The Sun Scoop Tracking Solar Cooker!

I made a tracking solar cooker, it's almost 1.5 sq meters in reflector size. It is a square section out of a parabolic dish, and it will allow the user to reach what you are cooking without moving the reflector or putting your eyes into dangerous levels of light rays. It is on equatorial mount so that as long as you point it at the sun in the morning and rotate it slowly at 15 degrees per hour, it will stay pointed at the sun all day.

Scissors, saw, drill, compass dividers protractor pencils, drawing board, drawing paper, 8 ft by 4 ft sheet material (ABS or corrugated plastic, like chloroplast), wood in 2 by 4 and 2 by 8 sizes, pots and pans, rebar, wood screws in various sizes, specular reflective tape.

I Went for a 15 cm (about 6 inch) diameter target where all the rays would come together as a ball of light. (I used a 1 to 5 scale so a circle of 15 cm was actually 3 cm wide). I drew a circle of 15 cm diameter for the target, then from the right edge of the target I drew 9 vertical lines 15 cm apart. Between the lines represents 15 cm vertical shafts of light. I then made horizontal lines from the top and bottom of the 15 cm ball. Where the top line intersects the last vertical line and the bottom line intersects the second-last vertical line are the ends of the first "mirror strip" of the parabolic dish. To get the ends of the next mirror strip, you draw a line from the bottom of the first mirror strip to the top of the target ball, and you draw a parallel line from the bottom of the ball to the 3rd vertical line, Where it intersects the 3rd vertical line is the end of the second mirror strip. You continue with this method until you have your 8 mirrors drawn. Next you draw a vertical line down from the center of the target ball to the bottom of the page. A line continued from the first mirror strip will intercept that line. Measure its length to both ends of the mirror strip and write them down, do this with all the other mirror strips too. These are the radii of the strips of plastic or metal that you will cut for your reflector!

Why make a model? It is because if there are mistakes in my design, they might show up in the model and I can correct it before wasting a load of material. A 1/5 model has 25 times less material in it so it is like taking out insurance policy against making big wasteful mistakes! And my reflector is very different from everyone else's! When most people make DIY parabolic dishes, they cut "petals" to form a reflector dish which is a bit like an umbrella. My reflector is like what you would get if you made a much larger umbrella and then took a big square "bite" out of it. The bite being my reflector! You cannot easily make the "bite" from "petals" because 3 or 4 of the petals would be full length, then petals would have pieces cut off them (so as to make the square shape. every shorter petal would have a different size chunk cut from it. But if you use the "cone" method, you can approximate my "bite" paraboloid with 8 cone pieces! There are just tiny strips of waste and an 8 ft by 4 ft piece of plastic sheet can be used to make 2 paraboloid Sun Scoops! I made the first one just under 1.5 sq meters (I chose that size because there are other solar cookers of that size and so people might be able to compare like with like. I still have a piece of the plastic sheet left. Big enough for another reflector of about 1 sq meter size. I am holding it in reserve because I might make the second one a bit differently than the first one. (For instance, I might use 10 cm slices (instead of the 15 cm (6 inch)) slices in my 1.5 sq meter reflector. This would give a much hotter concentrated area. the remaining piece is something like 3 ft by 4 ft. I might use 3 ft as the width. This makes it easier for me to make the frame with reclaimed pallet wood.

I made a frame last year for my previous solar project and this year I just modified and extended it to accept the wider reflector for this year. I only have 3 supports to keep the paraboloid rigid and I feel that 5 supports would keep its shape much better. I used wood, but if I had been able to bend aluminum pipe accurately, I would have gone with it.

I used a 4 ft by 8 ft sheet of 1/8 inch thick ABS plastic because it has a very smooth surface. (I could have used much cheaper corrugated plastic but it has a slightly "wavy" surface and that will spread the light in unknown ways. This is the "prototype" so I couldn't take the risk of using suspect material. (Corrugated plastic might be good enough). Anyway, i drew a line down the middle, 2 ft in from each side. I put the center mark for the first curved strip at the correct distance from the far edge, Then marked it at the edge of the plastic, then from the same center make the next arc of the circle. This is the bottom edge of your strip. Next, from where the bottom arc meets the center line, measure off the distance from the outer edge of the second mirror strip on you diagram and that is your next center point. Do you 2 arcs again, and continue the process with the next mirror strip till you are done. When I came to my last center point, I just drew a line from there across my plastic sheet and cut off the "waste piece" to use later for another project. This project uses approximately 5 ft by 4 ft of the plastic sheet. Next I cut the arcs, and ended up with pile of 8 arcs and a small pile of cuts from between the arcs. After that, I pulled the arcs tightly together and taped them together with TucTape. Nothing else worked last year, all the other tapes allowed the material to pull apart, so I went with the proven Tuctape. After that, I glued on the reflective material.

I ended up putting up a temporary frame on the side to hold the pot. That way, I have good access to the pot. The main job in the first part of the year was steaming soil to kill weed seeds and pathogens. Last year, I showed that steamed soil has huge benefits when you grow lettuce. (20% faster growth rate when I transplanted lettuce seedlings into steamed soil!) so, if it could be done efficiently and quickly, steam sterilization by solar cooker, could become a "thing". I had a 14 liter pot, that I painted black to absorb the heat. First, I put damp soil in it. But it took a long time to get hot and steamy, and also, the soil at the bottom burnt. Next I put lawn mower cuttings at the bottom, with some water, then soil above, and some of the lawn mower clippings burnt and steaming was still slow! So, I was accidently making biochar! The outside of the pot at the bottom where the light rays struck it was getting very hot, (above 200 C sometimes) and I thought that was a bad thing because metal at 200 C is losing heat quickly to the air by convection and radiation. How to fix that? So, I made a little wooden support "cross" about 5 cm (2 inches) tall and over that I put diamond mesh and a splash guard for a frying pan that fit snugly in the 14 liter pot. Then I filled it up to the wire with water. I was afraid that soil would fall through into the water, so I added a layer of lawn mower clippings first, then a shovel of soil, then another layer of lawn mower clippings, then another shovel of soil, and layered it to the top. Last layer was soil. It worked! The metal of the pot at the bottom got to 110 C max, now. and that meant that the water quickly took the heat away. After a few minutes, I could hear bubbling. Eventually steam came out of the top. I used an oven thermometer to check the temperature at the top in the middle down a couple of cm. If this was above 95 C, I deemed the soil "steamed" and took out the batch. I had another pot of almost the exact same size and for a while I exchanged pots. But here is the thing. Roughly half the water in the chamber boils off and ends up in the damp hot soil, after a couple of hours when the soil is steamed, you have significant hot water down there in the chamber. I found a better option was to empty the soil out of the pot, top up the hot water in the chamber, and layer lawnmower clippings and soil back into the same pot. That mean I kept the heat of the water and the bottom of the pot for the next batch. On a sunny day, it ended up that I could steam about 4 batches of soil (roughly 40 liters). I filled planters with it, and used steamed soil in some sections of my greenhouse, I also dug out soil to about 15 cm deep (6 inches) in one area, and I topped up a planter for the top 9 cm (3 1/2 inches). This was to see the effects of doing it different ways. In all cases there was a massive reduction in the amount of weeds that emerged. And plants generally did really well

I fried an egg (before I had things leveled properly, so it went to one side). After that I was steaming grass to use as mulch, (and potentially as mushroom substrate). I will also be steaming soil to kill bugs, seeds and diseases in the soil. That worked great last year. This year, its faster and the pots are bigger so I will get more done.

I want to build a "box cooker" that is big enough to hold my biggest pot. It will go on the stand, and I will have access via a door at the back. The sunlight will come in through a glass lid at the bottom front and hopefully a lot of heat will be trapped inside. I am hopeful that this will increase the capabilities of the solar cooker by a large factor. I also plan to have the box cooker double as a solar dehydrator or have a separate solar dehydrator that I can put on instead of the box. so it will be like having attachments.

I hope that in a few years, solar cooker manufacturers across the world will be making these thing with nice metal frames and in various sides. (It depends on the materials they have available). And also, the size of my reflector is similar in size and shape to that of a car bonnet or hood. So, maybe in a few years, industrial companies will stamp out "bonnet" reflectors and "hood" reflectors from metal blanks while others will be making them in fiberglass and in plastic. The industrial production can have much tighter focuses than mine, so they can probably send heat through a tiny hole into a box, meaning they can transfer more heat, while letting less out. So yeah, hopefully the future can be mighty!