Design, development and performance testing of a ...

Energy 27 (2002) 579–590 www.elsevier.com/locate/energy

Design, development and performance testing of a new natural convection solar dryer Dilip R. Pangavhane 1,∗, R.L. Sawhney 2, P.N. Sarsavadia 2 1

2

Department of Mechanical Engineering, K.K. Wagh College of Engineering, Amrutdham, Panchvati, Nashik-422 003 (M.S.), India School of Energy and Environmental Studies, Devi Ahilya Vishwa Vidhyalaya, Takshashila Campus, Khandwa Road, Indore-452 001 (M.P.), India Received 21 July 2000

Abstract Mechanical drying of agricultural products is an energy consuming operation in the post-harvesting technology. Greater emphasis is given to using solar energy sources in this process due to the high prices and shortages of fossil fuels. For these purposes, a new natural convection solar dryer consisting of a solar air heater and a drying chamber was developed. This system can be used for drying various agricultural products like fruits and vegetables. In this study, grapes were successfully dried in the developed solar dryer. The qualitative analysis showed that the traditional drying, i.e. shade drying and open sun drying, dried the grapes in 15 and 7 days respectively, while the solar dryer took only 4 days and produced better quality raisins.  2002 Elsevier Science Ltd. All rights reserved.

1. Introduction Open sun drying is the most commonly used method to preserve agricultural products like grains, fruits and vegetables in most developing countries. Such drying under hostile climate conditions leads to severe losses in the quantity and quality of the dried product. Mechanical drying is an energy consuming operation in the post-harvesting technology of agricultural products, so more emphasis is given on using solar energy sources due to the high prices and shortages of fossil fuels. Solar dryers are now being increasingly used since they are a better and more energy efficient option. Sun shines in India over an average 3000–3200 h per year, delivering about 2000 kWh/m2-year of solar radiation on the horizontal surface[1]. This abundantly available

∗

Corresponding author. Tel.: +91-253-512-876; fax: +91-253-511-962.. E-mail addresses: [email protected]; [email protected]; [email protected] (D.R. Pangavhane).

0360-5442/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 2 ) 0 0 0 0 5 - 1

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Nomenclature Ac ABin Cp F1 FR GT ˙a M R2 T Ta,d Ta,w Tco,d Tinlet T1,d T1,w T2,d T3,d T4,d T5,d T5,w UL

Collector area, m2 Outer surface area of the bin between the measurement points (entrance of the plenum chamber and the fifth tray), m2 Specific heat of air, J/kg 0C Collector efficiency factor Collector heat removal factor Solar irradiance on the collector surface, W/m2 Mass flow rate of circulated air, kg/s Statistical parameter, coefficient of determination Temperature of the drying air, °C Ambient air dry bulb temperature, °C Ambient air wet bulb temperature, °C Collector outlet air dry bulb temperature, °C Collector inlet air dry bulb temperature, °C Dry bulb temperature at first tray, °C Wet bulb temperature at first tray, °C Dry bulb temperature at second tray, °C Dry bulb temperature third tray, °C Dry bulb temperature fourth tray, °C Dry bulb temperature at fifth tray, °C Wet bulb temperature at fifth tray, °C Overall heat loss coefficient of the collector, W/m2-°C

Greek letters β (τα)e

Constant of the stack (Eq.1) Effective absorptance transmittance coefficient or the optical efficiency of the collector

solar energy can be used for the drying of agricultural products. Although for commercial production of raisins (or any other agricultural product), the forced convection solar dryer provides better control of required drying air conditions, the natural convection solar dryer does not require any other energy during operation. Hence, the natural convection solar dryer may become a more suitable proposition for the rural sector and other areas in which electricity is scarce and in irregular supply. Based on the guidelines provided by earlier researchers [2–6] and the results obtained in drying kinetic studies carried out for grapes by Pangavhane et al. [7] and Sawhney et al. [8], a natural convection solar dryer was designed and fabricated. This dryer consists of a flat plate solar air heater, flexible connector, reducer-cum-plenum chamber and drying chamber with chimney and

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a supporting stand. In the present work, the thermal performance of the solar air heater and the drying chamber was evaluated for the natural convection mode under no load conditions and the complete solar drying unit was also tested under grape load conditions. 2. Dryer details A natural convection solar dryer was manufactured and installed at the School of Energy and Environmental Studies, Devi Ahilya Vishwa Vidhyalaya, Indore (India). It consists of a solar flat plate air heater, flexible connector, reducer-cum-plenum chamber with chimney and a supporting stand as shown in Fig. 1(a). The solar air heater (Fig. 1b) consists of an absorber (painted matte black) with fins, glass cover, insulation and frame. The air duct beneath the absorber was made from an aluminium sheet (0.5 mm thick and 1.95 m×0.73 m×0.03 m in size) through which air was passed. The U-shaped corrugations (11 in number) were placed in the absorber plate parallel to the direction of airflow. Aluminium fins (a matrix foil 0.15mm thick) were fitted to the back of the absorber. At the lower end of the collector (air inlet), shutter plates 4 mm thick and 0.08 m×0.4 m in size, were also provided to stop the air flowing during the night. At the upper end (air outlet) of the collector, a flange portion was provided to connect the flexible connector with nuts and bolts. The air duct was made leak-proof with a good quality sealing material. The entire unit was placed in a rectangular box made from a galvanized iron sheet 0.9mm thick. The gap between the bottom of the air duct and the box was filled with glass wool insulation. The toughened glass plate (4 mm thick, 1.82 m×0.76 m in size) was fixed on the frame of the rectangular box at a distance of 0.04 m above the absorber surface. The glass was fitted to the rectangular box frame along with the U-channel rubber and sponge rubber with the help of galvanized iron angles and screws. The U-channel rubber and sponge rubber were used to prevent contact between the air duct and the rectangular box wherever required. To prevent insects from entering the dryer, wire mesh was fixed at the inlet side of the collector. As the collector was designed for variable tilt adjustment, for connecting the collector outlet to the inlet of plenum chamber, a flexible connector made from special fibre reinforced plastic foam (stable up to 200°C) was provided. The inlet and outlet ends of the connector were made by using two rectangular cross-sectional ducts each with outer flanges between which two layers of fibre reinforced plastic foam were secured with industrial adhesive to increase the strength of the connector. The whole unit was covered with a galvanized iron sheet enclosure. Glass wool was placed between the flexible connector and the galvanized iron enclosure as an insulating material. The plenum chamber was used to connect the outlet of the flexible connector to the inlet of the drying chamber. The cross section of the plenum chamber was increased gradually from inlet to outlet, which helps in maintaining the uniform distribution of air in the drying chamber. The inner side of the plenum chamber was made from an aluminium sheet and enclosed in a galvanized iron sheet enclosure between which the glass wool was used as an insulating material. The interior of the drying chamber (with inner dimensions 0.35 m×0.35 m×0.70 m) was made from an aluminium sheet 0.5 mm thick (Fig. 1c). From the base to the ceiling of the drying chamber, five aluminium trays (on which the product was placed) were stacked evenly at distances of 0.09 m apart. The frame of each tray (0.33 m×0.33 m) was made from an aluminium angle of 0.025 m. The tray was made from an aluminium wire mesh (2.5 mm×2.5 mm in size) fixed

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Fig. 1. (a) Sectional details of natural convection solar dryer. (b) Solar air heater. (c) Drying chamber.

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to an angle frame. Wire mesh was also placed at the bottom of the drying chamber and the outlet of the plenum chamber for uniform distribution of air in the drying chamber. The entire unit was enclosed in a galvanized iron sheet box of size 0.45 m×0.45m×0.7m and glass wool was filled in between the galvanized iron box and the aluminium box. For loading and unloading of the trays in the drying chamber a door was also provided with locking arrangement. The door was lined with sealing sponge rubber to make it airtight. At the bottom and top ends of the drying chamber an extended outer rib was provided to connect the reducer-cum-plenum chamber and chimney, respectively. Sponge rubber with 6 mm nuts and bolts was used for connecting all the parts. A chimney was made from a galvanized iron sheet and fitted at the top of the drying chamber. 3. Instrumentation for the natural convection solar dryer In the fabricated solar dryer, the RTD Pt-100 sensors (accuracy ± 0.1°C) were fixed at the outlet of the solar air collector (Tco,d) and at each tray in the drying cabinet (T1;d, T2;d, T3;d, T4;d, T5;d) to measure the dry bulb temperature at these locations (Fig. 1a). In addition, RTD Pt-100 sensors with wet wicks were fixed in the first (T1,w) and fifth (T5,w) trays of the drying chamber for measuring the wet bulb temperatures at these locations. The ambient, dry (Ta,d) and wet bulb (Ta,w) temperatures were also measured near the solar dryer under the shade. A radiation pyranometer (accuracy ± 2%) was placed in the plane of the collector for measuring the global solar irradiance. Wind speed was measured with a hot wire anemometer (accuracy ± 0.01 m/s) and air flow rate in the collector was determined by measuring the air velocity at the collector outlet. For measuring the weight loss of the sample, an electrical balance (200 g capacity and 0.0001 g accuracy) was used. No load tests were performed on sunny days and hourly values of the air temperatures, air velocity and solar irradiance were recorded for the five sunny days. In the drying experiments, Thompson seedless grapes were used as the test samples in the dryer. Drying experiments were performed during the period March–May 1998. During the drying experiments, the weather was generally sunny and no rain appeared. The fresh Thompson seedless grapes were harvested manually and transported in boxes to the experimental site. Before drying experiments, spoiled grape berries were discarded in order to prevent infection of the intact grapes by fungi or bacteria. The grape bunches were dipped into a solution of 2.0% commercial dipping oil and 2.5% K2CO3 solution at ambient temperature for 3 min, which reduces the drying time of grapes by improving the water permeability of the skin[7]]. After the pretreatment, grape bunches were cut into smaller sizes and spread out uniformly on the trays inside the dryer. In order to compare the performance of the solar dryer the samples were also dried by traditional methods, i.e. open sun drying and shade drying. The reduction in moisture content was determined by weighing the sample at every hour. 4. Result and discussion During the five days of the experiment, the diurnal variation of the ambient air temperature, solar irradiance, collector outlet air temperature and dryer outlet air temperature for the solar

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Fig. 2. Diurnal variation of the solar irradiance —, Collector outlet air temperature -䊐-, Dryer outlet air temperature ⌬-, asmbient air temperature (dry) -䊊- and ambient air temperature (wet) -∗-, for the natural convection solar dryer under no load conditions during 8/4/1998.

dryer were plotted and are shown for the two typical days of April and May in Figs. 2 and 3 respectively. From these figures it is observed that the rise in air temperature due to the generated air flow rate in the collector were sufficient for the purpose of grapes drying. For the inlet air temperature of 38°C, the maximum air temperature at the dryer inlet at no load conditions was recorded as 69.5°C at the solar irradiance level of 909 W/m2. The maximum temperature gain in the collector during the peak afternoon hours on all the days was around 30°C (varying between 25.9 and 33.5°C). During the 5 days, the daily mean values of air temperature at the dryer inlet vary from 51.9°C to 64.6°C and for solar radiations it varies from 605 to 673 W/m2.

Fig. 3. Diurnal variation of the solar irradiance —, Collector outlet air temperature -䊐-, Dryer outlet air temperature ⌬-, ambient air temperature (dry) -䊊- and ambient air temperature (wet) -∗-, for the natural convection solar dryer under no load conditions during 4/5/1998.

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The mass flow rate of the drying air in the thermosyphon mode of the collector depends on the prevailing wind conditions, ambient air temperature, incident solar radiation and the collector design. It was observed that, when the wind velocity was more or less uniform throughout the day, the air velocity in the collector shows definite dependence on the stack temperature difference (Tco,d⫺Ta,d). This indicates that the heated drying air in the collector, in addition to by wind pressure being driven, is also thermosyphon driven. In the thermosyphon mode, the mass flow ˙ a in a stack is given by: rate of the air M ˙ a ⫽ b冑(Tco,d⫺Ta,d). M

(1)

The various curves were plotted to find the correlation of mass flow rate of drying air with ˙ aCp(Tco,d⫺Ta,d). Assuming that the driving force (Tco,d⫺Ta,d)1/2, (Tco,d⫺Ta,d), (Tco,d⫺Ta,d)2 and M comes from the energy gained by the air stream rather than from the temperature difference, the ˙ aCp(Tco,d⫺Ta,d)] in the collector and the mass flow rate of correlation between energy gained [M the drying air is shown in Fig. 4. From this figure it is observed that the correlation of mass flow rate of drying air with the absorbed energy by the air stream (R2=0.93) is still better than the correlation between mass flow rate with the square of the temperature difference (R2=0.85), mass flow rate with temperature difference (R2=0.82) and mass flow rate with the square root of the temperature difference (R2=0.75), respectively. As in the thermosyphon mode, air velocity in the collector is not constant and is governed by the instantaneous conditions of the ambient air and solar radiation. The normal method of determining the collector constants FRUL and FR (τα)e from the efficiency versus (Tinlet⫺Ta,d)/GT curve is not possible, because collector inlet temperature of air (Tinlet) in this case is equal to the ambient temperature of air (Ta,d). For determining the collector constants operating in thermosyphon mode, an indirect method has been tried.

˙ aCp(Tco,d⫺Ta,d), for no load condition of the natural Fig. 4. Variation of mass flow rate of the drying air with M convection solar dryer.

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The energy balance equations of the collector can be written as[9]]: ˙ aCp(Tco,d⫺Ta,d) ⫽ F1Ac{(ta)eGT⫺UL [[(Tco,d ⫹ Ta,d) / 2]⫺Ta,d]} M

(2)

or ˙ aCp (Tco,d⫺Ta,d) ⫽ FRAc(ta)eGT. M Solving Eq.(2) for (Tco,d⫺Ta,d), one gets: ˙ aCp ⫹ (AcUL / 2)]. (Tco,d⫺Ta,d) ⫽ [(ta)eGTAc] / [M

(3) (4)

And using Eq. (1), one can write: ˙ a ⫽ b冑[(ta)eGTAc] / [M ˙ aCp ⫹ (AcUL / 2)] M

(5)

˙ 3aCp ⫹ M ˙ 2a(AcUL / 2) ⫽ b2(ta)eGTAc. M

(6)

or An effort was also made to find the correlation between the thermosyphon driven velocity ˙ a and the driving force GT (Fig. 5). From this figure, it was observed that the correparameter M ˙ a and GT is not better (R2=0.77) than the correlation between M ˙ a and the energy lation between M 2 ˙ gained by the air in the collector, i.e. MaCp(Tco,d⫺Ta,d) where R =0.93. The efficiency of the air collector is defined by: ˙ aCp(Tco,d⫺Ta,d) / (AcGT)]. (7) h ⫽ [M It may also be pointed out that different authors[10–12]] have used different values for the collector areas (absorber area, cover area and gross area) for the estimation of the collector efficiency. In our case, the gross area of the air collector has been used for calculations of the

Fig. 5. Variation of mass flow rate of the drying air with the solar irradiance (GT.), for no load condition of the natural convection solar dryer.

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solar dryer performance. It was found that the daily mean efficiency of the solar air collector varies from 48 to 56% during the experimentation. Similarly the hourly efficiency of the collector varies from 26 to 36% in the morning and evening hours and 38 to 65% in the afternoon hours. It is also noticed that the collector efficiency for the lower mass flow rates of the drying air (in the morning or evening hours) is lower and is higher for higher mass flow rates (Fig. 6). It is well known that for the forced flow case, the temperature rise of the air in the collector decreases and efficiency of the collector increases with increased mass flow rate of the fluid. In the thermosyphon mode, it is observed that the efficiency of the collector increases and the air mass flow rate also increases with increasing temperature difference of the air in the collector. Again, simplifying Eq.(2) and using Eq.(7), one gets: h ⫽ F1(ta)e⫺[(F1UL / 2GT)(Tco,d⫺Ta,d)]

(8)

h ⫽ FR(ta)e

(9)

and where ˙ aCp) / (AcUL)]{1⫺exp[(⫺F1AcUL) / (M ˙ aCp)]}. FR ⫽ [(M Eq.(9) can be combined with Eq.(7) and written as: ˙ aCp) / (AcULF1)}{1⫺exp[(⫺F1AcUL) / (M ˙ aCp)]} ˙ aCp(Tco,d⫺Ta,d) / (F1GTAc(ta)e)] ⫽ {(M [M

(10) (11)

and: ˙ aCp(Tco,d⫺Ta,d) / (F1GTAc(ta)e)] ⫽ FR / F1 [M

(12)

It may be pointed out that F1(τα)e, the collector constant, is independent of the mass flow rate while FR/F1, another collector constant is dependent on the air mass flow rate. Taking F1(τα) to

Fig. 6. Variation of the collector efficiency with mass flow rate of the drying air, for no load condition for the natural convection solar dryer.

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be equal to 0.7 (F1, τ and α are equal to 0.9), one can estimate F1UL from the above equations. The values of the expression in the left hand side of Eq.(11) can be obtained from the measured data. For calculating F1UL, we have to choose that value of the single parameter ˙ aCp) / (AcUL) in the right hand side, which makes (FR/F1) equal to the value of the left hand (M side obtained from the experimentally measured data. For this purpose, the values of the right ˙ aCp / AcF1UL is calculated. hand side expression of FR/F1 for different values of single parameter M 1 ˙ Once for the given experimental points, the value of MaCp / AcULF is obtained one can calculate the desired value of the collector parameter F1UL. From the values of (F1UL)collector obtained for each data point of the 5 days, it is seen that the value of (F1UL)collector varies between 3.6 and 19.4 W/m2-°C. The average value of (F1UL)collector for all the data points comes out to be equal to 13.5 W/m2-°C. The variation of F1UL with the mass flow rate of the drying air is also shown in Fig. 7. From this figure, it is observed that, the value of F1UL decreases with increased air mass flow rate. In forced flow case, it is well known that the value of F1UL of a collector is independent of the mass flow rate. But in thermosyphon mode, the value of F1UL decreases with increase in the mass flow rate of the drying air. From this figure it is also observed that there is a widespread data between mass flow rate and F1UL, this is due to the continuous variation in ambient air temperature and wind speed. Similarly this is also due to the natural convection current, as no maximum useful heat is carried out away from the collector by the drying air and because of heat loss may be as large as the solar heat absorbed. For determining the bin loss coefficient UL,Bin of the drying chamber. The energy balance equations of the drying chamber may written as: ˙ aCp(Tco,d⫺T5,d) ⫽ ABinUL,Bin{[(Tco,d ⫹ T5,d) / 2]⫺Ta,d}. (13) M Where, ABin is the outer surface area of bin between the measurement points (entrance of plenum chamber and the fifth tray). From the values of UL,Bin obtained for all the data points, it is observed that the values of UL,Bin varies between 0.2 and 3.5 W/m2°C with the average value

Fig. 7.

Effect of mass flow rate of the drying air on ULF1, for the natural convection solar dryer.

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of 1.14 W/m2°C. From these values obtained for UL,Bin, it can be said that the drying chamber is very well insulated. The drying tests were carried out for the solar dryer with manually harvested fresh Thompson seedless grapes. For qualitative analysis, the samples were also dried in shade drying and open sun drying. The grapes have to be dried up to the final safe moisture content of at least 17% dry basis. During the experimentation, the ambient data (solar radiation, temperature and relative humidity, etc.) as well as inside temperature of the drying chamber and mass of the grapes (for all the three methods) were recorded from 8.00a.m. to 5.00 p.m. every hour. During the rest period (night), the air duct shutters (damper) were kept closed. The open sun drying and shade drying samples were also kept in the closed containers during the night period. The drying was continued till the samples achieved the desired final moisture content. The variation of moisture ratio with time is shown in Fig. 8, for the solar dryer, open sun drying and shade drying. It is observed that the moisture content was reduced from 349.59 (%db) to 17 (%db), in 15 days for the shade drying, 7 days for the open sun drying while the solar dryer took only 4 days. From the sensory evaluation and chemical analysis result, it is observed that the organoleptic qualities of the grape dried in the solar dryer was better than the shade drying or the open sun drying, however more browning was observed in the open sun drying as compared to the shade drying and the solar dryer. This more browning was due to the direct exposure of the grapes to solar radiations for a longer drying time. 5. Conclusions The developed natural convection solar dryer is capable of producing the average temperature between 50 and 55°C, which was optimum for dehydration of the grapes as well as for most of the fruits and vegetables. This system is capable of generating an adequate flow of hot air to

Fig. 8. Variation of moisture ratio with drying time for sub bunch grapes drying case, ⌬ — shade drying, 䊐 — open sun drying, and 䊊 — solar dryer drying.

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enhance the drying rate. The drying air flow rate increases with increase in ambient temperature by the thermal buoyancy in the collector. The collector efficiencies of this natural convection solar dryer were ranged between 0.26 for 0.0126 kg/s and 0.65 for 0.0246 kg/s; which were sufficient for heating the drying air. The drying time of the grapes is also reduced by 43% compared to the open sun drying. 6. Acknowledgments The authors gratefully acknowledge the Directors of Karmaveer Kakasaheb Wagh College of Engineering, Nashik (MS), for the study leave granted to Professor Dr Dilip R. Pangavhane. Financial support of Council of Scientific and Industrial Research, (CSIR) New Delhi, is also acknowledged. References [1] Mani A. Handbook of solar radiation data for India 1980. Madras: Allied publishers, private limited publication, 1981. [2] Eissen W, Muhlbauer W, Kutzbach HD. Solar drying of grapes. Drying Technology 1985;3(1):63–74. [3] Garg HP, Mahajan RB, Sharma VK, Acharya HS. Design and development of a simple solar dehydrator for crop drying. Energy Conversion Management 1984;24(3):229–35. [4] Garg HP, Sharma VK, Mahajan RB, Bhargave AK. Experimental study of an inexpensive solar collector cum storage system for agricultural uses. Solar Energy 1985;35(4):321–31. [5] Sodha MS, Dang A, Bansal PK, Sharma SB. An analytical and experimental study of open sun drying and a cabinet type dryer. Energy Conversion Management 1985;25(3):263–71. [6] Lawrence SA, Pole A, Tiwari GN. Performance of a solar crop dryer under PNG climatic conditions. Energy Conversion Management 1990;30(4):333–42. [7] Pangavhane DR, Sawhney RL, Sarsawadia P. Effect of various dipping pretreatment on drying kinetics of Thompson seedless grapes. Journal of Food Engineering 1999;39:211–6. [8] Sawhney RL, Pangavhane DR, Sarsawadia P. Drying kinetics of single layer Thompson seedless grapes under heated ambient air condition. Drying Technology:An International Journal 1999;17(1&2):215–36. [9] Duffie JA, Beckman WA. Solar engineering of thermal process. New York: Wiley Interscience Publication, 1988. [10] Indrajit, Bansal NK, Garg HP. An experimental study on a finned type and non-porous type solar air heater with a solar simulator. Energy Conversion Management 1985;25(2):135–8. [11] Lawrence SA, Pole A, Tiwari GN. Performance of a solar crop dryer under PNG climatic conditions. Energy Conversion Management 1990;30(4):333–42. [12] Pande PC, Thanvi KP. Design and development of a solar dryer cum water heater. Energy Conversion Management 1991;31(5):419–24.