Green Roofs - Living Architecture Monitor - Winter 2009 - (Page 25) FIGURES 1&2 COST IN WATER PER TON OF AIR CONDITIONING BY A GREEN ROOF THERMOELECTRIC GALLONS OF WATER PER TON AC OR PER 84.4 kWh COST OF 33 GAL WATER-TON OF AC GREEN ROOF COST OF 84.4 kWh -TON AC $14.00 $12.00 $10.00 $8.00 $6.00 $4.00 $2.00 $0.00 NEW YORK PHILADELPHIA CHICAGO SEATTLE 90 80 GALLONS OF WATER 70 60 50 40 30 20 10 0 NEW YORK PHILADELPHIA CHICAGO SEATTLE WINTER 2009 FIGURE 1 COST IN U.S. DOLLARS OF ELECTRICITY USED IN AIR CONDITIONING UNITS IN BLUE; THE COST OF POTABLE WATER USED BY VEGETATION ON GREEN ROOFS TO PRODUCE A TON OF COOLING CAPACITY IN GREEN. FIGURE 2 QUANTITY OF WATER USED IN ELECTRICITY PRODUCTION FOR AIR CONDITIONING IN BLUE; THE QUANTITY OF POTABLE WATER USED BY VEGETATION ON GREEN ROOFS TO PRODUCE A TON OF COOLING CAPACITY IN GREEN. than reversed in vegetated areas and on green roofs. Local temperatures here can drop below ambient because the majority of heat flow is partitioned into latent vs. sensible heat. Evaporating water, together with shade production, interact in the export of latent heat and cool air, lowering local temperature. These effects have the additional potential of decreasing input air temperature into ac/hvac systems, thereby increasing efficiencies and decreasing electrical usage and energy costs. In sum, electric costs for air conditioning are 51 times greater in the City of New York than water costs for an equivalent quantity of heat removal by green roof systems using the drinking water supply. Full-cost accounting may increase this differential, since the quantities of water needed to produce a given amount of cooling capacity through electric power are similar or larger than the quantities of water needed to directly cool urban areas. It is critical to note here, however, that costs drop precipitously, and may become negative, where stormwater, greywater, treated wastewater, or other process waters are used. D Dr. Paul S. Mankiewicz, executive director of the Gaia Institute, received his Ph.D. from City University of New York/New York Botanical Garden Joint Program in Plant Sciences. He holds patents on a modular composting system and an ultralightweight green roof soil. Past president of the Torrey Botanical Society & board member of the NYC Soil & Water Conservation District, he is presently completing the first greywater treatment green roof in NYC. Peter Spartos received his B.Sc in Environmental Science from the University of Vermont in 2007 where he participated in research programs with Drs. John Todd, William Bowden, and Aleksandra Drizo addressing petroleum alternatives, stormwater runoff, and farm effluent treatment. He is project designer manager at the Gaia Institute. Eric Dalski graduated from Washington College with an Economics degree, focusing on Natural Resource Economics, and is currently enrolled in a Certificate Program in Conservation Biology at Columbia University and now works with the Gaia Institute as project planning and integration manager. ASSUMPTIONS The electricity for a ton of air conditioning (ac) in New York costs $13.50. The evaporation of 33 gallons of water produces a ton of ac. In New York City, thirtythree gallons of water costs 26 cents. Every ton of air conditioning requires 84 kWh to produce; in four major cities, this quantity of electricity takes between 24 and 89 gallons of water to generate. All costs in u.s. dollars (usd). LEARN MORE: The full technical paper for this article will be presented at the 7th Annual Greening Rooftops for Sustainable Communities Conference, Awards, and Trade Show June 3-5, 2009 in Atlanta, Georgia. Visit www.greenroofs.org for more details. LIVING ARCHITECTURE MONITOR 25 http://www.greenroofs.org
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