"Rock Bank" Keeps Greenhouse Warm

"Rock Bank" Keeps Greenhouse Warm BUILDINGS Miscellaneous 34-2-34 Steve Schwen markets vegetables nearly year round. Thanks to his "rock bank" greenhouse, he can do this in Minnesota with no outside energy costs. The 60 by 30-ft. deep greenhouse collects extra heat during winter days and stores it in river rock beneath 5-ft. wide growing beds. "We try to plant the beds to cold hardy crops for late winter, early spring sales," says Schwen. "It will get close to freezing during the night, but not enough to kill the plants, even when it's coldest outside." As winter crops are harvested, tomatoes and peppers are planted for late spring sales, months ahead of outside production. Schwen designed the greenhouse with a 15-ft. deep workroom behind it. Living quarters for farm interns take up second floor space. During the day, heat naturally rises to a collection area above the peak of the greenhouse. Here it's collected in a black, 8-in. collection duct. A solar-powered fan pulls it down and into a 10-in. dia. manifold buried down the center of the greenhouse. A series of 4-in. dia. perforated loops disperse the air to either side through 1 to 1 1/2-ft. deep beds of 1-in. smooth rock. The hot air heats the rock and moves upward through 2-ft. deep planting beds of sand/soil/compost mix. "We covered the rock beds with used Remay row cover to help keep the planting mix from filtering in over time," explains Schwen. Six 4-in. standpipes are situated around the greenhouse to release heat into the air on cold nights. Sensors in beds monitor soil and rock bed temperatures. A thermostat in the roof peak triggers the fan when the temperature there is higher than bed temperatures. However, if soil bed temperatures get too high, the fan stops, and excess heat in the roof peak is vented into the living quarters or through the roof. Schwen notes that the greenhouse is important to the farm in the summer as well. This past year, tomatoes and peppers in the greenhouse started producing in late spring and continued late into the fall. "We've sold a lot of tomatoes, peppers and cucumbers from the greenhouse when they weren't very good out of the field," he says. The entire greenhouse with thermal banking cost about $45,000, not counting labor. A $5,000 grant helped cover some of the costs. At the time he estimated that energy savings over conventional heating would pay for the building in about 10 years. Since it was built more than 10 years ago, current savings go right to the bottom line. Contact: FARM SHOW Followup, Steve Schwen, 67011 Hwy. 63, Lake City, Minn. 55041 (ph 507 753-2080; earthenpath@riverland.org; www.oakcentergeneralstore.com).

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