Jun 212014

I have been doing a lot of reading on cooking with solar. Having played with the dish a few years ago, I know and believe that there is massive untapped power in the sun. I know that I am not saying anything new. Anyone with a solar lamp on their sidewalk knows that we can make some form of altered energy from the sun. Sure you probably have seen roofs full of panels making electricity to power washing machines and air conditioners. But conversion of light to electricity is not by any means the most efficient use of solar. Heat is the natural conversion of choice here folks.

I have been reading about the idea, that with nothing more than a couple of cardboard boxes, some aluminum foil, and a plate of glass, one can cook whole meals. The theory is that with 2 cardboard boxes (one inside the other), with some sort of insulation placed in between, it can trap heat rather well. The insulation can be as simple as crunched up newspaper. The inside walls are painted black to absorb as much of the light as possible. The walls then acts as a heat radiator. Placing a piece of  glass over the top allows light in, the light hits the black surfaces and heats (sends infrared radiation) back out all directions. The aluminum foil and glass reflect the infrared radiation back into the box, so the effect is accumulative. In my investigation of this, I have seen stories of solar ovens reaching 360 degrees. That would make a fully capable solar oven. Clearly… This was something I had to try.

For some reason, I am prone to overkill. Using newspaper made sense, but I had some left over 2 inch thick fiberglass insulation from when I was making the sound panels for the studio. This stuff should do a wicked job in holding the heat. After measuring and cutting the fiberglass, I covered it with heavy duty aluminum foil. This was held in place with a liberal coating of 3M Spray 90. I placed the covered fiberglass pieces into the first box making a second box. I then placed a second cardboard box inside this and cut it to size. Clear as mud? Right then… The inside of the second cardboard box was then spray painted with rustoleum high heat black. I used strips of aluminum tape on the box flaps for reflectors.

In my reading someone made a really good point. You probably do not want to be eating food that has been sitting in spray paint gassed air. As the oven is heated, the paint will gas off. This is also true of the spray 90 as well as all the other ‘man made’ materials (fiberglass and cardboard). So the suggestion was to ‘run’ the oven, without food, to get as much of the bad stuff out as possible. I decided to do a test run. I used only 1 reflector for the test. I put quart of water in a Dutch oven and placed it in the solar oven at 11:40 am. Using my wireless oven thermometer, the water was 76 degrees. The water temperature went up 42 degrees (118 measured) by 12:40 pm. It then got quite cloudy, but even with the clouds it went up another 19 degrees (137 measured) by 1:40 pm. It then got much more overcast. I decided to let it go through the afternoon, but was going to pull it if it started to rain as cardboard and water don’t coexist so well. At 2:40 it was up another 10 degrees for 147 degrees. At 3:10 I pulled it inside because it started to rain. It was 155 degrees at this point for a total accumulation of 79 degrees.

For reference, water is pasteurized 160 degrees, and can start cooking food once it gets above 145 degrees and real baking starting around 180. While I did not reach this on the previous run, it was a great proof of concept. The following weekend, I decided to try to actually cook something. I wanted to try something really dense, so I chose a sweet potato. I sliced it into 4  quarters. I lightly coated it with olive oil and sprinkled it with some sea salt. This time I put a standard oven thermometer in the oven area and the wireless in with the food again. This way I could see the relation between the oven and the food temps. I used all 3 reflectors for this run. I read one thread that made the suggestion to ‘preheat’ the oven. So I put it out at 11:10am. It was 76 degrees outside.

Time  – Oven – Int – Accumulation
11:40 –  148  – 75   – N/A
12:50 – 200 –  156  – 81
2:00  – 225  –  202  – 127

This was roughly where it stayed for the rest of the afternoon. The internal reached a high of 210 at 2:50. Probably being way too cautious, I waited until 5pm to pull the food. I generally feel like a pretty intelligent person. But I will admit, I have the occasional lapse of seeing the reality of the situation. There is a reason why an oven is called an oven. Just because it is solar does not make it not believable nor powerful. Even though I saw the temperature, I assumed that it would not be that hot. Well, it was. I lifted the glass and was met with a wave of wicked hot air (clue number 1 numb nuts!).  I reached in and grabbed the lid of the Dutch oven. Mmmmyeahhh. Lets just say that the lid was put back rather quickly. This was so impressive and so much more confirmation of just how powerful solar can be. I ran in the house, grabbed my cell phone, and recorded one of those  videos I swore I would never do. You know the one… the shaky, point of view horror shows called “I just gotta show you this thing I’m doing”. But hey, I was excited. After this, I felt I was ready for an official meal with the thing.

The following weekend I decided to make a meatloaf. While I have gained a full respect of the oven, I am still not ready to try chicken yet, but I figured that meatloaf was pretty safe. I wanted to push the idea, so I made a 2+ pound load, plus 3 sweet potatoes. I also wanted to make some roasted garlic, but I did not want it interfering with the meatloaf, so made a separate cooking vessel. I painted a spaghetti sauce jar black (the outside). I lightly coated the garlic with olive oil, and put them into the jar. I did puncture the top of the jar so it would not blow up. I also added a few pieces of string so the reflective flaps would stay put. They flopped around a bit in the wind on the previous test. Once again the oven temperature took off like crazy and stayed nice and hot all day. My wife was sleeping through the day as she needed to work that night. One of the things I have been reading is that the food will not burn, and will stay quite moist regardless of how long it sat. I put the food in the oven about 12pm. While I was watching both temps, for food safety, my primary concern was the temperature of the meat, so the wireless oven probe was placed inside the meatloaf. I was shocked, at 2:40 pm the meatloaf was completely done. It was sitting at a comfortable 170+ degrees. I let it cook until my wife was up and ready for some food.

Being smarter about my food removal, I used some hot pan holders this time… heh heh. When I walked outside I could smell the roasted garlic. It was really intense. I lifted off the top glass and again was hit with a wave of hot air, but this time it was a wave of hot really really GOOD smelling air. I took everything inside and cut into the loaf. It sliced like butter. I am not sure that meat should slice like butter, but it was tender for sure. The only thing I want to sort out is how to brown the food. The flavor was amazing. I have a theory about why too. See… The Dutch oven is a small tight space, so the flavors have no where to go. Being somewhat bored, the flavors seek out the meat and attack it. Several hours later, they are so dizzy drunk with meaty goodness, they have no desire to leave. It is at this point that we want to eat it, and there is no time for escape… yeah… well. perhaps not.

 Posted by at 10:03 pm
May 152008

I wanted to see if I could get a solid stream of steam. If not steam, at least a good flow of hot water. Perhaps a insanely large tea maker. My hope was that if I could get steam, I can turn a small turbine. I looked again at the feed horn. I decided to pull off all the stuff that might melt. It was a actually a rather nice looking piece of machined aluminum. If I wasn’t so intent on making this solar thingmabob, I could see it being a very Star Wars candle holder or wine goblet. Since this piece fits perfectly into the holder (as it was sort of designed to be there), it will be used as the mount for my steam chamber. My thought is this, if I can get the water to flow slowly enough, and get a tube hot enough, once the water hits it, it will flash to steam. I bought some 1/4 in coper tubing, some brass fittiings and a 6 inch 1/2 in. diameter metal pipe. I made a coil with the brass tubing that flows in a spiral towards the center of cone on the horn. The idea being that the cool water starts on the outside and warms up as it moves towards the center. Then water is then directed up into the larger metal pipe. The pipe will is in the focal point, so it should be the hottest object on there. Man this is hard to write as nothing has a real name.

The pain here was coiling the tubing without kinking it. All in all I did pretty well, I only ended up with one kink. Once the coil was finished, I fitted it to the horn and attached the pipe. I pushed the coil down into the horn so it followed the contour of the piece. With everything in place it held together remarkably well. I then spray painted the thing with black grill paint. I don’t know if it will get grill hot or not, but if it was how enough to boil water, I figured I should use something a little more robust than standard spray paint.

I took everything over to the dish and screwed it on and got it attached to the hose. I had an interesting problem. I had a hard time getting a gentle steady stream. Even at a low flow it was almost too fast. I figured that I could use 2 taps for the flow adjustment. I wanted to use one for the coarse adjustment, then one for the fine. I really had not thought this through I guess, but once the water built up between the first and second tap, it had the same force through the contraption. There was too much pressure from the house I suppose. They say electronics is like water flowing through a pipe. I suppose I could have just built a water divider and send  the rest to um… the ground (via a sprinkler).

Once I got the stream slow enough I aimed the dish towards the sun. My method for testing the tempature was highly scientific. I did not have a thermometer for the metal so I would occasionally touch it (Bio temp sensor). Once it got hotter than I wanted to touch I knew it was on the right track. The water that was coming out was getting warmer and warmer. It too eventually got hotter than I wanted to touch. I let it go for a while.  I turned off the water and after a couple of minutes it started to sputter and dribble, then I got steam. I turned the water back on. While there was steam coming off of the water, I never got just a jet of steam. I think that this is mainly due to the pressure of the water. I will do more testing at some point in the future. This is a fun experimenters toy.

In the future I will sand out all the texture that was built into the dish. I think that the combination of the texture and the somewhat sloppy job of  putting the aluminum on is softening the focus of the dish. Also, the dish was designed to allow a wide vertical capture area. This means that the focus point  is more of an oval than a round point.

 Posted by at 5:26 pm
May 112008

With the success of the first tests on the collector, I decided to do more. Really, it is an excuse to go outside and melt stuff. I expanded the holder so I could hold a can. My goal… Boil a can of water. Like the bottle, I spray painted the can black. I grabbed a thermometer out of the kitchen. I got it all hooked up and pulled the dish out into the sun. The thermometer was reading about 82° (ambient) when I put it in the can. The temp dropped to about 73° due to the water. I tried to check on it fairly often. I wanted to check every 5 minutes, but I get distracted fairly easily, and lets just say it didn’t happen. Fortunately digital cameras embed the time of the shots.

I put the can out at about 3:10pm. The temperature came up slower that I had hoped for. But it was a little cloudy, so I was not getting constant heat. At around 3:52 it hit a light boil (205°). So in about 40 minutes the temperature went up 132°. Not bad. I am concerned that the texture that is on the dish may be dispersing some of the light as the foil conformed to it. Also being that the foil is not completely flat from when I glued it down. This is also probably causing a little bit of loss as well. Lastly, the paint I used was a satin, so it had a little bit of a sheen. This probably caused a little bit of loss due to reflection. But again this is an experiment, and why do we experiment?

82° at 3:09pm 78° at 3:15pm 145° at 3:27pm
190° at 3:35pm 201° at 3:40pm 205° at 3:52pm
 Posted by at 10:50 pm
May 042008

One night I stumbled on the subject of solar collectors. The guy was using several mirrors to bounce sun light to a single spot. The result was obviously the combined power if all the mirrors. He was aiming it at a solar panel and was able to increase the output fairly significantly. I started looking around for anyone who had done similar experiments. I found so many cool tributary ideas. The range included “death rays”, cookers, ovens, air heaters, water heaters, stirling power.

I have always been interested in solar, but my scope was limited to electrical panels. I have been in the process of bulding my own solar panel, but it has been slow going, and met with a few setbacks. I suppose in hind sight that it makes sense, but there is so many uses for solar energy. The more I learn, the more I am shocked that solar is not playing more of an important role in this energy crisis we find ourselves in. One of the things I was really intrigued by were solar collectors. Using a parabolic reflective surface to focus the light to a single point for maximum power. I like the dish idea because it is a fairly controlled experiment surface. Using a set of mirrors presents a danger of shooting somewhere untended… catching something on fire.

I work at a TV station and there is a small yard full of “obsolete” satellite dishes. I talked to our engineer and he said I could take one. I took one of the smaller ones, a 1.8 meter dish. Once I got it home I cleaned it as it was covered in years of crud. I took the feedhorn off as it was not necessary. I got a roll of heavy duty aluminum foil and some spray adhesive. I covered the inside of the dish with the foil shiny side out. It became immediately obvious that the reflection idea was working. In just garage light, the focus spot was very bright. I used a piece of PVC pipe in the feedhorn holder hole to hold my test subjects I covered the holder with foil to keep it from melting.

I started with a piece of paper as I figured that it would be an easy thing to get lit. I wadded it up and stuck it in the end of the PCV tube. I pulled the dish outside and aimed it at the sun. I kept a wrench in my pocket at I was constantly having to adjust the angle of the dish. I got it aimed and angled and was very excited. I waited…and waited… but nothing was happening. I was scratching my head. This thing should have been lit already. I put my hand near the holder and could feel heat. Then the reality smacked me. Duh! White paper is highly reflective. I replaced the paper with a piece of cardboard. As soon as I got it set it started smoking. It smoked more and more and finally ignited. Muahhh ahhh ahhh ahhhhhh!!! I have the power… Yes,  you bet I was excited.

My neighbor came over to see what I was doing. He was concerned that it would hit his house and catch it on fire. I assured him that, no, it was focused to a point that was sort of restricted by the shape of the dish. I am not sure that he believed me.

My next experiment… water. I grabbed a coat hanger and fashioned a holder. I got a water bottle from the recycle bin. Having learned from the paper, I spray painted the bottom of the bottle black. I filled the bottle about a third of the way up with water I shoved the hanger bottle holder into the PVC tube and watched. Rather quickly I started to see condensation inside the bottle. I figured the bottle would be a safe thing based on the concept where one can boil water in a leaf. It did start to melt above the water line so I covered the top with foil. Within minutes the water started to bubble, and soon after it was at a fairly strong boil.

 Posted by at 5:45 pm