Solar Cooker Lab Instructor s Guide Motivation: The purpose behind this solar cooker lab is to increase STEM education and to promote social awareness. Students will learn the basic principles of heat transfer and how to take temperature measurements with the Arduino, all while building a tool that could greatly increase someone s quality of life. Solar cookers are great in that they are built from readily available materials (cardboard, plastic wrap, aluminum foil, etc.), they are powered by the sun (a free and unlimited energy source), and they showcase all three types of heat transfer (radiation, convection, and conduction). Furthermore, the students can gain an understanding of basic circuitry, Arduino programming, and data collection when gathering temperature data from the solar cooker with thermistors (a type of temperature sensor) and the Arduino Uno microcontroller. Layout of the lab: This lab is meant to be semi-open ended in order to develop the students creativity and problem solving skills. That is why the lab procedure is made of general statements and questions instead of step by step instructions of how to build a solar cooker. It is suggested that the teacher present the students with the brief background of solar cookers and why they are useful. Then, offer them a variety of materials (different sized boxes, plastic sheeting/plastic food wrap/plexiglas, different colored paints, etc.) and ask the students to build their own solar cooker using the general steps provided in the lab procedure. After they have built a solar cooker, they can test how well it works with the Arduino microcontroller and the thermistors. Testing the solar cooker involves building the circuit that connects the thermistors with the Arduino, writing the Arduino code that will collect the temperature data from the thermistors, and recording the temperature data while the solar cooker is cooking. Pre-Requisites: Students should have previous experience with the Arduino Uno microcontroller, Arduino coding, and basic circuitry. A background in heat transfer would help in the understanding of the lab but is not necessary to complete the lab.
Time Breakdown: Pre-lab 20-30 minutes Lab Procedure Build a solar cooker 30 minutes Build circuit for thermistors 10-20 minutes Write code for the Arduino 20 minutes Test cooker Several hours to let the cooker heat up in the sun and to gather the transient temperature data Post Lab 1-2 hours Parts List: Parts should be found from common, everyday items to ensure a low cost lab. If items must be purchased, they should be as affordable as possible which is why some prices are not listed (cost may vary). Materials: Boxes (2 per solar cooker)- 1 large and 1 smaller box Newspapers- any type of paper/insulator can be used Aluminum foil Black paint/paint brushes Plexiglas- glass or plastic food wrap can also be used Arduino Uno [$25] 10 KΩ thermistors (Part number NTCLE100E3103JB0)[$0.75 each] 10 KΩ Resistors (1 per thermistor)[$0.05 each] Breadboard (1 per Arduino) [$3] 8 Jump Wires per solar cooker circuit (depends on number of thermistors used) Duct Tape [$3] Recommended Tools: Computer Scissors Optional: Heat lamp (for poor weather)[$250]
Pre-Lab Answers: Heat Gain Greenhouse Effect: The trapping of the sun s warmth in the lower atmosphere due to the fact our atmosphere lets visible radiation from the sun through easier than infrared radiation coming off the earth s surface Glass orientation: The glass is important as it allowed the visible sunlight to come through, but helps trap heat inside the box. The glass should seal the top of the box in order to avoid heat from leaking out. The more directly the glass faces the sun, the more solar heat gain achieved by the solar cooker. Reflectors: the reflectors are there to try to concentrate sunlight into the box. This way, the light is used more efficiently and the box can heat up faster. Heat Loss Convection: The way heat is transferred due to the movement of matter. i.e hot air rising, cold air falling. Radiation: Transfer of energy through electromagnetic waves. Infrared, visible light, UV rays. Conduction: When heat moves from a heat source to a heat sink. The different between convection and conduction is that matter doesn t move in conduction. The heat transfers through contact. Heat Storage Insulation: It s better to have multiple layers. This is more efficient at trapping heat in because it leaves air in between the layers where heat can stay trapped instead of having just one layer which will lose any heat that passes through it. Heat absorption: Black is the color that absorbs the most heat. This is because black absorbs all wavelengths and reflects nothing. Question: Write code for the final Rt equation. Rt = R*( 1023.0 / Vo - 1.0 ); Question: Write code for the final temperature equation. T = 1.0/(log(Rt / Ro)/B + (1.0 / (To + 273.15)));
Fill in the blanks of the code. #define THERMISTORPINX A0 //student determines number of sensors used (x sensors) #define THERMISTORNOMINAL 10000 //resistance at reference temperature of thermistor #define TEMPERATURENOMINAL 25 //reference temperature of thermistor #define BCOEFFICIENT 3977 //determined from thermistor specifications #define SERIESRESISTOR 10000 //resistance of second resistor (R) void setup() { Serial.begin(9600); // open serial port and set data rate to 9600 bps Serial.print("Volta\tResista\tTempA*C\n"); } void loop() { int Vox; // Integer value of voltage reading float Rtx; // Computed resistance of the thermistor Vox = analogread(thermistorpinx); //take the voltage reading float Vox; Rtx = SERIESRESISTOR*( 1023.0 / Vox - 1.0 ); //calculate the resistance of the thermistor Serial.print(Vox); Serial( \t ); Serial.print(Rtx); Serial( \t ); float steinhartx; steinhartx = log(rtx / THERMISTORNOMINAL)/BCOEFFICIENT + (1.0 / (TEMPERATURENOMINAL + 273.15)); steinhartx = 1.0 / steinhartx; // Invert steinhartx -= 273.15; // convert to C Serial.print("steinhartx\n"); delay(1000); }
Post-Lab Answers: LAB Data 1. List the specific materials you used in the construction of your solar cooker. Will vary by student. 2. Draw a schematic of your solar cooker design. Include dimensions. Will vary by student. 3. Show your code for the Arduino temperature sensor. Will vary by student. 4. Show a plot with temperature of the solar cooker with respect to time. Also show resistance with respect to temperature. Choose a large enough time interval to show all significant temperature changes in your solar cooker. 90 80 Temperature vs. Time Temperature (degrees C) 70 60 50 40 30 20 Ambient Temp. Solar Cooker Temp. 10 0 0 500 1000 1500 2000 2500 3000 3500 Time (seconds)
Temperature vs. Resistance Temperature (degrees C) 90 80 70 60 50 40 30 20 10 0 0 2000 4000 6000 8000 10000 12000 14000 Resistance (Ohms) Thermodynamics 5. By how many degrees did the solar cooker heat up greater than the environment? Explain how this happened. Answers will vary by students. The test group saw the box heat up 30 F (17 C). This happened because the Plexiglas allows the light to enter, the dark materials convert the light to heat energy and then the Plexiglas, which as an insulator, keeps the heat inside the cooker. 6. What components of your solar cooker increased the temperature inside the cooker? Explain how each component contributed to the temperature increase. Plexiglas- allows light to enter but traps heat energy because it is an insulator. Dark materials- convert light to heat energy. Foil- concentrates light to the inside of the solar cooker. Newspaper- helps insulate the heat, trapping it inside of the solar cooker. Social Applications 7. Would your solar cooker be a viable device to cook food? If the cooker reaches 212 F (100 C) then the cooker can boil water, allowing rice and other foods to be cooked. However the cooker can still heat/warm some food, such as chicken, at lower temperatures like 165 F (74 C). 8. If yes, how would you implement it? If no, what would make it a feasible option?
The cooker could be implemented by showing community members how to make it. The basic principles could be discussed so that those who choose to use the cooker can tweak it to provide the results that they want. The cooker could be improved by better insulation or more light concentration. The improvements depend on the individual students labs. 9. Discuss the pros and cons of construction cost, material availability, fuel cost, pollution and cook time of gas, wood, charcoal and solar cookers. Fill out the table below. Which is the most plausible for daily use? Why? Which would be best suited for your community? Construction/Purchase Cost Fuel Cost/Pound Material Availability Gas $100 ~$1 Easily found in stores Charcoal $50 ~$1 Easily found in stores Wood Free/Minimal Free Readily Available Solar Free/Minimal Free Readily Available Cook Time for Pollution piece of chicken 8-10 Minutes Minimal 8-12 Minutes More than gas 20-25 Minutes Moderate 3-4 Hours None Bonus: 10. Compare your cooker to the other students. Which cooker got the highest temperature? What made this cooker record the highest temperature? Will vary by class. 11. Try to cook a marshmallow with your cooker. Did it work? Will vary by student 12. Out of all the cookers constructed which was the biggest that could heat the marshmallows? Which was the smallest? Which size do you think would be the best for use in a community? Why? First two answers will vary by class. The best for a community would be either one that is very large and can effectively cook a large amount of food or one that is small but very effective. The first would be good because it could help feed a community, especially if several were present. The second would be better suited for use by individuals or families as it would take less resources to make but would still work well.