Sadb O'Conner

Help? We have a question for the engineers/physics gurus out there with a good understanding of heat transfer into air:

My partner and I are building a Jean Pain style compost heater (biomeiler). It's going to be enormous (about 20' diameter circle, 6' tall) and mostly carbon materials for slow heat (70% C, and about 30% N). We're experienced composters, and we live in the humid, rainy mountains of Virginia. There's no difficulty getting compost piles here to very high temperatures for a good long period, so that's the easy part for us. We know from experience that it will get hot enough (over 120°F) for at least 3 months, which is plenty for our needs.

The difficulty is calculating how much air ducting we need (we're planning on using non-perforated drain tile or spiral tubing of some variety, about 4" diameter). We want to run the air into the window of our house directly, which is why it's important that the tubing be fully sealed. The intake can either come from our basement or just the outside air (secured by hardware cloth against pests). Winter air temperatures here rarely drop below 20°F, and never for more than a week or two in a year.

We can't find much discussion online of heating AIR this way, even though there are plenty of mentions of people using it successfully. Most information focuses on using water, or a working fluid heat exchanger. We specifically do not want to do that style; we want to heat air directly and feed it straight into the house. It's only to supplement our woodstove, and smooth out the rollercoaster of hot/cold cycles between times when we're running the woodstove.

So we have two main questions:

1) how do we calculate thermosiphoning in an air system? We'd like to calculate that for small passive solar air heaters as well. Yes, we'll probably end up using a fan to move air through the heater, but it would be nice to understand the math involved in thermosiphoning for air first.

2) how do we calculate the rate of heat exchange from the compost pile/heater into air, through plastic ducts/tubing? If we have no air flow, we know the air in the tubing will heat up, but if we have a blower forcing the air to move through it, how fast should it be blowing, or how long should it run in intermittent cycles? Or, if it blows at a constant rate of airflow, how do we calculate the heat harvest per linear foot of tubing running through the heater?

Example:
If the interior of the pile remains around 140°F, and we have 80' of 4" drain tubing coiled in a vertical spiral through the layers of the pile, and we use a computer fan to move the air at 50 cubic feet per minute, and the air temperature is 25°F at the intake, what will the outflow air temperature be?

Any help with understanding the calculations, or suggestions for which equations we should use, would be very welcome! Thanks in advance!

Patrick Freeburger wrote:...maybe a series of permanent structure that could access 4+ pastures at a time?  If I didn't want to bring them back to a central location, I think I would look into something like that.

That actually was our initial plan, back before we had electric nets. We put up 7 strand wire fences to divide permanent pastures and planned to build shelters from which the fences radiated outward: 6 or 7 paddocks radiating from one shelter.

The problem with that, before the shed even existed, was that weeds grew like mad on the electric fence lines. The goats, sheep and pigs wouldn't eat the weeds too close to the zapping wires, so we had to spend huge amounts of time and energy weed-wacking under the fence wires (and repairing the inevitable broken lines). Additionally, the permanent locations for water troughs became barren of sod and dusty in dry weather, slick manure mud in wet weather. The areas farthest from the water trough were weedy and the areas closest were overgrazed. Putting up a permanent shelter would only exacerbate the problem.

We only had those permanent paddock division lines up for a few years, and yet you can still see the ridge of dirt and line of weeds where they were. We're gradually working to erase those by moving the nets across those locations. The nets also keep the goats contained better.

While we could put up permanent shelters, and run mobile electric net paddocks out from them in pie slices, the shelters themselves will still be sacrificing sod, with mud and weeds to deal with, and requiring maintenance. We'd also need to build a lot of them, to keep paddock sizes reasonable to prevent uneven grazing levels based on distance from shelter. I think it's more sustainable to have just one or two mobile shelters to maintain instead. & we don't lose any fodder-growing space to the mobile shelters, nor cause erosion, and the herd stays healthier by not sleeping in the mud or collecting parasites at a central location (scabies mites are endemic in local deer populations, for example).

Beautiful goats! They look big enough to hitch up if you wanted to harness them for moving the hutch ;-)

Jay Angler wrote: I sewed several 1 inch button holes in the high risk areas. For the size the OP is considering, installing grommets might be a more realistic approach. It might be possible to set things up with enough slope that this is never an issue, but gravity and water seem to interact negatively no matter how hard I try to direct them!

I love the grommet idea for drainage. We will definitely have the tarps set up sloped (it would be hard not to) and they'll probably be triangular, which tends to shed water better, but as you say, gravity and water don't seem to respect direction, nor care for tarps very much. I think we may also put ropes up under the center of the triangle, from tripod to tripod, to support the tarp better in the middle.

Andrew Cegielski wrote:How about a geodesic dome?

Unfortunately, domes are pretty unstable on uneven, steep hillsides. We get gusts of wind from 50 to 60 mph (that's not during hurricanes, just regular wind storms), and even very low to the ground, squat and decently heavy dogloos end up flying up in the air, then tumbling down the slope into the bog below.

There's also the amount of materials required to make a sturdy geo-dome big enough for the whole herd, or to make multiple domes. These need to be movable every day or two, by just one person. They also need to be rugged enough that 70 to 200 pound animals can scratch and rub on the structure all the time without making it fall apart (plastic connectors definitely cannot withstand it).

Andrew Cegielski wrote:How big do you want the structure to be?

Currently our whole herd of adults can squeeze together into one barn bay that is 10' x 20' but it's tight. When there are babies, a lot of them end up sleeping on top of the adult sheep to avoid being squished. (Side note, the goat kids do nest in the sheep wool like the ewes are their moms; it's very cute.) In summer heat, that's just too crowded; the lower status members of the herd would get bullied in such tight quarters, and we have to open other stalls for them to spread out. So we're looking at more than 200 sq ft area, at least 250 sq ft minimum, which would mean a dome with a diameter well over 17 feet and more than 8 feet tall.

We can do multiple shelters, especially with modular connection points, but domes don't tend to be modular; you can't leave one quarter dome in one spot and move two other quarters to a different paddock, for example. They also waste a lot of space in creating height that doesn't serve any purpose for the livestock below. Structurally, they're strongest when they're a true half-sphere, and they lose a lot of their selling points if you 'squash' them to have a semi-flat roof to use less material (and be lower profile in wind). Making several smaller domes instead of one large dome increases the total materials needed, without adding square footage to justify the extra materials.

I think a dome is a great design for a large, permanent barn structure. When it's mounted securely to the ground and has full walls reaching the ground, it becomes one of the best shapes for wind resilience. Airflow inside is excellent and you can build storage into that upper space, so it's not wasted. But I don't think it's the best thing for mobile, tarp-roofed livestock shelters on a slope.

Thank you, though!

Jay Angler wrote: If your tripod system had at least one leg each that is like a camera tripod with an adjustable height system, that might help compensate for the slope.

Great idea! Also, no, not many rocks, except the really large boulders jutting up a few places. The increasing depth of soil here means we're seeing less erosion and very few rocks of manageable size turn up in convenient locations. It's actually a bit sad for us; we had hoped to make stone walls in some locations, but finding rocks for it is a challenge.

Jay Angler wrote: Have you seen pictures of the small round-wood earth-bermed shelters that Sepp Holzer makes by digging right into the hill-sides of his farm (Mountain-sides is more accurate in his case)? Would a few of those spread around the land so you don't have to go as far if there's a bad storm, and something portable for shade have potential?

Yes and I drool over them! That's on our "someday" list, at least one 'cave' per slope. However, that's something we'd need heavy equipment for, or a whole lot of friends with shovels. It'll be a few years before our project list can include making earth-shelters. They're going to be exclusively for storm use, to avoid the problems of run-in sheds around here, but I think they'll save us a lot of herd- moving distance when, say, a hurricane hits the area.

Elizabeth Medgyesy wrote:Do all of you have structures available to your sheep (esp Am Blackbelly) every night? Do they use it? What type of sheep do you have?

Most farms in my area have permanent run-in sheds (three-sided open shelters facing south) in their pastures, for any livestock but cattle. But they also barely ever practice rotational grazing in this region, except for cattle, and if they do, it's only rotation through very large pasture enclosures, with a very low stocking rate. It's easy enough to have a handful of small sheds spaced out through the big permanent pastures. Of course, then they have terrible weed problems and erosion and manure buildup at those sheds. A lot of producers just won't bother with sheep and goats because they don't want to deal with providing shelter. Horse farms here have to put a lot of maintenance into the run-in shed areas (mucking and herbicides) or they face hoof rot and growth of poisonous plants like perilla and horse nettle.

Yes, we provide shelter to our sheep and goats, either by returning to the barn or setting up tarps on t-posts. The sheep definitely use them, especially for shade in summer sunshine, and shelter from sleet or cold rain in winter. We have Finns.

Dealy Blackshear wrote:At first thought I would be thinking that you have too many sheep. That’s a second thought as well.

Lol. We barely have enough sheep! Perhaps you're underestimating pasture growth in this climate. Our small herd can barely keep up with the overgrowth here throughout the growing season, and that's even with daily movement of the rotational paddock. If we had fewer, we'd need some kind of mechanical equipment to brushhog the pastures, because there's no way the herd could eat enough of it to keep briars and trees from taking over.

For reference, we get an average of 50 inches of rainfall annually, well spread out, and barely 3 months of winter (plenty of days in January and February are well above 40°F, so grasses do keep growing). The local extension office recommends 6 to 10 breeding ewes per acre, and that's with conventional pasture management, not intensive rotation. With 10 acres here dedicated to just the sheep and goats (and another 16 acres of pig-grazed pasture, mature hardwood forest and yards around the house and barn, all of which can supplement the herd if we needed more grazing, though we don't), we have far fewer sheep and goats than recommended for this lush of a growing zone.

Dealy Blackshear wrote: The next thought is, what breed are you raising?

Finnsheep; we've had our registered purebred flock for 7 years now. They absolutely need shade in our hot summer climate; putting them out on sunny pasture without shelter is not only cruel, but potentially fatal. It would also destroy their wool quality. They start panting from heat stress when it's only 75°F, and we frequently reach 100°F in summer.

The goats are mini-Oberhasli and pure Oberhasli, which need shade in summer and rain shelter year-round. Getting wet and not having a place to get dry can stress them to the point of extreme vulnerability to respiratory infections, as well as parasites. The purebreds especially need access to shelter; the hybrid minis are more resilient.

George Yacus wrote:...large triangular canvas or tarp among three tripods made of bamboo or other wooden spar material.

WOW firstly, I love your artistic talent in instantly producing such a great illustration of the design! Thank you!

I think we're going to try this design, although I worry that the wind will end up our enemy here. It's still worth a shot, and tarps are cheap enough that even if it does end up ripped by wind, we're not out much on the experiment. Your suggestion of 4 tripods to allow leapfrogging some of the time is particularly brilliant. We'll use dog tie-downs (clay soil here, they've worked fairly well for us in the past; buckets will be too subject to swinging in the wind and allowing the tripod to collapse sideways when the tarp acts as a sail).

Thanks again for your great suggestion! I will get some photos when we get this assembled (probably a month or so from now).

Thank you! Will do.

I was hoping to get their large aerial tuber variety, which they call Ranger:

"Ranger: Heavy producer of  larger than average aerial vine tubers up to 1 inch in size in clusters.  These are harvested in September. The roots are typical of the species up to 6 inches long."
https://oikostreecrops.com/products/perennial-vegetable-plants/edible-roots-bulbs-tubers/chinese-mountain-yam/

But really I would love to grow any large aerial edible variety, especially since our garden is part of our agritourism/educational showcase for permaculture mentoring.

I'd be delighted to plant any spare bulbils folks have on hand, though of course this is probably the wrong time of year for any to be unclaimed. But there's no rush; I'll definitely check back on this thread later in the year for harvest season and see what folks have available to sell, swap or share.

Thank you!