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Complete Biogas: Temperature and Biogas

If you want to make biogas, and you like things really, really simple—as simple as possible—Then you should move to the tropics.

Why? It's because the rate of biogas production—all else being equal—depends on the temperature of digestion. Within a certain range, the warmer the digester is, the better. By contrast: Sweater weather? No biogas from an unheated digester. So colder is not nearly as good. Bad, bad, bad, in fact, at least as far as biogas is concerned.

Now why should biogas production be so dependent on temperature? Well, the reason is that heat is just a kind of jittering motion of molecules. And the warmer they are, the faster and further they jitter. You can even see evidence of this if you have enough small particles (pollen, for example?) in a jar of water, and you look really closely. You’ll see them move in a sort of random dance, a jitter… bug? (It’s called Brownian motion. Back in 1905, Einstein proved that atoms existed by drawing certain conclusions based on that motion. You could look it up.)

So when these molecules are banging around, the faster and further they bang, the more likely they are to encounter other molecules, and to break apart and recombine to form new molecules, meanwhile (generally speaking) releasing just a bit of heat. Love at first sight happens more often in the tropics, no? (After all, some folks maintain it’s just chemistry.) Love at first sight, and biogas. In the tropics. Add in Tahiti and buy me a ticket, please.

But hey, I assume you’re like me, minus the beard. To be more specific, I assume you live in the U.S. or maybe Europe. For what concerns us here, it’s much the same, because it’s about climate.

For those of us who are living in a place where, at least some of the year, it actually gets cold enough that long pants, socks, and Pendleton shirts make sense, what that means is that we really can’t expect to put a simple, simple, simple biogas digester in the backyard and have it do very much in the winter. You’ll need insulation. You’ll need a heat source. And of course, as we explained in the series about food waste and biogas (part 1, part 2, and part 3), you’ll need enough of the stuff that makes good biogas.

• 95 degrees Fahrenheit/35 degrees Celsius: 100 percent
• 85 degrees Fahrenheit/30 degrees Celsius: 68 percent
• 75 degrees Fahrenheit/24 degrees Celsius: 46 percent
• 65 degrees Fahrenheit/18 degrees Celsius: 32 percent
• 55 degrees Fahrenheit/13 degrees Celsius: 21 percent
• Colder than 55 degrees Fahrenheit: zero percent

Just to keep it simple so as far as temperature is concerned, we’ll call the rate of biogas production at 95 degrees “100 percent,” and compare other (lower) temperatures to that. Every time Mother Nature drops the temperature by as little as 10 degrees, the rate of the production of biogas also drops, pretty steeply, by about a third. (See the table above)

Now, if you have a garden or farm, then you’re familiar with the USDA Plant Hardiness Zone maps, which revolve mostly around how cold it might get where you live. But at least first draft, what we want to know is how warm it might be for how long, and so for the purposes of biogas, the American Horticultural Society (AHS) Plant Heat Zone Map is what we want.

What this map or these maps tell us, according to the AHS website, is “…the average number of days each year that a given region experiences ‘heat days’—temperatures over 86 degrees (30 degrees Celsius)….”

Eighty six degrees ambient… Is that good enough?

Well, how about this: Let’s assume first of all that you carefully studied The Complete Biogas Handbook. That gave you all the tools you need so that you can design your digester to use one of those really good substrates (like food waste) and to be large enough so that when it’s warm and cozy at 95 degrees, it gives you 150 percent of your daily biogas needs: for cooking or whatever it is that you have in mind.

Well, it turns out that if that if you can get 150 percent of what you want at 95 degrees, then at 85 degrees, the rate of production will peg at just about 100 percent of what you want, just by sheer and astonishing coincidence. So if the average daily outdoor (ambient) temperature is 85 degrees or better, then without heating your digester on such days, you can make all the biogas you need, and maybe even a bit more, assuming you keep feeding your digester what it wants, what it’s designed to consume…. Got the picture?

Now I live in Oregon, between Portland and Salem—just above the 45th parallel—and the AHS map for Oregon tells me that, at very best, I should expect only 30 to 45 days a year with “…temperatures over 86 degrees…”. Right? In other words, if I expect to keep getting at least as much biogas as I had planned to get from my digester, I’d have to heat my digester for 335 days a year! (Ouch. That’s a bit discouraging, hey. Now where’s that Tahiti ticket when I need it?)

My digester would be better off in Florida, as you might expect. (I’d have a better tan as well. It’s a win-win, eh?) West and a little south of Miami there’s an area where I would experience better than 210 days a year of biogas weather, but that still means that I would need to heat the digester for in excess of 150 days a year…. The US isn't very “biogas friendly” in terms of climate, is it?

(Hey. Don’t lose hope now. It will be all right. We’ll get there. Together, if you keep reading.)

In any case, of course, all that AHS Plant Heat Zone stuff is far from the whole story, because you may not have to heat the digester very much even on colder days, particularly with proper insulation and the proper approach.

In fact (spoiler alert), on a day that is 55 degrees, the new digester I am working on (see the picture?) can be heated to 85 degrees using less energy in an hour than you generate as heat just by sitting down and watching internet videos for an hour… Even if you’re not laughing!

Want to learn more? Then keep reading… Part 2 will come along real soon now!

Photo:October first, 2014: Jeffrey Ironwood-Hunt tightens the main bolt on “The Compressor” a tool developed by David William House so that his new, low-cost, kittable & shippable, well-insulated biogas digester could utilize very low cost ‘bungs’ (holes in the wall of a container). These bungs David has developed cost less than a dollar apiece, and replace purpose-made bungs costing $20 or $30 each. The digester being built as shown here is larger than 2 m3, and the materials cost is less than $350.


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