Green Roofs - Living Architecture Monitor - Winter 2009 - (Page 23) TABLE A comparison of the quantity of water used to produce the electricity required for a ton of cooling capacity in an air conditioning unit versus the quantity of water required for the same cooling capacity on a green roof. Cost in water per ton of Thermoelectric Ratio of AC to air conditioning by a gallons of water green roof cooling green roof cost per ton AC or per 84.4 kWh New York Philadelphia Chicago Seattle Average 33 33 33 33 33 72 46 89 24 58 2.2 1.4 2.7 0.7 1.75 lent of a ton of air conditioning into latent heat production through evapotranspiration, with values for thermoelectric water varying from almost three times green roof values, to a slightly lower quantity, in the case of Seattle. In dense urban centers, however, there is an additional cost of air conditioning with HVAC units. The production of waste heat, under typical urban conditions of multiple units in close proximity, has a negative impact on the thermal load in neighboring air conditioning/hvac systems. Discharge heat from one unit contributes to the total heat input into adjacent and nearby units, increasing the amount of heat that a given unit needs to remove, proportionally increasing air conditioning costs in the process. Green roofs have the opposite effect. Cool air exported from green roofs acts to cool the surrounding spaces, decreasing the temperature of intake air of neighboring air conditioning units, increasing their efficiencies and decreasing costs. This effect depends on a sufficient quantity of water supplied to vegetation on a daily basis to allow the Bowen Ratio to achieve higher values, i.e. greater proportions of heat removed by moving water from liquid to vapor phase than by direct convective heat flow into the surrounding atmosphere and physical environment. THE METRICS OF HEAT REMOVAL One ton of refrigeration or air conditioning is linked to the quantity of heat required to melt one ton of ice in a day, the equivalent of removing 12,000 BTUs in one hour, or 288,000 in a 24-hour day. To convert to metric units, 1 btu = 252 calories. A ton of air conditioning is thus equivalent to 72,576,000 calories a day or 84.4 kWh. Since each gram of water that evaporates takes with it 580 calories, the heat loss equivalent to a ton of air conditioning is the evaporation of 125,131 grams or 33 gallons of water at 30°C (86°F). This, in turn, is the equivalent of the evaporation of a centimeter of water over 140 sq.ft., or about a third of a centimeter over 400 square feet. Analyses presented below assume: a theoretical 400 sq.ft. room; use of the conversion factors presented above; New York State average cost per kWh; and quantifying the average number of btus necessary for an efficient AC window unit to cool such a room. Thus, it takes, on average, a 10,000 btu/hr unit to cool an unoccupied 400 sq.ft. area to an average target room temperature. This equates to 240,000 btus/day, 60,480,000 cal/day, or 0.83 tons of ac. Using the September 2008 average cost per kWh for all sectors in New York (residential, commercial, industrial, transportation) at $0.1763/kwh, cooling the room http://www.nrca.net/nrcauniversity http://www.nrca.net/nrcauniversity
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