DIY · Card 85 · Energy · Light
An empty pizza box, some foil, black paper and cling film. Point it at the Sun and in half an hour the chocolate melts and the marshmallows go gooey, without any fire at all.
01The brief
Cook with nothing but sunlight.
1,000 W/m²
of sunlight reaching the ground on a clear day
60–90 °C
inside a well-made pizza-box oven in bright sun
1767
when Horace-Bénédict de Saussure built the first hot box
Every square metre of ground in full sunshine receives about as much power as a small electric heater. Most of it bounces off or blows away. A solar oven is a trap: it lets sunlight in, turns it into heat and keeps that heat from escaping.
A pizza box is all you need, and the reward is a warm s’more. It makes a good pair with the sundial and the melted crayon art.
02Kit
A pizza box and a raid on the kitchen drawer.
Tick off what you have
0 of 5 ready
03Method
A mirror flap, a black floor, a sealed window.
Step 1 of 8 · Minute 0
You need a clean pizza box, kitchen foil, black paper, clear plastic (cling film or a freezer bag), tape, glue, newspaper, a ruler, a craft knife, a skewer or stick, and something to cook: s’mores, nachos or a slice of bread with cheese.
It only gets lukewarm.
Why
Light or heat is escaping.
Fix
Seal every edge of the plastic, add more newspaper, and make sure the box faces the Sun squarely.
The window steams up.
Why
Moisture from the food is condensing on the cool plastic.
Fix
That’s fine: wipe it, or put the food on a dry plate. A second layer of plastic reduces it.
The flap won’t stay at the right angle.
Why
The cardboard hinge is floppy.
Fix
Prop it with a skewer or a pencil taped between the flap and the back of the box.
04Trapping sunlight
The oven warms up until the heat leaking out each second equals the sunlight coming in. Every part of the design either brings in more light or slows the leak.
Switch the parts on and off and watch the temperature:
Sunlight captured
109 W
Settles at
87 °C
Most of the way in
46 min
Hot enough that chocolate melts, marshmallows go soft and gooey, cheese melts on nachos, a ready-baked cookie warms through. The oven settles where the heat leaking out matches the sunlight coming in. Take away the window and the warm air just blows away; take away the black lining and most of the light bounces straight back out. A simple model, but real pizza-box ovens reach about 60 to 90 °C on a bright day. The amber line is the outside air, 22 °C.
The flap only helps if it is at the right angle. Find it:
Best flap angle
63°
Yours
70°
Reflected light
into the oven
A mirror sends light off at the same angle it arrives. To turn sunlight coming in at 35° above the horizon straight downwards, the flap must sit halfway between the two directions: 45° + 35 ⁄ 2 = 63° from the box top. Morning and evening, when the Sun is low, lean the flap further over the window; at midday, stand it more upright.
05The big idea
In equals out,
eventually.
Anything left in the sun warms up until it loses heat as fast as it gains it. To make something hotter, you can either let more energy in or let less out: there is no other way.
The same balance sets the temperature of a cup of tea, a house in winter and the whole planet. Earth’s atmosphere acts a little like your plastic window, which is why changing what is in it changes the temperature we live at.
06The math
1 · Sunlight captured
P = S · A · τ · α
S sunlight, A window area, τ and α the window and lining.
2 · Steady temperature
ΔT = P ⁄ (U · A′)
U the heat-loss rate per m² per °C, A′ the box surface.
3 · Best flap angle
45° + e ⁄ 2
e the Sun’s height above the horizon.
A 30 × 30 cm window in 1,000 W/m² sunshine lets in 0.09 × 1000 = 90 W. The plastic passes about 85 %, the black lining absorbs about 95 %, and the flap adds about half again: roughly 110 W, the power of a bright old-fashioned light bulb.
If the box loses about 1.7 W for every degree it is warmer than the air, it settles about 110 ⁄ 1.7 ≈ 65 °C above it: near 87 °C on a 22 °C day. With the Sun 40° high, the flap works best at 45 + 20 = 65°.
07Real life
From greenhouses and rooftops to a furnace in the Pyrenees.
Greenhouses
Glass lets sunlight in, the plants and soil turn it into heat, and the glass stops the warm air from blowing away. A greenhouse is a giant solar oven for tomatoes.
Hot cars
A parked car in the sun works exactly like your oven: sunlight through the windows heats the seats, and the closed car traps the heat. The inside can quickly become dangerously hot for children and pets.
Solar cookers
In sunny places where firewood is scarce, families cook with folding solar cookers made of reflective panels. They save fuel and spare people the smoke of cooking fires.
The Odeillo solar furnace
In the French Pyrenees, a huge curved mirror the size of an office block concentrates sunlight onto one spot. It can reach about 3,500 °C, hot enough to melt almost anything.
Rooftop water heaters
Black panels on sunny roofs, from Cyprus to China, heat water using the same tricks: dark surfaces to absorb light, glass to keep the heat in, and insulation behind.
Black and white clothes
A black T-shirt in strong sunshine absorbs more light and feels warmer than a white one. In hot, sunny places, people often choose light-coloured clothes for exactly this reason.
1767
The Swiss scientist Horace-Bénédict de Saussure wondered why it was colder on mountain tops, closer to the Sun. To study sunlight, he built a wooden box lined with black cork and covered with layers of glass.
Left in the sun, the inside of his ‘hot box’ reached around 87 °C, hot enough to cook fruit. It was the first solar oven, and the same principle explains how greenhouses work and, in part, why our planet is warm.
08Is and isn’t
A solar oven needs a hot day.
It needs bright sunlight, not warm air. A crisp, clear winter day can work better than a hot, hazy summer one.
The black paper makes heat.
It doesn’t make anything: it absorbs light and turns that energy into heat, where white paper would reflect it away.
The foil keeps the heat in.
The foil flap is a mirror that adds more sunlight. It is the plastic window and the newspaper that keep the heat in.
Archimedes set Roman ships on fire with mirrors.
A famous legend, but modern attempts to repeat it have mostly failed: aiming enough mirrors for long enough at a moving ship is very hard.
09Go further
A
Thermometer test
Put a cooking thermometer inside and record the temperature every 5 minutes. Does it follow the curve in the oven lab?
B
Take a part away
Build two ovens, one without the black lining. How much cooler does it stay?
C
Black and white cans
Fill two cans with water, paint one black, and leave both in the sun. Which warms faster?
D
A double window
Add a second layer of plastic with an air gap. Does the oven get hotter?
E
Track the Sun
Turn the oven every 15 minutes to face the Sun, and compare it with one you leave still.
10Questions
Cut a flap in the lid of a pizza box, leaving the back edge as a hinge, and cover the inside of the flap with foil. Line the box with black paper, tape rolled newspaper round the edges, and seal clear plastic over the opening. Put food inside, face it to the Sun and prop the flap so it reflects light onto the window.
On a bright, sunny day, usually about 60 to 90 °C inside. That is enough to melt chocolate, soften marshmallows and warm nachos, but not hot enough to bake bread or cook raw meat safely.
Yes, as long as the Sun is bright. It runs on sunlight, not warm air. A cold, clear winter day works better than a hot, hazy one, although cold air does steal heat a little faster.
Black surfaces absorb almost all the light that reaches them and turn it into heat. A white or shiny floor would reflect most of the light straight back out through the window.
S’mores, melted chocolate, nachos with cheese, cheese on toast and warm cookies from ready-baked dough. Keep to foods that are safe to eat warm rather than ones that need thorough cooking.
Safety · The inside can get hot enough to burn: use an oven glove to take food out. Never look at the Sun or at its reflection in the foil. Cook only foods that are safe warm, not raw meat or eggs.
All DIY cards →Keep wandering
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