Effects of Solar Radiation Reflection
Info: 8294 words (33 pages) Dissertation
Published: 12th Dec 2019
This particular research is intended to compare the thermal performance in means of reflectivity of facades materials with H/W ratios of urban canyons. Because it can see that contemporary urban facades moving away from traditional methods of making walls towards making false wall facades, mostly made by aluminum and other composite materials. considering about aluminum façade, variety material usage methods were very limited due to market availability and lack of knowledge.
In this research, it will test the how reflection of solar radiation affects upon pedestrian level and even on façade itself, lowering surface temperatures indeed reduce the cooling loads as well. So, having Retro-reflective materials will be the best option against increasing temperatures inside urban canyons, because nature of retro-reflective surfaces is to reflect incident radiation back to source. So, by applying on urban surfaces it will reflect the solar radiation back to sky and stop energy trapping inside urban canyons.
In dense urban areas, most solar radiation falls on vertical facades; therefore, keeping façade cooler would decrease the A/C loads in hot days. Contemporary building facades tend to have false wall façade in front of actual wall made by aluminum sheets, thus it is cheaper than making proper cement plastered façade.
Aluminum sheets can be coated with polyvinylidene fluoride (PVDF), fluoropolymer resins (FEVE), or polyester paint. Aluminum can be painted in any kind of color, and ACPs are produced in a wide range of metallic and non-metallic colors as well as patterns that imitate other materials, such as wood or marble. The core is commonly low-density polyethylene, or a mix of low-density polyethylene and mineral material to exhibit fire retardant properties.
Major issue of using such a material it is a diffuse reflective material so it will indeed reflect part of solar radiation of surrounding facades and canyon floor. Such a situation will thermally discomfort for pedestrians.
Chapter one – Background
An increase in a population in cities and towns versus rural areas. Urbanization began during the industrial revolution, when workers moved towards manufacturing hubs in cities to obtain jobs in factories as agricultural jobs became less common. (business dictionary, 2017)
126.96.36.199What is Urbanization?
Urbanization simply happen due to migration of rural population to cities where they can find better life, which was found as better solution at post- industrial ages to accelerate the economic growth but with growth of urbanity became threat at modern. Because of the facilities and infrastructure like better education, housing, health care, sanitation, business opportunities, and transportation, even in these days’ trend is there to rural to urban migration.
Urban growth across the world remains unstoppable and within the next two decades we will see a doubling of urban population over that at the start of this century (United Nation. 2010). So, as a byproduct of urbanization new habitable spaces were emerged all around globe and new infrastructure created to facilitate millions. Structures evolve through decades, from wooden to concrete and became Complex.
1.1.1 Origin and Evolution
The Industrial Revolution, which took place from the 18th to 19th centuries, was a period during which predominantly agrarian, rural societies in Europe and America became industrial and urban. Prior to the Industrial Revolution, which began in Britain in the late 1700s(“HISTORY.com,” n.d.), due to new found energy source “coal”. After that people were forced to move away from villages to where factories were established and trend was continuing still up to today,
In 1800, only 2% of the world’s population was urbanized
In 1950, only 30% of the world population was urban.
In 2000, 47%. of the world population was urban
More than half of the world’s population will be living in urban areas by 2008
By 2030, it is expected that 60% of the world population will live in urban areas.
Almost 180,000 people are added to the urban population each day
It is estimated that there are almost a billion-poor people in the world, of this over 750 million live in urban areas without adequate shelter and basic services. (“General Assembly, Special Session,”)
1.1.2 Contemporary Situation
During 20th century, next biggest revolution happens in human history, that was information revolution after the last day 1999, world has become a single village where people used to exchange their daily gossips with each other. That was happen through the information revolution. Now distance doesn’t matter for people who at the reach of active internet connection, with this new found technology people tend to be more concentrated on townships where this fascinating technology is available. Now leading cause of urbanization is transferred from gear wheals to digital codes, younger generations are showing much more interest in these high technologies than any other field, with start of second millennium it turns development of human kind and fate of world into new direction never like before.
Today, the most urbanized regions include
Northern America (82% living in urban areas in 2014),
Latin America and the Caribbean (80%),
Europe (73 %).
In contrast, Africa and Asia remain mostly rural, with 40% and 48% of their respective populations living in urban areas.(“wup2014-highlights.Pdf,” n.d.) it can see that where technology available it attract more people, northern hemisphere became richer and phosphorous during early industrial revolution and up to today they continue their dynasty by holding the power of technology. But even with the high technology they can’t overcome the one major problem, land scarcity became prominent issue in contemporary world due its nature of limited state and can’t be expand with human effort. Thus, it limiting the horizontal expansion of townships, so human brain is much clever in finding solutions so they start to expand their limits by conquering sky. Now Urbanization started to expand vertically as well, it was the emerge of Mega Cites with sky high mega structures to facilitate growing urban population.
Megacities are significant for their scale and concentrated economic power, so they are being home to about one in eight of the world’s urban dwellers. During late 90’s, there were only 10 cities with more than 10 million in population, but today, the number of megacities has nearly grown up to 28, the total population under them had grown up to nearly 453 million.Tokyo is the world’s largest city with a massive 38 million inhabitants and it hold its record for last few decades, followed by Delhi with 25 million, also Shanghai with 23 million close by.
By 2020, Tokyo’s population is 2030 with 37 million inhabitants, followed closely by Delhi, with population which projected to rise up to 36million.
New economic powers emerge around equator and their growth is unstoppable in way of urbanization and growth of population, Countries like China, India, Brazil will rise up to top and classical developed countries will overrun within next few decades.
Almost 90% of the global urbanization between now and 2025 will occur in countries of the developing world located mostly in tropical/ subtropical regions. Such statistics underscores the urgency of studying the human consequences of tropical urbanization. There is evidence that researchers are increasingly turning their attention to tropical urban climate issues (Emmanuel. 2005).
Such rapid growth contributes to both anthropogenic global climate change due to higher consumption of energy and materials (Emmanuel, 2011). Mostly these newly emerged cities are built with high rises, so independent structures which can support thousands of citizens, thus their nature of being detached from surrounding environment their energy consumption is relatively much higher than contemporary low rise buildings. Due to lack of passive strategies in order to have for providing ventilation, Lighting and cooling most of energy consumption will be reserved for maintaining building atmosphere habitable, so A/C and artificial Lighting will consume most of its energy budget.
To have such a density within small area it need to burn enormous amount of fossil fuels to energize those structures, although they have their own issues arise thorough their morphology, thus emitting CO2 became the one of the major issue we face today. So, it need to pay attention to what are the issues are about to happen, how to find alternatives or what are the mitigation strategies. Since the last few centuries there was an increased amount of problems throughout the world associated with the environment such as global warming, ozone depletion, urban heat island effects, destruction of rain forests, air pollution, acid rains etc. (Basnayake. 2011).
1.2.1 Global Climate
There are several different definitions by different people and organizations.
“Climate change is defined as statistically significant variation in either mean state of the climate or in its variability, persisting for an extended period. Climate change may be due to natural internal processes or external forcing or to persistent anthropogenic changes in the composition of the atmosphere or in land use” (IPCC,2001). But statement made by the IPCC is can be mention as through definition in understanding what is global climate.
The Intergovernmental Panel on Climate Change (IPCC) is the most prominent international organization to make an assessment of climate change. It was established by the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO) in 1988.
What Is Causing Earth’s Climate to Change?
Some causes are natural, like changes in Earth’s orbit and Solar radiation. Ocean changes and volcanic eruptions are also effecting the climate change. But after industrial revolution contribution of mankind is far greater than any other,
According to NASA, most scientists think that recent warming can’t be explained by nature alone. Most scientists say it’s very likely that most of the warming since the mid-1900s is due to the burning of coal, oil and gas. Burning these fuels is how we produce most of the energy that we use every day. This burning adds heat-trapping gases, such as carbon dioxide, into the air. These gases are called greenhouse gases. (NASA,” n.d.)
What Is the Difference Between “Climate Change” and “Global Warming”?
“Global warming” refers to the long-term increase in Earth’s average temperature. (NASA,” n.d.)
“Climate change” refers to any long-term change in Earth’s climate, or in the climate of a region or city. This includes warming, cooling and changes besides temperature.(NASA,” n.d.)
Global warming is the rise in the average temperature of Earth’s atmosphere and oceans since the late 19th century and its projected continuation. Earth’s mean surface temperature has increased by about 0.8° C with about two-thirds of the increase occurring since 1980 (NASA, 2011)., which can contribute to changes in global climate patterns, “global warming” often happen as a result of increased emissions of greenhouse gases from human activities. The increased volumes of carbon dioxide and other greenhouse gases released by the burning of fossil fuels, land clearing, agriculture, and other human activities, are believed to be the primary sources of the global warming that has occurred over the past 50 years. Also, it is mentioned that, unless action is taken, emission will continue to rise for decades to come. By 2100, global average temperature could be 6° C warmer than today (Basnayake, 2011). Global warming can be considered part of global climate change along with changes in precipitation, sea level, also.
1.2.3 Major urban issues and challenges
it is clear that the combined effects of urbanization and global climate warming can give rise to a decrease in urban air quality and an increase in urban heat island intensity, (Moonen, Defraeye, Dorer, Blocken, & Carmeliet, 2012) Also with the increment of temperature it will lead to thermal discomfort and thus it will increase the cooling loads for interiors as well, so more energy demand means for fossil fuel to burn so it will became a never ending dissonant cycle.
The individual building with its technical installation marks one end of the spectrum. At increasingly larger scales we have the effect of the urban morphology at neighborhood scale, the urban heat island effect at city-scale, the effect of topography at regional scale and finally the global effects of climate and climate change.(Moonen et al., 2012)
1.3 Urban Climatology
Urban climatology is a science shared between designers and climatologists.(Mills, 1997) The climatologists initially focused on the UHI (Urban Heat Island) issue in the macro-scale, and moved smoothly towards the meso-scale (i.e. urban geometry).(Tim R. Oke, 2004) The designers have gradually switched to urban environmental issues as the obstruction between the buildings and the urban environment.(Knowles & Knowles, 1985) Currently, the environmental quality of the urban street and open areas has turned out to be a major issue for both designers and climatologists, which can be observed through their regular scientific findings(Steemers, 2003) For example, they indicate that the surface and form characteristics of street and open spaces vary largely in providing thermal comfort. In particular, several parameters, such as geometry, properties of surfaces, and vegetation, affect the microclimate of these spaces(Ali-Toudert & Mayer,2006)
Considering about urban climatology there are large number of factors to be consider, as shown in figure above. so in order to have through understanding about urban climate behaviors it need to focus on all above areas mentions in figure above. But its scale would be massive so, it need to be done in steps and those steps need to have inter connections among them so, another researcher could start from where ones’ left.
Basically, urban climatology is depending on buildings and streets, so it need to understand what are the components of buildings which affect the urban atmosphere and which have the higher impact, considering about urban context it is not uniformly spread and there would be denser areas than others so it need divide area into several parts which were having similarities among them, so it would be helpful in case of selected area is too vast and difficult establish proper research method.
Buildings and its immediate context will generate its own mini scale atmosphere with its own climatic conditions, which refers as urban micro climate.
1.3.1 Urban Micro Climate
A Micro-climate refers as a local atmospheric zone where the climate differs from the surrounding area. Like,
Hill country- hilly areas have different climate conditions than surrounding lower levels. Temperature drops at rate of between 5 and 10 °C per 1000 m. also it depends on humidity and other various aspects also.
Microclimates can be found, near bodies of water that can cool the local atmosphere or in heavily urban area where it turns environment hot. Considering about urban context it also can be seen hot atmosphere than relative rural situations with its dense urban morphology of high-rises and massive structures. So it can define particular area with its unique characteristics there are several classifications like LCZ classification, so to get understanding urban climate in small scale it need to select common aspect which can express climatic conditions and its relation to urban geometry.
188.8.131.52 Urban Geometry
Urban geometry also plays a particular role in establishing building/urban behaviors(Zhun Min Adrian, Nyuk Hien, Marcel, & Steve Kardinal, 2013). Studying about urban geometry is important because it is the science of studying form, shape and structure of the cities, and their behavior when it comes to interaction with humans. Very existing of urban geometry is depending on human interaction and their requirement so, while catering human need urban geometry getting changed and it will visible through size and scale of buildings.
In dense urban context where land blocks were demarcated by streets, and filled with heavy structures which were built closely like high-rises. By analyzing build patterns in urban area it can identify the common elements/aspects which can used to express the urban geometry,
A selected urban area can have unique design features, such as;
Fraction of a land in a given urban area covered by buildings (land coverage)
Distances in between buildings, including streets width also.
Average height of buildings.
thus, having such a universal element to express urban geometry it will make it easy to understand and address issues easily. One such an aspect is urban canyon ratio.
184.108.40.206 Urban Canyon
Oke (1981) introduced the term “Urban Canyon” and presented detailed quantitative analysis of its energy balance and results of measurements. Studies about urban geometry is focus on analyze the energy budget and human interaction with buildings and streets within cities. But variations within selected area restricted particular research done due to its countless limit of variations. So identifying urban canyon will interpret the section cut of the selected area and it can account the building morphology and energy balance like anthropogenic heat and solar energy within the selected area. Tod hunter (as cited in Emmanuel. 1993) mentioned that, at the microscale, urban geometry is also important in explaining the spatial-temporal distribution of UHI as well as surface materials.
1.3.2Urban Micro Climate Modifications
According to (Seifu 2008) Street Orientation, Density of buildings, Street widths and Building heights, Impact of Shadow and Vegetation, Ground cover, Surface materials & Albedo. When it comes to climate modifications it need to identify what are the possibilities and what are providing the best benefits making urban canyons more habitable in means of thermal budget and energy consumption.
“The global radiation at each specific location and geographical latitude are affected by the street rotation and height-width that observed on daytime periods. It can be observed that east-west orientation has the capacity to make low the extremely temperatures in both locations, and this modification are desired in Tropical Climate.”(Abreu-Harbich, Labaki, & Matzarakis, 2014a) in tropics orientation is important because there is no recognizable seasonal change. So, solar radiation is generally same throughout whole year. So, orientation is the most important factor in urban canyon.
Givoni (1998) states “The orientation of streets affects the urban climate in several ways:
Wind conditions in the urban area as a whole;
Sun and shade in the street s and the sidewalks.
Solar exposure of buildings along the streets
Ventilation potential of the buildings along the streets
This street modification allows to form the street along with the sun path, so it be the most important factor for thermal comfortability inside street. Considering about the geographical situation and street orientation it will be the main contributor for the solar radiation will get into street. On the effect of street orientation to canyon air temperature, the northwest oriented streets are 1 to 2 °C cooler than the east-west oriented streets during day time and no difference by night.(Bourbia & Awbi, 2004) Bourbia and Boucheriba argued that larger SVF makes higher air temperatures, and, higher H/W ratio causes lower air temperatures.(Bourbia & Boucheriba, 2010)
220.127.116.11 Height to Width Ratio (H/W)
Building height and distance between buildings are matter when it comes to analyze the absorption of solar radiation like UV and emitting of IR as heat flux , “In Tropical Climates, when the height-width is more than 2, the constructions shade the facades and sidewalk and the temperatures are cooler than temperatures of urban station. When the height-width is low than 0.5, the buildings cannot shade the sidewalk, influencing the global radiation on surface.”(Abreu-Harbich, Labaki, & Matzarakis, 2014b)
So, buildings made with desired heights effect the net energy balance inside urban canyon, also it will make it easy to trap the radiation inside thus, it will negatively effect upon pedestrian level.
Changes happen in H/W ratio, it effects on SVF, so it has direct impact on solar radiation received by the street canyon. So, regarding the H/W ratio there are urban proposals and rules and regulations which should need to consider when constructing new buildings.
Sky View Factor
The sky view factor (SVF) means the fraction of sky visible from the ground up. SVF is a dimensionless value that ranges from 0 to 1. It can consider as a measurement of incoming radiation to street surface. It means higher the SVF higher the incoming radiation which contact with surface floor, canyon floor typically made with asphalt or material with heavy thermal mass so if it contacted with higher solar radiation it absorbs more energy than other materials thus, it will create heat flux upward.
Figure 15 sky view factor, capured from fish eye lense (“Measuring sky view factor of urban canyons” n.d.)
18.104.22.168 Density of Buildings
Contemporary urban context is highly congested due to lack of land and their price because of the high demand, modern urban structures are mostly made with heavy materials like concrete because of the demand modern urbanity has become a vertical spread than horizontal spread.
“If building density could be used as a daytime cooling strategy in the outdoors, a positive benefit may be derived from the aforesaid rapid urbanization.” (Emmanuel, R., Rosenlund. H & Johansson, E. (2007) it may be possible buildings can shade each other in order to reduce cooling loads and reduce the solar energy being contact with canyon floor surfaces. but it also has negative effects like reducing wind movement inside canyons, so it will increase the nocturnal heating thus, lead to creating Urban Heat islands
The compact buildings formation with less spacing in between them have much more energy consumption, reflection of long waves radiation are more than the sub urban or rural contexts. Due to have more surfaces which can absorb more solar energy it will heat up the urban atmosphere with heat release as long wave radiation.
22.214.171.124 Surface Materials
The materials used in the urban matrix play a key role in causing the distinctive heat gain ( Kannamma, 2012). There is evidence that urban geometry and thermal properties of surface materials in urban areas are the major cause of UHIs (Oke. 1987).
Instead of local natural materials which were practiced and developed over generations, new materials have arrived with so called urbanization. due to it required rapid construction and vary nature of urbanization is a vertical spread than horizontal spread so it requires new innovative quick resulted materials for build up to sky until it reaches limits of particular materials. It was happened due to evolution is happening due to development of technology. So, contemporary factors which use for selecting materials are durability, high strength, light weight, easy erection, low cost, low labor cost, aesthetic etc. but best possible materials which can fit into most of these categories (like Concrete) are have very low performance in handling solar radiation due to their high thermal mass and longer time lag of releasing heat back into atmosphere. This aspect will increase the nocturnal heating in urban atmosphere
Low albedo façade materials in Singapore led to a temperature increase of up to 2.5° C at the middle of a narrow canyon (Payadarshani, 2008). Materials like Concrete classify as low albedo materials because its reflectivity is low and also it can act as good thermal capacitor with high thermal mass due to its higher density.
The term ‘albedo’ describes the proportion of incident radiation reflected by a surface or object. A perfect reflector would have an albedo of 1, whereas a perfect absorber would have an albedo of 0. Albedo is sometimes used to describe the proportion of solar radiation reflected by the earth back into space, and this reflectance of solar radiation is also an important property in the built environment. Natural surfaces like Ice, Water even tree leaves also have component of reflectivity.
The method and quantity of heat release by the urban structures, depends on other controllable factor such as the sky view factor and building material. The design values of albedo are reported as one important factor in creating UHI (Giridharan et al., 2004). Interrelationship between building materials and SVF is highly important in maintaining proper thermal exchange between canyon and outer space, unless it would be effect negatively on thermal comfort on pedestrian levels. Lower albedo will be resulted in higher thermal storage in canyon surfaces thus lead to emit heat during night and with the effect of higher build masses it will resulted low wind speed in night so, with above mentioned two reasons it will caused to have heated night time atmosphere, will negatively effect upon thermal comfortability. Indirect solar gain in buildings can be a significant contributor to overheating. Solar radiation absorbed by the envelope of a building can be transmitted through the fabric of the envelope to the interior. In addition, the urban heat island effect, which refers to higher localized temperatures that are experienced in urban environments compared with surrounding green spaces
The word ‘albedo’ is derived from the Latin word for whiteness, and very broadly, white-colored surfaces have a high albedo, and can be effective in minimizing solar gain as they tend to be poor absorbers of solar radiation and good emitters.(Al-Homoud, 2005). However, color is not always a good indicator of the albedo of a surface as it is determined by reflectance of visible light, rather than other wavelengths of the spectrum, ‘white’ colored roofing shingles and galvanized steel can reach 35oC and 43oC hotter than air temperatures on a sunny day. Conversely, surfaces painted with red or green acrylic paint may be just 22oC hotter, even though they are not visibly bright.(Rosenfeld et al., 1995)
High albedo could make sunlit urban street canyons up to 1.2° C cooler Colombo (Emmanuel and Fernando, 2007). It means white color is not the key in mitigating higher thermal admittance but, it need to have other surface properties like fractioned surfaces and reflectivity by the material itself.
When the sunlight strikes a dark rooftop, about 15% of it gets reflected back into the sky but most of its energy is absorbed into the roof system in the form of heat. Cool roofs reflect significantly more sunlight and absorb less heat than traditional dark-colored roofs(Urban & Roth, 2010) so color is a material property which implemented on surface by its manufacturing process or may be in usage. It can be changed throughout the lifespan of particular material,
There are two properties that are used to measure the effects of cool roofs:
Solar reflectance, also known as albedo, is the ability to reflect sunlight. It is expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation, and a value of 1 represents total reflectivity.
Thermal emittance is the ability to emit absorbed heat. It is also expressed either as a decimal fraction between 0 and 1, or a percentage.
Thermal emittance is fact that won’t change based on usage and it is originated from the material structure itself.
2.1.2 Advantages in Using for Urban Context
It is estimated that pavements and roofs account for 60% of urban surfaces, roofs 20-25% and pavements approximately 40% (Akbari, Menon, & Rosenfeld, 2009). Presently these surfaces have relatively low albedo values and high thermal conductivities, typically absorbing and re-radiating around 90% of the total incident solar radiation (Wolf & Lundholm, 2008)This contributes to an urban heat island effect that can result in a rise in summer temperatures of 4oC-7oC (Wolf and Lundholm 2008).
Current situation is indeed benefitted in forming UHI and thus, effect negatively in thermal comfortability at pedestrian level. So, it need to move from contemporary martials to high albedo materials. Otherwise urban centers will no longer habitable in hot seasons.
it is estimated that changing the albedo of roofs and paved surfaces has the potential to increase the albedo of urban areas by 10%. If this was implemented globally across all urban areas, there would be a negative radiative force equivalent to offsetting 44Gt of CO2 emissions (24Gt by roofs, 20Gt by pavements, Gt=Gigatonn)(Akbari et al., 2009)
2.1.3 Disadvantages in Using for Urban Context
Even though high-albedo used recently as surface material but, 2012 research at University of California, San Diego‘s Jacobs School of Engineering into the interaction between reflective pavements and buildings found that, unless the nearby buildings are fitted with reflective glass or other mitigation factors, solar radiation reflected off light-colored pavements can increase the temperature in nearby buildings, increasing air conditioning demands and energy usage(Yaghoobian & Kleissl, 2012)
Compared to cool materials, the use of highly-reflective facades in urban canyons is less effective because multiple reflections between the walls and the ground (pavements) trap solar radiation in the urban canyon (Panão, Gonçalves, & Ferrão, 2007)Also according (Panão et al., 2007)Numerical studies have also confirmed that diffuse reflections happen inside the urban canyon increase the temperature urban surfaces.
This situation happens due to the very nature of materials reflectivity, most materials which were able to use as façade materials or even in roofs mostly reflected solar radiation at every direction away from the surface. So those reflected solar radiation can reflected multiple times between canyon walls and it caused radiation to trap inside canyon. So, surfaces will be getting heated up so, it will affect negatively in pedestrian level and cooling of building interiors as well.
Chapter three – Urban heat island
3.1 Origin and evolution of UHI
Writing in 1818, British scientist Luke Howard was probably the ﬁrst to note that air temperatures in cities such as, in this case, London, are higher than in undeveloped rural surroundings (Howard, 2007). One of the first scientists to propose that temperature records showed a global warming trend, and that a human-caused greenhouse effect might be the cause of that trend, was British engineer Guy Stewart Callendar (1898-1964). Even at early ages of industrial revolution it was able to identified there is strange phenomena happening in urban areas and also it was lucky to understand that there is human interaction which accelerate the process of global warming.
The existence of Urban heat islands has become a growing concern over the years. An urban heat island is formed when industrial and urban areas produce and retain heat. Much of the solar energy that reaches rural areas is consumed by evaporation of water from vegetation and soil. In cities, where there is less vegetation and exposed soil, most of the sun’s energy is instead absorbed by buildings and asphalt; leading to higher surface temperatures. Vehicles, factories and industrial and domestic heating and cooling units release even more heat. ( Park H.S. 1987)
Many urban and suburban areas experience elevated temperatures compared to their outlying rural surroundings; this difference in temperature is what constitutes an urban heat island. The annual mean air temperature of a city with one million or more people can be 1.8 to 5.4°F (1 to 3°C) warmer than its surroundings,1 and on a clear, calm night, this temperature difference can be as much as 22°F (12°C).2 Even smaller cities and towns will produce heat islands, though the effect often decreases as city size decreases.(T. R. Oke, 1982)
There are two types of heat island effects can be identified, surface and atmospheric urban heat islands These two heat island types differ in the ways they are formed, the techniques used to identify and measure them, their impacts, and to some points, the methods available to mitigate them.
3.1.1 Surface Urban Heat Islands
During hot seasons, urban heat islands are most intense when the sky is clear and winds are calm. Thus, Heavy cloud cover blocks solar radiation, reducing daytime warming in cities. Also, Strong winds increase atmospheric particle circulation in order to regulate the temperature, lowering the urban-rural temperature difference.
To identify urban heat islands, scientists use direct and indirect methods, numerical modeling, and estimates based on empirical models. Researchers often use remote sensing, an indirect measurement technique, to estimate surface temperatures. They use the data collected to produce thermal images,(Tim R. Oke, 1981)
3.1.2 Atmospheric Urban Heat Islands
Warmer air in urban areas compared to cooler air in nearby rural surroundings defines atmospheric urban heat islands. It can divide heat islands into two different types,
Canopy layer urban heat island exist in the layer of air where people live, from the ground to below the tops of trees and roofs.
Boundary layer urban heat island start from the rooftop and treetop level and extend up to the point where urban landscapes no longer influence the atmosphere. This region typically extends no more than one mile (1.5 km) from the surface.(Tim R. Oke, 1981)
Canopy layer urban heat islands are the most commonly observed and are often the ones referred to in discussions of urban heat islands. Atmospheric heat islands vary much less in intensity than surface heat islands. On an annual mean basis, air temperatures in large cities might be 1.8 to 5.4°F (1 to 3°C) warmer than those of their rural surroundings.
Urban climates are warmer and more polluted than their rural counterparts. This difference arises, in part, from the complex combination of surfaces woven together to make up the urban fabric. Among the most evident urban features are residential and commercial buildings, roadways, and parking lots. In the process of industrialization these structures have replaced natural features such as bare soil and vegetation.(Sailor, 1995) mostly modern cities were consist of mega structures which were built with materials with higher thermal masses. There is no going back to classical materials due to urbanity requires that vertical growth and they are spread as much as they can and land is limited.
Differences in the urban fabric are manifested in terms of modifications in the basic surface and substrate characteristics. In terms of the surface energetics, the important characteristics are albedo, moisture availability, vegetative cover, and roughness length. The substrate characteristics of importance are substrate moisture content, thermal conductivity, thermal diffusivity, and density. Each of these parameters plays a role in the heat, moisture, and momentum transfer between the surface and the atmospheric boundary layer.(Sailor, 1995)
Urban areas possess low values of albedo, vegetative cover, and moisture availability. These factors, along with the presence of high levels of anthropogenic heating, are associated with the phenomenon known as the urban heat island (UHI). That is, they explain why urban areas generally act as islands of elevated temperature relative to the natural areas surrounding them.(Sailor, 1995)
3.2.1Factors Causing UHI in Brief
Natural factors, such as changes in the sun’s intensity or slow changes in the Earth’s orbit around the sun
Natural processes within the climate system (e.g. changes in ocean circulation
Human activities that change the atmosphere’s composition (e.g. burning fossil fuels) and the land surface (e.g. deforestation, reforestation, or urbanization)
Considering heavy construction, urban canopy layer heat island is a result of many factors: mentioned by Oke et al., (1982).
Increased absorption of solar radiation (canyon geometry)
Increased long-wave counter-radiation (air pollution)
Decreased net long-wave radiation loss (canyon geometry)
Anthropogenic heat sources (releases into canyon air)
Increased sensible heat storage in urban fabric (materials – thermal admittance)
Decreased evapotranspiration (materials – vegetated area reduction and impervious surfaces)
Reduced Vegetation in Urban Areas
In rural areas, vegetation and open land naturally control the landscape and reduce air temperatures through evapotranspiration, in this process plants release water to the atmosphere, dissipating ambient heat. So, considering about urban areas are characterized by dry, Solid surfaces, such as conventional roofs concrete slabs, sidewalks, roads, and parking lots. More the city develops, more vegetation is lost, and more surfaces are about to covered with buildings. (Dwivedi & Khire, n.d.)
Properties of urban materials, in particular solar reflectance, thermal emissivity, and heat capacity, also influence urban heat island development, as they determine how the sun’s energy is reflected, emitted, and absorbed.(Dwivedi & Khire, n.d.)
Solar reflectance, or albedo, is the percentage of solar energy reflected by a surface. Much of the sun’s energy is found in the visible wavelengths; thus, solar reflectance is correlated with a material’s color. Darker surfaces tend to have lower solar reflectance values than lighter surfaces. Researchers are studying and developing cool colored materials, though, that use specially engineered pigments that reflect well in the infrared wavelengths. These products can be dark in color but have a solar reflectance close to that of a white or light-colored material. (Dwivedi & Khire, n.d.)
Urban areas typically have surface materials, such as roofing and paving, which have a lower albedo than those in rural settings. As a result, built up communities generally reflect less and absorb more of the sun’s energy. This absorbed heat increases surface temperatures and contributes to the formation of surface and atmospheric urban heat islands.(Dwivedi & Khire, n.d.)
Urban geometry influences wind flow, energy absorption, and a given surface’s ability to emit long-wave radiation back to space. In developed areas, surfaces and structures are often at least partially obstructed by objects, such as neighboring buildings, and become large thermal masses that cannot release their heat very readily because of these obstructions. Especially at night, the air above urban centers are typically warmer than air over rural areas. (Dwivedi & Khire, n.d.)
3.3.1 Contemporary UHI Mitigation Strategies
Heat Island mitigation is need for human comfort/outdoor comfort and for reduces global warming. The mitigation actions could be categorized as, related to reducing anthropogenic heat release, better roofing design, other design factors. (Humidification, increased albedo, photovoltaic canopies etc.)
These facts are the basis of most mitigation methods (Kannamma 2012),
The provision of:
shade & shelter (trees, awnings, narrow spaces)
high reflection or emission of radiation surfaces (light surfaces, surface films) surface moisture (water, vegetation, permeable covers)
good or poor heat storage (massive walls, roof insulation)
Although solar reflectance is the main determinant of a material’s surface temperature, thermal emittance, or emissivity, also plays a role. Thermal emittance is a measure of a surface’s ability to shed heat, or emit long-wave (infrared) radiation. All things equal, surfaces with high emittance values will stay cooler, because they will release heat more readily. Most construction materials, with the exception of metal, have high thermal emittance values. Thus, this property is mainly of interest to those installing cool roofs, which can be metallic.(“Reducing Urban Heat Islands,” n.d.)
Another important property that influences heat island development is a material’s heat capacity, which refers to its ability to store heat. Many building materials, such as steel and stone, have higher heat capacities than rural materials, such as dry soil and sand. As a result, cities are typically more effective at storing the sun’s energy as heat within their infrastructure. Downtown metropolitan areas can absorb and store twice the amount of heat compared to their rural surroundings during the daytime.(“Reducing Urban Heat Islands,” n.d.)
Albedo and emissivity are considered “radiative properties.” Heat capacity, on the other hand, is one of several “thermal properties” a material can possess. For thin materials like roofing, which is typically placed over insulation, reflectance and emittance are the main properties to consider, as the heat capacity of a well-insulated roof is low. For pavements, which are thicker than roofing products and are placed on top of the ground, which has its own set of thermal characteristics, designers and researchers need to consider a more complex set of factors that include radiative and thermal properties— such as heat capacity, thermal conductivity, and density.(“Reducing Urban Heat Islands,” n.d.)
Different mitigation measures are having huge financial and environmental benefits. Reduction of anthropogenic heat and planting vegetation on vertical walls of buildings could reduce air temperature up to 1.2 °C and reduce cooling energy demand up to 40% (Kikegawa et al., 2006).
Also, it need to highlight that, The study of the UHI phenomenon in Sri Lanka lags far behind that of even the other tropical countries. The first ever comprehensive historical survey of the UHI phenomenon in the Colombo Metropolitan Region (CMR) was completed only in 1999. More in depth studies for its bioclimatic changes are needed to advance the knowledge base (Emmanuel. 2005).
For quick understanding, it can be listed few researches and its methods used, regarding Highly reflective materials and there results for surface temperature changes.
Summary of influence of HR envelopes on the surface temperatures.
|Reference||Methodology||Surface temperature change|
|||Using CFD simulation and TRNSYS.||In a typical summer period, the surface temperature decreased up to 12°C.|
|||Using weather research and forecasting (WRF) v3.2.1 model.||Summer afternoon temperature in urban locations reduces by 0.11-0.53°C and some rural locations show temperature increases of up to 0.27°C.|
|||Monitoring roof surface units by experiment.||Daytime temperature of gray paint is almost 10°C higher than that of white paint in August.|
|||Installing a cooling roof and measuring temperature and so forth.||From April to September, temperature of roof decreased up to 20°C.|
|||Monitoring white and black roofs.||On a warm day, a reduction of 26°C by white roof in midafternoon occurs.|
|||Applying reflective roofs on two small nonresidential buildings and measuring temperature and so forth.||Average reduction of about 10°C in daytime during the summer period occurs.|
|||Using energy balance model.||Temperature reduction of up to about 15°F in July occurs.|
|||Using TRNSYS software.||Temperature reduction of up to 25°C during summer occurs.|
Table 1 Influence of HR materials on surface temperature (Yuan, Emura, & Farnham, 2016)
|Reference||Methodology||Building type/region||Energy consumption|
|||Using WRF modeling system.||Urban regional climate simulations in U.S.||Winter heating penalty can reduce and roll back or even exceed cooling energy savings in summer.|
|||Using DOE-2 model.||Residential and commercial simulation buildings of 11 U.S. metropolitan statistical areas.||The total savings for all 11 metropolitan statistical areas are annual electricity savings, 2.6 TWh; peak electricity demand savings, 1.7 GW.|
|||Using DOE2.1-E model.||Four commercial building prototypes simulations in 236 US cities.||3.3-7.69 kWh/m2 energy saving for annual cooling, 0.003-0.065 therm/m2 heating penalty for 236 cities in U.S.|
|||Applying the Temperatures of Urban Facets in 3D (TUF3D) model.||TUF3D model simulation domain with buildings and ground with 25 buildings in Southern California.||Overall building design cooling loads near artificial turf (AT) decrease by 15%-20%. The irrigation water conservation with AT causes embodied energy savings of 10 Wh/m2-day. Radiative energy from ground to wall increases with increasing the albedo of the nearby ground materials.|
|||Using EnergyPlus software.||Residential simulation buildings in different localities at Mediterranean latitudes.||Annual energy savings range from -13.7% to 41.7%.|
|||Applying Simplified Transient Analysis of Roofs (STAR) computer code.||Simulations in 16 California climate zones.||Cooling load decreased by about 38% per year; heating penalty increased by about 8.1% per year for Climate Zone 12.|
Table 2 of influence of HR envelopes on energy consumption. (Yuan et al., 2016)
Most methods expressing that using Highly Reflective materials will benefitted in saving energy. So, it will ensure the usage of HR materials as Contemporary mitigation strategy.
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