Cold-climate solar greenhouse

March 2, 2011

By Drs. Robert & Sonia Vogl
President and Vice President
Illinois Renewable Energy Association

The possibility of a cold-climate greenhouse appealed to us after seeing the hoop house our relatives near Ashland, Wis., acquired with a 40-acre homestead. Sitting around the the woodstove paging through books about cold-climate greenhouses stimulated our interest even more. Upon returning home, we searched the Internet and closely examined an example from India and another from China.

Solar greenhouse. (Photo by Dr. Sonia Vogl)

We discussed the concept with Vic Zaderej, who designs and builds solar homes. We had been discussing building a 1,000-square-foot, energy-efficient solar home, and soon determined we should build a solar greenhouse based on the same principles to further refine the design for a future project.

We decided to build a greenhouse that was 8 feet by 24 feet with a 10-foot-high sloped roof. A building that size could be transported on a large truck bed.
With one experienced carpenter and half a dozen volunteers, we estimated we could complete the project over a three-day weekend. Unfortunately, some key crew members were unable to stay for the third day. The building was enclosed, but it took several weeks for volunteers to finish.

Since our intent was to construct a greenhouse in which to grow winter vegetables using little to no supplemental heat, it is extremely well insulated with R-38 walls, floor and ceiling. Rafters and floor joists are constructed with 2-by-12s. Walls are 2-by-10s on 24-inch centers with openings for double-pane windows on the south and an insulated door on the east.

The entire structure rests on 4-inch-by-4-inch timbers, which should allow space to lift it and either move it or place it on a truck bed.

Two of the four 4-foot-by-5-foot windows on the south wall can be opened for fresh air or to cool the building. The roof, sides and interior are metal. The north wall is covered with phase-change material that melts at 72 degrees Fahrenheit in an attempt to limit heat buildup in fall and spring and release heat back into the room in winter.

Beets growing after cold winter nights. (Photo by Dr. Sonia Vogl)

DC LED lighting is hung above the planters growing four winter-hardy vegetables. Two solar panels provide electrical power that is stored in four sealed batteries in the building.

Four sheets of 2-inch foam insulation are pressure-fitted into the interior window space for nights and cloudy days. A 60-watt incandescent AC-powered light adds warmth.

Since the construction was delayed, our major focus has been on recording daily temperatures and making minor adjustments to establish its performance until May when outdoor gardens take over.

On some sunny October days, the interior temperature with closed windows reached 115 degrees Fahrenheit, clearly too high for plant growth. With appropriate exterior shading and opening windows, we should keep the temperatures from reaching that level.

When this winter’s coldest temperature reached -20 degrees Fahrenheit, the building’s interior temperature only dropped to 26 degrees Fahrenheit, suitable for winter vegetable survival. We intend to add more wattage to retain a higher temperature.

Next fall, serious efforts raising vegetables through winter will begin. Now, we will be satisfied nibbling beets, carrots, radishes and arugula.

Drs. Robert and Sonia Vogl are founders and officers of the Illinois Renewable Energy Association (IREA) and coordinate the annual Renewable Energy and Sustainable Lifestyle Fair. Dr. Robert Vogl is vice president of Freedom Field, and Dr. Sonia Vogl is a member of Freedom Field’s Executive Committee. E-mail sonia@essex1.com.

From the March 2-8, 2011, issue

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