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Development of advanced solar assisted drying systems
Renewable Energy 31 (2006) 703–709 www.elsevier.com/locate/renene
Development of advanced solar assisted drying systems M.Y.H. Othman a, K. Sopian b,*, B. Yatim a, W.R.W. Daud b a
Department of Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia b Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Available online 15 November 2005
Abstract The Solar Energy Research Group in the Universiti Kebangsaan Malaysia has been set-up more than two decades ago. One of the activities is in the field of solar thermal process, particularly in development of solar assisted drying systems. Solar drying systems technical development can proceed in two directions. Firstly simple, low power, short life, and comparatively low efficiency-drying system. Secondly, the development of high efficiency, high power, long life expensive solar drying system. The group has developed four solar assisted drying systems namely (a) the V-groove solar collector, (b) the double-pass solar collector with integrated storage system, (c) the solar assisted dehumidification system for medicinal herbs and (d) the photovoltaic thermal (PVT) collector system. The common problems associated with the intermittent nature of solar radiation and the low intensities of solar radiation in solar thermal systems can be remedied using these types of solar drying systems. These drying systems have the advantages of heat storage, auxiliary energy source, integrated structure control system and can be use for a wide range of agricultural produce. q 2005 Elsevier Ltd. All rights reserved. Keywords: Double-pass solar collector; V-groove solar collector; Solar dehumidification system; Photovoltaic thermal solar collector
1. Introduction Many commercial crop drying systems have been developed and mostly use fossil fuel especially diesel. Solar drying provides an alternative to the use of fossil fuel. However, the problems associated with the intermittent nature and the low intensities of solar radiation dampened the entry of these technologies in the market. However, the problems can be remedied by the use of heat storage, auxiliary energy source, control system and larger surface collector.
* Corresponding author. Tel.: C60 3829 6516; fax: C60 3826 5126. E-mail address: [email protected] (K. Sopian).
0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.09.004
704
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
Moreover, this will result in a high capital investment. Economic indicators such as cost of maintenance, payback period, internal rate of return can be use to calculate the economic feasibility of the system. The technical development of solar drying systems can proceed in two directions. Firstly, simple, low power, short life, and comparatively low efficiency-drying system. Secondly, high efficiency, high power, long life expensive drying system. The latter is characterized not only by an integrated structure, but also integrating in an energy system involving process other than drying such as hot water when the drying system is not in use. In addition, air based solar collectors are not the only available systems. Water based collectors can also be used. A water to air heat exchanger can be used. The hot air that will be used to dry the product can be forced to flow in the water to air heat exchanger. In addition, the hot water tanks of the farm can be used as heat storage of the solar drying system. This paper presents four solar assisted drying systems namely (a) the solar assisted drying systems with the V-grove collector and (b) solar assisted drying systems with the double pass collector (c) solar drying system with photovoltaic thermal solar collector and (d) solar assisted dehumidification system. The first, second and third systems are air based solar collector systems and the fourth is a water based system. 2. Solar assisted drying systems 2.1. Solar Assisted Drying System with the V-Groove Collector The use of V-grooved absorbers improve the heat transfer coefficient between the absorber plate and the air [1–3]. The first dryer uses collectors of the V-groove absorber type and a 10 kW auxiliary heat source for continuous operation. The configuration of the system is shown in with the V-groove collector is shown in Fig. 1. Air flows into the collector and carries away the heat received from the sun. Increasing the absorber surface area enhances the heat transfer area of the collector. The V-groove configuration enhances the heat transfer surface area and improves the performance of the system. All parts of the collector are kept in an outer case usually made of sheet metal. glass cover V–groove Collector
Air flow Air vent Drying Chamber
Glass cover
V–Groove Absorber
Air inlet Fig. 1. Schematic of the solar assisted drying system.
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
705
An average drying chamber temperature of 50 8C can be achieved with a flow rate of 15.1 m3/min and an average solar radiation of 700 W/m2 and ambient temperature of 27–30 8C. Green tea or herbal tea have been dried using this dryer. Herbal tea contains many organic compounds and the processing requirements differ depending on specifications on the types of tea to be produced. Discoloration of herbal tea will occur if the drying process is delayed. Hence, the fresh tealeaves are dried from an initial moisture content of 87% (wet basis) to 54% (wet basis) at a drying temperature of 50 8C and flow rate of 15.1 m3/min [4].The aroma of the herbal tea depends on the method of drying since chemical reaction continues during the drying process [5].The initial weight of the fresh tealeaves is 10.03 kg and the final weight is 2.86 kg. The drying process started at 8:00 and ended at 18:00. The total energy required to maintain a drying chamber temperature of 50 8C is 60.2 kWh. The auxiliary energy contribution is 17.6 kWh. Hence, solar energy contributed 42.6 kWh during the process and contributed approximately 70.2% of the overall energy requirement. 2.2. Solar Assisted Drying System with Double-Pass Collector Wijeysundra et al. [6] performed experimental and analytical analysis on a double-pass solar air collector with two glass covers and reported an increase of 10–15% in collector thermal efficiency. In addition, the use of a double-pass resulted in an increase in the pressure drop across the collector. Mohamad [7] conducted theoretical studies of a double-pass solar collector with porous media in the second channel. The solar collector has two glass covers. The air flows in the upper channel (passage between the first glass cover and the second glass cover) and into the second channel (passage between the second glass cover and the back-plate. The collector consist of two glass cover with addition of porous media under the second glass cover. Sopian et al. [8] performed the experimental studies on the performance of a double-pass solar collector with porous media in the second or lower channel. The double-pass solar collector with porous media has width and length are 120 and 240 cm. The upper channel depth is 3.5 cm and the lower depth is 10.5 cm. The schematic of the collector as shows in Figs. 2 and 3. The solar collector array consists of six solar collectors, arranged as 2 banks of 3 collectors each in series. Air enters the inlet of the upper channel in the first collector and flows in the lower channel. Next, the air flows to the second collector. The lower channel of the third collector is the air outlet from the first bank. In the second bank, air flows in the upper channel of the sixth collector and the outlet air is the lower channel of the fourth collector.
Fig. 2. The schematic of a double-pass solar collector with porous media.
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M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
upper
lower flow inlet
inlet outlet Fig. 3. The collector array for the solar drying system.
The outlet air from the third collector is mixed with the air outlet from the fourth collector. Centrifugal blower is used to induce the hot air. Fig. 4(a) shows the resulting outlet temperature if the flow rate is fixed at 0.0995 kg/s. The outlet temperature is about 90 8C at an average solar radiation level between 900 and 1000 W/m2. In general, the outlet temperature does not drop drastically when the solar radiation decreases due to passing clouds or rain as in any conventional solar air collector. The outlet temperature goes down gradually in the evening even at low solar radiation levels. The reason for this is because of the presence of porous media in the second channel that acts as the storage media for the system. The relevant data are average for an hour and the hourly thermal efficiency are shown in Fig. 4(b). The average thermal efficiency of the collector increases with the flow rate. 2.3. Photovoltaic thermal solar air heater The term PVT refers to solar thermal collectors that use PV cells as an integral part of the absorber plate. The system generates both thermal and electrical energy simultaneously. The number of the photovoltaic cells in the system can be adjusted according to the local load demands. In conventional solar thermal system, external electrical energy is required to circulate the working fluid through the system. The need for an external electrical source can be eliminated by using this hybrid system. With a suitable design, one can produces a self-sufficient solar collector system that required no external electrical energy to run the system. 100 Solar radiation
Outlet temperature
70 60
600
50 40
400
30 Ambient temperature
Air flow rate = 0.0995 kg/s
0 9:30 AM 11:00 AM 12:30 PM 2:00 PM
Inlet temperature
3:30 PM
Time of the day (5th November 1999)
100
100 OutletTemperature
80
800
200
(b)
90
20
Temperature (oC)
Solar radiation (W/m2)
1000
80
80
60
60 Inlet Temperature
40
40 Thermal efficiency
20
Temperature (oC)
1200
Thermal efficiency (%)
(a)
20
10
0 5:00 PM
0 9:00 AM
11:00 AM
1:00 PM
3:00 PM
5:00 PM
0
th
Time of the day (5 November 1999)
Fig. 4. (a) Temperature (outlet, inlet and ambient) and solar radiation for the day (b) hourly thermal efficiency and temperature (inlet and outlet) for the day.
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
707
Solar cell
Glass cover Air in CPC
Insulation
Fin
Fig. 5. A double-pass photovoltaic thermal solar collector system with CPC and fins.
The first multiple-pass concept for the air cooled PVT systems suitable for drying system was introduced by Sopian et al. [9].The double-pass flow enhanced cooling of the photovoltaic cells and thus increasing the efficiency of the systems. The double-pass concepts was later extended to include heat transfer augmentation features such as a fins for enhancing heat removal by convective and conductive heat transfers and also compound parabolic collectors for solar radiation booster [10]. Fig. 5 shows the basic design of the double-pass photovoltaic thermal solar collector with compound parabolic concentrator and fins. The air flowed through the first channel formed by the glass cover and the photovoltaic panel. Next, it enters the second channel formed by the back plate and the photovoltaic panel. The first channel has compound parabolic concentrators to concentrate solar radiation. The second channel has fins that transfer the heat from the photovoltaic panel. This flow arrangement and the compound parabolic concentrators as well as the fins will increase heat removal from the photovoltaic panel and will enhanced the efficiency of the system. Fig. 6 shows the performance characteristics curves of the various configuration of PVT collectors. The PVT collector with CPC and fins has better performance compared to others. The characteristics of the collector are measured at solar radiation level of 700 W/m2 and air flow rate at 0.1 kg/s with inlet temperature ranging from 30 to 54 8C. Thermal efficiency of the system is between 48 and 63.6% with air temperature output of 35.5–58 oC.
80
Overall efficiency (%)
70 60 50
Double pass with CPC & fins
40
Double pass with fins
30
Double pass with flat plate
20
Single pass with flat plate
10 0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035
(Ti - Ta)/S
Fig. 6. Perfomance Characteristics Curves of the CPC photovoltaic thermal air heater and other configurations.
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Fig. 7. Solar Assisted Dehumidification System.
2.4. Solar assisted dehumidification system The used of medicinal herbs as alternatives to modern medications are safe and have virtually no side effects. Drying process is required for the preservation of the product. However, tremendous amount of energy is required for conventional commercial scale drying systems. Traditional methods of drying such as open air drying, where the product to be dried is exposed directly to the sun has many disadvantages such degradation by wind-blown debris, rain, insect infestation, human and animal interference which will result in contamination of the product. The solar assisted dehumidification system is an effective and viable alternative to the many present drying techniques. The system consists of evacuated tube solar collectors, dehumidification system and the drying chamber as shown in Fig. 7. A temperature of 40 8C and air at relative humidity of 20–30% can be achieved in the drying chamber and the operated continuously since an auxiliary heater has been installed in the storage tank. High quality medicinal herbs can be produced using this system [11]. 3. Conclusions The design and performance of four advanced solar assisted forced convective solar dryers have been presented. The first drying system used the V-groove type solar collector and the second drying system used the double-pass solar collector with porous media in the second channel. The second drying system uses the photovoltaic thermal solar collector with heat transfer augmentation features. The fourth dryer is a water based solar collector dehumidification system. The drying chamber for all of the s is the cabinet type. The V-groove collector is suitable for agricultural produce such as chilies, green tea, and dried fruits. The double-pass solar collector with integrated storage system produces a much higher outlet temperatures compared to the first drying system. The connections for the V-groove solar collector is straightforward and simple while the connections for the double-pass solar is a little complicated. The connections for the double-pass solar collector involve internal manifolding. The drying system is used to dry animal feedstock such as oil palm fronds and other higher temperature drying processes. The PVT system generates both thermal and electrical energy simultaneously. The number of the photovoltaic cells in the system can be adjusted according to
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
709
the local load demands and the need for an external electrical source can be eliminated by using this hybrid system. With a suitable design, one can produces a self-sufficient solar collector system that required no external electrical energy to run the system. There are several advantages of using water based solar collector since water has a larger heat capacity than air. In addition, components that uses water as working fluid are smaller in size compared with components that uses air as the working fluid. The use of solar dryers will enhanced the environment, wealth creation and nation building as well as sustainable development for the country.
References [1] Goel VK, Chandra Ram, Raychaudhuri BC. Experimental investigations on single-absorber solar air heaters. Energy Convers Manage 1987;27(4):343–9. [2] Choudhury C, Anderson SL, Rekstad J. A solar air heater for low temperature applications. Solar Energy 1988;40: 335–44. [3] Hafiz MR, Othman MY, Salleh MM, Sopian K. Design and indoor testing of the V-groove back pass solar collector. Renew Energy 1999;16:2118–22. [4] Saijo R, Harada T. Volatile and non volatile forms of aroma compound in tea leaves and their changes due to injury. Agric Biol Chem 1973;37:1367–73. [5] Kirsi M, Julkunen R, Rimpilainen T. The effect of drying method on the aroma of the herbal tea plant (Rubus idaeus). Flavors and off-flavors. Proceeding of the sixth international flavor conference; Crete, Greece; 1989. [6] Wijeysundera NE, An LL, Tyioe LE. Thermal performance study of two pass solar air heaters. Sol Energy 1982; 28(5):363–70. [7] Mohamad AA. High efficiency solar air heater. Solar Energy 1997;60(2):71–6. [8] Sopian K, Supranto WR, Daud W, Yatim B, Othman MY. Thermal performance of the double-pass solar collector with and without porous media. Renew Energy 1999;18/4:557–64. [9] Sopian K, Liu HT, Kakac S, Veziroglu TN. Performance of a double pass photovoltaic thermal solar collector suitable for solar drying systems. Energy Convers Manage Int J 2000;41:353–65. [10] Othman MohdYusof, Yatim Baharudin, Sopian Kamaruzzaman, Abu Bakar Nazari. The development of hybrid photovoltaic-thermal (PV/T) solar air heater systems. Proceedings of the world renewable energy congress VII, 29 June–5 July 2002, Cologne, Germany; 2002. [11] Yahya M, Sopian K, Yatim B, Othmanand MY, Daud WRW. Design of a solar assisted dehumidification system for medicinal herbs. Proceedings of the second Asian Oceania drying conference, Batu Feringhi, Pulai Pinang, Malaysia, 20–22 August 2001. p. 383–92.
Development of advanced solar assisted drying systems M.Y.H. Othman a, K. Sopian b,*, B. Yatim a, W.R.W. Daud b a
Department of Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia b Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Available online 15 November 2005
Abstract The Solar Energy Research Group in the Universiti Kebangsaan Malaysia has been set-up more than two decades ago. One of the activities is in the field of solar thermal process, particularly in development of solar assisted drying systems. Solar drying systems technical development can proceed in two directions. Firstly simple, low power, short life, and comparatively low efficiency-drying system. Secondly, the development of high efficiency, high power, long life expensive solar drying system. The group has developed four solar assisted drying systems namely (a) the V-groove solar collector, (b) the double-pass solar collector with integrated storage system, (c) the solar assisted dehumidification system for medicinal herbs and (d) the photovoltaic thermal (PVT) collector system. The common problems associated with the intermittent nature of solar radiation and the low intensities of solar radiation in solar thermal systems can be remedied using these types of solar drying systems. These drying systems have the advantages of heat storage, auxiliary energy source, integrated structure control system and can be use for a wide range of agricultural produce. q 2005 Elsevier Ltd. All rights reserved. Keywords: Double-pass solar collector; V-groove solar collector; Solar dehumidification system; Photovoltaic thermal solar collector
1. Introduction Many commercial crop drying systems have been developed and mostly use fossil fuel especially diesel. Solar drying provides an alternative to the use of fossil fuel. However, the problems associated with the intermittent nature and the low intensities of solar radiation dampened the entry of these technologies in the market. However, the problems can be remedied by the use of heat storage, auxiliary energy source, control system and larger surface collector.
* Corresponding author. Tel.: C60 3829 6516; fax: C60 3826 5126. E-mail address: [email protected] (K. Sopian).
0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.09.004
704
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
Moreover, this will result in a high capital investment. Economic indicators such as cost of maintenance, payback period, internal rate of return can be use to calculate the economic feasibility of the system. The technical development of solar drying systems can proceed in two directions. Firstly, simple, low power, short life, and comparatively low efficiency-drying system. Secondly, high efficiency, high power, long life expensive drying system. The latter is characterized not only by an integrated structure, but also integrating in an energy system involving process other than drying such as hot water when the drying system is not in use. In addition, air based solar collectors are not the only available systems. Water based collectors can also be used. A water to air heat exchanger can be used. The hot air that will be used to dry the product can be forced to flow in the water to air heat exchanger. In addition, the hot water tanks of the farm can be used as heat storage of the solar drying system. This paper presents four solar assisted drying systems namely (a) the solar assisted drying systems with the V-grove collector and (b) solar assisted drying systems with the double pass collector (c) solar drying system with photovoltaic thermal solar collector and (d) solar assisted dehumidification system. The first, second and third systems are air based solar collector systems and the fourth is a water based system. 2. Solar assisted drying systems 2.1. Solar Assisted Drying System with the V-Groove Collector The use of V-grooved absorbers improve the heat transfer coefficient between the absorber plate and the air [1–3]. The first dryer uses collectors of the V-groove absorber type and a 10 kW auxiliary heat source for continuous operation. The configuration of the system is shown in with the V-groove collector is shown in Fig. 1. Air flows into the collector and carries away the heat received from the sun. Increasing the absorber surface area enhances the heat transfer area of the collector. The V-groove configuration enhances the heat transfer surface area and improves the performance of the system. All parts of the collector are kept in an outer case usually made of sheet metal. glass cover V–groove Collector
Air flow Air vent Drying Chamber
Glass cover
V–Groove Absorber
Air inlet Fig. 1. Schematic of the solar assisted drying system.
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
705
An average drying chamber temperature of 50 8C can be achieved with a flow rate of 15.1 m3/min and an average solar radiation of 700 W/m2 and ambient temperature of 27–30 8C. Green tea or herbal tea have been dried using this dryer. Herbal tea contains many organic compounds and the processing requirements differ depending on specifications on the types of tea to be produced. Discoloration of herbal tea will occur if the drying process is delayed. Hence, the fresh tealeaves are dried from an initial moisture content of 87% (wet basis) to 54% (wet basis) at a drying temperature of 50 8C and flow rate of 15.1 m3/min [4].The aroma of the herbal tea depends on the method of drying since chemical reaction continues during the drying process [5].The initial weight of the fresh tealeaves is 10.03 kg and the final weight is 2.86 kg. The drying process started at 8:00 and ended at 18:00. The total energy required to maintain a drying chamber temperature of 50 8C is 60.2 kWh. The auxiliary energy contribution is 17.6 kWh. Hence, solar energy contributed 42.6 kWh during the process and contributed approximately 70.2% of the overall energy requirement. 2.2. Solar Assisted Drying System with Double-Pass Collector Wijeysundra et al. [6] performed experimental and analytical analysis on a double-pass solar air collector with two glass covers and reported an increase of 10–15% in collector thermal efficiency. In addition, the use of a double-pass resulted in an increase in the pressure drop across the collector. Mohamad [7] conducted theoretical studies of a double-pass solar collector with porous media in the second channel. The solar collector has two glass covers. The air flows in the upper channel (passage between the first glass cover and the second glass cover) and into the second channel (passage between the second glass cover and the back-plate. The collector consist of two glass cover with addition of porous media under the second glass cover. Sopian et al. [8] performed the experimental studies on the performance of a double-pass solar collector with porous media in the second or lower channel. The double-pass solar collector with porous media has width and length are 120 and 240 cm. The upper channel depth is 3.5 cm and the lower depth is 10.5 cm. The schematic of the collector as shows in Figs. 2 and 3. The solar collector array consists of six solar collectors, arranged as 2 banks of 3 collectors each in series. Air enters the inlet of the upper channel in the first collector and flows in the lower channel. Next, the air flows to the second collector. The lower channel of the third collector is the air outlet from the first bank. In the second bank, air flows in the upper channel of the sixth collector and the outlet air is the lower channel of the fourth collector.
Fig. 2. The schematic of a double-pass solar collector with porous media.
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M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
upper
lower flow inlet
inlet outlet Fig. 3. The collector array for the solar drying system.
The outlet air from the third collector is mixed with the air outlet from the fourth collector. Centrifugal blower is used to induce the hot air. Fig. 4(a) shows the resulting outlet temperature if the flow rate is fixed at 0.0995 kg/s. The outlet temperature is about 90 8C at an average solar radiation level between 900 and 1000 W/m2. In general, the outlet temperature does not drop drastically when the solar radiation decreases due to passing clouds or rain as in any conventional solar air collector. The outlet temperature goes down gradually in the evening even at low solar radiation levels. The reason for this is because of the presence of porous media in the second channel that acts as the storage media for the system. The relevant data are average for an hour and the hourly thermal efficiency are shown in Fig. 4(b). The average thermal efficiency of the collector increases with the flow rate. 2.3. Photovoltaic thermal solar air heater The term PVT refers to solar thermal collectors that use PV cells as an integral part of the absorber plate. The system generates both thermal and electrical energy simultaneously. The number of the photovoltaic cells in the system can be adjusted according to the local load demands. In conventional solar thermal system, external electrical energy is required to circulate the working fluid through the system. The need for an external electrical source can be eliminated by using this hybrid system. With a suitable design, one can produces a self-sufficient solar collector system that required no external electrical energy to run the system. 100 Solar radiation
Outlet temperature
70 60
600
50 40
400
30 Ambient temperature
Air flow rate = 0.0995 kg/s
0 9:30 AM 11:00 AM 12:30 PM 2:00 PM
Inlet temperature
3:30 PM
Time of the day (5th November 1999)
100
100 OutletTemperature
80
800
200
(b)
90
20
Temperature (oC)
Solar radiation (W/m2)
1000
80
80
60
60 Inlet Temperature
40
40 Thermal efficiency
20
Temperature (oC)
1200
Thermal efficiency (%)
(a)
20
10
0 5:00 PM
0 9:00 AM
11:00 AM
1:00 PM
3:00 PM
5:00 PM
0
th
Time of the day (5 November 1999)
Fig. 4. (a) Temperature (outlet, inlet and ambient) and solar radiation for the day (b) hourly thermal efficiency and temperature (inlet and outlet) for the day.
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
707
Solar cell
Glass cover Air in CPC
Insulation
Fin
Fig. 5. A double-pass photovoltaic thermal solar collector system with CPC and fins.
The first multiple-pass concept for the air cooled PVT systems suitable for drying system was introduced by Sopian et al. [9].The double-pass flow enhanced cooling of the photovoltaic cells and thus increasing the efficiency of the systems. The double-pass concepts was later extended to include heat transfer augmentation features such as a fins for enhancing heat removal by convective and conductive heat transfers and also compound parabolic collectors for solar radiation booster [10]. Fig. 5 shows the basic design of the double-pass photovoltaic thermal solar collector with compound parabolic concentrator and fins. The air flowed through the first channel formed by the glass cover and the photovoltaic panel. Next, it enters the second channel formed by the back plate and the photovoltaic panel. The first channel has compound parabolic concentrators to concentrate solar radiation. The second channel has fins that transfer the heat from the photovoltaic panel. This flow arrangement and the compound parabolic concentrators as well as the fins will increase heat removal from the photovoltaic panel and will enhanced the efficiency of the system. Fig. 6 shows the performance characteristics curves of the various configuration of PVT collectors. The PVT collector with CPC and fins has better performance compared to others. The characteristics of the collector are measured at solar radiation level of 700 W/m2 and air flow rate at 0.1 kg/s with inlet temperature ranging from 30 to 54 8C. Thermal efficiency of the system is between 48 and 63.6% with air temperature output of 35.5–58 oC.
80
Overall efficiency (%)
70 60 50
Double pass with CPC & fins
40
Double pass with fins
30
Double pass with flat plate
20
Single pass with flat plate
10 0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035
(Ti - Ta)/S
Fig. 6. Perfomance Characteristics Curves of the CPC photovoltaic thermal air heater and other configurations.
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M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
Fig. 7. Solar Assisted Dehumidification System.
2.4. Solar assisted dehumidification system The used of medicinal herbs as alternatives to modern medications are safe and have virtually no side effects. Drying process is required for the preservation of the product. However, tremendous amount of energy is required for conventional commercial scale drying systems. Traditional methods of drying such as open air drying, where the product to be dried is exposed directly to the sun has many disadvantages such degradation by wind-blown debris, rain, insect infestation, human and animal interference which will result in contamination of the product. The solar assisted dehumidification system is an effective and viable alternative to the many present drying techniques. The system consists of evacuated tube solar collectors, dehumidification system and the drying chamber as shown in Fig. 7. A temperature of 40 8C and air at relative humidity of 20–30% can be achieved in the drying chamber and the operated continuously since an auxiliary heater has been installed in the storage tank. High quality medicinal herbs can be produced using this system [11]. 3. Conclusions The design and performance of four advanced solar assisted forced convective solar dryers have been presented. The first drying system used the V-groove type solar collector and the second drying system used the double-pass solar collector with porous media in the second channel. The second drying system uses the photovoltaic thermal solar collector with heat transfer augmentation features. The fourth dryer is a water based solar collector dehumidification system. The drying chamber for all of the s is the cabinet type. The V-groove collector is suitable for agricultural produce such as chilies, green tea, and dried fruits. The double-pass solar collector with integrated storage system produces a much higher outlet temperatures compared to the first drying system. The connections for the V-groove solar collector is straightforward and simple while the connections for the double-pass solar is a little complicated. The connections for the double-pass solar collector involve internal manifolding. The drying system is used to dry animal feedstock such as oil palm fronds and other higher temperature drying processes. The PVT system generates both thermal and electrical energy simultaneously. The number of the photovoltaic cells in the system can be adjusted according to
M.Y.H. Othman et al. / Renewable Energy 31 (2006) 703–709
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the local load demands and the need for an external electrical source can be eliminated by using this hybrid system. With a suitable design, one can produces a self-sufficient solar collector system that required no external electrical energy to run the system. There are several advantages of using water based solar collector since water has a larger heat capacity than air. In addition, components that uses water as working fluid are smaller in size compared with components that uses air as the working fluid. The use of solar dryers will enhanced the environment, wealth creation and nation building as well as sustainable development for the country.
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