Saturday, 28 May 2016

08:59 – Barbara is due back sometime this afternoon. I’ve told Colin, but he doesn’t seem to understand what I’m telling him. Or perhaps he’s like Duncan. When Barbara was gone overnight or longer, Duncan used to shun her when she returned.

I’ve been thinking about cooking and baking in a grid-down situation. Obviously, if that happens during cold weather, we’d be running our wood stove. The flat top of that is more than hot enough to use for frying, boiling water, etc. and our Coleman Camp Oven would allow us to bake on it as well. Granted, that oven has only a 10-inch (25.4 cm) square rack, so it’d be limited to baking one standard loaf of bread or perhaps two smaller ones at a time, but it would be useful. As long as the wood stove was burning, we could turn out one oven load every hour or so, 24 hours a day if necessary. For that matter, given fuel, we could cook/bake on our propane grill, Coleman propane stove, or Coleman dual-fuel stove. Or we could build a Rocket stove from concrete blocks.

Because the ability to cook and bake is so important, I want to have backups to our backups to our backups. So I’ve been researching solar ovens. I’ve read several books and webpages devoted to them, and I conclude that it’d be more accurate to call them solar crockpots than solar ovens. Short of an finicky parabolic or Fresnel-based focusing oven, which requires constant adjustment to keep the beam focused, standard solar box ovens top out at around 350F (~177C), and that’s only with $250+ commercial models under ideal conditions. Realistically, figure 250F to 275F, if you’re lucky. Still, that’s hot enough to boil water, cook meat, make casseroles, and even to bake (and brown) bread. It’s just that everything takes a lot longer to cook or bake. Instead of popping dinner in the oven an hour before you intend to eat, you pop it into the solar slow-cooker in the morning to have it finished by dinner time. Not that overcooking is an issue at all. At the temperatures reached in a solar box oven, something may be cooked after four or five hours, but it doesn’t hurt to leave it cooking for several more hours.

I’m using this page as a starting point, and more particularly, this model. I’d make a few changes to the design. I don’t want a cardboard solar cooker. A sudden cloudburst could destroy the oven. Instead, I’d make it with 1X12 boards, glued and screwed, for the sides, with masonite or thin plywood for the bottom and reflector(s). I may never have time to actually build one, short of a disaster, but I do want to have everything I need on hand to build several of these if we ever need them. I’d prefer a wooden structure, but I’d use cardboard boxes if necessary. Doing it that way, one can make a usable solar oven with two or three dollars’ worth of materials. A lot of the stuff you’d need can be found around most homes–cardboard boxes, glue, aluminum foil, and so on–but there are a couple items I’d want to keep on hand specifically for these ovens.

First, although I could make field-expedient black paint (or even just rub the inner surfaces of the oven with charcoal), I’d want to have a few cans of flat black high-temperature spray paint. In addition to blackening the inside surface of the oven, this stuff can be used for blackening the exterior surfaces of pots and other vessels so they’ll absorb the heat instead of reflecting it. Cast iron cookware is preferred for use in solar cookers, but ordinary aluminum, stainless steel, or chromed pots work just as well if you blacken the exterior surfaces. In fact, I’d probably spray paint the cast iron stuff as well. I’d run a new solar cooker and freshly-painted empty vessels for a full day in the sun just to drive off any residual chemicals from the paint. Incidentally, this kind of spray paint is also useful for blackening the outside of soft drink bottles to prevent algae growth if you’re using them for container gardening.

The best material for the glazing is double glass panels with an insulating air gap, but ordinary window glass works almost as well, as does heat-resistant plastic. I could probably scrounge enough window glass to make several solar cookers, but turkey-size oven bags are inexpensive, heat-resistant to 400F, a good size for a solar cooker, and reasonably durable. In a pinch, you could probably substitute the clear 4-mil plastic sheeting sold as disposable drop clothes, but it probably wouldn’t be as durable.