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11Статья поступила в редакцию 03.12.10. Ред. рег. № 912
The article has entered in publishing office 03.12.10. Ed. reg. No. 912
ПРОИЗВОДИТЕЛЬНОСТЬ ТЕМПЕРАТУРНОГО КОНТРОЛЛЕРА ДЛЯ КОСВЕННЫХ СОЛНЕЧНЫХ ОСУШИТЕЛЕЙ, ПРИМЕНЯЕМЫХ НА ФЕРМАХ В ТРОПИЧЕСКИХ РАЙОНАХ
12 3
М. Камта , Г.Б. Чая , С. Капсу
Университет Нгаундере, Факультет электротехники п/я 455, Нгаундере, Камерун E-mail: martin_kamta@yahoo.fr ^Университет Маруа, Факультет возобновляемых источников энергии п/я 46, Маруа, Камерун E-mail: tchayaguy@yhoo.com 3Университет Нгаундере, Факультет технологии производства п/я 455, Нгаундере, Камерун E-mail: kapseu@yahoo.fr
Заключение совета рецензентов: 23.12.10 Заключение совета экспертов: 30.12.10 Принято к публикации: 05.01.11
В данной работе представлены результаты исследований температурных колебаний, вызванных вентиляторами, запи-тываемыми постоянным током от фотогальванических элементов, в косвенном солнечном осушителе. Температурные колебания зарегистрированы и показаны как функция времени при различных погодных условиях. Профили энергетической освещенности и температуры совпадали без регулировок. С другой стороны, температура контролировалась в узком диапазоне 2 °C в области заданного значения. Оценка производительности температурного контроллера показана с помощью равномерного экспонентного затухания кинетики сушки ломтиков сладкого картофеля (ipomaea batatas) толщиной 3 мм с коэффициентом корреляции не менее 0,997.
Ключевые слова: температурный контроллер, косвенные солнечные осушители, кинетика сушки.
PERFORMANCE OF TEMPERATURE CONTROLLER FOR INDIRECT SOLAR DRYERS USED ON FARMS IN TROPICAL AREA
M. Kamta1, G.B. Tchaya2, C. Kapseu3
'University of Ngaoundere, IUT, Department of Electrical Engineering P.O. Box 455, Ngaoundere, Cameroon E-mail: martin_kamta@yahoo.fr 2University of Maroua, ISS, Department of Renewable Energies P.O. Box 46, Maroua, Cameroon E-mail: tchayaguy@yhoo.com 3University of Ngaoundere, ENSAI, Department of Process Engineering P.O. Box 455, Ngaoundere, Cameroon E-mail: kapseu@yahoo.fr
Referred: 23.12.10 Expertise: 30.12.10 Accepted: 05.01.11
In this paper, the variations of temperature in an indirect solar dryer provided with photovoltaic-powered d.c. fans have been recorded and plotted as a function of time, under various weather conditions. The profiles of both irradiance and temperature were almost the same without regulation. On the other hand, the temperature was controlled within a narrow bandwidth of 2 °C, in the vicinity of set point. The validation of the performance of temperature controller was shown by a monotonous decay exponential of drying kinetics of thin layer of sweet potato (ipomaea batatas) slices of 3 mm thick, with a correlation coefficient of at least 0.997.
Keywords: temperature controller, indirect solar dryers, drying kinetics.
Organization: National School of Agro-Industrial Sciences (ENSAI), University of Ngaoundere, Cameroon.
Education: Doctorate in physics of semiconductor devices, 1998. Cycle Doctorate in physics of semiconductor devices, 1989. Post-graduate, 1984. Master's degree in physics, 1983. Graduate in physics and chemistry, 1982. Faculty of sciences, University of Yaounde, Cameroon.
Experience: Senior lecturer in electronics, (ENSAI/IUT). Research activities: Cirad of Montpellier, February and March, 2007. Lab. of ESR spectroscopy, Louis Pasteur's University of Strasbourg, France, (1987-1989) and (1992-1995). Lab. of solid state chemistry, University of Nancy 1, France, 1986.
Main range of scientific interests: Physics of semiconductor devices. Electronics and Photovoltaic systems.
Publications: 3 articles in electron spin resonance (ESR) of defects in III-V materials, 2 articles in superconducting materials, 1 article and 6 communications about photovoltaic systems.
Martin Kamta
Guy Bertrand Tchaya
Organization: National School of Agro-Industrial Sciences (ENSAI), University of Ngaoundere, Cameroon.
Education: Post-graduate, 2009. Master's degree in electronics, electrotechnics and automation (EEA) 2006. Graduate in EEA, 2005. ENSAI and Faculty of sciences, University of Ngaoundere, Cameroon.
Experience: Lecturer in electronics, (High Institute of Sahel, University of Maroua, Cameroon). Research activities: Optimization and Modelling of indirect solar drying by the regulation of certain parameters, application to shea nuts.
Main range of scientific interests: Solar energy. Publications: 3 communications about solar energy.
Introduction
The open sun drying is the oldest traditional method of food preservation. Moreover, it is cheaper cost than the modern drying techniques [1]. However, it depends on weather conditions such that the nutritional and texture quality of dried products are unpredictable [2]. In the literature, diverse drying techniques based on modelling and architectural design of solar dryers are a matter of sustainable scientific research, with the aim to improve its performance [3-7]. Indeed, the improvement of the performance of several solar dryers consists of using fans to force the circulation of heated air through the drying unit [5]. As a result, the use of indirect forced-convection solar dryers may avoid the over drying of products which often leads to discolouration, loss of germination power and nutritional changes [2].
Up to date, few studies are undertaken on the solar drying at controlled temperature. It is in this context that this work is done, consisting to design and carry out a temperature controller to be incorporated in an indirect solar dryer for use on farms in tropical area, where there is no conventional electrical grid.
The objective of this work is to control the temperature in an indirect solar dryer provided by photovoltaic-powered d.c. Fans, when it is emptied or loaded, under various weather conditions.
Principle of the proportional band regulation of temperature
The proportional band regulation of temperature is based on the principle of 'on-off commands' in a feedback control system as is shown in Fig. 1. The blocks of Fig. 1 (i.e.: sensor+signal conditioner, comparator, switch, VCO and speed controller) represent the electronic circuit which has been designed and carried out in laboratory so that the steady-state error should be small. The edges of the temperature band to be controlled in a steady-state are equivalent to Vcmax and Vcmin respectively. Therefore, the desired value in terms of voltage is given by
- Control signal
- Power signal
) Heated air ) Temperature
Vc =
Vc max- Vc min 2
(1)
Рис. 1. Схема системы регулирования температуры с простой замкнутой петлей Fig. 1. Temperature control system block diagram with a simple closed-loop
Under solar radiations, the solar collector converts into heat a portion of solar energy received on its surface and the photovoltaic module converts into electricity the light intensity received on its surface. At the beginning, the switch block is in open position, corresponding to an upper set point Vcmax. Fans are supplied by the photovoltaic module through a speed controller (see Fig. 1) and are running slowly. The heated air is so then forced in the solar drying unit. As a result, the temperature in the drying unit increases until the measured value Vm matches the upper set point Vcmax. At this point, the switch block is closed automatically, changing the set point into Vcmin. Therefore, the voltage controlled oscillator (VCO) generates a control signal with a different duty cycle, so that fans are running at a high speed. As a result, the temperature in the drying unit decreases until the measured value Vm matches the lower set point Vcmin. At this other point, the switch block is opened automatically, changing the set point into Vcmax. Then, a control signal with an initial duty cycle is generated, and fans are running at the initial low speed. A new cycle starts again.
However, the principle of the proportional band regulation of temperature is best understood by the flowchart in Fig. 2. In fact, since the rise in temperature in the indirect solar dryer depends on the weather as well as on the nature of the solar collector, the regulation of
International Scientific Journal for Alternative Energy and Ecology № 11 (91) 2010
© Scientific Technical Centre «TATA», 2010
temperature should be taking place only if the measured value Vm reached the set point Vcmax. In other words, if the temperature inside the drying unit is less than the set point because of the weather, then the regulation should not take place. Let Vcmin be the lower edge of the controlled temperature band; the proportional bandwidth is given in terms of voltage by
P. B. = Vcmax - Vcmin.
(2)
plate was provided with three axial fans of 12 V, 4 W each one, for the forced-convection heat exchange.
The temperature controller was designed and carried out in laboratory so that its use doesn't need any special qualification. The controller electronic circuit was packaged in a wooden box of dimension 0.20x0.20x0.10 m, so that it could be screwed on the body of the drying unit. The controller could be switched on or off by pressing on a contact-breaker. The set point could be selected in the range of 40 °C to 55 °C by a linear potentiometer.
Рис. 2. Диаграма пропорционального регулирования
температуры Fig. 2. Flowchart of the proportional band regulation of temperature
Experimental Material and Methods
Experimental material The architectural structure of indirect solar dryer is shown in Fig. 3. This picture shows three main parts as followed: (6) the solar collector, which is a black body of 2.20x0.48 m area, embedded in a rectangular wooden frame of 10 cm thick and covered by glass; (4) the drying unit of dimension 0.5x0.5x0.6 m, which is built in wood of 10 cm thick, and covered with sheet steel, (2) the chimney of dimensions 0.3 m long and 0.10x0.10 m area. The whole structure is supported by a metal frame at 0.70 m above the ground.
The indirect solar dryer was designed and built by Bup N. D. [8] in our laboratory at the University of Ngaoundere, Cameroon, located at 7.3 °N latitude and 13.3 °E longitude. The tilt angle of the solar collector plate was fixed at 10°. The bottom of the solar collector
Рис. 3. Конструкция косвенного солнечного осушителя:
(1) выход воздуха, (2) труба, (3) дверца, (4) осушаемая среда, (5) поддон, (6) плита солнечного коллектора, (7) пропеллеры, (8) солнечное излучение, (9) стекло,
(10) датчик температуры Fig. 3. Architectural structure of indirect solar dryer: (1) air exit,
(2) chimney, (3) access door, (4) drying medium, (5) tray, (6) solar collector plate, (7) fans, (8) solar radiations, (9) glass,
(10) temperature sensor
A data acquisition unit (ALMEMO 23 90-5 S) was used to record the variations of both irradiance and temperature with a time increment of 2 Min during sunny days and cloudy days, using a NTC thermistor as temperature sensor and a FLA613-GS type pyranometer (1200 W/m2, 400-1100 nm) as sunlight sensor. All of fans, controller electronic circuit and data acquisition unit were supplied by a NP50G type photovoltaic module of 16.7 V, 3 A, 50 Wc.
Experimental methods
Generally for the experimental methods, data were recorded every 2 Min by a data acquisition unit. The experiments were carried out under various weather conditions in three stages: Firstly the drying chamber was unloaded and the controller was switched off. Secondly the drying chamber was unloaded and the controller was switched on. Thirdly, the drying chamber was loaded with thin layer of sweet potato (ipomaea batatas) slices of 3 mm thick and the controller was switched on.
An electronic digital weighing balance (SARTORIUS, ±0.001 g) was used to weigh the sweet potato slices before and during the drying operation. Before the drying, samples were weighted and distributed uniformly on the tray. During the drying, samples were unloaded from the drying chamber, and then weighted and loaded again. The period of measurements was of 15 min. The moisture content on water basic (w.b.) was the expressed by
Moisture content = —- x 100 (% w.b.), (3) m„
where mt is the weight of the samples at each period of measurements and m0 the initial weight of fresh samples.
Results and Discussion
Fig. 4 shows the variations of both irradiance and temperature as a function of time during a sunny day on march 2010, with high sky index of cleanness. Without regulation, it is obvious that the profiles of both irradiance and temperature are almost the same from 9 am to 13 pm. However, in the afternoon, the irradiance decreases more quickly than the temperature. This phenomenon may depend on the heat transfer coefficient of solar collector used [8]. The maximum temperature monitored in the drying chamber was 51±1 °C from 12:30 am to 13:00 pm, corresponding to a maximum irradiance of 1000 W/m2. The ambient temperature was in the range of 30-35 °C on that day. This result allows to discuss the cases where the temperature is controlled, the profile of the irradiance as a function of time being an indicator of sky index of cleanness.
Fig. 5 shows the profiles of both irradiance and controlled temperature as a function of time during a sunny day on march 2010, with a high sky index of cleanness. The temperature controller was switched on and the set point was selected at 50 °C by just acting on a potentiometer. At the beginning, the temperature curve reached the desired value after few minutes. This is the rise time of temperature, which can be estimated to 10 min. After the rise time of temperature, the profile of temperature remains flat without pumping phenomenon in the vicinity of 50±1 °C. The bandwidth of 2 °C was fixed by analysing the electronic circuit of controller. In the afternoon when the irradiance is low, the profile of temperature remains flat for few hours. In fact, the black
body of solar collector has the property to stock heat that can be restored even if the irradiance shifts. The design of the controller electronic circuit has been carried out so that the pumping phenomenon did not occur at any weather conditions.
Рис. 4. Профили энергетической освещенности и температуры в ситуации, когда косвенный солнечный осушитель не загружен и контроллер выключен в течение солнечного дня Fig. 4. Profiles of both irradiance and temperature when the indirect solar dryer is unloaded and the controller is switched off during a sunny day
Рис. 5. Профили энергетической освещенности и температуры в ситуации, когда косвенный солнечный осушитель не загружен и контроллер включен на 50 °С
в течение солнечного дня Fig. 5. Profiles of both irradiance and temperature when the indirect solar dryer is unloaded and the controller is switched on with a set point of 50 °C during a sunny day
It was interesting to know what is happening on cloudy days, using the temperature controller. On a cloudy day of April 2010, the variations of the irradiance shows that the sky index of cleanness was varied stochastically, (see Fig. 6). However, the profile of the controlled temperature remains flat, in the vicinity of the set point of 45±1 °C. A similar experiment was carried out by Sreekumar A. et al. [5], using an indirect solar dryer provided with two fans, both running with a constant speed during the drying operation. The profile of the temperature obtained was not flat at all, as is observed in this work.
International Scientific Journal for Alternative Energy and Ecology № 11 (91) 2010
© Scientific Technical Centre «TATA», 2010
Рис. 6. Профили энергетической освещенности и температуры в ситуации, когда косвенный солнечный осушитель не загружен и контроллер включен на 45 °С
в течение облачного дня Fig. 6. Profiles of both irradiance and temperature when the indirect solar dryer is unloaded and the controller is switched on with a set point of 45 °C during a cloudy day
Рис. 7. Профили энергетической освещенности и температуры в ситуации, когда косвенный солнечный осушитель не загружен и контроллер включен на 40 °C,
45 °C, 50 °C, 55 °C, 45 °C в течение солнечного дня Fig. 7. Profiles of both irradiance and temperature when the indirect solar dryer is unloaded and the controller is switched on with set points of: 40 °C, 45 °C, 50 °C, 55 °C, 45 °C, during a sunny day
110 100 90 80 70 60 50 40
30
10
11
12 13 14 Time (h)
15
16
Рис. Fig.
8. Кинетика сушки ломтиков сладкого картофеля
(ipomaea batatas) 3 мм толщиной 8. Drying kinetics of sweet potato (ipomaea batatas) slices of 3 mm thick
It was also interesting to know how the kinetic drying of agricultural products can be influenced by changing the set point in situ. Fig. 7 shows the regulation of temperature at desired set points during a sunny day on December 2008. A potentiometer was just used to select the set point. This test was also repeated on February 2009. Fortunately, the same result was observed [9]. Furthermore, there is no pumping phenomenon during the transient phases. So, this result proves the performance of the temperature control system (i.e. photovoltaic-powered fans through an electronic circuit of command, see Fig. 1) incorporated in an indirect solar dryer [10].
For the experimental validation of the performance of temperature control system designed and carried out, drying kinetics of thin layer of sweet potato (ipomaea batatas) slices of 3 mm thick have been measured and plotted as it is shown in Fig. 8. Close to 870 g of fresh sweet potato was used [11]. The test was performed on sunny days, with a maximum ambient temperature and irradiance closed to 40 °C and 1000 W/m2 respectively, as it is shown in Table. According to Fig. 8, it is obvious that the drying kinetics follows a monotonous decay exponential law given by
Y (t ) = Y0 + ae
-b( t -t0)
(4)
where Y0 is lowest moisture content and t0 is the beginning time of measurements which have been fixed to 10.50 hours in this work.
Experimental solar drying parameters of sweet potato slices Экспериментальные параметры солнечного осушителя при сушке ломтиков сладкого картофеля
Parameter Set point Tc, °C
40 50
Ambient temperature range, °C 35-38 35-40
Minimum irradiance, W/m2 600 652
Maximum irradiance, W/m2 1000 1000
Solar collector surface, m2 1.05 1.05
Surface of a tray, m2 0.25 0.25
Initial mass of fresh product, g 878 870
Sweet potato slices thickness, mm 3 3
Fitting parameters
Y0, %(w.b.) 2.06 ± 0.63 30.38 ± 0.55
a, %(w.b.) 52.30 ± 0.35 61.50 ± 0.71
b, h-1 0.369 ± 0.007 0.567 ± 0.009
correlation coefficient 0.999 0.997
Beginning time of measurements t0, h 10.50 10.50
Time constant of drying kinetic t, h 2.71 1.76
The theoretical model given by eq. (4) fits suitably the experimental data with a correlation coefficient of at least 0.997. The fitting parameters Y0, a and b are given in table 1 for the set points of 40 °C and 50 °C respectively. Similar experiments were carried out using indirect solar dryers provided with fans [5], and the profile of drying kinetics was not as monotonous as is observed in this work. Obviously, the values of time constants of drying kinetics depend on set points, i.e. the more is the value of set point, the more is the drying speed. Indeed, drying kinetics depend also on the thickness of the slices of product [8]. It could be deduced from the fitting parameters that, close to 62% of the product's total water content could be evacuated after 5 time constants for a set point of 50 °C, i.e. during 8.8 h of drying operation.
Conclusion
A temperature controller has been designed and constructed, the aim being to improve the performance of indirect solar dryers provided with photovoltaic-powered d.c. fans for farms in tropical area, where there is no conventional electrical grid. This temperature controller was designed so that its use doesn't need any special qualification. Indeed, the set point could be chosen by acting on a potentiometer during a drying operation. Its efficiency has been tested by controlling the temperature in a solar dryer when it was unloaded or loaded, under various weather conditions.
Results showed that without regulation, the profiles of both irradiance and temperature in the solar dryer were almost the same, the irradiance being considered here as an indicator of the sky index of cleanness. Otherwise, while the controller was switched on, the profile of temperature in the solar dryer was flat in the vicinity of set point, within a narrow bandwidth of 2 °C, the sky index of cleanness being whether high or low. The validation of the performance of this controller was shown by a monotonous decay exponential of drying kinetics of thin layer of sweet potato (ipomaea batatas) slices of 3 mm thick, with a correlation coefficient of at least 0.997. For set points of 40 °C and 50 °C, time constants of 2.71 h and 1.76 h were obtained respectively, by a suitable fitting of experimental data with an exponential function. Then, after 5 time constants (i.e. 8.8 h), 62 % of product's total water content could be evacuated for a set point of 50 °C.
Acknowledgements
The authors would like to acknowledge Dr. Bup N. D. for the design and construction of indirect solar dryer used in this work.
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International Scientific Journal for Alternative Energy and Ecology № 11 (91) 2010
© Scientific Technical Centre «TATA», 2010
