Innovative system uses composting process to heat high tunnel
By Audrey Alwell, MOSES
High tunnels allow vegetable farmers to extend the growing season, but often require supplemental heat to protect plants during spring and fall cold spikes. An innovative project by Practical Farmers of Iowa (PFI) uses the heat generated by aerobic composting to help warm the soil in a high tunnel. This prototype system could become a viable way for fruit and vegetable farmers to cut costs and better manage fickle early- and late-season temperature shifts.
PFI Energy Consultant Rich Schuler developed the compost heat extraction system that was installed late last fall at TableTop Farm in Nevada, Iowa. During the mid-April cold snap, the system effectively increased soil temperatures by 5 degrees, equivalent to more than 5 gallons of liquid propane for the 300-square-foot test plot.
“Five degrees can be the difference between a plant living and dying,” said farmer Sally Gran. Gran had used propane heat the year before, spending $1,400 on the heater and another $1,200 on propane. “The cost was unsustainable for the farm budget and didn’t really fit with our energy-use philosophy,” she added.
The compost heat system is made mostly from common lumber yard materials: 2x4s, plywood, foam insulation board, PVC pipes, and rubber hoses. Other components include the pumps, blower, stainless steel tubing and four 275-gallon food-grade plastic totes.
The water-filled totes act like a 1,100-gallon battery trickle-charged by heat extracted from the compost chamber. Heat from the composting process, which reaches an average of 145 degrees, warms water in two “heat exchangers”—a stainless steel tube placed in the center of the compost pile, and a set of copper tubes in the chamber’s “exhaust system.” A blower forces air into the bottom of the compost chamber, and as it passes through the compost, the air picks up heat and humidity. As that warm, moist air exits the chamber, it transfers heat to the copper tubes in the exhaust system. Both heat exchangers convey heated water to the tanks.
When heat is needed in the high tunnel during a cold snap, a pump moves hot water from the tanks through tubing in the soil beneath the high tunnel—essentially working like an in-floor heat system.
“The tanks are the energy storage system which allows a gush of heat to be delivered in a short time from the continuous trickle of heat extracted from the aerobic bacterial colony,” Schuler explained. The pump is turned off at the end of the cold period, and trickle charging of the water tanks resumes. The entire system is powered by a 140-watt solar PV panel.
Materials for the prototype system cost roughly $7,000—a price tag Schuler is determined to reduce in order for the system to be affordable for farmers. His goal is to build a system with 80 percent of the performance of the prototype at 20 percent of the cost.
“In short, I want the design to be ‘right’ before the system is published as a DIY compost/heat system for farmers,” he explained. “I view the system as an onion with layer after layer—each one affecting the next. Since it’s a prototype, I’m still working out the details between the layers.”
The prototype’s compost chamber is 5 cubic meters. It was “scaled” from a 1/2-cubic meter proof-of-concept model. Both sizes worked well, Schuler said. The amount of heat captured is directly related to the volume of the compost chamber. “A 50 yd3 system would capture and store roughly 500 times the heat energy of a 0.5 yd3 system (in the water tank),” he added.
Besides providing heat, the compost chamber also produces high-quality compost directly on the farm with less labor, machinery and fuel than most methods, Schuler said. He explained that aerobic composting requires the right mix of ingredients (carbon, nitrogen and water); the right conditions (temperature and oxygen); and proper management to yield high-quality, nutrient-rich and biologically-diverse compost. If a compost pile or windrow is not periodically turned to allow oxygen to enter, the entrained oxygen will be depleted, and it will become anaerobic.
“The PFI compost heat system is designed to eliminate the need to turn the compost by forcing air through the insulated compost box with a solar-powered blower,” Schuler explained. The blower provides oxygen to the compost and works with two heat exchangers to stabilize the compost temperature between 131 and 150 degrees. After the pile completes its warmest phase, it is moved outside to finish curing before being applied to the farm’s fields. Then the chamber gets reloaded with fresh material, and the process starts over again.
For TableTop Farm, the system means a more sustainable way of farming.
“It’s important for us to be as self-reliant as possible,” Gran said. “Any time we can use re-sources available on our own farm, it lowers our ecological footprint. Saving money on compost and high tunnel heat purchases is an added bonus.”
Funding for the prototype came from The Ceres Foundation, Soper Farms and PFI. Schuler said similar systems could be funded through the USDA’s Rural Energy for America Program (REAP). The deadline for the current year is July 7 for REAP grants, and July 31 for loans.
Schuler is interested in setting up additional prototypes on other Iowa farms. His email is firstname.lastname@example.org.
Audrey Alwell is the Communications Director for MOSES, and Managing Editor of the Organic Broadcaster.
From the July | August 2014 Issue