Int. J. Renewable Energy Technology, Vol. 1, No. 1, 2009
29
Performance evaluation of exhaust air recirculation system of mixed mode solar dryer for drying of onion flakes Surendra Kothari, N.L. Panwar* and Saurabha Chaudhri Department of Renewable Energy Sources, College of Technology and Engineering, Maharana Pratap University of Agriculture and Technology, Udaipur, 313 0001, Rajasthan, India Fax: +91 294 2471056 E-mail:
[email protected] E-mail:
[email protected] E-mail:
[email protected] *Corresponding author Abstract: This paper deals with the performance of mixed mode type solar dryer for drying onion flakes. During the experiments, no load and full load test were conducted to find out the effectiveness to the system with or without recirculation of exhaust air. The drying and thermal efficiencies and heat utilisation factors were recorded as 21%, 74% and 31% respectively, more compared to recirculation of exhaust air test. The quality of dried onion flakes without recirculation of exhaust air test was superior. The recirculation of exhaust air was founded feasible only with use of desiccant material. Keywords: mixed mode solar dryer; drying of onion flakes; exhaust air recirculation in solar dryers. Reference to this paper should be made as follows: Kothari, S., Panwar, N.L. and Chaudhri, S. (2009) ‘Performance evaluation of exhaust air recirculation system of mixed mode solar dryer for drying of onion flakes’, Int. J. Renewable Energy Technology, Vol. 1, No. 1, pp.29–41. Biographical notes: Surendra Kothari graduated from College of Technology and Agriculture Engineering, Udaipur in 1982. He received his MTech and PhD in Energy Studies from Indian Institute of Technology, Delhi in 1989 and 1999 respectively. Presently, he is working as an Associate Professor in the Department of Renewable Energy Sources, College of Technology and Engineering, Udaipur. He has about 22 years of experience in teaching, research and extension. He has more than 32 research publications in national and international journals. He has also authored five books. He has been involved in organising 20 summer/winter schools, five national seminars/symposiums and four international trainings. His scientific interest is in the fields of solar greenhouse, drying and energy management. N.L. Panwar graduated from Faculty of Engineering, JNVU, Jodhpur in Mechanical Engineering and obtained a Masters degree in Renewable Energy Engineering from MPUAT, Udaipur. He is working as an Assistant Professor in the Department of Renewable Energy Sources, College of Technology and Engineering, Udaipur. He is also working as a Research Engineer in the AICRP on renewable energy sources (thermo chemical conversion technology). His
Copyright © 2009 Inderscience Enterprises Ltd.
30
S. Kothari et al. scientific interests are in the fields of new and renewable energy sources, energy conservation and application of energy in agro-based industries. He has contributed more than 20 technical papers and five books on renewable energy sources. Saurabha Chaudhri is a graduate of Agricultural Engineering from Br. Balasaheb Sawant Konkn Krishi Vidyapeeth, Dapoli, Maharashtra and obtained his Masters degree in Renewable Energy Engineering from Maharana Pratap University of Agriculture and Technology University, Udaipur, Rajasthan. His major research area is in forced convection solar dryer. He has experience in solar thermal air conditioning, industrial solar drying system and hot water system for process heat. He has keen interest in solar thermal and biomass conversion technologies. At ITPI, he is an Engineer with Energy Group and is involve in projects related to renewable energy and appropriate technology in health.
1
Introduction
Onion is the major vegetable crop of India. It is grown in almost all countries in the world. India is one of the leading countries in the world producing onions of high quality and exports considerable quantities, earning substantial foreign exchange. India ranks first in the world with over 435.28 thousand hectare accounting for about 21% of the world area planted with onion. Leading onion production countries are China, India, USA, Turkey and Iran respectively. The production of onion in India was 5942.50 thousand tonnes in 2004–2005, with share of around 14% of world’s production. Onion is a perishable commodity due to its higher moisture content (about 82% wb). The quality, appearance, colour, flavour and texture deteriorate due to spoilage caused by microorganisms, enzymes, vinegar flies, etc. Microorganisms grow, multiply and thrive in presence of moisture and oxygen to degrade the harvested onions. The bruises caused during mechanical harvesting and handling, accelerate the process and cause internal contamination of onions. Therefore, the harvested onions should be marketed, processed or preserved as early as possible. To overcome this problem, drying is a prerequisite process for proper storage of onion. Drying enhances the shelf life, reduces weight and volume of foods substantially, and in addition, it minimises packaging, storage and transportation costs. Usually in India, open sun drying is preferred, however in this method, products become contaminated with dust, dirt, rainfall, animals, birds, rodents, insects and microorganisms. Under these conditions, losses can be as high as 40%–60% of total quantity (Mangaraj et al., 2001). Presently, drying of agricultural commodities in the food processing industry is being done with the help of various types of mechanical dryers. There may be about 2500–3000 of such dryers in operation (Shukla and Singh, 2003). Most of the agricultural products are dried at temperature range of 45°C–75°C. Solar energy can be used to heat air up to this range of temperature needed for drying of most of the agricultural products, efficiently and economically without compromising in quality of final product. Even in the cases where higher temperature than those possible with simple solar air heater is needed, preheating can be done and this results in substantial saving of conventional fuels and assures good results. In India, the average solar radiation available is 5 kWm–2 day for 250–300 days in a year with approximately eight to ten full sunshine hours.
Performance evaluation of exhaust air recirculation system
31
There are many types and designs of solar dryers available for drying of agricultural commodity, which mainly include natural convection and forced circulation solar dryer based on array of flat plate air heaters. The natural convection solar drier appears to have potential for adoption and application in the tropics and subtropics. It is suitable at a household (Sharma et al., 1995) level for drying of 10–15 kg of fruits and vegetables. But the natural convection solar dryer suffers from limitations due to extremely low buoyancy induced airflow inside the dryers (Bala and Woods, 1994). In addition, comparatively high investment, limited capacity and the risk of crop spoilage during adverse weather conditions have up to now prevented the wide acceptance of natural convection solar dryers (Schirmer et al., 1996). A forced circulated indirect solar drier for commercial drying of red chillies was designed and installed at Sardar Patel Renewable Energy Research Institute (SPRERI), Gujarat, India. The system consisted of 12 m2 at plate air heater, a 0.75 kW electric blower and a drying chamber to hold 60–65 kg of chillies per batch. The system dries red chillies with 12% (db) initial moisture content to 4% (db) final moisture content at an operating temperature of 60°C (Philip et al., 1993). The greenhouse concept with packed bed thermal storage can also be used for drying agricultural product. The greenhouse size 6 m × 4 m with 0.287 kg/s air mass flow rate, with 0.25 m height of packed bed can dry 2,280 kg of onion from moisture content 6.14 kg to 0.21 kg, water/kg of dry matter in 24 hours (Jain, 2005). The solar dryer with forced convection mode, the 100 kg of fresh onion flakes can dry from 85% to 8% moisture content in four to seven hours (Joshi and Philip, 1997), and this type of unit with electric backup was found quite useful for drying agricultural product at commercial scale (Philip, 1999). There is a scope of energy conservation in crop drying through solar energy and this can be achieved by recirculating exhaust air in dryer, by this, 27% energy can be saved (Schoenau et al., 1999; Jain et al., 2003). Further, solar photo voltaic operated forced convection dryer for food dehydration was also found suitable for developing countries (Saleh and Sarkar, 2002). The main drawbacks of forced circulation type are requirement of more ground space under air heating array, high power requirement to circulate the air and the high initial cost of installation. These drawbacks of existing dryers stimulated to design and develop the mixed mode shallow solar dryer with corrugated plate solar air heating unit with increased retention time of air in collector, packed bed thermal storage, exhaust fan to increase moisture removal rate and the exhaust air recirculation unit to improve the heat utilisation factor (HUF) with conservation of energy. Maximum energy savings of 27% and 17% was recorded with exhaust air recirculation during fall and summer dryer operation respectively (Schoenau et al., 1999). The recirculatory tray dryer was designed and tested by Shawik et al. (2001) with 5 kg/batch using central air distribution system. The dryer was tested for blanched potato chips at constant air flow rate of 1.5 m3/min and 65°C temperature. For removing moisture from 85.69% (db) to 9.89% (db), the observed drying time was three hours. The HUF and thermal heat efficiency was found to be 18.94% and 22.16% respectively.
2
Materials and methods
The solar dryer in which solar energy collection takes place in both air heater and drying chamber and but drying takes place only in the drying chamber is called mixed mode type solar dryer. The dryer has inline rock bed storage to supply heat in the afternoon.
32
S. Kothari et al.
The performance of recirculation unit is not affected by the inline thermal storage. The dryer consists of the following components: 1
solar air heating unit or solar collector
2
rock bed thermal storage
3
drying chamber
4
air circulation cum exhaust air recirculation unit.
The dryer was designed by keeping in view the climatic parameters of Udaipur (27° 42’ N, 75° 33’ E) Rajasthan, India. The parameters considered for designing the present dryer are summarised in Table 1 and its components are shown in Figure 1. Figure 1
Exhaust air recirculation arrangement in dryer (see online version for colours) Exhaust air gate
PV operated exhaust fan Exhaust air recirculation gate
3
4
2
5 6 1 7
Notes: 1 = Air inlet, 2 = Air heater, 3 = Packed bed storage, 4 = Drying unit 5 = Exhaust fan, 6 = Exhaust air out, 7 = Supporting leg
Performance evaluation of exhaust air recirculation system Table 1
Design considerations of mixed mode solar dryer
1
Capacity
10 kg fresh onion flakes
2
Initial moisture content
87% (db)
3
Final moisture content
7% (db)
4
Loading rate
5 kg m–2
5
Solar insolation
620 W m–2 hr (January)
6
Ambient temperature
30°C
7
Drying temperature
60°C
8
Ambient relative humidity
50%
9
Drying time
seven hours
Table 2 1
Technical specifications of mixed mode solar dryer
Surface area Air heating unit
2.16 m2
Drying chamber
2 m2
Thermal storage
0.8 m2
Total area
4.8 m2
2
Loading capacity
3
Air heating unit
4
5
33
10 kg fresh onion flakes
Absorber material
22 swg corrugated GI sheet painted with black board paint
Baffles
4 nos. @ 0.50 m spacing arranged in zigzag manner
Thermal storage Back up time
two hours
Volume
0.133 m3
Drying chamber Dimension
2.5 × 0.8 m
Product holder
GI wire mesh fitted in MS angle of 25 × 25 × 3 mm
6
Glazing material
200 micron UV stabilised plastic sheet
7
Air flow rate
120–180 m3 h–1
8
Air flow arrangement
12 V DC fan operating on 18 Wp SPV panel
9
Exhaust air recirculation unit Exhaust air gate
0.017 m2
Recirculation gate
0.018 m2
10
Partition between air heating unit and drying chamber
11
Insulation
22 swg corrugated GI sheet painted with black board paint
Bottom
2.5 cm glass wool
Sides
2.5 cm thermacol
34
3
S. Kothari et al.
Performance of developed dryer
The tests were conducted to know the performance of developed solar dryer from 12th to 25th of March, 2006 at solar yard, Department of Renewable Energy Sources, CTAE, Udaipur. The full load performance of dryer without recirculation and with recirculation of exhaust air was evaluated thrice and then averaged. At the time of evaluation of dryer the recirculation ratio was maintained approximately at 60%.
3.1 No load test 3.1.1 Without recirculation of exhaust air The maximum temperature attained at the end of collector and dryer was 60.5°C and 63.9°C at 13.00 hrs, when the insolation was also high, i.e., 744 W m–2. During the test, the mass flow of air varies between 122 and 172 kg h–1 depending upon the insolation, as the fan was operated on SPV panel. The average temperature at the collector outlet, dryer inlet and dryer outlet were 51.25°C, 51.81°C and 55.28°C respectively. During the test, the average mass flow rate of exhausted air was 149 kg h–1. From Figure 2, it can be inferred that the temperature rise is the function of solar insolation and ambient temperature and time dependent. Figure 2
Performance curve for no load without recirculation (see online version for colours) Ta wn Tdi wn Rho wn I wn
70
600
50
Radiation, W/m 2
Temperature, 0C
Relative humidity, %
60
Tco 800 Tdo wn Rhi wn 700
500
40
400 30
300
20
200
10
100
0
0 10:00
11:00
12:00
13:00 14:00 Time
15:00
16:00
17:00
3.1.2 With recirculation of exhaust air The maximum temperature attained at the end of collector and dryer was 70.1°C and 73.5°C at 14.00 hrs, when the insolation was 802 W m–2 (see Figure 3). During the test, the mass flow of air exhausted and recirculated air was 51–71 kg h–1 and 75–105 kg h–1 respectively, depending upon the insolation. The average temperature at collector outlet, dryer inlet and dryer outlet were 58.12°C, 58.43°C and 62.96°C respectively.
Performance evaluation of exhaust air recirculation system Performance curve for no load with recirculation (see online version for colours) Ta Tdi RHi Radiation
80
Tco Tdo RHo
900 800
C
0
Temperature,
Relative humidity, %
70
700
60
600
50
500 40 400 30
300
20
Radiation, W/m 2
Figure 3
35
200
10
100
0
0 10:00
11:00
12:00
13:00
14:00 Time
15:00
16:00
17:00
3.2 Full load testing 3.2.1 Without recirculation of exhaust air The full load testing without recirculation of the exhaust air was repeated thrice and average performance is presented in Figure 4. It was observed that during the full load without recirculation test, the maximum temperature at ambient, collector outlet, dryer inlet and dryer outlet was 34.63°C, 60.83°C, 61.03°C and 58.80°C respectively. The maximum solar insolation was 974 W/m2 at 13.00 hrs. The average temperature at collector outlet, dryer inlet and dryer outlet were 53.88°C, 54.44°C and 52.08°C respectively. The mass flow of air was in the range of 93–179 kg/hr with an average of 125 kg h–1 depending on solar insolation. It was observed that the dryer outlet temperature was lower than that of dryer inlet temperature and the relative humidity of the dryer outlet was more than the dryer inlet. This was due to addition of water vapours into heated air. In the last hours of drying, the difference between dryer inlet and outlet temperature and relative humidity decreased. It was an indication of completion of drying of onion flakes. Performance curve for full load without recirculation (see online version for colours) Ta Tdi RHi I
70 60
Tco Tdo RHo
1100 1000 900 800
50
700 40
600 500
30
400 20
300 200
10
100
0
0 10:00
11:00
12:00
13:00 14:00 Time
15:00
16:00
17:00
Radiation, W/m 2
Temperature, 0C Relative humidity, %
Figure 4
36
S. Kothari et al.
3.2.2 With recirculation of exhaust air The full load testing with recalculating the exhaust air was conducted thrice and averaged data are represented in Figure 5. During the full load with recirculation test, the maximum temperature at ambient, collector outlet, dryer inlet and dryer outlet were 33.60°C, 64.50°C, 64.20°C and 59.10°C respectively. The maximum solar insolation was 974 W m–2 at 13.00 hrs. The average temperature at collector outlet, dryer inlet and dryer outlet were 56.84°C, 56.93°C and 52.03°C respectively. During the test, the mass flow rate of exhausted and recirculated air was 54–75 kg h–1 and 75–116 kg h–1 respectively. It was observed that the average temperature at various points during this test was more than that of full load without recirculation test. This was due to the partial recirculation of preheated air and low mass of air exhausted from dryer. Performance curve for full load with recirculation (see online version for colours) Ta Tdi RHi I
90
C
0
Temperature,
Relative humidity, %
80
Tco Tdo RHo
1000 900
70
800
60
700
50
600
40
500 400
30
300
20
200
10
100
0
0 10:00 11:00
4
1100
Radiation,W/m 2
Figure 5
12:00
13:00 14:00 Time
15:00
16:00 17:00
Performance evaluation of drying unit
4.1 Variation in moisture content Average moisture content (db) with the recirculation of exhaust air and without recirculation of exhaust air test is presented in Figure 6. Within seven hours, the onion flakes were dried from moisture content of 85.5% to 6.77% (db) and 9.56% in without recirculation and with recirculation test respectively. This shows that the amount of moisture removed within seven hours in recirculation test was less than that with recirculation test. It can be inferred that the recirculation of exhaust air in dryer reduces the drying potential of air irrespective to temperature.
Performance evaluation of exhaust air recirculation system Figure 6
37
Variation in moisture content with respect to time 7
Wav Rav
6
MC (d.b.)
5 4 3 2 1 0 0
60
120
180
240
300
360
420
Time, min
4.2 Variation in hourly drying efficiency The average hourly drying efficiency in recirculation of exhaust air and with recirculation of exhaust air test is represented in Figure 7. In without recirculation air test, the drying efficiency varied in the range of 83% to 2% with an average of 23%. During the recirculation test, the drying efficiency varied in the range of 55% to 3% with an average of 19%. Figure 7
Variation in hourly drying efficiency with respect to time 90
Wav
Hourly drying efficiency
80
Rav
70 60 50 40 30 20 10 0 10:00
5
11:00
12:00
1:00 2:00 Time
3:00
4:00
5:00
Overall thermal efficiency
Overall thermal efficiency helps to find out effectiveness of dryer in actual use. The average thermal efficiency during without recirculation and with recirculation is presented in Figure 8. It was observed that during the without recirculation test, the
38
S. Kothari et al.
average thermal efficiency varied in the range of 18%–29% with an average of 23% while there is an average thermal efficiency range between 11%–15% with an average of 14% seen during with recirculation. The thermal efficiency in the case of without recirculation was 74% more than that of with recirculation. This was due to the lower temperature difference between collector inlet and dryer outlet and low mass flow rate of air due to recirculation of exhaust air. Figure 8
Variation in thermal efficiency with respect to time 35 Rav Thermal Efficiency, %
30
Wav
25 20 15 10 5 0 10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
Time, hr
6
Variation in HUF
The evaluation of recirculatory dryer can be done on the basis of HUF. HUF is the ratio of heat utilised to heat supplied. The variation of HUF with respect to time is shown in Figure 9. It was observed that during the without recirculation of exhaust air test and with recirculation of exhaust air test, the average HUF varied in the range of 0.08–0.17 with an average of 0.12 and 0.14–0.25 with an average of 0.19 respectively. The HUF in recirculation was 31% more than that of without recirculation. From the above figures, it can be inferred that though the drying efficiency decreases in recirculation, the HUF was increased. Variation in HUF with respect to time HUF Wav
HUF Rav
0.30 0.25 HUF
Figure 9
0.20 0.15 0.10 HUF Rav HUF Wav
0.05 0.00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 Time
Performance evaluation of exhaust air recirculation system
7
39
Quality evaluation of dried onion flakes
7.1 Water activity (aW) The water activity was measured with the Hygrolab 3 water activity meter.
7.2 Colour measurement The colour of dried flakes was measured with colourflex Hunterlab Colorimeter. From Tables 3 and 4, it can be inferred that there is no large difference in quality of onion flakes when dried without recirculation of exhaust air and with recirculation of exhaust air. Table 3
Water activity (aw) of dried onion flakes
Sr. no.
Test
Aw
1
W1
0.317
2
W2
0.357
3
W3
0.308
4
R1
0.460
5
R2
0.438
6
R3
0.414
Table 4
L-lightness colour values of dried product
Sr. no.
Test
L
1
W1
75.72
2
W2
75.50
3
W3
75.10
4
R1
75.08
5
R2
75.42
6
R3
74.98
8
Conclusions
1
Onion flakes were dried within seven hours from the moisture content of (db) 85.50% to 6.76% and 9.56% in the without recirculation and with recirculation test respectively. In other words, with recirculation of exhaust air, less moisture was removed per unit time.
2
The drying efficiency during without recirculation test and with recirculation was 23% and 19% respectively. The drying efficiency in without recirculation of exhaust air test was 21% more than that of during with recirculation of exhaust air test.
3
The thermal efficiency in without recirculation test and with recirculation was 23% and 14% respectively. The thermal efficiency in case of without recirculation was 74% more than that of with recirculation.
40
S. Kothari et al.
4
The HUF in recirculation was 31% more than that of without recirculation at an average recirculation ratio of 60%. So, the recirculation may be feasible for drying of low moisture content products.
5
The quality of dried onion flakes in without recirculation of exhaust air test was superior in terms of colour.
6
The recirculation of exhaust air is feasible only with use of desiccated material.
References Bala, B.K. and Woods, J.L. (1994) ‘Simulation of the indirect natural convection solar drying of rough rice’, Solar Energy, Vol. 53, No. 3, pp.259–266. Jain, D. (2005) ‘Modeling the performance of greenhouse with packed bed thermal storage on crop drying application’, Journal of Agricultural Engineering, Vol. 41, No. 3, pp.6–12. Jain, R.K., Shrivastav, P.P. and Das, H. (2003) ‘Dehydration characteristics of spinach in air recirculatory tray dryer’, Journal of Food Engineering, Vol. 31, pp.35–36. Joshi, S. and Philip, S.K. (1997) ‘Dehydration of onion using solar energy’, SESI, Vol. 7, No. 2, pp.95–102. Mangaraj, S., Singh, A., Samuel, D.V.K. and Singhal, O.P. (2001) ‘Comparative performance evaluation of different drying methods for chillies’, Journal of Food Science and Technology, Vol. 38, No. 3, pp.296–299. Philip, S.K. (1999) ‘Dehydration of agricultural products using solar energy’, Agricultural Engineering Today, Vol. 23, Nos. 3–4, pp.39–50. Philip, S.K., Sharma, S. and Rao, C.S. (1993) ‘Installation and performance monitoring of a commercial solar drier for chillies’, Proceedings of the National Solar Energy Convention, Roles of Renewable Energy held at Gujarat Energy Development Agency, Vadodara, Gujarat, December 11–13. Saleh, T. and Sarkar, M.A.R. (2002) ‘Performance study of a PV operated forced convection solar energy dryer’, A paper presented at the technical session of the 8th International Symposium for Renewable Energy Education (ISREE-8) held at Orlando, University of Florida, USA from August 4–8. Schirmer, P., Janjai, S., Esper, A., Smitabhindu, R. and Mühlbauer, W. (1996) ‘Experimental investigation of the performance of the solar drier for drying bananas’, Renewable Energy, Vol. 7, No. 2, pp.119–129. Schoenau, G.J. Arinze, E.A., Sokhansanj, S. and Trauttmansdorff, F.G. (1999) ‘Evaluation of energy conservation potential by exhaust air recirculation for a commercial type heated air batch type dryer’, Journal of Renewable Energy, Vol. 9, pp.676–681. Sharma V.K., Colangelo, A. and Spagna, G. (1995) ‘Experimental investigation of different solar dryers suitable for fruit and vegetable drying’, Energy, Vol. 6, No. 4, pp.413–424. Shawik, D., Tapash, D., Srinivasa, R. and Jain, R.K. (2001) ‘Development of an air recirculating tray dryer for high moisture biological materials’, Journal of Food Engineering, Vol. 50, No. 4, pp.223–227. Shukla, B.D and Singh, G. (2003) Drying and Dryers (Food and Agricultural Crops), Jain Brothers, New Delhi-11005.
Performance evaluation of exhaust air recirculation system
41
Nomenclature μm
Micron
DR
Drying rate, g of moisture evap./g of dry matter hr
G. I.
Galvanised iron
H1
Humidity ratio of ambient air, kg of water vapour per kg of dry air
H2
Humidity ratio of exhaust air, kg of water vapour per kg of dry air
It
Solar insolation, w m-2
m
Meter
MC
Moisture content
Rav
Average of parameters during recirculation air mode test
RH
Relative humidity (%)
RHi
Relative humidity of inlet air, %
RHo
Relative humidity of outlet air, %
Ta
Ambient temperature, °C
Tco
Temperature at collector outlet, °C
td
Assumed drying time, hours
Td
Drying temperature, °C
Tdi
Temperature at dryer inlet, °C
Tdo
Temperature at dryer outlet, °C
HUF
Heat utilisation factor
