The science idea
The Sun gives out an enormous amount of energy, and on a clear day in most parts of India roughly a kilowatt of it falls on every square metre of ground at midday. A solar cooker is a box designed to collect some of that energy and keep it. It has no fuel and no flame, which is why solar cookers are used in places where wood and gas are scarce, and why they are a favourite topic for environment and renewable-energy projects.
Three ideas of physics work together. The first is absorption. Light that falls on a dark, matt surface is mostly absorbed and its energy appears as heat, which is why a black car seat is hotter than a white one. The inside of the cooker is black so that it absorbs as much sunlight as it can. The second is reflection. A shiny foil flap acts like a mirror and sends extra sunlight, which would have missed the window, into the box. The flap can increase the energy entering the box by up to about a third or more, depending on its angle and on the height of the Sun.
The third idea is the control of heat losses. Heat moves in three ways: conduction (through solids, from particle to particle), convection (by the movement of warm air or liquid) and radiation (as infrared rays). A box that is open to the air loses its heat quickly because the warm air rises and is replaced by cool air. A clear plastic cover keeps that hot air inside, while still letting sunlight in. Glass would also block much of the infrared radiation given out by the hot black surface, which is the “greenhouse effect” at work; thin plastic wrap blocks the moving air more than the infrared. A layer of crumpled newspaper is a poor conductor, because it traps air, so it slows the conduction of heat out through the walls and floor of the box.
How hot will it get? On a clear, sunny day around noon a simple pizza-box cooker can reach about 70 to 100 degrees Celsius inside, and sometimes more. That is hot enough to warm water, melt chocolate or cheese and warm food that is already cooked, but it is slow, and it is not a replacement for a stove. Do not expect to boil water or cook raw food with the pizza-box version. On a cloudy or hazy day it will hardly warm at all, because a solar cooker needs direct sunlight.
You can turn the project into an investigation by changing one thing at a time: with and without the flap, with and without the cover, with a black or a white floor, or at different times of day. You can also calculate the energy gained by the water: the heat needed to warm a mass m of water by a temperature rise ΔT is Q = m × c × ΔT, where the specific heat of water c is about 4200 J per kg per °C.
Materials and tools
Cost: Free
A pizza box, foil, newspaper, cling film and black paper are all household items. You need a thermometer: borrow a kitchen or school one.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Pizza box or any shallow cardboard box about 30 to 35 cm square (lid must be hinged or flat) | 1 | Ask a pizza shop for a clean box, or use a shoe-box lid on a box. |
| Aluminium foil (kitchen foil) | About 1 m | Used foil is fine if it is not torn. Use the shiny side. |
| Black paper or black chart paper (matt) | 1 sheet, 35 cm square | Black paper from a gift wrap, an old black bag or card painted with soot-free black poster colour. Avoid shiny black plastic. |
| Clear plastic film (cling film or a transparent oven bag) | 2 pieces, 35 cm square | From the kitchen. It must be clear and wrinkle-free. |
| Newspaper | Several sheets | Old newspaper, crumpled into rolls or balls. |
| Strong tape and glue | 1 roll, 1 tube | Parcel tape, and glue stick or flour paste. |
| Stick, ruler or straw to prop the flap | 1 | A ruler or a bent wire coat hanger. |
| Small dark steel pot or tin (katori) with a lid, wrapped in black paper | 1 | A thin steel katori or an old tin lid. A thin black pot heats faster than a thick one. |
| Thermometer (kitchen or lab, 0 to 110 °C or more) | 1 | Borrow from school, a kitchen or a medicine box. Use a digital or alcohol thermometer, not a mercury one. |
Tools
- Craft knife or strong scissors (adult cuts the cardboard)
- Ruler and pencil
- Cutting board
- Oven gloves or a thick cloth
- Stopwatch or phone timer
- Sunglasses
Safety first
Read this before you start
- The foil flap sends bright sunlight into the box and also out of it. Do not look at the foil in the Sun or aim its reflection at anyone’s face or at cars. Wear sunglasses when you adjust it.
- The box, the pot and the water can become hot enough to burn you. Use oven gloves or a thick cloth to touch the pot, and open the cover away from your face because hot air will come out.
- Keep the cooker flat and flat-reflecting. Do not use a curved or dish-shaped reflector made of foil, or a magnifying glass, near paper: concentrated sunlight can start a fire.
- Do not use the cooker for raw meat, eggs, milk or leftovers. Use it only for warming water, melting chocolate or cheese, or warming food that is already cooked. Do not eat anything that touches foil or paper unless it is in a proper clean container.
- Do the test on a stable surface in the open, and do not leave the cooker unattended near small children or animals. Do not stand on a roof edge or a wall to place it.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Pick the box and mark the flap
Close the lid and draw a square window on the top of the lid, 5 cm in from the front, left and right edges. The back edge of the window will be the fold line. For a 35 cm lid the window is about 25 cm × 25 cm.
Tip: Mark the window with a ruler and check its corners with a set-square or the corner of a book.
Cut the flap
With a craft knife (an adult cuts), cut along the left, front and right lines of the window and fold the cut piece back along the fourth line. You now have a flap hinged at the back of the lid. Run the back of the knife along the fold line first so that it folds cleanly.
Common mistake: Cutting all four sides. The flap must stay attached along the back edge so that it can stand up as a reflector.
Line the flap with foil
Spread glue thinly over the inside face of the flap (the face that will face the window) and press on a sheet of foil, shiny side out, smoothing out the wrinkles with a soft cloth. Trim the edges.
Tip: A smooth foil surface reflects sunlight in a clean beam. Wrinkles scatter it in all directions.
Make the window cover
Lift the lid. Tape a sheet of clear plastic over the opening from the underside of the lid, pulled tight and sealed on all four sides. Add a second layer a little below the first, if you can, to trap a layer of still air.
Common mistake: Leaving a gap or a hole at an edge. Hot air escapes through even a small gap, and the temperature stays low.
Insulate the box
Roll or crumple newspaper into strips and push them along the inner edges and bottom of the box so that they fill the space between the walls and the pot. Tape them in place. Leave a clear space in the middle for the pot.
Tip: The more still air the newspaper traps, the better the insulation. Crumpled is better than flat.
Line the floor
Glue the sheet of black paper on the floor of the box, over the newspaper, as a smooth matt surface, pressed flat with no wrinkles.
Common mistake: Using shiny black plastic. A shiny surface reflects some sunlight instead of absorbing it.
Make the flap prop
Cut a notch near the end of a ruler or stick, or bend a coat-hanger wire, so that it can hold the flap leaning back at about 100 to 110 degrees from the lid. It should hold the flap steady in a breeze.
Tip: Test the prop indoors: the flap should not fall forward on the window.
Prepare the pot and thermometer
Wrap the pot in black paper, or use a dark pot. Put in 100 mL of water and place it in the middle of the box. Stand the thermometer beside the pot or hang it in the water with the bulb not touching the metal.
Common mistake: Putting the bulb of the thermometer against the hot metal. Then it reads the metal, not the water.
Place the cooker in the Sun
Choose a flat, open spot in direct sunshine. Do the test between 11 a.m. and 2 p.m. Turn the box so that the flap is at the back and the front faces the Sun, and tilt it slightly towards the Sun by raising the back with a book. Adjust the flap until its shadow is almost gone and the reflected light fills the window.
Tip: If your own shadow covers the box, move. Even a thin shadow from a wire, a tree or a wall cuts the heating.
Record the temperatures
Every 10 minutes for an hour, read the temperature inside the cooker, the temperature of the water and the temperature of the air in the shade. Write them in the table. Re-aim the box towards the Sun each time.
Common mistake: Taking readings while the lid is open. Read through the plastic, so that the heat does not escape.
Compare variations
On another sunny day, or at the same hour, run the test again with one change: with no foil flap, with the cover removed, or with a white paper floor. Keep the rest the same and record the same readings.
Tip: Keep a record of the date, the time and whether the sky was clear. These change from day to day.
Warm something and calculate
Use 100 mL of water (0.1 kg). Calculate the energy gained from Q = m × c × ΔT with c = 4200 J/(kg °C). For a rise from 28 °C to 63 °C, Q = 0.1 × 4200 × 35 = 14 700 J. Divide by the time in seconds to get the average heating power in watts.
Tip: Compare your power with a 1 kW stove or a 60 W bulb, and explain why the cooker is slow.
How it should look when it is finished
A square pizza box that looks like a flat laptop: the lid has an opening covered with clear plastic and a foil-lined flap leaning back on a stick. Inside there is a black floor edged with rolled newspaper and a small black pot with a thermometer beside it. Placed in strong sunlight, the thermometer climbs steadily, and after an hour the inside is clearly hotter than the air in the shade.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
Sunlight enters through the clear plastic window. The foil flap sends more light in. Most of the light strikes the black floor and the black pot and is absorbed, so its energy appears as heat and the temperature rises.
The heat can leave in three ways, and the cooker reduces each. Conduction through the walls is slowed by the newspaper, which traps still air. Convection, the loss of warm air, is blocked by the sealed plastic cover. Some heat leaves as infrared radiation, and a glass cover would block more of it than thin plastic.
So the box warms until the energy that goes in equals the energy that leaks out. A better cooker (more reflectors, a glass cover, thicker insulation) reaches a higher temperature because it either collects more energy or loses less.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The temperature hardly rises | The sky is hazy or cloudy, the cooker is in shade, or the plastic is leaking hot air. | Test on a clear day around noon, move into full sunlight and re-tape the cover. |
| It gets hot but only to about 50 °C | The cover or the box leaks heat, or the flap is not reflecting well. | Seal all gaps with tape, add a second plastic layer, smooth the foil and re-aim the flap. |
| The plastic fogs up with droplets | Water has evaporated from the pot and condensed on the cooler plastic. | Cover the pot with a lid. Wipe the plastic only after opening, and keep the plastic clean. |
| The flap falls forwards | The prop slips. | Notch the stick, or tape it to the box and to the flap. |
| One side of the box is much cooler | A shadow falls across it or the box is not facing the Sun. | Turn the box every 15 minutes and keep shadows away. |
| The thermometer reads very high suddenly | The thermometer bulb is touching the hot metal. | Hold it in the water or in the air beside the pot. |
Test and record
Start when the cooker is in the Sun. Read the temperatures through the plastic every 10 minutes. Write the date, the time of day and the sky (clear, hazy or cloudy) above the table.
| Time (min) | Air in shade (°C) | Inside the cooker (°C) | Water in pot (°C) | Notes (for example flap moved) |
|---|---|---|---|---|
| 0 | ||||
| 10 | ||||
| 20 | ||||
| 30 | ||||
| 40 | ||||
| 50 | ||||
| 60 |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- Why is the inside of the cooker black?
A matt black surface absorbs most of the sunlight that falls on it and turns it into heat. A white or shiny surface reflects much of the light.
- What is the job of the foil flap?
It acts as a mirror and reflects extra sunlight into the box, so more energy enters.
- Why is a clear cover used?
It lets sunlight in but traps the hot air inside, which stops heat loss by convection. Glass would also block much of the infrared radiation.
- Why is newspaper used inside?
It is a poor conductor of heat because it traps air, so less heat leaks out through the walls and floor (conduction).
- Name the three ways heat travels and give an example from this project.
Conduction (through the box walls), convection (warm air rising and escaping), and radiation (the Sun’s rays reaching the floor).
- Why is the temperature highest around noon?
The Sun is highest in the sky, so its rays pass through less air and fall more directly on the box, so more energy arrives each second.
- How would you calculate the energy the water gained?
Q = m × c × ΔT, where m is the mass of the water, c is about 4200 J/(kg °C) and ΔT is the temperature rise.
- What are the advantages and limits of a solar cooker?
It uses a free, clean energy source and needs no fuel, but it works only in direct sunlight, it is slow, and it cannot be used at night or on cloudy days.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a box solar cooker and to study how the colour of the floor, the reflector and the cover change the temperature inside.
- Apparatus
- A pizza box, foil, black paper, clear plastic film, newspaper, tape, glue, a thermometer, a black pot, a stopwatch.
- Procedure
- Describe how you cut the flap, lined it with foil, covered the window, insulated the box and set it in the Sun, and how you recorded the temperature every 10 minutes. State what you changed in the second test.
- Observation
- Your table of temperatures and a graph of temperature against time for each test.
- Result
- The temperature inside the cooker rose to ... °C in 60 minutes while the shade air stayed at ... °C. The water gained ... J. Describe the effect of each change.
- Conclusion
- A black absorber, a cover, insulation and a reflector help a box trap solar energy. Solar energy is free and clean but depends on direct sunshine.
Take it further
- Compare black and white paper floors: repeat the test and plot both curves on one graph.
- Add a second foil flap on one side and measure the extra heating.
- Find the best angle of the flap by trying 90, 100, 110 and 120 degrees at the same time of day.
- Compute the area of the window and the energy per second falling on it, assuming about 700 W per square metre of direct sunlight. How efficient is your cooker?
- Make a solar water heater from a black bottle in the box and measure how long it takes to warm 500 mL by 15 °C.
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