Designing Urban Heat Island Free Neighbourhoods
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Bartlett School of Planning MSc Urban Design and City Planning Urban Heat Island Supervisor: Ming Cheng Designing Urban Heat Island Free Neighbourhoods - Design Guidance for low Urban Heat Island Neighbourhood Yunwing Yeung Uncontrollable Causes Controllable Causes Anticyclone Conditions Building Materials For Vegetation andBody Water Body, vegetation coverage and evaporation fraction are the two key Vegetation and Water Season Introduction AA factors indicating the effectiveness of their cooling effect. Cities usually have a low vegetation Urban Geometry Urban Heat Island (UHI) is a global phenomenon. This term describes the phenomenon when air/ Natural Wind coverage which lead to a lower surface albedo, a lack of shading against direct solar radiation and Anthropogenic Factors surface temperature of an urban area is higher in comparison to rural areas. The key research Criteria for a Low UHI Neighbourhood Anthropogenic decreased evaporation fraction, thus a Cloud higherCover value (a) Heat on the left side and lower value Qev on the question of this project is: How to design a low UHI neighbourhood in high dense urban centres. Solar RadiationHeat right side of Oke’s Anthropogenic Vegetation and Water Body energy equation. Building Materials The overall aims of this project are listed below: Sensible Heat Solar Radiation Sensible Heat Heat Sensible 1. Investigate how tackling UHI could help to achieve “Smart Cities” 2. Establish a simplistic framework for urban designers to evaluate the UHI condition of Surface aAlbedo given area. Vegetation Coverage Evaporation Fraction Urban Insensible heat (Stored in Sensible Heat water vapour) Heat Island Insensible heat (Stored in Wind water vapour) Downwash Wind Wind Air Pollutant Downwash Wind Anthropogenic Factors Urban Geometry 3. Develop a set of design guidance to help to translate the understanding provided by the Uncontrollable Causes Controllable Causes above framework into practice. Wind Speed SVF 4. Develop a suggested masterplan to illustrate how the above framework and design Air Pollutant AHF API Materials Figure 7: LowBuilding vegetation coverage guidance work in real life situation. Figure 8: High vegetation coverage Anticyclone Conditions Meanwhile, cities Vegetation and Wateralso Bodyhave a low evaporation Seasonfraction. This indicates a decreased rate of evapAnthropogenic Heat is transformed into insensible heat (water vapour). Criteria for a Low UHI Neighbourhood oration carried out in the area, thus less heat Anthropogenic Urban Geometry Natural Wind Heat Solar This leads to a lower value Qev on the right Anthropogenic sideRadiation of Oke’s energy equation. Solar Radiation Heat Vegetation and Water Body Building Materials The Significance of UHI Anthropogenic Factors Cloud Cover Sensible Heat Sensible Heat Solar Radiation UHI has significant negative impact on living and working environment in cities. High intensity of Surface of heat stroke UHI could significantly decrease the efficiency of workers and increase the chance Vegetation Coverage Albedo for people working outdoors (Girdharan et al., 2004). Meanwhile UHI has strong links with in- Evaporation Fraction Wind Insensible heat (Stored in Wind water vapour) Downwash Wind Downwash Wind Uncontrollable Wind Causes Air Pollutant Air Pollutant Downwash Wind Anthropogenic Factors Urban Geometry creased mortality rate in cities (figure 2). Meanwhile, UHI also has significant impact on peoples’ Controllable Causes behaviour on water and energy usage. From a study on Pheonix City household water usage, Guhathakurta and Gober (2007) found there is an average increase of 1.7% household water usage Wind Speed SVF Sensible Heat Sensible Heat Heat Sensible Insensible heat (Stored in Insensible heat (Stored in water vapour) Sensible Heat water vapour) Urban Heat Island Air Pollutant Criteria for a Low UHI Neighbourhood API Vegetation and Water Body Figure 9: Low evaporation fraction Season AHF o when temperature increased by 1 C. Meanwhile the study conducted by USEPA (2008) found the Vegetation and Water Body dicate how urban geometry is contributing to the UHI intensity of an area. SVF is a factor describ- ing from 20 to 25oC (figure 3). Anthropogenic Factors Cloud Cover ing the amount of sky you can observe on ground floor. Cities usually have a low SVF. This traps Surface the long-wave Albedo It is clear that tackling UHI is crucial to increase energy and resource efficiency and to deliver bet- Vegetation Evaporation solar radiation within urban Coverage Fractionfabrics. This leads to a lower value of Q1 on the right sideUrban of Oke’s energy equation. Anthropogenic Factors Geometry ter living and working environment in cities. Anthropogenic Heat Heat Anthropogenic Solar Radiation Radiation Solar Urban Heat Island AHF Wind Speed SVF Sensible Heat Heat Sensible API Sensible Heat Heat Sensible Insensible heat heat (Stored (Stored in in Insensible water vapour) water vapour) Wind UncontrollableWind Causes Causes Criteria for a Controllable Low UHI Neighbourhood Downwash Wind Wind Downwash Anticyclone Conditions Building Materials Vegetation and Water Body Building Materials Air Pollutant Pollutant Air Vegetation Figure 11: Low SVF and Water Body Figure 2: Temperature vs Mortality Rate Graph (Wong et al. 2013) Figure 3: The relation between electric load and maximum daily temperature (EPA, 2008) Season Figure 12: High SVF Anthropogenic Heat Meanwhile, urban structures in cities could restrict Urban Geometry Natural Wind wind flow thus lower wind speed. This has de- Surface Albedo Vegetation Evaporation Radiation creased the rate of heatFraction loss by convection inSolar cities, thus lower value Qac on the right side of Oke’s Coverage Anthropogenic Factors energy Urban Geometry Cloud Cover equation. Anthropogenic Factors Sensible Heat Heat Anthropogenic Anthropogenic Heat Urban Heat Island Sensible Heat Solar Radiation Radiation Solar Insensible heat (Stored in Sensible Heat Sensible Heat water vapour) Energy Basis of UHI SVF AHF Wind Speed API Controllable Causes The key to understand the formation of UHI lies on the understanding of energetic balance between Wind Sensible Heat Sensible Heat UncontrollableInsensible Causes heat Downwash Wind(Stored in Insensible heat (Stored in water vapour) vapour) water Air Pollutant Anticyclone Conditions Wind Wind Building Materials Downwash Wind Downwash Wind Season Vegetation and Water Body cities and its surrounding environment. The ener- Air Pollutant Pollutant Air Criteria for a Low UHI Neighbourhood Urban Geometry Figure 13: Restricted wind flow (low wind speed) gy on Earth is largely determined by the amount of Natural Wind For Anthropogenic Factors, anthropogenic flux (AHF) and air pollution index (API) are the Anthropogenic Factors Cloudheat Cover stant. Therefore the problem of UHI is the concen- two key factors that could indicate how anthropogenic factors are contributing to the UHI intensi- tration of this solar energy in a form of heat energy could clearly illustrate this concept: ty of an area. AHF is a factor describing the amount of waste heat released by human activities. Surface Albedo in cities. Oke’s (1982) energetic balance equation Figure 4: Oke’s energy equation Vegetation Coverage Cities Evaporation Fraction usually have a high AHF as a result of Anthropogenic high population density. This leads to a higher value Heat of Qa on theFactors left side Anthropogenic Urban Geometry of Oke’s energy equation. Solar Radiation Urban Heat Island Sensible Heat In cities, the value (a) and Qa on the left side are higher while Q1, Qac and Qev are lower. As the result. Qslab is higher to balance out the equation. This higher value of Qslab represents UHI formation SVF Anthropogenic Sensible Heat Heat AHF Wind Speed in cities. There are four controllable causes Building Materials Urban Heat Island which responsible for this energetic Vegetation and Water Body imbalance in cities. Urban Geometry Anthropogenic Factors Uncontrollable Causes Controllable Causes Insensible heat (Stored in Solar Radiation vapour) Uncontrollablewater Causes Sensible Heat Wind Building Materials Criteria for a Low UHI Neighbourhood Controllable Causes API Controllable Causes Urban Heat Island Conceptual Framework Building Materials Uncontrollable Causes Surface Albedo Season Natural Wind Urban Geometry Cloud Cover Insensible heat (Stored in Air Pollutant Vegetation and Water Body Vegetation and Water Body Urban Figure 15: High AHF Geometry Anticyclone Conditions Anticyclone Conditions Downwash Wind Sensible Heat Season water vapour) Wind Speed SVF Criteriaby forbuildings. a Low UHICities Neighbourhood absorbed usually have a low surface Anthropogenic Heatalbedo which lead to high solar energy Vegetation and Water Body Season Figure 16: Low AHF Downwash Wind Meanwhile, air Factors pollution index (API) is aCloud factor the degree of air pollution in an area. Anthropogenic Air describing Pollutant Cover Vegetation Coverage Cities usually have a high API because of heavy traffic and various other human activities. This Evaporation Fraction Heat traps short wave heat raidation thus leads to aAnthropogenic higher value of (a) on the left side of Oke’s energy Anthropogenic Factors equation. Solar Radiation Sensible Heat Sensible Heat AHF API absorption thus a higher value (a) on the left side of Oke’s energy equation. Vegetation and Water BodySolar Radiation Building Materials Insensible heat (Stored in water vapour) Wind Anthropogenic Heat Natural Wind Downwash Wind Sensible Heat Solar Radiation Anthropogenic Factors Wind Natural Wind For Building Materials, surface albedo is the Conditions critical factor affecting the amount of solar energy Anticyclone Building Materials Urban Geometry Figure 14: Effective wind flow (high wind speed) Vegetation and Water Body Building Materials solar energy received, which is more of less con- Air Pollutant Cloud Cover Sensible Heat Sensible Heat Surface Albedo Urban Geometry Wind Speed SVF Figure 5: Low albedo material Vegetation Coverage Insensible heat (Stored in Evaporation Sensible Heat water vapour) Fraction Anthropogenic Factors AHF API Anthropogenic Heat Anthropogenic Heat Figure 17: High API Insensible heat (Stored in Wind water vapour) Downwash Wind Wind Air Pollutant Downwash Wind Solar Solar Radiation Radiation Air Pollutant these seven Sensible Heat Sensible Heatfactors, we could understand Figure 6: High albedo material For Vegetation and Water Body, vegetation coverage and evaporation fraction are the two key Criteria for a Low UHI Neighbourhood Building Materials Vegetation and Water Body the UHI condition in that given area. The coverage which lead to a lower surface albedo, a lack of shading against direct solar radiation and a low UHIheat neighbourhood Insensible (Stored Insensible heat (Stored in inshould perform in water water vapour) thesevapour) seven factors. decreased evaporation fraction, thus a higher value (a) on the left side and lower value Qev on the Wind Wind right side of Oke’s energy equation. Downwash Downwash Wind Wind factors indicating the effectiveness of their cooling effect. Cities usually have a low vegetation Figure 18: Low API By assessing how an area is performing on Sensible Heat Sensible Heat criteria framework on the right tells us how Air Air Pollutant Pollutant W W S A D DA S S AA I w S Figure 10: High evporation fraction Materials ForBuilding Urban Geometry, sky view factor (SVF) and wind speed are the two key factors that could inUrban Geometry Natural Wind electricity demand for cooling increase of 1.5-2.0% for every 0.6oC increase in temperature, start- S AA S SS In w SS W SS A D I In A S ww S D I w Anticyclone Conditions Building Materials S Surface Albedo Vegetation Coverage Urban Geometry SVF Wind Speed Evaporation Fraction Anthropogenic Factors AHF API D Bartlett School of Planning MSc Urban Design and City Planning Supervisor: Ming Cheng Designing Urban Heat Island Free Neighbourhoods - Design Guidance for low Urban Heat Island Neighbourhood Yunwing Yeung Criteria for a Low UHI Neighbourhood Design Guidance for a low UHI neighbourhood Criteria for a Low UHI Neighbourhood Proposal Vegetation and Water Body Building Materials Base on this criteria framework, from case studies and literature review, a set of design guidance Vegetation and Water Body Building Materials A suggested masterplan is produced are drawn according to the seven criteria. This is the toolkit for urban designers to respond to the comprising mixed design guidance understanding they gained from the framework. responding to all seven factors. It aims to illustrate how the design guidance could be integrated as part Criteria for a Low UHI Neighbourhood Anthropogenic Factors Anthropogenic Factors API Criteria for a Low UHI Neighbourhood of an urban regeneration project. Vegetation and Water Body 65% Vegetation and Water Body tively increase vegetation coverage. Vegetation and Water Body Dry fountain and pervious pave- Criteria for a Low UHI Neighbourhood Vegetation Coverage 1. Cool surfaces, green walls and green roofs are effective methods to increase energy efficiency Urban Geometry of buildings 2. Seperate residential landuse from landuse with high waste heat create Speed SVFoutput andWind green buffer between them. 1. Cool surfaces, green walls and green roofs are effective methods to increase energy efficiency of buildings 3. Arrange gas pipes in a way to minimize waste heat exhaust at street level (at least above 4 storeys) 2. Seperate residential landuse from landuse Anthropogenic Factors Surface Albedo AHF API Wind Speed SVF Criteria for a Low UHI Neighbourhood Vegetation and WaterBuilding Body Materials Criteria for a Low UHI Neighbourhood Criteria for a Low UHI Neighbourhood Vegetation and Water Body Building Materials AHF 30m 30m 6m 6m 4. Buildings should be lower than 30m to allow 4.Cool Buildings should be lower than 30m to allow 2. Seperate residential landuse frominlanduse 1. surfaces, green walls green roofs are 3.and Arrange gas pipes a way to minimize effective downwash effect effective downwash effect with high waste heat output and create effective methods to increasewaste energy efficiency heat exhaust at street level (at least of buildings green buffer between above 4them. storeys) API Vegetation and Water Body 0.15-0.30 283-289 Wm-2 Building Materials Urban Geometry SVF 3. Streets should be allign with major wind 3. Streets should be allign with major wind Cool surfaces, green walls and waste green roofs are 2. Seperate residential landuse from landuse direction cool roofs 1.could effectively minimize heat exhaustdirection effective methods increase efficiency withto high waste energy heat output and create buildings SVF green buffer between AHF ed on APIstreetoflevel. Wind Speed them. Evaporation Fraction Anthropogenic Factors Vegetation Coverage Evaporation Fraction Evaporation Fraction API Wind Speed Very low Elimination of back alleys together with green roofs and Wind Speed 2. Seperate residential landuse from landus with high waste heat output and create green buffer between them. 1.Pedestrianization Plant street trees or create green buffer along roads with heavy traffic (API above could help to de55-60) crease the amount ofDecrease traffic within the of traffic in areas w 2. the amount high API (55-60) site. 5. Always avoid walls of tall building (over 5. Always avoid walls of tall building (over 30m). Leave (at1.least 6m 3.openings Arrange gas pipes instreet awide) way to minimize 2. Seperate residential landuse from landuse Plant trees or create green buffer 30m). Leave openings (at least 6m wide) between tall buildings. waste exhaust at street level (at least with high waste heat heat output and create along roads with heavy traffic (API above between tall buildings. above 4them. storeys) green buffer between 55-60) Conclusion 60-70 0-0.4ms-1 Very low Vegetation Coverage Evaporation AHF SVF Vegetation Surface Coverage Albedo 0% 283-289 Wm-2 0.15-0.30 Anthropogenic Factors Vegetation Coverage Anthropogenic Factors Anthropogenic Factors Urban Geometry 0% Vegetation and Water Body 0-0.4ms-1 Vegetation and Water Body 1. Cool surfaces, green walls and green roo effective methods to increase energy effic of buildings Vegetation shouldfeatures be Surface arranged with respect to the wind analysis and design guidance to Evaporation Evaporation 2.Buildings Reduce overstreet on building walls 3. Arrange gas pipes in a way to minimize 2. Fraction Reduce overstreet features on building walls Albedo Coverage Fraction 1. Cool surfaces, green walls and green roofs are waste heat exhaust at street level (at leas maximize wind speed. Friction of wall tofeatures imposing on wind should be minimized. effective methods increase energy efficiency above 4 storeys) Anthropogenic Factors Anthropogenic Factors Geometry of Urban buildings Mongkok SVF 1. With the avoid same mass,ofbuild taller with large set 5.back Always tall building (over (podiums)walls is the optimal built form back is the optimal built 30m).(podiums) Leave openings (at least 6mform wide) API Surface Albedo Urban Geometry 3. The volume of water body should be higher than 20m3 to have measurable cooling effect Vegetation Coverage Figure 19: Location of the Sai Yeung Choi Street site Evaporation Wind Speed Fraction Vegetation and Water Body 0.15-0.35 6m AHF Building Materials Vegetation and Water Body 1. With the same mass, build taller with large set Building Materials WindAPI Speed Surface Albedo Urban Geometry Urban Geometry Very low 4. should lower than 30m to allow 5. Always walls of tallwith building (over 3. Buildings Streets avoid should bebeallign major wind 5.effective Always avoid walls of tall building (over downwash effect 30m). Leave openings (at least 6m wide) direction 30m). Leave openings between tall buildings. (at least 6m wide) between tall buildings. between tall buildings. Evaporation Fraction SVF 3. Streets should be allign with major wind 2. direction Reduce overstreet on building walls 4. Buildings should befeatures lower than 30m to allow 4. effective Buildingsdownwash should be effect lower than 30m to allow effective downwash effect 20m3 AHF SVF 0.15-0.35 Building Materials 0.15-0.30 Vegetation Coverage API 2. Besides vegetation, other techniques that could hold run down water such as pervious pavement should be applied 5. Always avoid walls of tall building (over 4. 30m). Buildings should be lower than 30m to allow Leave openings (at least 6m wide) effective tall downwash effect between buildings. Criteria for a Low UHI Neighbourhood Criteria for a Low UHI Neighbourhood 0% 0% AHF 3. The volume of water 1. Water body should streaming or body should be h 20m3be constantly 20m3the to have measurable cooling dispersed like a fountain to than maximize effect cooling effect Wind Speed 6m 30m Sai Yeung Choi Street site Vegetation and Water Body 3. The volume of water body s 20m3leaves 6. Choose plants with deciduous The volumet 2. Besides vegetation, other techniques than 20m3 to3.have measura 20m3 (species that store more water) to could hold effect run down waterthan such20m3 as per effect pavement should be applied Surface Evaporation Albedo Fraction 30m 30m 2. With Reduce overstreet features on building walls 1. theshould same mass, build taller with large set 3. Streets be allign with major wind (podiums) is the optimal built form 3.back Streets should be allign with major wind direction direction 5. Intensive green roof is more effective2.than 3. The volume Besides other o te 20m3 vegetation, extensive green roof. Soil1.moisture should be 2. Besides veg 3. The volume of water body than 20m3 to s Water should be run constantly stream hold down water 20m3 bodycould kept at a high level at all time (65% moisture is could hold ru than 20m3 to have effect dispersed like a fountain to maximize th pavement should be measu applied optimum) pavement sh effect cooling effect the design guidance into account to maximize API SVF Wind Speed Very low Surface Albedo API Anthropogenic Factors The project site, Sai Yeung Choi Street site is located in Mongkok, Hong Kong. ItVegetation is an extremely and Water Body 0.15-0.35 S 1. With the same mass, build withshould large settake Vegetation Evaporation Building massing and taller height Coverage (podiums) is the optimalonbuilt form walls 2.back Reduce overstreet features building Fraction 2.SVF Reduce features on building walls onoverstreet ground floor level. 30m 6m 6m Vegetation Coverage AHF 1. 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Ensure vegetation is maintain on a regularcool surface openand ground with less s basis, providing sufficient irrigation sunlight to optimize the cooling effect SVF Evaporation Fraction API Criteria for a Low UHI Neighbourhood HI Neighbourhood Building Materials S tion and Water Body BuildingVegeta Materials Vegetation and Water Body W 5. Always avoid walls of tall building (over Urban Geometry 30m). Leave openings (at least 6m wide) between tall buildings. Evaporation Vegetation Coverage Evaporation Fraction 65% 6m Evaporation Vegetation FractionCoverage S 4. Green facades should place on walls with sufficient sunlight (facing south and west) 3. Implement green roofs and green walls in Windopen Speed areas with less availableSVF space for green Wind Speed SVF spaces 5. Intensive green roof is more effective than extensive green roof. Soil moisture should be kept at a high level at all time (65% moisture is 4. Green facades should place on walls with optimum) sufficient sunlight (facing south and west) Surface Albedo Vegetation and Water Body API Building Materials 2. Place street trees on paved open ground with less shading and along roads 1. Ensure vegetation is maintain on a regular basis, providing sufficient irrigation and sunlight to optimize the cooling effect 3. Implement green roofsSurface and green walls in Surface Albedo areas with less available space for green open Albedo spaces 2. Place street trees on paved open Urbanground Geometry with less shading and along roads Urban Geometry Vegetation Coverage API Vegetation and Water Body Anthropogenic Factors AHF AHF AHF 5. Always avoid walls of tall building (over Old buildings are suggested to be renovated. Building 30m). Leave openings (at least 6m wide) Neighbourhood between tall buildings. and rooftops should be covered with cool surfaces. 3. Streets should be allign with major wind Criteria for a Low UHI direction Building Materials 1. Ensure vegetation is maintain on a regular basis, providing sufficient irrigation and sunlight 30m to optimize the cooling effect Criteria for a Low UHI Neighbourhood Criteria for a Low UHI Neighbourhood 4. Buildings should be lower than 30m to allow API downwash effect Vegetation and effective Water Body Building Materials W AHF 6m Evaporation Evaporation Fraction Fraction Anthropogenic Factors Anthropogenic Factors ometry ometry Vegetation Coverage 4. Buildings should be lower than 30m to allow effective downwash effect 2. Reduce overstreet features on building walls API AHF Vegetation Vegetation Coverage Coverage Wind Speed Wind Speed 30m Vegetation and Water Body Vegetation and Water Body AHF Wind Speed aterials aterials Surface Albedo ia for a Low UHI Neighbourhood ia for a Low UHI Neighbourhood Anthro Urban Geom etrypogenic Factors 3. Streets should be allign with major wind direction 1. With the same mass, build taller with large set back (podiums) is the optimal built form 3. Cool surface are most effective in low-rise neighbourhood (below 4 storeys) Criteria for a Low UHI Neigh bourh oodUHI Neighbourhood Crite ria for a Low formAHF could maximize canopy API level wind speed. Wind Speed Wind Speed SVF Different models illustrating how different urban Wind Speed SVF SVF Literature Review: Ng et al. (2008), etc. 2. Reduce overstreet features on building walls Evaporation Fraction Anthropogenic Factors Building Materials 2. Implement cool surfaces on over 70% of roofs to have measurable effect on outdoor temperature Surface Albedo Anthropogenic Factors Urban Geometry API API Vegetation Coverage Urban Geometry Urban Geometry 70% AHF AHF Surface Albedo SVF Wind Speed Wind Speed SVF SVF Vegetation Evaporation 1. With the same mass, build taller with large set Coverage back (podiums) is Fraction the optimal built form Evaporation Fraction Anthropogenic Factors Anthropogenic Factors 1. Implement cool surfaces on roofs and Surface paved Albedo open ground with less shading Urban Geometry Urban Geometry Vegetation Coverage 3 Scenarios illustrating how different urban form Vegetation and Water Body AHF API Wind Speed AHF affecting API Wind SpeedSVF. Building Materials SVF Evaporation Evaporation SVF Fraction Fraction Surface Albedo Vegetation Vegetation Coverage Coverage Anthropogenic Factors Surface Surface Albedo Albedo Vegetation and Water Body Building Materials Literature Review: Santos et al. (2003), etc. Urban Geometry Urban Geometry Urban Geometry AHF Vegetation and Water Body Vegetation and Water Body Evaporation Evaporation Fraction Fraction Wind Speed Building Materials Building Materials Vegetation Vegetation Coverage Coverage SVF Surface Surface Albedo Albedo Criteria for a Low UHI Neighbourhood Criteria for a Low UHI Neighbourhood This 3. Arrange gas1.pipes a waytrees to minimize Plantinstreet or create buffer 2. Decrease thegreen amount of traffic in areas with waste heat exhaust at street level (at least along roads with heavy traffic (API above project provides a simplistic toolkit for urban designers to respond high API (55-60) above 4 storeys) 55-60) to UHI in future urban de- sign projects. Nevertheless, urban design quality of these design guidance haven’t been explored in depth in 1. this project. Further consideration Plant street trees or create green buffer on this is required when applying this toolkit. 2. Decrease the amount of traffic in areas with along roads with heavy traffic (API above high API (55-60) 55-60) AHF 2. Decrease the amount of traffic in areas with high API (55-60)
