Solar Greenhouse Uses Underground Heat Sink

http://www.solaripedia.com/13/401/YMCA+Solar+Greenhouse+Heats+Up+in+Virginia.html

As food security becomes an issue everywhere, more and more people are opting for the ability to raise their own food year-round. In the Appalachian Mountains, in Blacksburg, Virginia, the local YMCA has built a solution for climates with cold winters.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. 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 on the north side. 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 solar greenhouse was designed by retired physics professor, Dave Roper. At an estimated cost of $35,000USD in 2010, the greenhouse project could seem out of reach to the average homeowner, but Roper says the design can be adapted to smaller spaces and could be built using recycled materials, which could bring costs down.