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EXPERIMENTAL ANALYSIS FOR PERFORMANCE EVALUATION

1. 8 Figure 2 2 Rate of moisture 1088 2 2 4 4 eren eodein ib eere nente 9 Figure 2 3 Drying rate with time 9 Figure 2 4 Typical drying rate curve 10 Figure 2 5 Representation of drying process essen ener 12 Figure 2 6 Structure of a cabinet 14 Figure 2 7 Green house type solar 15 Figure 2 8 Shelf type dryer with separate collector eene 17 Figure 2 9 Cross section of chimney type dryer sss 18 Figure 2 10 Angles describing the direction of a direct solar 19 Figure 2 11 Cross section the solar dryer flat plate 21 Figure 2 12 Absorption of solar radiation by absorber plate under a cover system 22 Figure 2 13 Instantaneous efficiency diagram of a flat plate collector 25 Figure 3 1 Schematic figure of the free convection dryer eese 20 Figure 3 2 Schematic diagram of test enne enne 3 Figure 3 3 Data Logger front e toe tee ine ee e neqne tiges ttes 32 Figure 3 4 A programming group of the Ls2Win 35 Figure 3 5 Connections properties dialog 35 Figure 3 6 Save DL2 control pa
2. m Air a out LS VST VS 5 VST ICT 5 IA m 3 SF P P7 Air in ES Figure 2 6 Structure ofa cabinet dryer The drying material 2 is spread in a thin layer on a tray 3 The bottom plate of the tray is perforated Air flows through the holes by natural convection through the material and finally leaves through the upper part of the cabinet Figure 2 6 The design of the dryer is simple and its cost is low It is suitable for drying small quantities 10 20 kg of granular materials e g for individual farmers The products dried in cabinet dryers are mainly agricultural products vegetables fruits spices and herbs Drying of the material can be made more even by periodic turning over of the material It is employed chiefly in tropical countries but during the warm months it can be used in the temperate climates as well The usual size of the drying area is 1 2 m 13 2 3 1 3 Green House Type Solar Dryer This dryer appears to look like a small greenhouse Figure 2 7 where there are two parallel long drying platforms made of wire mesh and are covered with slanted long glass roof with 14 long axis along the north south direction There is a metallic cap at the top of the glass roof leaving some space in between through which moist warm air can go out creating partial vacuum inside and therefore fresh outside air is sucked through holes provided on the side walls facing east and west below the drying platform
3. 10 41 20 79 9 1904 39 7824 19 44 44 4 0 94686 32 6 28 97 10 51 21 1 8 7488 41 6768 19 21 45 76 0 97282 33 53 29 32 11 01 21 38 8 7168 42 0352 19 18 46 16 0 98661 33 77 30 36 11 11 21 73 8 4416 36 352 19 71 46 56 0 99676 33 8 31 37 11 21 22 12 8 2624 39 5264 19 79 47 12 1 00974 34 58 31 13 11 31 22 47 8 416 35 4304 19 73 46 24 1 02434 34 26 30 78 11 41 2217 7 672 35 328 19 86 47 92 1 02556 34 75 31 17 11 51 23 28 7 7504 36 2496 20 77 47 44 1 03124 35 71 31 75 12 01 23 79 7 5648 37 0688 20 71 47 16 1 0357 35 21 31 5 12 11 24 26 7 3408 29 696 20 88 48 96 1 03286 37 65 33 79 12 21 24 12 7 0592 30 5664 20 91 50 1 02678 37 93 33 4 12 31 24 96 7 5456 28 5184 20 43 47 44 1 0288 37 9 33 85 12 41 24 97 7 2768 28 3648 21 47 48 88 1 01014 36 91 34 12 12 51 25 2 7 3152 27 904 21 29 48 56 1 01095 37 33 36 13 01 25 41 7 264 26 4192 21 43 48 88 1 00487 37 42 34 04 13 11 25 63 7 1424 25 1968 21 31 49 44 0 99392 37 41 34 72 18 21 25 81 6 6688 28 2624 21 95 51 36 0 98377 38 49 34 85 13 31 26 04 7 2704 26 5216 21 76 48 96 0 95781 38 76 35 69 13 41 26 1 7 5072 25 6512 22 03 48 72 0 94158 37 59 35 22 13 51 26 13 7 008 27 9552 22 45 50 32 0 92779 39 13 35 42 14 01 26 29 7 5904 23 904 21 91 49 12 0 90345 39 02 36 46 14 11 26 38 7 7568 23 1424 22 05 48 72 0 85517 38 81 36 99 74 14 21 26 44 7 7056 24 5376 22 36 48 8 0 83611 40 37 36 69 14 31 26 51 8 448 2
4. 35 4 25 A te e 27 20 i 6 15 4 10 1 35 887 344 55 40714 Lr 5 E 0 T T T 1 0 02 0 025 0 03 0 035 0 04 T TJI CC m W Thermal Efficiency Linear Thermal Efficiency Figure 5 3 Collector Instantaneous efficiency The plot is made with the beam radiation nearly normal to the collector so that the transmittance absorptance product for the tests condition is approximately the normal incidence value of the normal incident angle It was seen that the data points were scattered around the linear fit line This is because of the fact that during the tests the wind speed was not constant For the data the straight line that best approximates them is given by the following equation with correlation coefficient 0 29 7 35 887 344 55 T T 5 1 T 56 In equation 5 1 7 is the efficiency The first term on the right is equivalent to the efficiency when there is no heat loss and the second term represents the heat loss coefficient 5 2 Relative Humidity and Capacity of the Air The relative humidity is affected by the air temperature Heating the air decreases the relative humidity and respectively increases the capacity of the air to carry away moisture during a drying process The extent to which this is achieved depends on the weather conditions namely the absolute humidity and the temperature of the ambient air The relative humidity and temperature of t
5. Incident solar Cover system 0 1 0 xo 1 0 2 Absorber Pa plate 2 Figure 2 12 Absorption of solar radiation by absorber plate under a cover system The fraction of energy absorbed after all the reflection is given as TA 1 a p 3 2 5 TA T 2d Pal done where p refers to the reflectance of the cover system for diffuse radiation incident on the bottom side and the absorptance of the absorber plate 2 5 Drying Efficiencies The efficiency of solar drying can be studied under two contexts Collection efficiency Ne and the system efficiency ns 20 Collection efficiency measures how effectively the incident energy on the solar collector is transferred to the air flowing through the collector and is given as the ratio of the useful energy output over a specified time period to the total radiation energy Ir available during the same period The thermal performance of the solar collector is determined by obtaining values of instantaneous efficiency using the measured values of incident radiation ambient temperature and inlet air temperature This requires continuous measurement of incident solar radiation on the solar collector as well as the rate of energy addition to the air as it passes through the collector all under steady state or quasi steady state conditions 13 22 Q 2 6 where A is the collector surface area Eq 2 6 can be inte
6. A Air temperature after Tray 9 Air temperature after Tray2 Figure 5 8 Temperature variations with respect to the vertical distance from the drying chamber bottom 62 5 4 Metrological Data during the Test The weather through out the experiments was clear and hot at the daytime On November 2 2004 the maximum temperature reached 22 65 C at 2 41pm and radiation reached 1 0357 kW m around the noon The sunrise was at 7 00 am and sunset was at 5 30 PM This is shown in Figure 5 9 30 ed 5 25 1 E 3 S 20 0 8 E 2 5 154 06 E E 10 4 04 m 3 3 E 5 0 T T T 0 7 8 9 10 11 12 13 14 15 16 17 Time of the day hour Ambient air Temp Solar Radiation 63 Figure 5 9 Weather data for the test period measured total solar radiation and ambient temperature obtained from the pyranometer and temperature sensor 64 Chapter 6 Conclusions and Recommendations for Future Work 6 1 Conclusions 1 Solar energy was utilized to dry potato slices in the shelf or batch type solar dryer with 2m flat plate collector It produced temperatures of 14 29 from 8 30AM 4 00 PM higher than the ambient air temperature in a clear day and dried 6 3kg m of tray area 7mm x 10mm x 49 7mm slices on a sunny day from 8 30 am to 10 am of the next day The final moisture content of the potato of 2 62 2 93 which was observed after 13 sunny hours The collector performance is e
7. Logger Panel shows the general information about the state of the logger 3 3 1 5 Logger Panel It inspects the logger status and start logging The Program Name is the name of the logging program currently stored in the DL2 The program states can be e Standing by not logging not logged data e Armed Awaiting a start trigger but NOT awaiting first TIMED data e Logging actively recording data or awaiting first TIMED data e Stopped not logging logged data exists 3 3 1 6 Sensors Panel 37 On clicking the sensors tab it will show Figure 3 8 The sensor panel is used to test whether each connected sensor is working properly or not Since the reading may be a wrong value therefore each reading should be checked against the expected value This is useful for inspecting the sensors before logging started Click select All and either Enable Read continuously or Read Now to see the sensor reading the display below shows the sensors being read continuously 38 DL2 Control Panel Thes Figure 3 8 Sensors panel showing real time readings from temperature sensors 3 3 1 7 Dataset Panel It is used to retrieve the dataset The dataset panel displays the information about readings stored in the logger Clicking Datasets and then Refresh one obtains DL2 Control Panel dryerdata 39 Figure 3 9 Dataset control panel displaying the information about the readings stored in the DL2 Inspect the dialog most
8. 28 25974 13 50 0 19 0 018848 0 030039 22 45 50 32 0 927789 0 52792 28 45045 14 00 0 2 0 01984 0 030118 21 91 49 12 0 903448 0 542546 30 02638 14 10 0 18 0 017856 0 031187 22 05 48 72 0 855172 0 478601 27 9827 14 20 0 19 0 018848 0 031623 22 36 48 8 0 836105 0 500833 29 95034 14 30 0 19 0 018848 0 029194 22 15 45 6 0 803245 0 444196 27 65005 14 40 0 18 0 017856 0 031152 22 65 45 84 0 744422 0 416151 27 95129 14 50 0 18 0 017856 0 029605 21 81 43 44 0 730629 0 388156 26 56317 15 00 0 17 0 016864 0 031811 22 49 44 48 0 691278 0 372694 26 95685 15 10 0 16 0 015872 0 029781 21 93 41 2 0 647059 0 307383 23 7523 15 20 0 14 0 013888 0 035263 21 81 40 55 0 53144 0 261562 24 60883 15 30 0 16 0 015872 0 029332 22 08 38 87 0 572414 0 267823 23 39421 15 40 0 15 0 01488 0 028798 22 02 37 78 0 547262 0 235681 21 53278 15 50 0 15 0 01488 0 030969 22 04 36 89 0 479513 0 222073 23 15607 16 00 0 13 0 012896 0 03645 21 32 34 17 0 352535 0 166542 23 62062 16 10 0 13 0 012896 0 02893 21 23 31 37 0 350507 0 131419 18 74702 16 20 0 12 0 011904 0 030615 21 1 30 39 0 303448 0 111141 18 31302 16 30 0 12 0 011904 0 0332 20 73 29 04 0 250304 0 099417 19 8592 69 Continued 16 40 0 11 0 010912 0 034695 20 57 27 4 0 196856 0 074902 19 02447 16 50 0 07 0 006944 0 036897 20 29 26 2 0 160446 0 041314 12 87472 17 00 0 06 0 005952 0 031799 20 07 25 33 0 165416 0 031464 9 510595 17 10 0 05 0 00496 0 041982 20 13 24 49 0 103854 0 0
9. 3117 10 30 0 2 0 01984 0 025274 18 83 42 32 0 929412 0 468372 25 19722 10 40 0 19 0 018848 0 026361 19 44 44 4 0 946856 0 472798 24 96675 10 50 0 19 0 018848 0 027292 19 21 45 76 0 972819 0 502916 25 8484 11 00 0 18 0 017856 0 027346 19 18 46 16 0 986613 0 484164 24 53667 11 10 0 19 0 018848 0 026937 19 71 46 56 0 996755 0 508599 25 51276 11 20 0 2 0 01984 0 027066 19 79 47 12 1 009736 0 544938 26 98419 11 30 0 22 0 021824 0 02588 19 73 46 24 1 024341 0 581447 28 38152 11 40 0 21 0 020832 0 027361 19 86 47 92 1 025558 0 587469 28 64142 11 50 0 23 0 022816 0 025862 20 77 47 44 1 031237 0 611545 29 65104 68 Continued 12 00 0 23 0 022816 0 026118 20 71 47 76 1 0357 0 620259 29 94394 12 10 0 22 0 021824 0 027187 20 88 48 96 1 03286 0 615882 29 8144 12 20 0 21 0 020832 0 028331 20 91 50 1 026775 0 609033 29 65757 12 30 0 23 0 022816 0 026254 20 43 47 44 1 028803 0 619341 30 10009 12 40 0 23 0 022816 0 027135 21 47 48 88 1 010142 0 628513 31 11016 12 50 0 22 0 021824 0 026975 21 29 48 56 1 010953 0 598116 29 58179 13 00 0 21 0 020832 0 027317 21 43 48 88 1 004868 0 574698 28 59567 13 10 0 21 0 020832 0 028302 21 31 49 44 0 993915 0 588934 29 627 13 20 0 2 0 01984 0 029895 21 95 51 36 0 983773 0 586412 29 80423 13 30 0 21 0 020832 0 028398 21 76 48 96 0 957809 0 569464 29 7274 13 40 0 2 0 01984 0 028346 22 03 48 72 0 941582 0 532177
10. 500800 37 020000 17 770000 33 510000 0 838134 28 010000 02 Nov 10 01 56 19510000 11 251200 18 110000 33 410000 0 866531 28 230000 02 10 11 56 19 820000 9 715200 18 820000 42 240000 0 890061 32 010000 02 Nov 10 21 56 20 140000 10 886400 18 300000 40 200000 0 912779 30 680000 02 Nov 10 31 56 20 420000 9 939200 33 540000 18 830000 42 320000 0 929412 30 860000 02 Nov 10 41 56 20 730000 9 190400 41 440000 19 440000 44 400000 0 946856 32 500000 O2Nov10 51 56 21 100000 8 748800 42 560000 19 210000 45 760000 0 972819 33 530000 02 Nov 11 01 56 21 380000 8 716800 42 360000 13 180000 46 160000 0 986613 33 770000 _ OAM AAR 7 0nnn 441000 40200000 AN 7310n0nn 4 nnnoscec T cIojn ojojojljjojjjojjoo 411 Figure 3 11 Dataset retrieved from the logger to file The dataset viewer is also available as an icon on the desk top The Dataset Viewer offers the following commands 42 Open Command file menu opens and displays a DL2e dataset file Save As command file menu saves the dataset which is currently open in the dataset viewer as a data format file a comma to separated ASCII format which is compatible with most data processing applications Date Format Command View menu allows you to select the Day Month order for interpreting ambiguous data file timestamps 3 3 2 AMB Moisture Balance AMB 50 AMB 110 and AMB 310 The AMB moisture balance is a laboratory instrument of the type AMB 310 I
11. can be classified basically into two types natural convection type dryers and forced circulation type dryers 14 2 3 1 Natural Convection Solar Dryers These dryers appear to be more attractive for use in developing countries since they do not use any fan or blower to be operated by electrical energy Moreover they are low in cost and easy to operate However the problems with these dryers are slow drying not much control on temperature and humidity small quantities can be dried and some products due to direct exposure to sun change colour and flavour In its simple form it consists of some kind of enclosure and a transparent cover The food product gets heated due to direct absorption of heat or due to high temperature in the enclosure and therefore moisture from the product evaporates and goes out by natural circulation of air 2 3 1 1 Rack Type Solar Dryer The dryer consists of racks of certain width length and some spacing made of wire mesh over which the drying material is placed and covered at the top by a metal or wooden roof to protect the material from rain and excessive sun 2 3 1 2 Solar Cabinet Dryer or Box Dryer The simplest solar dryers are the cabinet dryers Figure 2 6 Their main characteristic is that the heat needed for drying gets into the material through direct radiation and through a 13 south oriented transparent glass or foil wall 1 Other walls of the dryer are opaque and well insulated
12. can be taken as a future work e The improvement of air distribution in the drying chamber can be studied for the performance improvement of the dryer 66 1 2 3 4 5 6 8 References Ambrose Osakwe and Herbert Weingartmann Performance of an Indirect Forced Convection Solar Dryer with Porous Air Heater Dept of Agricultural Engineering Universitat fiir Bodenkultur A 1190 Vienna Carl W Hall P E Drying and Storage of Agricultural Crops AVI Publishing Company Inc Westport Delta T Device Ltd User Manual Dome Solarimeter and Albedometer 128 Low Road Burwell Cambridge CB5 OEJ U K 1996 Delta T Device Ltd User Manual for DL2e Data Logger Hardware Reference version 3 128 Low Road Burwell Cambridge CB5 OEJ U K 1992 Delta T Device Ltd User Manual for DL2e Data Logger Getting Started version 5 0 128 Low Road Burwell Cambridge CB5 OEJ U K 1996 Delta T Device Ltd User Manual Relative Humidity and Air temperature sensors 128 Low Road Burwell Cambridge CB5 OEJ U K 1996 Delta T Device Ltd User Manual Temperature Probes 128 Low Road Burwell Cambridge CB5 OEJ U K 1996 Design and Testing of a New Solar Tray Dryer http users auth gr karapant tdk Publications files Vlachos et al 2002 pdf Duffie and Beckman Solar Engineering of Thermal Processes 2 edition John Wiley amp Sons INC New York 1991 10 Field Performance of a Sol
13. dry basis equation 4 5 was applied Aw w At 4 5 kg kg min Wa t 1 where 1 and t are successive times corresponding to when two successive measurements of a drying material is made and Md 96 Another equation can be used for the determination of the drying rate dry basis Md Md RDd 100 At 4 6 Md Md H 100 t The final mass is determined as follows Final mass Initial mass x 1 initial moisture content wet basis w b 4 3 Procedure of the Test The first step was weighing of empty trays and 2 45kg 2 61 and 2 81kg sliced potato equal 6 21 kg m on each tray were uniformly loaded and spread over T2 and respectively to form a layer Then T1 and T2 were placed in the drying chamber and T3 control sample was left to the open sun and dried under natural conditions in the sun The next step was to check whether the data logger functions properly or not Wake the logger and see the status report like power supply level installed program and the functionality of each channel The power supply level of the data logger should be greater 51 than 7 This could be done by adjusting the power output lobe of the adapter to the required value If the status report is the required one press wake and then start to begin logging The mass of the trays with the potato were recorded every 30 minutes The potat
14. flow rate of the working medium and on the weather e g wind conditions In the value of U the temperature dependence of the heat transfer from the covering is strong One can interpret the value of U as cumulative effects of three coefficients heat transfer from top covering U from the bottom plate Uy and from the edges Ue U U U U 2 19 where the top heat transfer coefficient from the absorber plate to the ambient Ui by convection and radiation empirical equation is given by Klein 1975 24 1 U 0 33 C E T QNS T aK 2 2 T N f W m K 0 OT T T T e 005Na e 1 2 4 1 yy where N number of glass covers 1 N lt 3 f 1 0 04h 0 0005A 14 0 091N 2 21 27 C 365 9 1 0 008838 0 0001298 8 0 lt B lt 90 2 22 collector tilt degrees emittance of glass emittance of plate 0 1 lt lt 0 95 T ambient temperature 260 X T lt 310K T mean plate temperature 320 T lt 420K h wind heat transfer coefficient W m 0 lt 10 If the wind velocity is V m s then h 5 7 3 8V 2 23 The energy losses through the back of the collector are caused by conduction through the insulation and convection and radiation to the environment The convection and radiation part may often be assumed to be close to zero and may be neglected The back loss coefficient may be approximated to U 2 24 where kin is
15. in the first bottom second and open sun tray respectively During the second day the moisture content decreased gradually By 10 AM of this day the moisture content dropped to about 2 62 3 05 and 5 6 The final moisture contents at 1PM were 2 62 2 93 and 3 58 on wet basis and which are considered as the equilibrium moisture contents of potato These moisture contents indicate that the first tray reached the equilibrium moisture content at the end of the first day The potato in the open sun tray reached equilibrium moisture content after seven hours of the second drying day This means a reduction of the drying period of 3 to 4 hours was obtained using the solar dryer 58 compared to the traditional sun drying depending on the weather conditions Other advan z 90 tages 3 Tray 1 2 80 70 2 60 4 A Tray3 the M C 3 50 protec E40 oO E 30 4 tion o 20 again 2 10 gt 0 at 0 200 400 600 800 1000 direct Drying time minutes sunsh ine dust and insects Figure 5 5 Moisture content curves for potato in solar dryer and open air sun dryer During the night times the inlet and the exit of the dryer were closed and the control sample was placed in room to prevent the potato from moisture regain The sudden drop on Figure 5 5 shows there was moisture loss during the night This is because during the night there was loss of moisture to the air in the dryer and
16. is exposed to air it will absorb either moisture or desorb moisture depending on the relative humidity of the air The equilibrium moisture content EMC will soon reach when the vapour pressure of water in the material becomes equal to the partial pressure of water in the surrounding air 14 The equilibrium moisture content in drying is therefore important since this is the minimum moisture to which the material can be dried under a given set of drying conditions A series of drying characteristic curves can be plotted The best is if the average moisture content M of the material is plotted versus time as shown in Figure 2 2 Equilibrium moisture level Moisture content M Time t Figure 2 2 Rate of moisture loss Another curve can be plotted between drying rate i e dM dt versus time t as shown in Figure 2 3 But more information can be obtained if a curve is plotted between drying rate dM dt versus moisture content M as shown in Figure 2 4 constant rate drying phase First falling rate dM dt Second falling rate Drying rate Equilibridm moisture level Figure 2 3 0 Drying Time t rate with time curve constant rate Falling rate Critical point zs Non hygroscopic material E E Hygroscopic material Moisture content M Figure 2 4 Typical drying rate curve As is seen from Figure 2 4 for both non hygroscopic and hygroscopic materials there i
17. possible it is traditional to harvest most grain crops during a dry period or season and simple drying methods such as sun drying are adequate However maturity of the crop does not always coincide with a suitably dry period Furthermore the introduction of high yielding varieties irrigation and improved farming practices have led to the need for alternative drying practices to cope with the increased production and grain harvested during the wet season as a result of multi cropping Drying and preservation of agricultural products have been one of the oldest uses of solar energy The traditional method still widely used throughout the world is open sun drying where diverse crops such as fruits vegetables cereals grains tobacco etc are spread on the ground and turned regularly until sufficiently dried so that they can be stored safely However there exist many problems associated with open sun drying It has been seen that open sun drying has the following disadvantages It requires both large amount of space and long drying time The crop is damaged because of the hostile weather conditions contamination of crops from the foreign materials degradation by overheating the crop is subject to insect infestation the crop is susceptible to re absorption of moisture if it is left on the ground during periods of no sun and there is no control on the drying process This could lead to slow drying rate contamination and poor quality o
18. the thermal conductivity of the insulation and Lin is the thickness The evaluation of the edge loss is complicated But fortunately the losses are usually small The losses through edges should be referenced to the collector area If the edge loss coefficient area product is UA ecage then the edge loss coefficient based on the collector area A is UA 2 25 28 Chapter 3 Experimental Setup and Instrumentation 3 1 Dryer Setup The solar assisted indirect dryer discussed here consists of a solar air collector a drying chamber and appliances An outlay of the dryer is given Figure 3 1 Outer side black painted steel sheet of 0 8 mm thick and 4 mm thick glass are used for the construction of the chamber body warm humid Cap FAN Exhaust air D iN Trays Air in Figure 3 1 Schematic figure of the free convection dryer 29 The solar collector is parallelpiped shape with dimension of L 2 m x W zi mx 0 14 m having 80 mm channel depth 40 mm gap between the absorber plate and glass and on the bottom 20 mm thick fiber glass insulation In the present study its longitudinal axis is oriented along the N S direction The collector is inclined at an angle of 12 with the horizontal and the optimum angle for latitude of 9 The absorber plate consists of 1 mm thick steel flat sheet blackened on the sun facing side The cover material of the collector is 4 mm thick commercial glass The lower end fa
19. 1 1258 12 00816 21 82759 26 3274 0 486941 0 734123 0 81693 360 690 956 1164 9 722449 18 18774 22 98221 0 413162 0 65794 0 604657 71 390 657 876 1079 8 37551 15 12261 19 9573 0 243471 0 554055 0 546764 420 618 806 1010 6 783673 12 44061 17 50178 0 287738 0 484798 0 443844 450 601 768 952 6 089796 10 98467 15 43772 0 125424 0 263176 0 373086 480 582 732 899 5 314286 9 605364 13 5516 0 14018 0 249325 0 340924 510 571 710 873 4 865306 8 762452 12 62633 0 081157 0 152365 0 167246 540 562 693 849 4 497959 8 111111 11 77224 0 066401 0 117737 0 154381 570 559 687 841 4 37551 7 881226 11 48754 0 022134 0 041554 0 05146 Continued 600 525 599 810 2 987755 4 509579 10 38434 0 00209 0 005079 0 001662 630 523 591 791 2 906122 4 203065 9 708185 0 014756 0 055405 0 122218 660 519 578 742 2 742857 3 704981 7 964413 0 029512 0 090034 0 315194 690 518 567 708 2 702041 3 283525 6 754448 0 007378 0 076183 0 218706 720 517 562 687 2 661224 3 091954 6 007117 0 007378 0 034628 0 135083 750 516 561 669 2 620408 3 05364 5 366548 0 007378 0 006926 0 115785 780 516 560 651 2 620408 3 015326 4 725979 0 0 006926 0 115785 810 516 558 643 2 620408 2 938697 4 441281 0 0 013851 0 05146 840 516 558 628 2 620408 2 938697 3 907473 0 0 0 096488 870 516 558 622 2 620408 2 938697 3 69395 0 0 0 038595 900 516 558 619 2 620408 2 938697 3 587189 0 0 0 019298 F
20. 1 37 9 38 11 12 21 24 77 7 9424 14 726 20 94 25 18 1 0596 38 26 38 39 12 31 25 12 7 4304 14 112 21 49 25 29 1 0807 40 04 39 93 Continued 12 41 25 44 7 3472 13 286 21 59 25 7 1 0787 40 43 40 64 12 51 25 78 7 6864 13 171 21 45 26 03 1 0434 39 95 40 53 13 01 25 92 8 3072 13 165 21 34 25 86 1 056 39 37 40 24 77 78 Appendix B Dimension of the sliced potato and the trays used in the drying chamber Figure B 1 Sample of sliced potato Figure B 2 Dimension of trays used in the drying chamber T1 92 5 cm x 42 cm T2 96 cm x 44 cm T3 102 cm x 44 cm
21. 2 7072 22 15 45 6 0 80325 39 92 36 41 Continued 14 41 26 5 8 3328 19 9168 22 65 45 84 0 74442 40 53 36 58 14 51 26 4 9 0688 20 1728 21 81 43 44 0 73063 39 44 36 35 15 01 26 19 8 9472 22 5408 22 49 44 48 0 69128 37 67 37 31 15 11 26 18 10 4128 20 0896 21 93 41 2 0 64706 36 35 37 49 15 21 26 03 11 04 21 1776 21 81 40 55 0 53144 35 72 40 75 15 31 25 86 12 6144 19 3152 22 08 38 87 0 57241 34 79 39 39 15 41 25 73 13 8624 16 7232 22 02 37 78 0 54726 35 73 39 95 15 51 25 65 15 1488 17 344 22 04 36 89 0 47951 35 13 39 9 16 01 25 42 17 5296 17 984 21 32 34 17 0 35254 34 16 37 58 16 11 25 19 9488 19 4944 21 23 31 37 0 35051 31 44 35 72 16 21 24 68 21 4848 20 96 21 1 30 39 0 30345 30 49 33 87 16 31 24 34 23 296 22 784 20 73 29 04 0 2503 29 35 29 91 16 41 23 99 25 9456 26 08 20 57 27 4 0 19686 27 92 27 98 16 51 23 65 27 4944 29 1328 20 29 26 21 0 16045 26 75 26 81 17 01 23 27 28 8768 30 8224 20 07 25 33 0 16542 26 82 26 56 17 11 22 92 31 232 30 5664 20 13 24 49 0 10385 28 22 27 91 17 21 22 62 33 2288 30 976 19 95 23 95 0 06572 27 02 27 28 17 31 22 31 35 2256 34 304 19 58 22 82 0 04772 23 31 23 7 17 41 21 96 37 4272 38 0416 19 21 21 86 0 03357 21 92 21 74 17 51 21 58 39 168 42 1376 18 85 20 97 0 01623 21 01 20 39 18 01 21 21 40 3456 45 5168 18 38 20 23 0 00472 19 78 19 25 75 8 01 14 91 30 976 28 3136 15 75 24 96 0 37404 25 24 27 32 8 11 15 29 2
22. 21734 10 4636 17 20 0 04 0 003968 0 060864 19 95 23 95 0 06572 0 015951 12 13583 17 30 0 03 0 002976 0 067899 19 58 22 82 0 047718 0 00969 10 15386 70 Table A 2 Percentage Moisture content on wet basis and percentage drying rate on dry basis on 1 Tray2 and Tray3 Moisture Moisture Moisture Drying rate Drying rate Drying rate Drying Mass of Massof Massof contenton contenton contenton on dry of on dry on dry Time potato on potato on potato on wet basis wet basis wet basis basis of T1 basis of T2 basis of T3 minutes T1 gm T2 gm T3 gm T1 96 T2 96 of 3 96 96 96 0 2450 2610 2810 81 55918 81 55939 81 55872 30 2212 2484 2679 74 29388 76 7318 76 8968 1 313265 0 872637 0 84266 60 2081 2349 2554 66 49796 71 55939 72 4484 1 409178 0 934968 0 804065 90 1906 2213 2425 59 3551 66 42529 67 85765 1 291132 0 928042 0 829795 120 1715 2042 2262 51 55918 59 79693 62 05694 1 409178 1 198144 1 048501 150 1522 1884 2112 43 68163 53 7433 56 71886 1 423934 1 094259 0 964878 180 1345 1731 1966 36 45714 47 88123 51 52313 1 305888 1 05963 0 939148 210 1175 1573 1803 29 51837 41 82759 45 72242 1 254242 1 094259 1 048501 240 1013 1422 1649 22 90612 36 04215 40 24199 1 195219 1 045779 0 990609 270 897 1273 1498 18 17143 30 33333 34 86833 0 855836 1 031927 0 971311 300 812 1157 1385 14 70204 25 88889 30 84698 0 627121 0 80338 0 726875 330 746 105
23. 4 2 2 365 where n is the day of the year The following equation relates the angle of incidence to the other angles cos sin cos f sin cos sin cos y cos 5 cos cos D cos 0 6 sin 0 sin J cos y cos 2 3 cos 6 sin D sin y sin The cross section of the collector used in this research work is shown in Figure 2 11 The flow of air is under the absorber to reduce the convective heat loss of the air from the covering The channel depth is 80 mm Glass Absorber plate Bottom plate DDK DA DH 2992406240624000 0 55255560 Insulation Figure 2 11 Cross section the solar dryer flat plate collector It is convenient to define an average transmittance absorbtance as the ratio of the absorbed solar radiation S to the incident solar radiation Ir Thus S 1 2 4 This is especially convenient when direct measurements are available for Ir 2 4 2 Transmittance and Absorbtance When radiation is incident on the cover of a collector most of the radiation is transmitted through the cover and some reflected back The transmitted radiation easier absorbed by the absorber or reflected back to the cover Figure 2 12 demonstrates the situation where 7 15 the transmittance of the cover system at the desired angle and is the angular absorbtance of the plate The product should be thought as symbol representing a property of the cover absorber combination 9
24. 8 5696 27 136 15 72 26 15 0 42637 26 46 28 88 8 21 15 69 25 6704 26 2656 16 48 28 43 0 47708 27 72 36 14 Continued 8 31 16 13 22 4896 20 1984 16 51 30 38 0 53306 28 4 38 91 8 41 16 55 21 0112 17 12 16 63 31 62 0 5712 29 38 96 8 51 16 99 18 0032 14 6752 17 38 32 61 0 64219 30 09 36 65 9 01 17 5 15 7568 13 6768 17 85 34 61 0 68722 30 84 34 05 9 11 18 01 14 6496 14 816 18 24 36 41 0 7213 33 11 34 32 9 21 18 47 14 0736 16 1344 17 99 36 67 0 75619 33 91 33 89 9 31 18 9 13 632 16 5824 18 02 37 11 0 82962 33 66 33 84 9 41 19 28 11 7248 16 9984 18 79 39 66 0 83164 36 04 34 43 9 51 19 7 11 5072 17 0624 18 66 40 88 0 83489 39 17 35 4 10 01 20 03 11 9616 17 0688 18 44 38 81 0 87221 38 07 34 88 10 11 20 33 12 102 17 734 18 9 31 14 0 897 37 89 34 68 10 21 20 64 11 322 17 965 19 09 20 97 0 9262 36 02 34 7 10 31 20 9 10 746 17 613 19 06 21 14 0 9554 35 45 35 06 10 41 21 19 9 216 17 261 19 65 21 62 0 9663 36 45 36 08 10 51 21 55 8 9536 17 005 19 62 21 89 0 9728 37 15 37 16 11 01 21 87 9 0432 16 666 19 74 22 21 1 0012 37 17 37 2 11 11 22 16 8 5184 16 243 19 64 22 51 1 0178 37 21 37 28 11 21 22 48 8 0128 15 789 19 85 22 86 1 0292 38 09 38 08 11 31 22 91 8 128 15 053 20 28 23 4 1 0231 37 93 37 99 11 41 23 34 8 064 15 053 20 41 23 98 1 0511 38 11 38 08 11 51 23 78 8 4352 14 835 20 71 24 33 1 0377 37 97 38 09 12 01 24 2 8 0576 14 592 20 57 24 57 1 0531 38 06 38 07 76 12 11 24 52 8 4992 14 893 20 57 24 77 0 976
25. ADDIS ABABA UNIVERSITY SCHOOL OF GRADUATE STUDIES DEPARTMENT OF MECHANICAL ENGINEERING EXPERIMENTAL ANALYSIS FOR PERFORMANCE EVALUATION OF SOLAR DRYER By Aklilu Tesfamichael Approved by Board Examiners Dr Ing Demiss Alemu Chairman Department Graduate Committee Dr Ing Abebayehu Assefa Advisor Dr Rajendra Karwa External Examiner Dr Ing Demiss Alemu Internal Examiner Acknowledgments The completion of this thesis would not have been possible without the help of several people First I would like to thank my God for guiding me throughout the journey of my life My gratitude love and respect go out to all my caring and understanding family I also thank Dr Ing Abebayhu Assefa my advisor for this project and who has taught me many courses His advice and guidance for my research and contribution to my education has been invaluable I thank Dr Ing Demiss Alemu for the inspiration and encouragement to work on this project My thanks also go out to Dr Rajandar Karwa for his advice and availing articles I also thank the entire working group in mechanical workshop especially Ato Kassaye Negash and Ato Daniel Kefli who supplied me with weighing device Last but not least the equipment facilities presented by the Solar Energy Research and Development Group of Faculty of Technology AAU is acknowledged Table of Contents Acknowledgments oni NN i ee aoe a eG co vi List
26. SE INS Rein 48 Test Procedure and Computations 48 4 1 SAMPLE PREPARATION48 4 1 1 Characteristics of Potato Used in the 48 4 2 MOISTURE DETERMINATION 49 4 3 PROCEDURE OF THE TEST 51 4 4 EFFICIENCY ANALYSIS 52 Ch pter S un Ee O 54 Results and Discussion 54 5 1 COLLECTOR PERFORMANCE 54 5 1 1 Collector Efficiency esses eene 54 5 3 DRYING TESTS 58 5 4 METROLOGICAL DATA DURING THE TEST 63 Chapter Re E LO ot a ste E LEN DUCUNT 65 Conclusion and Recommendation for Future 65 6 1 CONCLUSION 65 6 2 RECOMMENDATION FOR FUTURE WORK 66 References S nme iuvenibus tbt eii aoc tos 67 Appendix AN REI Re CE UI RES 68 TABLE A 1 RAW DATA OF THE EFFICIENCY ANALYSIS 68 111 TABLE A 2 PERCENTAGE MOISTURE CONTENT ON WET BASIS AND PERCENTAGE DRYING RATE ON DRY BASIS ON TRAY1 TRAY2 AND TRAY3 71 TABLES A 3 RELATIVE HUMIDITY OF THE DRYING AIR AT THE EXIT OF THE COLLECTOR AND TRAY2 AND TEMPERATURE OF DRYING AIR AT THE EXIT OF THE COLLECTOR TRAY1 AND TRAY2 73 Appendix Pa Re 1 DIMENSION OF THE SLICED POTATO AND THE TRAYS USED IN THE DRYING CHAMBER 1 iv List of Table Table 4 1 Characteristics of fresh potato ready for drying purpose List of Figures Figure 2 1 Moisture in the drying
27. What type of units the raw readings are converted to e When or how logging should start e What happens when memory is full e How long a sensor shall receives power before taking a reading 3 3 1 9 The DL2 Program Editor Select the logger panel in the DL2 control panel and click retrieve The program will appear laid out as a table in a row of tab sheets in the DL2 program Editor 2 DL2 Program Editor dryer prog pg2 lol x m File Edi View Window Help m 8 x Program Name fs Dryer Password ch 145 cn 1630 ch 31 45 ch 46 60 ch 61 62 ch 6364 Sensor Library Input Card Type 15 channel y Ch Label Sensor Code and Type Electrical ctions n 10m Avg of Resistance cold jn TM1 Thermistor 2K type Fenwal UUA32J2 10s samples 20 exc BHin RH2 Relative Humidity Sensor types 10m Avg of DC Voltage RHT 2nl 02 RHT 2 02 humidity output 10s samples fix range Chan03 1 Thermistor 2K type Fenwal UUA32J2 M pe ad pales Amb temp 1 Thermistor 2K type Fenwal UUA32J2 eee angit 10m Avg of ThermocoL Ae Temp TCK Thermocouple Chromel Alumel type K 10s samples CM3 Copy of Dome Solarimeter types G51 10m Avg of DC Voltage Ittadia G 52 10s samples auto range y Ready 2 Figure 3 10 A copy of the dryer logging program in the DL2 is retrieved and displayed in the program Editor 41 3 3 1 10 Retrieving Logged Data to PC The Ls2Win progra
28. a particle to its surface are the temperature of the particle its moisture content the physical dimensions the internal structure and the composition of the material The change in moisture content with time is proportional to the change in moisture gradient across the particle from the interior to the surface In other words the rate of drying decreases with decrease in moisture content but increases with decrease in particle size Therefore the potato was washed repeatedly until it was clean and then sliced using a manual slicing machine to increase the rate of drying 4 1 1 Characteristics of Potato Used in the Experiment For determination of the average characteristics of the potato 10 sliced potatoes were taken 48 Table 4 1 Characteristics of fresh potato ready for drying purpose Characteristics Average Value of potato tested Shape Rectangular Potato weight per tray kg 2 45 T 2 2 61 T 2 2 81 Potato weight per unit area kg m 6 21 Potato slice length mm 49 8 Potato slice height mm 10 Potato slice width mm 7 Surface area cm 18 3 Average weight of each sliced potato g 3 3 Average volume of each sliced potato cm 3 48 Average density g cm 0 947 Moisture content potato slice wet basis 9 81 56 Moisture content potato slice dry basis 96 442 3 Moisture content of dried material of the potato slice 2 6 3 6 wet basis 9 Moisture content of dried material of the po
29. aa impor Edita Contra Panal viewer Me ized Figure 3 4 A programming group of the Ls2Win software 3 3 1 4 Create a New DL2 Control Panel Double click the New DL2 control panel desktop icon or select New DL2 Control panel from the start programs Ls2Win menu this will open the DL2 Control Panel DL2 Connection Properties COM1 3600 Connections box Figure 3 5 properties dialog On clicking OK a dialog box appears to save the connection properties as a DL2 control panel file A Save DL2 control panel dialog box is shown below Write the file name in this case dryer and click OK to accept the file name 35 DL2 Control Panel files 412 y Le Figure 3 6 Save DL2 control panel dialog box DL2 control panel now creates a short cut icon to it self on the disk top and retrieves and displays states of information from the logger You have now established communication with the logger If you select the incorrect connection setting select properties from the file menu change the setting in the DL2 connection properties dialog and click refresh to refresh DL2 control panel DL2 Control Panel dryerdata Figure 3 7 Logger Panel on the DL2 Control panel This figure shows the dialog box of the control panel after communicating with the logger There are four panels of information in the DL2 control panel you switch between them by clicking the Logger Sensors Datasets and Error buttons Figure 3 7
30. ame a common practice after World War II The increase in drying was coupled to the rapid increase in mechanization and increase in land labor productivity Large quantities of moist or wet products were produced at harvest requiring moisture removal to avoid loss during subsequent handling and storing Speed of operations from harvest to storage forced the consideration study and use of heated air for drying 2 Literature survey was outlined by S Soponronnarit 21 as follows Wibulswas et al 1977 found that the drying rate of wet cloth in a convection solar cabinet dryer was about 4 2 kg m day Watabutr 1981 found that the maximum drying efficiency occurred when the ratio of outlet area to solar receiving area was 11 per cent the inlet area was much greater than the outlet area and the slope of the glass cover was 14 yielding a drying rate of about 3 2 kg m day using box dryer Drying of banana in a solar cabinet took three days and better quality product was obtained as compared with that in the case of direct sun drying Anon 1979 Wibulswas and Thaina 1980 tested a mixed mode natural convection solar dryer and found that the drying rate of wet cloth was 5 kg m day The maximum drying efficiency occurred when the ratio of outlet area to absorbing area was 0 8 per cent Patranon 1984 conducted in field solar drying using dryers similar to that of Wibulswas and Thaina 1980 The products dried were banana fish mea
31. ample 6 Halogen Heater 2 200 w 7 Protection of Temperature Regulation 8 Power Supply switch 9 Cover 10 Sample Chamber 11 Adjustable feet Figure 3 12 AMB moisture balance description 3 3 3 Dome Solarimeter Pyranometer The Global radiation is monitored by Delta T Device Ltd of type GS1 dome solarimeter The instrument gives the instantaneous solar irradiance in kW m The pyranometer model is CM3 Sensitivity temperature deviation 10 to 40 C 6 relative to 20 C and its sensitivity 10 35 W m The connection of the pyranometer with the data logger is made according to the setting provided by the data logger supplier 3 44 3 3 4 Anemometer The anemometer is a versatile wind speed indicator which can be used wherever an accurate visual reading of wind speed is required The anemometer used is of type digital anemometer from Delta T Device Ltd It measures flow velocities in the interval 0 to250m s The display accuracy 15 1 degree and sensor accuracy is 0 5 degree 3 3 5 Ambient Air Temperature Sensor The temperature sensor used is of type AT2 052 a thermistr from Delta T Device Ltd It measures ambient temperature in the range 50 150 It has an accuracy of 0 1 C 3 3 6 Air Temperature Sensor The common features of AT2 air temperature sensor is the solar radiation shield that protects the sensors from solar radiatio
32. ar Tunnel Drier http wireO ises org wire doclibs KoreaConf nsf id DA314E2D0606C68BC12565A0 004ED110 SFile 1 817 pdf 67 11 12 13 14 15 16 17 18 19 20 21 22 Kreith Frank Principle of Solar Engineering Mc Graw Hill Book Company Washington 1978 Grain Storage Techniques http www fao org docrep T1838E T1838E00 htm Laszlo Imre Solar Drying in Handbook of Drying Garg H P Advances in Solar Energy Technology D Reidel Publishing Company Volume III Holland 1987 Yuncu H and Paykoc E Solar Energy Utilization Martinus Nijhoff Publishers Dordrecht Netherland 1987 Indirect Through pass Solar Food Dryer http www homepower com files fooddeh pdf Cicala L and Farina G Performance Analysis of Solar Air Heaters of Conventional Design Journal of Solar Energy Volume 41 No 1 PP 101 107 1998 Biondi P et al Performance Analysis of Solar Air Heaters of Conventional Design Journal of Solar Energy Vol 41 No 1 pp 101 107 1988 Proceeding of Energy Conference 2002 Energy in Ethiopia Status Challenges and Prospects UNCC Addis Ababa 21 22 March 2002 Solar Air Heating http www courses ait ac th ED06 22 course 1 lecs module3 m32098 html Solar Drying in Thailand http www ieiglobal org ESDVol2No2 dryingthailand pdf Janjai S Investigation of the Performance of a Solar Dryer for Lemon grass Kingmongkut s University of Technology Thon
33. buri International Symposium 68 23 Nejat T Veziroglu Alternative Energy Sources Hemispheric Publishing Company Volume I New York 1989 24 Klein S A Calculation of Flat Plate Collector Loss Coefficients Journal of Solar Energy Vol 17 pp 79 80 1975 69 Appendix A Raw data of the experiment is presented in the following tables Table A 1 Raw data of the efficiency analysis Air flow Air mass Collector Global Solar Useful Collector velocity flow rate To Ta Ir Ambient Exit temp radiation energy efficiency Hour m s kg s Cm W Temp T C TCC L kW m kW 8 30 0 12 0 011904 0 024404 16 68 30 58 0 569574 0 166293 14 59801 8 40 0 13 0 012896 0 025804 17 22 32 65 0 597972 0 19998 16 72155 8 50 0 13 0 012896 0 026921 17 64 34 83 0 63854 0 222791 17 44533 9 00 0 14 0 013888 0 026657 17 23 35 03 0 667748 0 248442 18 60299 9 10 0 14 0 013888 0 025196 17 17 34 7 0 69574 0 244674 17 58371 9 20 0 15 0 01488 0 026785 18 34 38 04 0 735497 0 294602 20 02739 9 30 0 17 0 016864 0 026154 17 83 38 53 0 791481 0 35083 22 1629 9 40 0 16 0 015872 0 025631 18 35 39 26 0 815822 0 333543 20 44215 9 50 0 18 0 017856 0 025939 17 77 39 51 0 838134 0 39013 2327315 10 00 0 21 0 020832 0 024581 18 11 39 41 0 866531 0 44594 25 73133 10 10 0 18 0 017856 0 026313 18 82 42 24 0 890061 0 420278 23 60954 10 20 0 22 0 021824 0 023993 18 3 40 2 0 912779 0 480335 26
34. ce of the collector 1 m x 0 08 m is the air inlet to the dryer whereas its higher face end is connected to the rectangular duct of the chamber The drying chamber is a long rectangular column consisting iron frame with its two sides the right and the front covered with glass the front facing south and the east with a glass door and the rest is covered by sheet metal Fresh outside air enters through the inlet of the air heater gets heated during its passage to the air heater The heated air rises through the drying trays in the drying chamber and leaves the chamber at the top through the exhaust opening Thus the material to be dried gets heated directly by absorbing heat through the glass walls and from heated air coming from solar air heater by natural convection The drying chamber has 1 2 m x 1 04 m x 0 55 m outer dimensions Inside the drying chamber three shelves were prepared but only two trays 1 and T2 were inserted on which the products to be dried are placed Each tray is 19 mm deep with wire mesh in the bottom and its area is Aj 0 39 m and 0 42 m with the two trays in the chamber the total area is 0 81 m The relative positions of these trays are the bottom tray tray T1 is 30 placed 0 18 m above the drying chamber s base hot air s entry point the middle tray tray T2 is located at 0 32 m and the top tray tray T3 at 0 46 m above the chamber s base respectively On the drying chamber holes were dril
35. cy in the function efficiency function an independent variable can be plotted as shown in Figure 2 13 Ata given operating point the utilized energy flow rate from the collector is Q 7 A T These considerations can be appropriately applied according to Eq 2 7 for expressing the long term efficiency by substituting time averages Ir ay Tp av and Ta av 5 Tey n ta U i r a 2 13 24 From Eqs 2 12 and 2 13 the threshold value of incident radiation flux can be determined with which the absorbed energy flow rate and loss heat flow rate equal and thus the efficiency is zero U LT av e sol th 2 14 TA From Ig using the appropriate metrological data the possible operation time of the correlation can be stated n TO n n f Figure 2 13 Instantaneous efficiency diagram of a flat plate collector The instantaneous efficiency of a collector can also be expressed from the known inlet temperature T of the working medium with the aid of the heat removal factor Fg Since the collector works as an open cycle drawing external air a configuration often utilized for air heaters only the inlet temperature coincides with the environmental one Ta Under this working condition the following is used to calculate the efficiency of the collector 17 25 san 2 15 n ra oa T Where Fr is the factor of heat removal referred to the outlet air temperature and can be
36. drying chamber DOOM oido 62 Figure 5 9 Weather data for the test period measured total solar radiation and ambient temperature obtained from the pyranometer and temperature sensor 64 vii Abstract An experimental set up has been developed to investigate the performance of natural convection solar dryer for drying of selected material Measurements of total solar radiation on the plane of the collector ambient temperature and humidity air flow rate temperature and relative humidity inside the dryer as well as solid s moisture loss in weight data are employed to study the performance of the dryer A data logger and a computer were employed for data acquisition First detailed diagnostic experiments were carried out with no drying material on the trays Next a number of experiments were conducted using potato slices For all the test conditions the material gets dried with system s efficiency of 15 9 The drying time compared to sun drying was reducing by about 19 The protection of the dried material against direct sunshine dust and insects results better quality product viii Chapter 1 Introduction 1 1 Overview and Objective of the Thesis Food scientists have found that by reducing the moisture content of food to between 10 and 20 bacteria yeast mold and enzymes are prevented from spoiling it The flavor and most of the nutritional value is preserved and concentrated 16 Wherever
37. e either in thin layer drying or deep layer drying In thin layer drying which is done in case of most of fruits and vegetables the product is spread in thin layers with entire surface exposed to the air moving through the product and the Newton s law of cooling is applicable in the falling rate region Most of the grains are dried in deep layer which can be considered as a series of thin layers and the temperature and the humidity varies from layer to layer 14 2 2 Air Properties The properties of the air flowing around the product are major factors in determining the rate of removal of moisture The capacity of air to remove moisture is principally dependent upon its initial temperature and humidity the greater the temperature and lower the humidity the greater the moisture removal capacity of the air The relationship between temperature humidity and other thermodynamic properties is represented by the psychrometric chart It is important to appreciate the difference between the absolute humidity and relative humidity of air The absolute humidity is the moisture content of the air mass of water per unit mass of air whereas the relative humidity is the ratio expressed as a percentage of the moisture content of the air at a specified temperature to the moisture content of air if it were saturated at that temperature The changes in condition of air when it is heated using the solar energy and then passed through a bed of moist product ar
38. e shown in Figure 2 5 The heating of air from temperature to T is represented by the line AB During heating the absolute humidity remains constant at whereas the relative humidity falls from 0 tog As air moves 11 through the material to be dried it absorbs moisture Under hypothetical adiabatic drying sensible heat in the air is converted to latent heat and the change in the condition of air is represented along a line of constant enthalpy BC Both absolute humidity and relative humidity increase from and and from to c respectively but air temperature decreases to T The absorption of moisture by the air would be the difference between the absolute humidities at C and B 0 0 If unheated air is passed through the bed the drying process would be represented by the line AD Assuming that the air at D to be at the same relative humidity as the heated air at C then the absorbed moisture would be 0 considerably less than that absorbed by the heated air 0 Lines of constant RH gt Lo 3 3 X 2 E 9p 2 Gees xt lo lt Figure i 2 5 Representation NN of drying process E op y ying proces A C B Air temperature 2 3 Types of Solar Dryers 12 There are a large variety of solar dryers These solar dryers have been classified in many ways Considering the operational modes and practicability of dryers they
39. ere are two basic mechanisms involved in the drying process the migration of moisture from the interior of an individual material to the surface and the evaporation of moisture from the surface to the surrounding air The drying of a product is a complex heat and mass transfer process which depends on external variables such as temperature humidity and velocity of the air stream and internal variables which depend on parameters like surface characteristics rough or smooth surface chemical composition sugars starches etc physical structure porosity density etc and size and shape of products The rate of moisture movement from the product inside to the air outside differs from one product to another and depends very much on whether the material is hygroscopic or non hygroscopic Non hygroscopic materials can be dried to zero moisture level while the hygroscopic materials like most of the food products will always have a residual moisture content This moisture in hygroscopic material may be a bound moisture which remained in the material due to closed capillaries or due to surface forces and unbound moisture which remained in the material due to the surface tension of water as shown in Figure 2 1 14 Bound Unbound moisture moisture EH Equilibrum moisture Free moisture Relative humidity Moisture content M Figure 2 1 Moisture in the drying material When the hygroscopic material
40. expressed as 0 2 16 where zx a Further instantaneous efficiency can be expressed directly as a ratio of useful heat flow rate coming into the working medium heat flow rate on the absorber E om cin 2 17 Where mis the mass flow rate of air through collector and is given by m p Q 2 18 p is density of air Q is volume flow rate of air C is specific heat of air In practice 77 versus or 7 versus T u T diagrams are used in place of the 7 versus f efficiency diagram For representation of the thermal behavior of collectors besides those above other practical diagrams such as 77 versus and 77 versus T function curves can also be used In these cases other factors in the 77 equation appear as the parameters of the efficiency curves 26 The simplified calculation method has several weak points One is that the value of T must be known to perform the calculation The temperature of the absorber plate changes in the flow direction of the working medium and T can be interpreted only as a mean temperature The greatest error appears in the application of the overall heat transfer coefficient U and its use as a constant value U models the overall effect of complex and nonlinear heat transfer processes Its value for a given collector depends on the local values of T on the sky temperature T in view of radiation on the mass
41. f dried products and loss in production Although the spreading of the crop on the ground or on a platform and drying it directly by the sun is cheap and successfully employed for many products throughout the world where solar radiation and climatic conditions are favorable because of the above mentioned factors of open sun drying process and a better understanding of the method of utilizing solar energy to advantage have given rise to a scientific method called solar drying Solar drying of farm crops offers the following advantages by permitting early harvest which reduces the field loss of products from storm and natural shattering The field conditions dry and fewer weeds are often better for harvesting earlier in the season planning the harvesting season to make better use of labor Farm crops can be harvested when natural drying conditions are unfavorable Long time storage with little deterioration Extended storage periods are becoming increasingly important with large amount of grain being stored and carried over through another storage year by the farmer government and industry and the farmer s taking advantage of higher price a few months after harvest although in some years there may be no price advantage By removing moisture the possibility of the grain heating with subsequent reduction or destruction of germination is decreased The farmer s selling a better quality product which is worth more to him and to those who must
42. g and roasted rice shell the latter playing the role of absorber The front surface 3 over the layer of the rice is also transparent The wall of the chimney 5 is made of black plastic foil The frame work of the dryer is wood and wire Manufacture of the unit is inexpensive and simple The air needed for drying amounts to 5 7 m min per rice The chimney is 5 m high Drying is not uniform so the rice in the static bed must be turned over at intervals The duration of the drying is 3 4 days in the case of 15MJ m day mean global sun radiation and 23 collector surface With the application of a large 36 m collector surface drying time can be reduced to 1 2 days in good weather As a rule of thumb the solar collector surface must be approximately three times the surface of the bed 13 17 Warm humid air out Warm humid air Fresh air inlet Figure 2 9 Cross section of chimney type paddy dryer 2 4 Key Elements of Solar Dryer The indirect free convection solar dryer used in this research work has the following major components The solar collector where the ambient air is preheated the drying chamber where the material to be dried comes in direct contact with the hot air from the collector and reduces its moisture and connecting ducts 2 4 1 Solar Collector The solar collector plays the part of primary energy source for a solar dryer Essentially it has functions of energy conversion and energy transfer The p
43. he ambient air are included for comparison Relative humidity kg vapor kg dry air Ambient temperature C 8 9 10 11 12 13 14 15 16 17 Time of the day hour RH ofar at the inlet of the chamber s RH of the air above the second tray x RH humidity of ambient air Ambient air temp 57 Figure 5 4 Time variation of the relative humidity in the dryer at the exit of the collector and just above the second tray The average relative humidity of the ambient air was 30 41 compared to the relative humidity of the air at the collector exit which has an average of 11 89 in the morning 8 7 8 9 between 11 15 hours with a minimum of 6 6 at 13 30 hour and an average of 18 97 in the late afternoon However the low relative humidity of the exhaust air shows that the potential of the drying air to remove moisture was not fully utilized this can be seen from the graph in the after noon This can be improved through proper utilization of the drying potential of the air by increasing the number of the drying trays 5 3 Drying Tests The experimental results obtained are shown in Figures 5 5 5 6 and 5 7 Figure 5 5 shows the moisture content of potato as a function of the drying time As may be expected tray T1 the one placed nearer to the hot air exhibits the most rapid drying By 6 PM of the first day the moisture content dropped to about 4 37551 7 881226 and 11 48754 wet basis for the potato
44. in the room Figure 5 6 shows the drying rate of potato as a function of the drying time As seen from the curves in the figure the drying rate for the first tray at the bottom of the drying chamber expectedly has the highest drying rate during the first 3 hours However as it gets dried its 59 drying rate decreases The drying rate of the potato on the second tray is larger than the first one after hours because the drying air absorbs less moisture from the first tray Even though there was moisture loss during the night in all the trays but the drying rates were nearly zero this is because the moisture loss was the entire night This effect is reflected in Figure 5 6 1 6 4 e Trayl Drying rate 96 kg kg dry product 30 130 230 330 430 530 630 730 830 930 Drying time minutes Figure 5 6 Drying rate curves plotted for potato on a dry basis During the initial stages of drying the rate of moisture migration is sufficient to maintain the surface in a completely wet condition Figure 5 7 Therefore during this period the rate of drying of the material is controlled by the rate of evaporation from the surface This is controlled by the condition of air adjacent to the surface Thus during this period the rate of drying is relatively constant as shown in Figure 5 7 which is known as the constant rate period The point where the drying rate starts to decrease is known as the critical moisture co
45. inal dry mass 451 8 481 3 518 2 72 Tables A 3 Relative Humidity of the drying air at the exit of the collector and tray2 and temperature of drying air at the exit of the collector 1 and Tray2 Relative Global humidity at 2 Nov 04 Airtemp at radiation on Cold theexitof 08 31 56Relative the exitof the collector Air temp Air temp Junction the collector humidity from Ambient air the collector plane atexit of at the exit Hour Temp C the exit of T2 Temp C KW m C of T2 C 8 31 15 44 19 6544 25 0304 16 68 30 58 0 56957 26 64 25 85 8 41 15 75 17 2608 31 3856 17 22 32 65 0 59797 24 48 23 17 8 51 16 41 15 264 33 1776 17 64 34 83 0 63854 23 34 21 44 9 01 16 99 15 0144 34 048 17 23 35 03 0 66775 23 54 22 25 9 11 17 47 15 008 35 9424 17 17 34 7 0 69574 26 31 25 24 9 21 17 91 13 248 43 4176 18 34 38 04 0 7355 27 09 26 28 9 31 18 41 12 7808 42 496 17 83 38 53 0 79148 27 92 26 73 9 41 18 82 12 2368 41 216 18 35 39 26 0 81582 29 74 27 33 9 51 19 21 11 5008 43 8272 17 77 39 51 0 83813 28 01 26 79 10 01 19 51 11 2512 43 008 18 11 39 41 0 86653 28 23 27 1 10 11 19 82 9 7152 39 8336 18 82 42 24 0 89006 32 01 28 43 10 21 20 14 10 8864 37 7344 18 3 40 2 0 91278 30 68 27 7 10 31 20 42 9 9392 39 0144 18 83 42 32 0 92941 30 86 27 76 73 Continued
46. led to accommodate rod shaped measuring probes The dimensions of the trays are shown in Appendix B of Figure B 2 3 2 Sensors Positioning The aim of the research work is experimental performance evaluation of the solar dryer For this reason a measuring system is installed in and around the dryer Figure 3 2 shows the location and types of the sensors applied Cap lt y NN Exhaust air warm humid Air temperature sensor Ambient air temperature sensor Humidity sensor Air speed sensor I Pyranometer Datalogger Figure 3 2 Schematic diagram of test set up 3 3 Instruments 31 3 3 1 Data Logger The DL2e logger unit contains all the hardware required for capturing and storing data from a wide variety of sensors under most environmental conditions It runs an internal logging program that is set up by the user and tells the logger how and when to acquire data ad DL2 Program Editor Version 3 Delta T Devices Ltd Figure 3 3 Data Logger front panel 3 3 1 1 Features of the Data Logger The DL2e Logger is a programmable data logging device capable of taking readings and storing data from a wide variety of sources It is independently powered capable of operating under wet conditions and at high and low temperatures 32 PC software Ls2Win is first used to program the logger specify what sensors are to be connected to the logger and how frequently to rec
47. lty of Technology In this paper the variables and mechanisms involved in the dehydration processes would be discussed The drying time of the dryer is also compared with the open sun drying time 1 4 Thesis Direction Earlier works on solar drying are outlined in the literature review The research to be described in the rest of this thesis proceeds as follows The second chapter focuses on the literature survey of different drying theories like drying mechanism drying air properties types of solar dryers and drying efficiency The third chapter deals with test set up and installation of the measuring devices such as thermocouples relative humidity sensors and pyranometer with the data logger The fourth chapter describes the procedure of the experiment The fifth and sixth chapters deal with the analysis of results and conclusion and recommendations respectively Chapter 2 Theory of Solar Dryer Solar drying refers to a technique that utilizes incident solar radiation to convert it into thermal energy required for drying purposes Most solar dryers use solar air heaters and the heated air is then passed through the drying chamber containing material to be dried The air transfers its energy to the material causing evaporation of moisture of the material 2 1 Drying Mechanism In the process of drying heat is necessary to evaporate moisture from the material and a flow of air helps in carrying away the evaporated moisture Th
48. m called DL2 control panel to e Display information about the logger and its datasets e Display data settings in real time e Retrieved logged data from the logger data from the logger to a PC disk file e Display the data on your PC screen 3 3 1 11 Dataset Viewer This is a separate program that automatically opens on the desk top when a dataset viewer in the DL2 control panel is retrieved This shows the reading of the connected sensors from the dryer at different locations at interval of 10 minutes The binary format dataset is displayed in the dataset viewer File View Options Help Channel 1 2 3 4 5 6 7 Label cold jn RHin 03 Ambtemp Ittadia Sensor Type TM1 RH2 TM1 TM1 TCK CM3 TEK Units degC degC degC degC degC 02 Nov 08 31 56 15 440000 19 654400 28 660000 16 680000 30 580000 0 569574 26 540000 02 Nov 08 41 56 15 750000 17 260800 30 490000 17 220000 32650000 0 597972 24 480000 02 Nov 08 51 56 16410000 15 264000 32 080000 17 640000 34 830000 0 638540 23 340000 02 Nov 09 01 56 16990000 15 014400 32530000 17 230000 35 030000 0 667748 23 540000 02 Nov 09 11 56 17 470000 15 008000 32520000 17 170000 34 700000 0 695740 26 310000 02 Nov 09 21 56 17 310000 13 248000 34950000 18 340000 38 040000 0 735497 27 080000 02 Nov 09 31 56 18 410000 12 780800 35 550000 17 830000 38 530000 0 791481 27 320000 02 Nov 09 41 56 18 820000 12236800 36 280000 18 350000 33 260000 0 815822 23 740000 02 Nov 09 51 56 13210000 11
49. mperature was recorded However for drying of the potato the temperature was increased by decreasing the inlet of the collector and outlet of the drier areas and a temperature within 14 29 C higher than the ambient air temperature was obtained and this variation is shown in Figure 5 1 54 Uy gt CA l l l O E Nua B peme 10 4 0 T T T T T T T T T 1 7 8 9 10 11 12 13 14 15 16 17 Time of the day hour Ambient air Temp Collector exit air Temp Figure 5 1 Shows the inlet and outlet air temperatures for the collector The instantaneous efficiency of the solar collector started to rise in the morning period and was relatively constant at 28 56 from 11 15 hours to 13 45 hours and dropped down in late afternoon The variation obtained is typical for a flat plate collector and indicates strong dependence of efficiency on the metrological data The daily efficiency averaged over 9 hours 8 30 to 17 30 comes out to be 25 6 35 4 30 4 25 4 20 4 15 4 Instantaneous efficiency 46 0 T T T T T T T T T 1 8 9 10 11 12 13 14 15 16 17 18 Time of the day hour Figure 5 2 Variation of instantaneous efficiency of the flat plate collector 23 Figure 5 3 shows a plot of the collector efficiency as a function of the normalized temperature rise As seen the collector efficiency curve complies with the standard curve 9 Instantaneous efficiency
50. n and rain when they are mounted outdoors The temperature sensors used to measure air inside the drying is of K type thermocouple from Delta T Device Ltd It measures temperature in the range of 120 70200 C with an accuracy of 0 1 C 6 3 3 7 Humidity Sensor To measure relative humidity sensors from Delta T Device Ltd of type RHT2n1 02 are used The accuracy at 23 C is 2 RH 5 to 95 RH 2 5 RH RH lt 5 and gt 95 6 45 3 3 8 Hot Wire Anemometer Air velocity is measured by hotr wire anemometer NTC anemometer from Testo instruments It measures flow velocities in the interval 0 to 20m s The accuracy is within 0 05 m s 596 of m 0 to2m s and 0 5 m s 596 of 2 to20m s 3 3 9 A Digital Platform Balance It is used to determine the weight loss of the dried product within the specified time interval The accuracy is within 0 16 3 4 Program Installed in the Data Logger First the program is prepared on the PC and later transferred to the logger via RS232 serial port DL2e programming editor is used to retrieve some of the sensors and adapt others from the DL2e library After configuring the each channel the recording action is set to 10 minutes The name of the program used for the data collection is SDryer which is shown in Figure 3 13 In the program the following terms are used for the name of the sensors Label Description Cold jn Cold junction temperature in the data logger Amb temp Ambien
51. ncouraging Thermal efficiency lies between 14 59 and 29 95 and temperature rise is between 14 and 29 C for air flow rates ranging from 0 00595 to 0 0114 kg s m of collector area The drying time required by traditional open sun drying is reduced by 3 hours about 19 in natural convection dryer under the existing environmental conditions Further more the drying material is protected from direct solar radiation infestation by insects and contamination by dust As a result the product quality is high Since when the moisture content reaches equilibrium moisture the drying rate is zero Potato is a hygroscopic material The system drying efficiency Ns or system efficiency is about 16 and dryer efficiency 77 is about 65 65 6 2 Recommendations for Future work e The conducted experiments will form possible bases for the future work and the study can be developed for other agricultural products and different seasons e Use of wind energy or photovoltaics to provide fan power can be considered where and when feasible e Modeling and simulation to investigate the design and optimization of the solar dryer and the dryer operation can be carried out e The effects of the location of trays and number of trays in the chamber and use of other designs of collector can be taken as a future work e The possible use of side reflectors to increase the amount of incoming solar energy to the unit must increase the dryer performance and hence
52. nel dialog 2 36 Figure 3 7 Logger Panel on the DL2 Control panel eee 37 Figure 3 8 Sensors panel showing real time readings from temperature sensors 39 Figure 3 9 Dataset control panel displaying the information about the readings stored in the vi Figure 3 10 A copy of the dryer logging program in the DL2 is retrieved and displayed in thes program Edi creauit 41 Figure 3 11 Dataset retrieved from the logger to file 42 Figure 3 12 AMB moisture balance 44 Figure 3 13 A copy of the logging program used in this project 122222 47 Figure 5 1 Shows the inlet and outlet air temperatures for the collector 55 Figure 5 2 Variation of instantaneous efficiency of the flat plate collector 55 Figure 5 3 Collector Instantaneous efficiency eese enne 56 Figure 5 4 Time variation of the relative humidity in the dryer at the exit of the collector and just above the second 1 58 Figure 5 5 Moisture content curves for potato in solar dryer and open air sun dryer 59 Figure 5 6 Drying rate curves plotted for potato on a dry 60 Figure 5 7 Drying rate Curves certe cet rre een tee Mene 61 Figure 5 8 Temperature variations with respect to the vertical distance from the
53. ntent Thereafter the period of drying is known as the falling rate period This is the period when the 60 surface of the material is not wetted completely by migration of moisture The drying rate tends to zero when the rate of evaporation from the surface equals the rate of absorption of moisture by the material and is known as the equilibrium moisture content Since the drying rate decreases to zero potato is a hygroscopic material Drying rate dry basis 0 T T T T T T T 1 0 10 20 30 40 50 60 70 80 Moisture content w b e Tray 1 Tray2 Tray 3 Figure 5 7 Drying rate curves Figure 5 8 displays the variation of air temperature with vertical distance from the bottom of the drying chamber Ambient air temperature is included in the graph for comparison A major drawback of the shelf type dryer is the uneven drying As a result of the migration of the drying front the materials at the entrance are dried while at the exhaust are under dried This problem can be alleviated by rotating the drying shelves In such a rotating operation the hot air from the collector is used to heat the product already in the latter 61 stages of drying falling rate period while the unsaturated air is used to remove moisture from product in the upper shelves 0 T T T T T T T T T 1 8 9 10 11 12 13 14 15 16 17 18 Time of the day hour Ambient air temperature a Collector exit air temperature
54. o slices were manually stirred randomly This would help to increase the temperature of potato slices and would ease the moisture diffusion through the potato slices The drying process was considered to be complete once the moisture content of the slices dropped to about 3 1 on wet basis Once the solar drying experiment was completed samples of the solar dried potato were compared with the potato that was dried by open sun The factor considered in this comparison was drying time The measured data from the collector and dryer air temperature at the outlet of the collector and over trays and T2 relative humidity at the inlet and exit of the chamber solar irradiance and ambient temperature were also recorded in the data logger at intervals of 10 minutes The data were scanned each 10 seconds and averaged for 10 minutes The values were stored in the data logger and by the end of the tests transferred to a personal computer 4 4 Efficiency Analysis The efficiency of the dryer is considered by taking into account the complete collector drier system for the solar energy input The measured values by the data loggers transformed into 52 physical meaningful values like solar radiation temperatures humidity air speed and weight The useful heat is then estimated from the formula me T T 4 7 where T Air temperature at exit of collector T Air temperature at inlet of collector equal to the ambient tempe
55. of fig re Astratt LER Chapter Loreen NAO 1 TIMO UICN E IEEE SERRE SRR ees 1 1 1 OVERVIEW AND OBJECTIVE OF THE THESIS 1 1 2 LITERATURE REVIEW 4 1 3 OBJECTIVES OF THE RESEARCH 6 1 4 THESIS DIRECTION 6 Chapter 2 ens Sek ica RR 7 Theory of Solar Dryer iii A an A eet 7 2 1 DRYING MECHANISM 7 2 2 AIR PROPERTIES 11 2 3 TYPES OF SOLAR DRYERS 12 2 3 1 Natural Convection Solar Dryers sss 13 2 3 2 Indirect Type Solar Dryers esses eene 15 2 4 KEY ELEMENTS OF SOLAR DRYER 18 2 4 T Solar Collector id AA A ee 18 2 5 DRYING EFFICIENCIES 22 2 5 1 System Drying Efficiency esses eene enne 23 Chapter Ju A ap e Ee HE eee cir eda 20 Experimental Setup and 20 3 1 DRYER SETUP 29 3 3 INSTRUMENTS 31 3 3 I Data OBRA vere catre ye i nee 0 32 3 3 2 AMB Moisture Balance AMB 50 AMB 110 and AMB 310 sess 43 11 3 3 3 Dome Solarimeter Pyranometer essen 44 J 5 4 AHOIBOELEF 7 srt e TR GI eee utate 45 3 3 5 Ambient Air Temperature Sensor essere 45 3 3 6 Air Temperature 45 3 3 7 H midity ENS ia 45 3 3 8 Hot Wire Anemomteter 46 3 3 9 A Digital Platform Balance essen eene 46 3 4 PROGRAM INSTALLED IN THE DATA LOGGER46 Chapter oe csc tec ee ee
56. of hot air in the dryer must be increased 2 3 2 1 Shelf Type Dryer In Figure 2 8 a schematic view of the so called shelf dryer is shown As can be seen in Figure 2 8 the material to be dried is placed on perforated shelves 1 built one above the other The front wall of the case faces south its top and sides 2 are covered by transparent walls glass or sheet and the back wall 3 is heat insulated The back wall and the floor are covered with a coating of black paint The ambient air is warmed in a flat plate collector 4 joined to the bottom of the case and it flows up to the space under the lowest shelf Moist air exits to the open through the upper opening of the casing 5 In the scheme shown in Figure 2 8 the chimney effect is ensured by the increased height of the dryer 2525256 bed POS f maim SOS sud m H logde m wm m y i SSR 16 Figure 2 8 Shelf type dryer with separate collector The experiments indicated that separation of the collector is only justified with a high efficiency collector This dryer is suitable for drying fruits and vegetables 13 2 3 2 2 Chimney Type Paddy Dryer For large amounts of material an appropriately high chimney has to be connected to the dryer housing Figure 2 9 gives cross section of chimney type dryer designed and built for drying 1000 kg rice Rice is placed in a static bed 1 in a 0 1 m thick layer The collector consists of a plastic coverin
57. of the indicated values are self explanatory Dataset A logger program can generate up to three datasets the TIMED dataset contains data recorded at regular recording intervals and TRIG 61 and TRIG 62 datasets which contain data recorded on detection of events on digital input channels 61 and 62 respectively Auto warp If selected when memory is full the most recent data overwrites the oldest readings The most recent data are retained the oldest data are overwritten by new data This option is only available for TIMED dataset Retrieve Retrieves the selected dataset to a PC disk file Delete retrieved Records Delete the most recently retrieved dataset records from the logger s memory This option is only available for the TIMED dataset For TRIG 61 and TRIG 62 datasets the alternative command is Delete All Records which deletes the entire contents of the selected dataset from the logger s memory Clear All Dataset Deletes the contents of all datasets from the logger s memory This command is only enabled when the logger is not logging Size Determines how used and available memory is played from a large drop down list of options 3 3 1 8 Programming the DL2e Logger 40 For real logging applications a logging program is needed A logging program specifies some or all of the following e What sensor types are connected to each of the logger s channels e How frequently readings are to be logged from each channel e
58. oof case it is equally suited for use as a laboratory instrument or for outdoor installation for remote locations Being modular and 33 programmable it is an extremely flexible tool and easily adaptable for a wide variety of application 3 3 1 2 Data Logger Software LS2Win Ls2Win enables a PC to communicate with the logger edit logging programs and collect the data It is supplied on CD ROM and the installation procedure is as follows 5 3 3 1 3 Items Required for Installation of LS2Win To operate the logger from your PC the following are need e A PC running windows 95 98 2000 or NT4 0 Service pack 4 or later e One free RS232 serial port e CD ROM drive required for installation e At least 16M RAM memory and 5Mb for hard disk space e Logger PC RS232 cable Type LRS1 available from Delta T or you can make up Setup installs a program group named Ls2Win on the program menu which contains the following items e New DL2 Control Panel For creating DL2 Control Panels which one can use to communicate with the data logger e DI2 Program Editor for creating and viewing logging programs e Dataset Viewer for inspecting the contents of data set files files containing logged data 34 e Dataset Import wizard for importing logged data in to Microsoft Excel Setup also installs desktop shortcuts that correspond to each item in the Ls2Win program group t mi na Ae GEN n DLzPingem Da
59. ord data Once the logger has been set up the PC can be disconnected and the logger can be left to operate on its own Data recorded from sensors is stored in the logger s own memory and periodically transferred to a PC or to any device which has an RS232 serial port for example a printer When data has been collected from logger it can be cleared from the logger s memory to make room for more data Only one PC is needed to operate any number of loggers and it only needs to be connected while setting up the logger and collecting the stored data The logger also has a front panel with keypad and display that can be used to check and control logger operation without using a PC The DL2e Logger is modular in design Depending on the input cards installed the logger can record data from up to 62 sensors Input cards are available for analogue and pulse output sensors Each logger has on board digital input channel for pulse counting or event detection and two relays for powering up sensors or simple control applications The logger has an internal clock and can be set up to record data at regular intervals This is known as timed data In addition it can also record data when events are detected known as event triggered data The logger can be powered from external DC power supply or from its own internal batteries It has extremely low power consumption and can operate for extended periods on a set of batteries With a rugged weatherpr
60. rature Measuring the collector flow exit area the flow velocity of air and the local density of the air the mass flow rate is calculated as m 4 8 The instantaneous efficiency of the collector is the calculated from the relation 0 La 4 9 The cumulative efficiency is calculated from the ratio of the sum of the useful heat and the solar radiation reaching the area pe 2 9 4 10 Y Al The system drying efficiency Ms or system efficiency is calculated from the ratio of the energy required to evaporate the moisture of the commodity to the heat supplied to the drier 4 11 where w is the mass of moisture evaporated L is the latent heat of evaporation of water at the dryer temperature 53 Chapter 5 Results and Discussion Four successful tests were conducted between October 11 and November 3 2004 and in this thesis work one of the test data was used to evaluate the collector efficiency drying curves humidity and temperature measurements in the dryer During the tests period the heated air was used to dry potato The test raw data from the experiment is tabulated in the appendix A 5 1 Collector Performance 5 1 1 Collector Efficiency The efficiency of the collector could be seen from difference in temperature of the exit and inlet of the air to the solar collector In this dryer by fully opening the inlet and the exit of the dryer a temperature within 10 20 C higher than the ambient air te
61. rediction of the solar collector performance requires information on the solar energy absorbed by the collector absorber 18 plate The solar energy incident on a tilted surface has three different components beam radiation diffuse sky radiation and diffuse ground reflected radiation and on hourly basis the absorbed radiation My is 1 cos B 21 I 1 R ta I 70 2 p I I a wher e dur m and D p are the view factors from the collector to the sky and from the collector to the ground respectively The subscripts b d and g represent beam diffuse and ground respectively The meaning of the angles in Eq 2 1 is described in Figure 2 10 which shows the relationship between the angles which describe the position of a surface on the earth and the position of the sun relative to the earth and the surface Zenith Normal to horizontal surface Figure 2 10 Angles describing the direction of a direct solar beam The formulae given in this chapter are taken from by Duffie and Beckman 1991 if not stated otherwise 19 The symbols have the following definitions 0 Latitude north positive 90 lt lt 90 Slope between the plane of the surface and the horizontal 0 lt 8 180 y Surface azimuth angle 180 y lt 180 Declination a Solar altitude angle Zenith angle The declination angle may be found using the equation 8 23 45 28
62. rpreted only as a transient value owing to the time dependence of the irradiance For definite period the long term efficiency of the collector can be expressed with the time integral of utilized and input energy flow rates 13 0 2 7 A 0 2 5 1 System Drying Efficiency The system drying efficiency Ns or system efficiency is the ratio of the energy required to evaporate the moisture of the commodity to the heat supplied to the drier 20 Therefore 2 8 where w is the mass of moisture evaporated L is the latent heat of evaporation of water at the dryer temperature A is the solar collector area and the dryer efficiency is given by 0 Nel Me 2 5 2 Calculation of Collector Efficiency 23 In Eq 2 6 of the instantaneous efficiency utilized heat flow rate Qu is the difference between the heat flow rate absorbed Q and the heat flow rate lost Q to the ambient air 0 0 0 2 9 where Q tal A 2 10 is the heat flow rate absorbed by the absorber from the irradiation getting through the covering and Q AU T 2 11 is the heat flow rate transferred to the ambient air from an absorber at temperature Tp In Eq 2 11 U is the overall heat transfer coefficient of the collector to the ambient air Substituting into Eq 2 6 the instantaneous efficiency of the collector is T T n 1a U 2 12 If 7 0 and are taken as constant values instantaneous efficien
63. s This cap doesn t allow rain and dust to enter the dryer and enhances the moisture evaporation from the product The inside of the dryer as well the trays are painted black Fresh air in the dryer enters through the openings through shutters provided in the lower portion of the walls below the glass roof and above the drying platforms Solar radiation penetrates through the glass roof heats the product directly and absorbed within the dryer increasing the inside temperature 14 Cap Exhaust air pai warm humid Shutter open Figure 2 7 Green house type solar dryer 2 3 2 Indirect Type Solar Dryers The capacity per unit area of cabinet dryer is limited by two conditions need for direct radiation on the drying materials and small airflow rate To dry large quantities of material the basic area of the dryer has to be increased To avoid this problem it is preferable to 15 place the material in several independent layers the necessary heat transfer is thus accomplished by convection The increase in mass flow rate of the air can be achieved by increasing the effects that produce natural convection These effects must also be increased if the air is to be circulated through a material laid in several layers one over the other or through a thick layer as in the case of the chimney type To keep up without using a ventilator for instance in a field the chimney effect must be exploited For this purpose the vertical flow
64. s a constant drying rate terminating at the critical moisture content followed by falling drying rate The constant drying rate for both non hygroscopic and hygroscopic materials is the same while the period of falling rate is little different For non hygroscopic materials in the period of falling rate the drying rate goes on decreasing till the moisture content become zero While in the hygroscopic materials the period of falling rate is similar until the unbound moisture content is completely removed then the drying rate further decreases and some bound moisture is removed and continues till the vapour pressure of the material becomes equal to the vapour pressure of the drying air When this equilibrium reaches then the drying rate becomes zero 14 The period of constant drying for most of the organic materials like fruits vegetables timber etc is short and it is the falling rate period in which is of more interest and which depends on the rate at which the moisture is removed In the falling rate regime moisture is migrated by diffusion and in the products with high moisture content the diffusion of moisture is comparatively slower due to turgid cells and filled interstices In most agricultural products there is sugar and minerals of water in the liquid phase which also 10 migrates to the surfaces increase the viscosity hence reduce the surface vapour pressure and hence reduce the moisture evaporation rate 14 Drying is don
65. sh produce are to be processed for the commercial market forced convection dryers should be used 1 One basic disadvantage of forced convection dryers lies in their requirement of electrical power to run the fan Since the rural or remote areas of many developing countries are not connected to the national electric grids the use of these dryers is limited to electrified urban areas Even in the urban areas with grid connected electricity the service is unreliable In view of the prevailing economic difficulties in most of these countries this situation is not expected to change in the foreseenable future The use of natural convection solar dryer could boost the dissemination of solar dryers in the developing countries 1 Therefore experimental performance of solar dryer has been evaluated in this thesis 1 2 Literature Review Drying of agricultural dates back to the beginning of civilization The use of the solar energy and air movement provided the major method of moisture removal in the field Crops for human consumption were occasionally dried in ovens or by hanging in heated rooms Between World War I and II several experimental mechanical drying units were built and a few commercial units were in operation Commercial dryers were primarily used for dehydration of fruits vegetables and hay drying of seed corn with heated air and drying hay in the barn usually with unheated forced air Commercial and large scale farm drying bec
66. t and coconut Exell 1980 developed a low cost mixed mode natural convection solar dryer for paddy drying Paddy could be dried safely in 2 3 days Solar air heaters which were integrated in natural convection solar dryers have also been investigated These were plastic film solar air heaters used in a solar rice dryer Exell 1980 and a flat plate solar air heater in a cabinet solar dryer Wibulswas and Haina 1980 Patranon 1984 Due to natural convection of air through the solar air heater the air flow rate varies Hence thermal efficiency varies throughout the day The solar collection efficiency is usually less than in the case of forced convection Some recommended drying temperatures are fruits and vegetables 38 55 C temperature over 65 C can result in sugar caramelization of many fruit products fish 60 66 C rice grains seeds brewery grains 45 C maximum temperature 16 1 3 Objectives of the Research The general objective of this thesis is experimental investigation of the components of free convection solar drying systems which involve consideration of solar collector and drier In this thesis the analysis will concentrate on the practical field test performance of the existing locally manufactured solar dryer using the state of the art equipment recently available for solar energy research and development program of the Faculty of Technology The experiment is conducted in the Addis Ababa University Facu
67. t is used to determine the initial and final moisture contents of the material to be dried and its measuring capacity is in the range of 4 to 310 g The AMB moisture balance is easy to use The user sets the drying parameters into memory puts the samples into the weighing chamber and then starts the test The temperature of drying is automatically regulated and the results elapsed time current temperature in the chamber and the mode are displayed during the test The user is told when the test has automatically stopped either due to the sample being dry and the weight no longer changing or the elapsed time reaching the limit the user has set The final values are held on the display until the user resets the balance The balance can be interfaced to a printer or computer The results are shown when the test progresses and after the test has finished a summary of the test can be sent to a PC or printer At the end of the test the following data are displayed over the digital displays of the AMB moisture balance 43 1 Percentage moisture or Percentage Solids ii Initial mass mg ili Final mass mg iv Drying temperature C v Elapsed drying time s vi Time interval between two successive measurements s 6 1 p gt 8 r Key OS m 1 Display and Keypad Ts J H 9 2 Balance 4 ONU PS 3 Pan Support oe A 10 4 Weighing Pan 5 Weighing S
68. t temperature Irradia Solar radiation at the surface of the collector Ae temp Air outlet temperature from the collector 46 Trl temp Air temperature over tray 1 Tr2 temp Air temperature over tray 2 Tr3 temp Air temperature over tray 3 Tto Temperature at the exit of the dryer chamber RH in Humidity of air inlet to the chamber RH out Humidity of air out from the chamber LACT 15 channel 1 coin TM1 Thermistor 2K Fenwal UUA32J2 A pais RH2 Relative Humidity Sensor types 10m Avg of DC Voltage RHT2n1 02 RHT 2 02 humidity output 10s samples fix range TM1 Thermistor 2K type Fenwal UUA32I2 co ee 4 Ambenp TM1 Thermistor 2K type Fenwal 2 2 aei palas 5 Thermocouple Chromel Alumel type K Tom Avg F Misi CM3 Copy of Dome Solarimeter types 551 10m Avg of DC Voltage 552 10s samples auto range J Lc Figure 3 13 A copy of the logging program used in this project 47 Chapter 4 Test Procedure and Computations 4 1 Sample Preparation Before the test was conducted with material to be dried the dryer was checked with no load The exit and inlet flow areas were reduced by closing the air flow channels at the expense of the air flow rate until a drying air temperature above 38 C was achieved which is required for the drying of fruits and vegetables 16 The important factors affecting the migration of moisture from the interior of
69. tato slice 2 7 3 73 dry basis 96 4 2 Moisture Determination 49 The initial moisture content on wet and dry basis of the potato used in the experimental work was determined by AMB AMB 310 moisture balance The AMB balance test was set at Mode 1 with strobe time interval of 2 seconds and drying temperature 160 C Then slices of potatoes samples were placed on the AMB moisture balance tray Samples of the potato of weight w were dried in the moisture balance at 160 C until the weight of the dried sample became stable The moisture content on wet basis of the potato used was 81 56 442 3 moisture content at dry basis The moisture content dry basis Md of the potato is expressed as 2 4 100 9 4 1 For the determination of the moisture content dry basis Md of the potato at time 6 during the drying process the following equation can be used Md 100 4 2 Wy or moisture content wet basis of the potato at any time ti during the drying process Mw 100 43 where w is the weight of the potato at time t The moisture content on dry basis and wet basis are related by Eq 4 4 2 Mw 0100 9 4 4 ewe VA The determination of the potato weight was done by weighing the drying tray with its load of potato at any time in the drying process 50 For the determination of the instantaneous drying rate RDd
70. use those products 2 Therefore by providing a sheltered drying area or chamber in which the crops to be dried and stored a stream of air is heated by solar energy to reduce its relative humidity which is then passed over the crops This form of solar drying could improve the quality of the crop to be dried reduce spoilage by contamination and local overheating reduce spillage losses speed up the drying process achieve better quality control and reduction in drying time The disadvantages of open sun drying need an appropriate technology that can help in improving the quality of the dried products and in reducing the wastage This led to the application of various types of drying devices like solar dryer electric dryers woodfuel driers and oil burned driers However the high cost of oil and electricity and their scarcity in the rural areas of most third world countries have made some of these driers very unattractive Therefore interest has been focused mainly on the development of solar driers 23 Solar dryers are usually classified according to the mode of air flow into natural convection and forced convection dryers Natural convection dryers do not require a fan to pump the air through the dryer The low air flow rate and the long drying time however result in low drying capacity Thus this system is restricted to the processing of small quantities of agricultural surplus for family consumption Where large quantities of fre

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