The case for building-integrated hydroponics

Cultivation of crops such as tomatoes or lettuce in a modern hydroponic growing system reduces water consumption, requires no soil, doubles the growth rate, and enables year-round production. The product is healthier than field agriculture, because pesticide use is often unnecessary, and contamination from soil or airborne pathogens is nearly eliminated. On Mediterranean islands, high import costs, poor soils, and limited water supplies further favor hydroponic vegetable production. Effective hydroponic cultivation requires intensive regulation of the greenhouse environment. Typically, a combination of evaporative cooling, fossil fuel heating, forced draft ventilation, and natural transpiration is employed in greenhouses to maintain interior temperature and relative humidity in the ranges 18-24C and 30-70%, respectively. By mounting a hydroponic greenhouse on the roof of a small (e.g. two-storey) building of the same plan area, considerable energy savings may be realized for the building, particularly in dry, warm climates where evaporative cooling is effective. In winter, solar heat gain in the greenhouse can be shared with the building, in some cases eliminating the need for additional heating. In summer, the building is shaded from solar gain, while an evaporative cooling system in the greenhouse serves both structures. A simplified spreadsheet model was constructed to estimate annual energy savings. The model requires specification of ambient temperature, humidity, and insolation for a diurnal cycle in each season of the year, together with various physical properties of the building and greenhouse. The model yields the daily heating and cooling loads and the evaporative water demand. For a climate roughly similar to that of southern Greece, the model predicts that the building cooling load would be less than 9% of the total cooling load for the combined structure. Various arrangements for meeting this load through evaporative cooling are discussed.