Solar oven

I thought it'd be fun to show my son how we could use energy from the Sun to heat some food in an improvised solar oven. I suggested the idea without really thinking through how we were going to do it (I never learn, it's always better to have a clear plan before I share an idea with the small boy as he gets excited and wants to try it immediately!), so the end result is far from pretty, but it worked and he was really enthusiastic about 'cooking' his own snacks.

We used the following materials to make our solar oven (or as my son prefers, his 'Sun oven'):
- Two cereal boxes, one a lot smaller than the other.
- Aluminium foil
- Cling film
- 2 pipe cleaners
- A fleece scarf
- Sticky tape (we used parcel tape as it was what I found first), a pen and scissors.

The first thing I did was to tape up the bigger cereal box at the top, lay it on one of its biggest sides and then put the smaller box on top in the centre. I drew around it with the pen then marked a line on one of the long sides half way between the edge of the big box and the outline of the smaller box. I used scissors to cut a flap which went along the lines drawn on the two short sides of the smaller box and the extra line I marked midway between the edge and the outline. My son helped me fold this upwards at a 90 degree angle. He then cut a piece of foil to cover this with his scissors - it ended up bigger than the flap and somewhat uneven at the edges as he's only just getting the hang of cutting with scissors, but it didn't matter. We just folded it around the edges of the flap, tucked the overlap on the fourth edge inside the box and secured it with a couple of pieces of tape. The bigger box is there to insulate the inner oven and provide a reflective flap which can be angled to direct more sunlight into the oven.

Cutting the foil

The small cereal box we used opened along one of the narrow, tall edges and I cut along two more sides to make a lid with a hinge along one of the long edges. I then cut a large window in this, leaving about 1cm around the edges for strength. We folded some cling film to give two layers and cut a piece to fit. Again the cutting was a bit wobbly, but we just taped the piece on and folded the overlaps over. The purpose of the cling film is to seal the oven so the heat doesn't escape, but to allow sunlight through to warm it up inside, basically like a mini greenhouse.

Foil-covered flap and scarf inside for insulation

We then cut some more foil to line the smaller box and taped a few bits of the box for strength. For insulation, we put a layer of fleece scarf inside the larger box at the bottom then twisted the rest of the scarf around the sides before placing the smaller box snugly inside with the lid opening the same way as the foil-covered flap on the outer box. I found if we twisted the inner box a tiny amount that the outer box would hold the lid pretty tightly closed.

Inner box with a cling film lid 

We taped a pipe cleaner to each side of the back of the foil covered flap, fairly near the top (i.e. furthest from the rest of the box), pointing outwards at 90 degrees. I made a couple of holes in the outer box at the side of the oven, right near the fold of the box and two holes immediately below these on the narrow edge (see pictures). It'd have been easier to do this before putting the scarf and inner box in, but I threaded the ends of the pipe cleaners through the holes and twisted the ends back on themselves to secure them. These make an adjustable holder for the flap so you can bend or tighten the pipe cleaners so the flap is positioned to reflect the sunlight.

Putting food into the solar oven

We put it outside on the grass. If you're not sure how to angle it, put your arm up and look at the direction of your shadow - this should be pointing towards the reflective flap. Depending on the angle of the sun, you may need to raise or lower the flap slightly so the reflected light goes in the oven - you can see by putting your hand over it and looking where the shadow is.


I suggested that we might try and warm a cup of water, and use a thermometer to see how hot it got.  However the small boy pointed out that ovens are used for cooking food, and he was hungry (as seems to be the case much of the time at the moment)... I therefore decided we'd try an edible experiment - seeing which melted first out of cheese, chocolate and a marshmallow.  To avoid mess and increase the time taken before the small boy required feeding again, we melted them on a cracker.  It wasn't a very scientific experiment, we used different quantities of each foodstuff, and the result would no doubt depend on the type of cheese and chocolate used and probably also their positions in the solar oven, but the small boy was very excited about the idea. 

Melted chocolate

He carefully placed a piece of cheddar cheese, a couple of chocolate buttons and some mini marshmallows on different crackers, and closed the lid.  I asked him to predict which would melt first, and he guessed the cheese, and then proceeded to run round the garden excitedly, checking on his oven with each lap (which answers the question about why he perpetually needs feeding, he uses a lot of energy!).  The cheese started to look sweaty after about 5 minutes, and he was convinced it was melted, so he opened the lid and gave it a poke.  It was soft, but I wouldn't describe it as melted.  However, the chocolate button was liquid, so we agreed that it had melted and could be eaten.  After a further 5 minutes (having let the hot air out by opening the lid when he removed the cracker and chocolate) he decided the cheese was melted enough to eat although I think this was a decision driven by hunger rather than the evidence in front of him, it definitely didn't look melted to me.  The marshmallow didn't properly melt even after another 10 or 15 minutes, it seemed softer but it still kept its shape and bounced back a bit when poked.  I was expecting the marshmallow to melt - I thought they had a melting temperature which was around body temperature, but this didn't seem to be the case as the thermometer I put in when he took the cheese cracker out was showing 58 degrees Celsius at the point we finally gave up on the marshmallows and the ate them anyway.

It's fair to say that this experiment was repeated several times over the course of the next few sunny days, with various combinations of cheese, chocolate and marshmallows (not all of which I'd recommend!).  The solar oven was definitely a hit, and we talked about why it got hot and why it doesn't work as well on a cloudy day so there was some science as well as snacking involved!


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