Green Roofs - Living Architecture Monitor - Winter 2009 - (Page 21) “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.” ENERGY FLOW AT THE EARTH’S SURFACE The earth’s surface receives, on average, about 200 watts per square meter. This quantity varies by orders of magnitude on a daily and/or seasonal basis for each area of the earth’s surface. Vegetative cover and the availability of liquid water, however, directly effect the partitioning of thermal energy virtually everywhere. Following basic conservation laws, energy input equals energy output. Within this context, though, it is critical to understand how energy is partitioned between absorption, re-radiation and latent heat loss locally. This partitioning shunts greater or lesser quantities of heat to storage in ground level structures and the lower atmosphere versus heat exported to the upper atmosphere. This partitioning directly impacts energy usage in buildings by controlling local temperature. The physical chemistry of water has long been known to play a fundamental role in controlling temperature at the earth’s surface. In terrestrial landscapes, the interaction of radiation with vegetation surfaces and water regulates the partitioning of the energy outputs. Where liquid water is not present, sensible heat flow is maximized, moving energy into natural and man-made structures and the lower atmosphere. Where liquid water is present, as modeling of green roofs has shown, a third to an eighth of incoming energy may heat surface materials, with the majority of energy flow dissipated out of the locale as latent heat . This partitioning of water into the vapor phase carries with it 580 calories per gram at 30°C (86°F). By this process, following a typical evaporation rate in the temperate zone of 6 mm of per day , a 10,000 square foot green roof, on an average basis, would partition three billion calories daily into the atmosphere through evaporation. This is enough to drop the temperature of about six thousand tons of material by 1°F. This conversion to latent heat can also be expressed in standard units, i.e., tons of air conditioning and kWh of electricity. In order to quantify the potential contribution of vegetated roofs and other landscapes to local climate control, the quantity of air conditioning that they provide can be expressed in terms of energy removed by the specific quantity of water evaporated as well as the cost in electricity and in the quantity of water required in electricity production. Here, in changing phase from liquid to gas, water moves heat out of the vegetated land surface and exports it into the surrounding environment. The specific quantity of energy removed from a physical surface depends on the quantity of water evaporated, with each gram of water moving to vapor phase carrying with it 580 calories at 30°C, as noted above. The advantage of using evaporative water loss as the metric is that it can be converted to quantitative units of electrical energy required to lower temperature through phase change in a confined fluid to distribute heat within an air conditioning or hvac system. Direct comparisons can thus be made between thermodynamically equivalent methods of thermal regulation. Direct costs of electricity and water for cooling via air conditioning units and green roofs will be compared here, together with the quantities of water required for electricity production. While it is recognized that indirect costs and benefits can be evaluated through full cost accounting, which includes all material and energy costs of hvac and green roof systems, from mining and manufacturing to installation and maintenance, in this case final costs – electricity versus water – will be compared in terms of equivalent quantities of heat removed. The cost of removing a specific quantity of heat with electricity is compared in the table below to the cost of removing a similar quantity through the evapotranspiration of water through plant communities. TABLE A comparison of the cost (in usd) of electricity used in air conditioning units to the cost of potable water used by vegetation on green roofs to produce a ton of cooling capacity. Cost of 33 gallons water (= 1 ton of AC) on a green roof New York Philadelphia Chicago Seattle Average $0.26 $0.20 $0.09 $0.13 $0.17 Cost per 84.4 kWh-ton AC $13.50 $7.93 $8.44 $5.57 8.86 Ratio of AC to green roof cooling cost 51 41 91 43 56 While a ton of air conditioning through an hvac system in these four cities ranges in cost between usd $5.57 and usd $13.50, this is between 40 and 90 times higher than the cost of potable water for the same quantity of cooling. There is also a significant water cost for the production of electricity. In thermoelectric generation plants, as noted in the table below, for the four cities compared the cost ranges between about 0.3 and about 1 gallon per kWh. As it takes 84.4 kWh to produce a ton of air conditioning, it therefore requires from between 24 to 89 gallons of water to produce this much cooling a day with electric power. It should be noted that this is in about the same range as the quantity of water required by a green roof to partition the equivaLIVING ARCHITECTURE MONITOR WINTER 21
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