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Performance of a solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices
Renewable Energy 30 (2005) 2085–2095 www.elsevier.com/locate/renene
Performance of a solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices S. Janjai*, P. Tung Solar Energy Research Laboratory, Department of Physics, Faculty of Science, Silpakorn University, Nakhon Pathom 73000, Thailand Received 1 December 2004; accepted 9 February 2005 Available online 19 March 2005
Abstract A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors was developed. The dryer is a bin type with a rectangular perforated floor. The bin has a dimension of 1.0 m!2.0 m!0.7 m. Hot air is supplied to the dryer from fiberglass-covered solar collectors, which also function as the roof of a farmhouse. The total area of the solar collectors is 72 m2. To investigate its performance, the dryer was used to dry four batches of rosella flowers and three batches of lemon-grasses during the year 2002–2003. The dryer can be used to dry 200 kg of rosella flowers and lemon-grasses within 4 and 3 days, respectively. The products being dried in the dryer were completely protected from rains and insects and the dried products are of high quality. The solar air heater has an average daily efficiency of 35% and it performs well both as a solar collector and a roof of a farmhouse. q 2005 Elsevier Ltd. All rights reserved. Keywords: Solar energy; Solar dryer; Herbs; Spices; Solar collector
1. Introduction Drying is one of the most important post-harvest operations for herbs and spices. It is mainly aimed to reduce the moisture content for preservation. For some spices such as chili and pepper, drying is not only for preservation purposes but also for modifying * Corresponding author and WREN member. Tel.: C66 34 270761; fax: C66 34 271189. E-mail address: [email protected] (S. Janjai).
0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.02.006
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the tastes and flavors in order to increase their market values. In developing countries, natural sun drying method is commonly used for drying herbs and spices. Although negligible investment is required by this method, products being dried are usually contaminated by insects, birds and dusts. In the case of Thailand, most of herbs and spices are still dried with the natural sun drying method. Due to rewetting of the products during drying by rain and too slow drying rate in the rainy season, toxic substances such as an alphatoxil produced by molds is often found in the dried products. This is one of the main problems obstructing the growth of exports of herbs and spices to international markets. Situated near the equator, Thailand receives relatively high solar radiation [1]. Consequently, the utilization of a solar drying technology is considered to be an alternative solution to the problem of drying agricultural products in this country. Although many types of solar dryers have been developed during the last two decades [2–8], their applications are still limited, mainly due to their unreliable performance and high investment cost relative to a production capacity. A reduction of losses, an improvement of quality of product and an investment cost are also important criteria dictating the adoption of the solar dryer. A number of solar dryers do not meet these criteria. Therefore, development of a well-performed solar dryer is of significant economic importance. As farmers usually have a farmhouse with galvanized iron sheets as a roof for use in various agricultural activities, with a proper design it is feasible to use this roof to produce hot air for drying agricultural products. Such a drying system will provide space for the solar collectors and reduce the total investment cost. The objectives of this work are to develop a solar dryer using hot air from roofintegrated solar collectors for drying herbs and spices and to investigate its performance.
2. Description of the dryer A farmhouse with a working area of 8!9 m2 was constructed at Silpakorn University located at Nakhon Pathom (13.88N, 103.88E), Thailand. The farmhouse is oriented in the east–west direction. The top structure of the farmhouse was designed in such a manner that N W
1
E S 2
2.5 m 3 4 8.0
m
5 4.5 m
4.5 m
Fig. 1. Schematic diagram of the solar dryer using hot air from a roof-integrated solar collectors: (1) south-facing solar collectors, (2) north-facing solar collectors, (3) horizontal air duct, (4) vertical air duct, (5) dryer.
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1 2
0.125 m 0.125 m 4.5 m
1.0 3
m
4
Fig. 2. Schematic diagram of the solar collector: (1) corrugated fiberglass cover, (2) iron frame, (3) back insulator, (4) air channel.
it can support two arrays of solar collectors, one on the south-facing structure and the other on the north-facing structure. Sixteen solar collectors, each of which has an area of 1.0!4.5 m2 were constructed and placed on the top structure of this farmhouse, as shown in Fig. 1. Each solar collector consists of iron frames, a back insulator and a transparent corrugated fiberglass cover, as shown in Fig. 2. The back insulator is made of glass wool sandwiched between two galvanized iron sheets. The upper sheets of the insulator are painted black to absorb solar radiation. Ambient air is sucked to pass through an air channel between the back insulator and the cover. These solar collectors function both as a hot air generator and a roof of the farmhouse. The solar collectors have a total area of 72 m2 with a tilted angle of 58. A rectangular air duct is placed horizontally at the middle of the roof to collect hot air from the two arrays of the solar collectors. Another cylindrical air duct is vertically connected to the middle of the rectangular air duct to transport the air from the solar collectors to the air distributor box of the dryer. As most of spices and herbs are bulky products, a bin-type dryer was selected for this work. The dimension of a product container of the dryer is 1.0 m!2.0 m!0.7 m. The bottom side of the container is made of a rectangular perforated plastic plate and the top side is open for loading the products to be dried and for allowing moist air to leave
0.7 m 4 1
0.5 m 3
2
2.0 m
5
1.0 m 1.0 m
Fig. 3. Schematic diagram of the dryer: (1) air duct from the collectors, (2) air distribution box, (3) fan, (4) product container, (5) space underneath the product container.
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the dryer. To obtain a uniform air flow distribution, a 2-HP axial fan is installed at the wall separating the air distribution box and the space underneath the product container (see Fig. 3.). The walls of the dryer are made of high-density foam sandwiched between two aluminum sheets to reduce heat losses. Hot air from the collectors is sucked by this fan into the box and then blown through the products in the dryer. 3. Investigation of the performance of the dryer The performance of the dryer was investigated by means of full-scale tests of the dryer. The instruments and the method used for the experiments are as follows. 3.1. Instruments To investigate effects of environmental and operating parameters on the performance of the dryer, various measuring devices were employed. Two pyranometers (Kipps and Zonen, model CM3 and CM5) were placed on the solar collectors to measure solar radiation, one on the north-facing side and the other on the south-facing side of the roof. Thermocouples of type K were used to measure air temperatures in the collectors, air ducts and drying bin. Hot wire anemometers (Airflow, model TA5) were employed to monitor the air speed in the collector and in the air ducts. This anemometer was also used to monitor the ambient wind speed. Relative humidities of ambient air and drying air were periodically measured with a hygrometer (Defensor, model MS1). Voltage signals from the pyranometers and thermocouples were recorded every 10 min by a 20-channel data logger (Yokogawa, model DC100). For relative humidity, it was manually read and recorded at 3-h intervals. The air speeds in the solar collectors and the air ducts were also manually read and recorded 2–3 times during the drying experiments. Samples of products in the dryer were weighed at 3-h intervals using a digital balance (Satorius, model E2000 D). Before the installations, the pyranometers were calibrated against a new pyranometer recently calibrated by Kipps & Zonen, the manufacturer. For the hygrometer, it was calibrated using standard saturated salt solutions supplied by Defensor, the producer. Before being used, the thermocouples were also tested by measuring the boiling and freezing temperatures of water to ensure the accuracy. 3.2. Materials Materials used for drying tests were rosella flowers and lemon-grasses. In general, dried rosella flowers are used to make rosella juice by boiling them with water. It is a medicinal plant whose constituents are believed to help reduce high blood pressure. Lemon-grasses are normally employed as spices, both in fresh and dried forms in many Asian foods. Recently, dried lemon-grasses have also been used as a spice in white sausages in Germany. Rosella plants are grown mainly in the central region of Thailand and their harvesting season is from November to December. These months are in the dry season with clear days and low relative humidity of ambient air. This weather condition is
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favorable for solar drying. For lemon-grasses, they are cultivated in all regions of the country and they are harvested all the year around. For the drying testes, 200 kg of fresh rosella flowers with an initial moisture content of about 90% (wb) was used for each drying test. Before drying, their seeds were manually taken out. For the lemon-grasses, only the stems were employed for the drying tests. They were cut with the thickness of about 2 mm. Approximately 200 kg of fresh lemon-grasses with an initial moisture content of 70% (wb) was prepared for the drying tests. 3.3. Method Four drying tests were carried out for rosella flowers in November–December 2002. For lemon-grasses, three tests were undertaken in February 2003. For all tests, the dryer was manually loaded with the products to be dried in the morning and the fan was started at about 8 a.m. and it was stopped at 5 p.m. The fan was started again in the next morning and the process was repeated until final moistures of 16% (wb) for rosella flowers and 6% (wb) for lemon-grasses were reached. During the drying test, 100 g of the product sample was taken from the dryer and weighed at 3-h intervals. At the end of the drying process, this sample was dried in an oven at 103 8C for 24 h to determine the moisture contents of the products. During the drying process, the products in the dryer were stirred manually two times per day to ensure uniform drying of the products.
4. Results and discussion 4.1. Performance of the solar collectors The transmittance of the corrugated fiberglass cover was measured by using two pyranometers placed above and underneath the fiberglass cover. The transmittance of a new fiberglass is 0.60. After 3 years of use, its transmittance decreased to 0.58. However, it needed to be cleaned from time to time to eliminate deposits of dusts. The typical temperature profile of air flowing in the collectors is shown in Fig. 4. The air temperature increased rapidly in the first half of the collector length and increased slightly in the second half. For a typical drying test, the average daily efficiency of the collectors was found to be 35%. As the collectors also function as a roof of a farmhouse, no leakage of water has been observed from these collectors during rain. 4.2. Performance of the solar dryer 4.2.1. Rosella flower drying Rosella flowers were dried in the months of November and December 2002. As these months are in the dry season, most of the days are sunny with sporadic clouds. The variations of solar radiation on the south-facing collectors and the north-facing collectors are shown in Fig. 5. The south-facing collectors received higher radiation than that of the north-facing collectors. This is due to the fact that in December the sun is in the south of the celestial equator, causing a smaller incident angle of solar radiation on the south-facing
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Temperature ( ˚C)
80
60
40 South-facing collector
20
0 0
1
2 3 Distance from the inlet (m)
4
5
Temperature (˚C)
80
60
40 North-facing collector
20
0 0
1
2
3
4
5
Distance from the inlet (m) Fig. 4. Temperature profile along the length of the collector at 12:00 a.m. on 28 December 2002.
Global radiation (w/m2)
1200 North-facing collectors
1000 800
South-facing collectors
26 Dec, 2002
27 Dec, 2002
28 Dec, 2002
29 Dec, 2002
600 400 200 0 800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
time Fig. 5. Global solar radiation incident on the south-facing collectors and the north-facing collectors for the drying test of rosella flowers.
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collectors compared to that of the north-facing collectors. With the orientation and tilted angle of these collectors, the north-facing collectors and the south-facing collectors receive alternatively more and less radiation, compared to each other during a year. However, the difference in the amount of energy received is small during most of the time in a year. This is because of the fact that Thailand is situated near the equator with the sun near the zenith year round and the tilted angle of the collectors is small. The small difference in incident radiation resulted in a slight difference in outlet air temperature from the collectors as shown in Fig. 6. It is observed that the temperature rise above the ambient air is in the range of 5–20 8C during 9 a.m.–5 p.m. When air from the collector outlet was forced through the air ducts to the perforated floor of the product container, its temperature was dropped by a few degrees, due to heat losses. For most cases, the relative humidity of the air flowing into the dryer varied in the range of 25–35% during 9 a.m.–5 p.m. Considering the temperature and the relative humidity of the air entering the drying bin, this drying air has higher drying potentials for drying the rosella flower, compared to ambient air. The drying air was forced through the rosella flower with an air speed of 0.1 m/s. The pressure drop of the drying air is in the range of 40–80 Pa, depending on the depth of the bed of the products. For 200 kg of fresh rosella flowers, its bed depth decreased from 80 cm on the first day to 50 cm on the fourth day or the last day of drying, due to a shrinkage of the rosella flowers. For clear sky weather conditions, the moisture content of the rosella flowers in the drying bin was reduced from an initial value of 92% (wb) to the final value of 16% (wb) within 4 days or with an effective drying time of approximately 30 solar hours as shown in Fig. 7. From Fig. 7, it was observed that the moisture content slowly decreased on the first and the second day, then rapidly decreased on the third day and slowly again on the fourth day. This can be explained as follows. The rosella flowers have a natural wax coated on their surfaces. This wax prevents most of the migration of moisture from the inside of the flowers into the drying air. After the surface is dried the wax is broken, the moisture from inside can be easily released, thus increasing the drying rate on the third day. For the last
Temperature (˚C)
80 Outlet air from the north-facing collectors Outlet air from the south-facing collectors Ambient air
60 40 20
26 Dec, 2002
27 Dec, 2002
28 Dec, 2002
29 Dec, 2002
0 8:00 11:00 14:00 17:00 8:00 11:00 14:00 17:00 8:00 11:00 14:00 17:00 8:00 11:00 14:00 17:00
time Fig. 6. Variation of the temperature of the outlet air from collectors and the temperature of ambient air for the drying test of rosella flowers.
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Moisture content, % (w.b)
100 28 Dec, 2002
80 26 Dec, 2002
29 Dec, 2002
27 Dec, 2002
60 40 20 0 800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
time Fig. 7. Variation of the moisture content of rosella flowers of the drying test during 26–29 December 2002.
day, the drying rate is slow again because most of water to be evaporated is in the monolayer or multi-layer water with a high binding energy [9]. The color of the dried rosella flowers, which is a common quality indicator, was measured by a color-view spectrophotometer. It was found that the color of the rosella flowers dried with this dryer is comparable to that of a high-quality dried rosella flowers in markets. 4.2.2. Lemon-grasses drying The dryer was also used to dry three batches of lemon-grasses in March 2003. This month is in the dry season with a lot of clear sky days. As in March, the sun is near the celestial equator, the south-facing collector received more radiation than that of the northfacing collector as depicted in Fig. 8. This difference in the incident radiation resulted in the difference in the outlet air temperature of both collectors as shown in Fig. 9. This hot and dry air from the collector was used to reduce the moisture content of lemon-grasses in the dryer from 70 to 6% (wb) within 3 days. A typical drying curve of the lemon-grasses is shown in Fig. 10. On the first day, the moisture decreased rapidly because the lemon-grasses in 1200
Global radiation (W/m2)
North-facing collectors
1000
South-facing collectors
800 600 400 15 Mar,2003
16 Mar,2003
17 Mar,2003
200 0 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00
time Fig. 8. Global solar radiation incident on the south-facing collectors and the north-facing collectors for the drying test of lemon-grasses.
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Temperature (˚C)
120
2093
Outlet air from the north-facing collectors Outlet air from the south-facing collectors Ambient air
100 80 60 40 20
15 Mar, 2003
16 Mar, 2003
17 Mar, 2003
0 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00
time Fig. 9. Variation of the temperature of the outlet air from the collectors and the temperature of ambient air for the drying test of lemon-grasses.
the drying bin had been cut into small pieces, allowing the moisture to be released easily on the first day. For the following days, the moisture content decreased as the lemon-grasses were in the falling rate phase of drying. Volatile oil, which is a main quality indicator, was extracted from samples of the lemon-grasses dried with this dryer. The average of the oil content of 2.8% (by weight) was obtained. This corresponds to a high-quality dried lemon-grasses. 4.3. Economic evaluation To evaluate the economics of the roof-integrated solar dryer, the dryer is assumed to be used for 10 months (January–October) for drying lemon-grasses and 2 months (November–December) for drying rosella flowers, with one batch of drying run per week. The economic benefit of the collectors functioning as the roof of the farmhouse was not taken into account for this evaluation. Based on the economic situation in Thailand, the costs and economic parameters are shown in Table 1. With these data the investment rate of return (IRR) and the pay-back period (PBP) were calculated using the method described in [10]. It was found that the IRR and PBP are
Moisture content, % (w.b)
80
60
40
17 Mar, 2003
16 Mar, 2003
15 Mar, 2003
20
0 8:00
11:00
14:00
17:00
8:00
11:00
14:00
17:00
8:00
11:00
14:00
17:00
time Fig. 10. Variation of the moisture content of lemon-grasses of the drying test during 15–17 March 2003.
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Table 1 Cost and economic parameters Material cost for the construction of the dryer and the structure of the farmhouse Labor cost for the construction Interest rate Life span of the dryer Electricity cost Salvage value of the dryer Labor cost for the preparation of raw materials (rosella flowers and lemongrasses) Cost of fresh rosella flowers Cost of fresh lemon-grasses Sale price of dried rosella flower Sale price of dried lemon-grasses
US$3125 US$200 7% 15 years US$0.075/kWh US$200 US$5/batch US$50/batch US$40/batch US$94/batch US$116/batch
70.3% and 3.9 years, respectively. This demonstrates that the investment in this dryer is economically promising for use in Thailand.
5. Conclusions A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors has been developed and tested. The dryer was used to dry rosella flower and lemon-grasses. With this dryer, 200 kg of rosella flowers and lemon-grasses can be dried within 4 and 3 days, respectively. The products being dried in this dryer are completely protected from rains and insects. Dried products of high quality are obtained. The solar collectors performed well as both the air heater and a roof of a farmhouse. The average daily efficiency of the collectors is 35%. However, the solar collectors have to be cleaned from time to time to eliminate the deposits of dusts. Based on the economic situation in Thailand, the IRR and the PBP of this dryer are found to be 70.3% and 3.9 years, respectively.
Acknowledgements The authors would like to thank the National Energy Policy Office (NEPO) for the financial support to carry out this research work. The authors would also like to acknowledge the assistance of Mr W. Makcharoen in conducting the drying experiments. Valuable advice from Prof. B.K. Bala is also gratefully acknowledged.
References [1] Janjai S, Laksanaboonsong J, Nunez M, Thongsathitya A. Development of a method for generating operational solar radiation maps from satellite data for a tropical environment. Solar Energy 2005 (in press). [2] Bala BK, Mondol MRA, Biswas BK, Chowdury BL, Janjai S. Solar drying of pineapple using solar tunnel drier. Renewable Energy 2003;28:183–90.
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[3] Schirmer P, Janjai S, Esper A, Smitabhindu R, Muhlbauer W. Experimental investigation of the performance of the solar tunnel dryer for drying bananas. Renewable Energy 1996;7(2):119–29. [4] Soponronnarit S, Assayo M, Rakwichian W. Performance of a solar banana dryer. RERIC Int Energy J 1991;13:71–9. [5] Sodha MS, Bansal NK, Kumar A, Bansal PK, Malik MAS. Solar crop drying. vol. 1. Boca Raton, FL: CRC Press; 1987. [6] Boon-Long P, Hirun A, Siratanapanta T, Siriplapla P, Sitthipong N, Sucharitakul P, Therdtoon P, Rerkkringkrai P. Solar-assisted tobacco curing. Proceedings of the regional seminar on solar drying, Yogyakarta, Indonesia; 1984. p. 85–95. [7] Wibulsawas P, Thaina S. Comparative performance of a solar cabinet dryer. Proceedings of a workshop on fuel and power. France: Universite de Bordeaux; 1980. p. 1–14. [8] Exell RHB, Kornsakoo S. A low-cost solar rice dryer. Appropriate Technol 1976;5:23–5. [9] Anon.. Principes generaux du sechage. Perspect Agricoles 1977;6:6–29. [10] Thuesen GJ, Fabrycky WJ. Engineering economics. New York: Prentice Hall; 2001.
Performance of a solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices S. Janjai*, P. Tung Solar Energy Research Laboratory, Department of Physics, Faculty of Science, Silpakorn University, Nakhon Pathom 73000, Thailand Received 1 December 2004; accepted 9 February 2005 Available online 19 March 2005
Abstract A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors was developed. The dryer is a bin type with a rectangular perforated floor. The bin has a dimension of 1.0 m!2.0 m!0.7 m. Hot air is supplied to the dryer from fiberglass-covered solar collectors, which also function as the roof of a farmhouse. The total area of the solar collectors is 72 m2. To investigate its performance, the dryer was used to dry four batches of rosella flowers and three batches of lemon-grasses during the year 2002–2003. The dryer can be used to dry 200 kg of rosella flowers and lemon-grasses within 4 and 3 days, respectively. The products being dried in the dryer were completely protected from rains and insects and the dried products are of high quality. The solar air heater has an average daily efficiency of 35% and it performs well both as a solar collector and a roof of a farmhouse. q 2005 Elsevier Ltd. All rights reserved. Keywords: Solar energy; Solar dryer; Herbs; Spices; Solar collector
1. Introduction Drying is one of the most important post-harvest operations for herbs and spices. It is mainly aimed to reduce the moisture content for preservation. For some spices such as chili and pepper, drying is not only for preservation purposes but also for modifying * Corresponding author and WREN member. Tel.: C66 34 270761; fax: C66 34 271189. E-mail address: [email protected] (S. Janjai).
0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.02.006
2086
S. Janjai, P. Tung / Renewable Energy 30 (2005) 2085–2095
the tastes and flavors in order to increase their market values. In developing countries, natural sun drying method is commonly used for drying herbs and spices. Although negligible investment is required by this method, products being dried are usually contaminated by insects, birds and dusts. In the case of Thailand, most of herbs and spices are still dried with the natural sun drying method. Due to rewetting of the products during drying by rain and too slow drying rate in the rainy season, toxic substances such as an alphatoxil produced by molds is often found in the dried products. This is one of the main problems obstructing the growth of exports of herbs and spices to international markets. Situated near the equator, Thailand receives relatively high solar radiation [1]. Consequently, the utilization of a solar drying technology is considered to be an alternative solution to the problem of drying agricultural products in this country. Although many types of solar dryers have been developed during the last two decades [2–8], their applications are still limited, mainly due to their unreliable performance and high investment cost relative to a production capacity. A reduction of losses, an improvement of quality of product and an investment cost are also important criteria dictating the adoption of the solar dryer. A number of solar dryers do not meet these criteria. Therefore, development of a well-performed solar dryer is of significant economic importance. As farmers usually have a farmhouse with galvanized iron sheets as a roof for use in various agricultural activities, with a proper design it is feasible to use this roof to produce hot air for drying agricultural products. Such a drying system will provide space for the solar collectors and reduce the total investment cost. The objectives of this work are to develop a solar dryer using hot air from roofintegrated solar collectors for drying herbs and spices and to investigate its performance.
2. Description of the dryer A farmhouse with a working area of 8!9 m2 was constructed at Silpakorn University located at Nakhon Pathom (13.88N, 103.88E), Thailand. The farmhouse is oriented in the east–west direction. The top structure of the farmhouse was designed in such a manner that N W
1
E S 2
2.5 m 3 4 8.0
m
5 4.5 m
4.5 m
Fig. 1. Schematic diagram of the solar dryer using hot air from a roof-integrated solar collectors: (1) south-facing solar collectors, (2) north-facing solar collectors, (3) horizontal air duct, (4) vertical air duct, (5) dryer.
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1 2
0.125 m 0.125 m 4.5 m
1.0 3
m
4
Fig. 2. Schematic diagram of the solar collector: (1) corrugated fiberglass cover, (2) iron frame, (3) back insulator, (4) air channel.
it can support two arrays of solar collectors, one on the south-facing structure and the other on the north-facing structure. Sixteen solar collectors, each of which has an area of 1.0!4.5 m2 were constructed and placed on the top structure of this farmhouse, as shown in Fig. 1. Each solar collector consists of iron frames, a back insulator and a transparent corrugated fiberglass cover, as shown in Fig. 2. The back insulator is made of glass wool sandwiched between two galvanized iron sheets. The upper sheets of the insulator are painted black to absorb solar radiation. Ambient air is sucked to pass through an air channel between the back insulator and the cover. These solar collectors function both as a hot air generator and a roof of the farmhouse. The solar collectors have a total area of 72 m2 with a tilted angle of 58. A rectangular air duct is placed horizontally at the middle of the roof to collect hot air from the two arrays of the solar collectors. Another cylindrical air duct is vertically connected to the middle of the rectangular air duct to transport the air from the solar collectors to the air distributor box of the dryer. As most of spices and herbs are bulky products, a bin-type dryer was selected for this work. The dimension of a product container of the dryer is 1.0 m!2.0 m!0.7 m. The bottom side of the container is made of a rectangular perforated plastic plate and the top side is open for loading the products to be dried and for allowing moist air to leave
0.7 m 4 1
0.5 m 3
2
2.0 m
5
1.0 m 1.0 m
Fig. 3. Schematic diagram of the dryer: (1) air duct from the collectors, (2) air distribution box, (3) fan, (4) product container, (5) space underneath the product container.
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the dryer. To obtain a uniform air flow distribution, a 2-HP axial fan is installed at the wall separating the air distribution box and the space underneath the product container (see Fig. 3.). The walls of the dryer are made of high-density foam sandwiched between two aluminum sheets to reduce heat losses. Hot air from the collectors is sucked by this fan into the box and then blown through the products in the dryer. 3. Investigation of the performance of the dryer The performance of the dryer was investigated by means of full-scale tests of the dryer. The instruments and the method used for the experiments are as follows. 3.1. Instruments To investigate effects of environmental and operating parameters on the performance of the dryer, various measuring devices were employed. Two pyranometers (Kipps and Zonen, model CM3 and CM5) were placed on the solar collectors to measure solar radiation, one on the north-facing side and the other on the south-facing side of the roof. Thermocouples of type K were used to measure air temperatures in the collectors, air ducts and drying bin. Hot wire anemometers (Airflow, model TA5) were employed to monitor the air speed in the collector and in the air ducts. This anemometer was also used to monitor the ambient wind speed. Relative humidities of ambient air and drying air were periodically measured with a hygrometer (Defensor, model MS1). Voltage signals from the pyranometers and thermocouples were recorded every 10 min by a 20-channel data logger (Yokogawa, model DC100). For relative humidity, it was manually read and recorded at 3-h intervals. The air speeds in the solar collectors and the air ducts were also manually read and recorded 2–3 times during the drying experiments. Samples of products in the dryer were weighed at 3-h intervals using a digital balance (Satorius, model E2000 D). Before the installations, the pyranometers were calibrated against a new pyranometer recently calibrated by Kipps & Zonen, the manufacturer. For the hygrometer, it was calibrated using standard saturated salt solutions supplied by Defensor, the producer. Before being used, the thermocouples were also tested by measuring the boiling and freezing temperatures of water to ensure the accuracy. 3.2. Materials Materials used for drying tests were rosella flowers and lemon-grasses. In general, dried rosella flowers are used to make rosella juice by boiling them with water. It is a medicinal plant whose constituents are believed to help reduce high blood pressure. Lemon-grasses are normally employed as spices, both in fresh and dried forms in many Asian foods. Recently, dried lemon-grasses have also been used as a spice in white sausages in Germany. Rosella plants are grown mainly in the central region of Thailand and their harvesting season is from November to December. These months are in the dry season with clear days and low relative humidity of ambient air. This weather condition is
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favorable for solar drying. For lemon-grasses, they are cultivated in all regions of the country and they are harvested all the year around. For the drying testes, 200 kg of fresh rosella flowers with an initial moisture content of about 90% (wb) was used for each drying test. Before drying, their seeds were manually taken out. For the lemon-grasses, only the stems were employed for the drying tests. They were cut with the thickness of about 2 mm. Approximately 200 kg of fresh lemon-grasses with an initial moisture content of 70% (wb) was prepared for the drying tests. 3.3. Method Four drying tests were carried out for rosella flowers in November–December 2002. For lemon-grasses, three tests were undertaken in February 2003. For all tests, the dryer was manually loaded with the products to be dried in the morning and the fan was started at about 8 a.m. and it was stopped at 5 p.m. The fan was started again in the next morning and the process was repeated until final moistures of 16% (wb) for rosella flowers and 6% (wb) for lemon-grasses were reached. During the drying test, 100 g of the product sample was taken from the dryer and weighed at 3-h intervals. At the end of the drying process, this sample was dried in an oven at 103 8C for 24 h to determine the moisture contents of the products. During the drying process, the products in the dryer were stirred manually two times per day to ensure uniform drying of the products.
4. Results and discussion 4.1. Performance of the solar collectors The transmittance of the corrugated fiberglass cover was measured by using two pyranometers placed above and underneath the fiberglass cover. The transmittance of a new fiberglass is 0.60. After 3 years of use, its transmittance decreased to 0.58. However, it needed to be cleaned from time to time to eliminate deposits of dusts. The typical temperature profile of air flowing in the collectors is shown in Fig. 4. The air temperature increased rapidly in the first half of the collector length and increased slightly in the second half. For a typical drying test, the average daily efficiency of the collectors was found to be 35%. As the collectors also function as a roof of a farmhouse, no leakage of water has been observed from these collectors during rain. 4.2. Performance of the solar dryer 4.2.1. Rosella flower drying Rosella flowers were dried in the months of November and December 2002. As these months are in the dry season, most of the days are sunny with sporadic clouds. The variations of solar radiation on the south-facing collectors and the north-facing collectors are shown in Fig. 5. The south-facing collectors received higher radiation than that of the north-facing collectors. This is due to the fact that in December the sun is in the south of the celestial equator, causing a smaller incident angle of solar radiation on the south-facing
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Temperature ( ˚C)
80
60
40 South-facing collector
20
0 0
1
2 3 Distance from the inlet (m)
4
5
Temperature (˚C)
80
60
40 North-facing collector
20
0 0
1
2
3
4
5
Distance from the inlet (m) Fig. 4. Temperature profile along the length of the collector at 12:00 a.m. on 28 December 2002.
Global radiation (w/m2)
1200 North-facing collectors
1000 800
South-facing collectors
26 Dec, 2002
27 Dec, 2002
28 Dec, 2002
29 Dec, 2002
600 400 200 0 800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
time Fig. 5. Global solar radiation incident on the south-facing collectors and the north-facing collectors for the drying test of rosella flowers.
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collectors compared to that of the north-facing collectors. With the orientation and tilted angle of these collectors, the north-facing collectors and the south-facing collectors receive alternatively more and less radiation, compared to each other during a year. However, the difference in the amount of energy received is small during most of the time in a year. This is because of the fact that Thailand is situated near the equator with the sun near the zenith year round and the tilted angle of the collectors is small. The small difference in incident radiation resulted in a slight difference in outlet air temperature from the collectors as shown in Fig. 6. It is observed that the temperature rise above the ambient air is in the range of 5–20 8C during 9 a.m.–5 p.m. When air from the collector outlet was forced through the air ducts to the perforated floor of the product container, its temperature was dropped by a few degrees, due to heat losses. For most cases, the relative humidity of the air flowing into the dryer varied in the range of 25–35% during 9 a.m.–5 p.m. Considering the temperature and the relative humidity of the air entering the drying bin, this drying air has higher drying potentials for drying the rosella flower, compared to ambient air. The drying air was forced through the rosella flower with an air speed of 0.1 m/s. The pressure drop of the drying air is in the range of 40–80 Pa, depending on the depth of the bed of the products. For 200 kg of fresh rosella flowers, its bed depth decreased from 80 cm on the first day to 50 cm on the fourth day or the last day of drying, due to a shrinkage of the rosella flowers. For clear sky weather conditions, the moisture content of the rosella flowers in the drying bin was reduced from an initial value of 92% (wb) to the final value of 16% (wb) within 4 days or with an effective drying time of approximately 30 solar hours as shown in Fig. 7. From Fig. 7, it was observed that the moisture content slowly decreased on the first and the second day, then rapidly decreased on the third day and slowly again on the fourth day. This can be explained as follows. The rosella flowers have a natural wax coated on their surfaces. This wax prevents most of the migration of moisture from the inside of the flowers into the drying air. After the surface is dried the wax is broken, the moisture from inside can be easily released, thus increasing the drying rate on the third day. For the last
Temperature (˚C)
80 Outlet air from the north-facing collectors Outlet air from the south-facing collectors Ambient air
60 40 20
26 Dec, 2002
27 Dec, 2002
28 Dec, 2002
29 Dec, 2002
0 8:00 11:00 14:00 17:00 8:00 11:00 14:00 17:00 8:00 11:00 14:00 17:00 8:00 11:00 14:00 17:00
time Fig. 6. Variation of the temperature of the outlet air from collectors and the temperature of ambient air for the drying test of rosella flowers.
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Moisture content, % (w.b)
100 28 Dec, 2002
80 26 Dec, 2002
29 Dec, 2002
27 Dec, 2002
60 40 20 0 800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
800
1100 1400 1700
time Fig. 7. Variation of the moisture content of rosella flowers of the drying test during 26–29 December 2002.
day, the drying rate is slow again because most of water to be evaporated is in the monolayer or multi-layer water with a high binding energy [9]. The color of the dried rosella flowers, which is a common quality indicator, was measured by a color-view spectrophotometer. It was found that the color of the rosella flowers dried with this dryer is comparable to that of a high-quality dried rosella flowers in markets. 4.2.2. Lemon-grasses drying The dryer was also used to dry three batches of lemon-grasses in March 2003. This month is in the dry season with a lot of clear sky days. As in March, the sun is near the celestial equator, the south-facing collector received more radiation than that of the northfacing collector as depicted in Fig. 8. This difference in the incident radiation resulted in the difference in the outlet air temperature of both collectors as shown in Fig. 9. This hot and dry air from the collector was used to reduce the moisture content of lemon-grasses in the dryer from 70 to 6% (wb) within 3 days. A typical drying curve of the lemon-grasses is shown in Fig. 10. On the first day, the moisture decreased rapidly because the lemon-grasses in 1200
Global radiation (W/m2)
North-facing collectors
1000
South-facing collectors
800 600 400 15 Mar,2003
16 Mar,2003
17 Mar,2003
200 0 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00
time Fig. 8. Global solar radiation incident on the south-facing collectors and the north-facing collectors for the drying test of lemon-grasses.
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120
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Outlet air from the north-facing collectors Outlet air from the south-facing collectors Ambient air
100 80 60 40 20
15 Mar, 2003
16 Mar, 2003
17 Mar, 2003
0 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00 8:00 10:00 12:00 13:30 15:00 17:00
time Fig. 9. Variation of the temperature of the outlet air from the collectors and the temperature of ambient air for the drying test of lemon-grasses.
the drying bin had been cut into small pieces, allowing the moisture to be released easily on the first day. For the following days, the moisture content decreased as the lemon-grasses were in the falling rate phase of drying. Volatile oil, which is a main quality indicator, was extracted from samples of the lemon-grasses dried with this dryer. The average of the oil content of 2.8% (by weight) was obtained. This corresponds to a high-quality dried lemon-grasses. 4.3. Economic evaluation To evaluate the economics of the roof-integrated solar dryer, the dryer is assumed to be used for 10 months (January–October) for drying lemon-grasses and 2 months (November–December) for drying rosella flowers, with one batch of drying run per week. The economic benefit of the collectors functioning as the roof of the farmhouse was not taken into account for this evaluation. Based on the economic situation in Thailand, the costs and economic parameters are shown in Table 1. With these data the investment rate of return (IRR) and the pay-back period (PBP) were calculated using the method described in [10]. It was found that the IRR and PBP are
Moisture content, % (w.b)
80
60
40
17 Mar, 2003
16 Mar, 2003
15 Mar, 2003
20
0 8:00
11:00
14:00
17:00
8:00
11:00
14:00
17:00
8:00
11:00
14:00
17:00
time Fig. 10. Variation of the moisture content of lemon-grasses of the drying test during 15–17 March 2003.
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Table 1 Cost and economic parameters Material cost for the construction of the dryer and the structure of the farmhouse Labor cost for the construction Interest rate Life span of the dryer Electricity cost Salvage value of the dryer Labor cost for the preparation of raw materials (rosella flowers and lemongrasses) Cost of fresh rosella flowers Cost of fresh lemon-grasses Sale price of dried rosella flower Sale price of dried lemon-grasses
US$3125 US$200 7% 15 years US$0.075/kWh US$200 US$5/batch US$50/batch US$40/batch US$94/batch US$116/batch
70.3% and 3.9 years, respectively. This demonstrates that the investment in this dryer is economically promising for use in Thailand.
5. Conclusions A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors has been developed and tested. The dryer was used to dry rosella flower and lemon-grasses. With this dryer, 200 kg of rosella flowers and lemon-grasses can be dried within 4 and 3 days, respectively. The products being dried in this dryer are completely protected from rains and insects. Dried products of high quality are obtained. The solar collectors performed well as both the air heater and a roof of a farmhouse. The average daily efficiency of the collectors is 35%. However, the solar collectors have to be cleaned from time to time to eliminate the deposits of dusts. Based on the economic situation in Thailand, the IRR and the PBP of this dryer are found to be 70.3% and 3.9 years, respectively.
Acknowledgements The authors would like to thank the National Energy Policy Office (NEPO) for the financial support to carry out this research work. The authors would also like to acknowledge the assistance of Mr W. Makcharoen in conducting the drying experiments. Valuable advice from Prof. B.K. Bala is also gratefully acknowledged.
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