- Email: [email protected]
dryers
ARTICLE IN PRESS
Renewable Energy 33 (2008) 2207–2211 www.elsevier.com/locate/renene
Design and development of efficient multipurpose domestic solar cookers/dryers Naveen Kumar, Sagar Agravat, Tilak Chavda, H.N. Mistry Sardar Patel Renewable Energy Research Institute, Vallabh Vidyanagar, Gujarat 388 120, India Received 7 June 2007; accepted 9 January 2008 Available online 3 March 2008
Abstract A truncated pyramid-type solar cooker is designed, fabricated and tested. The truncated pyramid geometry concentrates the incident light radiations towards the bottom and the glazing glass surface on the top facilitates the trapping of energy inside the cooker. One of the salient features of the proposed design is to completely eradicate the need for tracking the sun during cooking, as tracking of sun does not yield better performance. During testing, the highest plate stagnation temperature, under no-load condition, approached 140 1C and under full-load condition, water temperature inside the cooker reached 98.6 1C in 70 min. Two figures of merit, F1 and F2, were calculated and their values were 0.1171 C m2/W and 0.467 1C l, respectively, meeting the standards prescribed by the Bureau of Indian Standards for solar box-type cookers. Minor modifications in design are recommended to achieve higher temperatures and reduce cooking times. The design also allows trays to be retained for use as a household dryer. r 2008 Elsevier Ltd. All rights reserved. Keywords: Solar cooker; Solar oven; Truncated pyramid solar concentrator; Thermal performance; Dryer cum cooker
1. Introduction Solar box-type cookers (SBCs) and SK-14-type concentrating cookers are widely accepted as practical pollutionfree renewable energy solutions for cooking/baking [1,2]. Despite many advantages, however, there are some drawbacks and practical inconveniences, which do not allow these devices to become more popular. For example, in SBCs, it takes several hours to complete cooking. Whereas, in SK-14-type with aperture area 1 m2, cooking completes in 45 min, which is reasonably good, the concentrator needs to track the sun after every 15 min for getting higher performance. Besides, it requires very careful operations because of risks involved in handling high temperature. The high-intensity concentrated radiation can also potentially damage eyes. Though high temperatures are achieved in solar concentrators, utilization of the total captured energy is done very poorly. Energy losses are very high because the concentrated energy escapes (not trapped), Corresponding author. Tel.: +91 2692231332; fax: +91 2692237982.
E-mail address: [email protected] (N. Kumar). 0960-1481/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.01.010
whereas in SBCs it is trapped [3,4]. The typical highest stagnation plate temperature achieved in box-type cookers is 130 1C and in SK-14-type concentrators it is 280 1C. Thus, it is desirable that a solar cooker/oven should be designed and manufactured, which can achieve high temperatures [5], is safe in operation and reasonably low in cost. Additionally, the cost of SK-14-type concentrators prohibits their use at village levels. The market price of a SBC is around Rs. 1700/- and SK-14-type concentrator costs around Rs. 6000/-. Thus, ranges of solar devices to fill the gap (difference in prices) for these applications, with different efficiency and prices, are necessary for product diversifications. Through the present paper, a truncated pyramid-type solar cooker is described, which has features to achieve high temperatures, reduce cooking times and is low on cost. The proposed solar cooker price of about Rs. 2000/- and the cooking time could be reduced to 1.25 h. In addition a facility of food drying has been incorporated in the devices to make it multipurpose. This study aims to report the design, fabrication and testing of the concept of truncated pyramid geometry for developing a solar dryer
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cum cooker suitable for household applications. The proposed design is fabricated and the device is tested for solar cooking only. 2. Truncated pyramid-type solar cooker The concept of concentrating solar energy was utilized to achieve high temperatures, as done in SK-14-type concentrators and to trap the collected energy, as done in SBCs, to retain high temperatures over a considerable period of time to minimize losses. Combining these two advantages into a single design resulted in a new truncated pyramid-shaped solar cooker with enhanced performances. Owing to the geometry of the design (see Fig. 1), rays hitting the inner sidewalls (made of highly reflective anodized aluminum) of the truncated pyramid concentrator are reflected downward, so as to create a zone of high temperature at the bottom. In such geometries, a larger value of angle y leads to a smaller size of the bottom absorbing plate and hence higher concentration ratios. However, there is a restriction Sunrays Glazed surface
Bottom absorbing plate Fig. 1. Schematic diagram of a truncated pyramid-type solar cooker.
on the angle y and numbers of reflections in such geometries so that incident rays reach the bottom absorbing plate as given below [6]: 2ðn þ 1Þy þ ap90 ,
(1)
where n is the number of reflections and a the angle made by the sun (incident rays) on the glass glaze surface. If the incident radiations do not satisfy this condition, rays will reverse their path and exit out of the concentrator instead of reaching the bottom plate. For the time period of 7 h (typical average time period for sunshine hours in a day for cooking/drying), the maximum value of angle a would be 521. Assuming n ¼ 1, the maximum permissible value (using Eq. (1)) of the angle y would be E101. In our conceptual design the angle y ¼ 101, which also gives an approximate optimum size of the bottom absorbing plate. Thus, the pyramid model proposed here concentrates light on the vessel placed at the bottom, irrespective of the position of sun, thereby avoiding the need for tracking the sun. The fabricated model (see Fig. 2) was tested to confirm the concept and to find the experimental optimum correlation between the values of angles y and a and their impact on the shape and size of the bottom absorbing plate. The glazing surface is kept horizontal and a variable slant is provided to the glazing surface by adjustable angular tilts to the base of the proposed cooker to receive maximum insolation, despite seasonal variations in the position of the sun. The value of a can be changed by an appropriate tilt to the base of the device. The glazed surface of the fabricated cooker was made of glass of size 50 cm 50 cm, as was chosen in standard SBCs used in India. The size of the bottom absorbing plate was 32.6 cm 32.6 cm. This plate was made of aluminum and painted black to absorb maximum solar energy. The depth of the device (distance between the glazing surface and the bottom absorbing plate) was 49.2 cm (see Fig. 2(a)) whereas in SBCs, it was 8 cm only. The relatively larger depth facilitates the provision of its acting as a dryer with trays kept vertically inside the chamber (see Fig. 2(b)), thereby making it a multipurpose device.
Fig. 2. Just functional conceptual truncated pyramid dryer cum cooker realized by SPRERI: (a) Truncated pyramid type solar cooker, and (b) truncated pyramid type solar dryer.
ARTICLE IN PRESS N. Kumar et al. / Renewable Energy 33 (2008) 2207–2211
3. Methodology for testing In order to test the performance of the newly fabricated solar cooker, two figures of merit (FOM), viz. F1 and F2 are generally calculated [7,8]. The first FOM, F1, is defined as the ratio of optical efficiency to the heat loss factor by the bottom absorbing plate and is a measure of the differential temperature gained by the absorbing plate at a particular level of solar insolation. The second FOM, F2, is more or less independent of climatic conditions and gives an indication of heat transfer from the absorbing plate to the water in the containers placed on the plate. In this section, the procedure and methodology of determining F1, F2 and the standard boiling curve, generally adopted in the analysis of SBC [7,8], are briefly outlined. 3.1. Determination of F1 The value of F1 is determined by a stagnation test carried out under no-load condition and is mathematically expressed as F1 ¼
Z T pz T az ¼ , U LS Gs
2209
(t2t1) the time taken for heating water from TW1 to TW2 in seconds, Ta (1C) the average ambient temperature from the time period t1 to t2, G (W/m2) the average solar radiation from the time period t1 to t2 and (MC)w the product of the mass of water and specific heat (J/1C). The full-load test was started in the mornings between 10.00 a.m. and 10.30 a.m. of local solar time. For BIS certification, the required value of F2 for SBC is no less than 0.4 1C l. 3.3. Determination of standard boiling time After knowing F1 and F2, the time for sensible heating from the ambient temperature up to 100 1C, which is also known as the standard boiling time tboil [7,8] is calculated as F 1 ðMCÞw ð100 T a Þ tboil ¼ ln 1 . (4) F 1G 60AF 2 This time is a function of ð100 T a Þ=F 1 G and thus the plot of tboil versus ð100 T a Þ=F 1 G could be referred to as the characteristic curve of the cooker [7].
(2)
where Z is the optical efficiency, defined as the fraction of the incident solar radiation, which reaches the absorber and gets absorbed and ULS the heat loss factor at stagnation. The stagnation temperature of the absorbing plate (Tpz), ambient temperature (Taz) and solar insolation (Gs) are measured when a steady state is reached. According to guidelines of the Bureau of Indian Standards (BIS) for testing the SBC, steady conditions are defined as a 10 min period when (1) variation in the cooker absorbing plate temperature is +1 1C, (2) variation in the solar radiation is +20 W/m2, (3) variation in the ambient temperature is +0.2 1C, (4) solar radiation is greater than 600 W/m2. No-load tests were carried out on a clear sky and in order to get a stagnation temperature near solar noon, the testing was started before 10.00 a.m. For states like Gujarat (grade B category), where the experiment was conducted, the BIS-required value of F1 for SBC is not less than 0.11 1C m2/W. 3.2. Determination of F2 The F2 is obtained by heating the containers (full of water) placed on the absorbing plate, i.e. under a full-load condition and is mathematically expressed as F 1 ðMCÞw 1 ðT W1 T a Þ=F 1 G ln F2 ¼ , (3) 1 ðT W2 T a Þ=F 1 G Aðt2 t1 Þ where t1 is the time when water temperature reached TW1 in (1C), t2 the time when water temperature reached TW2 (1C),
4. Testing of the cooker The concept of solar concentration for application in solar cooking by realizing a truncated pyramid solar oven is designed, fabricated and tested. First, a test was conducted to measure the absorbing plate stagnation temperature when the glazing surface was facing the sun (the wooden base of the device was given a tilt so that the horizontal glazing surface faced the sun), and the highest absorbing stagnation plate temperature reached was 112 1C. The second test was conducted when the glazing surface was kept horizontal (i.e. the wooden platform rested on the ground) and the highest stagnation temperature of the absorbing plate reached was 119 1C. The lesser temperature of the absorbing plate achieved during the first test, as compared to the second test, might be attributed to rays reversing their path upward instead of reaching the bottom absorbing plate. It was also found that the tracking of the sun (east facing in the morning, south facing during the noon, etc.) did not give better performances compared to non-tracking. These tests were carried out under no-load conditions and without any insulation to the cooker, except that the bottom absorbing plate rested on the wooden platform. Calibrated thermocouples were used to measure the plate temperature after every 15-min interval. Thus, the data logger would average out the readings taken after an interval of 15 min to minimize the risk of accepting spurious data. The no-load test was also repeated (as test no. 3) when the device was insulated with 50 mm glass wool and thermocole sheets and the highest temperature approached 140 1C (see Fig. 3), which was sufficient for cooking/baking. It is worth noticing that the temperature of 130 1C was attained as early as at 11.20 a.m. and retained up to 2.30 p.m. Calibrated RTD temperature sensors were
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and readings were observed till 4.00 p.m. Solar insolation and temperatures were recorded after time intervals not more than 5 min apart. The variation in the ambient and water temperatures with time during the full-load test is shown in Fig. 4. After collecting the necessary data, the F2 was determined as discussed in the section on results and discussion given below. 5. Results and discussion
Fig. 3. Plate stagnation and ambient temperature variation with time for no-load testing for a truncated pyramid solar cooker.
The results of the first two tests showed that the larger value of concentration ratio (higher value of angle y) did not give the higher value of stagnation temperature because incident rays might have reversed their path, as was found during experimentation of the first two tests (discussed in the previous section). For the third test, carried out after properly insulating the cooker from outside, the steady state (as defined in Section 3) highest plate stagnation temperature (Tpz) measured was 138.46 1C at 1.40 p.m. The ambient temperature (Taz) and solar insolation at that time were 37.9 1C and 858.11 W/m2, respectively. By substituting these values in Eq. (2), the value of F1 obtained was 0.117 1C m2/W. Thus, this proved the efficacy of the design for solar cookers and met the requirements of the BIS. From the collected data of test 4 (carried out under fullload conditions), the initial and final temperature/time data pairs were located. The BIS norms of the initial temperature (TW1) to be 60 1C and the final temperature (TW2) to be 90 1C, prescribed for the Gujarat state, were used. The corresponding times t1 and t2 for TW1 and TW2 were found to be 10.55 a.m. and 12.05 p.m., respectively. Thus, it took 70 min to raise the water temperature from 60 to 90 1C. The average ambient temperature Ta and the average solar insolation G for the 70 min were found to be 34.80 1C and 866.03 W/m2, respectively. It can also be observed from Fig. 4 that the water temperature inside the vessel was
Fig. 4. Loaded water and ambient temperature variation with time when a truncated pyramid solar cooker was loaded with 2 l of water.
used for measuring the plate stagnation temperature and the time interval between two readings was set at 10 min. Encouraged by these results, the full-load test (as test no. 4) was carried out in accordance with the guidelines of the (BIS) [8]. According to the BIS standards, cooking pots should be filled with 8 l of water per square meter of aperture area. Two empty cooking pots of equal sizes were filled with 2 l of water because the aperture area of the cooker was 0.25 m2. The ambient temperature was equally distributed in both the pots. The temperature probe of the calibrated RTD sensor was placed in one of the cooking pots with the measuring tip submerged in water. The temperature probe lead was sealed where it left the pots and the cooker. The test was started in the mornings between 10.00 a.m. and 10.30 a.m. of Indian Standard Time
Fig. 5. Typical solar insolation variation with time on a full-load testing day.
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6. Conclusion A truncated pyramid-type solar cooker cum dryer was designed and fabricated so as to meet the requirements of local farming households. Cooking tests were carried out as per requirements of the BIS. The highest plate stagnation temperature, under no-load conditions, approached 140 1C and the water temperature inside the cooker reached 98.6 1C under full-load tests. It was also found that tracking of the sun did not give better performances. Two FOMs, F1 and F2, were calculated and their values were 0.117 1Cm2/W and 0.467 1C l, respectively, which meet the standards for SBC set by the BIS, thereby qualifying the device for efficient solar cooking. Acknowledgments Fig. 6. Typical characteristic curve for a truncated pyramid solar cooker.
retained at 90 1C even at 4 p.m. Fig. 5 shows a typical variation of solar insolation with time on the full-load test day. Thus, the value of F2 of the cooker, when 2 l of water was placed in it, was calculated (using Eq. (3)) to be 0.467 1C l. Based on these data, the value of standard boiling time was calculated and the characteristic curve for the oven is depicted in Fig. 6. Thus, the designed solar cooker is found to be suitable for cooking and meets the requirements of the BIS. The depth of the cooker was kept at 49.2 cm, for keeping trays that served as dryers. Higher plate stagnation temperatures under no-load conditions could be achieved if the depth was reduced to 15 cm, resulting in faster cooking. A booster mirror could definitely enhance its performance. If the device were to act as a cooker cum dryer, then the depth of the cooker would be high, and it would compromise on the cooking time. The truncated pyramid design proved to be an efficient geometry for solar cooking and also potentially suitable for drying, thereby making it a multipurpose device.
Authors gratefully acknowledge and thank Prof. B.S. Pathak, Director, SPRERI, for supporting this work. Authors would also like to thank Dr. K.C. Khandelwal, Ex. Advisor, MNRE, India, for helping in correcting the grammar of the manuscript. References [1] Patel NV, Philip SK. Performance evaluation of three solar concentrating cookers. Renew Energy 2000;20:347–55. [2] Chaudhuri TK. Estimation of electrical backup for solar box cooker. Renew Energy 1998;17:1–3. [3] Chaudhuri TK. Limitation of BIS designs specifications for solar box type cooker. J Sol Energy Soc India 1998;8(1):1–9. [4] Sonune AV, Philip SK. Development of domestic concentrating cooker. Renew Energy 2003;28:1225–34. [5] Patela R. Energy analysis of the solar cylinder-parabolic cooker. Sol Energy 2005;79:221–33. [6] Grise´ W, Patrick C. Passive solar lighting using fiber optics. J Ind Technol 2002;19(1):1–7. [7] Garg HP, Kandpal TC. Laboratory manual on solar thermal experiments. 1st ed. New Delhi: Narosa; 1999. [8] BIS. Indian standards solar-box type—specification, part 3: test method. IS 13429 (part 3), first revision. New Delhi: Bureau of Indian Standards; 2000.
Renewable Energy 33 (2008) 2207–2211 www.elsevier.com/locate/renene
Design and development of efficient multipurpose domestic solar cookers/dryers Naveen Kumar, Sagar Agravat, Tilak Chavda, H.N. Mistry Sardar Patel Renewable Energy Research Institute, Vallabh Vidyanagar, Gujarat 388 120, India Received 7 June 2007; accepted 9 January 2008 Available online 3 March 2008
Abstract A truncated pyramid-type solar cooker is designed, fabricated and tested. The truncated pyramid geometry concentrates the incident light radiations towards the bottom and the glazing glass surface on the top facilitates the trapping of energy inside the cooker. One of the salient features of the proposed design is to completely eradicate the need for tracking the sun during cooking, as tracking of sun does not yield better performance. During testing, the highest plate stagnation temperature, under no-load condition, approached 140 1C and under full-load condition, water temperature inside the cooker reached 98.6 1C in 70 min. Two figures of merit, F1 and F2, were calculated and their values were 0.1171 C m2/W and 0.467 1C l, respectively, meeting the standards prescribed by the Bureau of Indian Standards for solar box-type cookers. Minor modifications in design are recommended to achieve higher temperatures and reduce cooking times. The design also allows trays to be retained for use as a household dryer. r 2008 Elsevier Ltd. All rights reserved. Keywords: Solar cooker; Solar oven; Truncated pyramid solar concentrator; Thermal performance; Dryer cum cooker
1. Introduction Solar box-type cookers (SBCs) and SK-14-type concentrating cookers are widely accepted as practical pollutionfree renewable energy solutions for cooking/baking [1,2]. Despite many advantages, however, there are some drawbacks and practical inconveniences, which do not allow these devices to become more popular. For example, in SBCs, it takes several hours to complete cooking. Whereas, in SK-14-type with aperture area 1 m2, cooking completes in 45 min, which is reasonably good, the concentrator needs to track the sun after every 15 min for getting higher performance. Besides, it requires very careful operations because of risks involved in handling high temperature. The high-intensity concentrated radiation can also potentially damage eyes. Though high temperatures are achieved in solar concentrators, utilization of the total captured energy is done very poorly. Energy losses are very high because the concentrated energy escapes (not trapped), Corresponding author. Tel.: +91 2692231332; fax: +91 2692237982.
E-mail address: [email protected] (N. Kumar). 0960-1481/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.01.010
whereas in SBCs it is trapped [3,4]. The typical highest stagnation plate temperature achieved in box-type cookers is 130 1C and in SK-14-type concentrators it is 280 1C. Thus, it is desirable that a solar cooker/oven should be designed and manufactured, which can achieve high temperatures [5], is safe in operation and reasonably low in cost. Additionally, the cost of SK-14-type concentrators prohibits their use at village levels. The market price of a SBC is around Rs. 1700/- and SK-14-type concentrator costs around Rs. 6000/-. Thus, ranges of solar devices to fill the gap (difference in prices) for these applications, with different efficiency and prices, are necessary for product diversifications. Through the present paper, a truncated pyramid-type solar cooker is described, which has features to achieve high temperatures, reduce cooking times and is low on cost. The proposed solar cooker price of about Rs. 2000/- and the cooking time could be reduced to 1.25 h. In addition a facility of food drying has been incorporated in the devices to make it multipurpose. This study aims to report the design, fabrication and testing of the concept of truncated pyramid geometry for developing a solar dryer
ARTICLE IN PRESS N. Kumar et al. / Renewable Energy 33 (2008) 2207–2211
2208
cum cooker suitable for household applications. The proposed design is fabricated and the device is tested for solar cooking only. 2. Truncated pyramid-type solar cooker The concept of concentrating solar energy was utilized to achieve high temperatures, as done in SK-14-type concentrators and to trap the collected energy, as done in SBCs, to retain high temperatures over a considerable period of time to minimize losses. Combining these two advantages into a single design resulted in a new truncated pyramid-shaped solar cooker with enhanced performances. Owing to the geometry of the design (see Fig. 1), rays hitting the inner sidewalls (made of highly reflective anodized aluminum) of the truncated pyramid concentrator are reflected downward, so as to create a zone of high temperature at the bottom. In such geometries, a larger value of angle y leads to a smaller size of the bottom absorbing plate and hence higher concentration ratios. However, there is a restriction Sunrays Glazed surface
Bottom absorbing plate Fig. 1. Schematic diagram of a truncated pyramid-type solar cooker.
on the angle y and numbers of reflections in such geometries so that incident rays reach the bottom absorbing plate as given below [6]: 2ðn þ 1Þy þ ap90 ,
(1)
where n is the number of reflections and a the angle made by the sun (incident rays) on the glass glaze surface. If the incident radiations do not satisfy this condition, rays will reverse their path and exit out of the concentrator instead of reaching the bottom plate. For the time period of 7 h (typical average time period for sunshine hours in a day for cooking/drying), the maximum value of angle a would be 521. Assuming n ¼ 1, the maximum permissible value (using Eq. (1)) of the angle y would be E101. In our conceptual design the angle y ¼ 101, which also gives an approximate optimum size of the bottom absorbing plate. Thus, the pyramid model proposed here concentrates light on the vessel placed at the bottom, irrespective of the position of sun, thereby avoiding the need for tracking the sun. The fabricated model (see Fig. 2) was tested to confirm the concept and to find the experimental optimum correlation between the values of angles y and a and their impact on the shape and size of the bottom absorbing plate. The glazing surface is kept horizontal and a variable slant is provided to the glazing surface by adjustable angular tilts to the base of the proposed cooker to receive maximum insolation, despite seasonal variations in the position of the sun. The value of a can be changed by an appropriate tilt to the base of the device. The glazed surface of the fabricated cooker was made of glass of size 50 cm 50 cm, as was chosen in standard SBCs used in India. The size of the bottom absorbing plate was 32.6 cm 32.6 cm. This plate was made of aluminum and painted black to absorb maximum solar energy. The depth of the device (distance between the glazing surface and the bottom absorbing plate) was 49.2 cm (see Fig. 2(a)) whereas in SBCs, it was 8 cm only. The relatively larger depth facilitates the provision of its acting as a dryer with trays kept vertically inside the chamber (see Fig. 2(b)), thereby making it a multipurpose device.
Fig. 2. Just functional conceptual truncated pyramid dryer cum cooker realized by SPRERI: (a) Truncated pyramid type solar cooker, and (b) truncated pyramid type solar dryer.
ARTICLE IN PRESS N. Kumar et al. / Renewable Energy 33 (2008) 2207–2211
3. Methodology for testing In order to test the performance of the newly fabricated solar cooker, two figures of merit (FOM), viz. F1 and F2 are generally calculated [7,8]. The first FOM, F1, is defined as the ratio of optical efficiency to the heat loss factor by the bottom absorbing plate and is a measure of the differential temperature gained by the absorbing plate at a particular level of solar insolation. The second FOM, F2, is more or less independent of climatic conditions and gives an indication of heat transfer from the absorbing plate to the water in the containers placed on the plate. In this section, the procedure and methodology of determining F1, F2 and the standard boiling curve, generally adopted in the analysis of SBC [7,8], are briefly outlined. 3.1. Determination of F1 The value of F1 is determined by a stagnation test carried out under no-load condition and is mathematically expressed as F1 ¼
Z T pz T az ¼ , U LS Gs
2209
(t2t1) the time taken for heating water from TW1 to TW2 in seconds, Ta (1C) the average ambient temperature from the time period t1 to t2, G (W/m2) the average solar radiation from the time period t1 to t2 and (MC)w the product of the mass of water and specific heat (J/1C). The full-load test was started in the mornings between 10.00 a.m. and 10.30 a.m. of local solar time. For BIS certification, the required value of F2 for SBC is no less than 0.4 1C l. 3.3. Determination of standard boiling time After knowing F1 and F2, the time for sensible heating from the ambient temperature up to 100 1C, which is also known as the standard boiling time tboil [7,8] is calculated as F 1 ðMCÞw ð100 T a Þ tboil ¼ ln 1 . (4) F 1G 60AF 2 This time is a function of ð100 T a Þ=F 1 G and thus the plot of tboil versus ð100 T a Þ=F 1 G could be referred to as the characteristic curve of the cooker [7].
(2)
where Z is the optical efficiency, defined as the fraction of the incident solar radiation, which reaches the absorber and gets absorbed and ULS the heat loss factor at stagnation. The stagnation temperature of the absorbing plate (Tpz), ambient temperature (Taz) and solar insolation (Gs) are measured when a steady state is reached. According to guidelines of the Bureau of Indian Standards (BIS) for testing the SBC, steady conditions are defined as a 10 min period when (1) variation in the cooker absorbing plate temperature is +1 1C, (2) variation in the solar radiation is +20 W/m2, (3) variation in the ambient temperature is +0.2 1C, (4) solar radiation is greater than 600 W/m2. No-load tests were carried out on a clear sky and in order to get a stagnation temperature near solar noon, the testing was started before 10.00 a.m. For states like Gujarat (grade B category), where the experiment was conducted, the BIS-required value of F1 for SBC is not less than 0.11 1C m2/W. 3.2. Determination of F2 The F2 is obtained by heating the containers (full of water) placed on the absorbing plate, i.e. under a full-load condition and is mathematically expressed as F 1 ðMCÞw 1 ðT W1 T a Þ=F 1 G ln F2 ¼ , (3) 1 ðT W2 T a Þ=F 1 G Aðt2 t1 Þ where t1 is the time when water temperature reached TW1 in (1C), t2 the time when water temperature reached TW2 (1C),
4. Testing of the cooker The concept of solar concentration for application in solar cooking by realizing a truncated pyramid solar oven is designed, fabricated and tested. First, a test was conducted to measure the absorbing plate stagnation temperature when the glazing surface was facing the sun (the wooden base of the device was given a tilt so that the horizontal glazing surface faced the sun), and the highest absorbing stagnation plate temperature reached was 112 1C. The second test was conducted when the glazing surface was kept horizontal (i.e. the wooden platform rested on the ground) and the highest stagnation temperature of the absorbing plate reached was 119 1C. The lesser temperature of the absorbing plate achieved during the first test, as compared to the second test, might be attributed to rays reversing their path upward instead of reaching the bottom absorbing plate. It was also found that the tracking of the sun (east facing in the morning, south facing during the noon, etc.) did not give better performances compared to non-tracking. These tests were carried out under no-load conditions and without any insulation to the cooker, except that the bottom absorbing plate rested on the wooden platform. Calibrated thermocouples were used to measure the plate temperature after every 15-min interval. Thus, the data logger would average out the readings taken after an interval of 15 min to minimize the risk of accepting spurious data. The no-load test was also repeated (as test no. 3) when the device was insulated with 50 mm glass wool and thermocole sheets and the highest temperature approached 140 1C (see Fig. 3), which was sufficient for cooking/baking. It is worth noticing that the temperature of 130 1C was attained as early as at 11.20 a.m. and retained up to 2.30 p.m. Calibrated RTD temperature sensors were
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and readings were observed till 4.00 p.m. Solar insolation and temperatures were recorded after time intervals not more than 5 min apart. The variation in the ambient and water temperatures with time during the full-load test is shown in Fig. 4. After collecting the necessary data, the F2 was determined as discussed in the section on results and discussion given below. 5. Results and discussion
Fig. 3. Plate stagnation and ambient temperature variation with time for no-load testing for a truncated pyramid solar cooker.
The results of the first two tests showed that the larger value of concentration ratio (higher value of angle y) did not give the higher value of stagnation temperature because incident rays might have reversed their path, as was found during experimentation of the first two tests (discussed in the previous section). For the third test, carried out after properly insulating the cooker from outside, the steady state (as defined in Section 3) highest plate stagnation temperature (Tpz) measured was 138.46 1C at 1.40 p.m. The ambient temperature (Taz) and solar insolation at that time were 37.9 1C and 858.11 W/m2, respectively. By substituting these values in Eq. (2), the value of F1 obtained was 0.117 1C m2/W. Thus, this proved the efficacy of the design for solar cookers and met the requirements of the BIS. From the collected data of test 4 (carried out under fullload conditions), the initial and final temperature/time data pairs were located. The BIS norms of the initial temperature (TW1) to be 60 1C and the final temperature (TW2) to be 90 1C, prescribed for the Gujarat state, were used. The corresponding times t1 and t2 for TW1 and TW2 were found to be 10.55 a.m. and 12.05 p.m., respectively. Thus, it took 70 min to raise the water temperature from 60 to 90 1C. The average ambient temperature Ta and the average solar insolation G for the 70 min were found to be 34.80 1C and 866.03 W/m2, respectively. It can also be observed from Fig. 4 that the water temperature inside the vessel was
Fig. 4. Loaded water and ambient temperature variation with time when a truncated pyramid solar cooker was loaded with 2 l of water.
used for measuring the plate stagnation temperature and the time interval between two readings was set at 10 min. Encouraged by these results, the full-load test (as test no. 4) was carried out in accordance with the guidelines of the (BIS) [8]. According to the BIS standards, cooking pots should be filled with 8 l of water per square meter of aperture area. Two empty cooking pots of equal sizes were filled with 2 l of water because the aperture area of the cooker was 0.25 m2. The ambient temperature was equally distributed in both the pots. The temperature probe of the calibrated RTD sensor was placed in one of the cooking pots with the measuring tip submerged in water. The temperature probe lead was sealed where it left the pots and the cooker. The test was started in the mornings between 10.00 a.m. and 10.30 a.m. of Indian Standard Time
Fig. 5. Typical solar insolation variation with time on a full-load testing day.
ARTICLE IN PRESS N. Kumar et al. / Renewable Energy 33 (2008) 2207–2211
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6. Conclusion A truncated pyramid-type solar cooker cum dryer was designed and fabricated so as to meet the requirements of local farming households. Cooking tests were carried out as per requirements of the BIS. The highest plate stagnation temperature, under no-load conditions, approached 140 1C and the water temperature inside the cooker reached 98.6 1C under full-load tests. It was also found that tracking of the sun did not give better performances. Two FOMs, F1 and F2, were calculated and their values were 0.117 1Cm2/W and 0.467 1C l, respectively, which meet the standards for SBC set by the BIS, thereby qualifying the device for efficient solar cooking. Acknowledgments Fig. 6. Typical characteristic curve for a truncated pyramid solar cooker.
retained at 90 1C even at 4 p.m. Fig. 5 shows a typical variation of solar insolation with time on the full-load test day. Thus, the value of F2 of the cooker, when 2 l of water was placed in it, was calculated (using Eq. (3)) to be 0.467 1C l. Based on these data, the value of standard boiling time was calculated and the characteristic curve for the oven is depicted in Fig. 6. Thus, the designed solar cooker is found to be suitable for cooking and meets the requirements of the BIS. The depth of the cooker was kept at 49.2 cm, for keeping trays that served as dryers. Higher plate stagnation temperatures under no-load conditions could be achieved if the depth was reduced to 15 cm, resulting in faster cooking. A booster mirror could definitely enhance its performance. If the device were to act as a cooker cum dryer, then the depth of the cooker would be high, and it would compromise on the cooking time. The truncated pyramid design proved to be an efficient geometry for solar cooking and also potentially suitable for drying, thereby making it a multipurpose device.
Authors gratefully acknowledge and thank Prof. B.S. Pathak, Director, SPRERI, for supporting this work. Authors would also like to thank Dr. K.C. Khandelwal, Ex. Advisor, MNRE, India, for helping in correcting the grammar of the manuscript. References [1] Patel NV, Philip SK. Performance evaluation of three solar concentrating cookers. Renew Energy 2000;20:347–55. [2] Chaudhuri TK. Estimation of electrical backup for solar box cooker. Renew Energy 1998;17:1–3. [3] Chaudhuri TK. Limitation of BIS designs specifications for solar box type cooker. J Sol Energy Soc India 1998;8(1):1–9. [4] Sonune AV, Philip SK. Development of domestic concentrating cooker. Renew Energy 2003;28:1225–34. [5] Patela R. Energy analysis of the solar cylinder-parabolic cooker. Sol Energy 2005;79:221–33. [6] Grise´ W, Patrick C. Passive solar lighting using fiber optics. J Ind Technol 2002;19(1):1–7. [7] Garg HP, Kandpal TC. Laboratory manual on solar thermal experiments. 1st ed. New Delhi: Narosa; 1999. [8] BIS. Indian standards solar-box type—specification, part 3: test method. IS 13429 (part 3), first revision. New Delhi: Bureau of Indian Standards; 2000.