What is Cool Pavement
“Cool pavements refer to a range of established and emerging materials. These pavement technologies tend to store less heat and may have lower surface temperatures compared with conventional products. They can help address the problem of urban heat islands, which result in part from the increased temperatures of paved surfaces in a city or suburb. Communities are exploring these pavements as part of their heat island reduction efforts”.(EPA, 2008)
Why to Need Cool Pavement
The conventional pavement surface temperature is generally 20-30ºC [68-86°F] higher than the air/surrounding temperature due to pavement solar energy absorption during daytime, especially in the summer(Ongel and Harvey, 2000). Also, the absorbed heat energy from pavement surface, which stored in the pavement subsurface, will be re-released into the atmosphere after sunset, keeping heating the atmosphere at nighttime. Due to the large area covered by pavements in urban areas (Figure 1), they are an important element to consider in heat island mitigation. Cool pavements can help to address heat islands effect through reducing pavement and air temperature, if used in a large scale. Also the cool pavements with lower surface temperature could reduce the chance of heating stormwater as it runs off the pavement into the local waterways1. Besides these effects contributing to address the problem of urban heat islands and improve water quality, the lower temperature in the pavements also would reduce pavement damage/deterioration and improve pavement service life/durability(Pomerantz, Akbari, and Harvey, 2000).
How to Keep Pavements Cool
Potential Mechanisms for Keeping Pavements Cool
Increase Pavement Surface Reflectance
Solar reflectance, or albedo, is the percentage of solar energy reflected by a surface. Most existing research on cool pavements focuses on solar reflectance, which is the primary determinant of maximum pavement surface temperature(EPA, 2008). High albedo also could reduce pavement temperatures below the surface, because less heat is available at the surface to then be transferred into the pavement. Many opportunities exist to improve this property in pavements.
Increase Pavement Thermal Emittance
A material’s thermal emittance determines how much heat it will radiate per unit area at a given temperature, that is, how readily a surface sheds heat. Thermal emittance plays a role in determining a material’s contribution to urban heat islands. Research from 2007 suggests albedo and emittance have the greatest influence on determining how a conventional pavement cools down or heats up, with albedo having a large impact on maximum surface temperatures, and emittance affecting minimum temperatures4. Although thermal emittance is an important property, there are only limited options to adopt cool pavement practices that modify it because most pavement materials inherently have high emittance values.(Levinson, et al, 2002)
Increase Pavement Surface Convection
Pavement transfers heat to the near-surface air through convection as air moves over the warm pavement. The rate of convection depends on the velocity and temperature of the air passing over the surface, pavement roughness, and the total surface area of the pavement exposed to air. Some permeable pavements have rougher surfaces than conventional pavements, which increases their effective surface area and creates air turbulence/circulation over the pavement. While this roughness can increase convection and cooling, it might also reduce the surface’s net solar reflectance.(EPA, 2008)
Reduce Pavement Thermal Conductivity
Thermal conductivity is the ability or power of materials to conduct or transmit heat. It determines how fast and easily the heat would be conduct from high temperature object/part to low temperature object/part. Pavement with low thermal conductivity may heat up at the surface but will not transfer that heat throughout the other pavement layers as quickly as pavement with higher conductivity.(EPA, 2008)
Reduce Pavement Heat Capacity
Heat capacity is the amount of heat required to raise the temperature of one unit weight of a substance by one degree Celsius without change of phase. It determines how much energy would be absorbed and stored in the pavement at certain temperature. Many artificial materials, including pavement, can store more heat than natural materials, like dry soil and sand. As a result, built-up areas typically capture more of the solar energy—sometimes retaining twice as much as their rural surroundings during daytime6. The higher heat capacity of conventional urban materials contributes to heat islands at night, when materials in urban areas release the stored heat.
Evaporation would require heat energy to achieve the phase change from liquid to gas. It will absorb heat energy from surroundings and cool down them. Evaporation of water on the pavement surface or within the pavement void structure would draw out heat from the pavement and the near-surface air, thus cooling the pavement down. It is just similar to the evaporative cooling from vegetated land cover.
Shading pavements could reduce the sunlight on the pavement and thus directly reduce the heat sources (solar energy) coming into the pavement, thus reducing the pavement temperature.
External Active Mechanical Cooling
Besides the evaporation cooling, another cooling option is active mechanical cooling. The active mechanical cooling includes cooling of water circulating through pipes embedded in pavements(http://www.roadenergysystems.nl/)(dr.ir. A.H. de Bondt. R. Jansen)(Mallick, Chen, and Bhomick, 2009), and cooling through thermoelectric devices embedded in pavements(Hasebel, Kamikawa, and Meiarashi, 2006).
Potential Cool Pavement Types and Other Measures
Potential Cool Pavement Types
1. Conventional Asphalt Pavement
Conventional asphalt pavement consists of asphalt binder mixed with aggregate. It can be modified with high albedo materials, like using light-colored aggregate, colored asphalt by pigments or sealant, or using tree resin in place of asphalt. It also could be treated after installation to raise reflectance, like applying light-colored coating, or chip seals, whitetopping, ultra-thin whitetopping (UTW) and microsurfacing with light-colored aggregate and/or emulsified polymer resin for maintenance(Tran, et al, 2008). This pavement could be applied in a wide range of functions from parking lots to highways.
2. Conventional Concrete Pavement
Conventional concrete pavement is made by mixing Portland cement, water and aggregate. It can be used in a wide range of applications including trails, street road, parking lots, and highways. The concrete pavement generally has a higher reflectance than asphalt pavement. It can be modified to increase the reflectance by using white cement, or cement blended with light color slag.
3. Permeable Pavement
Permeable pavement contains more voids than convention pavement and is designed to allow water to drain through the surface into the sublayers and even ground below. Permeable pavements include porous asphalt pavement (including open-graded friction course (OGFC) or permeable friction course (PFC)(Alvarez, 2009), pervious concrete pavement, and brick or block pavers.
Besides these nonvegetated permeable pavements, there are also some vegetated permeable pavements, such as grass pavers and concrete gird pavers, which use plastic, metal, or concrete lattices for support and allow grass or other vegetation to grow in the interstices1. Unlike the nonvegetated permeable pavements, the typical use of vegetated permeable pavements is for lower traffic volumes such as alleys, parking lots, and trails and they may be best suited to climates with adequate summer moisture.
The permeable pavements have a rough surface and more void content, which can increase their effective surface area exposed to air and create air turbulence over the pavement. This will increase the convection between pavement and air, and thus help to reduce the pavement temperature. Besides this increased convection, the evaporative cooling of permeable pavements also could reduce pavement temperature through phase change of water when moisture exists on and/or within pavements.
Beyond reducing temperature, permeable pavements also could potentially reduce the air/pavement noise, and improve driving safety. Also, the full depth permeable pavement could reduce the stormwater runoff and improve the water quality.
Other Potential Measures
Pavement shading includes tree shading, vegetation shading, which especially could be used in parking lots. Besides tree shading and vegetation shading, another emerging option considered by some local governments and private firms is to install canopies that incorporate solar panels in parking lots, even along or on the highway(ODOT). Beyond shading pavement surface from incoming solar energy, these photovoltaic canopies also could generate electricity that can help power nearby buildings or provide energy for plug-in electric vehicles(GOlden, et al, 2006).
2. External Active Mechanical Cooling
Water circulation cooling system could directly supply the hot water to nearby buildings or use them to generate electricity. Thermoelectric devices are two-way devices(Nolas, 2003). The device can provide temperature differential if electricity is applied to it, which can be used for cooling or heating. For the same device embedded in the pavement, it also can generate electricity from the temperature differential in the pavement. The electricity generated by thermoelectric devices, if stored, could be used to power themselves to cool down or heat up pavements. With the high efficiency and low cost, thermoelectric devices also could be used to harvest the solar energy in pavements and provide energy for nearby buildings and plug-in electric vehicles, especially if in a large scale.
What are Potential Benefits and Costs of Cool pavement
- Mitigate urban heat island(UHI) effect;
- Reduce energy use and greenhouse emission;
- Improve water quality and reduce stormwater runoff (for permeable pavement);
- Increase pavement life/durability and waste reduction;
- Reduces pavement maintenance costs;
- Enhance nighttime illumination;
- Comfort improvements;
- Improve driving safety (for permeable pavement);
- Noise reduction.
- Cool pavement costs will depend on many factors including, but not limited to, the following(EPA, 2008)(Khan, et. al, 2007),:
- Current status and near future researcher & development outcomes of technologies;
- Its sustainability if used on a massive scale and potential application barriers;
- The region;
- Local climate;
- Time of year;
- Accessibility of the site;
- Underlying soils;
- Project size;
- Expected traffic;
- The desired life of the pavement.
Besides the benefits and costs listed above, the benefits and costs considerations should include environmental, social, and economic aspects, rather than just the economic one. The general initial construction cost for cool pavement might be higher than that for regular conventional pavement. However, life-cycle assessments (LCA), including life-cycle cost assessments (LCCA), can help in evaluating whether long-term benefits can outweigh higher up-front costs.
[]National Center of Excellence on SMART Innovations at Arizona State University. 2007. What Factors Influence Elevated Pavement Temperatures Most During Day and Night? Case Study 1(1).[]
[]Christen, A. and R. Vogt. 2004. Energy and radiation balance of a Central European city. International Journal of Climatology. 24(ii):1395-1421.[]
- EPA. 2008. Reducing Urban Heat Islands: Compendium of Strategies: Cool Pavements.↵
- Ongel, A. and Harvey, J.T. Analysis of 30 Years of Pavement Temperatures using the Enhanced Integrated Climate Model (EICM). Draft report prepared for the California Department of Transportation. Pavement Research Center, Institute of Transportation Studies, University of California Berkeley, University of California Davis. UCPRC-RR-2004/05↵
- Pomerantz, M., H. Akbari, and J. Harvey. 2000. Cooler Reflective Pavements Give Benefits Beyond Energy Savings:Durability and Illumination. Lawrence Berkeley National Laboratory, Berkeley, CA.↵
- Levinson, R., H. Akbari, S. Konopacki, and S. Bretz. 2002. Inclusion of Cool Roofs in Nonresidential Title 24 Prescriptive Requirements. Paper LBNL-50451. Lawrence Berkeley National Laboratory, Berkeley, CA.↵
- Energy from asphalt for heating and cooling buildings and roads. http://www.roadenergysystems.nl/↵
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- M. Hasebel, Y. Kamikawa and S. Meiarashi. Thermoelectric Generators using Solar Thermal Energy in Heated Road Pavement. 25th International Conference on Thermoelectrics, IEEE, 2006.↵
- Tran, N., B. Powell, H. Marks, R. West, and A. Kvasnak. 2008. Strategies for Design and Construction of High-Reflectance Asphalt Pavements. Under review for the 2009 Transportation Research Board Annual Meeting.↵
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