Prototype Solar Stoves
For the Peru Children’s Trust.
During the summer of 1999 I set about making our first prototype. It was made from 0.7 mm thick AnocoilÓ aluminium, which is almost mirror quality and some steel tubing. The reflector profile opens out like a fan, as each segment pivots about the middle. Around the edge is a rubber strip that fits over the edge of the aluminium and keeps it all together. We found that the 0.7 mm aluminium is a bit thin to support its own weight so it sagged in the middle. This was overcome by clamping a conical support underneath. We also found that it was a bit wobbly and the focal point would move if there were any wind. Despite this it does work well on sunny days and looks cool. It also packs up very well so is ideal for camping. It isn't ideal for developing countries, as it isn't that stable and the aluminium was too heavy for my suitcase. As each sheet overlaps the next the reflection tends to be at an angle, so the focal point is not as accurate as the other prototypes. It is 1.2 m in diameter, which is equivalent to 1m2 of sunlight but probably only 60% efficient.
The second cooker was designed from the book ‘Cooking with the Sun: How to Build and Use Solar Cookers’ by Beth and Dan Halacy. Despite taking a while to make it was much more efficient. The reflecting profile was made from 24 cardboard ribs located radially every 15 degrees. Each of the 24 reflective segments is then taped down to the ribs. We made the box and reflector 1 m x 1 m and this time the efficiency and stability were much better. On a sunny day it is easy to boil potatoes and fry sausages. To test the focal point we placed a sheet of newsprint where the pot should go. The heat was sufficient to make it burst into flames in seconds. Our only criticism is that it would be better if the reflector was made from fewer parts, and after 4 months in a damp garage the cardboard has started to sag and lose its shape.
Our third model was more stable, simpler to make and more efficient. We used the 'wine box' principle of interlocking cardboard to create the required shape for the reflector to fit into. This was a mini-version only 0.5 m x 0.5 m and designed the rib shapes on the computer. The reflector profile is spherical with a radius of 0.5 m (equal to the square size of the enclosure). This gives an almost parabolic shape with a focal point at 0.25 m from the reflector. Each rib is 0.05 m apart and 11 ribs are required to make the shape. Each rib has a different radius depending on its location.
The reflector was made from one piece of 0.3 mm thick AnocoilÓ aluminium. I marked on 15 degree angles and then cut through with a very sharp knife leaving an area in the middle uncut. Take care of your fingers!
We then placed the reflector on top of the card profile and cut the edges to length. The reflector was then taped down around the edges, which produced an accurate focal point.
The grill was made from a 16 mm steel tube and some strips of brass bar. A pivot point in the middle allows for variations in sun height.
This mini-stove has an area of only 0.25 m2 but was still able to boil water. It was also quite easy to make, so we decided it was this model that we would 'scale up' and make from wood in Peru.
The Solar Cookers we made in Peru were 700 x 560 mm. I have also designed a unit 1 x 1 m called Solar1000 that will give more heat.
Last updated on 20th November 99.