Compost mound for heating water-----Jean Paine---pack it tight or leave it loose?

I have posted on the Jean Paine mound before and got some contradcitory answers.  I built my mound 51 weeks ago and it only procuced heat for about 3 months.  I now have about 50 yards of wood chips, a lot of cow poop and old bedding straw and hay.  

When I layer this together do I pack it tight? Or leave space for air to get in?

Is the mound anaerobic or aerobic ?

This time I am goig to wet the layers as I build it, I did not do that last time.

Anybody out here actually made a jean Pain mound?  It is a strange thing that I cannot get good information from people who have bult one.  

Thanks
Zeek

Using water will definitely help you get a proper bacterial colony going. It doesn't have to be soaked but if it's consistently moist, that is what is needed for good composting.  Although I've seen some stuff online that was decidedly wet looking.  I don't build Jean Pain mounds, but I'm building some substantial compost piles.  You do not need to pack it tight.  The weight of the materials as well as the moisture will pack down the pile. From what I've read about Pain's piles, he used water, but he also shredded his woody waste, not just chipped it.  The difference is that his material, like bent up match sticks, had way more surface area that was exposed to moisture/bacteria/fungi and as such was much more available to be broken down.  The nature of the shape of his material also allowed for a certain about of air in the pile, I would think, but the sheer size of his piles would dictate that these were likely fairly dense with not much air flow.  He developed a methane digester for his pile as well.  Can you describe your system, and the size of your piles?

Right now all I have is a giant 40 to 50 yard pile of wood chips from mainly pine trees. What I planned on doing was layering the wood chips getting them wet putting some of the cow manure and straw and hay then the poly pipe that would be one layer. I would repeat that layer all the way to the top. I then have a circulation pump that keeps water always circulating through the compost pile for maximum heat transfer.

I did not intend on building another Mount of this year but I got home from work and there was a huge pile of chips so I figured I would use them.

What I wanted to do was just take the black pipe put it on the roof of the house facing south and use that to heat my hot water until it could no longer Heat like in the winter.

Thanks
Zeek

I have not built one and I'm not excessively knowledgeable but I've done some reading on them.  These are all impressions I got from reading and youtube videos so take this with a few grains of salt:

It should be wetter than you think.  
Jean packed them pretty tightly.
If using wood chips and natural convection for air movement through the pile, you don't want ingredients that will block airflow like leaves.
Coiling 100' of perforated pipe under the pile allows air in for the natural convection.
Pine is fine but if it's cedar or tamarack it won't rot and thus it won't compost.
Jean also had leaves from the branches in his chipped mix so it wasn't entirely Carbon, there was some Nitrogen at the start.

If you haven't read Gaelen Brown's book on compost heat, it has a lot of good information and is only a few years old.

a year late...

I've built one. it was neat. heated water to about 160° F for a year or so, then cooled off over a few months. I packed it as tight as I could by stomping on each layer while I sprayed water on it.

I want to point out, though, that these things are sources of methane, which is a potent greenhouse gas. much worse than carbon dioxide. they're also a lot of work to build, though it is kind of fun. anyway, I wouldn't go chipping up a bunch of wood to get what seems like guilt-free heat. from an environmental impact point of view, you're better off heating water by burning that wood in a clean and efficient stove like an RMH. if you've got a source of free wood chips, I would say the higher use is as mulch.

that said, if you were able to build a digester that could contain the whole pile and collect all the methane, that could be a clean source of energy. that's far from a simple undertaking, though.

Thanks for the report Tel!  What approximate dimensions were the wood chips when you made your pile?  The chips I get from the city seem to be an inch on the longest dimension and 1/4" on the thinnest and often shaped like a square or rectangle.  I think Jean Pain's chips were shaped more like toothpicks.

Regarding methane...  Does a wood chip pile generate methane in large quantities?  I assumed that compost piles with more animal manure and food scraps generated more methane but I was hoping that piles built primarily from wood chips might not.  But that's based on zero science or experience, just hopeful wishes...  

the chips were like you describe from the city. came out of the standard Vermeer chippers I see towed behind arborist trucks. a mix of conifers and hardwoods, including species that are otherwise considered roughly impossible to compost (cedar, juniper, walnut).

big wood chips piles, especially when they're packed tight and wet, are mostly anaerobic and a great habitat for certain methanogens (it's basically a lignocellulose digester, without the tight controls and methane collection). adding too much food or manure can cause a more standard compost pile to go anaerobic, but the problems then are usually ammonification and putrefaction, both of which stink. there's some methanogenesis, too, but piles with those problems are generally smaller and so have less capacity for methane production (but plenty of bad smell potential).

I'm not a big fan of most composting, but if you're composting for another reason already, it probably makes sense to run pipes through the pile to harvest some heat. I don't think building one just for heat is the best use of resources, though, at least in most cases.

another way to think of the difference between a Jean Pain pile vs combustion for heating: CH₄ contains 55 MJ/kg, which is a pretty high energy density. in any anaerobic system that doesn't capture that CH₄ (such as a Jean Pain pile), all that energy is wasted. worse, it is released into the atmosphere to act as a greenhouse gas. combustion captures that energy and creates CO₂ instead. it's still a greenhouse gas, but it's less potent than CH₄ and much more energy is extracted from the reaction.

a big old pile of wood chips does leave you some nice mulch after it's done heating water, which a fire does not. the cost:benefit ratio is pretty bad, though.

Jean Pain was a clever guy and found an interesting way to be self-sufficient without giving up modern conveniences. he likely didn't understand all the consequences of his actions, though, and I don't think wide adoption of his methods is a good idea.

Thanks Tel!  I was getting ammonia and methane mixed up in my memory, thanks for sorting me out

I'm trying to heat my greenhouse with wood chip compost in an enclosed chamber.  I'm assuming that capturing the CH4 would be very tricky.  I am aerating the pile with a fan so there is a pipe for the exhausted air/methane to travel through.

if you're pushing air through your pile, you'll get a lot less CH₄ and more CO₂ because you're providing oxygen. the fan consumes some energy, but you'll also be releasing more energy than an anaerobic pile would. the microbial metabolism of wood chips to CO₂ releases the energy that would remain in CH₄ in an anaerobic pile.

Sweet, so I'll get less CH4 and more energy from the pile?  Woo hoo!  

I thought the Jean Pain piles were quite tall and had some natural aeration from the height of the pile causing air to rise through it.

I'm sure that a little air would move through it, but not nearly enough to prevent anaerobic conditions.

Great info so far, thank you.

We tried to create a compost in-floor heat last winter, since the tile floor can be frigid when it's below 0 outside in the winter, even with the stove toasting up the house.  The radiant floor heat would also reduce our wood consumption and help us be able to leave the house without worrying about it freezing solid.

At first the pile was simply 40~ bales of partially decomposing straw, retired from trailer skirting.  That pile was STEAMIN'.   I forget why, but we dug the pile open and spliced in another 50-100ft of hose, probably to try and capture more heat.  I think we already had 50-100 feet in there.  We put the pile back together and it never really got hot again   So we started bringing in horse manure and adding more straw, but it never really took off again.  Someone suggested not using manure since it was most already processed from digestion.  So we opted for wood hips, chips, and chunks from the wood chopping area (this is all during winter), as well as anything else we could think of to compost.  We got a brief glimpse of floor heat before cracking it open, then it went dead and stopped melting the snow.  
Obviously we didn't pre-plan our composting methods.

I'm curious what a good recommendation for a compost heap size and make-up might be to do such a thing?  To heat the lines for 6~ months?  We have a small circulation pump and the hose is filled with non-toxic RV antifreeze mixed with the water.  We have a 'level check station' we can monitor and add water if needed (for the pump's sake) and also it allows any air in the lines to escape.  

Jen Fan wrote:
I'm curious what a good recommendation for a compost heap size and make-up might be to do such a thing?  To heat the lines for 6~ months?  We have a small circulation pump and the hose is filled with non-toxic RV antifreeze mixed with the water.  We have a 'level check station' we can monitor and add water if needed (for the pump's sake) and also it allows any air in the lines to escape.  

I'm gonna punt on answering that question (there would be a whole lot of guesswork even if I tried). instead, I suggest a wood-fired water heater. if you've got the space for a big compost pile, you've also got the space for a water reservoir. heat that water and run your antifreeze solution in pipes through it to heat your floor. the reservoir could be one big tank, or a whole bunch of barrels connected to each other.

I guess burning rotting bales of straw isn't really an option. I'm not sure how straw compares to wood chips for energy, but I believe the pile of chips I built was about 45 cubic yards.

tel jetson wrote:

Jen Fan wrote:
I'm curious what a good recommendation for a compost heap size and make-up might be to do such a thing?  To heat the lines for 6~ months?  We have a small circulation pump and the hose is filled with non-toxic RV antifreeze mixed with the water.  We have a 'level check station' we can monitor and add water if needed (for the pump's sake) and also it allows any air in the lines to escape.  

I'm gonna punt on answering that question (there would be a whole lot of guesswork even if I tried). instead, I suggest a wood-fired water heater. if you've got the space for a big compost pile, you've also got the space for a water reservoir. heat that water and run your antifreeze solution in pipes through it to heat your floor. the reservoir could be one big tank, or a whole bunch of barrels connected to each other.

I guess burning rotting bales of straw isn't really an option. I'm not sure how straw compares to wood chips for energy, but I believe the pile of chips I built was about 45 cubic yards.

We have wood heated water in the winter, but we can't really combine the two systems for a couple reasons; 1. being that the installed floor line was not designed to be pressurized like our house water is, 2. due to having outdoor exposure, the line must have anti-freeze in it, which obviously can't be connected to house water lines, 3. we need this as radiant passive heat for when we're not here to stoke the stove and keep the house from freezing, 4. adding another water-heating system to our little stove would strip it of its heat for cooking and warming the house, we can't support another hot water system on it

We've thought of that already

So the perks of compost would be 1; providing us with radiant passive heat that will work even if we're not home, and also not cost us more wood chopping and burning (in fact it would probably save us on that!) 2; the compost heap will be in the greenhouse next year, as we intend to build a greenhouse over the area where we've set this up, so it will help to warm the greenhouse during winter, 3; fresh, rich, organic compost when springtime rolls around, right there and ready to transfer into the garden beds

That's a lot of wood chips!  I wonder if there are alternative materials we could use?  We have an abundance of straw, hay, brush, and woody debris.  We might be able to borrow the neighbor's wood chipper if needed...

Jen Fan wrote:
We've thought of that already

So the perks of compost would be 1; providing us with radiant passive heat that will work even if we're not home, and also not cost us more wood chopping and burning (in fact it would probably save us on that!) 2; the compost heap will be in the greenhouse next year, as we intend to build a greenhouse over the area where we've set this up, so it will help to warm the greenhouse during winter, 3; fresh, rich, organic compost when springtime rolls around, right there and ready to transfer into the garden beds

ultimately, I think it would be a lot less work to build another wood-burning system. a simple heat exchanger would solve the anti-freeze and pressure problems. remember that if you're composting material for the heat, you're only able to harvest a small fraction of the energy that material contains. you wouldn't get all of it from a wood-burning system, either, but you would get a whole lot more than from a compost heat system.

but I do understand the appeal of the compost heat idea. it's on all the time without any attention from you after the initial setup. you get good mulch out of the deal. especially if you're chipping the wood or otherwise preparing the material yourself, though, I'm not sure it pencils out. if I built one again I would probably be a little faster, but I would guess that I spent over 40 hours building the pile. possibly as much as 80 hours. and I didn't have to chip any of it. I also live in a much milder climate than you, so there's no guarantee that what worked for me would work for you.

I'm really not intending to discourage you, but I do think it's wise to temper expectations. on first blush, heating with compost seems like such a no-brainer: free heat from something you would do anyway. in practice, though, I haven't found the benefits to outweigh the costs.

if you're absolutely determined to go the compost route, I would say go as big as you can manage. stick to mostly high-carbon material. the tighter you can pack it all together, the better. build it once and leave it; no tearing it apart to add more pipe or material. minimize external surface area: a sphere is the shape with the highest volume to surface area ratio, but a tall cylinder is more practical. maximize pipe surface area: a lot of small diameter pipe is better than a little large diameter pipe. don't put it next to anything flammable: if it's big and active enough to effectively heat water, there's a risk that it's big enough to catch on fire. plenty of barns have burned down because somebody stacked wet hay or straw in them. piles of wood chips can also heat up enough to burn.


here's another idea: solar water heating. how's the winter weather where you're at? if you get a lot of winter sun, a solar water heater could be a good option. even heating a large enough volume of water over the summer could work. again, you would use a heat exchanger to tie into your anti-freeze radiant floor system.

Mike Jay wrote:
I thought the Jean Pain piles were quite tall and had some natural aeration from the height of the pile causing air to rise through it.

The physics of heat transfer are interesting in these kinds of piles.  In other heat-producing objects like a stove, a fire or even a hot rock, the hot object heats up the air that is in close proximity.  That hot air, in turn, rises, as it is lighter than the air around it.  We all know that: hot air rises, as in up, up and away in my beautiful, my beautiful balloon.  Smoke, gets, in, your . . . eyeee-eyes.  Convection.

But heat itself doesn't rise.  It radiates outward in every direction.  So if you are using an infrared heater, (like the sun), the infrared waves radiate out from the source and whatever surface they hit (the sidewalk, my old head, the car in the driveway) are warmed.  THEN, once that sidewalk is warm, it warms the air around it and that hot air rises.  That's convection.  Technically speaking, we'd call this kind of heat transfer radiation.  

With a compost pile, there really isn't much air movement to speak of—certainly not enough that you'd feel the breeze moving through it like you do as air passes through a window screen.  The moist heat tends to "melt" the softer carbon stuff in the pile (leaves, grass clippings) and they clog up the space between the chips.  For an aerobic pile, this causes the microbes to quickly exhaust (as in "use up", not "expel") the available O2 and it starts to get stinky.  The bottom of the pile becomes this tightly layered mash of stuff with zero air-movement.  You don't get convection taking place (hot air rising, and cold air being pulled downward by gravity).

We've all had that experience of turning an compost pile and then coming out the next morning and seeing steam rising off of it into the cool morning air.  Cool story.  What's the point again?

Wood chips in and of themselves don't have the capacity to hold a lot of heat.  There is not much thermal mass.  They are a good insulator and a good food source for the microbial herd, but they don't hold onto the heat like a stone or a cast iron skillet would.  If you set that 5 lb. stone next to the fire for an hour, it will absorb the heat (primarily via conduction and radiation) and then continue to radiate it out for hours once you take it away from the fire.  A similar sized block of wood (or similarly weighted block of wood) would not do the same.  In fact, if you set that stone there for an hour, and then broke it open, the center of the stone will be very hot.  Not so for the block of wood -- it would self-insulate the center of the block and would be cool to the touch when you cut it in half.

So a couple of thoughts:

1.  Would it be smart to build you pile with a layer of stones or other thermal mass toward the exterior of the pile, thus capturing that heat and more effectively holding it?  Or just incorporate a random mix of stones throughout the pile -- perhaps 5% of the mass of the pile.  These heat sinks would serve as something of a battery to hold the energy.

2.  Based on my musings above, it would seem that aerobic piles transfer heat in part though convection.  Warm air (as you state) rises through the pile.  But anaerobic piles transfer heat via conduction.  The water in the pile would be the primary conduit for this transfer: trapped steam/highly moisture-saturated hot air.  Sealing the outside of the pile to minimize the escape of this steam would increase its effectiveness: a thick plastic sheet over the whole pile, which would then be covered by another foot of insulating wood chips.

3.  If you did "seal" the pile (point 2 above), then it would also be possible to capture the methane that has been raised as a concern with such systems.  Landfills do it all the time.  They aren't capturing the heat, however.  It's lost.  I'm not sure you would get enough methane from a compost pile to actually do anything with it (like create an eternal flame in honor of dearly departed Grandad) but a perfectly sealed pile would allow you to capture a harmful byproduct and utilize it.  The problem becomes a solution.  Commercial gas "digesters" do this efficiently, and the heat produced is often an unwanted byproduct—they exact opposite of your desire—oh the irony.

4.  This is quickly becoming the nerdiest active thread on the board.

I've got WAY too much time on my hands.

tel jetson wrote:

Jen Fan wrote:
We've thought of that already

So the perks of compost would be 1; providing us with radiant passive heat that will work even if we're not home, and also not cost us more wood chopping and burning (in fact it would probably save us on that!) 2; the compost heap will be in the greenhouse next year, as we intend to build a greenhouse over the area where we've set this up, so it will help to warm the greenhouse during winter, 3; fresh, rich, organic compost when springtime rolls around, right there and ready to transfer into the garden beds

ultimately, I think it would be a lot less work to build another wood-burning system. a simple heat exchanger would solve the anti-freeze and pressure problems. remember that if you're composting material for the heat, you're only able to harvest a small fraction of the energy that material contains. you wouldn't get all of it from a wood-burning system, either, but you would get a whole lot more than from a compost heat system.

but I do understand the appeal of the compost heat idea. it's on all the time without any attention from you after the initial setup. you get good mulch out of the deal. especially if you're chipping the wood or otherwise preparing the material yourself, though, I'm not sure it pencils out. if I built one again I would probably be a little faster, but I would guess that I spent over 40 hours building the pile. possibly as much as 80 hours. and I didn't have to chip any of it. I also live in a much milder climate than you, so there's no guarantee that what worked for me would work for you.

I'm really not intending to discourage you, but I do think it's wise to temper expectations. on first blush, heating with compost seems like such a no-brainer: free heat from something you would do anyway. in practice, though, I haven't found the benefits to outweigh the costs.

if you're absolutely determined to go the compost route, I would say go as big as you can manage. stick to mostly high-carbon material. the tighter you can pack it all together, the better. build it once and leave it; no tearing it apart to add more pipe or material. minimize external surface area: a sphere is the shape with the highest volume to surface area ratio, but a tall cylinder is more practical. maximize pipe surface area: a lot of small diameter pipe is better than a little large diameter pipe. don't put it next to anything flammable: if it's big and active enough to effectively heat water, there's a risk that it's big enough to catch on fire. plenty of barns have burned down because somebody stacked wet hay or straw in them. piles of wood chips can also heat up enough to burn.


here's another idea: solar water heating. how's the winter weather where you're at? if you get a lot of winter sun, a solar water heater could be a good option. even heating a large enough volume of water over the summer could work. again, you would use a heat exchanger to tie into your anti-freeze radiant floor system.

Lots of good points, thanks.  Though upgrading the heat system here isn't a very viable option.  We use a wood cook stove with a tiny fire box.  It will probably never leave this cabin due to its size, nor do we have any space or place for a second stove.  Nor do we have a second stove.  There's also only room for the water tank currently insulated in the attic.  There's nowhere up here we could bury one or more tanks easily or effectively.  We're on a rocky outcropping with giant 2-5' boulders for substrate (hence one of the attractions of actively creating more growing medium).  Much of our growing happens lower on the property, but it usually also 10-20º cooler in the lower areas year-round.  What's more, the cost and time of replacing, moving, pressurizing, and upgrading our piping and entire house heating system to accommodate it would probably blow the time and cost of building a compost pile out of the water.  I guess I'm not too concerned with wasted heat/energy.  It's not a terrible waste when it's providing heat in the greenhouse.  Our barn is basically a greenhouse, and having compost heaps in there during the dead of winter makes a huge difference in temp for the animals.  You'll usually find them sleeping on the piles and enjoying the steam!
Winter can see 7-15' of snow and consistently frozen temps for several months.  So anything outside has to be super insulated or buried 3-6'.  Our outside pressurized water lines are like 10' underground and it took a massive excavator to trench that line with all the boulders we have.  For the floor heat, we already have foam pipe insulation AND fiberglass insulation around the  floor pipes wherever it's exposed outside, we hope that that, combined with the antifreeze, would be enough to carry hot water to the house without losing too much heat.  When the cabin was built, it was built with piping in the floor so that someday it could be hooked up to a heat source.  10 years later we're just now trying it out

For lack of storage space for water, even if we tied more hot water into the current stove, it still requires us to be home and using the stove to heat that water.  When it's -20º and the water is having to circulate both indoors and out, I don't see any hot water tank we could accommodate staying very hot for very long without that stove actively heating it.  

So unless there's some knowledge I lack about stove-hot-water, I'm just not sure how it's a more viable option than compost heat.  And truly, I don't know a whole heck of a lot!  Just what I've experienced so far

I think you might be misunderstanding a few things, Jen. I'm not the most articulate, so I don't blame you. gonna try again.

pressure: you wouldn't need a pressurized system to heat with fire. the same system you're using for compost heat would work. you would maybe need to modify the heat exchanger, but maybe not. with a compost pile, your heat exchanger is the tube that runs through the pile to heat up your antifreeze mixture. for a wood-fired system, it could actually be the same tubing, but submerged in water you've heated up instead of a compost pile. so you're using hot water to heat your antifreeze solution in the tubing which you use to heat your floor.

the burner: I'm suggesting building a new one outside, not using your existing stove. there are designs floating around for rocket stove water heaters that are relatively simple to build. this would not require frequent firing, so it wouldn't be a lot of trouble. pressurizing them makes them more complicated and/or more dangerous, so I wouldn't do that. leave some opening to atmospheric pressure instead. if you're often gone for weeks at a time, a solar system might be better because it can operate without anybody around.

wasted energy: the waste from heating with compost is not heat energy, it's chemical energy. so building it in a greenhouse or barn won't help you capture it. methane gas will escape into the atmosphere unless you build some sort of sealed digester designed to collect it (not impossible, but rather ambitious). that methane represents A LOT of wasted energy that is inherent in Jean Pain style piles. an aerobic system would not have this drawback, but would be more complicated and probably require more energy input.


again, I don't want to talk you out of a compost heater. I do want you to go into it with the best information about costs and benefits and with reasonable expectations. your desire for finished compost is certainly a compelling case. if you're building a pile anyway, you might as well try out using it for heat.

and keeping a large volume of water warm: insulate it. pile straw around it. or dry wood chips. or branches. or manufactured insulation. or wool. or cattail down. if you've got the space, it's doable. 55-gallon steel drums daisychained together would do the trick, I think.

this is probably the biggest strength of the compost pile system, to be honest. once it's going, it'll keep going for months until it's exhausted. unless maybe -20° weather shuts it down. might take an exceptionally large pile to power through that. I really don't know.

I'm communicating from a point of perspective about how our very goofy system works, and failing to convey all the details, is I think what's happening

You've made a lot of good points, I appreciate your input!  We shall see what this winter brings.  And if we have time to even tackle the floor heat!  With the new greenhouse addition, there would be more use for a wood burning stove out there.  Lots of logistics to consider with trying to make a hot water 'battery bank', where to put it, how to insulate it, etc etc.

I know space is pretty dear in a greenhouse, but a thermal battery in there could be helpful. it would heat the greenhouse in the winter and moderate temperatures in the summer.

if you do go the compost-in-the-greenhouse route, I would recommend trying to keep it aerobic. carbon dioxide would benefit the plants. methane would not.

Jen Fan wrote:I'm communicating from a point of perspective about how our very goofy system works, and failing to convey all the details, is I think what's happening

You've made a lot of good points, I appreciate your input!  We shall see what this winter brings.  And if we have time to even tackle the floor heat!  With the new greenhouse addition, there would be more use for a wood burning stove out there.  Lots of logistics to consider with trying to make a hot water 'battery bank', where to put it, how to insulate it, etc etc.

Jen, there's lots of good information about DIY solar hot water heating/storage atbuilditsolar.com that might be applicable to your project, especially storage tanks one can even build in a crawlspace if needed.

I think some great points have already been made about the compost heat, and it's interesting the talk about the methane from the compost... when in Jean Pain's case the mound of wood chip (a by-product of his forestry work) was built around a methane digester, in order to provide warmth for that reaction to work, netting him the methane for cooking and motor fuel, and the hot water was the by-product (or at least a co-product along with the methane), and the compost a by-product... (or was it? I think it was all good for him). His mound was packed tightly around the perimeter at least, since there was no other means used to retain the pile, and the sides were quite a bit steeper than the "angle of repose" for loose materials in a heap.

I think anyone trying this needs at least 2 of 3 (4 if you add the digester) if not all the bases covered at home.
That is, (1) you have a feedstock that needs composting (the manure and/or the woodchips), (2) a need and a means to use the heat/hot water, and (3) a need for the resulting compost. It's a lot of work, and all the components (manure, compost, making heat) are more easily done on their own.

According to Gaelan Brown, the Btu's for composting the woodchips are higher than for burning an equal weight of wood. Plus, at the end you have compost instead of ashes.

Kenneth Elwell wrote:
According to Gaelan Brown, the Btu's for composting the woodchips are higher than for burning an equal weight of wood.

I'm just not sure how that could be. it's contrary to what I know about chemistry, microbiology, and thermodynamics*. I'm certainly open to being convinced. and that isn't to say that there aren't real advantages to using compost over combustion for heating, because there are.


*take what you hear from strangers on the internet (me) with a grain of salt, obviously. these are all things I've studied in recent years, but not enough that I would claim any sort of authority.

from "The Compost-Powered Water Heater" by Gaelan Brown, 2014, p.14:

According to Jean Pain's documented results, he was able to capture more energy by composting then digesting the woody material than he would have captured by simply burning all the wood. Many seemingly knowledgeable people, including college professors and engineers, have told me that this claim must be impossible-- that there's only so much energy to be harnessed in a given mass of wood chips.
   The Compost Power Network's experiments have at least partially validated Jean Pain's claim in this regard. more research is under way, but in a nutshell: Microbial respiration and digestion seem to be more energy-abundant processes than combustion. Microbes are more efficient at turning food into energy than is a woodstove. That should not be hard to believe , in my opinion.

Taking a look at the Rocket Mass Heater as another example of efficiency as compared to conventional woodstoves, the escaping flue gasses are MUCH cooler than the 600*F exhaust of a woodstove. Yet, the combustion is actually hotter in the RMH... the difference being the charging of the mass and holding in all that heat and releasing it slowly... NOW, instead, compost the wood slowly for a similar slow-release, instead of furiously burning it in a RMH or an 80% efficient woodstove (or something like 80% of 80% according to Ernie and Erica's explanation). Composting drops the temperatures involved to ~140*F, so no "boom-squish" to worry about, and it is running 24/7 without tending like a stove.

sure, except that no laws of chemistry or thermodynamics have to be suspended for a RMH to be more efficient than a conventional wood stove.

anyway, color me skeptical. if the claim is that more heat energy was captured by composting and digesting the chips than would have been using a fireplace or real inefficient wood stove, I would still want some more evidence and at least a credible hypothesis regarding what's going on, but I could almost see that penciling out. if the claim was that a space was kept more comfortable with a compost heater than with a wood stove, I would have no trouble believing that.

the theoretical amount of energy that could be captured from either combustion or microbial metabolism is the same. capturing the great majority of that heat is easier with a RMH than a compost system for a couple of reasons: a compost pile is constantly losing energy to the atmosphere both as radiated and conducted heat and in the form of gaseous products of microbial respiration. some of that gas can be captured if a sealed reactor is included, but much will be lost even then. putting the pile in a structure that needs heating does solve the lost heat problem, but takes up a whole lot of indoor space. it's also more difficult to build a system where every last bit of woody biomass is metabolized than it is to build a system where it's all combusted.

you're absolutely right about a compost system just humming along without further attention once it's set up, which is a real advantage. having the mulch at the end is also real nice and shouldn't be discounted. but I've built one of these things. it worked well and it was fun, but it was also a lot more work than occasionally feeding an RMH would have been. having hot water on demand was great. I sunk a couple of bathtubs into the top of the thing and set up an outdoor shower next to it. those were all real nice. just lovely, in fact. I just don't want to oversell the actual benefit, especially considering the methane released.

I guess I'm also applying Carl Sagan's aphorism about extraordinary claims requiring extraordinary evidence.

I think the claims of efficiency versus combustion are against a conventional woodstove or maybe a wood boiler? (What was available at the time Pain was doing this?), not a RMH, so there's that...

Neither of these "reactions" are happening in a closed system, so the inputs aren't tightly controlled... but the composting has water as an input that the combustion is missing...
The composting is "alive", so the "growth" of the microbes maybe adding to the system from air and water inputs?

Then there's the leftovers, ash compared to compost, exhaust gasses in both. I imagine all the minerals in the ash, are also remaining in the compost? But comparing weights at the end, more mass remaining in the compost? (weight of microbes? remains of wood?)
There's a point where the flue gas can't get cooler and still exhaust properly (without help), and there's a point where the compost isn't as hot, say 90*F and we give up on trying to capture more heat? so ho much is "lost" and how much is "written off"?

Like I said before though, it isn't for everyone. It is a lot of work and if it's not ticking ALL, or all-but-one of the boxes for you (manure, woodchips, hot water, biogas, compost) then the payoff is diminished.
I think the RMH stuff is MUCH more promising for widespread adoption. (and also not the standard for comparing the combustion of wood to composting mentioned in that quote from the book)

Kenneth Elwell wrote:Neither of these "reactions" are happening in a closed system, so the inputs aren't tightly controlled... but the composting has water as an input that the combustion is missing...
The composting is "alive", so the "growth" of the microbes maybe adding to the system from air and water inputs?

the added water also removes energy from the compost pile. it's more mass that has to be heated up and quite a bit of it evaporates (visible on a cold day), which is another energy loss. water is already completely oxidized. there's no more energy to get out of it. it's necessary for all that microbial action to take place, but it doesn't add any energy to the system. if a pile gets rained on, that's yet another loss to the environment.

the growth of microbes is what the chemical energy in the wood is used for. their respiration creates heat as a byproduct, just like my respiration does. that microbial biomass eventually dies and provides fuel for another round of microbes, but it's all limited by the initial energy present in the form of the wood chips or other compost material.


by now, it's clear to me that we mostly agree about all this, Kenneth. heating with compost is a great idea in certain situations, but it's got some real drawbacks that I think folks should know about before jumping into a project. the amount of work involved isn't necessarily a problem, because we've got these bodies so we might as well use them. the methane creation is a bigger deal as far as I'm concerned, because it could exacerbate a problem many people are trying to solve by using alternative sources of energy.