Research on the Urban Heat Island (UHI) Effect: Current Status and Future Development

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1 Technical Report Research on the Urban Heat Island (UHI) Effect: Current Status and Future Development Hideki Takebayashi Architecture Department, Graduate School of Engineering, Kobe University Abstract This paper introduces the current status and future development of research on urban heat island (UHI) mitigation technologies. The following are presented as future challenges and possibilities: planning of outdoor spaces; improvement of thermal comfort through design; evaluation methods that consider post-construction maintenance; research on adaptive cities that adapt to possible future climate change; and presentation of workshops to encourage municipalities to devise their own measures. This paper introduces a standard for the accelerated soiling test for roofing materials to determine their solar reflectance and infrared radiation as stipulated in the U.S.A. for the purpose of evaluating decreased reflectance due to the soiling of highly reflective materials (ASTM D7897) and compares this method with the accelerated soiling test conducted in Japan. Our experimental accelerated test method revealed issues with QUV testing after application of soiling agents and the potential for recovery of reflectance once the coating has deteriorated due to excessive QUV testing. Keywords: urban heat island, UHI, heat stroke, mitigation measure, adaptation measure, evaluation method, soiling, accelerated test, solar reflectance, adaptive city 1. Introduction The urban heat island (UHI) effect is a phenomenon in which a city s central district tends to become hotter than its suburbs, resulting in temperature contour lines that form the outline of an island. As a result of global warming and urbanization, this phenomenon has a significant influence on large cities and has caused human health problems including increased incidence of heat stroke. In 2004, the Japanese government outlined UHI countermeasures that have been promoted by relevant ministries and agencies. Four specific measures were proposed: improvement of ground cover, reduction in anthropogenic heat, improvement of urban design, and lifestyle changes. Following a review in 2013, measures intended to reduce the influence on human health were added. This paper briefly introduces these measures and describes future challenges and possibilities. Test Navi Report No. 19 (Vol. 85) 1

2 2. Framework of UHI Countermeasures The UHI phenomenon (manifested as higher air temperatures near the ground surface) can be prevented in two ways: reducing the heat supplied to the ground surface; and diffusing the heat that accumulates at the ground surface. Improving the ground cover, reducing anthropogenic heat, and changing lifestyles are effective for the former; improving urban design is effective for the latter. Each measure is outlined below. Improving ground cover Manmade materials such as concrete and asphalt are easily heated by solar radiation and thereby add much heat to the ground surface. Substituting these with materials that do not become heated to high temperature by solar radiation can be effective. Table 1 shows the specific methods that have been suggested: using highly reflective materials, adding greenery, and utilizing evaporation (1). Reducing anthropogenic heat Air conditioners transfer indoor heat to outside air. Consequently, the more air conditioners that are in operation, the more heat is supplied to the ground surface. Suggested countermeasures include restricting the operation of air conditioners and replacing them with cooling systems that do not release heat outside but transfer it to water or underground. Changing lifestyles The more electrical appliances such as lights, refrigerators, TVs, and computers that are in use, the greater the cooling load, requiring even more air conditioner operation. In order to reduce air conditioner operation, lifestyles must change. The use of energy-efficient devices also contributes to a reduced cooling load. Improving urban design In urban areas with a high density of buildings, multiple reflections promote absorption of solar radiation and hinder radiative cooling. Moreover, buildings and other structures interfere with wind movement, impeding the diffusion of heat near the ground surface. Designing a well-ventilated city requires a change to the arrangement of buildings. 3. Mitigation and Adaptation Measures Global warming is addressed by restricting CO2 emissions and the UHI phenomenon is addressed by reducing the heat supplied to the ground surface and facilitating heat diffusion. Since these measures might not be implemented effectively, an alternative approach is to reduce the influence of global warming and the UHI phenomenon as much as possible even if they worsen. The former are called mitigation measures and the latter are called adaptation measures. Test Navi Report No. 19 (Vol. 85) 2

3 Promoting adaptation measures to reduce the influence on human health When a person is exposed to direct solar radiation (or strong reflection) and the heat is not properly carried away from the body through sweat or convection due to poor ventilation, high temperature or high humidity, not only will the person feel uncomfortably hot, but the body temperature rise also increases the risk of heat stroke. Specific adaptation measures include screening solar radiation with roadside trees or other structures and improving ventilation by adapting the layout of surrounding features. 4. Future Challenges and Possibilities As Table 1 shows, the performance of UHI mitigation technology is understood. The following future challenges and possibilities can be expected: Planning of outdoor spaces and improvement of thermal comfort through innovative design Detailed research is required on the permissible scope of use of highly reflective walls (as they are unsuitable for walls close to the ground); the combined effect of screening solar radiation using roadside trees and cooling by means of mist; and the thermal design of outdoor spaces with consideration for children and the elderly. Conditions such as east-west roads, north-south roads, road widths, building heights, solar elevation, and time of day should also be considered. Evaluation methods that consider post-construction maintenance Figure 1 shows an apparatus for accelerated soiling tests adopted as the standard for the accelerated soiling test for roofing materials to determine their solar reflectance and infrared radiation as stipulated in the U.S.A. (ASTM D7897) (2). It is used to evaluate the decrease in reflectance due to soiling of highly reflective materials. Similar exposure tests have been conducted in China and India. From the viewpoint of international competition in such technologies, Japan needs to conduct exposure tests and standardize soiling acceleration methods. The methods should be discussed together with other issues, including efflorescence of water-retentive materials; evaluation of performance degradation of greenery due to tree death; and need for and effect of maintenance such as washing, fertilizing, and pruning. Research on adaptive cities capable of adapting to possible future climate change Appropriate measures should also be taken in the area of construction and urban design in preparation for possible changes in climate. Figure 2 illustrates the positioning of mitigation and adaptation measures in urban spaces as suggested by Prof. Dr. J. Baumuller of the University of Stuttgart (3). It identifies the thermal environment and flooding as issues requiring specific consideration. Test Navi Report No. 19 (Vol. 85) 3

4 Conducting workshops to encourage municipalities to devise their own measures Figure 3 is a framework for the Kobe Smart City Plan created by the Kobe City Planning Office (4). It suggests a plan for a sustainable and eco-friendly city. The city plans to hold workshops in cooperation with Prof. Dr. J. Baumuller in order to encourage municipalities to study specific measures. Municipal representatives and climate specialists will attend the workshop, which will be held for several days in each city. The main topics will include the thermal environment in summer (the Cool Spot plan) and changes in precipitation (water storage capacity). The framework of the argument comprises planning for land use, traffic, energy, green spaces, and water surfaces as well as management (criteria and guidelines). 5. Research on Accelerated Soiling Tests in Japan Table 2 shows the results of a document review concerning accelerated soiling testing. The Public Works Research Institute (PWRI) method (5) is intended mainly to evaluate soiling of civil engineering structures such as roads, while the JIS draft (6) and Kitsutaka method (7) are intended primarily to evaluate soiling of the walls of buildings. Only the method established by M. Sleiman et al. (8) as ASTM D7897 is intended to evaluate a decrease in reflectance. This method uses a QUV machine to accelerate surface deterioration. The following is an outline of the experimental accelerated testing conducted by the author's team. It was founded on the PWRI method and adopts the method of Sleiman et al. Measure the gloss, luminance, and solar reflectance of a test plate (steel plate). Expose the test plate to QUV conditions for the following periods: 8 hours ([UV/60 C for 4 hours, condensation/50 C for 4 hours] x 1 cycle) 64 hours ([UV/60 C for 4 hours, condensation/50 C for 4 hours] x 8 cycles) 296 hours ([UV/60 C for 4 hours, condensation/50 C for 4 hours] x 37 cycles) (QUV testing for about 60 hours and 250 hours is equivalent to real exposure for 3 months and 1 year, respectively.) Measure the gloss, luminance, and solar reflectance of the test plate after the QUV testing. Spray a suspended solution of 5% carbon black (FW-200) and 95.0% deionized water with an air spray gun until the entire surface is uniformly covered. Test Navi Report No. 19 (Vol. 85) 4

5 Dry the plate for an hour at 60 C and cool it at room temperature. Wash the plate with flowing water and gauze. Measure the gloss, luminance, and solar reflectance of the test plate after the washing. These experimental accelerated tests revealed the following issues: Excessive QUV testing causes deterioration of the coating on the surface layer. The deteriorated coating comes off along with the soiling, resulting in recovery of reflectance. The effectiveness of washing with gauze varies with the operator. From a machine contamination standpoint, it is difficult to conduct QUV testing after soil application in the same way as with the method of Sleiman et al. 6. Summary This paper introduces the current status and future challenges of conducting research on urban heat island mitigation technologies. The following are presented as future challenges and possibilities: Planning of outdoor spaces; improvement of thermal comfort through design; evaluation methods that consider post-construction maintenance; research on adaptive cities that adapt to possible future climate change; and presentation of workshops to encourage municipalities to devise their own measures. In the U.S.A., the standard for the accelerated soiling test for roofing materials to determine their solar reflectance and infrared radiation (ASTM D7897) has been established and used for evaluating decreased reflectance due to the soiling of highly reflective materials. Similar exposure tests have been conducted in China and India. From the viewpoint of international competition in such technologies, Japan needs to conduct exposure tests and standardize soiling acceleration methods. An experimental accelerated test method revealed issues with QUV testing after the application of the soiling agents and the possibility of recovery of reflectance when the coating deteriorates due to excessive QUV testing. Test Navi Report No. 19 (Vol. 85) 5

Table 1: Effect of UHI mitigation from coverings in an urban area Figure 1: Accelerated soiling test apparatus stipulated in the standard for the

6 Table 1: Effect of UHI mitigation from coverings in an urban area Figure 1: Accelerated soiling test apparatus stipulated in the standard for the accelerated soiling test for roofing materials for determining their solar reflectance and infrared radiation (ASTM D7897) Test Navi Report No. 19 (Vol. 85) 6

measures in urban spaces Figure 3: Framework of Kobe Smart City Plan Table

7 Climate Protection Mitigation Adaptation Industry Traffic Househeating Urban influences Heat Floods Figure 2: Positioning of mitigation and adaptation measures in urban spaces Figure 3: Framework of Kobe Smart City Plan Table 2: Comparison of accelerated testing methods Test Navi Report No. 19 (Vol. 85) 7

8 Bibliography (1) Architectural Institute of Japan, Cool Roof Guidebook (2014), Chijinshokan. (2) ASTM D7897, Standard Practice for Laboratory Soiling and Weathering of Roofing Materials to Simulate Effects of Natural Exposure on Solar Reflectance and Thermal Emittance (2015). (3) J. Baumueller, N. Baumueller, Global warming ready? German Planning Laws and Climate Change (2015), Lecture material prepared by the Kobe City Housing and Urban Affairs Department. (4) Kobe City Planning Office, Kobe Smart City Planning: Toward a Sustainable and Environmentally Friendly City (2012), Kobe City. (5) Public Works Research Institute of the Ministry of Construction of Japan, Final report of development of anti-pollution technology of the structure (1999). (6) JIS (draft), Test method for accelerated dirt collection on exterior building wall materials and finishes. (7) Yoshinori Kitsutaka, Accelerating Test Method for Soiling on Finishing Materials of External Walls: Study on an evaluation method for the soiling on finishing materials of external building walls (Part 3), Journal of structural and construction engineering, Architectural Institute of Japan, Vol. 404 (1989), (8) M. Sleiman, T.W. Kirchstetter, P. Berdahl, H.E. Gilbert, S. Quelen, L. Marlot, C.V. Preble, S. Chen, A. Montalbano, O. Rosseler, H. Akbari, R. Levinson, H. Destaillats, Soiling of building envelope surfaces and its effect on solar reflectance Part II: Development of an accelerated aging method for roofing materials, Sol. Energy Mater. Sol. Cells, 122 (2014) Test Navi Report No. 19 (Vol. 85) 8

Field Observation of Cooling Energy Savings Due to High-Reflectance Paints

Buildings 215, 5, 31-317; doi:1.339/buildings5231 Article OPEN ACCESS buildings ISSN 275-539 www.mdpi.com/journal/buildings/ Field Observation of Cooling Energy Savings Due to High-Reflectance Paints Hideki