Using Cool Roofs to Reduce Energy Use, Greenhouse Gas Emissions, and Urban Heat-island Effects

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Cool roofs, cool pavements, and urban vegetation reduce energy use in buildings, lower local air pollutant concentrations, and decrease greenhouse gas emissions from urban areas. This report summarizes the results of a detailed monitoring project in India and related simulations of meteorology and air quality in three developing countries. The field results quantified direct energy savings from installation of cool roofs on individual commercial buildings. The measured annual energy savings potential from roof-whitening of previously black roofs ranged from 20 - 22 kWh/m2 of roof area, corresponding to an air-conditioning energy use reduction of 14 - 26% in commercial buildings. The study estimated that typical annual savings of 13 - 14 kWh/m2 of roof area could be achieved by applying white coating to uncoated concrete roofs on commercial buildings in the Metropolitan Hyderabad region, corresponding to cooling energy savings of 10 - 19%.

With the assumption of an annual increase of 100,000 square meters of new roof construction for the next 10 years in the Metropolitan Hyderabad region, the annual cooling energy savings due to whitening concrete roof would be 13 -14 GWh of electricity in year ten alone, with cumulative 10-year cooling energy savings of 73 - 79 GWh for the region. The estimated savings for the entire country would be at least 10 times the savings in Hyderabad, i.e., more than 730 - 790 GWh. We estimated that annual direct CO2 reduction associated with reduced energy use would be 11 - 12 kg CO2/m2 of flat concrete roof area whitened, and the cumulative 10-year CO2 reduction would be approximately 0.60 - 0.65 million tons in India. With the price of electricity estimated at seven Rupees per kWh, the annual electricity savings on air-conditioning would be approximately 93 - 101 Rupees per m2 of roof. This would translate into annual national savings of approximately one billion Rupees in year ten, and cumulative 10-year savings of over five billion Rupees for cooling energy in India.

Meteorological simulations in this study indicated that a reduction of 2°C in air temperature in the Hyderabad area would be likely if a combination of increased surface albedo and vegetative cover are used as urban heat-island control strategies. In addition, air-temperature reductions on the order of 2.5 - 3.5°C could be achieved if moderate and aggressive heat-island mitigation measures are adopted, respectively. A large-scale deployment of mitigation measures can bring additional indirect benefit to the urban area. For example, cooling outside air can improve the efficiency of cooling systems, reduce smog and greenhouse gas (GHG) emissions, and indirectly reduce pollution from power plants — all improving environmental health quality.

This study has demonstrated the effectiveness of cool-roof technology as one of the urban heat-island control strategies for the Indian industrial and scientific communities and has provided an estimate of the national energy savings potential of cool roofs in India. These outcomes can be used for developing cool-roof building standards and related policies in India. Additional field studies, built upon the successes and lessons learned from this project, may be helpful to further confirm the scale of potential energy savings from the application of cooler roofs in various regions of India. In the future, a more rigorous meteorological simulation using urbanized (meso-urban) meteorological models should be conducted, which may produce a more accurate estimate of the air-temperature reductions for the entire urban area.

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