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How green can you go? A poop-powered kitchen

Any tinkering addicts, basement inventors out there looking for a good project? I’ve had a vision of the eco-kitchen future, and I want you to help make it into a demonstrable reality. I don’t have the tinkering skills, or time away from the toddling kids, to work on it myself. The vision is this: cooking and refrigeration powered by methane produced from household waste (including bodily waste). Can such a utopia be possible. You betcha! It works (in the fevered theory of my mind) like this: When anaerobic bacteria feed on organic wastes (like poop, food scraps, paper, etc.) they produce methane as one of their waste products. This is the basis for “landfill gas,” in which the methane emerging from all the trash buried in landfills is captured and put to good use—or at least burned off. (If you can’t do anything useful with it, it’s a good idea to at least burn off methane that would otherwise go into the atmosphere. Methane is a super-strong greenhouse gas, moreso than the carbon dioxide that results from burning the methane. So burning it and converting it to carbon dioxide reduces the impact on global warming. It is still adding to global warming when you do this, just not as much as if you let the methane escape unburned.) A biodigester is a contraption that takes advantage of these anaerobic bacteria to process wastes and capture the useable methane. The physical solids that remain can also be a good fertilizer in gardening and farming. Biodigesters can be either large or small in scale. Large scale versions are now cropping up on dairy farms to power electric generators. Here in Vermont, one of the utilities is pursuing this under the name “Cow Power.” It’s being done in Michigan as well, and probably elsewhere. Meanwhile, in another part of the eco-universe, is the absorption refrigerator, long a sturdy workhorse for those living off-grid and who power their refrigerators with propane. Happily, these refrigerators—which are marvelously efficient—can also run on methane. Treehugger has an article describing a few that are “dual fuel” and can run on either methane or electricity. That’s a handy feature to keep in mind. Okay, so enough background. Here’s my proposal: install a biodigester in your home, perhaps like this one designed by Robert Crosby. Route the resulting methane to your absorption refrigerator. Voila! A fossil-fuel free refrigerator! (And, as mentioned before, high quality fertilizer for your garden.) If you have excess methane coming out of the biodigester, you could use it to preheat domestic hot water. It’s probably not a good idea to try to store any quantity of the methane as that would be a serious fire hazard, so don’t make plans to cook with it. That’s why the refrigerator is, in theory, a good use for the methane; a biodigester emits methane in a more-or-less slow-and-steady manner, and the absorption refrigerator uses its fuel in a slow-and-steady manner. They were made for each other! But what if you don’t produce enough methane? Or if you go away on vacation and don’t add wastes to the biodigester for a week? That’s where the dual-fuel methane/electric refrigerators come in. Problem solved. Does Crosby’s biodigester produce enough methane to power a home’s refrigerator? I emailed Bob and asked him that very question. Doing some quick calculations, he determined that they should indeed be pretty well matched up.
A quick google search looks like a typical propane refrigerator might go through about 1/4 gal of propane per day, or 91,000/3 about ~23,000 Btu/day (or ~30,000 Btu/Day in a warmer climate). At a heat value of, say, 600 Btu/Cu Ft biogas, that would be equivalent to ~ 40 Cu Ft/day. Plugging some “what-if” values into my biogas calculator <http://biorealis.com/wwwroot/digester_revised.html> to see what volume and type of waste would be needed to provide that amount, it looks like it could be done if fed the waste from 4 people plus maybe 5 lbs/day of leaves, veggie scraps, grass clippings etc.
Let’s summarize the benefits: 1) you are recycling your bodily and other wastes, 2) you get climate-friendly refrigeration, 3) you get high quality, home made fertilizer for your garden, and 4) you prevent methane that might otherwise be produced in a landfill or at a wastewater facility from escaping into the atmosphere, which would exacerbate climate change. So what do you say? Any eco-conscious high-schoolers out there with really liberal parents who want to do a science project sure to win first place at the regional science fair? If you do try this out, definitely let us know so we can give you credit on the blog. And, more importantly,

be safe!

Don’t start producing methane in your house until you’ve figured out how to do it without hurting yourself or starting any fires. Seriously—for all my glib tone, I do not want to find out that I inspired some overeager yahoo to play with fire. [Photo courtesy of NatalieMaynor]


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