Cool roofs can help reduce the heat island effect and also help improving the energy performance of the buildings. A cool roof can reflect the sun’s heat and emits absorbed radiation back into the atmosphere at a higher rate than standard materials. The cool roofs technology has been used for more than 20 years (EPA, 2012). The cool roof basically helps in reflecting sunlight and heat, thus reducing the temperature of the roofs. 20-‐25% of the urban surface is reported to be occupied by roof surface. This provides huge scope of providing passive cooling to enhance energy performance of the modern buildings (Zinzi & Angoli, 2012).
The effect of global warming and climate change are matters of concern for the global community, and impacts both rural and urban areas. The world population is expected to increase by 2.3 billion between 2011 and 2050 (United Nations, 2011) and the population living in urban areas is projected to increase from 3.6 billion in 2011 to 6.3 billion 2050. In Asia, it is expected that half of the population will live in urban areas by 2020, while Africa is likely to reach a 50% urbanization rate in 2035 (United Nations, 2012).
One of the more effects in the urban areas is due to the Urban Heat Island (UHI) effect. The UHI can impact issues like public health, environmental hazards, quality of life, etc. The relative warmth of city compared to the rural areas is called as UHI. The changes in landscape i.e. buildings, roads, and other infrastructure replace open land and vegetation causes the surfaces to become impermeable and dry. The UHI effect tends to be strongest during the day when the sun is shining. On a sunny/hot day, the urban space (roofs, pavements) are exposed to sun which can bring the temperature of these areas to around 27–50°C hotter than the air.
The GHG emission reduction potential by the cool roofs is by energy saving which leads to reduced energy demand and hence cutting down the emissions. Akbari et al., 2011 found that changing the black roof to white roof can help in saving of 0.066-‐0.072 kWh/m2/day. The normalized energy saving for per unit of reflectance reduction will be 0.11-‐0.12 kWh/m2/day. It is estimated that white roofs increase solar reflectance by 0.40 helping reducing 100t/100m2 of CO2 emissions. Similarly, colored roofs can help mitigate 5t/100m2 of CO2 emissions which has solar reflectance of about 0.20 (Akbari et al., 2009).
Cool roofs are still a very new technology in Asia except for Japan. There are no policies and regulations promoting this in Asia. It is not a very well established energy efficiency technology in this region. The cool roofs are generally of three types, namely, inherently cool roofs, coated roofs and green roofs (Mathews, 2012).
- Inherently cool roofs The roofs made of thermoplastic white vinyl have highest reflectance and emittance that roofing materials are capable of.
- Coated roofs The existing roof is transformed to cool roofs by applying solar reflective coatings. The coatings are durable and help suppression of algae and fungal growth, and also have ability to self-‐wash under normal rainfall (EPA, 2008).
- Green roofs The green roofs have plants on the roof tops which acts as thermal mass layer that reduces the flow of heat into the buildings. The solar reflectance is about 0.3-‐0.5, though it depends on plant types.
The cool roofs requirements depend on the slope of the roof. Table 1 displays the various requirements for the roofs. The requirements are by meeting the solar reflectance and thermal emittance values or to meet the Solar Reflectance Index (SRI) values.
The color of the cool roofs (blue, green, grey, etc.) can reflect sunlight. Figure 1 shows different colored roofing materials. A colored surface that reflects much of the invisible sunlight is a called a cool dark color. The cool dark color roofs have potential to reflect more sunlight than a similar-‐looking conventional dark color. For example, a conventional dark colored surface might reflect 20% of incoming sunlight, whereas, a cool dark colored surface reflects 40% (Urban & Roth, 2010).
Currently, there are several building energy-‐efficiency standards, which includes ASHRAE 90.1, ASHRAE 90.2, the international energy conservation code and California’s title 24 have cool-‐roof credits or requirements.
governments, research institutes have explored small as well as large scale implementation of the cool roofs. Cool roof is a well-‐developed technology is U.S.A. In U.S. cool roofs is a part of energy code in many of their states. In Europe, however, the technology is as developed as that in U.S. Europe launched a cool roofs promotion plan called The Cool Roof project, for implementation of Action Plan for promotion, market transformation and changing behavior towards cool roof technology (Synnefa and Santamouris, 2009).
Japan, Hong Kong, Singapore, and Malaysia are conducting research on cool roofs use. India has also an energy code that which mentions that if a project used the prospective method of the code it has to use the high reflective roof. Ahmedabad, Mumbai, New Delhi and Bangalore are few Indian cities using this technology (Tetali, 2011).
One of the study based in Hyderabad, India in year 2011 mentions that, with the assumption of an annual increase of 100,000 square meters of new roof construction for the next 10 years, the annual cooling energy savings due to whitening concrete roof would be 13 -‐14 GWh of electricity and cumulative cooling energy savings of 73 – 79 GWh for the region (Akbari et al., 2011).
Asia Pacific Economic Cooperation (APEC) has done a study in this region which focused on implementation of cool roof technology. The study mentions that total annual HVAC saving potential through implementation of cool roofs is 134,628.570 GWh. In south East Asia alone the saving is 38,527.678 GWh. The potential saving is a result of overwhelming demand of cooling in the tropical environment (APEC, 2011).
Cool roof technology is well established in western world but a very new technology in developing world. Though it is considered that one side coating is well known practice for decoration and protection in all part of the world, there is lack of awareness and experience considering the benefits of cool roof technology. The cool roof market is very new and there is a need of characterized market leaders for this technology.
There are many benefits of cool roof technology. Comfort and quality improvement of indoor environments is one major factor. An improvement in health and low indoor pollution is achieved. Apart from that, savings in energy cost is another social benefit that is achieved by the implementation of this technology.
The economic benefits for the technology are both direct and indirect. Direct saving are the reduced energy costs and reduced electricity generation cost (due to reduced energy demand) for the cool roofs user. The indirect economic benefits are from reduced infrastructure development for transmission and distribution service. The Table 2 gives illustrates the energy saving by few buildings in U.S. Using cool roofs.
A study in India suggests that annual direct CO2 reduction from white coating of the flat roofs in city of Hyderabad would be 11-‐12 kg CO2/m2 . The cumulative emission reduction in 10 years would be 0.6-‐0.65 million tons of CO2/m2. With the price of electricity estimated at 7 Rupees per kWh, the annual electricity savings on air-‐ conditioning would be approximately Rs. 93 – 101 per m2 of roof (Akbari et al., 2011).
The cool roofs are well known technology to mitigate UHI effect. Apart from this environmental benefit it helps reducing GHG emissions; reduce air pollutants, aids in earth cooling. The reduced energy demands leads to reduced energy generation which further leads to less use of fossil fuels and hence reducing harmful air pollutants like NOx. The reduced NOx provides better quality air and also helps preventing smog formation which is a major issue in mega cities recently. Similarly, reduced demand means reduced electricity generation and hence less GHG emissions like CO2. One of the studies in Houston and Baton Rouge estimated that there is a possibility of CO2 reduction by 6-‐7% from reduced energy use in the buildings (Konopacki and Akbari, 2002). A study by Akbari et al., 2009 shows the CO2 offset by cool roofs for low sloped and steep sloped roofs, given in the table below.
CO2 offset of cool roofs (Source: Akbari et al., 2009)
- Low sloped roof: -‐100 kg CO2/m2
- Steep sloped roof: -‐63 kg CO2/m2
Similarly, when the carbon offset of cool roofs are compared with the emission from cars it is found that if current dark colors roofs are changed to white roofs, it is possible to offset 34 billion tons of CO2. If the year to implement this is taken to be 10 years, annual CO2 offset will be 2.4 billion t/yr, which is equivalent to 600 million cars for ten years. In the same way, if the years to implement this is taken to be 20 years, annual CO2 offset will be 1.2 billion t/yr, which is equivalent to 300 million cars for 20 years (Akbari & Rosenfeld, 2008).
The cool roofs are considered as low cost technology that can make a huge economic and environmental difference. Initial cost of the cool roofs are comparable to the traditional roofing materials; some cost less than traditional ones and few more than the traditional roofing materials. The maintenance cost is also low as the coatings are reapplied in every 10-‐15 years. However, in the long run, combining initial cost, maintenance cost and energy saving cost cool roofs are better than the traditional roofing materials. Table 3 illustrates the cost of different type of low sloped cool roofs.
In the international CO2 market, the cool cities can yield carbon equivalent offset of 40-‐160 Gt, which is valued up to $1-‐4 trillion, based on $25/tCO2 (Akbari and Matthews, 2012). The current CO2 market has no carbon trading for CO2-‐equivalent, such as cool cities, as current CO2 market is based on direct CO2 reduction.
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Akbari, H., Menon, S. and Rosenfeld, A. (2009). Global cooling: increasing world-‐wide urban albedos to offset CO2. Climatic Change. 95 (3–4), doi: 10.1007/s10584-‐ 008-‐9515-‐9.
Akbari, H., and Matthews, H.D. (2012). Global cooling updates: Reflective roofs and pavements. Energy and Buildings.55, pp. 2-‐6.
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Synnefa, A. and Santamouris, M. (2009). ‘Promotion of Cool Roofs in the EU-‐The Cool Roofs Project’, Group Building Environments Studies, Greece. Accesses at: 20 December 2012. Available at: http://heatisland2009.lbl.gov/docs/231120-‐synnefa-‐doc.pdf
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