Pocket Solar Cooker

*Information below provided by the designer
I'm proud to introduce this compact, portable, 3D-printed solar cooker prototype. I started thinking about this idea after seeing solar cookers being used at a mountain hut while hiking. Using the powerful summer sunlight in the Alps was smart, but those large parabolic reflectors are not convenient to carry, so I decided to make one that fits in a backpack and can still boil some water or coffee (successfully tested making coffee on the 20th of November).
==Detailed features==
The thing that I designed is a parabolic reflector of approximately 80cm diameter (0.5m²) made of 37 hexagonal tiles. Tiles have a maximum radius of 8cm, a thickness of 3mm, and can be stored as a stack which is approximately 12.5cm high, fitting in a custom designed box. The maximum power hitting the parabolic surface is roughly 500W under ideal conditions, but the actual heating power will be affected by the mirror film reflectance, the absorptance of the pot's coating and the surface accuracy. The sunlight is reflected mostly upward (only a small part horizontally or downward if the sun is below 40°), and the focal point is at the center of the pot's bottom surface, so it is only necessary to paint the bottom surface in black.
The center tile, which is thicker, is used to hold the cooker using a tripod, and also as a base to support the pot. The support system is composed of two circular rails holding a carriage for leveling the pot for a sun height ranging between 30° and 70°. The rails are connected to the center tile by three beams. The top of the three beams is printed together with the rails for maximum stiffness, and the bottom of each beam as a separate part. Beam sections are assembled by a hook system and connected to the center tile with nuts.
The interface with the pot is made of two stainless steel blades of half hexagon shape whih can be stored in the same box as the tiles. The carriage just slides on the rails, and the steel blades are nested in one another and then secured on the carriage with four folded nails.
It takes about 30mn to assemble all parts, mostly connecting the tiles together with the (many) small clips. Maybe a little too long for the picnic in a single day hike, but this can be suitable for a bivouac. The "two seconds" version is left for future work.
==Advanced design characteristics==
The core idea is the use of an hexagonal tiling, resulting in a convenient hexagon shaped surface. The tiling is computed using a mapping between a plane and the paraboloid which preserves the distance between each point and the center of the paraboloid. The resulting tiles thus have approximately the same maximum diameter which optimizes their storage as a stack.
The paraboloid has a focal length of 35cm, and the reflected light forms roughly a cone of 60° aperture (half-angle). The center tile is shifted by a 20° angle with respect to the axis of revolution. This way, most of the light hits the bottom of the pot, and the weight is placed nearly above the center tile, thus minimizing the mechanical stress on the beams. The value of 20° is a tradeoff which does not tilt the reflector too much with respect to the perpendicular orientation, so that the amount of light received for a given reflector surface is stays close to the maximum.
The edges between tiles are determined by computing a plane orthogonal to the surface (at the center of each edge) and the strips and clip anchors are aligned with the surface.
The support is designed to minimize shadows, both on the reflector and on the pot. Thus, the three beams that hold the rails are aligned with the focal point, and they are also relatively flat in a plane aligned with the sun. The rails structure and the carriage are also aligned with the focal point, as well as the steel parts, which are only 1mm thick and stay at a good distance from the focal point to minimize the shadow on the pot.
==Construction process==
This thing is mostly 3D printed. The tiles are printed in PLA on one of their edges, ensuring a smooth reflector surface. They are sanded before applying an adhesive mirror film (I used a cheap one found in a supermarket). For sanding the tiles, I printed specific sanding blocks whose curvature matches the paraboloid surface and mounting them on a vibratory sander. A perfectly smooth surface is perhaps not required if the mirror film is thick enough, but the adhesive does not seem as efficient without at least some sanding. I also sanded the back to obtain a smooth surface and avoid damaging the mirrors when stacking the tiles, and also to reveal the small embossed numbering which were painted before sanding. I glued all strips at the last step, after re-cutting the mirror film following the edges with scissors.
I printed the support in polycarbonate as I feared that it could receive enough sunlight to exceed the temperature limits of PLA, especially when approaching the focal point. Dark colors must be avoided of course. Despite that, the support seems somewhat flimsy during use, so a more heat-resistant material would be better. I even slightly melted one corner of the carriage, probably due to a misalignment with the sun.
To hold the reflector, my first idea was to screw a camera tripod directly in the center tile (which has a threaded hole to this end), but the stability was not very satisfactory, and I was afraid that the pot could exercise to much traction on the thread. So I finally replaced the connector part of the tripod with a 3D printed T-shaped part which is attached to the center tile by reusing the nuts which hold the support beams. This part is specific to the camera tripod that I had at hand, but it should not be too difficult to reuse the design to work with a different stand.
==Conclusion==
Building this project is quite a lot of work ! Not only the printing time (about 10 hours for each tile with 0.1mm layer height at slow speed), but it also requires playing with tolerance parameters and print settings to get a good fit between parts and also a lot of sanding work.
Apart from that, the cost boils down to two spools of PLA, one square meter of adhesive reflective film and the laser-cut parts which I ordered online (those could be replaced by a simpler, easily to cut shape, at the expense of a minor loss of efficiency, and of course, look).
The OpenScad design is largely generic, so this can be modified in many ways. I can provide additional details and printing advice to anyone interested.
==External links==
*https://www.thingiverse.com/thing:5137843
==Contact==
Category:Solar cooker designs
Category:Solar array cooker designs