Solar Water Disinfection (SODIS)

Via Afrigadget, a visual explanation of how to disinfect water with just a PET plastic bottle. The diagram, developed by Unicef, pretty much speaks for itself. Too much gunk in the water? Let it settle and filter through some cloth. At least six hours of sunlight will be enough UV to kill bad buggies. Using solar water disinfection, or “SODIS”, replaces the need to boil water, thus reducing deforestation to supply fire wood.

Obviously, this is not a long term solution. Drinking water out of heated plastic bottles can’t be a good thing. But in a pinch . . .

More info here.

Build a Solar Dehydrator


Like many of you, I suspect, we’ve got a few too many tomatoes at this time of the year. One of our favorite ways to preserve our modest harvest is with our solar dehydrator. There’s nothing like the taste of sun dried tomatoes, but unless you live in a very dry desert climate like Phoenix, Arizona you can’t just set fruit out in the sun and expect it to do anything but go moldy. In most places in the world, including here in Los Angeles, the relative humidity is too high to dry things out in the sun. Solar dehydrators work by increasing air flow to dry out the food. The one we built uses a clever strategy to get air moving without the use of electric fans such as you’d find in your typical store bought electric dehydrator.

Our solar dehydrator is constructed out of plywood and consists of a heat collector containing a black metal screen housed in a box with a clear plastic top. This screen heats up on a sunny day and feeds hot air into a wooden box above it. Vents at the bottom and the top of the contraption create an upward airflow through natural convection (hot air rises). You put the food on screen covered trays in the upper box. With sliced tomatoes it takes about two full days of drying and you have to take the food indoors at night to prevent mold from growing (a minor inconvenience). We built our dehydrator several years ago and have used it each season for tomatoes, figs and for making dried zucchini chips.

You can find plans for this “Appalachian Dehydrator”, designed by Appalachian State University’s Appropriate Technology Program, in the February/March 1997 issue of Home Power Magazine. The February/March 1999 issue of Home Power features a refinement of this plan, but we just built the original design and it works fine. The original plans and improvements to those plans are split between two articles: Issue #57 and Issue #69. You need to download and read through both before building this dehydrator. . Alternatively, the always excellent Build it Solar website has a whole bunch of solar dehydrator designs, including a nice cardboard version. And while you’re in the library there’s also a book by Eben Fodor, The Solar Food Dryer.

As an added bonus to the tinkerers out there, take almost any of these designs, remove the top box, stick it in a window and you’ve got a passive solar room heater.

Appropriate Technology

Our rocket stove, pictured above, was mentioned on BoingBoing last week and we thought we’d use the occasion for a brief roundup of similar “appropriate” technology concepts. The term appropriate technology evolved out of political economist E. F. Schumacher’s book Small is Beautiful and is easier to show rather than describe. While not always low-tech, appropriate tech concepts feature elegantly simple (but not simplistic) design, efficient use of energy and are usually aimed at poor countries. What Homegrown Evolution would like to prove is that these technologies have a place in developed western countries as well. Here’s three of our favorite appropriate tech ideas and websites:

1. Rocket Stoves: our brick rocket stove and a link to a video on how to make a simple metal version.

2. The glorious Solar Cooking Archive which has links to dozens of simple solar cooker plans that you can build yourself. We built our cardboard and aluminum foil Pavarti cooker with plans from the solar cooking archive.

3. AfriGadget. The subtitle of this blog says it all, “solving everyday problems with African ingenuity.”

Our Rocket Stove

 
  • Editor’s note: we have a new design for a portable rocket stove here.

Low-tech is the new high-tech, and the best example of the low-tech revolution is the miraculous rocket stove–a stove that makes it possible to cook with small twigs–no logs needed! Best of all rocket stoves are easy to build. We liked the idea so much that we decided to build a permanent one just off our back deck for entertaining and as a backup to our gas stove should an emergency take out our utilities.

The rocket stove was developed for use in poor nations where wood used for cooking has led to the vast, wholesale, deforestation of large swaths of the earth’s surface. Rocket stoves can be built out of metal or masonry and consist of a L shaped tube, at the bottom of which you place your wood. The chimney effect creates a highly efficient, largely smoke-free burn. There’s no need to cut down a tree to cook your dinner–all you need is a few small branches or twigs.

Before we built the rocket stove we considered making a cob oven, a mud domed wood fired oven in which you can cook bread and pizza. There’s a trend in the eco-world to build cob ovens and we felt a certain pressure to keep up with the eco-Joneses. We started to build the base for one and then began to think about how often we would actually build a fire, especially considering that it has to burn for several hours before a cob oven gets hot enough to cook in. Also, where would we get the logs? And how good is it to burn such a fire and contribute to Los Angeles’ already smog choked air?

Staring at the bricks we had scavenged to build the base of cob oven, we realized that we could re-purpose them for a permanent backyard rocket stove that we would actually use. Furthermore we realized that our rocket stove could burn some of the palm fronds that regularly tumble down from the iconic palm trees that line our old L.A. street.

Here’s the materials we used:

36 bricks
4-inch galvanized steel stove pipe elbow
4-inch stove pipe
ash (scavenged from park BBQs)
1 tin can
50 pound bag of premixed concrete for the base
mortar mix
grill (scavenged)

The first step was to make a small foundation for the rocket stove. We fashioned a 18 by 18-inch by 4-inch slab with 2 x 4 lumber and a bag of premixed cement. Folks in cold places will need to make a deeper foundation to avoid frost heave.

Next we built a brick cube, leaving a small hole for the bottom of the stovepipe. For advice on how to build with brick we recommend taking a look at this. As you can see our masonry could use some more practice, but the results are not too bad–we like to think of our stove as being a bit “rustic”. You can avoid the hassle of brickwork by making a simpler rocket stove–check out these two instructional videos, one for a metal model, and another version using bricks. We chose brick largely for aesthetic reasons and we’re satisfied with the results.

Drawing from Capturing Heat

The next step is to put the pipe together fitting the elbow up into the longer pipe, and sized so that the top of the pipe is just below the bottom of the grill. Check out our earlier post for a video that can help with this part of the assembly. Serendipitously, on a bike ride, we found a grill in the middle of Sunset Boulevard that fit the opening in our brick rocket stove exactly.

You pour the ash into the completed brick cube to fill the space between the pipe and the inside wall. The ash acts as insulation to increase the efficiency of the stove. You could also use vermiculite but note that sand or soil will not work. Insulation works because of small pockets of air between particles, hence the need for ash or vermiculite, which are also non-combustible. We used a piece of scrap sheet metal with a 4-inch circular hole cut in it to keep the ash from spilling out the gap between the pipe and the squarish opening at the bottom.

Lastly you use a tin can sliced down the side and flattened out to form a shelf which you insert into the elbow at the bottom of the stove. Note the drawing above for the shape of the shelf. You put your twigs and kindling on this shelf and start the stove up with newspaper underneath the shelf. As the twigs burn you push them in over the edge to keep the fire going.

Our first test run of the stove was very successful–we boiled a pot of water and cooked some eggs in a a pan. The fire burned cleanly with little smoke except during start up. For more info on rocket stoves check out the Aprovecho Research Center.

And please people don’t burn wood inside and watch out for embers. Make sure you put the fire out completely when you are done cooking!