Building-integrated greenhouse systems for low energy cooling

Installation of an evaporatively cooled hydroponic greenhouse on the roof of building can yield net energy savings for the combined structure, when compared to conventional air conditioning, and can conserve space by adding productive capacity to the rooftop. The proposed system offers energy and water savings far exceeding the levels achieved by traditional green roofs, but requires circumstances that favor co-location of a technically sophisticated agricultural facility with the building. Compared with field agriculture, the hydroponic cultivation of crops in greenhouses sharply reduces water consumption, requires no pesticides, uses several times less land, and doubles the growth rate. Integration of hydroponics into the built environment also sharply reduces the distance from farm to consumer, saving transport costs, reducing waste, and increasing product quality. A 120 m2 low-energy greenhouse was constructed in 2006 on a barge platform in the Hudson River Estuary, a temperate eastern North American climate. A staged cooling algorithm escalates from passive ventilation to active ventilation to evaporative cooling as a function of internal and external temperature. In a Mediterranean climate, a temperature differential as large as 15 C can be achieved by evaporative cooling in arid summer conditions. Placed on a building, the greenhouse could serve as a complete HVAC system, although higher air velocities will require customized design of building ventilation systems. A hypothetical 720 m2 building of two stories in a Mediterranean climate is analyzed using a highly simplified spreadsheet model. Results indicate that this building adds only 9% to the summer cooling load for a rooftop greenhouse of the same planform area. Required inputs include water (550 tons per year) and power for fans (23 MWh per year), but these inputs for the combined system are lower than the energy cost to cool the building alone using conventional air conditioning. For this purpose, it is assumed that the greenhouse is independently viable as an agricultural venture.