For the last two weeks, work has continued, quite unsteadily, on the extension to the outer shell, or “mantle,” of the biomeiler. Over these two weeks I have grown increasingly pleased that I decided to make only an upgrade and did not attempt a complete rebuild of the pile.
The second trailer load of shavings did much to fill the empty volume, but, as I expected, it was necessary to add a third. One problem with filling the mantle extension was that the pile shrank as more weight was added. The situation recalled to my mind a lesson in one of my astronomy classes in which we calculated the radius of a collapsing star as more mass is added. Counterintuitively, the star gets smaller as mass is added, because the force of gravity increases and pulls the star’s surface closer to the center. I was fairly certain there was no risk of creating a black hole in my back yard, but decided to err on the side of safety by quitting after three trailer loads of shavings.
The last of the shavings filled the pile to within a few inches of the top of the perimeter fence. I overcame my disinclination for leaf raking and used leaves from the yard to bring the pile level with the top.
The weather then turned rainy before I could add enough leaves to make a conical peak. The rain cloud had a silver lining, though. I was saved from adding water to the pile myself. Evidence that the pile extension material was already composting came in the form of a steady release of steam from the top and the clear feeling of heat when I put my hand on top. This is the first year in which I have found such obvious proof of heat generation. Generally, to the touch, the exterior of the pile is not perceptibly warmer than the outdoor air.
Between rain showers, I was able to fasten some Reflectix around the pile. This particular type of insulation is ideal for use on a biomeiler. It can reflect infrared heat back into the pile, reduce the cooling and drying effects of air blowing over the surface, prevent rain or melting snow from entering the pile, and discourage squirrels from digging in.
If you consider also that it is lightweight, easy to cut, and can be fastened with duct tape and twist ties, you must admit there is no better material available.
Today, after church, I added a final load of leaves to the center of the pile and covered the top with Reflectix. At this stage, it looked as if I had constructed an enormous “Jiffy Pop” container. The final dimensions of the pile are: 12’ in diameter, 6’4” high at the perimeter, and 7’ tall in the center.
I finished the job by covering it all with a tarp (seen in the main picture for this post). The rougher texture of the tarp will make it easier for snow to accumulate on top of the pile. The Reflectix is too slippery to allow for a stable snow cap.
There are a few details to tidy up, but the pile is now completely functional for the winter ahead. Since the internal temperatures of the pile are at record highs, there is plenty of justification to experiment with extracting heat through the PEX piping buried in the pile. However, that line of research will have to wait while I cross off some tasks waiting above it on my to-do list.
Whew! You made it to the end before snow sets in for the winter.
I see only a smattering of duct tape here and there holding down the top tarp. Will heat escape from beneath the tarp and be wasted?
The tarp only serves to help hold down the Reflectix before the snow comes,
and to be a rougher surface to help keep the snow from sliding off the pile.
The tarp has no insulative value.
There is no way to avoid losing some heat to the environment.
The mantle, the cap of leaves, and the Reflectix are all there to insulate the cores.
That is the best that can reasonably be done to minimize wasted heat.