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Solar Growing Dome

A passive-solar dome greenhouse enables early transplant production without burning fuel for heat

Vegetable transplants for the KPU Farm at the Garden City Lands are produced in a solar greenhouse, which allows seeding to begin in winter without burning fuel for heat.

The greenhouse is a 13 m diameter dome, built from a kit in October 2018 (kit available from Canadian and US-based suppliers). Dome under construction
The greenhouse is designed to operate off-grid. Its dome shape reduces surface area (and associated heat loss) by about 40% relative to a standard rectangular greenhouse enclosing the same area.   Dome in snow
It is glazed with translucent, multi-walled polycarbonate panels (72% light transmission, R3 insulation value). The panels are much stronger than glass, allowing the structure to withstand stronger winds and heavier snow loads than are ever expected at the site. Standing on dome
The north-facing quarter of the dome is opaque, due to additional fiberglass insulation with a silver backing, intended to reflect solar radiation into an 8,500 L pond that serves as a heat sink to stabilize interior temperatures. Curing garlic
The dome sits on a 1 m insulated wall with framed raised beds around most of the interior. Earth tubes (air pipes) with solar-powered fans draw air from the north to the south of the dome, through the soil of the raised beds. Seeding exterior bed
Solar-powered fans attached to a thermostat draw cool outside air into the dome when the interior air temperature exceeds a set threshold. Three large air vents at the top of the structure are attached to wax-filled cylinders, which open and close the vents automatically due to thermal expansion and contraction of the wax. Open vents at top of dome
Early season germination occurs in a chamber constructed of a re-purposed commercial refrigerator with glass doors, heated by a water-filled electric slow cooker on a thermostat. Trays of germinated seeds are then placed on a table covered with electric heat mats. Both of these grid-tied electric heating devices allow targeted warming of small volumes of soil and air, instead of warming the entire enclosed volume of the greenhouse. Germination chamber

Seeding typically begins in late January, and the greenhouse is full of transplants between February and June.

Dome interior with transplants

In summer and fall, the greenhouse provides a warm and dry space for curing farm products, such as garlic, storage onions, sweet potatoes, dried herbs, and winter squash. Curing squash in dome

The greenhouse also offers a unique and versatile gathering space for classes, workshops, and meetings.

Meeting of the Garden City Conservation Society, June 2019

Lilly pads in dome pond Tomatoes in dome Transplants in dome

Dome behind mustard