- Email: [email protected]
Experimental study of a double exposure solar cooker with finned cooking vessel
Available online at www.sciencedirect.com
Solar Energy 82 (2008) 287–289 www.elsevier.com/locate/solener
Experimental study of a double exposure solar cooker with finned cooking vessel Arezki Harmim *, Mebarek Boukar, M’hamed Amar Unite´ de Recherche en Energies, Renouvelables en Milieu Saharien, P.O. Box 478, Adrar, Algeria Received 27 March 2007; received in revised form 15 July 2007; accepted 31 October 2007 Available online 26 November 2007 Communicated by: Associate Editor M. Grupp
Abstract A comparative experimental study of a box type solar cooker with two different cooking vessels was conducted, the first one conventional and the second one identical to the first in shape and volume but its external lateral surface provided with fins. Fins are shown to improve the heat transfer from the internal hot air of the cooker towards the interior of the vessel where the food to be cooked is placed. This reduces the cooking time considerably. The tests were carried out on the experimental platform of the Research Unit in Renewable Energies in Saharan Medium of Adrar, located at 27°53 0 N latitude and 0°17 0 W longitude in the Algerian Sahara. Ó 2007 Published by Elsevier Ltd. Keywords: Double exposure solar cooker; Finned cooking vessel; Cooking time
1. Introduction The heat transfer into the pot in solar cookers of different types was the subject of a number of experimental and theoretical studies (Grupp et al., 1991; Binark and Turkmen, 1996; Nahar, 2001; Amer, 2003; Ekechukwu and Ugwuoke, 2003; Narasimha Rao and Subramanyam, 2003; Negi and Purohit, 2005; Petela, 2005). It was observed that typical cooking times are about 2–3 h for box types and 1–2 h for concentrating types. For a given type of solar cooker it is possible to reduce the cooking time by carrying out modifications on the shape of the cooking vessel. These modifications can improve heat transfer to the food through the pot walls. In this objective Gaur et al. (1999) proposed a cooking vessel provided with a concave lid. Their experimental study showed a reduction of 10–13% in cooking time compared to an ordinary cooking vessel under the same conditions.
*
Corresponding author. Tel.: +213 49 96 51 68; fax: +213 49 96 04 92. E-mail address: [email protected] (A. Harmim).
0038-092X/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.solener.2007.10.008
The most recent work in this field by Narasimha Rao and Subramanyam (2005) showed that cooking vessels provided with a central annular cavity increase the effective area of heat transfer into the pot content and reduce the cooking time. The objective of the present work is to investigate a finned cooking vessel in order to increase efficiency of solar cookers and to reduce cooking time. The test pot is an ordinary cylindrical kitchen pot whose lateral external surface is provided with fins distributed around the circumference. This configuration increases the surface of heat transfer towards the interior of the vessel and keeps an adequate volume to contain the food to be cooked. 2. Description of the solar cooker and the vessel The solar cooker used in the present investigation is of the double exposure type. A schematic sketch of this cooker is shown in Fig. 1. It consists of a box type solar cooker with a double glazed bottom which allows the absorber to receive solar radiation on its lower side with the help of a parabolic reflector disposed under the cooker. The cooker
288
A. Harmim et al. / Solar Energy 82 (2008) 287–289 Hole for installation of thermocouple
Plan reflector
Lids
Vessel Door
Absorber
Vessels Parabolic reflector
Fins
Conventional cooking vessel
Finned cooking vessel
Fig. 3. Photograph of the two cooking vessels used in our comparative study.
Fig. 1. Schematic sketch of the double exposure solar cooker.
vessels is provided with fins made of aluminium painted black. Fins are of rectangular constant cross section (5.5 cm by 0.05 cm) and have a length of 2.2 cm; they are spaced at 1.5 cm. The finned cooking vessel and the conventional one are shown in Fig. 3.
120 800
Temperature (°C)
100 600
80 60
Solar radiation Finned vessel Conventional vessel Ambient
40
400
200
Solar radiation (W/m²)
box is equipped with three glass mirrors (43 cm by 26 cm) encased in sheet metal shell fixed using hinges to the framework of the upper side of the cooker. In this manner, the absorber is exposed to solar radiation from two sides. When the cooker is not used, the glass mirrors are folded and are used as a cover for the box cooker. The absorber consists of a galvanised steel sheet painted black of a surface of 40 cm per 22 cm and 0.15 cm thickness. A door is provided on a lateral side of the box to access the cooking vessel. The parabolic reflector is a section of a linear parabolic concentrator with a focal length of 30 cm. It is made of rectangular glass mirrors fixed on a wooden framework of 80 cm length and 45 cm width. The photograph of the double exposure solar cooker prototype is shown in Fig. 2. For the present study, two cooking vessels were used. They are made of aluminium painted black, are cylindrical in shape and have flat base. Both the cooking vessels have an identical lid, they have a diameter of 14 cm and a height of 7 cm. The lateral external surface of one of the cooking
20 0
0 10.5
11.0
11.5
12.0
Local time (hours)
Fig. 4. Water heating test. Comparison between water temperature in the finned cooker vessel and water temperature in a conventional cooker vessel. Experiment was conducted on February 25th, 2007.
120 800
600
80
60
Solar radiation Finned vessel Conventionalvessel Ambient
40
400
200
Solar radiation (W/m²)
Temperature (°C)
100
20 0 10.0
10.5
11.0
11.5
Local time (hours)
Fig. 2. Photograph of the experimental setup of the double exposure solar cooker.
Fig. 5. Water heating test. Comparison between water temperature in the finned cooker vessel and water temperature in a conventional cooker vessel. Experiment was conducted on March 13th, 2007.
A. Harmim et al. / Solar Energy 82 (2008) 287–289
289
Table 1 Results of water heating test (0.8 l in each vessel) Date
Initial water temperature (°C) Mean ambient temperature (°C) Mean solar radiation (W/m2) Time for boiling temperature with finned vessel (min) Time for boiling temperature with conventional vessel (min) Reduction in using finned vessel (min) Difference in power (W)
3. Experimental study Thermal performance testing of the solar cooker was conducted in Adrar, Algeria. Adrar is located at 27°53 0 N latitude and 0°17 0 W longitude. During each test, both cooking vessels were placed side by side on the absorber of the solar cooker and loaded with the same mass of water at the same temperature. The temperatures of the water in each vessel, of the absorber, of the air in the cooker as well as ambient temperature and horizontal irradiation were recorded at 1 min intervals using a data logger system. Solar irradiation was measured by a class 2 CM11 type pyranometer. All temperatures were measured by copperconstantan thermocouples. The thermocouple used for the temperature measurement of water inside the vessel is introduced through a small hole at the lid center. Once both the vessels filled with water were placed in the cooker, the door was closed until test end. During tests, the cooker was manually oriented according to azimuth at an interval of 15 min in order to collect a maximum of solar radiation. 4. Results and discussion It was found that the temperature of the water in the finned cooking vessel was always higher than the temperature of water in the conventional cooking vessel. Fig. 4 presents the temperature–time history of the water in both cooking vessels under the same test conditions on February 25th, 2007. The time taken for attaining boiling temperature (99 °C) by the two cooking vessels was 91 min for the finned, and 103 min for the conventional vessel. Fig. 5 shows the temperature–time history of the water in both cooking vessels when testing was started with a different initial water temperature on March 13th, 2007. The initial water temperature in the finned cooking vessel and in the conventional one was 25 and 30 °C, respectively. After 17 min, water in both the vessels was at the same temperature (44 °C). The water in the finned cooking vessel attained boiling temperature nearly 14 min earlier than the water in the conventional cooking vessel. The finned
25/02/07
13/03/07
19/03/07
25/03/07
15.6 22.2 711 91 103 12 (11%) 5.95
44.0 21.9 732 59 73 14 (19%) 9.96
14.1 24.1 742 86 104 18 (17%) 9.52
27.2 21.5 661 108 123 15 (12%) 4.52
cooking vessel took about 12–18 min less time than the conventional one for the same temperature rise in water in both the vessels. Results of the water heating test are given in Table 1. 5. Conclusion It was experimentally demonstrated that cooking time can be reduced by using a finned cooking vessel. This reduction is consistent with the increase of the heat transfer surface area by fins attached to the external surface of the cooking vessel. The average difference in power is 7.49 W. In future work, dimensions and geometry of the fins should be studied in more detail in order to optimize the performances of this kind of cooking vessel. References Amer, Emad H., 2003. Theoretical and experimental assessment of a double exposure solar cooker. Energy Conversion and Management 44, 2651–2663. Binark, A.K., Turkmen, N., 1996. Modelling of hot box solar cooker. Energy Conversion and Management 37, 303–310. Ekechukwu, O.V., Ugwuoke, N.T., 2003. Design and measured performance of a plane reflector augmented box-type solar-energy cooker. Renewable Energy 28, 1935–1952. Gaur, A., Singh, O.P., Singh, S.K., Pandey, G.N., 1999. Performance study of solar cooker with modified utensil. Renewable Energy 18, 121–129. Grupp, M., Montagne, P., Wackernagel, M., 1991. A novel advanced boxtype solar cooker. Solar Energy 47, 107–113. Nahar, N.M., 2001. Design, development and testing of a double reflector hot box solar cooker with a transparent insulation material. Renewable Energy 23, 167–179. Narasimha Rao, A.V., Subramanyam, S., 2003. Solar cookers – Part I: cooking vessel on lugs. Solar Energy 75, 181–185. Narasimha Rao, A.V., Subramanyam, S., 2005. Solar cookers – Part II: cooking vessel with central annular cavity. Solar Energy 78, 19–22. Negi, B.S., Purohit, I., 2005. Experimental investigation of a box type solar cooker employing a non-tracking concentrator. Energy Conversion and Management 46, 575–604. Petela, R., 2005. Exergy analysis of the solar cylindrical-parabolic cooker. Solar Energy 79, 221–233.
Solar Energy 82 (2008) 287–289 www.elsevier.com/locate/solener
Experimental study of a double exposure solar cooker with finned cooking vessel Arezki Harmim *, Mebarek Boukar, M’hamed Amar Unite´ de Recherche en Energies, Renouvelables en Milieu Saharien, P.O. Box 478, Adrar, Algeria Received 27 March 2007; received in revised form 15 July 2007; accepted 31 October 2007 Available online 26 November 2007 Communicated by: Associate Editor M. Grupp
Abstract A comparative experimental study of a box type solar cooker with two different cooking vessels was conducted, the first one conventional and the second one identical to the first in shape and volume but its external lateral surface provided with fins. Fins are shown to improve the heat transfer from the internal hot air of the cooker towards the interior of the vessel where the food to be cooked is placed. This reduces the cooking time considerably. The tests were carried out on the experimental platform of the Research Unit in Renewable Energies in Saharan Medium of Adrar, located at 27°53 0 N latitude and 0°17 0 W longitude in the Algerian Sahara. Ó 2007 Published by Elsevier Ltd. Keywords: Double exposure solar cooker; Finned cooking vessel; Cooking time
1. Introduction The heat transfer into the pot in solar cookers of different types was the subject of a number of experimental and theoretical studies (Grupp et al., 1991; Binark and Turkmen, 1996; Nahar, 2001; Amer, 2003; Ekechukwu and Ugwuoke, 2003; Narasimha Rao and Subramanyam, 2003; Negi and Purohit, 2005; Petela, 2005). It was observed that typical cooking times are about 2–3 h for box types and 1–2 h for concentrating types. For a given type of solar cooker it is possible to reduce the cooking time by carrying out modifications on the shape of the cooking vessel. These modifications can improve heat transfer to the food through the pot walls. In this objective Gaur et al. (1999) proposed a cooking vessel provided with a concave lid. Their experimental study showed a reduction of 10–13% in cooking time compared to an ordinary cooking vessel under the same conditions.
*
Corresponding author. Tel.: +213 49 96 51 68; fax: +213 49 96 04 92. E-mail address: [email protected] (A. Harmim).
0038-092X/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.solener.2007.10.008
The most recent work in this field by Narasimha Rao and Subramanyam (2005) showed that cooking vessels provided with a central annular cavity increase the effective area of heat transfer into the pot content and reduce the cooking time. The objective of the present work is to investigate a finned cooking vessel in order to increase efficiency of solar cookers and to reduce cooking time. The test pot is an ordinary cylindrical kitchen pot whose lateral external surface is provided with fins distributed around the circumference. This configuration increases the surface of heat transfer towards the interior of the vessel and keeps an adequate volume to contain the food to be cooked. 2. Description of the solar cooker and the vessel The solar cooker used in the present investigation is of the double exposure type. A schematic sketch of this cooker is shown in Fig. 1. It consists of a box type solar cooker with a double glazed bottom which allows the absorber to receive solar radiation on its lower side with the help of a parabolic reflector disposed under the cooker. The cooker
288
A. Harmim et al. / Solar Energy 82 (2008) 287–289 Hole for installation of thermocouple
Plan reflector
Lids
Vessel Door
Absorber
Vessels Parabolic reflector
Fins
Conventional cooking vessel
Finned cooking vessel
Fig. 3. Photograph of the two cooking vessels used in our comparative study.
Fig. 1. Schematic sketch of the double exposure solar cooker.
vessels is provided with fins made of aluminium painted black. Fins are of rectangular constant cross section (5.5 cm by 0.05 cm) and have a length of 2.2 cm; they are spaced at 1.5 cm. The finned cooking vessel and the conventional one are shown in Fig. 3.
120 800
Temperature (°C)
100 600
80 60
Solar radiation Finned vessel Conventional vessel Ambient
40
400
200
Solar radiation (W/m²)
box is equipped with three glass mirrors (43 cm by 26 cm) encased in sheet metal shell fixed using hinges to the framework of the upper side of the cooker. In this manner, the absorber is exposed to solar radiation from two sides. When the cooker is not used, the glass mirrors are folded and are used as a cover for the box cooker. The absorber consists of a galvanised steel sheet painted black of a surface of 40 cm per 22 cm and 0.15 cm thickness. A door is provided on a lateral side of the box to access the cooking vessel. The parabolic reflector is a section of a linear parabolic concentrator with a focal length of 30 cm. It is made of rectangular glass mirrors fixed on a wooden framework of 80 cm length and 45 cm width. The photograph of the double exposure solar cooker prototype is shown in Fig. 2. For the present study, two cooking vessels were used. They are made of aluminium painted black, are cylindrical in shape and have flat base. Both the cooking vessels have an identical lid, they have a diameter of 14 cm and a height of 7 cm. The lateral external surface of one of the cooking
20 0
0 10.5
11.0
11.5
12.0
Local time (hours)
Fig. 4. Water heating test. Comparison between water temperature in the finned cooker vessel and water temperature in a conventional cooker vessel. Experiment was conducted on February 25th, 2007.
120 800
600
80
60
Solar radiation Finned vessel Conventionalvessel Ambient
40
400
200
Solar radiation (W/m²)
Temperature (°C)
100
20 0 10.0
10.5
11.0
11.5
Local time (hours)
Fig. 2. Photograph of the experimental setup of the double exposure solar cooker.
Fig. 5. Water heating test. Comparison between water temperature in the finned cooker vessel and water temperature in a conventional cooker vessel. Experiment was conducted on March 13th, 2007.
A. Harmim et al. / Solar Energy 82 (2008) 287–289
289
Table 1 Results of water heating test (0.8 l in each vessel) Date
Initial water temperature (°C) Mean ambient temperature (°C) Mean solar radiation (W/m2) Time for boiling temperature with finned vessel (min) Time for boiling temperature with conventional vessel (min) Reduction in using finned vessel (min) Difference in power (W)
3. Experimental study Thermal performance testing of the solar cooker was conducted in Adrar, Algeria. Adrar is located at 27°53 0 N latitude and 0°17 0 W longitude. During each test, both cooking vessels were placed side by side on the absorber of the solar cooker and loaded with the same mass of water at the same temperature. The temperatures of the water in each vessel, of the absorber, of the air in the cooker as well as ambient temperature and horizontal irradiation were recorded at 1 min intervals using a data logger system. Solar irradiation was measured by a class 2 CM11 type pyranometer. All temperatures were measured by copperconstantan thermocouples. The thermocouple used for the temperature measurement of water inside the vessel is introduced through a small hole at the lid center. Once both the vessels filled with water were placed in the cooker, the door was closed until test end. During tests, the cooker was manually oriented according to azimuth at an interval of 15 min in order to collect a maximum of solar radiation. 4. Results and discussion It was found that the temperature of the water in the finned cooking vessel was always higher than the temperature of water in the conventional cooking vessel. Fig. 4 presents the temperature–time history of the water in both cooking vessels under the same test conditions on February 25th, 2007. The time taken for attaining boiling temperature (99 °C) by the two cooking vessels was 91 min for the finned, and 103 min for the conventional vessel. Fig. 5 shows the temperature–time history of the water in both cooking vessels when testing was started with a different initial water temperature on March 13th, 2007. The initial water temperature in the finned cooking vessel and in the conventional one was 25 and 30 °C, respectively. After 17 min, water in both the vessels was at the same temperature (44 °C). The water in the finned cooking vessel attained boiling temperature nearly 14 min earlier than the water in the conventional cooking vessel. The finned
25/02/07
13/03/07
19/03/07
25/03/07
15.6 22.2 711 91 103 12 (11%) 5.95
44.0 21.9 732 59 73 14 (19%) 9.96
14.1 24.1 742 86 104 18 (17%) 9.52
27.2 21.5 661 108 123 15 (12%) 4.52
cooking vessel took about 12–18 min less time than the conventional one for the same temperature rise in water in both the vessels. Results of the water heating test are given in Table 1. 5. Conclusion It was experimentally demonstrated that cooking time can be reduced by using a finned cooking vessel. This reduction is consistent with the increase of the heat transfer surface area by fins attached to the external surface of the cooking vessel. The average difference in power is 7.49 W. In future work, dimensions and geometry of the fins should be studied in more detail in order to optimize the performances of this kind of cooking vessel. References Amer, Emad H., 2003. Theoretical and experimental assessment of a double exposure solar cooker. Energy Conversion and Management 44, 2651–2663. Binark, A.K., Turkmen, N., 1996. Modelling of hot box solar cooker. Energy Conversion and Management 37, 303–310. Ekechukwu, O.V., Ugwuoke, N.T., 2003. Design and measured performance of a plane reflector augmented box-type solar-energy cooker. Renewable Energy 28, 1935–1952. Gaur, A., Singh, O.P., Singh, S.K., Pandey, G.N., 1999. Performance study of solar cooker with modified utensil. Renewable Energy 18, 121–129. Grupp, M., Montagne, P., Wackernagel, M., 1991. A novel advanced boxtype solar cooker. Solar Energy 47, 107–113. Nahar, N.M., 2001. Design, development and testing of a double reflector hot box solar cooker with a transparent insulation material. Renewable Energy 23, 167–179. Narasimha Rao, A.V., Subramanyam, S., 2003. Solar cookers – Part I: cooking vessel on lugs. Solar Energy 75, 181–185. Narasimha Rao, A.V., Subramanyam, S., 2005. Solar cookers – Part II: cooking vessel with central annular cavity. Solar Energy 78, 19–22. Negi, B.S., Purohit, I., 2005. Experimental investigation of a box type solar cooker employing a non-tracking concentrator. Energy Conversion and Management 46, 575–604. Petela, R., 2005. Exergy analysis of the solar cylindrical-parabolic cooker. Solar Energy 79, 221–233.