Misc. The "Sun Scoop" Tracking solar cooker!

[Brian in Victoria BC]

TL;DR Summary

I did a one to 5 scale model earlier in the year, then scaled it up, and it has been good.

My solar cooker is a DIY proof of concept. Ideally it should be made with a metal frame but I can't do that so it is wood with protective aluminum foil. The idea is to concentrate the light in a 6 inch diameter ball near the bottom of the cooking pot. It is called Sun Scoop because even though it is a part of a paraboloid, it looks like a big scoop.

It is on "equatorial mount" that just means it rotates on an axis that points exactly in line with the earths axis. (In my case, that means it is pointing north and tilted up at exactly 48.5 degrees (my latitude). This time of year, I won't be cooking food, there is a lot of sun with clouds, and cloudy spells and days with no sun. So it is not safe unless I do something quick like frying an egg. Instead I steam soil to start seeds in and I steam cut grass to use as mulch. (It rots way quicker when it is steamed and also the steaming kills weed seeds and bugs).
The main page for the project will be https://www.instructables.com/The-Sun-Scoop-Tracking-Solar-Cooker/

Here is a playlist about it. . I really hope people try this. It would be nice to see people do some mad things in the next few years, with improved versions of this. For instance, making quicklime with the sun might be possible with formed paraboloids on this type of frame!

 

[berkeman]

I didn't watch the video so apologies if you explain it there. How did you calculate and build the backing and joining to get a good parabolic shape for the reflective foil? Inquiring minds want to know...

 

[Brian in Victoria BC]

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!

 

[Brian in Victoria BC]

So, I started with the strip for mirror 1 and did its outer and inner radius, I drew the outer radius of mirror strip to the inside edge of mirror 2, then its inner edge, and repeated until I had the 8 mirror strips marked. Then I took a strong

scissors and cut them out.

 

[Brian in Victoria BC]

I ended up with the strips here and a tiny amount of wasted material, in the little pile at front. Then I used strong construction tape called Tuck Tape to stick them together.

 

[Brian in Victoria BC]

I was steaming soil in the solar cooker (kills off weed seeds, etc.) and 20% growth boost for lettuce seedlings. I have heavy clay soil, so I added grass clippings, etc. for a bit more organic matter and to leave space for water at the bottom of the cook pot. But, the focus gets hot, and it burns the grass at the bottom while heat transfers very slowly to the top of the pot. I made a "water chamber" at the bottom of the pot and this has sped up the steaming a lot. The heat of the sun quickly brings the water to the boil, and steam rises through the soil and we get sterilization more quickly than before. For those who solar cook food, this might work for some soups too on parabolic dishes, a chamber at the bottom will let the watery parts of the soup down but the big stuff that is more likely to burn will be kept out. (Success might depend on the type of soup) Video of the new water chamber technique

 

[Rive]

...making quicklime with the sun...

In theory it's possible, but to reach 1000+ degree temperature with a homebrew setup is not really trivial.

...concentrate the light in a 6 inch diameter ball near the bottom of the cooking pot.

I would add an iron stove plate there for convenience (and for protection).

 

[Brian in Victoria BC]

Hi, I have done 3 "boil tests" over the last few months. You heat 10 kg of water to over 90C and measure the speed of heating. First one I didn't have the sun well focused on the pot (pot was too high and I had to rebuild the frame that it was on. Second one, I used a fancy thermometer, but it is FAULTY. It was reading 16 degrees too high! I tried calibrating it against another thermometer but the calibration "line" wasn't straight. So those results were garbage! Last test, even though weather reports were for perfect sunny day, there were some high wispy clouds I used a meat thermometer and it was fine but only measures to 1 degree C accuracy.

But anyway, I took the results. At its best, it delivered 670 Watts to the water. and when it was near the end the water was taking about 215 Watts (meaning that about 455 Watts were being conducted, convected, evaporated and radiated from the pot). I am still using the pot to steam batches of soil and lawn mower chopped weeds. The most I have done in a day is 4 batches. (about 40 liters of soil). I get it up above 90 C all the way through and that is good enough for me.

Last year in a test lettuce seedlings grew roughly 20% faster in cooked soil than in non cooked soil. (I was not adding the chopped up weeds in that test. For me, it is a joy to not have to do weeding and if I sow experimental seeds, there is no guesswork as to which seedlings are weeds and which are veggies.

Anyway, that is the update for now. I have a bit more soil to steam and then I am done. I might do another boil test, and after that, I will make the "solar oven" part, with a "window" at the front, maybe Pyrex, maybe a piece of glass from a microwave turntable. and the door at the back, and start drying fruit and herbs in it. (And I will make it big enough so I can still cook pots of soil in the solar oven). One measurement is faulty, but that is ok, the trend is the thing.

The test was on the 7th July. I would like to point out that 670 Watts is 90% of a horsepower. I don't have to feed my little horse or clean up after it! But the horse only works hard on sunny days. Thanks Brian

 

[Brian in Victoria BC]

I have done a lot of solar steamed soil over the last few months, and I've got a procedure and technique going now. Maybe solar steamed soil (and weeds) can be a bit like a replacement for "slash and burn agriculture". The advantage is that it saves the biomass. I don't like the concept of "biochar" but currently it is a bit of a craze in some online communities. I hear that people get money from "carbon offset" schemes to build biochar processing facilities. You could do biochar with a solar cooker, or perhaps better yet, people could process the material like I do, mix it with soil and sell it as potting compost. Another thing that I have thought of is wood chips. Landscape garden companies often grind down woody material from their clients. Well! if you steam that stuff at 5 psi (or maybe atmospheric pressure will do), and bag it, it is a "mushroom block"! all ready to go to grow lions mane or oyster mushrooms! Anyway, here is the procedure in my latest video. Solar slash and burn alternative!

 

[Brian in Victoria BC]

In theory it's possible, but to reach 1000+ degree temperature with a homebrew setup is not really trivial.


I would add an iron stove plate there for convenience (and for protection).

Thanks, I have been spending the last few months steaming soil. I just kept it simple, and used the 14 liter pot with about 2 liters of water in a chamber in the bottom to make the steam. Last year, my pot was 7 liters, it was in a turkey bag and I left it the whole day on the solar cooker. This year, my pot is 14 liters (10 usable), there is no oven bag (its too big for one) and when it is sunny, I do up to 4 batches of steamed soil a day! Progress!

Now my apples and plums are nearly ripe, so I am making a big box cooker to go on top instead of the pot! I have the glass from an old fireplace and I will put it at the bottom front to allow the light and heat in, the box will have a heavy metal base and it will be aluminum sheeting on the sides and probably on top too. The oven door at the back will probably be iron sheet. The whole thing will be insulated with 3 1/2 inch fiberglass batts. (except the window).

I'm hoping to start and finish it tomorrow. I'm making the box cooker big because its first job is to dry fruit. I probably will put a hotplate inside it. for later. Oh, I won't try making quicklime this year. (I am happy with my level of concentration) but it probably is possible. A swiss guy Bruno Kägi, who I have texted with did a proper fiberglass parabolic dish on a mold so he got very good concentration. And in 2011 a guy called Markus Kayser did "The solar sinter" with a Fresnel lens where he 3D printed glass objects from desert sand with solar heat.

The thing is my set up allows the user to have a stationary workstation while the reflector moves along concentrating the sunshine. With Bruno's technique but using my dish dimensions, I think that quicklime is very much a possibility. (I am aware that that it is very difficult, and I wouldn't be able to do it, but some of the better technicians out there certainly could.

Thanks Brian

 

[Brian in Victoria BC]

Here it is in dehydrator mode. I am still working things out.

I have been going chunky to dehydrate (saves a lot of fiddling about. In this case, the beans became really tiny, but still were nice, and the plums needed more time. But things didn't last long so I think they were good!

 

[Brian in Victoria BC]

I finally got in supports for large trays today. I have enough stainless steel wire for 4 trays, plus I can use the 2 little trays I already have, so there should be a vast productivity increase! Just have to build those trays! (I think they are 2 to 3 times the area of the trays I already have!)