SOLAR COOKER

FOR IRIS NATIONAL FAIR 2010 Project Code: 52 Age Category: 1 Subject Category: Environmental Science Guides: Mr. Suryanarayanarao S.R., Mr. Amrit Ambirajan Sishu Griha Montessori and High School, Bangalore, India Participant: Daya Ambirajan, VI Std. CONTENTS SYNOPSIS ................................................................................................................................................... 3 INTRODUCTION....................................................................................................................................... 4 What is a box type solar cooker? .............................................................................................................. 4 Genesis of the Project ............................................................................................................................... 4 Objective ................................................................................................................................................... 4 Problem ..................................................................................................................................................... 4 Hypothesis ................................................................................................................................................ 4 Literature Review...................................................................................................................................... 4 Experiments .............................................................................................................................................. 4 METHODOLOGY, EXPERIMENTS AND RESULTS ......................................................................... 5 MATERIALS............................................................................................................................................ 5 COOLING RATE EXPERIMENTS......................................................................................................... 5 EXPT. 1: MELTING POINT STUDIES OF DIFFERENT WAXES ....................................................... 5 EXPT 2: TEMPERATURE OF WATER WITH MELTED WAX .......................................................... 7 EXPT 3: BIG POT VS SMALL POT ....................................................................................................... 8 SOLAR COOKER EXPERIMENTS........................................................................................................ 9 EXPT. 4: EMPTY BOX TEMPERATURE .............................................................................................. 9 EXPT.5: TEMPERATURE OF WATER IN THE SOLAR COOKER .................................................. 10 EXPT 6: WAX IN WATER (NO PREHEATING) ................................................................................ 11 EXPT 7: IN THE SOLAR COOKER WITH PREHEATING ................................................................ 12 DISCUSSION ............................................................................................................................................ 13 COOLING RATE EXPERIMENTS....................................................................................................... 13 SOLAR COOKER EXPERIMENTS...................................................................................................... 14 CONCLUSIONS ....................................................................................................................................... 14 FUTURE APPLICATION ....................................................................................................................... 14 POSSIBLE FUTURE WORK ................................................................................................................. 14 ACKNOWLEDGEMENTS ..................................................................................................................... 15 BIBLIOGRAPHY ..................................................................................................................................... 15 Daya Ambirajan Page 2 SYNOPSIS I was looking for a method by which food can stay warm for a longer time in a box type solar cooker. Literature survey shows that: Heat is released when waxes freeze. Solar cooker studies use water instead of food. Choice of wax: Of the waxes studied, Carnauba wax type 1 (melting point around 80oC) was the most suitable. Wax keeps water hot for longer time: Melted carnauba type 1 wax in small pots distributed around a large container of hot water, increased cooling time of water compared to cooling by a bigger pot of wax placed in the centre. Experiments using the solar cooker: Peak solar cooker temperature was 125oC. The temperature of water never went more than 80oC even though wax temperature reached 118oC. A pot of wax in a large container of water was placed in the solar cooker with temperatures recorded every hour. The wax did not melt because the water kept its temperature low. Next, wax was preheated/ melted in the solar cooker before placing it in the water. The water container with pots of wax was 6oC higher than the water container with pots of water. Conclusions: Wax keeps food warm longer in a solar cooker when preheated. It is preferable to use small pots distributed around a large container than have the same amount of wax in the centre. These conclusions suggest that capsules of wax could be preheated and dropped into the food container in a solar cooker. Daya Ambirajan Page 3 INTRODUCTION What is a box type solar cooker? The solar cooker works as a trap for sunlight. Light enters the box through the glass sheet and is absorbed by the black surfaces inside, causing them to heat up. The heat radiated from these surfaces cannot leave the box because it is reflected back by the glass sheet. This is how it cooks the food. Genesis of the Project My mother uses the solar cooker to cook rice. So, for the IRIS project, I thought of extending the cooking time of the solar cooker so that the food in it can be warm for longer. Objective I am looking for a method by which the food can stay warm or cook for a longer time in a box type solar cooker. Problem A solar cooker does not retain heat in the absence of bright sunlight (in the evenings and sometimes in cloudy weather). Hypothesis This project extends the cooking time in the solar cooker by using the latent heat of fusion of a substance like wax. In the absence of bright sunlight, both the food and the wax cool down. But when the wax starts to solidify, it gives out latent heat which keeps the food warm longer. Literature Review In ref. 1, the author put a transparent wax between the two windows. In references 2 and 3, the authors put the wax in an extra cavity at the bottom of the food container, filled with wax. Both were done to store latent heat. Rather than putting the wax in an extra cavity at the bottom of the food container or putting the wax in between the windows, I wanted to find a way of putting the wax closer to, or in the food. That was the goal of my study. References 2, 3 and 4 gave me the idea of using carnauba wax for this project. I used water instead of food according to the international standard procedure for testing solar cooker performance. (Reference 5) Since I did not want to cool the wax by opening the lid to check the reading shown on the thermometer, I searched the internet for another method. Reference 5 gave me the idea of using a thermocouple reader (digital thermometer) to take the reading instead. Experiments There were two types of experiments- cooling rate experiments (experiments 1 to 3) and solar cooker experiments (experiments 4 to 7). Cooling rate experiments: In experiment 1, I studied the melting points of 3 different waxes. Experiment 2 was to check whether wax slows down the rate of cooling of hot water. Experiment 3 was to decide if it is better to use one big pot of wax or distribute the same amount of wax among smaller containers. Daya Ambirajan Page 4 In the solar cooker experiments, all experiments involved monitoring temperature over the whole day. During the experiments the solar cooker readings were taken every hour. The direction of the solar cooker was adjusted every hour to track the sun. In experiment 4, I measured the empty box temperature over a full day. After this, experiment 5 was to measure the temperature of containers of wax and water in a solar cooker over a full day. Experiment 6 was carried out with a pot of wax placed inside a container of water. It was done to see if the wax melted and temperatures were not checked. Experiment 7 involved preheating the wax before putting it in the container of water. METHODOLOGY, EXPERIMENTS AND RESULTS MATERIALS Solar cooker Four T-type Thermocouple wires (copper and constantan) Omega HH 25TC digital thermometer Ten 50ml stainless steel cups Two 250ml stainless steel cups Paraffin wax Carnauba wax - Type 1 Carnauba wax – Type 2 Measuring cup 2 stainless steel containers with lids (diameter= 18cm, height= 5.5cm) Thermometer (First I used an alcohol thermometer but it couldn’t be used in the solar cooker because the upper limit was too low. Then a mercury thermometer was used but it broke. Finally I used a digital thermometer.) COOLING RATE EXPERIMENTS EXPT. 1: MELTING POINT STUDIES OF DIFFERENT WAXES In this experiment I wished to determine which wax was most suitable for further experiments. This was studied by seeing how long the waxes took to cool. Based on availability, I chose paraffin wax. Based on an internet survey I also studied two types of carnauba wax. Methodology: 3 kinds of wax were used: Paraffin, Carnauba type 1 and Carnauba type 2. The wax (250ml) was first melted on a stove. Then it was placed on the base of a lab stand with a thermometer clamped on. The temperature was checked every minute until the temperature reached 40oC. Daya Ambirajan Page 5 Lab thermometer Wax melted on the stove Figure 1 : Setup for melting point study (photograph included) Result: 140 Temperature, C 120 100 80 oC for 60 mins 80 77 oC for 20 mins 60 56 oC for 30 mins 40 20 0 0 20 40 60 80 100 120 140 Time, minutes Type 1 Type 2 Paraf f in Figure 2 : Cooling of the waxes Ambient temperature was around 22oC The melting point of carnauba wax type 1 was around 80oC, and it stayed there for 60 minutes. The melting point of carnauba type 2 was close to type 1, but it stayed there for only 20 minutes. The melting point of paraffin was 56- 57oC, but it stayed there for around 30 minutes. Conclusions: Looking at all the above observations, and keeping in mind that food cooks at higher temperatures, it was decided that carnauba wax type 1 was the best wax for the solar cooker. Daya Ambirajan Page 6 EXPT 2: TEMPERATURE OF WATER WITH MELTED WAX The goal of this experiment was to check whether wax does indeed slow down the cooling of water in a container. This experiment would help support the hypothesis that the cooking vessel would stay hotter longer by using the latent heat of wax. Methodology: 2 large containers were filled with 700ml of water at 90oC. 5 small containers with melted carnauba wax type 1(50ml each) were placed in one (set up A), 5 small containers with water at 90oC (50ml each) were placed in the other (set up B), used as a control. The temperature of water was recorded every minute. Figure 3: set up for measuring temperature of water with melted wax Results: 100 Temperature, oC 90 80 70 A (wax) 60 B (Water) 50 40 0 20 40 60 80 100 TIME , MINS Figure 4 : Results of experiment 3 Temp. A (with wax) was higher than temp. B (with water) throughout Maximum difference between A and B was 8oC At the end, temp. A was higher by 6oC Conclusions: As the wax started freezing, it gave out latent heat which maintained the water at a higher temperature. The container with the pots of water (the control) was 6oC cooler than the container that had the pots of wax. Therefore I found out that carnauba wax type 1 can be used to keep water hotter longer. Daya Ambirajan Page 7 EXPT 3: BIG POT VS SMALL POT In this experiment we studied the effect of pot size (containing wax) and distribution on the cooling of water in the container. This would give us a clue as to whether larger or smaller wax containers are better. Methodology Five small pots (50ml each) and one big pot (250ml) of melted carnauba wax were placed in the two large containers of 700ml hot water. A thermocouple wire was placed in each one. The temperatures were compared to see which one holds water hotter longer. Set up B Set up A 700ML WATER 50ml WAX 250ml wax 700ml WATER Figure 5: Big Pot vs. small pots (with photo of the experiment). Result T2 (small pots) was 7oC higher than T1 (big pot) after 45minutes. 100 90 80 TEMP,OC 70 60 50 T1oC(big pot) 40 T2oC(small pots) 30 T4oC(ambient) 20 10 0 0 10 20 30 40 50 TIME, MINS Figure 6 : Temperatures from experiment 11 (big pot versus small pots) Daya Ambirajan Page 8 Conclusion It is preferable to use small pots distributed through the container rather than a big pot placed in the centre. SOLAR COOKER EXPERIMENTS EXPT. 4: EMPTY BOX TEMPERATURE The goal of this experiment was to determine how hot the box in a box type solar cooker gets over a day. This information can then be compared with the melting points of the waxes. Methodology: The thermometer was placed inside the solar cooker from 8:00am to 5:00pm. Readings were taken hourly. The solar cooker and mirror were oriented to face the sun for efficiency. The thermometer was placed in the box and temperature recorded every hour. Figure 7: set up for empty box temperature Result: 140 temperature, oC 120 100 80 60 40 20 8 9 10 11 12 13 14 15 16 17 18 19 time, hours Figure 8: empty box temperature Daya Ambirajan Page 9 `On a sunny day, the box temp is above 80oC from 10 am to 4 pm The highest temperature was 125oC Conclusions: Since on a sunny day, the box temp is above 80oC from 10 am to 4 pm, wax can be used to keep the food warm after 4 pm. EXPT.5: TEMPERATURE OF WATER IN THE SOLAR COOKER This experiment was to see how the temperature of wax and water change during a day in the solar cooker. The wax and water were kept in separate vessels. Methodology: Two small vessels (250ml), one with wax and one with water, were placed in the solar cooker. Their temperatures were measured every one hour. MIRROR AIR BOX TO DIGITAL THERMOMETER WATER WAX Figure 9: Setup for experiment 4. Result 140 TEMPERATURE 120 100 80 T1(wax) 60 T2(water) 40 T3(box) T4(ambient) 20 0 8 10 12 14 16 18 20 TIME Figure 10: Temperature of water and wax in the solar cooker Daya Ambirajan Page 10 From 11am to 4 pm the temperature of the carnauba wax was above its melting point. The highest temperature of the water was 80oC from 12 to 1 pm. After 5pm the box temperature was much less than the wax temperature whereas the water and box temperatures were almost the same. Conclusions: It is clear that if the wax is to be fully melted, it must not be heated with the water because the water keeps it from reaching temperatures higher than the melting point. This shows that the wax should be preheated for melting before putting it in the water. The next two experiments study the effect of preheating. In Expt. 5 wax was kept in a cup in the water. In Expt. 6 preheating of the wax and water was carried out until the wax melted and then it was transferred into the water. EXPT 6: WAX IN WATER (NO PREHEATING) This experiment observes the effect of wax on water in the solar cooker. In this experiment the wax pot is placed in the container with water. The goal is to see if the wax melts. Methodology 135g carnauba wax in a steel vessel (250ml) was placed in a larger steel vessel in the solar cooker. Control: 250ml water was placed in a larger vessel containing 750ml water MIRROR Wax container Water container Figure 11: Wax in water no preheating Result Wax did not melt completely but more water evaporated from the wax container (setup A) than from the one with water (setup B). Daya Ambirajan Page 11 Conclusion This proves that if the wax is to be fully melted, it must not be heated with the water because the water keeps it from reaching temperatures higher than the melting point. This suggests that the wax must be preheated before putting in the vessel. EXPT 7: IN THE SOLAR COOKER WITH PREHEATING In this experiment we preheated the water and wax separately. This experiment tested the hypothesis that wax will keep the water in the container warmer than the control (pot with water) for a longer time in a solar cooker. Methodology First we preheated two containers each with 700ml water and five small containers with totally 250ml water and 250ml wax separately. At around 3:00pm (at box temperature 85oC), the water was poured into the dishes containing the wax and water pots. Temperatures were recorded in the log book for every one hour. Room temp Setup B (with water) BOX Setup A (with wax) PREHEATING POTS Figure 10: Small pots in the solar cooker (a photograph of the experiment is also shown). Daya Ambirajan Page 12 Result 110 100 Temperature, oC 90 80 70 Wax 60 Water 50 Box ambient 40 30 20 14:00 15:00 16:00 17:00 Time, hours 18:00 19:00 Figure 12: Small pots in the solar cooker graph • The container with small pots of wax was of higher temperature than the container with small pots of water from the beginning to the end of the experiment. • The maximum difference between temp A and temp B was 26oC just after pouring in the preheated water. The temperature difference came down to 6oC by the end of the day. Conclusions Carnauba wax type 1 can be used to keep food warm DISCUSSION The box type solar cooker keeps the food warm longer than other types because of the insulated box with a glass lid. There have been attempts to extend cooking time using the latent heat of wax. In one such attempt a transparent wax was put in between the two glass sheets of the solar cooker, however this would reduce the heat entering the box. In other attempts carnauba wax has been placed in an extra cavity in the food container. When it solidified, it gave out latent heat that kept the food warm longer. I thought that it would be much more efficient if the wax was in the food so the latent heat could be given out directly into the food, rather than travel through air to reach the food. COOLING RATE EXPERIMENTS Experiment 1 clearly showed that carnauba type 1 was better than carnauba type 2 and paraffin wax. Not only did carnauba type 1 have a higher melting point than the other waxes, but also stayed hot for the longest time(of the waxes studied in this experiment). Based on this experiment carnauba wax was used in all future experiment. Experiment 2 clearly shows that wax does increase the time that a food container stays hot. Experiment 3 showed that carnauba wax type 1 in small pots increases the cooling time better than large pots of wax. Daya Ambirajan Page 13 SOLAR COOKER EXPERIMENTS Experiment 4 showed the peak empty box temperature was around 125oC which is higher than the melting point of carnauba type 1 wax. Experiment 5 was carried out to see how the temperatures of a container of wax and a container of water changed during the day when placed in the solar cooker. The temperature of water never went more than 80oC (close to the melting point of carnauba type 1 wax) even though the box temperature reached 118oC. However, the wax temperature reached about 118oC. This experiment suggests that if a pot of wax is placed in a container of water it will not melt. To test this we carried out experiments 6 and 7. In experiment 6, a pot of wax in a large container of water was placed in the solar cooker with temperature recorded every hour. The wax did not melt because the water did not allow it to. This is why, for the next experiment (7) wax was preheated. In this experiment the container containing pots of wax was 6oC higher than the container with water. Carnauba wax is usually the wax of choice for such experiments because of its unique properties such as the highest melting point amongst commercially available waxes and a high flash point (ref.4). It is usually used for producing food products, sweets, dental floss and chewing gum since it is non toxic (ref. 4). It has also been used in a plate warmer. (ref.2) Clearly, carnauba wax can be used to keep food warm longer in the absence of bright sunlight. Instead of pots of carnauba wax, capsules could also be used. CONCLUSIONS 1. Carnauba type 1 wax was chosen because it has a melting point close to 80oC and also because it stays hot longer than the other waxes. 2. Wax keeps food warm longer in a solar cooker when preheated. 3. It is preferable to use small pots distributed all over the large container than have the same amount of wax right in the centre. FUTURE APPLICATION The above conclusions suggest that capsules of wax could be preheated and distributed inside the food container in the solar cooker. POSSIBLE FUTURE WORK 1. Measure the temperature of water in a container with a black lid (in the solar cooker). 2. Make small sealed cubes with the wax inside and use them in the food container. Daya Ambirajan Page 14 ACKNOWLEDGEMENTS Firstly I would like to thank Mr. S.R.S. Rao, a teacher in my school, for continuous encouragement and guidance throughout. Next, I would like to thank my parents for guiding me through all the hurdles I had to pass. Then, my thanks go to Darren and Sharon for allowing me to use their terrace in Bangalore. My grandparents also provided a great deal of support including their terrace in Chennai. I am thankful for the support and guidance received from my school including: Ms. Sujatha Mohandas, Principal The Management of Yenkay Education Society The teachers of Sishu Griha Montessori and High School BIBLIOGRAPHY 1. V.V.S. Murty and Preeti Kanthed, "THERMAL PERFORMANCE OF A SOLAR COOKER HAVING PHASE CHANGE MATERIAL AS TRANSPARENT INSULATION," http://www.fskab.com/Annex17/Workshops/EM4%20Indore%202003-03-21-24/Presentations/VVS_murty.pdf . 2. http://www.freepatentsonline.com/5052369.html “Heat retaining food container,” United States Patent 5052369 3. Edward Vandas, "PLATE WARMER," US Patent 4,246,884, Jan.27 1981. 4. http://en.wikipedia.org/wiki/Carnauba_wax , “Carnauba wax” 5. Paul A. Funk, "EVALUATING THE INTERNATIONAL STANDARD PROCEDURE FOR TESTING SOLAR COOKERS AND REPORTING PERFORMANCE," Solar Energy, Vol.68, No.1, pp.1-7, 2000. Daya Ambirajan Page 15