The temperatures in the biomeiler pile are measured with thermocouples buried in several locations. Nine of them are found at three heights (1’, 2’, and 3’ above the ground) in the inner core, outer core, and mantle of the pile. The graph in today’s post shows the temperatures measured by thermocouples at 2’ height. These sensors are buried 2’ below the upper surface of the pile. A sheet of Reflectix, a tarp, and some snow lie above the top of the pile.
Interpreting this graph presents a challenge. The temperatures measured in different places were often identical, with the result that the lines on the graph frequently overlap. To make better sense of the situation, look at the order of the lines shown in the legend of the graph. The lines listed lower are drawn after the upper lines, and will overwrite them if the values are the same.
In the last post, I disclosed the discovery of an ice cap lying below the upper surface of the pile, covering the inner core, outer core, and half of the mantle layer of the pile.
In previous updates, I have described how the general trend of the pile temperatures has been the inverse of the outdoor air temperatures. The remarkably-steady temperature in the air vent was a prominent indicator directing me toward the conclusion that water in the outer layers of the pile was freezing during cold spells and the phase change from liquid to solid was releasing heat into the air vent and inner regions of the pile.
That explanation was sufficient to explain the cold-spell half of the cycle of temperature changes. However, it did not explain why the deeper regions of the pile were cooling during warm spells. Heat lost to the air from the surface during cold spells should be replaced by heat from the air near the surface during warm spells, rather than pulled from the depths of the pile, through a great thickness of insulating wood chips. Discovering the existence of the ice cap allows me, now, to form a hypothesis about the warm-spell half of the temperature cycle.
It was my presumption that, inside the pile, heat moves primarily by conduction and that radiative heat transfer is minimal due to the low temperatures of the materials. There seemed to be no possibility for convective heat transfer because the wood chips and other solid materials cannot move to carry heat to new locations.
However, the size of the ice cap indicates that the use of mechanical aeration has led to a considerable flow of moisture upwards through the pile. It is not convection in the usual sense of the term, but it has the equivalent effect.
Aerating the pile from below has been lifting water vapor to the surface of the pile where it encounters temperatures cold enough to freeze it. When the outdoor temperatures rise, or when the internal temperature of the pile rises, some of the ice melts. Meltwater drains into the pile from above, absorbing heat and cooling the internal temperatures as it moves downward. The spring temperatures are causing the ice cap to melt so fast that the composting process has been stopped. The pile is unlikely to begin composting again until the ice cap has melted away and enough heat has been absorbed from the environment to bring the internal temperatures high enough for the bacteria to thrive.
The ice cap, then, functions as a throttle for the biomeiler! When the outdoor air temperature drops or the pile temperature drops, ice forms and releases heat into the pile. When the outdoor air temperature rises or the pile temperature rises, ice melts and cold water runs down through the pile, cooling it.
I will continue monitoring temperatures in the pile to investigate my new hypothesis as the throttling action of the ice cap adds a number of considerations to the use of the biomeiler.
