NEVADA, Iowa — More specialty crop producers are using high tunnels to protect crops and to expand the growing season to capitalize on early- and late-season markets.
High tunnel owners may pay for supplemental heat to protect early- and late-season crops. For many fruit and vegetable farmers, these added costs are another drain on the budget at a time when cash flow is tight.
What if farmers could use one system to get both compost and heat? An innovative on-farm project by Practical Farmers of Iowa uses waste heat from composting to protect crops inside high tunnels during cold spikes. The project uses "trickle charging" to heat soil inside a high tunnel.
The system started heating a portion of the high tunnel soil shortly after the crop was planted in early April at TableTop Farm near Nevada. The system increased soil temperatures by 5 degrees during the mid-April cold snap. Sally Gran, who runs TableTop Farm, said the boost was a nice in comparison to her heating costs last year. Such a system could become viable for other fruit and vegetable farmers to cut costs and better manage temperature swings.
"Last year we installed a $1,400 propane heater in the high tunnel and spent an additional $1,200 on LP," Gran said. "The cost was unsustainable for the farm budget and didn't really fit with our energy use philosophy."
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How the system works
Rich Schuler, energy consultant with PFI, said trickle charging the soil in a high tunnel works the same way as in a car battery: A large battery delivers a flood of current to an electric motor when the engine is started; the depleted battery is then charged by the alternator when the car is driven. The compost system operates the same way. An insulated 1,100-gallon water tank acts as the battery, which is trickle-charged by the heat extracted from the aerobic composting process over time. When heat is needed in the high tunnel during a cold snap, a pump moves hot water from the tank into tubing in the soil. The pump is turned off at the end of the cold period, and trickle charging of the water tank resumes.
Most materials for the system are common hardware store items: plywood, foam board insulation and PVC pipes. Other components include the pumps, blower, stainless steel tubing and four 275-gallon food-grade plastic totes.
Eco-friendly energy
"The system was designed to use very little electricity," Schuler said, adding that it's been operating off-grid on a single 140-watt solar PV panel. "An important goal of the project is to put alternative hardware in the field that is not nearly the cost of other renewable technologies. Materials for the prototype system cost roughly $7,000, including the solar PV components. The next goal will be to build a system with 80 percent of the performance of the prototype, at 20 percent of the cost."
Another benefit of the project, he said, is that it produces high-quality compost directly on the farm with less labor, machinery and fuel than most methods. Aerobic composting requires the right mix of ingredients; the right conditions; and proper management to yield high-quality compost.
A blower provides an adequate supply of oxygen to the compost and works with two heat exchangers to stabilize the compost temperature between 131 and 150 degrees.
"After the warmest phase of the composting process, the material in the compost chamber is moved outside to finish curing before field application," Schuler said. "The system is then loaded with fresh material, and the process starts over again."
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PFI's Energy Program is designed to help farmers reduce off-farm inputs and carbon footprints. The compost heat project is funded by The Ceres Foundation, Soper Farms and PFI members.
