Field Day Report: Compost heat at TableTop Farm
How many field days greet you with a bluegrass band? When you nest a PFI field day within a Farm Cruise, some unexpected things can happen! 30 people came out to TableTop Farm to see the compost heat extraction system and hear how it integrates with the farm’s high tunnel production and compost process. Sally Gran, owner and operator of TableTop Farm, and Rich Schuler, PFI’s energy consultant and project engineer, led the field day.
Gran kicked things off describing the history and details of production at TableTop, including their past experience trying to heat the high tunnel for late season production. They had installed a propane furnace that used a lot of energy – both in BTUs and Gran’s patience. It was installed high on the wall of the tunnel, which allowed a lot of the heat to quickly escape through the ceiling. This year they removed the furnace and removed the large circulation fan over the door, instead replacing it with a vent.
After discussing the use of the high tunnel for winter greens production, Schuler took over the field day to explain the design goals, construction, and use of the compost heat extraction system. As reported on the Practical Blog and in the Practical Farmer (here and here and here and here), the compost heat extraction system uses heat generated from the composting process to heat water, which then is pumped through pex tubing under an insulated area of soil in the high tunnel. In the initial design, the compost was aerated only from blowers on the bottom of the chamber (simulating turning). Schuler, however, could not keep the middle of the pile from overheating, and eventually had to install a coil to move heat from the center of the pile.
During the cool spring of 2014, the compost unit was “switched on” proving a five-degree bump to the soil temperature. According to Schuler, the system is an example of using “sips of energy” rather than “gushes” that a furnace or boiler would use. It takes longer to re-heat the water after it has circulated through the soil, but the energy is truly free, renewable, and is generating a useful product for the farm from their own vegetable scraps (and calf bedding from the farm next door). A win-win-win for energy, the environment, and TableTop’s bottom line. Schuler emphasized, however, that this model is a “Cadillac prototype”. It’s fancy, and was expensive to build. He hopes the next iteration of this system will provide 80% of the heating potential at 20% of the cost of the current system.