Solar dehydrator

A recent discussion on the Yahoo SolarCooking forum brought up the topic of solar dehydrators. A few clicks later brought me to this section of builditsolar and this explanatory video.

I have a suitably large piece of glass that’s been sitting idle for years and plenty of scrap plywood, so I thought I’d research a bit more to see what else was needed. That lead me to the trays and various liners. Here is what the ones from an Excalibur brand dehydrator looks like.

Both materials need to be food grade quality. (Yes, I know folks have used wood and window screens for years, but this is just the road I choose to take. YMMV) The Excalibur trays are made of hard polycarbonate and the polyscreen of “FDA approved food-grade materials.” You can buy them directly from Excalibur for $10 and $3.75 respectively. If I
ordered 5 of each, it comes out to ~$69 +tax & shipping. That’s getting close to the price of one of their basic 4 tray dehydrators.

So I searched for alternatives. An hour later and the best I could find were these cooling racks & silicone baking sheets.

For $40 I would have 6 trays and 4 sheets. Many things I plan to dehydrate would fit fine on the racks themselves without any danger of falling through the gaps. But things like fruit leather (fruit roll-ups), corn, peas, etc. need something like the silicon sheets.

Somewhere around this time my young niece and nephew saw me browsing through solar dehydrator designs on my iPad. Of course they wanted to know what they were. I did my best to explain and they (mostly my niece) listened with mild interest. But when I mentioned that we could make fruit leather…well, the interest level rocketed and plans to mix all sorts of fruit flavors started flying. Needless to say this project moved up a few levels.

I knew my lack of woodworking skills was going to mean a long development (thinking through) time, so I went back to shopping the 4 tray Excalibur. There are less expensive dehydrator brands out there, but my gut reaction was they wouldn’t work as well. JMO. Another hour going through eBay, Amazon, etc. resulted in finding a company in AZ that would ship it to CA for $106 total. I also ordered four of the $5 silicon sheets needed to make fruit leather. I’m eagerly awaiting the start of a new cooking adventure. It won’t be totally solar-less because I plan to use my parabolic cooker to gently cook some of the fruit to be used in the fruit leather.

If you’re like me and don’t know much about dehydrating food, these videos might be of interest to you.

Solar Box Oven: an easy DIY project

The amount of documentation available on the ‘net these days about solar ovens & cookers is absolutely staggering. People have been messing around with solar cooker design literally for centuries – yes, I mean hundreds of years. So go ahead and search the ‘net and browse away. Just set aside a specific time for research before going hands-on. If you don’t, you could spend months reading all the pros and cons of various designs and not get around to actually making something. Here are two sites to get you started:

After the first hour of reading, stand up and walk around your basement, attic, storage shed, anywhere you stash your stuff and look around for items that might be useful in a solar cooker. By now you’ll know what many of these things are, but don’t worry about it. Even stuff you don’t think will be useful may turn out to be needed for some crazy design you decide you want to try. Keep an open mind.

Since I had decided on a box oven design that would mimic the commercially manufactured Global Sun Oven (GSO, see photo on left side of my header), the first thing that I looked for was a sturdy box that fit the dimensions spec’d on some website:

“The GSO inside is cut and either machine stamped or machine formed out of sheet aluminum. It is anodized black inside. 17″ X 17″ square, with the top sloping with the front (low end) 9″ high and the rear (high end) 12″ high.”

I got lucky and quickly found an old laser printer box that measured 18″ x 18″ square with 13″ high sides. I also had plenty of large sheets of plain cardboard that could provide multiple layers for insulation. I’d seen people stuffing things like newsprint or perlite between cardboard “walls”, but I decided to keep it simple and add two layers of cardboard on the inside of the thick-walled box.

But I’m skipping ahead a bit. My first decision was to make the angle of the glazing (the window) steeper than the GSO’s. Instead of a 9″ front wall, I made mine ~6″. I left the back wall the full 13″ height of the box. This resulted in a glazing angle of ~23º when the box was flat on its bottom.

In cutting down 3 sides of the box to get that shape, I set the box flaps aside to use as support for the floppy (note bamboo pieces used as support) aluminum-lined Reflectix that I decided to use for reflectors. It’s what I had lying around. Coroplast (plastic cardboard) lined with reflective mylar would definitely be better, but I didn’t have those and I didn’t want to wait to get them.

The Reflectix I had was 24″ wide, so I cut four 18″ sections and glued 3 of them to the 3 box flaps. I velcroed the 4th section to the box flap that was still attached to the box. One by one I clipped the ends of the Reflectix together until I got a rough approximation of the angles of the GSO reflectors. I can adjust it as needed.

I lined the inside walls with plain old aluminum foil – stuff I had in the kitchen. I chose not to use glue since it might stink up the box, so I just folded it around the edges and wedged it in as best I could. It’s not perfectly flat, so watered down Elmer’s glue might’ve been a better option. I also poked holes through the foil and all the cardboard layers, then threaded an unfolded paperclip through everything to “sew” the loose cardboard sheets to the main box. I used 2 paperclips per side. I have some binding rivets somewhere that would do a neater job, but I couldn’t find them.

For the glazing I had a couple of choices. I had some fairly thick (3/16-1/4″?) glass that a glass shop a few blocks away could have cut to size for $5-10, but I decided not to risk the common glass cracking issues that many people had reported. Instead I went with 1/8″ polycarbonate, a Lexan substitute. Fortunately there’s a Tap Plastics nearby and it only cost ~$18 with tax.

The day after I finished making the box oven I put an empty black graniteware pot in the oven and set up the reflectors at 9 am. It was a clear day with highs in the low 80s. Within
30 min. the oven temp was 275ºF and stayed there until I opened the oven to add the ingredients for beef stew into the pre-heated pot at 10:40 am.

I rotated the oven to follow the sun about once an hour. The oven temps stayed in the 225-265º range. The wind occasionally picked up and knocked my rickety reflectors askew, so I had to adjust those a few times.

I tasted the stew after 5 hrs. The meat, mushrooms and carrots were nicely done, but the potatoes and onions needed just a bit more time. I had decided to use medium sized potatoes & onions whole instead of quartered or sliced. Another hour in the oven helped, but I think I’ll slice those ingredients next time.

I have a few refinements in mind to try and get higher oven temps, but for the next few months this will function very well as a slow cooker.

Pressure cookers: small ones to fit in a solar cooker

There are many sites about pressure cookers that explain the basics of how they work and why they conserve energy. For solar cooker users, the latter reason isn’t really critical, but it does help to have a pressure cooker that is dark colored to better absorb the sun’s energy. Most solar cookers also have limited space, so I looked for pressure cookers in the 3-4 qt. range.

There were not many that fit my requirements and I chose the Hawkins Futura line (3 litre version) primarily because Hawkins has many years of experience in building pressure cookers. I emailed them to confirm that the anodized aluminum Futura cooked at 15 psi and quickly received an affirmative reply. I had been advised that some pressure cookers (especially aluminum ones) only reached 10-12 psi. Spec sheets across the board rarely listed the psi rating.

A similarly priced option was the 2.8 L Halulite made by GSI Outdoors, the company famous for its lightweight camping gear.

This is probably also a safe option, but it might be a bit harder to find a replacement gasket in the future. Since GSI usually focuses on light weight gear, it’s no surprise that their pressure cooker at 2.75 lbs. weighs less than half of the similarly sized Futura at 6.2 lbs. I did not verify the psi spec, so you’ll want to do that if you’re interested in this one.

The additional thermal mass of the Futura would actually be a benefit with a parabolic solar cooker because it would more evenly distribute the intense energy usually generated. In fact, a standard 4 qt. stainless steel pressure cooker would probably work just fine on a 4-5 ft. parabolic cooker. However, if you intend to use the pressure cooker in a Parvati solar oven (a parabolic funnel), the thinner walled Halulite would probably be the better choice.

Hawkins has a unique design that requires a modified version of closing & opening the lid. You should read these instructions carefully. Browse their excellent website for more info on pressure cookers.

The Long Road to a Parabolic Cooker

Yesterday I placed an order for the 1.5 m diameter (1.77 m² collection area) Solar Burner parabolic cooker from Cantina West. Although it’s only been a couple of weeks since I jumped into the solar cooking pond with a DIY box cooker, I’ve spent countless hours reading the archives of the Yahoo SolarCooking forum and other online sources. I decided to share some thoughts about the process of selecting a parabolic cooker.

The parabolic cooker (paracoo for short) is the one style of solar cooker that best simulates a traditional stove. If the collection area is large enough and there is sufficient sunlight, the level of heat concentrated at the focal point is just as capable of burning your hand as a gas or electric stove. For that reason, similar safety procedures need to be used when cooking with a paracoo.

I started my journey down the paracoo rabbit hole the same way I think many people will: let’s see if I can build it myself. So I searched satellite dish conversions and mylar reflective material. People were giving away unused satellite dishes, but they were usually in the 18-22″ range. These will work, but it’d take nearly an hour to boil a quart of water. Larger sizes were not available to me, but even if they were, I discovered the 4-5 ft. dishes were very heavy.

Then I stumbled on a discussion about the Solar Sizzler, a 3 ft. plastic parabolic dish covered with mylar.
I discovered the inventor and seller had recently died and the product was no longer available. I also discovered around this time that the mounting frame and aiming mechanism for these parabolic dishes were just as important as the dish itself. In the above photo, the dish is propped up on the ground and the cooking platform is hanging from a campfire tripod. That doesn’t make for very convenient or accurate aiming, which in turn makes the cooker less effective.

Unlike box, funnel, Parvati, etc. solar ovens, a paracoo requires more frequent re-orientation to the sun as it changes position in the sky. Not only that, you need to pay attention to both the vertical and horizontal changes in the angle of the sunlight. Now you should be doing that with the other solar ovens also, but the necessity for frequent daily vertical adjustments are just not as critical for the other ovens as it is for the paracoo.

Basically, even if I could find a used Sizzler or a similar standalone shiny dish, I’d still need to build the supporting frame for the dish, a frame for the cooking platform (to hold a pot or pan) and the aiming mechanisms. These cost money for raw materials (wood or tripods) and some construction skills. The result would be a finicky cooker with decent cooking power. Maybe you could boil a quart of water in half an hour on a clear, windless day if you moved the Sizzler frequently enough.

By this time I had belatedly given some thought to the type of cooking I was hoping to achieve with a paracoo. My box oven was great for the slow cook style, but it couldn’t do (easily) any of the stovetop stuff like fry, boil and pressure cook. So now my parcoo requirements had progressed to a 3-5 ft. diameter dish built on a frame that allowed simple adjustments and which had a stable cooking platform to hold my pots & pans.

Here’s where I started to go nuts chasing some of the high-end products. With the “help” of the good folks on the Yahoo forum (Mark, James, Steve, Luther et al…sorry if I missed anyone), I learned about the fabled SK-14 with its sleek, efficient, German-engineered polished aluminum reflectors. I also learned that these came with price tags in the hundreds. Although the shiny ones caught my eye first, I’m going to tell the rest of the story in reverse…starting with the least shiny of the lot.

The Solar Burner (SB) at $170 (all prices in USD, include shipping to U.S.) is the least expensive of the commercially available paracoos currently offered. It is made of painted steel and the reflective surface is mylar. It is solidly built with so-so fit & finish. At 1.5 m it is also the largest of the paracoos I considered, so even though the mylar is not as reflective as polished aluminum, it packs considerable power. The horizontal adjustment is easy, but the vertical adjustment requires hand loosening a water valve-like bolt and tilting the dish before re-tightening.

For twice the price at $350, there’s the Solar Parabolic Boiler (SPB), previously known as the Tinytech.

Made in India, it comes with a 45″ x 45″ square parabola (1.30 m²) of polished aluminum sheets made in Germany. That’s a significant step up in quality for the reflectors, so the smaller collection area compared to the SB doesn’t really result in much reduction of real-life power output. The smaller parabola means a shorter reach to the cooking platform. That would be an important consideration for folks with shorter arms than mine. The other critical difference is that the parabola rotates up & down, which makes the vertical adjustment very simple. But in order to adjust horizontally, it seems like (maybe SPB owners can confirm) you have to lift one side of the cooker and pivot to turn. The framing is also painted steel, probably of equal fit and finish to the SB.

Next step up in price is the $400 Pika Hunt.
Made in Japan, it not only matches the polished aluminum sheets from Germany, it also sports a tripod stand and frame made of aluminum. The dish rotates freely around both vertical and horizontal axes, so making aiming adjustments is as easy as can be. At 80 cm in diameter (0.50 m²), it qualifies as a portable paracoo and is sold with a convenient carrying bag. Unfortunately, the price for that portability is anemic power. The Google Translate version of their website states “When sunny, 38 minutes makes a lot of excitement in 1L of water.” If the price was a bit lower, I would have asked an uncle who speaks Japanese to call and confirm that it was powerful enough to get a pressure cooker up to 15 psi in a reasonable timeframe.

Finally we get to the legendary SK series from Germany.
Now re-named Premium 14 (1.4 m, 1.54 m²) and Premium 11 (1.1 m, 0.95 m²), these polished aluminum reflectors are mounted on galvanized steel frames. Like the Pika, these parabolas rotate freely around both axes. With the high quality of reflector, even the smaller Premium 11 is well capable of providing convenient cooking power. The kicker is the premium prices of $460 and $550. But considering that these are likely to last decades (if properly maintained), those are really not unreasonable prices.
UPDATE: A company in Barcelona apparently still manufactures the older SK-10 design. You’d want to verify that the website is accurate, but 170 € for the unit + shipping could be ~$350. Aside from the lower price, I prefer the seemingly more stable stand. Although it is not as convenient as the freely rotating stand of the current model, the ^-shaped legs make for a very simple aiming device; just line up the shadows of the legs. Select K10 in the left column:

If you’ve read this far, you can understand that it was a long and winding route before I finally decided to go with the $170 Solar Burner. Portability would have been fantastic, but for at least the next year or so I was unlikely to use the paracoo away from home. If money was no object, I would probably order a matching set of SKs. As with all buying decisions, personal budgetary constraints and intended use will influence the final choice. Hopefully some of the information shared here will help you in your solar cooking journey.

Parabolic cooker designs

UPDATE 2: The German maker of the SK series is now selling on eBay as “cooksolar”. The Premium 11 costs $448 shipped to the U.S. as of Aug. 2011.
eBay listing for Premium 11

UPDATE 3: The 1.1 meter diameter “Premium Solar Cooker” is $378 as of Sep. 2012. The “Solar Burner” is now $150. If I were making a purchase decision today (9/25/12), I would spend the extra money for the German made unit. The Solar Burner still functions well enough, but rust and mylar degradation after a year does not bode well for longevity. The cooking surface is also not very stable when rotating the parabolic dish, so I’ve lost a couple of meals when heavy pots took a tumble. My guess is the more expensive unit is more stable and easier to use on a daily basis.