Experimental Analysis on Solar Air Dryer

Transcription

1 International Journal of Engineering, Management & Sciences (IJEMS) ISSN: , Volume-1, Issue-12, December 2014 Experimental Analysis on Solar Air Dryer Arvind Kumar Singh, Pushpendra Kumar Sharma, Harimohan Sharma Abstract An indirect forced convection solar dryer is designed and fabricated to investigate its performance under the hot and dry climatic condition of Jaipur, India. The system consists of flat plate collector and drying chber with two shelves and a chimney. Dimensions of collector and drying chber are 1.21 m x 0.87 m and 0.5 x 0.5 x 0.5 m respectively. There are four baffles of dimension 40cmx3cmx7cm fitted between absorber and back plate. The distance between back plate and absorber plate is 8cm. The experiments were performed on green chilies. The drying time is 10 hours. Collector is inclined at 45 with horizontal and oriented in the South - North direction. During experimentation 11 temperatures, relative humidity, air velocity and solar intensity were measured hourly in the month of May and July, 2010 for drying of green chilies and potato chips. Solar intensity varied between W/m 2 and maximum temperature rise was 20.6 o C. Hot air at 61 o C from the collector is supplied through 3.5 cm dieter PVC pipe at the bottom of the drying cabinet. The moisture content and weight of chilies was reduced from 90% to 13% and 2 kg to kg in three days respectively. Result of the present study shows that the drying time is reduced and quality of the final products are superior. Index Terms Solar Power, thermal efficiency, heat loss coefficient, solar intensity, etc.. I. INTRODUCTION Energy in various forms has played an increasingly important role in worldwide economic progress and industrialization. In view of the world s depleting fossil fuel reserves, which provide the major source of energy, the develoent of non-conventional renewable energy sources has received an impetus. Sunlight available freely as a direct and perennial source of energy provides a non-polluting reservoir of fuel. The simplest and the most efficient way to utilize solar energy is to convert it into thermal energy for heating applications by using solar collectors. Solar air heaters, because of their inherent simplicity are cheap and most widely used collection devices. The main applications of solar air heaters are space heating; these can also be effectively used for curing/drying. The solar air heater occupies an important place ong solar heating system because of minimal use of materials and the direct use of air as the working substance reduces the number of required system components. Manuscript received December 09, Arvind Kumar Singh, THDC Instiute of Hydropower Enginering and Technology, Tehri, Utrakhand, India Pushpendra Kumar Sharma, THDC Instiute of Hydropower Enginering and Technology, Tehri, Utrakhand, Harimohan Sharma, Apex group of Institutions, Jaipur, Rajasthan II. SOLAR AIR HEATER Solar air heater is a type of energy collector in which the energy from the sun, solar insolation, is captured by an absorbing medium and used to heat air. Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications. A simple solar air collector consists of an absorber material, sometimes having a selective surface, to capture radiation from the sun and transfers this thermal energy to air via conduction heat transfer. This heated air is then ducted to the building space or to the process area where the heated air is used for space heating or process heating needs. Solar air collectors can be commonly divided into two categories: Glazed (reticulating types) Unglazed (bient air heaters -transpired type) III. GENERAL DESCRIPTION AND PRINCIPLE OF SOLAR DRYER Drying or dehydration of material means removal of moisture from the interior of the material to the surface and then to remove the moisture from the surface of drying material. Drying of seeds prevents germinations and growth and fungi and bacteria. The traditional age old practices of drying food crops in developing countries like India, Bangladesh etc. is spreading food products in open sun termed as open sun drying or natural sun drying.this natural sun drying is simple and economical but suffers from many drawbacks such as there is no control over the drying rate discoloration.there is no uniform drying. Classical drying or natural sun drying has several disadvantages like spoilage of product due to adverse climatic condition like rain, wind, moist, and dust, storm may dage the crops, loss of material due to birds and animals, deterioration of the material by decomposition, insects and fungus growth etc. in open sun drying. Also the process is highly labor intensive, time consuming and requires large area. Artificial mechanical drying and Classical drying or natural sun drying process is highly energy intensive and expensive which ultimately increases product cost. Thus solar drying is the best alternative as a solution of all the drawbacks of natural drying and artificial mechanical drying. Solar dryers used in agriculture for food and crop drying,for industrial drying process, dryers can be proved to be most useful device from energy conservation point of view. It not only save energy but also save lot of time, occupying less area, improves quality of the product, makes the process more efficient and protects environment also. Solar dryers circumvent some of the major disadvantages of classical drying. Solar drying can be used for the entire drying process or for supplementing 1

2 Experimental Analysis on Solar Air Dryer artificial drying systems, thus reducing the total ount of fuel energy required. There are several advantages of controlled drying of grains such as product quality, storage capability and hygiene improvement, reduced wastage, time and space improved transportability. Applications of Solar Dryer: Agriculture crop drying Medicinal plants Drying Pharmaceutical Drying Food processing industries for dehydration of fr uits, potatoes, onions and other vegetables, Dairy industries for production of milk powder, casein etc. Seasoning of wood and timber. Screen print Drying Textile industries for drying of textile materials. To dry materials a supply of heat energy is essential to evaporate the water and supply of air to carry away the water vapour produced. For control drying of food we use fuels like electricity, natural gas or coal etc. Due to scarcity of fossil fuels and many environmental problems associated with these uses energy engineers are searching of alternating sources of energy. Solar energy is best solution or appropriate alternative sources of energy. Solar energy is clean, safe and abruptly available. An improved technology in utilizing solar energy is the use of solar dryers where the air heated in a flat plate solar collector and then heated is pass through drying chber. A. Types of Solar Dryers Basically solar dryers can be broadly classified into two types: 1. Direct type solar dryers or Natural Convection. 2. Forced Circulation type solar dryers. Natural convection dryers are generally of a size appropriate for on-farm use. The dryer consist of three components, solar collector, drying chber and solar chimney. In this the air is passed naturally through a bed of dryer. The forced convection solar dryers can be considered as a conventional mechanical drying system in which air is forced through a bed of dryer but the air is heated by a solar collector rather than by more conventional means. Forced Circulation or Active type solar dryers employ motorized fans for circulation of drying air. The integral type dryers are simple in both construction and operation and require little maintenance. However, they are likely to operate at lower efficiency due to their simplicity and there is less control of drying operation. The distributed types are much elaborate structures so require great investment in materials and running cost, but have higher efficiency and as a result product quality is generally higher. The solar dryer has two significant disadvantages: a limited ability to process crops when the weather is poor; and drying can only occur during daytime. This not only limits production and extends the drying time but also may have an adverse effect on production and product quality particularly fleshy crop products such as roots and stems that typically have drying time of several days. This limitation has led to the develoent of hybrid system with auxiliary heating system such as burner using biomass, biogas or fossil fuels. Alternatively, to achieve more efficient energy use, some active solar dryers are designed with thermal storage devices (mainly rocks or gravels) to extend drying time during the night time and in periods of low sunshine. Solar air heater mainly of three types: I. Flat plat solar air heater II. Finned solar air heater III. V-corrugated solar air heater Solar air heater also can further classified on the basis of box material, absorber plate material, size, blower capacity. To fabricate this system used wood and for solar radiation collector used GI sheet. Dimensions of wooden block for solar air heater are 127cmX93X10cm and the dimension of GI sheet is 127cmX87cm. The Solar Air Heater where an air stre is heated by the back side of the collector plate. Baffles attached to the plate increase the contact surface. The most favorable orientation, of a air heater, for heating only is facing due south at an inclination angle to the horizontal equal to the latitude plus 15 o. Air in air heater passed through a space between the absorber plate and insulator with baffles arranged to provide a long (zig-zag) flow path COMPONENT OF SAH A. Absorber Plate The primary function of the absorber plate is to absorb as much as possible of the radiation reaching to plate, loose as little heat as possible upward to the atmosphere and downward through the back of the container.. In general, absorption of solar energy impinging on an absorber plate should be as high as possible, but re-emission (loss) outward from the collector should be minimized. Absorber plates are usually given a surface coating (which may be a black paint) that increases the fraction of available solar radiation absorbed by the plate (its absorptance α). These surfaces must be able to withstand repeated and prolonged exposure to high temperatures without appreciable deterioration or out gassing. As a absorber plate we can use various material sheet on the basis of their Solar absorptance, Infrared emittance and Reflectance. The emittance of a surface varies with its temperature and its roughness. If it is a metal, it depends also on its degree of oxidation. Highly polished metals have low emittance but more reflectivity. Selective absorbers often consist of a very thin black metallic sheet metal base. The sheet is thick enough to act as a good absorber, with α =

3 Table gives values of absorptance and infrared (IR) emittance for various Materials; it alsogives values of reflectance. It is noteworthy that many common building materials have excellent emitting surfaces for long wave radiation. B. Baffles A flat board or plate, deflector, guide, or similar device constructed or placed in flowing air systems to cause more uniform flow velocities to absorb more energy and to divert, guide the air. These baffles provide more area of contact by diverting or deflecting the air flow. Due to this reason heat transfer takes place more and air take more heat. Main purpose of baffles in solar dryer is to provide more contact area to get more heat. Main applications of baffles are Solar Dryer, Heat Exchangers, Flow Channels etc. International Journal of Engineering, Management & Sciences (IJEMS) ISSN: , Volume-1, Issue-12, December 2014 Thermal Efficiency The thermal efficiency of the solar collectors (η) is defined as the ratio between the energy gain and the solar radiation incident on the collector plane: η = mc p ΔT/I.A t where I is solar radiation incident to the outer plate W/m2, A t is plates area, Cp is specific heat capacity of air [kj/kg c] at air mean temperature, ΔT is temperature difference between inlet and outlet airflow ( C) The heat loss calculations for solar collector. The solar thermal efficiency depends essentially on thermal losses from outer surfaces of the collector. These losses are classified as. Heat Loss Coefficient: IV. FABRICATION OF THE SOLAR AIR HEATER: Fabrication of a Solar Air Heater carried out by wood, blower, PVC pipe, fiber coated rubber pipe, GI sheet and GI pipe and clips etc. First step is that made a wooden box of 127cmX93cmX10cm without cover on the top with two holes for inlet and outlet air. After made baffles of 40cmX7cmX3cm and fix them inside the wooden box at gap between two baffles is 3o cm. Thermocouples attached at different 9 point at uniform distance on the back side of Galvanized iron sheet by using of shoulder and attach this sheet on the top of the wooden box with certain arrangement of nut and bolts. Attach GI pipe at inlet and outlet holes of solar air heater. Also attach the blower at the inlet pipe with fiber coated rubber pipe. Assemble wheels on the bottom to make it portable. Fig.- shows the Solar air heater Drying Chber: Design of solar drying chber is most important part of the solar drying system because air flow through chber also depends on design. Most common designs are cylindrical chber with a cone of 40-60º, drying chbers with a flat bottom, horizontal box type drying chbers. Drying Systems should be designed to be the most efficient and economical. It is square shaped drying chber with dimension of 5ocmX50cmX50cm. Drying chber take hot air from the Solar Air Heater through a pipe and remove the humidity or moisture content from the product and dry the product. Hot air leaves in atmosphere through solar chimney Air Mass Flow Rate m = ρa out V Where V is average outflow air velocity in channels, A out is outflow duct area and ρ is the outflow air density at outflow temperature. The Top Heat Loss Coefficient This coefficient determines the sum total of energy lost from the absorber to the bient by the combined processes of convection and radiation U 1 t C Tp Ta hw Tp N f 2 2 N 1()() Tp Ta Tp Ta cos c.24 L d p N 1 p 9 30 Ta f N 2 w w h h w v e.252 h 1 2N f 1 N d g Where Ut= top loss coefficient N= number of glass cover Tp= plate temperature Ta= bient air temperature h w = air convective heat transfer coefficient δ = Stefan s Boltzmann constant ε p = emissivity of plate ε g = emissivity of glass L = length of plate The bottom heat loss coefficient Energy dissipation from the bottom of the collector is the collective effect of conduction from the absorbing surface to the insulator at the bottom and convection and radiation from the outside wall to the bient surroundings. Thermal loss coefficient from the bottom could be calculated as follows 3

4 Experimental Analysis on Solar Air Dryer Ub x i x k i 1 w 1 w hw k Where U b =Bottom loss coefficient Ki and kw are thermal conductivity of insulating material and base plate Xi and Xw are thickness of insulation and wooden plate Total heat loss coefficient U= Ub+Ut Q=UAΔT V. EXPERIMENTAL SET UP It is the assembled view of all three parts (Solar Air Heater, Drying chber, Solar chimney) of solar dryer. v 15.6 m/s hw x 15.6 ρ 1.2kg/m 3 Cp 1.005KJ/kgk Area of plate A p = 1.21x.87 Dieter of outlet pipe 3.6cm Ao 4 d Xw, thickness of wood 2 2cm Kw, conductivity of wood 0.17 w/mk Energy input IxA p 490x.87x1.21 Energy output Ut = 64.98, Ub = 7.5, Ut= = 72.5 Q L = UApΔT = 305.2W Effect of other pareters 1. If we vary the velocity of air then efficiency decreases as the velocity of air increases. 2. If Δt increases then efficiency increases. 3. If solar intensity increases then efficiency first increases and then decreases. Table 2 Absorber plate temperatures Performance Analysis Figure 1 Solar Air Dryer Absorber Plate temperature of nine points taken Table 1 Absorber plate temperature T1 T2 T3 T4 T5 T6 T7 T8 T9 9: N=0, as there is no cover Day 1 May 17 9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 Absorber Plate temperatures, o C t1 t2 t3 t4 t5 t6 t7 t8 t T1= (t1+t2+t3+t4+t5+t6+t7+t8+t9)/9 temperature at absorber plate, collector outlet, inlet to dryer, at first and second shelves, and chimney outlet, solar intensity, relative humidity and velocity at inlet and outlet, weight of the product and efficiency Mean plate temperature (T1+T2+T3+T4+T5+T6+T7+T8+T9)/9 Mean temperature 34 C Solar Intensity 490W/m 2 Ambient temperature 30 C ε p

5 International Journal of Engineering, Management & Sciences (IJEMS) ISSN: , Volume-1, Issue-12, December 2014 Table 3 Absorber plate temperatures Day1 17 may T1 T2 T3 T4 T5 T6 Tb I η 9: : : : : : : : Fig 3 time of day vs. temperature VI. RESULT AND DISCUSSION The graph of the figure 4.1 is plotted between solar intensity, efficiency and time of day and data taken from table 1. The graph shows hourly variation of solar intensity and efficiency of the solar air heater, as the day progresses solar intensity increases with maximum at noon, and efficiency is maximum at the morning. CONCLUSION Experiments were performed on solar air heater, the maximum temperature rise is 20 c and maximum solar intensity was 890w/m 2. Maximum efficiency of 50.2 is obtained and there are top and bottom losses as there is no glass cover on the top, further efficiency can be improved by providing insulation at the bottom Effect of other pareters 1. If we vary the velocity of air then efficiency decreases as the velocity of air increases. 2. If Δt increases then efficiency increases. 3. If solar intensity increases then efficiency first increases and then decreases. Fig 2 solar intensity vs. efficiency vs. time in hour The graph 2 which is bar chart shows relation between time of day vs. temperature. The two temperatures shown are bient temperature and plate temperature and from the bar chart it is clear that the difference of the temperature is between 15 c to 20 c, also maximum temperature difference is 20.6 c. REFERENCES [1] 1. Garg H.P., Prakash J., Solar Energy Fundentals and Applications, Tata Mcgraw-Hill Publishing Co. Ltd., New Delhi, [2] 2. Barnwal P., Tiwari G.N., 2008, Grape Drying by using Hybrid Photovoltaic-Thermal (PV/T) Green House Dryer: An Experimental Study, Solar Dryer, 82(12), [3] 3. Tiwari G.N., Nayak S., Dubey S., Solanki S.C., Singh R.D., 2009, Performance Analysis of a Conventional PV/T Mixed Mode Dryer Under No Load Condition, International Journal of Energy Research, 10(33), [4] 4. Sharma A., Chen C.R., Lan V., 2009, Solar Energy Drying Systems: A Review, Renewable and Sustainable Energy Reviews, 13(6-7), [5] 5. Forson F.K., Nazha M.A.A., Rajakaruna H., 2007, Modeling and Experimental Studies on a Mixed-Mode Natural Convection Solar Crop Dryer, Solar Energy, 81(3), [6] 6. Janjai S., Llert N., Intawee P., Bala B.K., Boonrod Y., Mahayothee B., 2009, Solar Drying of Peeled Longan using a Side Loading Type Solar Tunnel Dryer, Drying Technology, 27(4), [7] 7. Boughali S., Benmoussa H., Bouchekima B., Mennnouche D., Bouguettaia H., Bechki D., 2009, Crop Drying by Indirect Active Hybrid Solar- Electrical Dryer in The Eastern Algerian Septentrional Sahara, Solar Energy, 83(12), [8] 8. Romano G., Kocsis L., Farkas I., 2009, Analysis of Energy and Environment Pareters during Solar Cabinet Drying Of Apple and Carrot, Drying Technology, 27(4),

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