The outdoor air temperature in the month of May was in the upper 40sF for the first week of the month, then jumped to the 50s and low 60sF. The biomeiler has fully switched to air conditioner mode and has steadily chilled the incoming air to 37F for the entire month. In consequence, the coop temperature has generally been only 10-15 degrees warmer than the outdoor air temperature.
The chart below shows the temperatures measured in the biomeiler at a height of 4’ above the ground. This height marks the boundary between the pre-existing biomeiler and the recent extension.
The data were collected by a thermocouple buried in the pile at that height at the very center of the inner core, another placed halfway between the inner and outer walls of the outer core, and a third similarly placed between the inner and outer walls of the mantle. The air temperature was measured on a thermometer in the open air.
The chart indicates that composting has all but ended. The chart looks rather empty because there was no aeration to show and some of the thermocouples showed the same temperatures, leading to the lines covering each other.
In the post “April Experimental Results, part 2,” I expressed my desire to investigate the biomeiler to determine whether or not an ice layer has formed at the top. Recently, I was able to uncover the top of the pile.
It was immediately clear that the outer core had subsided well below the elevation of the mantle. The inner core had done the same, but to a lesser extent. I suspect the difference is partly explained by the materials having originally been piled to a greater height in the center.
After making a few excavations it became clear that an ice layer is, indeed present at a depth of approximately 9” below the surface over the inner and outer cores, but not over the mantle or crust. In the image, the thermometer dial indicates 32F.
The process of composting can create its own convection as warmer gases rise up through the pile and fresh air is drawn in from the sides to balance the air pressure. However, it is a moderate surprise that it happened in the biomeiler. I would have guessed that the thick, insulating mantle and crust layers, composed of sawdust and wood shavings, would greatly reduce or eliminate any natural convection. There were a few occasions during the winter in which I used mechanical aeration, but the amount of time was hardly sufficient to form the volume of ice present. This observation leads me to believe that mechanical aeration is not necessary for a biomeiler of sufficient volume.
Discovery of the ice layer means that another observation can now be better interpreted. That the same cessation of composting has taken place two years in a row at precisely the time of year when the outdoor air temperature has warmed enough to make the heat unnecessary adds support to my hypothesis about a throttling process taking place within the biomeiler.
The process can be described as follows:
- Composting heats the gases within the pile and their reduced density causes them to rise to the top of the pile.
- Moisture is carried upwards with the warm gases and condenses under the insulation covering the surface of the pile.
- The liquid water freezes at the surface of the pile, forming an ice layer.
- When the biomeiler heats up too much or when the outdoor air temperature gets high enough, ice melts and cools down the composting.
- When the biomeiler cools down or the outdoor air temperature drops, more ice is formed and the latent heat of fusion is released into the pile, warming the interior.
Line 4 explains why the biomeiler changes to air conditioning mode in the spring. It will take further experimentation to discover if line 5 would lead to an automatic restart of the heating mode in the fall. To date, I have never let the biomeiler run unaltered for two winters in a row. It has always been too tempting to dissect the pile or make an update to the experiment. It is currently an open question as to whether or not I will show sufficient restraint this fall.
