to birds, insects and rodents (pests); totally dependent on good weather and very slow drying rate with danger of mould growth thereby causing deterioration and decomposition of the products. The process also requires large area of land, takes time and highly labour intensiv. With cultural and industrial development, artificial mechanical drying came into practice, but this process is highly energy intensive and expensive which ultimately increases product cost. Recently, efforts to improve sun drying have led to solar drying. In solar drying, solar dryers are specialized devices that control the drying process and protect agricultural produce from damage by insect pests, dust and rain. In comparison to natural sun drying, solar dryers

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generate higher temperatures, lower relative humidity, lower product moisture content and reduced spoilage during the drying process. In addition, it takes up less space, takes less time and relatively inexpensive compared to artificial mechanical drying method. Thus, solar drying is a better alternative solution to all the drawbacks of natural drying and artificial mechanical drying. The solar dryer can be seen as one of the solutions to the worlds food and energy crises. With drying, most agricultural products can be preserved and this can be achieved more efficiently through the use of solar dryers.

Solar dryers are a very useful device for: Agricultural crop drying. Food processing industries for dehydration of fruits and vegetables. Fish and meat drying. Dairy industries for production of milk powder. Seasoning of wood and timber. Textile industries for drying of textile materials, etc. Thus, the solar dryer is one of the many ways of making use of solar energy efficiently in meeting mans demand for energy and food supply. Air is commonly used as a heat transfer fluid in many types of energy conversion systems. In drying applications and space heating solar energy can take part in a major role because which can be done with warm air alone. Nearly any black surface which is heated by the sun will transfer heat to air when the air is blown over it. Air is

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distributed over the black radiation-absorbing surface and the air stream should be in contact with the complete collector surface to achieve higher temperatures. Air collector is usually over-laid by one or more transparent covers to reduce the heat loss. A good review of solar air heaters and their applications has been reported. Conventional, fuel-operated artificial dryers are more efficient, providing uniform high quality products. But such units are beyond the reach of the farmers with limited crop volume and high requirements of financial resources with respect to the cost of equipment. The increasing rate of fuel consumption in agriculture has made it necessary not only to save energy by intensifying the drying processes and improving their designs and where these solar energy systems can play a major role.

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Chapter 2 Literature Survey SunlightSunlight, sunlight in the broad through sense, is the total frequency and solar spectrum of electromagnetic radiation given off by the Sun. On Earth, is filtered the Earth's atmosphere, radiation is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off of other objects, it is experienced as diffused light. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter. Sunlight may be recorded using a sunshine

recorder, pyranometer or pyrhelio meter. Sunlight takes about 8.3 minutes to reach the Earth. Direct sunlight has a luminous efficiency of about 93 lumens per watt of radiant flux, which includes infrared, visible, and ultraviolet light. Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth's surface.

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Sunlight is a key factor in photosynthesis, a process vital for life on Earth.

CalculationTo calculate the amount of sunlight reaching the ground, both the elliptical orbit of the Earth and the attenuation by the Earth's atmosphe- re have to be taken into account. The extraterrestrial solar illuminance (Eext), corrected for the elliptical orbit by using the day number of the year (dn), is given by

where dn=1 on January 1; dn=2 on January 2; dn=32 on February 1, etc. In this formula dn-3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and therefore the maximum Eext occurs around January 3 each year. The value of 0.033412determined knowing that the ratio between perihelion. (0.98328989AU) squared and the aphelion (1.016710033 AU) should be approximately 0.935338. The solar illuminance constant (Esc), is equal to 128103 lx. The direct normal illuminance (Edn), corrected for the attenuating effects of the atmosphere is given by:

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where c is the atmospheric extinction coefficient and m is the relative optical airmass.

Solar constantThe solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to the Earth). When solar irradiance is measured on the outer surface of Earth's atmosphere, the measurements can be adjusted using the inverse square law to infer the magnitude of solar irradiance at one AU and deduce the solar constant. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1.366 kilowatts per square meter (kW/m).

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Sunlight intensity in the Solar SystemDifferent bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun. A rough table comparing the amount of light received by each planet on the Solar System follows -

Planet

Perihelion - Aphelion distance (AU)

Solar maximum (W/m) 14,446 6,272 2,647 2,576 1,413 1,321 715 492 55.8 45.9 16.7 13.4 4.04 3.39 1.54 1.47 and

radiation minimum

Mercury 0.3075 0.4667 Venus Earth Mars Jupiter Saturn Uranus 0.7184 0.7282 0.9833 1.017 1.382 1.666 4.950 5.458 9.048 10.12 18.38 20.08

Neptune 29.77 30.44

The actual brightness of sunlight that would be observed at the surface depends also on the presence and composition of an atmosphere. For example Venus' thick atmosphere reflects more than 60% of the solar

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light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds". Sunlight on Mars would be more or less like daylight on Earth wearing sunglasses, and as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus it would give perceptions and "feel" very much like Earth daylight. For comp