Cost benefit optimization of cistern volume and green roof area in the florida showcase green envirohome (FSGE)

Most water conservation and energy savings strategies for residential homes have higher first cost (more capital) than traditional spending. However, the added benefits of these greening strategies can outweigh the increase of initial capital cost at the end of the house life time. A cost-benefit optimization model was formulated to demonstrate the optimal design strategy with a typical Florida residential home (shingle and metal roofs) to synergistically integrate a green roof with gray water/stormwater reuse system. The objective of this model is to find the optimum green roof area without making the home cost more over a specific life time, such as 50 years, than a traditional one through energy saving and water conservation. We found out that when the energy savings was varied, the area of the green roof increased to an upper limit due to the irrigation requirement of the green roof. Due to the variability of the rainfall, a chance constraint was added to the model to address stormwater harvesting uncertainty. Research finding proved such a concept. As the risk level is varied, the area of green roof only changed by about 4.4% and 5.3% for the shingle and metal roofs respectively.