Complete Biogas: Temperature and Biogas, Part 2

Reader Contribution by David William House
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So basically, the thought we left you with last time (here, part 1) was that the US climate is not “biogas-friendly.” Recall? Even at the very best “biogas weather” places in the continental US, to attain anything close to optimal temperature for better than 150 days a year, we have to heat the digester. 

Even here, in semi-tropical Oregon, with the highest happiness index in the US (well actually… I just made that up), I cannot escape certain realities. For me, it’s 335 days of heating the digester, and based on what I see outside my window and the weather for the last week, I’m starting that countdown….

Now, some might ask: isn’t it true that you can produce biogas at colder temperatures? And, yes, sure, not only is it possible, but millions of people around the world do just that, every day. (This is not a hyperbole. It’s really, truly millions in India, China, Nepal and other countries. I can prove it to you if you like: You should always ask for proof.) And the vast majority of those folks have underground digesters, which are seldom very warm at all. (Think of lying directly on the ground to sleep. Even if it was dry, you’d get pretty cold, most of the year. Maybe even in Tahiti….)

So how come millions do it… Yet I’m saying that’s not the way to go, huh?

Well, in a biogas digester, as the previous post said, the rate of digestion is dramatically affected by the temperature of the digester. Colder is slower, so a given volume of slurry will produce less biogas for a given period when it is colder. Right? So if you know that, then it seems obvious that the way all those millions compensate for the colder digesters is to make them bigger. Much bigger. Five or ten times bigger.

And bigger, all else being equal, is more expensive, right? (Of course, again, yes.) But in developing countries, larger digesters are necessary because adding a solar heating system and providing good insulation would be (well… might be) more expensive than building that larger digester.

What’s very cool for us (or very warm, actually), is that the economics of large vs. small digesters are generally pretty different in the US and Europe than in developing countries. Things that are relatively cheap for us, such as some kinds of building materials, are relatively expensive in those other countries. And vice versa: labor is very cheap in most developing countries, but not so much in the US and Europe. These things push the economics of large vs. small around differently in these different places.

Different economics? Want an example? Well, imagine you want to have a small (low-cost) swimming pool at your house in Burbank, CA. (Why would you live in Burbank, anyway? Never mind.) Is it cheaper to go and buy a small above-ground (mostly plastic) pool in the US, or cheaper to dig a hole and build a (mostly concrete) pool in the ground?

The market shows the answer. No one looking for a suitable but low cost small pool builds one out of concrete in the US, largely because it takes quite a bit of expensive labor and even special equipment (for gunite?) to do it. So in the US, using concrete to build pools is usually more expensive than using plastic. Of course you want something that will last for ‘long enough,’ something that has a low hassle factor. Something that works for you. But if you don’t have to, why send a large torpedo fishtailing its merry way into your bank account?

Volumetric Efficiency
95 degrees F (35 degrees C) 1 volume
85 degrees F (30 degrees C) 1.5 volumes
75 degrees F (24 degrees C) 2.2 volumes
65 degrees F (18 degrees C) 3.2 volumes
55 degrees F (13 degrees C) 4.7 volumes
45 degrees F (35 degrees C) 6.9 volumes
…And do you recall that I promised
I would give you an Excel spread-
sheet for calculating temperature?
Well, you can find that here….

The difference is even more pronounced with biogas digesters because, as we just indicated a few paragraphs ago, a warmer digester is a smaller digester. So, at least as I see it, the most reasonable economic comparison has to be made on the basis of the amount of biogas being produced. That is, you’ve got to compare the cost of a colder larger digester against the cost of a warmer smaller digester, based on the digester volume required to produce the same daily biogas output. Make sense?

In fact, if you recall that table we showed you in part 1 (here’s a better version), it should be evident that, on the basis of daily output, a unit volume of digester at 95 degrees F is equal to almost five unit volumes of digester at 55 degrees F. So you’d best not compare a cubic meter to a cubic meter, or a cubic foot to a cubic foot of digester space, at least when they’re running different temperatures. You’ve got to compare the two digesters on the basis of what we might call “temperature-based volumetric efficiency.” (Say that five times fast, eh?)

Want a real-world example? I estimate that the press-plastic digester that I am working on completing right now will have about $350 in materials in it by the time I’m finished. It’s got 2” of rigid polystyrene on every one of its six sides, and it will hold a bit more than 2.5 cubic meters of slurry. Based on my calculations at present, on a 40 degrees F day, keeping all the slurry in the digester at a biogas-comfortable 85 degrees will take about the same constant input of heat energy as you, personally, output as heat energy while standing and having a relaxed talk with a neighbor over the backyard fence. Even if your neighbor is boring. In other words, it doesn’t take much energy at all to keep it warm. (Left image: The completed hot water heating coil for the press-plastic digester.)

Now, you likely don’t know this, but a standard 5 or 6 cubic meter underground digester, built in the tropics and running at ambient temperature underground — we’ll call it 55 degrees Fahrenheit — will also cost about $350 in materials. (Well, actually, for materials and labor, so to comparison is not exact. But still.)

And here’s the thing. If both the small warm and the large cold are fed the same diet, then the US-ready, polystyrene-insulated digester I hope to show you how to build might even produce five times the biogas as the underground digester, if we keep it warm, and feed it better and faster. (But for the sake of argument, let’s say they produce the same amount every day…)

In that case, on the one hand, we have a US-ready, heated digester which at minimum is producing the same amount of biogas as an underground, cold, equatorial belt, 5 cubic meter digester. Without having to dig the hole. 

So which one would you want?

In sum? Well ultimately it’s about getting enough biogas, right?

#1: You’ve got to have enough stuff…

Capiche? We made this abundantly clear in Biogas and Food Waste, part 3 (here). Many people will be stopped by the fact that they don’t have enough (and also choose not to get enough) stuff to put in the digester.

I know there’s been a lot of talk about home biogas: but think about it. According to the United Nations Environment Programme, using figures provided by the US Department of Agriculture and the Natural Resources Defense Council, the average American produces less than a pound of food waste a day. Sure, yes: that’s a lot of stuff when we multiply it by more than 300 million of us, but for one household, that’s just not! enough stuff — even if you add in feces and urine, toss in your lawn clippings and ask your dog and cat to contribute, as we’ll cover in an upcoming blog post.

Let’s be real about this, OK? Biogas is great, and for my money it offers a good many of us the opportunity to actually become carbon neutral, maybe even carbon negative. The possibilities are present and important. If you educate yourself, if you invest the time and energy, then you can pretty well make as much as you want. (Just look in the back of your local fast food restaurant.)

At the very same time, the reality is that the great majority of us will not have enough feedstock within several feet of our front door, and not everyone is going to have enough interest to go get what is out there, just a bit further away. You and I are going to educate ourselves, but will the rest?

And besides, not everyone has the right situation. Is someone really going to try to put a two-ton digester in their apartment? On the fifth floor? (If they do, and you live on the fourth floor, try very hard to make sure your dining room table is not directly underneath the digester….)

Hey: it’s not heresy to say that biogas is not for everyone. So what? More for the rest of us.

#2: Keep it warm…

The newest digester on which I’ve been working (the poly-panel digester) uses the digester insulation as the digester container.

This insulation-as-container idea not only keeps the cost per volume low, but it insures that added heat will not be wasted, and thus that it’s going to be practical to heat the digester. Here in the US. Even in rainy, presently-bracing-and-chilly Oregon.

You want to know more about the digester I’m building (the press-plastic digester), or that even newer and more exciting (poly-panel) digester project we’ve got going? Well… keep reading, ‘cause I’m going to keep posting.

Get Temperature and biogas production: a spreadsheet. Find a table that shows how biogas production is impacted by changes in temperature. Or, to look at it another way, get a table that shows how, at lower temperatures, you will need a larger digester. And hey: those links offer some pretty cool pictures, too.

Do you want to keep up-to-date on progress on this new digester? Well, if you sign up for our newsletter at The Complete Biogas Handbook’s website,we’ll let you know what’s happening with biogas, with these new digesters, and with upcoming Beginner’s Biogas workshops, and Build-a-Kit Biogas workshops.

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