Wood chip trench in a polytunnel...

Hello one and all, long time lurker here making my first post! Big thanks to Paul and all the rest of you wonderful folks who make such great contributions. I wonder if anyone could give me a few pointers...

I'm currently working on a permaculture farm in the north of Scotland, it's early days for the place - coming up to the end of the first year. We've got a polytunnel to extend our growing season and we've been scratching our heads over how to heat it...

My initial plan was to build a rocket mass heater of the half-barrel bell variety and run it along the length of the tunnel, using the beds inside as the mass.

Then I remembered about Jean Pain and his amazing water heating woodchip piles and thought I'd investigate those a bit more. My thinking was to run a kind of closed circuit piping system through the beds to heat them. Or, to drip feed into water butts inside the tunnel which would act as thermal batteries, and then irrigate the polytunnel with the water...

It was then pointed out to me the benefits of capturing the CO2 given off from the heap and piping it into the tunnel; the plants would love it.

I researched a bit more and came across http://www.builditsolar.com/Projects/Sunspace/NewAlchemycompost.pdf (possibly on your forum) about using compost to heat and raise CO2 levels in a greenhouse. They run a perforated pipe through a large compost pile set against their north wall and blow heat and CO2 through their veggie beds. Moisture from the bed was also carried along and condensed, keeping the roots hydrated. Seemed promising!

So, my ponderings now are... digging a big ol' trench along the middle of polytunnel where the central walkway would be, filling it up with woodchips and other good stuff, running perforated pipe through it and up into the veggie beds. It would mean a fair bit of digging and shifting but all the CO2/heat given off from the pile would be put to use and it being in the polytunnel and in the ground would insulate it. And if it's lower than the veggie beds hopefully convection should circulate everything for me..?

Any thoughts..? Is it all possible? Would a trench variation on the Jean Pain pile work or does it need to be in a big ol' mound for proper operation? Will I just keep thinking of different approaches and never actually get anything done?

I've attached a little sketch, hope you can make sense of it. The tunnel is 18.5 by 5.5 metres... 2.5 metres high. The trench in the sketch is 16cubic metres, not sure if this is too small or overkill.

Any composting experts out there?

Many thanks

Sean

I would be concerned about adding excessive moisture during the winter. With the materials so spread out, there's going to be more surface area. It's also tough to get compost to heat along the edges. So, for the first time in a permaculture discussion, I think you have too much edge.

Raising CO2 is good to a point, but is also possible to reach lethal levels. You'll need to have a means of controlling interaction between the pile and the greenhouse air. A plastic covering would help control moisture and gas exchange.

Dale Hodgins wrote:I would be concerned about adding excessive moisture during the winter. With the materials so spread out, there's going to be more surface area. It's also tough to get compost to heat along the edges. So, for the first time in a permaculture discussion, I think you have too much edge.

Raising CO2 is good to a point, but is also possible to reach lethal levels. You'll need to have a means of controlling interaction between the pile and the greenhouse air. A plastic covering would help control moisture and gas exchange.

Thanks for the speedy reply Dale. Too much edge! This is my worry too. I was hoping that the insulation from being set into the ground would help it on it's way. I've a pile outside which isn't much more than a cubic metre and that's getting pretty hot...

The plastic sheeting makes a lot of sense, that would have to be done - thanks. I don't think CO2 levels would get too high; the polytunnel isn't completely sealed and we'll be opening the doors to get in and out (we're highly skilled) which should exhaust a fair bit. This is wishful thinking though... maybe we'll just have to hold our breaths/breath through a line which leads outside.

The dangerous CO2 levels are for the plants. We can stand much higher levels. I think it may have to do with nighttime resperation when plants need oxygen.

I tried a similar thing some years ago in my greenhouse, and with all the soil contact, the pile did not heat up as desired. It stayed cool, and became a stinky anaerobic mess. I think that the pile needed more air, and less soil contact.

The other consideration is that excess woody material in my greenhouse has caused explosions of roly-poly population. Aka sow bugs. It took years to balance back out their overpopulation, in the meanwhile they voraciously ate all my basil seedlings, girdled lots of peppers, bored holes in my radishes, etc, etc. Now I am very cautious about excess woody material in the greenhouse.

But as always, it all depends. Give it a try. Make sure you report back how it worked. This is how we all keep learning.
good luck!

Sean

Tried this in our commercial 40 mtr polytunnels If dug into the tunnel it went into a mush without heating up. Next put kerb stones as edging on top of black membrane and infill with woodchip. Again gave no frost protection but at least the woodchip did not go to mush and made a good footpath. Solved the problem by using a sawdust burner made from 2 x 45 gallon drums and an industrial size engine air filter to hold the sawdust. Now light a 11 pm and the fire is normally still going at 9 am providing frost protection to the whole tunnel due to the slope of the tunnel.

Thanks for your replies folks, it's great to hear your experiences. Shame you didn't have much luck! Anyone think that running a perforated pipe through the middle drawing air in from the outside would help? Clutching at straws here, think it's time to move on to a different approach

I have another notion of using a Jean Pain pile to drip feed into a 1000 litre tank inside the polytunnel, which would sub irrigate the beds with warm water and act as a thermal mass. Has anyone tried anything similar?

Sean Kettle wrote:Thanks for your replies folks, it's great to hear your experiences. Shame you didn't have much luck! Anyone think that running a perforated pipe through the middle drawing air in from the outside would help? Clutching at straws here, think it's time to move on to a different approach

I have another notion of using a Jean Pain pile to drip feed into a 1000 litre tank inside the polytunnel, which would sub irrigate the beds with warm water and act as a thermal mass. Has anyone tried anything similar?

I use black 1000 ltr IBC containers filled with water down the centre of the tunnel with a bench above as a germination bench. At times of severe frost we also put a cloche over the gemination bench to give a polytunnel within a polytunnel.