A little while ago, I was inspired to reduce my energy consumption in the kitchen by Ed Begley, Jr. Mr. Begley is a well-known environmentalist, the frequent butt of jokes, and an actor who played a supporting role (Stan Sitwell) in Arrested Development, one of the funniest sitcoms in TV history, in my opinion. In an interview on KCRW's Good Food program, Mr. Begley talked about two ways that he conserves energy in his kitchen. The first was a bicycle-powered toaster, a gizmo that requires peddling at a full sprint to crisp and darken two slices of bread. I like bicycling, but peddling to make a piece of toast is a little too much for me.
The second device Mr. Begley talked about was a solar oven. That sounded like a lot more fun.
Commercial ovens are available from many retailers, but I thought it would be more interesting (and a lot cheaper) to build my own. I easily found illustrated step-by-step instructions at the public library in Cooking with the Sun (by Lorraine Anderson and Rick Palkovic). Other plans can be found online here, here and here.
For me the solar oven would be a curiosity of sort. In places with poor energy infrastructure, like the rural parts of developing nations or refugee camps, however, solar cooking could lead to dramatically better lives. Much of the cooking in these places is done over wood fires, using twigs and branches, the gathering of which requires more and more time each day as nearby scrub dwindles from overharvesting. In addition, indoor cooking over a wood stove severely degrades air quality in kitchen (something described in great detail by Professor Kirk Smith of UC Berkeley). A solar oven can avoid these problems. Organizations like Solar Cookers International are working to spread knowledge about solar cooking (and solar water pasteurization) around the world.
Building a solar oven
It didn't take much material to build a solar oven: just two boxes (one smaller than the other, enough for a several-inch gap), a few sheets of cardboard, all-purpose glue (you want a glue, like Elmer's white glue, that will not release gases at high temperature), a brush for spreading the glue, aluminum foil, newspaper, a piece of glass the size of the small box (I used one from a picture frame), a coat hanger, silicone sealer, and various tools (a sharp utility knife, scissors, wire cutters). After acquiring all of the necessary parts, I think I spent five or six hours building the oven.
The next section in this post describes how I built the oven, followed by a scientific explanation of how it works.
The photo below shows the two boxes after one of the most time-consuming and potentially frustrating steps: covering the inside of the large box, and the inside and outside of the small box with foil (although aluminum foil boxes have tearing guides, I could never manage to get a good, clean tear). I used an inner box that was 12" x 18" x 9" and an outer box that was 18" x 24" x 11". The aluminum foil is attached to the outside surface of the inner box and the inside of the outer box to help isolate the cooking chamber (and its warm contents) from the outside world.
After the boxes were coated with foil, I built a platform out of cardboard stacks in the larger box for the small box (left photo below). For insulation, I put crumpled cardboard in the spaces between the cardboard stacks. Next, I glued the small box onto the platform and filled the inter-box gap with crumpled and rolled newspaper (right photo below).
To provide insulation on the top of the oven, I built a frame to hold a piece of glass (left photo). Next, I covered a large piece of cardboard with foil to be a reflector panel. Finally, I built a mechanism to hold the cover open. There are many possibilities for this — I used two pieces of corrugated cardboard and a coat hanger. One piece of cardboard is glued to the reflector, one piece to the oven's cover. (You can barely see them in the right-hand photo below; they're the two long, thin rectangles on the right side of the box.) The coat hanger, which is bent in a Z-shape, fits into the corrugations on the two cardboard rectangles. It is a decent reflector prop but somewhat unstable in the wind (more on this in part two of this series).
How It Works
The fundamental principle that allows two cardboard boxes, aluminum foil, and a piece of glass to be converted into an oven is the same one that allows life to exist on this planet: the greenhouse effect.
Here's how the effect works in a solar oven: energy in the form of sunlight comes through the glass cover and is either directly absorbed by the cooking pot or reflects off the aluminum foil. Some of the reflected energy hits the pot, some of it exits through the window. The glass cover serves two purposes. First, it separates the pot from the ambient environment, thus preventing air current from cooling the pot. Second, the optical properties of the glass keep the heat of the pot inside the cooking chamber.
When an object is at a different temperature than its surroundings, it loses or gains heat by one of three modes: conduction (heat transfer through solid contact, like your hand touching a hot surface), convection (heat transfer through fluid contact, like a cold breeze) or radiation (heat transfer by electromagnetic waves, like sunlight or the heat lamps in outside dining areas). In case of a solar oven, radiation is the main mode of interest. Without getting into all of the details, the essential idea is that the pot absorbs energy at many wavelengths, but only emits long wavelength energy (Planck's law explains this). In fact, almost all of the radiant energy emitted by the pot has a wavelength longer than 3 micrometers, wavelengths for which glass is opaque. Newport Optics has a diagram that shows the spectral transmissivity of various types of glass. Note the steep drop around 3 micrometers. Thus, the glass cover prevents the hot pot from losing heat to the surroundings. If you want to read more about this, I posted over at my Flickr account.
In part 2, I'll show how my oven performed and describe some of my early tests.