FACULTY OF ENGINEERING AND SUSTAINABLE DEVELOPMENT The Urban Heat Islands Analysis Factors of Building Surface, Green Area and Lighting Hanyue Liu Qian Liu Supervisor: Mats Sandberg Examiner: Björn Karlsson October 2012 Master’s Thesis in Energy Systems 1 Summary The development of urbanization and industrialization has improved the comfort level and the quality of human’s life effectively. But at the same time, it also caused many global environment problems, such as air pollution, greenhouse effect and urban heat island (UHI). The environment cannot keep the balance because of lots of industrial activities, anthropogenic heat and building surface effect. Almost 3 billion people, who are living in cities, have to face this situation, and the quantity is still increasing. The authors use literature research methodology, case study, comparative study and trend analysis study while writing. UHI have a harmful effect (such like photochemical smog and increasing energy use) on urban ecological environment, daily life and health. As the product of urbanization and industrialization, UHI has become one of the most important climate issues. Urban green space is helpful to reduce urban heat islands. London has a better urban green space system to face UHI and Hong Kong still has a long way to improve the green environment for reduce UHI. This article consider that roof greening is a suitable method to help Hong Kong gets rid of urban heat islands. And the lighting and population could indicate the level of urbanization, also can reflect the effect of urban heat islands preliminary. Over-capitalized lighting will exacerbate urban heat islands. 2 Content Summary ........................................................................................................................ 2  1.  Introduction ............................................................................................................ 4  1.1  Background ...................................................................................................... 4  1.2  Reasons ............................................................................................................ 4  1.3  Study Cases ...................................................................................................... 5  1.4  Purpose ............................................................................................................. 6  2.  Methods .................................................................................................................. 6  2.1  Literature Research Methodology.................................................................... 6  2.2  Case Study ....................................................................................................... 6  2.3  Comparative Study........................................................................................... 6  2.4  Trend Analysis Study ....................................................................................... 7  3.  Results and Discussion ........................................................................................... 7  3.1  Building Surface .............................................................................................. 7  3.1.1  Height to floor ratio .................................................................................. 8  3.1.2  Different hard surface materials ................................................................ 8  3.1.3  Building surface analysis ........................................................................ 10  3.1.4  Radiation ................................................................................................. 11  3.1.5  Reflection and absorption ....................................................................... 11  3.2  Green Space ................................................................................................... 14  3.2.1  The Remission of Urban Heat Islands by Green Plants .......................... 14  3.2.2  The Effects of Reducing Temperature and Increasing Humidity by Different Green Lands in Urban Area .................................................................. 16  3.2.3  London .................................................................................................... 19  3.2.4  Hong Kong .............................................................................................. 20  3.3  The Lighting................................................................................................... 23  4  Conclusion ............................................................................................................ 32  5  Reference .............................................................................................................. 33  3 1. Introduction 1.1 Background Urban construction is the strongest human intervention on natural environment. It will make the biggest changes on natural environment, and these changes would be almost irreversible. The population and city area are growing with the development of urbanization. An urban heat islands (UHI) phoneme means the temperature in a metropolitan area is significantly warmer than its surrounding rural areas. It was investigated and described firstly by Luke Howard in the 1810s, although he was not the one who names this phenomenon.1 2 1.2 Reasons There are several reasons to cause urban heat islands. The major reasons are human activities and heat storage of artificial constructions which come from solar radiation. At first, UHI effect is mainly caused by the solar radiation. There are a lot of artificial constructions such as concrete, asphalt and various building surfaces etc in the city. The artificial constructions can change the thermodynamic properties of underlying surface. The altered underlying surface could absorb heat quicker and has a lower heat capacity. Under the identical conditions of solar radiation, artificial constructions warming faster than the green area, water and so on. Therefore, the surface temperature of artificial constructions is higher than natural significantly. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. Another major influence is from artificial heat source. Factory production, motor vehicles and the live of residents will emission a lot of heat and produce nitrous oxides, carbon dioxide and dust in quantity. They could cause the atmospheric pollution, these substances would absorb heat radiation of underlying surface, cause the greenhouse effect, thereby warming the atmosphere. Besides, the decrease of greenbelt, forest and water is also a reason. Along with the development of 1 Luke Howard, The climate of London, deduced from Meteorological observations, made at different places in the neighborhood of the metropolis, 2 vol., London, 1818-20 2 Wikipedia, http://en.wikipedia.org/wiki/Urban_heat_island#cite_note-0, 2012-08-15 4 urbanization and population growth, the buildings, squares and roads etc. in city are decreases massively, the ability of reduce heating island effect will be weaken. Urban heat islands have detrimental effects on urban living environment and becomes more and more obviously. 1.3 Study Cases The study of United Kingdom Meteorological Office (UKMO) shows that in the summer at the end of this century, the temperature will be 10℃ higher than surrounding suburbs in the cities of UK. Because of the global warming, people have to use the high-energy-consuming equipments to keep cool inside. On account of the complicated building material exerted great influence on the standard temperature in city, particular those artificial materials which have the complicated spectral characters3. By learning more about urban heat island, the buildings and the green area have been found as the most important factors that due to urban heat island. London is located at 51°30’N, 0°07’W4, is one of the four world-class cities.5 As an international metropolis, London has more than 8 million populations.6 The total area of London is about 1570km2, and the population density is about 5000 people per square kilometer. Hong Kong is located between 22°08' N to 22°35'N and 113°49'E to 114°31'E, which is a megacity in the Pearl River Delta region. With high-density populations and high-rise buildings, the effect of urban heat islands is obvious.7 3 W.Y. Liu, A.D. Gong, J. Zhou and W.F. zhan, Investigation on Relationships between Urban Building Materials and Land Surface Temperature through a Multi-resource Remote Sensing Approach, Remote sensing information, 2011. TP79. 116-0046-08 4 GeoHack, http://toolserver.org/~geohack/geohack.php?language=zh&pagename=%E4%BC%A6%E6%95%A6&params=51 _30_25_N_0_07_39_W_type:city, 2012-08-17 5 Loughborough University, The World According to GaWC 2008, Globalization and World Cities Study Group and Network (GaWC), 2009-05-15 6 Office for National Statistics, http://www.ons.gov.uk/ons/rel/mro/news-release/census-result-shows-increase-in-population-of-london-as-it-tops- 8-million/censuslondonnr0712.html, 2012-08-15 7 Hong Kong survey and mapping office, http://www.landsd.gov.hk/mapping/en/publications/map.htm, 2012-07-19 5 The total land area of Hong Kong is only 1104.41km2, but its population is about 7 million, there are more than 6000 people per square kilometer.8 9 The commercial and economic activities in Hong Kong are frequent, and there are many high-rise building in the city. There is no doubt that the local climate of Hong Kong is impacted by urbanization. 1.4 Purpose In this project, the author mainly discussed the influence by the reflection, radiation and absorption from the building surface, aim to analysis the extent of the influence to UHI from five factors. The factors include height to floor ratio(HA), kinds of hard surface materials, building surface radiation, reflection and absorption. And in the parts of green area, the authors aim to prove that the green area is beneficial to control the urban heat islands, and during analysis present situations of green area in London and Hong Kong, a suitable suggestion will be provide by authors to help Hong Kong gets rid of urban heat islands. At last, the article try to find out the correlation between lighting, population and the level of urban heat islands. 2. Methods For this thesis, the main literature and other resources are searched by online library. 2.1 Literature Research Methodology Depending on literature research, the authors get knowledge on the case and current situation of urban heat islands. It is very helpful and useful to get a preliminary and deeper knowledge of the framework. 2.2 Case Study This article chooses London, Hong Kong and some other cities to analyze the impact from building surface, green space and lighting. 2.3 Comparative Study Compare various situations in different cities to analyze urban heat islands, summarize strengths and weaknesses parts to develop and improve. 8 Hong Kong census and Statistics Department, 2011 population census-summary results, 2011, p7 9 Hong Kong survey and mapping office, http://www.landsd.gov.hk/mapping/en/publications/map.htm, 2012-7-19 6 2.4 Trend Analysis Study According to the current situation of urban heat islands, this article proposes development tendency for the future. 3. Results and Discussion This article attempts to be divided into three parts to analyze urban heat islands, they are building surface, green space and lighting. 3.1 Building Surface In the city, downtown and residential area takes the high-density buildings. UHI has been found to have a closely related with this situation. The cities will get more solar radiation energy than surrounding area during the day because of the effects of multiple reflections and absorption between the walls and walls, walls and grounds. At night, the cities not only have the back radiation from the atmosphere, but also have the long wave back radiation downward from the walls and roof. So that the loss of the long wave back radiation is lower than surrounding suburbs. All of above will lead to a higher temperature than surrounding. Along with advance in research into urban climate, some new theory and research method has developed. Many mathematics and physical method has been applied across the research. In 1987, Oke came up with the theory definite the activity interface. Based on this activity interface, there has the energy balance function of building-air-ground volume shows below Q +Q =Q +Q +△Q +△Q n F H E S A Where Q is the net radiation, n Q is anthropogenic heat, F Q is sensible heat, h Q is laten heat fluxes. E △Q is temperature changes of the heat storage of internal underlying surface. E △Q heat level fluxes changes A 7 The temperature field is influence by substrate. Of this, the heat transformation process can be divided into three broad categories  Heat Convection The processes of heat transfer and air movements  Radiation The short wave solar radiation and long wave radiation between different surfaces  Heat Conduction The processes of heat conduction and heat storage of the solid surfaces For example, ground, walls and vegetation. All three transfer processes influence and interact with each other. 3.1.1 Height to floor ratio Because of a shortage of territorial area, building to continuously develop, gradually top in recent years. Passing to check to see the data, it is obvious to find that the heat absorbing area is increasing with the height of the building in case of the same space area. It is defined that height to floor ratio (HA) as the value shows that the ratio of the radiation between the building and the ground. Usually, the value can be 5-6 times as more as the radiation which the ground can be absorption. Wenyu L10 measures the temperature in Beijing in2010. The author used Proba Chris and Landsat5 TM to analysis the result. It shows that in summer, the outer surface temperature of the building can be 71℃. But in the same case, the temperature of the lawn is only 38℃.So that, the absorption of the building outer surface directly cause the temperature of the out surface and surrounding air grows up. It would eventually that the problem of UHI will become more severe. 3.1.2 Different hard surface materials Hard surface include road, square, building surface and so on. The temperature is mainly affect by long wave radiation. In summer, after absorbing the solar radiation, 10 W.Y. Liu, A.D. Gong, J. Zhou and W.F. zhan, Investigation on Relationships between Urban Building Materials and Land Surface Temperature through a Multi-resource Remote Sensing Approach, Remote sensing information. 2011. TP79. 116-0046-08 8 these hard surfaces will increased the average radiation temperature of the surrounding space, which in turn may affect the sensation of heat of the body.11 The average temperature of the green area and building materials exist obvious difference. Usually, the value of the green area is lower than 305.5K and the value of the building materials is larger than 305.5 K. The maximum temperature difference is 3.0K. The typical hard surface material is brick, metal, concrete, cement, brick and glass. Those five different hard surface materials will have wider variations, wide distribution and discrepancies in physical properties. The three central important surface characteristics which would be impacts on land surface temperature are reflectivity, thermal conductivity, and thermal inertia. Figure 1: Physical map of hard surface materials 11 Dexuan Zhu. (2003), Control study of building environment, Nanjing Southeast University Press 9 1. The Residential portion of the city has a lot of artificial construction like concrete, which is easy to absorb heating but have a small heat capacity. In the same case of the radiation, the temperature increased more quickly than natural underlying surface. 2. The glass skin had found wide application in high-rise commercial Buildings. The UHI effect is further enhanced. The glass curtain wall has led to a great drop in their surface temperature. It is about 3.3K-4K lower than other material. It has a great reflectivity of blue and green light. 3. The metal hard surface has a lower surface temperature. The reflective is higher in the near infrared. 4. The walls and top of most buildings take concrete and cement as the material. The reflectivity of these two materials is lower, only about 0.2-0.4.The average temperature of concrete, cement and associated with the asphalt is almost same. Those materials are one of the major resources of a striking thermal environment in the city. 5. From the measurement, the brick and tile-roof building take the highest surface temperature. The reflectivity is much lower than other materials. 3.1.3 Building surface analysis When normal object is exposured by light it will express as three reactions, they are reflect, absorb and perspective. If it defines that ρ is the reflectivity, ε is absorptivity and τ is transmissivity, the function will shows below ρ+τ+ε=1 Because most of the object is opaque, the transmissivity τ is equal to zero. So that the function will changed as, ρ+ε=1 The Absorptive capacity of atmosphere for infrared radiation which is between 2.5-5μm and 8-13.5μm is relatively weak. In this case, usually the transmissivity τ is more than 80 percents. Consequently, in order to keep the objects decreasing the temperature, it needs to bounce the heat absorbed back into space by using these two fields. 10