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Solar Cooking

Cooking progress

22 Sept 09 - an email from the designer 1 Jan 09 sunny but windy - rice undercooked 30 Dec 08 overcast - chick peas undercooked 29 Dec 08 sunny & patchy cloud - rhubarb cooked 28 Dec 08 sunny and calm - rice overcooked

27 Dec 2008

Building a solar cooker step by step

Here's how I built a solar cooker for use in Lower Hutt, New Zealand

Feel free to ask me about it.

Cheers, Paul

Find a design

There seem to be a million different designs for solar cookers. I've chosen to experiment with this solar reflector design by Philip King. It looked simple and used materials I already had, or could easily find. And it is scalable to whatever size piece of cardboard you can find.

Other solar cooker plans.

Materials

1. Cardboard bike box - I went down to the local bike shop and asked if they had any spare bike boxes. They pointed me in the direction of the bin out the back. I rummaged through that looking for the best one. [Note the Recycle symbol on the top left - does this project count as up-cycling? There's a lot of pun potential in this.]
2. Aluminium sisilation. I still have some reflective underfloor insulation left over from by previous insulation efforts.
3. PVA Glue. Used to glue the sisilation on to the cardboard.
4. Black pot. I bought a second hand pot for $5 at the market and spray painted it black.
5. Black paint
6. 1/2 height glass AG jar. Used to raise the pot a bit (so that it benefits from some light on the bottom of the pot too). We happened to have a few (thanks Timbo!).
7. The plans [PDF] download and print out.

Tools

• Ruler and/or tape measure (I used both, but didn't need to)
• Long straight bit of wood (for drawing long lines)
• Knife
• Pencil
• Black felt pen
• Screw driver and pliers (to remove staples form box)
• Calculator (to work out the biggest size that would fit on my bike box)

Prepare the box

Cut the box into two halves - down the corners that have joins.

Remove all the staples.

Select the nicest of the two pieces.

Pick a scale, draw the grid

The nice thing about this design is that it will scale to any size, based on a 8 x 6 square block grid (the purple squares shown on the image at the top of this page).

So - I can maximize the size of the cooker to fit this bike box. (Or any other box I might find.)

To find the biggest "block" that will fit on your piece of cardboard:

1. Measure the longest edge of the piece of cardboard (I got 1477mm)
2. Divide that number by 8 (I got 184.6mm)
3. Round that number down to the nearest easy number (180mm)
4. Measure the short edge (I got 1152mm)
5. Divide that number by 6 (I got 192mm)
6. Round that number down to the nearest easy number (190mm)
7. Pick the smaller of those two results - ie 180mm. That's going to be the size of the square "block".

Draw the grid:

1. Start from the straightest edge
2. Measure the middle of the side and start creating the 180mm grid out from the middle (You'll notice I didn't do that - not sure what I was thinking.)
3. As long as the measurements are within a couple of mm it's OK (you don't need to bother with pin-point accuracy)
4. I used a T-square to get a nice right-angle

Draw the shape

Mark off all the required quarter marks (the orange squares on the image at the top) as shown on the plan.

With the plan in front of me, I duplicated each line. Then I went over the outline with a black pen - so that it was blindingly obvious before I started cutting.

It takes a while to do this - but you only need to do it once. Slow and steady wins the race.

Cut it out

I used as Stanley knife (or as the American's would call it a "box cutter").

I also went over the fold lines with the felt pen.

And another one

There is a chance I might want to make more of these so I decided to designate this first cut-out a template.

The nice thing about having a template is that I was able to position the shape to avoid rough bits of the bike box.

To help locate the fold lines I also added little "cut-ins" (I wonder what the technical terms for that is?) at the end of the fold lines.

I ran a pen around the template and cut the second piece out. This is quick and easy.

Rushing ahead

At this point I got so excited I stopped taking photos. If I make another one I promise to take photos and update this page.

1. I cut out a piece of aluminium foil (slightly larger than the cardboard shape).
2. Glued the foil to the cardboard.
3. Waited overnight for the PVA glue to dry.
4. Next morning I folded over the extra trim and taped down.
5. Folded the fold lines using the pointy end of a spoon along a ruler. [If I had been smarter I would have creased the fold lines before step 2, and pre-folded the cardboard, then glued the foil onto the side that was creased. Corrugated cardboard folds much better with a crease on the inside of the fold. You don't want to try creasing the foil side after you've glued it because you'll probably tear it.]
6. Rushed outside to test it.

First test!

OK - here's the first test. It's a pot of rice set out just before 10am in full sun.

It's a funny angle to look from - but it's an odd shape which ever way you look at it.

I put it on a wooden frame to keep it out of reach from my 14 month old son.

I put the pot on a half-height AG preserving jar. This allows the black pot to receive some light/heating on it's bottom too.

I added a bungee cord hooked from the middle of the base to keep it steady in a light Wellington breeze.

I turned the cooker towards the sun every few hours.

I left it in the sun til about 4pm because I was doing something else and forgot about it. The rice was over cooked but not burnt (I had too much water in the pot). We had it for dinner in a rice salad.

Lessons learned:

• It works!
• It cooks very slowly so you don't have to worry about timing so much.
• My pot has a black handle so it absorbs heat too - but not enough to burn my hand.

---

1 Jan 2008

It was sunny but windy today. I put out another pot of rice - this time long grain brown rice. By the end of the day it still wasn't cooked. I expect that was because the wind was sucking away the heat.

So I'm thinking about build a simple solar oven.

---

22 Sept 2009

Just got this email from the cooker designer:

Sender : Philip King
Sent : 22 September 2009 22:10:00
Hi, I've just seen on your web site that you've tried the solar cooker design which I worked up a couple of years ago. Thanks for giving it a go, it's good to see that the plans worked!

I just wanted to say something about your last comment about the brown rice not cooking (I know this was a long time ago, but I've only just seen it), and it may be that you would get better results if you enclosed the pot in a heat-proof plastic bag. This is the technique recommended when these cookers are used in Africa, and in my experience it makes a huge difference to the efficiency of the cooker. These plastic bags are usually sold in supermarkets as turkey roasting bags or oven bags, and when used with the cooker the pot is placed inside and the bag is closed with a clothes-peg. It also helps to raise the pot a little off the base of the cooker, about an inch or so is plenty, and this can be done by using three small stones or sticks.

Best wishes,

-= Philip

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Why?

I like making things.

A solar cooker is both easy to make and cheap! It may even be useful - time will tell.

I'm trying to reduce my carbon footprint and energy consumption.

It could also add to my resilience - if the power was out (in summer) I'd still be able to cook some food. [Note to non-New Zealand readers; mid-winter is our peak electricity period when everyone turns their heaters on. So we're more likely to experience a power outage in winter... except... ]

A solar cooker is potentially an energy efficient way to preserve summer veges.

See also Preserving Produce without Heat