Methane Midden: 2 Week Update

When we left the Methane Midden 2 weeks ago it was 75% finished and consisted of 2 55 gallon steel drums destined to be batch methane digesters wrapped in 290′ of garden hose for temperature management.  Around these digesters we then placed 4000#’s of freshly chipped green brush that had been soaked for 2 days in some giant totes and into this soaking wet brush we layered another 240′ of hose which is intended to absorb heat from the pile and complete it as a mini version of Jean Pain’s epic methane and hot water producing brush compost piles.  Much has happened in the past 2 weeks, so lets catch up!

First off, we needed to finish the pile with another 18″ of material.  Alas, we were out of brush, so we needed to be a bit creative to find another 2000#’s of material in a jiffy. So Kevin and I hooked up the trailer to the Golf and headed back to the municipal yard to see what we could find.  While not ideal, we were able to scrounge up about 1200#s of grass clippings (likely sprayed with Chemlawn) and then went back for a load of about 800#’s of wood chips.  The thought was to shred the wood chips in my Bio-80 shredder to reduce particle size and then to mix this with the grass clippings.  The result was a very good looking mix that had decent moisture content, plenty of nitrogen and available carbon.  My primary concern is that the grass clippings will mat depsite our attempts to mix them well with the shredded chips; time will tell.  Here is the pile of about 6000#’s of material (and another 2 tons of soaked up water):

5 tons of material: 3 tons green brush, 1 ton grass/chips. 1 ton water.

The added material gave us about 6″ of mulch on top of the drums and raised total pile height to a bit over 40″ which is my preferred pile height.  Next up was a top layer of straw to prevent excessive moisture evaporation and to insulate the pile somewhat to keep temps up.  We want the BTU’s in the water not the air after all.  Took about 2 bales:

All tucked in! about 3-4" covering the entire pile. In winter I would add another 6-8". Makes it look nice for the neighbors to boot.


Steel Drum Heating/Cooling

This was all finished within a week of the last post.  Pile temps at this point were stuck at 123, but within days of adding the top we shot up to 132-136 in many spots so we began to start testing.  First, I filled one drum with water to test how long it took to get it to 100 degrees (city water is 52 degrees).  Within 4 days the drum was up to 100 degrees, gaining about 8 degrees every 24 hours.  Of course, this slowed as the temperature variation decreased between the pile and the water, but was still 118 within 8 days total, about the time the pile was a steady 135.  Max measured temp in the drum was 124 after 12 days, though there were cooling runs in there.   System proof #1 was locked in: we could heat water to over 100 degrees, though this was never in much doubt.  Next up we needed to see if we could cool the water in the drum.  Using 52 degree municipal water with the tap set at about 8 gallons per minute (gpm) (fill a 5 gallon bucket and time it with a stop watch and then do the math) I was able to drop the drum from 118 to 112 in an hour.  This was very encouraging and I am confident that I can get the drums to under 100 in less than a day.

Water Heating

While this was going on, I was also taking temps of the water as it exits the pile.  This was far less encouraging.  Initially, the first 10 gallons of water were wicked hot – essentially pile ambient of 124 or so.  However, very soon after temps dropped quickly and by the time 20-30 gallons had gone through the pile water temps at pile exit were down to 64.  Oi!  However with only a few minutes of thinking this through several glaring design flaws were identified — a direct result of my planning this on the fly.  First off – 530′ of hose sounds like ALOT of hose.  And it is, but given the small internal dimensions, even at 530′ total water in the hose is under 30 gallons, and probably closer to 24.  That means at the 8gpm, fresh water only had 3 minutes in the pile.  That is not nearly enough time to pull in much heat as was proven by the small temp gain.  Also, using rubber coated garden hose was an expedient, but not well thought out, tubing choice.  The rubber is almost certainly acting as an insulator, reducing the piles ability to conduct heat into the water.  Finally, starting with 52 degree water means I have a long way to go to get to my goal of 120.

Initial Conclusions

  • Midden will heat 110 gallons of water to well over the 100 degrees needed for methane production
  • Water cooling is sufficent to regulate drum temps to within 95-100 degrees despite being in a 130 degree pile
  • Current configuration will not heat water sufficiently for domestic water use.

Due to some upcoming events – a tour of our home gardens by the Madison Permaculture Guild and my workshop at the MREA I am postponing charging the methane digesters with material until 6/22 at the earliest.  That said, there is still enough time for some tinkering, so I decided to play with the hot water side as those results were really disappointing.

Modification #1: Closed Loop Water Heating

So, while the results of the outlet temps are discouraging, there are some nuggets in there to be excited about upon reflection.  First off – I am getting 10 degree rise in a matter of minutes at a relatively high flow rate.  There is an immense amount of heat energy in the pile – sitting near it on a still day you can feel it radiate off– I juts need to find a better way to capture it.  An obvious choice is to close the water system by pumping it right back into the pile.  To accomplish this I need to get one of the pumps I bought for the Appleseed Biodiesel processor from Patrick’s house.  A quick call explaining what I am up to was more than enough to have Patrick excited enough to come over.

Meet Patrick, he builds underwater robots. Yeah, thats right.

Patrick forgets more about electronics over breakfast than most of us ever knew.  His day job is building and piloting underwater remote operated vehicles – like the ones that found the Titanic and are trying desperately to staunch the leak in the gulf.  Patrick has also been to Antarctica a few times to help them fix their equipment.  On top of all that, he is wicked smart and wants to save the world.  He also has a pole barn that he lets me store alot of my stuff in.  Good guy to have around.  I called Patrick to see if I could get one of my pumps, and he came over the next day, not only with my pump, but with it mounted to a base and with a slick motor controller so we would play with flow rates.  Did I mention I like Patrick?

The pump is our “do anything” pump that we use from everything from ethanol to biodiesel to syngas cooling to this.  $40 from Harbor Freight, close to indestructible, and will pump up to 650 gph with its .5 hp motor.  We have 5 or 6 laying around or in use at any time.  However, 650 gph (10.8 gpm) is too fast hence the motor controller – which is essentially a dimmer switch for electric motors.

Motor controller set at about 66% or 7gpm. Plug the controller into the outlet, and the motor into the controller. Slick.

With the controller on and the system on closed loop pumping into the pile, then into a half filled 55 gallon drum, and then back into the pile we were able to run it continuously to see how high we could ge the temps.  66% gets us about 7 gpm and we ran it on closed loop overnight and got a steady 94 degrees come morning.   That is a huge improvement over 62 degrees, and enough for space heating, but I still want 120.  So then we let the pile warm up for a day, and ran another test at 30% or about 4 GPM.  This is really too slow for the pump and will burn it out over time, so we only did it for about a much shorter time.  Intake water was 80 degrees at the start.  After 45 minutes here was the result:

Actually we had 104 for 30 minutes and then the sensor shifted and we lost 3 degrees for some reason.

Now were talking!  That is very encouraging given that even at 4 gpm we only have about 7 minutes in the pile.

Next Steps

As I said, in 2 weeks we will run the methane experiment.  Once we prove / disprove the ability to make burnable gas in the system I will disassemble the methane portion to reduce the chance of serious accident.  At that point we will really focus in on getting that temp up to 120.  How?

With 2 55 gallon drums in the pile I already know that there is enough energy to bring them up to over 120 degrees – what is unknown is the recharge rate without draining the pile’s heat.  I also strongly suspect that with only 25 gallons of water in the pile in the current system there is not enough time to bring the water up past 104 degrees.  Answer?  Plumb the drums into the water heating system – water flows into drum #1 from the pump, then works it way into drum #2 and then through the 530′ of hose.  In one swoop we increase the waters time in pile from 7 minutes (at 4 gpm) to about 35 minutes by jumping from a system volume of 25 gallons to 135.  That should make a HUGE difference.

Second we need a better pump.  The clearwater 650gph is a GREAT pump, but simply too strong for this.  Luckily there are incredibly awesome pumps available for radiant floor applications made by Grundfos.  How awesome?  How about going from .5hp to .04hp and still maintaining 2-17gpm?  What about having 3 spd settings from the factory?  What about only having a AMP rating of .75 – yes this pump will only use 90 watts!  Thats like a quarter a day to run nonstop.  Plus, if anything, it is even better made.  $100 at Menard’s.

Third, we may very well need to swap out the garden hose.  If options 1 and 2 don’t get us to 120 we will need to get serious and buy PEX tubing.  Again, looking to the radiant floor applications here.  PEX is designed to be heat conductive, is flexible, takes heat ranges from 35-230 degrees, and is as inert as any plastic (insert cynicism here).   It is more expensive, but has alot of upside and comes in 500′ rolls.  Also, swapping tubing would mean a complete tear down of the pile, something I am loathe to do.  Again, hopefully a new pump and plumbing in the drums solves the issues.

All in all the first 2 weeks with The Methane Midden have been awesome.  This is the most exciting project I have been on since we first started on the gasifier.  Space heat+hot water+compost with almost no moving parts other than an nearly indestrucible pump.  Most importantly, the only skills needed are how to run a pitchfork and screw in a garden hose. Total price, even with pump, will be under $300 including stakes, drums, hose and straw.   If I can get the water to 120, or even close, consistently then I plan on building a new pile in the garage this winter, doubling the insulation, and plumbing in a heat exchanger (old car radiator) to be put in the hallway of our first floor.  Yep – space heating a house in an HOA with compost on under a quarter a day in electricity while sequestering carbon from garbage.

Want to help fund the Methane Midden?  Pledge to my Kickstarter project!

Be the Change.


8 Responses

  1. Since I don’t see myself getting into methane production (yet, anyway), I’d put water into the barrels, let that 110 gal heat up, then pump it straight from the barrels into my shower. Just need a midden-top shower stall, or insulated pipe from the midden to the house…

  2. Pretty awesome, Rob. I keep posting something as trite and understated as this, but frankly, sometimes your posts leave me at a loss for words. I was going to suggest PEX and radiant heat components, but you got there on your own. PEX is supposed to be very easy to work with, though I do know there’s a special PEX tool for either cutting or joining, can’t remember which. I dislike single-use tools, or more specifically the expense of them. Perhaps you can put the word out in your network for such a tool if you go the PEX route. Know any plumbers?

  3. I’ve done a little research on this topic since you’ve blogged on it before. I’ve considered different options for a hot water system previously, including one that involves a coil in a wood stove. The concept for the coil is what I would like to see if it could be duplicated with the compost pile. I’ll see if I can explain.

    The system includes a hot water tank (a re-purposed electric hot water tank can work) with cold input and hot output connections. The water is heated by flowing via thermal convection though the coil in the wood stove, input and exit points through where the heating coils used to be. The tank needs to be above the coil so the hot water can flow into it. As hot water enters the tank via thermal convection, cooler water exits into the coil to be heated. Hot water is then readily available when needed (cold water entering the tank when hot is drawn out to be used in the house).

    My thought is to use a similar system with the compost pile. Let thermal convection move the water rather than a pump, flowing it through the tank and continually heating it. Hot water will be readily available just as with a conventional hot water heater, just not necessarily with the same rapidity of recovery.

    Might this work for your experiment?

    • DP – I have thought about this as well and see a lot of upside in simplicity and total system efficiency as you say. My concern is that in our winters the storage tank would be in ambient temps of -10 (F) or so which would lead to potentially unacceptable heat loss from the tank, if I am picturing the system as you are.

      I do hope to use a coil heat exchanger to transfer the heat to our domestic supply – if I can get pile temps to over 118 or so. If not, we would do a pre-heat as demand calls for it, and then use the rest of the heat for space heating.

  4. Thanks for sharing. This is really useful for me. Good luck with your work!

  5. Hello Rob, Good luck with your experiment, as a fellow french gardener I can only support your attempt at using french technologie in south central wisconsin.

    Everytime, I see picture of your home, there is always the same question coming back … what kind of relation do you have with your neighbours !!! after all you and your experimetation ( aka growing something else than grass) does not really fit in what they expected from their neighbours when they bought their lots

    • Yeah, I’m “that guy” for the neighborhood. And as we are on the freeway we have achieved even greater notoriety. “Oh you mean THAT house…”

      There is so little community in suburbs these days its hard to tell. This project is definitely pushing the limits. One neighbor did ask what the heck we were doing – I should really offer a tour for them. Not sure if them knowing everything would help or hurt though…

      Thanks for the good wishes!

  6. […] Methane Midden is a good example of this thinking.  While significantly on the energy/fertility side with its 4-6 […]

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