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Performance analysis of solar drying system for red chili
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 99 (2014) 47–54 www.elsevier.com/locate/solener
Performance analysis of solar drying system for red chili Ahmad Fudholi a,⇑, Kamaruzzaman Sopian a, Mohammad H. Yazdi a, Mohd Hafidz Ruslan a, Mohamed Gabbasa a, Hussein A. Kazem b a
Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi Selangor, Malaysia b Faculty of Engineering-Sohar University, PO Box 44, Sohar PCI 311, Oman Received 24 July 2013; received in revised form 30 September 2013; accepted 17 October 2013 Available online 26 November 2013 Communicated by: Associate Editor I. Farkas
Abstract This study is concerned with performance analysis of solar drying system for red chili. Red chili was dried to final moisture content of 10% w.b from 80% w.b in 33 h using this system. In this study, energy and exergy analyses of the solar drying process were performed for red chili. Using the first law of thermodynamics, energy analysis was carried out to estimate the useful energy gained from the collectors. However, exergy analysis during solar drying process was estimated by applying the second law of thermodynamics. The specific energy consumption (SEC) was 5.26 kW h/kg. The values of evaporative capacity and improvement potential were from 0.13 kg/s to 2.36 kg/s and 0 W to 135 W, respectively. The efficiencies of the solar collector, drying system, pick-up, and exergy were 28%, 13%, 45%, and 57% respectively, at an average solar radiation of 420 W/m2 and a mass flow rate of 0.07 kg/s. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Energy analysis; Exergy analysis; Improvement potential; Specific energy consumption; Solar drying; Red chili
1. Introduction Red chili is traditionally dried directly under the open sun. Open sun drying requires a large open space and long drying times. Although this traditional method requires only a small investment, open sun drying is highly dependent on the availability of sunshine and is susceptible to contamination from foreign materials (dust and sand) as well as insect and fungal infestations, which thrive in moist conditions. Such contaminations render the products unusable. Most agricultural and marine products require drying to preserve the quality of the final product, but open sun drying results in low-quality products. Therefore, solar drying has become one of the most attractive and promising applications of solar energy systems as an alternative to open sun drying. ⇑ Corresponding author. Tel.: +60 132924765.
E-mail address: [email protected] (A. Fudholi). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.10.019
Several studies reported on the solar drying systems for agricultural and marine products (Bala and Janjai, 2012, 2005; Belessiotis and Delyannis, 2011; Fudholi et al., 2010; Bala et al., 2005; Bala and Mondal, 2001). Several studies specifically investigated solar drying systems for red chili. Janjai et al. (2011) reported the use of a solar greenhouse dryer for the commercial drying of 1000 kg of fruits or vegetables in Champasak, Lao People’s Democratic Republic. The researchers also reported the installation of six units of greenhouse dryers at agro-industrial sites in Thailand between 2008 and 2009. Lhendup (2005) conducted a technical and economic performance analysis of solar drying red chili in Bhutan. Hossain and Bala (2007) studied a mixed-mode forced convection solar tunnel dryer for drying red chili in Bangladesh. Hossain et al. (2005) then used a simulation model to evaluate the technical and economical performance of the solar tunnel dryer.
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Nomenclature collector area (m2) specific heat of air (J kg1 °C1) evaporative capacity (kg/h) exergy (W) solar radiation (W/m2) relative humidity (%) absolute humidity of air leaving the drying chamber (%) hi absolute humidity of air entering the drying chamber (%) has absolute humidity of the air entering the dryer at the point of adiabatic saturation (%) IP improvement potential (W) L latent heat of vaporisation of water at exit air temperature (J/kg) M moisture content (%) Mf final moisture content fraction on wet basis (%) Mi initial moisture content fraction on wet basis (%) mo initial weight of product (kg) air mass flow rate (kg/s) m_ P power (W) S saving in drying time (%) SEC specific energy consumption (kW h/kg) SMER specific moisture extraction rate (kg/kW h) Ac C E Ex G H h0
Exergy is defined as the maximum amount of work that can be produced by a system or a flow of matter or energy to reach equilibrium with a reference environment. Energy and exergy analyses of the drying process should be performed to determine the energy interactions and thermodynamic behavior of drying air throughout a drying chamber. Exergy analysis allows for a more efficient energy resource use because the analysis enables the determination of the locations, types, and true magnitudes of the losses. Therefore, exergy analyses can reveal where and by how much designing more efficient thermal systems is possible by reducing the sources of existing inefficiencies. Increased efficiency can often contribute in an making these processes environmentally friendly by directly reducing the irreversibilities (where exergy is destroyed) that might otherwise occur. Therefore, exergy is one of the most powerful tools in providing optimum drying conditions. In the past few decades, thermodynamic analysis, particularly exergy analysis, has become an essential tool in the system design, analysis, and optimization of a thermal system (Chowdhury et al., 2011). The energy analysis method is widely used in evaluating the performance of the food drying system, but studies on exergy analysis remain relatively limited. Several studies were conducted on the exergy analyses of food drying. Midili and Kucuk (2003) performed energy
T t tOS
temperature (°C) drying time (h) time taken for drying the product in open sun (h) tSD time taken for drying in solar drying (h) v volumetric airflow (m3/s) W mass of water evaporated from the product (kg) Xa ambient absolute humidity (%) Xd dryer outlet absolute humidity (%) XR uncertainty in results x1,x2,xn uncertainty in the independent variables q density of air (kg/m3) g efficiency (%) Subscripts a ambient c chamber dci inflow of drying chamber dco outflow of drying chamber f fan h heater i inlet o outlet t total
and exergy analyses of the drying process of shelled and unshelled pistachios by using a solar drying cabinet. Akpinar (2004) performed energy and exergy analyses in drying red pepper slices by using a convective type dryer. Dincer and Sahin (2004) developed a new model for the thermodynamic analysis of the drying process. Akpinar et al. (2006) conducted first and second law analyses of the thermodynamics of the pumpkin drying process. Colak and Hepbasli (2007) performed exergy analysis on the thin layer of a green olive in a tray dryer. Corzo et al. (2008) performed energy and exergy analyses of the thin layer drying of coroba slices at three different air temperatures. Ozgener and Ozgener (2009) examined the exergy variation during the drying process in a passively heated solar greenhouse. Akpinar (2010) performed energy and exergy analyses of the solar drying process of mint leaves. Exergy efficiencies were derived as a function of the drying time and temperature of the drying air. Akpinar (2011) reported on the energy and exergy analyses of the solar drying of parsley leaves and the variations of the exergy inflow, outflow, and loss with the drying time. Chowdhury et al. (2011) also reported that exergy inflow, outflow, and exergy loss follow similar pattern. The variations in the exergy inflow, outflow, and loss in solar drying are caused by variations in daily solar radiation. However, no study has reported on the exergy analysis of the solar drying
A. Fudholi et al. / Solar Energy 99 (2014) 47–54
system for red chili. Therefore, the main objective of this study is to perform energy and exergy analyses of the solar drying system for red chili. 2. Material and methods Samples of chili (C. annuum L.) were obtained from the farm of Universiti Kebangsaan Malaysia, Selangor, Malaysia. A total of 0.4 kg of fresh red chili was used in each experiment. About 0.4 kg of red chili were taken and dried in an oven at a temperature of 120 ± 1 °C until a constant weight was reached. The initial and final masses of the red chili were recorded using an electronic balance. The procedure was repeated at 1 h intervals until the end of the drying process. The average moisture content was 80.2% (w.b). The solar drying system was installed in the Solar Energy Research Park, Solar Energy Research Institute, Universiti Kebangsaan Malaysia. The solar drying system consists of a finned double-pass solar collector, a blower, and a flat bed drying chamber. The drying system is classified as a forced convection indirect type. A schematic diagram of the solar dryer is shown in Fig. 1. The width and length of the collector are 1.2 and 2.4 m, respectively. The solar collector array consists of four solar collectors. The total area of the collector is 11.52 m2. The collector has a glass cover, and the sides are insulated and painted black on an aluminum absorber plate. The upper channel depth is 3.5 cm, and the lower depth is 7 cm. The bottom and sides of the collector are insulated with 2.5 cm thick fiberglass wool to minimize heat losses. Air initially enters the collector through the first channel formed by the glass that covers the absorber plate and then through the second channel formed by the back plate and the finned absorber plate. The drying chamber is 2.4 m in length, 1.0 m in width, and 0.6 m in height. The drying process was conducted from 9:00 AM to 5:00 PM. The solar dryer was shut down at night. The drying process was continued until the next day, and the process was repeated until the required equilibrium moisture content was reached. For the experiments, the solar dryer was loaded to its full capacity of 40 kg of red chili, which was divided and equally distributed on eight trays.
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The red chili was also placed in a small tray positioned at the center of the dryer to determine the moisture loss by using a Camry R9364 digital electronic balance that was placed on the top center of the drying chamber. The balance has an accuracy of 0.01 g. The air temperature (ambient, collector inlet, and collector outlet temperatures), radiation intensity, and air velocity were measured. The air temperatures before entering, inside, and outside the dryer chamber were also measured. Relative humidity sensors were installed in the inlet, middle, and outlet sections of the drying chamber. An air flow DTA 4000 anemometer was used to determine the air flow velocity in the solar collector. T-type thermocouples and a Li-200 pyranometer with accuracies of 0.018 °C and 1%, respectively, were used. During the drying process, the temperature and relative humidity in the solar dryer were recorded at 1 min intervals by using the ADAM Data Acquisition System, which is connected to a computer. An experimental uncertainty analysis was also performed (Fudholi et al., 2013a). The uncertainty estimation was calculated using (El-Sebaii et al., 2011; Akpinar, 2010): 2
2
2 1=2
X R ¼ ½ðx1 Þ þ ðx2 Þ þ . . . ðxn Þ
ð1Þ
The schematic illustration of the drying system with the input and output terms is shown in Fig. 2. The figure shows the four major points to consider, namely, (1) input of drying air to the drying chamber to dry the products, (2) input of moist products to be dried in the chamber, (3) output of the moist air after removing the evaporated moisture from the products, and (4) output of the dried products. The moisture contents are reduced to the level required for each commodity of the product. In the analysis, the thermodynamic balance equations for the mass, energy, entropy, and exergy of the drying system as a control volume are first written for the product, air, and moisture content in the air (Fig. 2). Comprehensive details of such an analysis were provided by Dincer (2011). Using exergy calculations of drying process, Exergy Band Diagram is shown in Fig. 3. 3. Performances analyses The performances of solar drying systems for red chili have been reported by Fudholi et al. (2013a), such as
Double-pass solar collector with finned absorber Air inlet
15o
Auxiliary heater Blower
Drying chamber
Fig. 1. The schematic of solar drying system.
Fig. 2. The schematic illustration of the drying system (Dincer, 2011).
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Exdco [11.7-489.7 W]
collector. System drying efficiency is a measure of the overall effectiveness of a drying system. The system drying efficiency can be obtained using the following equation:
Exloss [1-238.4 W]
gd ¼
Drying chamber
Fig. 3. Band diagram of exergy balance.
saving in drying time (S), specific moisture extraction rate (SMER), and evaporative capacity (E). Fudholi et al. (2013a) evaluated the time savings when drying chili by comparing solar and open sun drying. The performance of solar drying compared with that of open sun drying was calculated using the following equation: ð2Þ
ð3Þ
In this study, the specific energy consumption (SEC) of the solar drying system was obtained using Eq. (4), as reported by Fudholi et al. (2012): SEC ¼
Pt W
ð4Þ
Evaporative capacity was used as a performance measure for solar dryers. The weight of the water that can be extracted by air flow from the products to be dried was defined by Jannot and Coulibaly (1998) as: _ d X aÞ E ¼ mðX
ð5Þ
3.1. Energy analysis The useful energy gained from the collector was calculated using the magnitude of solar radiation. The thermal efficiency of a solar collector is the ratio of useful heat gained to the solar radiation incident on the plane of the collector. This thermal efficiency is expressed as (Fudholi et al., 2013a,b,c): gc ¼
_ Q mCðT o T iÞ ¼ 100% Ac G Ac G
gp ¼
ð6Þ
System drying efficiency is defined as the ratio of the energy required to evaporate moisture to the heat supplied to the dryer. The heat supplied to the dryer for the solar collector is the solar radiation incident on the solar
h0 hi W ¼ has hi vqtðhas hi Þ
ð8Þ
Which the mass of water removed (W) from a wet product can be calculated: W ¼
The specific moisture extraction rate (SMER), which is the energy required to remove 1 kg of water, was calculated using Eq. (3), as reported by Fudholi et al. (2013a): W SMER ¼ Pt
ð7Þ
Pick-up efficiency is useful for evaluating the actual evaporation of moisture from the product inside the drier. It is a direct measure of how efficiently the capacity of air to absorb moisture is used. The pick-up efficiency is defined as the ratio of the moisture picked up by the air in the drying chamber to the theoretical capacity of the air to absorb moisture. Mathematically it can be expressed by the following equation (Banout et al., 2011):
Exdci [12.7-505.7 W]
tOS tSD S¼ 100 tOS
WL Ac G þ P f þ P h
mo ðM i M f Þ 100 M f
ð9Þ
3.2. Exergy analysis The exergy values were calculated using the characteristics of the working medium from the first-law energy balance. For this purpose, the general form of the exergy equation applicable for a steady flow system can be expressed as (Akbulut and Durmus, 2010): T _ ðT T a Þ T a ln Ex ¼ mC ð10Þ Ta For exergy inflow of drying chamber: T dci _ ðT dci T a Þ T a ln Exdci ¼ mC Ta For exergy outflow of drying chamber: T dco _ ðT dco T a Þ T a ln Exdco ¼ mC Ta
ð11Þ
ð12Þ
However, during the solar drying process, the exergy losses are determined using the following equation: Exloss ¼ Exdci Exdco
ð13Þ
The exergy efficiency can be defined as the ratio of energy use (investment) in the drying of the product to exergy of the drying air supplied to the system. However, it is explained as ratio of exergy outflow to exergy inflow for drying chamber. Considering this definition, the exergy efficiencies of drying chamber can be determined. Thus, the general form of exergy efficiency is written as (Akpinar, 2010; Akbulut and Durmus, 2010): gEx ¼
Exdco Exloss ¼1 Exdci Exdci
ð14Þ
A. Fudholi et al. / Solar Energy 99 (2014) 47–54
IP ¼ ð1 gEx ÞExloss
ð15Þ
4. Results and discussion
Solar radiation (W/m 2)
During the 5 d (33 h) experimentation period, the daily mean values of the drying chamber air temperature, drying chamber relative humidity, and solar radiation ranged from 28 °C to 55 °C, 18–74%, and 104 W/m2 to 820 W/m2, respectively, with corresponding average values of 45 °C, 30%, and 420 W/m2, as shown in Fig. 4. The drying temperature and relative humidity under solar drying continuously varied with increasing drying time. The results revealed that the drying temperature in solar drying was greater than the ambient temperature, whereas the relative humidity in this system was lower than the ambient relative humidity. The drying temperature and relative humidity values also significantly differed at 15 °C and 30%, respectively, during the 33 h drying period. The efficiency of the collector ranged from 11% to 74% with an average value of 28% at a drying air flow rate of 0.07 kg/s. The thermal efficiency rates during the 5 d of drying are shown in Fig. 5, which illustrates the increase in the thermal efficiency of the collector at a low solar radiation. During the solar drying process, the useful energy gained from the collector ranged from 399 W to 1978 W, as shown in Fig. 5. The results of the drying curve of the red chili via open sun and solar drying are shown in Fig. 6. The drying curve revealed the profile change in the moisture content (M)
versus drying time (t). The final drying levels of the red chili were obtained after 33 h in the solar drying system but took about 65 h in the open sun drying system. A 49% saving in drying time was obtained for solar drying compared with open sun drying. Fig. 6 clearly indicates that the drying rate in the solar drying system under forced convection can be much higher than that of the open sun drying, as reported by Akpinar (2010). The drying time obtained in the present study was compared with the results obtained in previous studies. Fudholi et al. (2010) reported that the moisture content of fresh chili decreased from 80% (w.b) to 5% (w.b) in 48 h of solar drying. Banout et al. (2011) compared the use of a double-pass solar dryer with a cabinet dryer via open sun drying of red chili in Central Vietnam. Drying 40 kg of red chili by using a double-pass solar dryer reduced the moisture content from 90% (w.b) to 10% (w.b) in 32 h (including nights). Mohanraj and Chandrasekar (2009) reported that 40 kg of chili by using a forced convection solar drier integrated with gravel as heat storage material reduced the moisture content from 73% (w.b) to 9% (w.b) in 24 h with drying efficiency of 21%. Janjai et al. (2011) reported the use of a solar greenhouse dryer to dry 300 kg of red chili. In this dryer, the moisture content was reduced from 75% to 15% in 3 d. Kaewkiew et al. (2012) investigated the performance of a large-scale greenhouse dryer to dry red chili in Thailand. Drying 500 kg of red chili by using this dryer reduced the moisture content from 74% to 9% in 3 d. Kaleemullah and Kailappan (2005) studied the drying kinetics of red chili in a rotary dryer. They conducted drying experiments at a temperature range of 50 °C to 65 °C for 19 h to 33 h and observed that the quality of dried red chili and drying time increased at a low drying temperature. By contrast, the quality of dried red chili and drying time decreased with increasing drying temperature. However, Kaleemullah and Kailappan (2005) concluded that
1000
100
800
80
600
60
400
40
200
20
0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 10 :3 0 12 :3 0
30
:3 11
0
9:
0
:3 15
0 :3
:3 13
9:
0 11
30
0
Temperature, Humidity ( o C, %)
The maximum improvement in the exergy efficiency for a system or process is obviously achieved when the exergy loss (Exloss) is minimized. The concept of an exergetic “improvement potential” (IP) can be considered as a useful tool in analyzing systems or processes more effectively. The IP of a system or process is given by (Fudholi et al., 2013b; Akpinar, 2010):
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Time of the day Solar radiation Ambient temperature Ambient relative humidity
Chamber temperature Chaber humidity
Fig. 4. Temperatures (ambient and chamber), relative humidity of chamber, ambient relative humidity, and solar radiation from March 16, 2012 to March 20, 2012.
A. Fudholi et al. / Solar Energy 99 (2014) 47–54 100
2000 1800
80
1600 1400
60
1200 1000
40
800 600
20
400 200
0 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 10 :3 0 12 :3 0
9: 30
11
:3 0 15 :3 0
:3 0
11
13
:3 0 9: 30
:3 0
15
13
11
:3 0
0 9: 30
Thermal efficiency (%)
Solar radiation (W, W/m 2)
Energy gained from the collector,
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Time of the day Solar radiation
Energy gained from the collector
Thermal efficiency
Fig. 5. Energy gained from the collector, thermal efficiency and solar radiation with drying time from March 16, 2012 to March 20, 2012.
Moisture content, wb (%)
90 80 Solar drying Open sun drying
70 60 50 40 30 20 10 0 0
8
16
24
32
40
48
56
64
72
Drying time (h) Fig. 6. Moisture content (wet basis) variation with drying time.
S = 420 W/m2 to Eq. (7) yielded a drying efficiency of 12.7%. Eq. (8) and a psychometric chart determined the pick-up efficiency to be 44.9%. The specific energy consumption (SEC) of 5.26 kW h/kg was calculated according to Eq. (4). The evaporative capacity, which ranged from 0.13 kg/h to 2.36 kg/h with an average of 0.97 kg/h, was
600
100
500
80
400 60 300 40 200 20
100
Exergy efficiency (%)
Improvement potential, Exergy (W, W))
the performance of red chili dried at 55 °C was the best in terms of drying time and quality of dried red chili. The experimental results showed that solar drying 40 kg of dry red chili without auxiliary heating to reduce the moisture content of 80–10% within 33 h (5 d of drying). Adding L = 2407 kJ/kg (668 W h/kg), t = 33 h, and
0
9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 10 :3 0 12 :3 0
0
Time of the day Exergy inflow Improvement potential
Exergy outflow Exergy efficiency
Exergy loss
Fig. 7. Improvement potential, exergy efficiency, and exergies (inflow, outflow, and loss) variation with drying time.
A. Fudholi et al. / Solar Energy 99 (2014) 47–54 Table 1 Performance evaluation of the solar drying system. Parameters
Unit
Value
Initial weight of product (total) Final weight of product (total) Initial moisture content (wet basis) Final moisture content (wet basis) Air mass flow rate Average solar radiation Average ambient temperature Average drying chamber temperature Average ambient relative humidity Average drying chamber humidity Drying time Blower energy Solar energy Evaporative capacity Specific energy consumption Overall heat collection (thermal) efficiency Overall drying efficiency, up to 10% wb Pick-up efficiency, up to 10% wb Overall exergy efficiency, up to 10% wb Overall improvement potential, up to 10% wb
kg kg % % kg/s W/m2 °C °C % % h kW h kW h kg/h kW h/kg % % % % W
40 8 80 10 0.07 420 30 44 62 33 33 4.13 160.43 0.97 5.26 28 13 45 57 47.29
calculated using Eq. (5). Evaporative capacity increased with increasing solar radiation. Using exergy analysis of drying process, Exergy Band Diagram was obtained as shown in Fig. 3. Minimum and maximum the exergy inflow, outflow, and loss of 12.7 W and 505.7 W, 11.7 W and 489.7 W, and 1 W and 238.4 W, respectively, was observed. The exergy inflow, outflow, and loss variation with respect to time are given in Fig. 7. Exergy inflow, outflow, and loss follow similar patterns as similarly reported by Chowdhury et al. (2011) and Akpinar (2011). The variations in the exergy inflow, outflow, and loss of the solar drying process were caused by variations in the daily solar radiation. Midilli and Kucuk (2003) reported similar findings, where exergy efficiency decreased with increasing drying air temperature. During the solar drying process, the exergy efficiency was calculated using Eq. (14), which revealed a range of 43–97% as shown in Fig. 7. The exergy efficiency values varied between 43% and 97% with an average of 57%. The values of the improvement potential ranged from 0 W to 135 W with an average of 47 W, as shown in Fig. 7. The summary of the experimental results and observations are given in Table 1. The collector, drying system, and pick-up efficiencies were 28%, 13%, and 45%, respectively, at an average solar radiation of 420 W/m2 and an air flow rate of 0.07 kg/s. 5. Conclusion The energy and exergy analyses of the solar drying system for red chili were performed in this study. Given the results from these analyses, the following remarks may be concluded: Drying red chili via solar drying reduced the moisture content from 80% (w.b) to 10% (w.b) in 33 h.
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The solar drying system was compared with open sun drying. A 49% saving in drying time was obtained for the solar drying system compared with that of the open sun drying. An SEC of 5.26 kg/kW h was obtained. The evaporative capacity of the solar drying system ranged from 0.13 kg/h to 2.36 kg/h with an average of 0.97 kg/h. The solar collector, drying system, and pick-up efficiency rates were 28%, 13%, and 45%, respectively, at an average solar radiation of 420 W/m2 and an air flow rate of 0.07 kg/s. Maximum and minimum collector efficiencies of 52% and 11%, respectively, were observed. The drying temperature varied between 28 °C and 55 °C with an average of 44 °C. The values for improvement potential ranged from 0 W and 135 W with an average of 47 W. The values for exergy efficiency varied between 43% and 97% with an average of 57%. The variations of exergy with drying time were showed to determine when and where the minimum and maximum values of the exergy losses took place during the drying process. Exergy analysis is a useful method in establishing strategies for the design and operation of solar drying systems, where the optimal use of energy is important. Therefore, exergy analysis should be used to conduct performance evaluations of solar drying systems with the highest possible thermodynamic efficiencies. Acknowledgements The authors would like to thank the Yayasan Felda for funding this research grant (RMK9 RS-DL-001-2007), and the Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia for support. References Akbulut, A., Durmus, A., 2010. Energy and exergy analyses of thin layer drying of mulberry in a forced solar dryer. Energy 2010 (35), 1754– 1763. Akpinar, E.K., 2004. Energy and exergy analyses of drying of red pepper slices in a convective type dryer. International Communications in Heat and Mass Transfer 31, 1165–1176. Akpinar, E.K., 2010. Drying of mint leaves in solar dryer and under open sun: modeling, performance analyses. Energy Conversion and Management 51, 2407–2418. Akpinar, E.K., 2011. Drying of parsley leaves in a solar dryer and under open sun: energy and exergy aspects. Journal Food Process Engineering 34, 27–48. Akpinar, E.K., Midilli, A., Bicer, Y., 2006. The first and second law analyses of thermodynamic of pumpkin drying process. Journal of Food Engineering 72, 320–331. Bala, B.K., Janjai, S., 2005. Solar drying of fish (Bombay Duck) using tunnel dryer. International Energy Journal 6, 91–102. Bala, B.K., Janjai, S., 2012. Solar drying technology: potentials and developments. Energy, Environment and Sustainable Development, 69–98. Bala, B.K., Mondal, M.R.A., 2001. Experimental investigation of solar drying of fish using solar tunnel drier. Drying Technology 19 (2), 1–10.
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Fudholi, A., Sopian, K., Ruslan, M.H., Othman, M.Y., 2013b. Performance and cost benefits analysis of double-pass solar collector with and without fins. Energy Conversion and Management 76, 8–19. Fudholi, A., Sopian, K., Ruslan, M.H., Othman, M.Y., Bakhtyar, B., 2013c. Energy analysis and improvement potential of finned doublepass solar collector. Energy Conversion and Management 75, 234–240. Hossain, M.A., Bala, B.K., 2007. Design of hot chilli using solar tunnel drier. Solar Energy 81, 85–92. Hossain, M.A., Woods, J.L., Bala, B.K., 2005. Optimization of solar tunnel drier for drying of chilli without color loss. Renewable Energy 30, 729–742. Janjai, S., Intawee, P., Kaewkiew, J., Sritus, C., Vathsana, K., 2011. A large-scale solar greenhouse dryer using polycarbonate cover: modeling and testing in a tropical environment of Lao People’s Democratic Republic. Renewable Energy 36, 1053–1062. Jannot, Y., Coulibaly, Y., 1998. The evaporative capacity as a performance index for a solar-drier air-heater. Solar Energy 63 (6), 387–391. Kaewkiew, J., Nabneaan, S., Janjai, S., 2012. Experimental investigation of the performance of a large-scale greenhouse type solar dryer for drying chilli in Thailand. Procedia Engineering 32, 433–439. Kaleemullah, S., Kailappan, R., 2005. Drying kinetics of red chillies in a rotary dryer. Biosystem Engineering 92 (1), 15–23. Lhendup, T., 2005. Technical and financial feasibility of a solar drier in Bhutan. Energy for Sustainable Development IX (4), 17–24. Midilli, A., Kucuk, H., 2003. Energy and exergy analysis of solar drying process of pistachio. Energy 28, 539–556. Mohanraj, M., Chandrasekar, P., 2009. Performance of a forced convection solar drier integrated with gravel as heat storage material for chili drying. Journal of Engineering Science and Technology 4 (3), 305–314. Ozgener, L., Ozgener, O., 2009. Exergy analysis of drying process: an experimental study in solar greenhouse. Drying Technology 27, 580– 586.
ScienceDirect Solar Energy 99 (2014) 47–54 www.elsevier.com/locate/solener
Performance analysis of solar drying system for red chili Ahmad Fudholi a,⇑, Kamaruzzaman Sopian a, Mohammad H. Yazdi a, Mohd Hafidz Ruslan a, Mohamed Gabbasa a, Hussein A. Kazem b a
Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi Selangor, Malaysia b Faculty of Engineering-Sohar University, PO Box 44, Sohar PCI 311, Oman Received 24 July 2013; received in revised form 30 September 2013; accepted 17 October 2013 Available online 26 November 2013 Communicated by: Associate Editor I. Farkas
Abstract This study is concerned with performance analysis of solar drying system for red chili. Red chili was dried to final moisture content of 10% w.b from 80% w.b in 33 h using this system. In this study, energy and exergy analyses of the solar drying process were performed for red chili. Using the first law of thermodynamics, energy analysis was carried out to estimate the useful energy gained from the collectors. However, exergy analysis during solar drying process was estimated by applying the second law of thermodynamics. The specific energy consumption (SEC) was 5.26 kW h/kg. The values of evaporative capacity and improvement potential were from 0.13 kg/s to 2.36 kg/s and 0 W to 135 W, respectively. The efficiencies of the solar collector, drying system, pick-up, and exergy were 28%, 13%, 45%, and 57% respectively, at an average solar radiation of 420 W/m2 and a mass flow rate of 0.07 kg/s. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Energy analysis; Exergy analysis; Improvement potential; Specific energy consumption; Solar drying; Red chili
1. Introduction Red chili is traditionally dried directly under the open sun. Open sun drying requires a large open space and long drying times. Although this traditional method requires only a small investment, open sun drying is highly dependent on the availability of sunshine and is susceptible to contamination from foreign materials (dust and sand) as well as insect and fungal infestations, which thrive in moist conditions. Such contaminations render the products unusable. Most agricultural and marine products require drying to preserve the quality of the final product, but open sun drying results in low-quality products. Therefore, solar drying has become one of the most attractive and promising applications of solar energy systems as an alternative to open sun drying. ⇑ Corresponding author. Tel.: +60 132924765.
E-mail address: [email protected] (A. Fudholi). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.10.019
Several studies reported on the solar drying systems for agricultural and marine products (Bala and Janjai, 2012, 2005; Belessiotis and Delyannis, 2011; Fudholi et al., 2010; Bala et al., 2005; Bala and Mondal, 2001). Several studies specifically investigated solar drying systems for red chili. Janjai et al. (2011) reported the use of a solar greenhouse dryer for the commercial drying of 1000 kg of fruits or vegetables in Champasak, Lao People’s Democratic Republic. The researchers also reported the installation of six units of greenhouse dryers at agro-industrial sites in Thailand between 2008 and 2009. Lhendup (2005) conducted a technical and economic performance analysis of solar drying red chili in Bhutan. Hossain and Bala (2007) studied a mixed-mode forced convection solar tunnel dryer for drying red chili in Bangladesh. Hossain et al. (2005) then used a simulation model to evaluate the technical and economical performance of the solar tunnel dryer.
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Nomenclature collector area (m2) specific heat of air (J kg1 °C1) evaporative capacity (kg/h) exergy (W) solar radiation (W/m2) relative humidity (%) absolute humidity of air leaving the drying chamber (%) hi absolute humidity of air entering the drying chamber (%) has absolute humidity of the air entering the dryer at the point of adiabatic saturation (%) IP improvement potential (W) L latent heat of vaporisation of water at exit air temperature (J/kg) M moisture content (%) Mf final moisture content fraction on wet basis (%) Mi initial moisture content fraction on wet basis (%) mo initial weight of product (kg) air mass flow rate (kg/s) m_ P power (W) S saving in drying time (%) SEC specific energy consumption (kW h/kg) SMER specific moisture extraction rate (kg/kW h) Ac C E Ex G H h0
Exergy is defined as the maximum amount of work that can be produced by a system or a flow of matter or energy to reach equilibrium with a reference environment. Energy and exergy analyses of the drying process should be performed to determine the energy interactions and thermodynamic behavior of drying air throughout a drying chamber. Exergy analysis allows for a more efficient energy resource use because the analysis enables the determination of the locations, types, and true magnitudes of the losses. Therefore, exergy analyses can reveal where and by how much designing more efficient thermal systems is possible by reducing the sources of existing inefficiencies. Increased efficiency can often contribute in an making these processes environmentally friendly by directly reducing the irreversibilities (where exergy is destroyed) that might otherwise occur. Therefore, exergy is one of the most powerful tools in providing optimum drying conditions. In the past few decades, thermodynamic analysis, particularly exergy analysis, has become an essential tool in the system design, analysis, and optimization of a thermal system (Chowdhury et al., 2011). The energy analysis method is widely used in evaluating the performance of the food drying system, but studies on exergy analysis remain relatively limited. Several studies were conducted on the exergy analyses of food drying. Midili and Kucuk (2003) performed energy
T t tOS
temperature (°C) drying time (h) time taken for drying the product in open sun (h) tSD time taken for drying in solar drying (h) v volumetric airflow (m3/s) W mass of water evaporated from the product (kg) Xa ambient absolute humidity (%) Xd dryer outlet absolute humidity (%) XR uncertainty in results x1,x2,xn uncertainty in the independent variables q density of air (kg/m3) g efficiency (%) Subscripts a ambient c chamber dci inflow of drying chamber dco outflow of drying chamber f fan h heater i inlet o outlet t total
and exergy analyses of the drying process of shelled and unshelled pistachios by using a solar drying cabinet. Akpinar (2004) performed energy and exergy analyses in drying red pepper slices by using a convective type dryer. Dincer and Sahin (2004) developed a new model for the thermodynamic analysis of the drying process. Akpinar et al. (2006) conducted first and second law analyses of the thermodynamics of the pumpkin drying process. Colak and Hepbasli (2007) performed exergy analysis on the thin layer of a green olive in a tray dryer. Corzo et al. (2008) performed energy and exergy analyses of the thin layer drying of coroba slices at three different air temperatures. Ozgener and Ozgener (2009) examined the exergy variation during the drying process in a passively heated solar greenhouse. Akpinar (2010) performed energy and exergy analyses of the solar drying process of mint leaves. Exergy efficiencies were derived as a function of the drying time and temperature of the drying air. Akpinar (2011) reported on the energy and exergy analyses of the solar drying of parsley leaves and the variations of the exergy inflow, outflow, and loss with the drying time. Chowdhury et al. (2011) also reported that exergy inflow, outflow, and exergy loss follow similar pattern. The variations in the exergy inflow, outflow, and loss in solar drying are caused by variations in daily solar radiation. However, no study has reported on the exergy analysis of the solar drying
A. Fudholi et al. / Solar Energy 99 (2014) 47–54
system for red chili. Therefore, the main objective of this study is to perform energy and exergy analyses of the solar drying system for red chili. 2. Material and methods Samples of chili (C. annuum L.) were obtained from the farm of Universiti Kebangsaan Malaysia, Selangor, Malaysia. A total of 0.4 kg of fresh red chili was used in each experiment. About 0.4 kg of red chili were taken and dried in an oven at a temperature of 120 ± 1 °C until a constant weight was reached. The initial and final masses of the red chili were recorded using an electronic balance. The procedure was repeated at 1 h intervals until the end of the drying process. The average moisture content was 80.2% (w.b). The solar drying system was installed in the Solar Energy Research Park, Solar Energy Research Institute, Universiti Kebangsaan Malaysia. The solar drying system consists of a finned double-pass solar collector, a blower, and a flat bed drying chamber. The drying system is classified as a forced convection indirect type. A schematic diagram of the solar dryer is shown in Fig. 1. The width and length of the collector are 1.2 and 2.4 m, respectively. The solar collector array consists of four solar collectors. The total area of the collector is 11.52 m2. The collector has a glass cover, and the sides are insulated and painted black on an aluminum absorber plate. The upper channel depth is 3.5 cm, and the lower depth is 7 cm. The bottom and sides of the collector are insulated with 2.5 cm thick fiberglass wool to minimize heat losses. Air initially enters the collector through the first channel formed by the glass that covers the absorber plate and then through the second channel formed by the back plate and the finned absorber plate. The drying chamber is 2.4 m in length, 1.0 m in width, and 0.6 m in height. The drying process was conducted from 9:00 AM to 5:00 PM. The solar dryer was shut down at night. The drying process was continued until the next day, and the process was repeated until the required equilibrium moisture content was reached. For the experiments, the solar dryer was loaded to its full capacity of 40 kg of red chili, which was divided and equally distributed on eight trays.
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The red chili was also placed in a small tray positioned at the center of the dryer to determine the moisture loss by using a Camry R9364 digital electronic balance that was placed on the top center of the drying chamber. The balance has an accuracy of 0.01 g. The air temperature (ambient, collector inlet, and collector outlet temperatures), radiation intensity, and air velocity were measured. The air temperatures before entering, inside, and outside the dryer chamber were also measured. Relative humidity sensors were installed in the inlet, middle, and outlet sections of the drying chamber. An air flow DTA 4000 anemometer was used to determine the air flow velocity in the solar collector. T-type thermocouples and a Li-200 pyranometer with accuracies of 0.018 °C and 1%, respectively, were used. During the drying process, the temperature and relative humidity in the solar dryer were recorded at 1 min intervals by using the ADAM Data Acquisition System, which is connected to a computer. An experimental uncertainty analysis was also performed (Fudholi et al., 2013a). The uncertainty estimation was calculated using (El-Sebaii et al., 2011; Akpinar, 2010): 2
2
2 1=2
X R ¼ ½ðx1 Þ þ ðx2 Þ þ . . . ðxn Þ
ð1Þ
The schematic illustration of the drying system with the input and output terms is shown in Fig. 2. The figure shows the four major points to consider, namely, (1) input of drying air to the drying chamber to dry the products, (2) input of moist products to be dried in the chamber, (3) output of the moist air after removing the evaporated moisture from the products, and (4) output of the dried products. The moisture contents are reduced to the level required for each commodity of the product. In the analysis, the thermodynamic balance equations for the mass, energy, entropy, and exergy of the drying system as a control volume are first written for the product, air, and moisture content in the air (Fig. 2). Comprehensive details of such an analysis were provided by Dincer (2011). Using exergy calculations of drying process, Exergy Band Diagram is shown in Fig. 3. 3. Performances analyses The performances of solar drying systems for red chili have been reported by Fudholi et al. (2013a), such as
Double-pass solar collector with finned absorber Air inlet
15o
Auxiliary heater Blower
Drying chamber
Fig. 1. The schematic of solar drying system.
Fig. 2. The schematic illustration of the drying system (Dincer, 2011).
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Exdco [11.7-489.7 W]
collector. System drying efficiency is a measure of the overall effectiveness of a drying system. The system drying efficiency can be obtained using the following equation:
Exloss [1-238.4 W]
gd ¼
Drying chamber
Fig. 3. Band diagram of exergy balance.
saving in drying time (S), specific moisture extraction rate (SMER), and evaporative capacity (E). Fudholi et al. (2013a) evaluated the time savings when drying chili by comparing solar and open sun drying. The performance of solar drying compared with that of open sun drying was calculated using the following equation: ð2Þ
ð3Þ
In this study, the specific energy consumption (SEC) of the solar drying system was obtained using Eq. (4), as reported by Fudholi et al. (2012): SEC ¼
Pt W
ð4Þ
Evaporative capacity was used as a performance measure for solar dryers. The weight of the water that can be extracted by air flow from the products to be dried was defined by Jannot and Coulibaly (1998) as: _ d X aÞ E ¼ mðX
ð5Þ
3.1. Energy analysis The useful energy gained from the collector was calculated using the magnitude of solar radiation. The thermal efficiency of a solar collector is the ratio of useful heat gained to the solar radiation incident on the plane of the collector. This thermal efficiency is expressed as (Fudholi et al., 2013a,b,c): gc ¼
_ Q mCðT o T iÞ ¼ 100% Ac G Ac G
gp ¼
ð6Þ
System drying efficiency is defined as the ratio of the energy required to evaporate moisture to the heat supplied to the dryer. The heat supplied to the dryer for the solar collector is the solar radiation incident on the solar
h0 hi W ¼ has hi vqtðhas hi Þ
ð8Þ
Which the mass of water removed (W) from a wet product can be calculated: W ¼
The specific moisture extraction rate (SMER), which is the energy required to remove 1 kg of water, was calculated using Eq. (3), as reported by Fudholi et al. (2013a): W SMER ¼ Pt
ð7Þ
Pick-up efficiency is useful for evaluating the actual evaporation of moisture from the product inside the drier. It is a direct measure of how efficiently the capacity of air to absorb moisture is used. The pick-up efficiency is defined as the ratio of the moisture picked up by the air in the drying chamber to the theoretical capacity of the air to absorb moisture. Mathematically it can be expressed by the following equation (Banout et al., 2011):
Exdci [12.7-505.7 W]
tOS tSD S¼ 100 tOS
WL Ac G þ P f þ P h
mo ðM i M f Þ 100 M f
ð9Þ
3.2. Exergy analysis The exergy values were calculated using the characteristics of the working medium from the first-law energy balance. For this purpose, the general form of the exergy equation applicable for a steady flow system can be expressed as (Akbulut and Durmus, 2010): T _ ðT T a Þ T a ln Ex ¼ mC ð10Þ Ta For exergy inflow of drying chamber: T dci _ ðT dci T a Þ T a ln Exdci ¼ mC Ta For exergy outflow of drying chamber: T dco _ ðT dco T a Þ T a ln Exdco ¼ mC Ta
ð11Þ
ð12Þ
However, during the solar drying process, the exergy losses are determined using the following equation: Exloss ¼ Exdci Exdco
ð13Þ
The exergy efficiency can be defined as the ratio of energy use (investment) in the drying of the product to exergy of the drying air supplied to the system. However, it is explained as ratio of exergy outflow to exergy inflow for drying chamber. Considering this definition, the exergy efficiencies of drying chamber can be determined. Thus, the general form of exergy efficiency is written as (Akpinar, 2010; Akbulut and Durmus, 2010): gEx ¼
Exdco Exloss ¼1 Exdci Exdci
ð14Þ
A. Fudholi et al. / Solar Energy 99 (2014) 47–54
IP ¼ ð1 gEx ÞExloss
ð15Þ
4. Results and discussion
Solar radiation (W/m 2)
During the 5 d (33 h) experimentation period, the daily mean values of the drying chamber air temperature, drying chamber relative humidity, and solar radiation ranged from 28 °C to 55 °C, 18–74%, and 104 W/m2 to 820 W/m2, respectively, with corresponding average values of 45 °C, 30%, and 420 W/m2, as shown in Fig. 4. The drying temperature and relative humidity under solar drying continuously varied with increasing drying time. The results revealed that the drying temperature in solar drying was greater than the ambient temperature, whereas the relative humidity in this system was lower than the ambient relative humidity. The drying temperature and relative humidity values also significantly differed at 15 °C and 30%, respectively, during the 33 h drying period. The efficiency of the collector ranged from 11% to 74% with an average value of 28% at a drying air flow rate of 0.07 kg/s. The thermal efficiency rates during the 5 d of drying are shown in Fig. 5, which illustrates the increase in the thermal efficiency of the collector at a low solar radiation. During the solar drying process, the useful energy gained from the collector ranged from 399 W to 1978 W, as shown in Fig. 5. The results of the drying curve of the red chili via open sun and solar drying are shown in Fig. 6. The drying curve revealed the profile change in the moisture content (M)
versus drying time (t). The final drying levels of the red chili were obtained after 33 h in the solar drying system but took about 65 h in the open sun drying system. A 49% saving in drying time was obtained for solar drying compared with open sun drying. Fig. 6 clearly indicates that the drying rate in the solar drying system under forced convection can be much higher than that of the open sun drying, as reported by Akpinar (2010). The drying time obtained in the present study was compared with the results obtained in previous studies. Fudholi et al. (2010) reported that the moisture content of fresh chili decreased from 80% (w.b) to 5% (w.b) in 48 h of solar drying. Banout et al. (2011) compared the use of a double-pass solar dryer with a cabinet dryer via open sun drying of red chili in Central Vietnam. Drying 40 kg of red chili by using a double-pass solar dryer reduced the moisture content from 90% (w.b) to 10% (w.b) in 32 h (including nights). Mohanraj and Chandrasekar (2009) reported that 40 kg of chili by using a forced convection solar drier integrated with gravel as heat storage material reduced the moisture content from 73% (w.b) to 9% (w.b) in 24 h with drying efficiency of 21%. Janjai et al. (2011) reported the use of a solar greenhouse dryer to dry 300 kg of red chili. In this dryer, the moisture content was reduced from 75% to 15% in 3 d. Kaewkiew et al. (2012) investigated the performance of a large-scale greenhouse dryer to dry red chili in Thailand. Drying 500 kg of red chili by using this dryer reduced the moisture content from 74% to 9% in 3 d. Kaleemullah and Kailappan (2005) studied the drying kinetics of red chili in a rotary dryer. They conducted drying experiments at a temperature range of 50 °C to 65 °C for 19 h to 33 h and observed that the quality of dried red chili and drying time increased at a low drying temperature. By contrast, the quality of dried red chili and drying time decreased with increasing drying temperature. However, Kaleemullah and Kailappan (2005) concluded that
1000
100
800
80
600
60
400
40
200
20
0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 10 :3 0 12 :3 0
30
:3 11
0
9:
0
:3 15
0 :3
:3 13
9:
0 11
30
0
Temperature, Humidity ( o C, %)
The maximum improvement in the exergy efficiency for a system or process is obviously achieved when the exergy loss (Exloss) is minimized. The concept of an exergetic “improvement potential” (IP) can be considered as a useful tool in analyzing systems or processes more effectively. The IP of a system or process is given by (Fudholi et al., 2013b; Akpinar, 2010):
51
Time of the day Solar radiation Ambient temperature Ambient relative humidity
Chamber temperature Chaber humidity
Fig. 4. Temperatures (ambient and chamber), relative humidity of chamber, ambient relative humidity, and solar radiation from March 16, 2012 to March 20, 2012.
A. Fudholi et al. / Solar Energy 99 (2014) 47–54 100
2000 1800
80
1600 1400
60
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40
800 600
20
400 200
0 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 10 :3 0 12 :3 0
9: 30
11
:3 0 15 :3 0
:3 0
11
13
:3 0 9: 30
:3 0
15
13
11
:3 0
0 9: 30
Thermal efficiency (%)
Solar radiation (W, W/m 2)
Energy gained from the collector,
52
Time of the day Solar radiation
Energy gained from the collector
Thermal efficiency
Fig. 5. Energy gained from the collector, thermal efficiency and solar radiation with drying time from March 16, 2012 to March 20, 2012.
Moisture content, wb (%)
90 80 Solar drying Open sun drying
70 60 50 40 30 20 10 0 0
8
16
24
32
40
48
56
64
72
Drying time (h) Fig. 6. Moisture content (wet basis) variation with drying time.
S = 420 W/m2 to Eq. (7) yielded a drying efficiency of 12.7%. Eq. (8) and a psychometric chart determined the pick-up efficiency to be 44.9%. The specific energy consumption (SEC) of 5.26 kW h/kg was calculated according to Eq. (4). The evaporative capacity, which ranged from 0.13 kg/h to 2.36 kg/h with an average of 0.97 kg/h, was
600
100
500
80
400 60 300 40 200 20
100
Exergy efficiency (%)
Improvement potential, Exergy (W, W))
the performance of red chili dried at 55 °C was the best in terms of drying time and quality of dried red chili. The experimental results showed that solar drying 40 kg of dry red chili without auxiliary heating to reduce the moisture content of 80–10% within 33 h (5 d of drying). Adding L = 2407 kJ/kg (668 W h/kg), t = 33 h, and
0
9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 15 :3 0 9: 30 11 :3 0 13 :3 0 10 :3 0 12 :3 0
0
Time of the day Exergy inflow Improvement potential
Exergy outflow Exergy efficiency
Exergy loss
Fig. 7. Improvement potential, exergy efficiency, and exergies (inflow, outflow, and loss) variation with drying time.
A. Fudholi et al. / Solar Energy 99 (2014) 47–54 Table 1 Performance evaluation of the solar drying system. Parameters
Unit
Value
Initial weight of product (total) Final weight of product (total) Initial moisture content (wet basis) Final moisture content (wet basis) Air mass flow rate Average solar radiation Average ambient temperature Average drying chamber temperature Average ambient relative humidity Average drying chamber humidity Drying time Blower energy Solar energy Evaporative capacity Specific energy consumption Overall heat collection (thermal) efficiency Overall drying efficiency, up to 10% wb Pick-up efficiency, up to 10% wb Overall exergy efficiency, up to 10% wb Overall improvement potential, up to 10% wb
kg kg % % kg/s W/m2 °C °C % % h kW h kW h kg/h kW h/kg % % % % W
40 8 80 10 0.07 420 30 44 62 33 33 4.13 160.43 0.97 5.26 28 13 45 57 47.29
calculated using Eq. (5). Evaporative capacity increased with increasing solar radiation. Using exergy analysis of drying process, Exergy Band Diagram was obtained as shown in Fig. 3. Minimum and maximum the exergy inflow, outflow, and loss of 12.7 W and 505.7 W, 11.7 W and 489.7 W, and 1 W and 238.4 W, respectively, was observed. The exergy inflow, outflow, and loss variation with respect to time are given in Fig. 7. Exergy inflow, outflow, and loss follow similar patterns as similarly reported by Chowdhury et al. (2011) and Akpinar (2011). The variations in the exergy inflow, outflow, and loss of the solar drying process were caused by variations in the daily solar radiation. Midilli and Kucuk (2003) reported similar findings, where exergy efficiency decreased with increasing drying air temperature. During the solar drying process, the exergy efficiency was calculated using Eq. (14), which revealed a range of 43–97% as shown in Fig. 7. The exergy efficiency values varied between 43% and 97% with an average of 57%. The values of the improvement potential ranged from 0 W to 135 W with an average of 47 W, as shown in Fig. 7. The summary of the experimental results and observations are given in Table 1. The collector, drying system, and pick-up efficiencies were 28%, 13%, and 45%, respectively, at an average solar radiation of 420 W/m2 and an air flow rate of 0.07 kg/s. 5. Conclusion The energy and exergy analyses of the solar drying system for red chili were performed in this study. Given the results from these analyses, the following remarks may be concluded: Drying red chili via solar drying reduced the moisture content from 80% (w.b) to 10% (w.b) in 33 h.
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The solar drying system was compared with open sun drying. A 49% saving in drying time was obtained for the solar drying system compared with that of the open sun drying. An SEC of 5.26 kg/kW h was obtained. The evaporative capacity of the solar drying system ranged from 0.13 kg/h to 2.36 kg/h with an average of 0.97 kg/h. The solar collector, drying system, and pick-up efficiency rates were 28%, 13%, and 45%, respectively, at an average solar radiation of 420 W/m2 and an air flow rate of 0.07 kg/s. Maximum and minimum collector efficiencies of 52% and 11%, respectively, were observed. The drying temperature varied between 28 °C and 55 °C with an average of 44 °C. The values for improvement potential ranged from 0 W and 135 W with an average of 47 W. The values for exergy efficiency varied between 43% and 97% with an average of 57%. The variations of exergy with drying time were showed to determine when and where the minimum and maximum values of the exergy losses took place during the drying process. Exergy analysis is a useful method in establishing strategies for the design and operation of solar drying systems, where the optimal use of energy is important. Therefore, exergy analysis should be used to conduct performance evaluations of solar drying systems with the highest possible thermodynamic efficiencies. Acknowledgements The authors would like to thank the Yayasan Felda for funding this research grant (RMK9 RS-DL-001-2007), and the Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia for support. References Akbulut, A., Durmus, A., 2010. Energy and exergy analyses of thin layer drying of mulberry in a forced solar dryer. Energy 2010 (35), 1754– 1763. Akpinar, E.K., 2004. Energy and exergy analyses of drying of red pepper slices in a convective type dryer. International Communications in Heat and Mass Transfer 31, 1165–1176. Akpinar, E.K., 2010. Drying of mint leaves in solar dryer and under open sun: modeling, performance analyses. Energy Conversion and Management 51, 2407–2418. Akpinar, E.K., 2011. Drying of parsley leaves in a solar dryer and under open sun: energy and exergy aspects. Journal Food Process Engineering 34, 27–48. Akpinar, E.K., Midilli, A., Bicer, Y., 2006. The first and second law analyses of thermodynamic of pumpkin drying process. Journal of Food Engineering 72, 320–331. Bala, B.K., Janjai, S., 2005. Solar drying of fish (Bombay Duck) using tunnel dryer. International Energy Journal 6, 91–102. Bala, B.K., Janjai, S., 2012. Solar drying technology: potentials and developments. Energy, Environment and Sustainable Development, 69–98. Bala, B.K., Mondal, M.R.A., 2001. Experimental investigation of solar drying of fish using solar tunnel drier. Drying Technology 19 (2), 1–10.
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