Square Foot Solar Cooker DIY

You can get two portable panel cookers from one 18-by-24-inch piece of corrugated plastic. Using a tape measure and marker, divide the plastic into six 8-by-9-inch sections. Then, cut the plastic into two even sections of three panels each. (Follow the cutting guide above.) I use a single-edge razor blade, but you can use sturdy scissors, a table saw, a hacksaw blade, or a utility knife.

Then, carefully cut only halfway through the plastic along each base line, so the sides can be folded up.

Corrugated plastic, like cardboard, is made with rows of internal channels to give it strength. Cutting across the channels requires more pressure than cutting along the channels. Many of the cuts for this project are made only halfway through the material, just enough to allow it to bend. It’s easy to make these half-cuts across channels using moderate pressure; the ribs in the plastic will keep your blade from slicing all the way through.

Making half-cuts along the channels is more difficult. If you use too much pressure, you’ll cut all the way through. Also, the tip of your blade can catch on the edge of the channel and throw off the blade line. To prevent this, place the plastic on a lighted table or illuminate it from the back, which will allow you to see exactly where the channels are. Alternatively, push a small metal rod or wooden dowel inside a channel, which will act as a guide for the blade and allow for a quick cut.

Once you’ve separated the two panels and made half-cuts along the base lines, mark and half-cut another line (“final cut”) about 1/8 inch inside both edges of the base lines. (It’s easiest to push a dowel through the adjacent channel for this cut.) This final cut will allow the cooker to fold up neatly for transport.

To secure the sides of the panel while not folded, I use brass upholstery tacks, but small nails or screws work too. (For wood panels, use glue and nails or screws to attach the sides.) Mark the holes for pins on your panel before you add the Mylar in the next step.

Mylar works well to provide the reflective surface needed for a solar cooker. You can also use mirrors, which work well for a larger stationary unit.

Once your panel template is cut, use it to create a template for the Mylar. Place your Mylar shiny-side down, and then place your template firmly over the Mylar. Mark around the template, and then cut out the Mylar. Attach the Mylar to the plastic using glue, contact cement (use a roller to ensure wrinkle-free application), tape, or staples. Because  Mylar stains easily, it’s helpful to use a mirror for the base of the cooker instead. You should always back glass mirrors with duct tape or clear packing tape to prevent them from shattering if broken. (Mirrors break over time. Some consider it a sign of good use.)

Once complete, the cookers can be folded up to measure 8-by-9-by-1/2-inch. They’re perfect for a large purse or small backpack.

After building the reflector, the next project is assembling a cooking vessel. I like to use glass canning jars. A wide-mouth pint and half-pint canning jar are the perfect combo for a mini-cooker. It won’t provide a huge volume of food, but it will make 8 ounces of rice in under 80 minutes, completely unattended. With six hours of good sunlight, that’s easily a full 32 ounces of food.

First, paint the outside of the half-pint jar with flat black paint and let it cure for a day or so in the solar cooker. Then, place the half-pint ring right-side up inside the pint lid. Next, place the half-pint lid on top of the half-pint ring, so they make a platform to set the half-pint jar on. Place the half-pint jar (with food contents) on top of the half-pint lid and ring, and then put the pint jar upside-down over the top so it can screw into the pint lid and ring. Once assembled, it’s a shock-resistant, mess-free system. Other-sized nesting jars work also, so experiment.

Jars and food contents will get hot, so handle them carefully.

Also, always wear eye protection when working around bright, reflective lighting.

Pasteurizing Water in Solar Cookers

These solar cookers can also work to pasteurize water. Most microorganisms won’t survive in water that’s been heated to 160 degrees Fahrenheit for at least 45 seconds, or 145 degrees for 30 minutes. This won’t sterilize the water, which requires a much higher temperature, but it’s adequate to kill most germs and can be used in emergency situations when other methods of water decontamination aren’t available. (For more information, visit the Centers for Disease Control and Prevention and read “Making Water Safe in an Emergency”, as well as the World Health Organization website for research on boiling water.)

A recycled brown quart glass bottle is perfect for pasteurizing water. My latest test showed a quart of water heated in the solar cooker went from 80 degrees to 162 degrees in just under 1 hour, 45 minutes. Of course, this was with ideal sunny conditions. After the bottle reaches temperature, screw on the top and remove it from the cooker; the bottle will form a vacuum. It’s possible to do 4 or 5 quarts on a sunny day. This method won’t remove chemical contamination from water.

Solar Still DIY

Solar cookers also work well as a solar still DIY setup for distilling aromatics and hydrosols.

You can make a simple distiller using the “BIJUJ” (“bottle in a jar under a jar”) method. Fill a 12-ounce brown or green bottle (the source) with water and aromatic herbs, and place it inside a wide-mouth quart canning jar (the collector). Then, cover the top with an upside-down small-mouth pint or quart jar (the condenser), and set the whole thing in the solar cooker. As the liquid heats up, it will create vapors that condense, eventually forming droplets that will fall into the collector jar. To increase the yield, use a fan to blow over the upper jar, or cover the jar with some kind of re-reflective material to help cool it down.

You can smear a small amount of valve-grinding compound where the two jars meet, and with a bit of work and turning, it will grind down imperfections in the glass for a better seal. This process makes a lot of loud, squeaky noise, and you’ll need to wash the jars thoroughly afterward.

It’s also possible to make essential oils and other distillates using this technique. (Though it’s illegal to distill spirits at home without a federal license.) I’ve made some excellent hydrosols with sage, rosemary, jasmine, and other herbs.

Instead of a bottle, you can use a wide-mouth 8-ounce jar as the source, and if you put the panel on a slight tilt, the droplets will fall to the side rather than back into the jar. This makes an excellent dehydrator.

Solar Cooker DIY Wax Melter

You can use a solar cooker DIY setup for melting wax for candles and covering homemade jams with paraffin. A 12-ounce aluminum can works well for melting wax. For a faster melt in cold weather, cover the can with a clear outer jar. After it’s melted, you can pour the wax into candle molds or drop a wick or two directly in the can and let the wax cool. If the wick is too thin, the candle will tunnel and not burn completely. If it forms a wide pool of wax, the wick is just right.

One great use for melted wax is to make emergency candles. They work well for lighting, heating, and cooking. Cut a tin can down to 1 inch. Roll a length of 1-inch-wide cardboard into a spiral, like a cinnamon roll. Put it in the tin can, and then fill the can with wax. (Leave the center of the cardboard roll a bit taller to act as a wick starter.) This will produce a small candle of about 3 ounces. My recent test showed a burn time of over one hour for each candle. One candle brought a quart teapot of water to a boil, with probably enough burn time to do a second quart.

These candles do produce a smoky burn and will smoke a lot when blown out, so don’t use them to heat a nice pan, and only use them in a well-ventilated area.

Don’t use these candles with a plastic-based Mylar cooker, because it will get too hot.

Resources for Solar Cooker Designs

Some great panel cooker designs have been done by Jim La Joie, Roger Haines, Sharon Clausson, Matteo Muccioli, and others.

For a more in-depth look at solar cookers, check out the YouTube channel “Big Blue Sun Museum of Solar Cooking” by Luther Krueger. It contains more than 60 interviews with designers.

A comprehensive source of information can be found through the outstanding Solar Cookers International Wiki. This herculean project, up-dated by Paul Hedrick and Tom Sponheim, is the go-to for solar cooking.

Craig Bergland is the operator of the Facebook page Secret Solar Institute of Northern Nevada. All materials there are available for free dissemination. Credit is nice, but not necessary. This project and others like it were done with help, tools, and encouragement from his local makerspace, Bridgewire, in Reno, Nevada.