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The YMCA Solar Greenhouse in Blacksburg, Virginia, USA, uses a novel way to store energy collected from the sun: a subterranean heat sink of soil, rocks and water beneath its interior planting beds. This system is called the Subterranean Heating and Cooling System (SHCS), and it collects solar energy and stores it for use when the sun is not shining. When outside conditions are very cold, heat is stored during the day in the ground and walls of the greenhouse and released during the night to keep the greenhouse air warm. Other solar greenhouses typically store the sun’s energy in water barrels and/or rocks along the north wall inside the greenhouse - taking up valuable floor space. Conventional greenhouses are also constructed on a long north/south axis, with glazing on both slopes of the roof; conventional greenhouses tend to overheat when the sun is shining and get too cold during winter nights. This 18'x32' solar greenhouse is constructed with its long axis running east and west, instead of north/south; it employs a heavily insulated north roof as opposed to a transparent or translucent roof.
The south glazing is 10-mm x 4' x 20' double-walled polycarbonate. The passive solar greenhouse must capture enough solar radiation for the photosynthesis process for the plants, with interior climatic conditions that support growing vegetables year-round. During winter, the greenhouse collects enough solar energy through the south-facing translucent polycarbonate in the subterranean thermal mass during the day to ensure that the vegetables do not freeze at night. The goal is to minimize the temperature fluctuation between day and night to reduce thermal stress to the plants. Overheating during the day is prevented using venting windows that are located high on the east and west ends of the building. Ventilation also helps regulate humidity which, in turn, helps limit diseases and pests in the greenhouse. As far as is known, this is the first large solar greenhouse using the SHCS east of the Mississippi in the United States. Rainwater is collected off the south side roof into a 2090-gallon cistern; a pressure tank is fed from the cistern to irrigate plants. The goal of this greenhouse is to encourage homeowners, neighborhoods and local farmers to boost production of fresh, local food.
Basic features for a functional, efficient solar greenhouse:
1. Collection of the maximum amount of solar radiation during the day to facilitate plant growth
2. Efficient storage of solar heat collected from the sun’s radiation during the day in the subterranean heat sink
3. Slow release of the heat to the interior of the building during the night through natural thermal radiation
4. Reduction of heat losses by insulation of the solid surfaces of the greenhouse, including a soil berm that runs the length of the north side
5. Ventilation capabilities to exhaust hot air if needed to prevent overheating and moisture build-up
Subterranean Heating and Cooling System
Water and heat energy are stored under the 16-inch-deep planting beds in a three-foot layer of rocks with three layers of slotted 4-inch pipes in the rocks. When the air is hot and humid (>70º) a fan on the west end pushes the SGH air into the pipes where the water condenses into the rocks with its heat of condensation and the air comes out at the east end cool and dry. When the air is cold and dry (<50º) the fan pushes the SGH air into the pipes where water evaporates into the air using the heat stored in the rocks, and the air comes out at the east end warm and humid. Thus the temperature and humidity are kept in a range beneficial for plant growth during winters. For extra cooling when needed, there are small vent windows high on the east and west ends that open when the temperature exceeds 90 degrees. An exhaust fan at the west window turns on when the temperature there exceeds 95 degrees.
The South side picks up the largest amount of solar energy. The polycarbonate panels are set onto wood 2” x 6” framing; the surface is sloped for maximum solar gain at Blacksburg’s latitude. The panels cover an area 20 feet long by 10-mm thick double-walled polycarbonate with ribs separated by 1 cm. The polycarbonate panels are translucent, providing a diffuse light, allowing them to transmit the majority of incident solar radiation into the greenhouse. This warms the interior space, the insulated walls, the ground and heat sink below the planting beds, and is absorbed by the vegetables.
The foundation walls in the ground are insulated concrete forms (ICF) comprised of a sandwich of rigid insulation on the outsides and eight inches of concrete inside, as well as waterproofing. The north wall (ten feet tall) and portions of the east and west walls are buried into a hillside soil berm that mitigates temperature extremes. Walls on the east, west, and north sides are covered with Hardie Board fiber-cement panels inside and out, and, insulated. The south foundation wall is six feet tall. Greenhouse designer David Roper says that in future iterations of this solar greenhouse, he would not insulate the inside of the concrete walls; instead, he would double the insulation on the outside to provide more thermal mass inside the greenhouse.
Wall & Ceiling Color
The inside of the west, east and north wall, and north ceiling are painted white to maximize reflection of solar radiation during the day.
The north roof joins the south face at a central ridge line; it has a steeper slope than the south side. In winter, when the sun has a low elevation angle, this angle optimizes the solar radiation absorption on the inside surface. The roof is comprised of 2"x10" rafters ad 2” x 8” ends with closed-cell spray foam insulation in the voids and Hardie Board inside and out. This roof also helps mitigate cold air infiltration against northerly winter winds.
Door & Ventilation
The door is located on the east wall; this helps reduce cold air infiltration as prevailing winds are generally northwesterly in winter. On sunny days, the air in the greenhouse can become too warm, and overheating can damage the vegetables and encourage diseases and pests. To mitigate overheating, operable, thermostat-controlled venting windows are located high on the east and west sides of the building so that as warm air rises, it is exhausted through these small openings. Greenhouse designer David Roper says that one lesson learned would be to install much larger vent windows for greater cooling capabilities.
According to Roper, there were several additional lessons learned; for example, in future designs he would connect the ends of the four-inch slotted drain pipes closer to each other at the 24” end pipes, better brace the Hardie Board that separates the rocks under the walkway from the planting soil, and place soil inside the greenhouse after the entire inside of the structure is completed. He also recommends that the structure be built before the heat sink is built, to protect the heat sink from inclement weather
The YMCA at Virginia Tech Community Gardens
The YMCA at Virginia Tech in the Town of Blacksburg, Virginia, maintains a 15-acre area for the YMCA Community Gardens, designed by Steve Somick of Anderson and Associates of Blacksburg, Virginia. Eventually the growing space inside the solar greenhouse will be rented to gardeners, similar to the renting out of garden space in the rest of the Hale-YMCA Community Gardens. The solar greenhouse was designed by L. David Roper, and the architectural drawings were made by Colley Architects. Engineering calculations were done by Truesdell Engineering. The construction was managed by Green Valley Builders, with several other construction companies and many volunteers helping.
YMCA Solar Greenhouse Slides 2012 (6,624 kb)
YMCA Solar Greenhouse Handout (125 kb)