Thermal Performance of an Indirect Solar Dryer

World Renewable Energy Congress Ii"1(WREC2000) © 2000 Elsevier Science Ltd. All rights reserved. Editor: A.A.M. Sayigh

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THERMAL PERFORMANCE OF AN INDIRECT SOLAR DRYER G./~lvarez, E. Sima and L. Lira Centro National de Invmtigaci6n y Dmerrollo Temol6gico (CENIDET-DGIT-SEP). ,~_P. 5-164, Oammvaca Morelos Cp 62050, M6xico. Tel. and Fax (7) 312-76-13, gaby(~cenidetedu.mx

ABSTRACT

In this paper we present a procedure to evaluate the thermal performance of an indirect solar drier of small capacity (~20K~day) developed at the Zacatepec Institute of Technology (Mexico). The main components of the indirect solar drying system are the solar air collector, the drying chamber, where the drying takes place, and the air-handling unit, which consists of a blower. Many factors determine the thermal response of a solar dryer. The procedure presented considers the thermal evaluation of the solar air collector, the drying chamber and the complete solar drying system. The absorber surface of the solar air collector was made of aluminium disposable cans. For the thermal performance of the solar air collector the ASHRAE 93-77 standard was used. The heat losses of the drying chamber were evaluated and the thermal efficiency of the solar drying system was measured. The time constant was 9 min, the maximum efficiency was 54.5% and a figure of the modified angle of the solar air collector is presented, as well as the temperature and moisture content of the shelled corn in the chamber during operation. The system dries a 25.7 Kg of shelled corn in 10 hrs. The thermal efficiency measured of the solar dryer system was 32.5%. KEYWORDS: Solar drying, solar air collector, solar dryer.

INTRODUCTION Cereal grains are a major source of food for humans and for animals. The production of cereal grains have been increased rapidly and has at least tripled in the past 20 years, largely as a result of new fertilisers and weed and insect control measures. This increase in production necessitates continued emphasis on harvesting, handling and drying to economically preserve the crop product. Drying and preservation of agricultural products has been one of the oldest uses in solar energy. The traditional method, still widely used in Mexico, is open to sun drying where crops like cereal grains are spread on the ground and turned regularly until dried so they can be stored safely. However there are high levels of dust and atmospheric pollution, spoilage and intrusion of animals, thus there is no control of the drying process. In order to archive better quality control and reduce losses, artificial drying should be employed. Conventional artificial dryers used fuels like electricity, coal and fossil fuel to heat the ambient air for drying purposes. However, due to the high costs, short supply and highly polluted, a drying with solar energy is proposed. Several studies [1,2,3], have been shown that the drying systems with ambient air and solar energy are adequate, efficient and economic alternative. In this paper, a procedure to evaluate the thermal performance of an indirect drier of small capacity (~20K~day) is presented. The solar drier was built in the Zacatepec Institute of Technology [4]. SOLAR DRYER

An indirect solar drier is basically the same of that of conventional fuel type dryers with the only difference is that solar collectors replace the heat source. Usually this kind of solar dryer is recommended for grain drying [5], because it is possible to have control on the t e x t u r e and humidity. The components of the solar dryer considered are one bin dryer, one air solar collector made of disposable cans, one small fan, ducts and one electronic circuit for flow control. Figure 1 shows the components of the solar dryer. The bin dryer is made of steel painted black and composed of two parts: The first part consists of a top open cylinder of 0.92 m diameter and 0.90 m height. The grain product is introduced through an acrylic window of 0.40 m x 0.73 m.. The second part is located at the top ofthe bin and has a shape of a bell of 0.92 m at the bottom 0.88 m height and 0.20 m at the top. The solar collector box is made of insulated steel sheet 24 of 2 m long, 1 m wide and 0.13 m height. This box

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Figure 1.- Solardryer contains an absorber surface and a transparent cover. The total area of the collector is 5 m2 and the collector aperture area is 2 m2. The absorber surface consists of 15 ducts made of aluminium disposable cans of 0.07 m diameter joint together with plastic cement and painted black. This absorber surface is cover with a 5 mm glass. The head tubes located at the top and bottom are 0.1 m diameter. The air blower used has the following characteristics: Model: CEB800, 1/20 hp, voltage 127 v, current 0.9 A, velocity 1550 rpm, maximum flow rate 800 m3/hr. Finally, an electronic circuit was developed to maintain the inlet air temperature of the bin constant. The electronic circuit varies the input voltage of the fan to control the air velocity through the grain product. THERMAL PERFORMANCE To determine the thermal performance of the solar collector the ASHRAE93-7716] standard was use to measured the time constant, the efficiency and the modified angle. Also a procedure was developed to determine the overall heat loss coefficient of the bin. Finally, the thermal performance of connected components of the bin plus solar collector of the solar dryer was determined. All the tests were carry out in the city of Cuernavaca, Morelos, Mexico at a latitude of 18.5 °, from 9:45 to 15:45 hrs solar time from December 24 of 1998 to January 15 of 1999. Thus, the ASHRAE 93-77 standard was applied under the climatic conditions of Mexico. The time constant, the efficiency of the collector and the modified angle was determined. The time constant measured was 9 min. The maximum efficiency was 54.5% + 3% [7]. Figure 2 shows the efficiency data measured and its fitted curve. Figure 3 presents the modified angle measured and its fitted curve. 0.60 1.1

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To calculate the overall heat loss of the bin dryer, a thermal resistance was placed inside the bin. Ambient temperature and average inside bin temperature was recorded using T-24 thermocouples. Figure 4 shows the position of the thermocouples inside the bin and the thermal resistance. The test was performed under laboratory conditions. The procedure used to determine the time constant of the bin dryer was the following: the average inside bin temperature was increased by varying the air flow rate that circulates through the dryer,

1167 until the difference between the interior and ambient t ~ m r e were 10,12,14 y 18.50(2. The imerior and ambient temperatures, the voltage and current of the thernal resistance were recorded each 10 seconds by 25 minutes. After that time the resistance power was reduced to zero and all the t ~ m r e s were recorded until it reaches [T~(t)-T&]/[T~(0)-T~]<0.368. The time constant equation calculated as a function of the tempcraa~ difference was TCffi-83.09+49.70 AT-0.87 AT2. To determine the heat loss coefficient of the bin dryer, an energy balance is perforn~ as Qsupplied'l- Qm=Q~t + Q~o~ + Qsto,~. At steady state QmffiQst~, thus the bin heat loss can be calculated by w~ - m~ C~Ts - r~ )

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Figure 5.-Overall heat loss coefficient vs the temperature difference.

The thermal efficiency of the complete solar dryer system was calculated considering the useful energy gain divided by the solar irradiance received on the collectors Tlss =

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Figure 6 shows the schematic diagram of the instrumentation of the solar dryer system. The solar incident radiation on the collector, the inlet and outlet air velocity, humidity and temperature of the system, the ambient temperature and the interior air temperature of the bin dryer were measured. Figure 7 shows the incident solar radiation measured and the instantaneous thermal efficiency calculated using equation (2). In this case the thermal efficiency was calculated with no load in the bin dryer. The average maximum peak efficiency was 57.5% and was measured at 12:30. e_

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Figure 7.- History of the instantaneous thermal efficiency and

1168 Figure 8 presents the histories of the incident solar radiation and the air mass flow rate. It can be noted that the air mass flow rate supplied during the test to the grain is proportional to the incident solar radiation. As the set point temperature of the electronic controller circuit was fixed at 49°C, the air blower begin to work when the incident solar radiation reaches 650 W/m 2. Afterwards, the bin dryer was loaded with 25.7 Kg of shelled corn. The humidity content of the shelled corn was 28% and the height ofthe grain bed was of 3.5 cm. The grain was exposed to the heated air delivered from the solar collector through the grain dryer. After a period of 10 hrs, (2 days) the grain reaches 12.1% humidity and weight 18.5 Kg. Figure 9 shows the measured incident solar radiation, the average temperature and humidity of the ~ The humidity of the gr~o" decreases from 26% to 16% and the temperature of the grain increases from 19-C reaches a maximum of 35 C and the decreases to 30°C. After 16:30 the test was intem~pted because there was not enough irradiance to keep set point temperature of the mass flow rate of the bin dryer. At the next day the grain started with almost the same humidity of the previous day and it took 3 hours to reach the 12% humidity. 80.0

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Figure 9.- Histories of the incident solar radiation and the temperature and humidity of the grain during two days.

Finally, figure 10 shows a comparison of the efficiency curves when the solar dryer system was loaded and unloaded with shelled corn. The thermal efficiency of the solar dryer without corn is higher than the thermal efficiency of the solar dryer with corn as was expected. The average efficiency of the solar dryer with corn is 32.5%, meanwhile the one without corn was 44.9%.

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1169 CONCLUSIONS A procedure to describe the thermal perfonmnce of a solar dryer was presented. The maximum thermal efficiency o f the collector was 54.5% and the time constant was 9 minutes. The overall maximum heat loss coefficient of the bin dryer was 4.3 W/m2 °C. The average thermal efficiency of the solar dryer system loaded with corn was 32.5%. To set the inlet bin air temperature 49"C, the air flow rate must be proportional to the incident solar radiation and the air blower starts to blow the air when the minimum incident solar radiation is 650 W/m2. One of the problems presented was that the temperature of the grain bed at the centre was higher than the temperature on the sides. Thus, the grain at the centre dries faster than the grain on the sides. Finally, after a period of 10 hrs, an amount of 7.20 Kg of water was evaporated. Aelmowledgements The authors want to thank M.C. Freddy Chart Puc for the advice on the design of the electronic circuit control for the air blower.

NOMENCLATURE A Arcs Cp Ctcs hE hs I

Solar angle Bin dryer area, m2 Specificheat, W/m K Timeconstant, min Enthalpyof the inlet air, J/kg Enthalpyof the outlet air, J/kg OaTent,amp

TC Time constant of the solar collector, min Tlss Solar dryer efficiency Air mass flowrate, m3/s m TA Ambient temperature,°C T~ Interior air temperature of the bin, °C Outlet air temperatureof the collector, °C Ts Voltage, v V Ucs Overall heat loss coefficient, W/m2 °C

REFERENCES

1. Maekawa, T.; K. Toyoda; K. Matsumoto. "Computer Simulation of Solar Energy for Grain Drying in Japan". In Do,ing "82PP. 145-150. 1982 2. Bello, A. "Aplicaci6n de la Energia Solar en el Secado de Granos". Ingenieria de Investigaci6n. Vol. 2, No. 1, pp. 64-68. 1983. 3. Dominguez, J.; A. Parra; L. G. Villa. "Simulaci6n Matemfitica y Optimizaci6n del Secado de Productos Agropecuarios con Aire Natural y Energia Solar". _Ingenieria de Investigaci6n. Vol. 2, No. 2, 1983, pp. 57-62 4. Acosta, A.1L; J. J. Vfizquez. "Disefio, Construcci6n y Caracterizaci6n de un Secador Solar Prototipo de Productos Agricolas" 5. Sodha, M.S. and R. Chandra. "Solar Drying Systems and their Testing Procedures: A Review". Energy Convers. Vo135 No. 3, pp 219-267. 1994. 6. ASHRAE Standard 93-86. "Methods of Testing to Detenv~e the Thermal Performance of Solar Collectors". American Society of Heating and Air Conditioning Engineers, Inc., pp 1910-1958. 1977. 7. Alvarez, G.; E. Simfi y L. Lira. Proceedings ANES 98, Mexicali, B.C, M6xico, pp. 118-122. 1998.